Ap-t65 06 Asphalt Pavement

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AP-T65/06

AUSTROADS TECHNICAL REPORT

Asphalt Paving

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Asphalt Paving

Asphalt Paving First Published November 2006

© Austroads Inc. 2006 This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without the prior written permission of Austroads.

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Asphalt Paving ISBN 1 921139 62 5

Austroads Project No. TP1054 Austroads Publication No. AP–T65/06

Project Manager Gary Liddle, VicRoads Prepared by John Rebbechi

Published by Austroads Incorporated Level 9, Robell House 287 Elizabeth Street Sydney NSW 2000 Australia Phone: +61 2 9264 7088 Fax: +61 2 9264 1657 Email: [email protected] www.austroads.com.au

Austroads believes this publication to be correct at the time of printing and does not accept responsibility for any consequences arising from the use of information herein. Readers should rely on their own skill and judgement to apply information to particular issues.

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Asphalt Paving

Sydney 2006

Austroads profile Austroads is the association of Australian and New Zealand road transport and traffic authorities whose purpose is to contribute to the achievement of improved Australian and New Zealand road transport outcomes by: ƒ ƒ ƒ ƒ ƒ

undertaking nationally strategic research on behalf of Australasian road agencies and communicating outcomes promoting improved practice by Australasian road agencies facilitating collaboration between road agencies to avoid duplication promoting harmonisation, consistency and uniformity in road and related operations providing expert advice to the Australian Transport Council (ATC) and the Standing Committee on Transport (SCOT).

Austroads membership

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Austroads membership comprises the six state and two territory road transport and traffic authorities and the Australian Department of Transport and Regional Services in Australia, the Australian Local Government Association and Transit New Zealand. It is governed by a council consisting of the chief executive officer (or an alternative senior executive officer) of each of its eleven member organisations: ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ

Roads and Traffic Authority New South Wales Roads Corporation Victoria Department of Main Roads Queensland Main Roads Western Australia Department for Transport, Energy and Infrastructure South Australia Department of Infrastructure, Energy and Resources Tasmania Department of Planning and Infrastructure Northern Territory Department of Territory and Municipal Services Australian Capital Territory Australian Department of Transport and Regional Services Australian Local Government Association Transit New Zealand

The success of Austroads is derived from the collaboration of member organisations and others in the road industry. It aims to be the Australasian leader in providing high quality information, advice and fostering research in the road sector.

Asphalt Paving

SUMMARY In 2004, Austroads commenced a review of all technical publications with a view to compiling existing information into a series of publications using a common format. Part 4B of the Guide to Pavement Technology provides an overview of asphalt materials, manufacture and placing. This technical report is one of a series that supports Part 4B as follows: Guide to Pavement Technology – Part 4B: Asphalt Appendix A

Asphalt mix design procedures

Appendix B

Framework specification for asphalt and asphalt recycling

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Technical Reports AP-T62/06

Introduction to asphalt mix design

AP-T63/06

Asphalt characterisation for pavement design

AP-T64/06

Asphalt manufacture

AP-T65/06

Asphalt paving

AP-T66/06

Asphalt recycling

AP-T67/06

Maintenance of asphalt surfacings

This document is intended to provide readers with a working knowledge of paving operations and is considered best practice at the time of writing. Text used in this document is based primarily on information originally prepared for the Austroads Asphalt Guide (AP-G66/02) (2002). A section on planning the paving operation provides information on the development of traffic control plans and the need for paving trials particularly for large projects. The problems associated with working in an urban environment are discussed with emphasis on night work. The preparation of the existing surface can have a significant effect on the quality and durability of the asphalt laid. Advice is given on cleaning and correction of existing defects before placing the new asphalt. Cold milling and pre-treatment of granular and concrete surfaces are covered. A brief discussion of the risks associated with paving in adverse weather is provided. It is considered best practice to apply a tack coat to all but granular (or rock) surfaces prior to the placement of fresh asphalt and a brief discussion on this topic is included. The transport and transfer of asphalt to the paver is discussed with a view to achieving the best outcome whilst being aware that the scale of the project will influence some of the options. Materials transfer vehicles have been shown to reduce the roughness of the asphalt layer but can substantially raise the price of the project. An extensive section is provided on paver operation. The correct operation of the paver is critical in achieving the best outcome and the adjustments and procedures necessary to obtain an acceptable outcome are discussed in detail. Handwork and joints are examined as these often have a large impact on the overall quality of a project.

Austroads 2006 — i—

Asphalt Paving

Compaction equipment is covered. Compaction temperature is an important factor in attaining an adequate level of compaction and guidance is provided on the number and types of roller required for various situations. Special mention of differences in compaction techniques for stone mastic asphalt, open graded asphalt and thin and deep lift asphalts is provided. The finished properties of the asphalt mat are discussed and these cover tolerances on shape, mat thickness, ride quality and compacted density.

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Extensive checklists are provided at the rear of the document covering typical activities in an asphalt paving operation. The document concludes with advice on identifying and rectifying problems.

Austroads 2006 — ii —

Asphalt Paving

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CONTENTS 1

PLANNING AND PREPARATION FOR ASPHALT PAVING ...............................................1

1.1 1.2 1.3 1.4 1.5 1.6

General...................................................................................................................................1 Planning..................................................................................................................................1 Paving Trials...........................................................................................................................2 Traffic Control Plans ...............................................................................................................2 Night Work..............................................................................................................................3 Audit and Surveillance of Asphalt Paving Contract Work.......................................................3

2

PREPARATION OF PAVEMENT SURFACE ........................................................................4

2.1 2.2 2.3 2.4 2.5 2.6 2.7

Cleaning .................................................................................................................................4 Correction of Defects in Bituminous Surfaces ........................................................................4 Correction of Defects in Concrete Surfaces ...........................................................................5 Cold Milling .............................................................................................................................5 Pre-treatment of Granular Pavements....................................................................................7 Pre-treatment of Concrete Surfaces.......................................................................................8 Public Utilities .........................................................................................................................8

3

CLIMATIC CONDITIONS.......................................................................................................9

3.1 3.2 3.3 3.4

General...................................................................................................................................9 Pavement Temperature..........................................................................................................9 Moisture..................................................................................................................................9 Risk Management...................................................................................................................9

4

TACK COATING ..................................................................................................................10

4.1 4.2 4.3

General.................................................................................................................................10 Application ............................................................................................................................10 Precautions...........................................................................................................................11

5

TRANSPORT OF ASPHALT ...............................................................................................12

5.1 5.2 5.3 5.4

General.................................................................................................................................12 Vehicles ................................................................................................................................12 Release Agents ....................................................................................................................13 Organisation .........................................................................................................................13

6

TRANSFER OF ASPHALT FROM TRUCKS TO PAVER ...................................................14

7

PAVER PERFORMANCE AND SCREED OPERATION .....................................................15

7.1 7.2 7.3

General.................................................................................................................................15 Tractor Unit...........................................................................................................................16 Screed Unit...........................................................................................................................17 7.3.1 Angle of inclination .................................................................................................18 7.3.2 Volume of material in front of screed ......................................................................19 7.3.3 Paving speed..........................................................................................................20 7.3.4 Primary compaction by screed unit ........................................................................21 7.3.5 Crown adjustment of screed...................................................................................21 7.3.6 Screed extensions ..................................................................................................21 7.3.7 Care of screed ........................................................................................................22 Automatic Sensing and Levelling Equipment .......................................................................22 Paver Stoppages ..................................................................................................................22

7.4 7.5

Austroads 2006 — iii —

Asphalt Paving

8

SPREADING OPERATIONS ...............................................................................................22

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8.1 8.2 8.3 8.4 8.5 8.6 8.7

General.................................................................................................................................22 Setting Out............................................................................................................................22 Spreading by Paver ..............................................................................................................22 Spreading by Grader ............................................................................................................22 Spreading by Hand...............................................................................................................22 Layer Thickness ...................................................................................................................22 Joints ....................................................................................................................................22 8.7.1 Longitudinal joints...................................................................................................22 8.7.2 Wedge joints...........................................................................................................22 8.7.3 Transverse joints ....................................................................................................22 8.8 Paving Output.......................................................................................................................22 8.9 Automatic Level Control .......................................................................................................22 8.9.1 Joint matching shoe................................................................................................22 8.9.2 Levelling (or averaging) beam ................................................................................22 8.9.3 Fixed wire ...............................................................................................................22 8.9.4 Crossfall control......................................................................................................22 8.9.5 Computerised level control .....................................................................................22 8.9.6 Laser control...........................................................................................................22 8.10 Service Fittings .....................................................................................................................22 8.11 Safety During Paving Operations .........................................................................................22 9

COMPACTION .....................................................................................................................22

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14 9.15 9.16 9.17 9.18

General.................................................................................................................................22 Compaction Equipment ........................................................................................................22 Mix Temperatures for Placing...............................................................................................22 Determination and Use of Temperature Profiles ..................................................................22 Roller Numbers and Speed ..................................................................................................22 Rolling Procedures ...............................................................................................................22 Rolling of Transverse Joints .................................................................................................22 Rolling of Longitudinal Joints................................................................................................22 Initial Rolling .........................................................................................................................22 9.9.1 Unsupported edges ................................................................................................22 Intermediate Rolling..............................................................................................................22 Final Rolling..........................................................................................................................22 Special Techniques ..............................................................................................................22 Rolling Pattern ......................................................................................................................22 Hand Compaction.................................................................................................................22 Compaction of Open-graded Asphalt ...................................................................................22 Compaction of Deep Lift Asphalt ..........................................................................................22 Compaction of Stone Mastic Asphalt....................................................................................22 Placing and Compaction of Ultra Thin Open-graded Asphalt Surfacing...............................22

10

FINISHED PAVEMENT PROPERTIES................................................................................22

10.1 10.2 10.3 10.4 10.5

General.................................................................................................................................22 Thickness and Level.............................................................................................................22 Shape ...................................................................................................................................22 Riding Quality .......................................................................................................................22 Compacted Density ..............................................................................................................22 10.5.1 General...................................................................................................................22 10.5.2 Testing of density using nuclear density gauge......................................................22

Austroads 2006 — iv —

Asphalt Paving

11

FIELD OPERATIONS CHECK-LIST....................................................................................22

12

TROUBLE SHOOTING GUIDE............................................................................................22

FURTHER READING ....................................................................................................................22

TABLES

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Table 2.1: Table 8.1: Table 9.1: Table 9.2:

Guide to selection of profiler .......................................................................................7 Typical asphalt layer thickness .................................................................................22 Asphalt spreading temperatures ...............................................................................22 Typical temperatures for placing and compacting of asphalt containing various binders............................................................................22 Table 9.3: Number of rollers.......................................................................................................22 Table 9.4: Typical rolling sequence............................................................................................22 Table 10.1: Typical permissible tolerances in shape ....................................................................22 Table 10.2: Typical in situ air voids (dense graded asphalt).........................................................22 Table 10.3: Typical relative compaction (bulk density) .................................................................22

Austroads 2006 — v—

Asphalt Paving

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FIGURES Figure 2.1: Figure 2.2: Figure 4.1: Figure 5.1: Figure 6.1: Figure 7.1: Figure 7.2: Figure 7.3: Figure 7.4: Figure 7.5: Figure 7.6: Figure 7.7: Figure 7.8: Figure 7.9: Figure 8.1: Figure 8.2: Figure 8.3: Figure 8.4: Figure 8.5: Figure 8.6: Figure 8.7: Figure 8.8: Figure 8.9: Figure 8.10: Figure 8.11: Figure 8.12: Figure 8.13: Figure 9.1: Figure 9.2: Figure 9.3: Figure 9.4: Figure 9.5: Figure 9.6: Figure 9.7: Figure 9.8: Figure 10.1: Figure 10.2:

Rotary road broom ...................................................................................................4 Shape correction ......................................................................................................5 Tack coat sprayer in operation ...............................................................................10 A ‘Flocon’ non-tipping delivery truck.......................................................................12 Materials transfer device ........................................................................................14 Typical paver ..........................................................................................................15 Typical paver components .....................................................................................15 Flow of material through paver...............................................................................16 Components of screed unit ....................................................................................17 Factors influencing vertical position of the free-floating screed..............................18 Response to change in angle of attack ..................................................................19 Screed response to a step-function disturbance ....................................................19 Head of material .....................................................................................................20 Paver with hydraulic screed ...................................................................................22 Handwork ...............................................................................................................22 Overlapping of longitudinal joints in successive courses .......................................22 Overhang of outer edge .........................................................................................22 Poor roller placement .............................................................................................22 Poor roller place .....................................................................................................22 Paving in echelon (hot joints) .................................................................................22 Cutting disc attached to steel roller ........................................................................22 Correct overlap of longitudinal joint ........................................................................22 Transverse joint construction .................................................................................22 Mix deliveries required to match paving machine speeds for various compacted depths ................................................................................22 Joint matching shoe ...............................................................................................22 Multiple skid beam .................................................................................................22 Fixed wire ...............................................................................................................22 Vibratory steel wheeled roller .................................................................................22 Pneumatic-tyred roller ............................................................................................22 Vibratory plate ........................................................................................................22 Asphalt temperature profile curve ..........................................................................22 Rolling of transverse joint .......................................................................................22 Compacting longitudinal joint .................................................................................22 Operation of rollers.................................................................................................22 Typical rolling pattern .............................................................................................22 Laser profiler ..........................................................................................................22 Nuclear density testing ...........................................................................................22

Austroads 2006 — vi —

Asphalt Paving

1

PLANNING AND PREPARATION FOR ASPHALT PAVING

1.1 General Good organisation is required to ensure efficiency and smoothness of asphalt paving work. Equipment must be suitable to the task and personnel trained and skilled to achieve the required standards of workmanship. Surfaces must be adequately prepared. Transport, spreading and compaction must be completed while materials are sufficiently hot to achieve the required standards of density and surface finish.

1.2 Planning

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Preliminary planning activities for an asphalt job should include: ƒ

site inspection to assess the site conditions and the basic requirements of the job

ƒ

arrangement of contracts for supply of asphalt, where appropriate

ƒ

arrangement of paving trials, if appropriate

ƒ

planning of traffic control arrangements

ƒ

advertising and advance warning to occupiers of adjoining properties, if necessary

ƒ

issuing of instructions to supervisory personnel

ƒ

assessment of plant requirements

ƒ

assessment of field organisation required.

The site should be inspected well in advance of the proposed start date for the work to confirm: ƒ

the appropriate treatment: − − −

type(s) of mix nominal size(s) of mix layer thickness(s)

ƒ

any remedial treatment required to restore the pavement to a condition suitable for the asphalt treatment

ƒ

profiling requirements

ƒ

the manner of executing the work

ƒ

staging of work

ƒ

the traffic control requirements for the work

ƒ

the need for public utility adjustments and treatment adjacent to the utility

ƒ

tie-in to existing work.

The site should be inspected again a day or two before commencement to ensure the pavement has been satisfactorily prepared. Where possible, paving should be scheduled to take advantage of daylight and daytime temperatures and to avoid peak hour traffic restrictions and early morning frosts. When paving at night, adequate artificial lighting must be provided.

Austroads 2006 — 1—

Asphalt Paving

Detailed site planning should include: ƒ

correct position of longitudinal joints

ƒ

planning of optimum length of each run and minimisation of transverse joints; method of level control

ƒ

optimum use of labour, equipment and delivery trucks

ƒ

adoption of good practices in all aspects of work

ƒ

sketches and/or plans of the work, if appropriate.

1.3 Paving Trials

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For larger works, particularly project work using transportable mixing plants, it may be appropriate to carry out paving of trial areas. This can assist in assessing: ƒ

suitability of proposed mix in terms of workability, ease of compaction, and surface texture (including check for segregation)

ƒ

adequacy of mixing plant to supply asphalt at the rate required for continuous paving

ƒ

adequacy of the proposed transport to ensure mix arrives on site at the required temperature and rate of supply

ƒ

adequacy of spreading equipment, and associated techniques to achieve the required rate of paving and to produce the required quality of asphalt pavement

ƒ

suitability of the spreading and rolling pattern including location of joints

ƒ

suitability of compaction equipment and procedures

ƒ

allowance in thickness to be made for roller compaction.

1.4 Traffic Control Plans The work site should be adapted to minimise traffic constraints and serve the needs of road users while at the same time providing adequate safety for the work crew. Where required, a traffic control plan should be prepared for each job. The plan should ensure that control of traffic is carried out in accordance with appropriate standards such as AS 1742.3 and associated field guides, or Transit New Zealand’s Code of practice for temporary traffic management 2004, as well as any specific requirements. The traffic control plan elements include: 1.

determination of the most appropriate method of traffic control, such as: ƒ

single lane for 2 way traffic

ƒ

2 lanes for 2 way traffic

ƒ

side track or detour around the site.

2.

arrangements for the supply and use of all necessary signs, traffic control devices, lights and pilot vehicles where necessary

3.

detailed layout of all signs and devices (with diagrams as necessary)

4.

arrangements for the required number of traffic controllers

Austroads 2006 — 2—

Asphalt Paving

5.

informing the public of the work by letterbox drop, advertising, advance warning signs and other appropriate means well before the worksite

6.

removal of parked cars

7.

arrangements for site to be left in a safe condition overnight, especially if traffic has access to partially finished work.

Wherever possible, the full width of roadway should be paved each day and a ramp provided at the end of each day's work for the smooth passage of traffic. If it is not possible to pave the full width, traffic should be prevented from crossing the exposed longitudinal edge.

1.5 Night Work Asphalt paving at night can significantly reduce problems associated with traffic disruption. Diversion or restriction of traffic can also provide a safer work site by reducing the proximity of traffic and improving site access. This can speed up the whole operation.

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Disadvantages of night work can include: ƒ

low pavement and ambient temperatures

ƒ

difficulty in achieving the same quality of surface finish, joints, etc.

ƒ

the need for good artificial lighting

ƒ

noise and disruption to residents

ƒ

additional labour costs

ƒ

potential for errors to be undetected.

1.6 Audit and Surveillance of Asphalt Paving Contract Work A guide to audit and surveillance of asphalt paving contract works is included in the asphalt manufacture part of this series Austroads (2006b) and includes checklists for paving activity as well as general guidelines for the preparation of quality plans, inspection and test plans and manufacture of asphalt. Detailed checklists for asphalt paving activity and a troubleshooting guide are included in Sections 11 and 12 of this document.

Austroads 2006 — 3—

Asphalt Paving

2

PREPARATION OF PAVEMENT SURFACE

2.1 Cleaning Before the asphalt is placed, the existing surface should be dry, and thoroughly swept to remove any loose stones, dirt and foreign matter. Sweeping should be carried out with a rotary road broom or suction cleaner (Figure 2.1). It should extend at least 300 mm beyond each side of the area to be paved.

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Any foreign matter adhering to the pavement and not swept off by broom should be removed by other means. Any areas affected by minor oil contamination should be cleaned by an approved method. Any area significantly affected by oil, and which has softened to an appreciable degree or has ravelled, should be removed and reinstated with asphalt.

Figure 2.1: Rotary road broom

2.2 Correction of Defects in Bituminous Surfaces Prior to commencement of the paving operation, any defects should be corrected, as follows: ƒ

filling of potholes and depressions with asphalt or approved patching material. Cold-mix should not be used as it may lead to bleeding or flushing

ƒ

removal of excess binder from fatty patches

ƒ

crack filling

ƒ

repair of edge breaks

ƒ

cleaning and repairing any joints

ƒ

removal and replacement of unstable materials

ƒ

removal and replacement of cold-mix patches

ƒ

shape correction.

Where the surface is badly out of shape, a corrective (regulation) course of asphalt should be applied first (Figure 2.2). This will reduce the effects of differential compaction of subsequent layers and enable the best possible riding quality to be achieved. Alternatively cold milling may be used to correct surface shape, as well as remove any unsuitable or unstable materials.

Austroads 2006 — 4—

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Asphalt Paving

Figure 2.2: Shape correction

2.3 Correction of Defects in Concrete Surfaces Remedial action may be required to ensure that slabs are firmly supported and that the joints are in good condition. Joints and cracks in the slabs will be reflected in the asphalt overlay. Slabs should be firmly supported by the sub-base, and if necessary corrected by mud jacking, grout injection or other appropriate means. Joints should be sealed with suitable hot bituminous filler. Techniques for control of reflection cracking include the use of a ‘bandage’ of fabric impregnated with bituminous materials over the joints, or saw cutting of the asphalt overlay and formation of a sealed joint in the asphalt.

2.4 Cold Milling Planing or milling of the surface is used to remove existing asphalt that is unstable, poorly shaped or where the new asphalt must match existing road levels, kerb and gutter, etc. Common cold milling applications include: ƒ

removal of surface that is uneven or rough

ƒ

removal of rutted, unstable or fatty materials

ƒ

restoring desired profile by removing excess crown or cross fall

ƒ

excavation of areas to be patched

ƒ

creation of tapers for smooth transition or matching of levels of adjoining work

ƒ

texturing as a bonding technique or for improving surface texture depth.

Austroads 2006 — 5—

Asphalt Paving

The benefits of cold milling include: ƒ

old pavement material is removed, eliminating build-up.

ƒ

the need for levelling or regulating courses or material may be reduced or eliminated.

ƒ

differential compaction problems from uneven bases are eliminated.

ƒ

it may provide a source of recyclable Reclaimed Asphalt Pavement (RAP).

ƒ

it can be done quickly with minimum inconvenience to the traffic flow.

ƒ

it allows neat and speedy excavation for patching.

The depth of milling (removal of material) will depend on: ƒ

the purpose

ƒ

the material in the existing pavement.

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Depths of up to 150 mm are possible in one pass, except for the smallest machines (cutting width less than 500 mm) that are generally limited to a maximum depth of 100 mm. If only the wearing course is to be replaced, the usual depth is in the range 25 to 50 mm, which corresponds to the layer thickness for a 10/14 mm asphalt mix. Milling used for functional purposes such as improving skid resistance, drainage, or rehabilitation of the wearing surface generally requires only minimal milling depth. Where possible, the size of the profiler should be matched to the size and productivity of the job. Table 2.1 provides a general guide to suitable profiler size based on typical production rates for a depth of cut of up to about 80 mm and appropriate job size that is suited to the productivity of the machine. Other considerations include: ƒ

The depth of the existing asphalt layer should be determined so that unnecessary exposure of the granular base to possible damage and moisture ingress, by removal of the whole depth of asphalt, is avoided.

ƒ

After milling, care should be taken to avoid leaving thin layers that are likely to be stripped off by traffic. The minimum thickness of asphalt in the old layer left in place should be approximately 30 mm.

ƒ

Good control of the milling is required to ensure a consistent finished profile and a smooth riding surface.

ƒ

Care should be taken where there are numbers of service pits and fittings in the milling area.

ƒ

The presence of geotextiles in the existing pavement can cause problems during milling.

ƒ

Profilers with rear conveyor discharge can reduce traffic disruption by operating within a narrower width.

ƒ

Ramping the ends of the work with temporary materials (e.g. cold-mix) may be required to reduce traffic hazard risk.

Austroads 2006 — 6—

Asphalt Paving

A cold milling machine can be used as an alternative to ripping and crushing equipment, especially where asphalt is to be removed for recycling. The particle size of the RAP obtained from cold milling is determined by: ƒ

depth of cut

ƒ

forward speed of the machine

ƒ

quality of material

ƒ

age and condition of the pavement surface

ƒ

ambient temperature.

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Table 2.1: Guide to selection of profiler Width of profiler (mm)

Production rate (m2/h)

150

10–30

350

40–60

500

60−80

600

70–100

1000

200–500

1900

300–600

2000

400–700

2100

500−900

Suitable job size (m2) 0−500

500−1000

1000−2000

>2000

2.5 Pre-treatment of Granular Pavements On crushed rock or natural gravel pavements, a sprayed bituminous prime or primerseal should be applied before asphalt is placed. Although desirable, a primer or primerseal is not always necessary where the asphalt thickness is in excess of 100 mm. A prime is used to penetrate the surface to protect the base against weather and assist in achieving a bond between the granular pavement and the asphalt layer. A primerseal is used where it is desired to run traffic on the granular pavement for a period of time before placing asphalt. Sufficient time should be allowed for curing of primerseals prior to paving. A cutback bitumen primerseal may require up to about 12 months curing to allow evaporation of the cutter oil. Bitumen emulsion primerseals contain little or no cutter and do not require extended curing periods. Without proper curing, cutter oils may bleed though asphalt surfaces causing softening of the layer or carrying of binder to the surface.

Austroads 2006 — 7—

Asphalt Paving

2.6 Pre-treatment of Concrete Surfaces For overlays on concrete pavements the use of a primer is generally recommended to ensure a bond is achieved with the asphalt. A tack coat may also be required, but by itself will often lack the penetration into the concrete necessary to provide a good bond. Concrete bridge decks require priming or sealing using a variety of treatments before overlaying with asphalt. Cutback bitumen treatments must be left for sufficient time for cutters to evaporate.

2.7 Public Utilities

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The levels of public utility surface fittings (covers, access points, etc.) should be adjusted prior to paving to match the proposed surface levels or masked and clearly marked and recorded for immediate recovery after the asphalt work (see also Section 8.10).

Austroads 2006 — 8—

Asphalt Paving

3

CLIMATIC CONDITIONS

3.1 General Ideally, paving operations should be planned for the daytime when the weather is warm to hot and when rain is not expected before completing the compaction. Asphalt cools rapidly in thin layers and when pavement and ambient temperatures are low. Wind and excessive moisture will also increase the cooling rate. Unless quickly and adequately rolled, these conditions will result in a low degree of compaction being achieved and a subsequent reduction in the service life of the asphalt.

3.2 Pavement Temperature

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Generally, asphalt layers of less than 40 mm thickness should not be placed when pavement temperatures are less than about 10°C (15°C for open graded asphalt and mixes containing polymer modified binders). Higher minimum pavement temperatures are desirable where cooling rates are increased by wind. Paving at lower pavement temperatures will generally be satisfactory for layers of 40 mm and thicker. However, mix temperatures and/or compaction densities should be closely monitored to ensure that compaction standards are achieved (see also Section 9).

3.3 Moisture When paving, the pavement should be dry, except for granular surfaces that may be slightly damp. Work may be permitted on a slightly damp bituminous surfaced pavement, provided it was previously tack coated. Paving should not proceed if rain appears imminent.

3.4 Risk Management Ideal conditions do not always exist and at times decisions have to be made as to whether paving should proceed. In these instances a risk management approach should be adopted. Paving under adverse conditions will involve some risk of poorer density and surface finish, particularly where handwork is involved. This can lead to a reduction in the life expectancy and the early replacement of the asphalt pavement. In such circumstances, particular attention must be paid to planning and execution of work to ensure that required standards of density, joint construction and handwork are achieved. The additional effort involved, and potential risks, should be balanced against the additional benefits to the community, from completing the work at that time. Reasons for proceeding may include: ƒ

minimisation of disruption to road users

ƒ

political or other time constraints

ƒ

critical path constraints when part of a larger project.

Austroads 2006 — 9—

Asphalt Paving

4

TACK COATING

4.1 General A tack coat is a light application of bituminous binder that provides a bond between the existing surface and the new asphalt layer. Generally, rapid setting cationic bitumen emulsion is used for tack coating although medium setting grades and anionic emulsions may be used in dry conditions. Medium curing cutback bitumen may also be used for tack coating. A curing period for evaporation of the cutter is necessary. Surfaced pavements require tack coating before commencing asphalt paving except that tack coat may be omitted when placing asphalt over a freshly placed, untrafficked, asphalt or clean primed surface.

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A tack coat should not be applied directly to natural gravel or crushed rock surfaces because of its inability to penetrate the surface and the likelihood of pick-up of tack coat and underlying granular material by vehicles and paving equipment. These surfaces must be first primed or primersealed. Tack coating should not be applied if the pavement surface is wet.

4.2 Application The tack coat should be applied by a mechanical sprayer with a spray bar to ensure an even application (Figure 4.1). Hand spraying or brushing may be used but this is undesirable and should be limited to small irregular shaped areas inaccessible to a mechanical sprayer.

Figure 4.1: Tack coat sprayer in operation

All contact surfaces should be tack coated, including cold joints. Generally, tack coat is applied at a rate of 0.15 to 0.30 L/m2 of residual bitumen.

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Asphalt Paving

If it is necessary to reduce the viscosity of the emulsion, it may be diluted with water or warmed to not more than 50°C. Dilution must be performed with care to avoid premature breaking. When diluting, water must be added slowly to the emulsion. Asphalt should not be placed until the emulsion has fully broken; i.e. the tack coat has turned from its original brown colour to a shiny black. Some tack coating material may be picked up on truck tyres. If the pick up is excessive, a light application of coarse sand or 5 mm aggregate should be broadcast across the areas traversed by construction traffic.

4.3 Precautions

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A number of precautions are required when tack coating: ƒ

Avoid over-application of tack coat. Surplus binder from tack coating may lead to flushing, shoving or instability of the finished work. Over-application may occur in surface depressions and any excess should be removed or dispersed by brushing.

ƒ

Protect adjacent property, kerbs and gutters, guard rails, bridge handrails, etc., against splashing by masking or screening.

ƒ

The work area should be cordoned off, and pedestrians and vehicular traffic kept well clear, to protect against direct contact with tack coat material and over-spray which can be carried substantial distances by wind.

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Asphalt Paving

5

TRANSPORT OF ASPHALT

5.1 General Asphalt should be transported in such a way as to minimise the loss of heat, segregation of the mix or contamination by foreign matter. The mix should be delivered at a uniform rate, within the capacity of the spreading and compacting equipment, to enable a continuous paving process. To reduce the cooling of the mix, deliveries should be made by the shortest practical route, and waiting time and delays on site should be minimised.

5.2 Vehicles

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Delivery trucks should have clean, smooth, metal bodies and a minimum capacity of 8 tonnes. The size and number of trucks is important to both the smooth running of the job and the quality of the work.

Figure 5.1: A ‘Flocon’ non-tipping delivery truck

Delivery trucks usually have tipping bodies and can include semi-trailers and dog trailers. Nontipping, ‘Flocon’ type bodies, may also be used (Figure 5.1). Truck bodies should have sufficient overhang (about 0.5 m) to enable tipping into the paver hopper without spillage. During transportation, the asphalt should be covered with canvas or other similar waterproof cover. Transporting asphalt over long distances may require heavy duty covers and, in some instances, insulation of truck bodies using oiled plywood or other suitable material, to minimise heat loss. Trucks should be driven by experienced personnel. Care should be taken when dumping the mix into the paver hopper to avoid spilling mix onto the pavement in front of the paver, or jarring the paver. Trucks should reverse to a position just short of the paver to allow the paver to make contact with the stationary truck and push it forward. Trucks should only apply brakes sufficiently to maintain the truck in contact with the paver.

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Asphalt Paving

5.3 Release Agents Internal surfaces of truck bodies should be clean so as to prevent the mix from adhering. Release agents may be used sparingly to facilitate easy unloading of mix. Any excess release agent should be removed before loading asphalt, particularly if it contains diesel or flux oil.

5.4 Organisation

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Transport operations should be organised to ensure continuous paving operations, taking into account: ƒ

the number of trucks required and their availability

ƒ

meal breaks

ƒ

haulage distance/travelling time

ƒ

the times at which the mix can be placed, due to site availability

ƒ

site conditions and access restrictions that may limit the size of truck that can be used (e.g. overhead cables).

Suitable communications between the mixing plant and the site are essential for effective control and organisation. Preferably, each truck should be in radio contact with the mixing plant and the site.

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Asphalt Paving

6

TRANSFER OF ASPHALT FROM TRUCKS TO PAVER

For most applications, asphalt is dumped directly from trucks into the front hopper of the paver. Where project circumstances are suitable, a materials transfer device may be used to improve control over feeding asphalt materials to the asphalt paver. In this case, delivery trucks dump asphalt into a materials transfer device that then feeds it into the paver hopper by means of a conveyor belt.

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Materials transfer devices (Figure 6.1) generally hold about 20 to 25 tonnes of asphalt.

Figure 6.1: Materials transfer device

Use of materials transfer devices provides advantages in terms of: ƒ

minimising the paver being bumped by trucks

ƒ

acting as a surge bin for asphalt delivery to minimise unplanned paver stops resulting from interruptions to asphalt supply

ƒ

reducing segregation of the asphalt in the paver hopper

ƒ

the ability to incorporate remixing facilities to reduce influence of mix segregation and temperature variation during loading and transport.

If used correctly, materials transfer devices can lead to improved uniformity and smoothness of the paved finish.

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Asphalt Paving

7

PAVER PERFORMANCE AND SCREED OPERATION

7.1 General Pavers operate on the floating screed principle, spreading material in a uniform layer to a desired thickness and longitudinal and transverse shape.

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The paver also provides primary compaction of the mix (which is completed by subsequent rolling).

Figure 7.1: Typical paver

The principal components of a self-propelled paving machine (Figure 7.2) are: ƒ

tractor unit

ƒ

screed unit

ƒ

automatic sensing and levelling equipment.

Figure 7.2: Typical paver components

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Asphalt Paving

7.2 Tractor Unit The tractor unit provides the power to drive the paver and includes: ƒ

a traction unit (usually pneumatic tyred)

ƒ

a receiving hopper

ƒ

a feed (slat) conveyor system

ƒ

lateral spreading augers

ƒ

controls.

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The asphalt in the hopper is transferred by slat conveyors, through adjustable flow gates, to the two individually controlled augers that spread the material laterally and evenly in front of the screed (Figure 7.3).

Figure 7.3: Flow of material through paver

The basic adjustments to components of the tractor unit that affect paver performance and output are: ƒ

Paver speed (see Section 7.3.3).

ƒ

Flow control gates. Ideally, the flow gates should be adjusted so that the augers and conveyors run for the entire operating time (and not less than 80% to 90%) to provide as constant a head of material as possible.

ƒ

Slat conveyor speed.

ƒ

Auger height adjustment. Generally, the bottom of the augers should be about 50 mm to 75 mm above the finished mat surface to allow for correct feeding of material.

ƒ

Auger feed. Generally, auger feed is controlled automatically using paddle arms, which sense the level of material at the augers. The paddle switches also regulate the conveyor speed.

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Asphalt Paving

ƒ

Auger box size. On some pavers larger auger boxes can be used to increase the quantity of material for paving of deep lifts.

Overfeeding of augers can result in the following mat deficiencies: ƒ

ripples

ƒ

auger shadow (variable texture)

ƒ

short and long waves

ƒ

other mat blemishes due to cooling of the mix

ƒ

premature auger wear.

Underfeeding or a fluctuating feeding of augers will cause a poor quality riding surface.

7.3 Screed Unit

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The screed unit consists of a screed plate, compaction device(s) (i.e. tampers and/or vibrators), and thickness controls. It is connected to the tractor unit of the paver by towing (levelling) arms, which are pin-jointed.

Figure 7.4: Components of screed unit

The screed is supported by the mix and automatically rides up and down seeking the level that is parallel to the line of pull. This arrangement gives the screed a floating action and allows it to spread to a relatively uniform surface despite irregularities in the underlying pavement. Several factors such as paving speed, head of material, asphalt mix consistency, pre-compaction and screed angle of attack, all influence the vertical position of the screed. Variation in any of these factors will cause a change in mat depth, density and/or texture.

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Asphalt Paving

The three primary factors that influence the vertical position of the free-floating screed (Figure 7.5) are: ƒ

Factor F1 – angle of attack

ƒ

Factor F2 – head of material

ƒ

Factor F3 – paving speed.

Understanding the inter-relationship and controlling these three variable factors is essential to producing high quality asphalt paving. Where possible, all other factors should be kept constant. SCREED DEPTH CRANK

TOW POINT

F3 PIVOT POINT

F2

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F1 Figure 7.5: Factors influencing vertical position of the free-floating screed

7.3.1

Angle of inclination

The thickness of the spread mix is changed by altering the angle of inclination (attack F1 in Figure 7.5) of the screed plate. Altering the angle of attack is effected by: ƒ

adjusting the angle of the screed plate in relation to the levelling arms, using the depth adjustment control (e.g. turnbuckle, crank, winder etc.)

ƒ

raising or lowering the tow (pivot) point of the levelling arms.

Changing the angle of attack allows more or less mix to pass under the screed plate. This causes an imbalance in the vertical forces and the screed plate rises or falls until a new equilibrium position is reached when the vertical motion stops. A gradual change in thickness is thus achieved (Figure 7.6). The response of the screed to a change in the angle of attack is not immediate. It takes a distance of approximately four to six times the length of the levelling arm (i.e. the natural paving length) for the screed to reach a new equilibrium position ( Figure 7.7). Therefore, it is desirable that: ƒ

adjustments to the angle of attack, to produce changes in thickness, be made only as a result of unusual conditions at the paver, and not indiscriminately by the screed operator

ƒ

the natural paving length be allowed to reflect one change before making another.

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Asphalt Paving

Tow point

MANUAL CONTROL WITH SCREED DEPTH CRANKS

Tow point

AUTOMATIC CONTROL OF TOW POINT

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Figure 7.6: Response to change in angle of attack

Screed path

Step function disturbance Transient response of screed 63% Direction of paving

87%

95%

98%

99%

100%

Tow point path STEP AMPLITUDE = 100%

0

1L

2L

3L 4L 5L L = LEVELLING ARM LENGTH

6L

Figure 7.7: Screed response to a step-function disturbance

7.3.2

Volume of material in front of screed

The balance of forces on the screed, and hence the quality of the mat, is assisted by maintaining a constant head of material in front of the screed (F2 in Figure 7.5, also see Figure 7.8). Control of the volume of material in front of the screed (i.e. in the auger chamber) is achieved by: ƒ

proper setting of the adjustable flow gates

ƒ

uniform operation of slat conveyor system

ƒ

operation of the augers (which are generally controlled by sensors on the surface of the mix).

The volume of material in the auger chamber should be maintained at 50% to 75% of the height of the auger. Increasing the volume increases resistance, which causes the screed to rise. Decreasing the volume allows the screed to fall. Similarly, changes in the consistency in the mix due to changes in temperature or composition, also increase or decrease the resistance, resulting in changes to level, density and texture.

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Asphalt Paving

CORRECT DEPTH OF MAT MAINTAINED

CORRECT HEAD OF MATERIAL

SCREED RISES DUE TO EXCESS MATERIAL FORCED UNDER NOSE OF SCREED

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EXCESSIVE HEAD OF MATERIAL

SCREED SETTLES DUE TO INADEQUATE SUPPORTING MATERIAL INSUFFICIENT HEAD OF MATERIAL

Figure 7.8: Head of material

7.3.3

Paving speed

Changes in the forward speed of the paver (F3 in Figure 7.5) can significantly affect the mat quality, in terms of: ƒ

the thickness of the mat

ƒ

the compaction achieved by the screed.

Changes in paving speed directly affect the head of material and consequently, the angle of attack. Increasing paver speed will decrease mat depth. Decreasing paver speed will increase mat depth. If the paver speed is increased, the mix flowing under the screed is exposed to compactive forces for a shorter period, producing a mat with reduced density and reduced compacted thickness. Therefore, any interruption in the paver speed will be reflected in the quality of the finished riding surface, and should be avoided. Changes in the paver speed can be caused by: ƒ

poor planning of supply of asphalt to the paver

ƒ

trucks bumping the paver when tipping loads

ƒ

trucks holding their brakes while paver is attempting to push, resulting in loss of paver traction.

The effect of trucks on paver progress can be avoided through experience and co-operation between truck driver and paver operator.

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Asphalt Paving

7.3.4

Primary compaction by screed unit

The design of the screed enables primary compaction of the mix. This is achieved by incorporating vertically oscillating tamper blade(s), immediately ahead of the screed plate, or vertical vibration of the screed plate. The degree of primary compaction may be varied by adjusting either the amplitude and/or frequency of vibration. To achieve consistent compaction, these screed variables should also be set to suit the layer depth. Screed compaction of deep lifts requires: ƒ

low frequency vibration

ƒ

high impact forces (high amplitude).

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Screed compaction of thin layers requires: ƒ

high frequency vibration

ƒ

low impact force (low amplitude).

The degree of compaction will vary with the speed of the paver, with higher densities achieved at lower paver speeds. Pavers typically achieve compaction to about 85% of relative density although this will vary with the design and weight of the screed assembly. Screed assemblies using single or multiple tampers will generally achieve higher densities than vibration alone. 7.3.5

Crown adjustment of screed

The crown adjustments at the centre of the screed enable control of the transverse profile of the finished surface. Separate adjustment is provided for the leading and trailing edges of the screed. The forward adjustment is the ‘lead crown’ which controls the flow of material beneath the screed. The lead crown should always be slightly greater than the tail crown. The rear adjustment or ‘tail crown’ controls the finished contour of the mat. The crown adjustments should be checked daily before paving. This can be carried out by raising the screed and using a string line to measure both the lead and tail crowns. The measurements of the lead and tail crowns (of the raised, unsupported screed) will, through experience and familiarity with the particular paver, indicate the crown that will be produced in the finished mat. 7.3.6

Screed extensions

The width of paving may be varied using extensions to the screed. These may be rigid boxes that are bolted to the screed, or hydraulically operated extensions (Figure 7.9). Paving widths of up to 8 m are possible using screed extensions, although widths greater than 5 or 6 m are not commonly used due to the difficulty of maintaining a consistent head of material across the full width of the screed.

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Asphalt Paving

When using screed extensions, care should be taken to: ƒ

ensure alignment of extensions matches that of the main screed

ƒ

ensure auger extensions are fitted to provide an adequate head of material in front of the screed extensions

ƒ

ensure tamping/vibration is consistent across whole width of mat.

7.3.7

Care of screed

The condition and operation of the screed will directly affect the quality of the finished pavement.

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The screed should be lifted and checked before commencing paving each day for: ƒ

loose or worn screed plates

ƒ

poor shape or distortion that can be a result of overheating of screed

ƒ

misalignment of screed extensions and main screed

ƒ

incorrect crown adjustments.

Figure 7.9: Paver with hydraulic screed

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Asphalt Paving

7.4 Automatic Sensing and Levelling Equipment This equipment is used to control the operation of the screed unit in a pre-determined relationship to either: ƒ

the existing surface

ƒ

an adjoining finished surface

ƒ

a fixed reference line.

These controls are used to maintain levels, mat thickness and crossfall within required limits. A sensor box controls the angle of attack by changing the vertical position of the tow point. The sensor box may be used with a variety of devices for achieving the specified surface levels (Section 8.9).

7.5 Paver Stoppages

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When the paver stops the equilibrium conditions of the screed can change due to: ƒ

a slight settling of the screed

ƒ

cooling of the material in front of the screed.

When the paver starts moving again, the screed will rise or fall to achieve a new equilibrium position. These effects can be eliminated or minimised if the paver moves continuously. Sometimes it is not practical to keep the paver moving continuously. In these cases the stopping and accelerating of the paver should be achieved quickly but smoothly. During prolonged paving stoppages, the settling of the screed may cause a permanent depression of unacceptable depth in the pavement surface. In such circumstances the mat should be cut back to remove the depression and a transverse joint constructed as described in Section 8.7.

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Asphalt Paving

8

SPREADING OPERATIONS

8.1 General Asphalt should be spread and compacted uniformly in order to: ƒ

limit segregation

ƒ

produce a homogeneous product

ƒ

achieve a density that delivers the intended design performance of the asphalt

ƒ

provide the specified thickness of asphalt

ƒ

achieve the specified riding quality.

Spreading may be carried out by self-propelled paver, grader or hand methods. Wherever possible self-propelled pavers should be used as they provide greater control during spreading and a superior surface finish. Paver operations are also quicker and more economical. The use of such equipment forms the main focus of this Chapter.

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8.2 Setting Out The work should be set out in advance with the order of runs and the position of joints for each layer clearly defined. Setting out will be influenced by the desirable location of joints, minimisation of handwork, level control procedures and provision for traffic.

8.3 Spreading by Paver Spreading is designed to be a continuous operation. The rate of delivery of the asphalt should be arranged so that the paver can operate at a uniform speed. Paving should not commence until sufficient asphalt is on site to ensure continuous operation. At the start of each paving run, and at each cold transverse joint, the paver operation should be as follows: ƒ

check crown adjustment

ƒ

position the screed on wooden blocks at the height of the uncompacted layer

ƒ

set the tow points at the required uncompacted mat thickness

ƒ

heat the screed plate to the temperature of the mix

ƒ

set hopper gates to ensure constant flow of mix.

After laying a short length of mat, the paver set up should be checked as follows: ƒ

flow gates delivering suitable flow of material

ƒ

screed heat sufficient to avoid dragging of surface

ƒ

settings of tamper(s) and/or vibrators

ƒ

screed depth control and thickness of uncompacted asphalt

ƒ

functioning of automatic controls.

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Asphalt Paving

A 3 m straight edge should be used at frequent intervals to check surface shape and smoothness, particularly at joints and paver stops. The spread material should be examined constantly for faults in texture. Any faults such as segregation or tearing of the surface should be corrected before compaction. To increase output and eliminate or reduce cold joints, two or more pavers may be operated in echelon. The pavers should work close enough so that the temperature of the uncompacted edge behind the advance paver is not less than 100°C (120°C for PMBs) when the following paver matches the longitudinal joint.

8.4 Spreading by Grader Spreading by grader may be used only in special circumstances. It should be discouraged because of difficulty in achieving compaction and ride quality requirements and promotes segregation of the asphalt. The method should only used for applications such as: ƒ

temporary access roads

ƒ

patching work where it is not practical to operate a paver.

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Points to be observed in placing asphalt by grader include: ƒ

prompt spreading is essential to avoid cooling and loss of workability

ƒ

placing of thin layers may be difficult

ƒ

grader operation is helped by uniform spreading from trucks; a slight excess of material allows a head of material in front of the blade

ƒ

handling of asphalt should be kept to a minimum to avoid segregation, more rapid cooling, and the effect of partial compaction by the grader wheels.

8.5 Spreading by Hand Hand spreading should always be kept to a minimum. Hand placing of open graded asphalt, ultra thin mixes and asphalt containing PMBs is particularly difficult and requires extra care. Shovelling, raking or other disturbances of the surface after machine spreading should be kept to the minimum and completed quickly, before the mix cools below the minimum temperature for effective compaction. Asphalt should be placed in full shovelfuls and not cast or thrown over the new mat or area to be paved. Generally, a slight excess of material is placed that is then screeded to level. Wooden lutes are most commonly used for hand screeding. Their light weight enables smooth screeding of hot materials. Where practicable, the screeding should be done with a head of material in front of the lute, and using a single pass that leaves a uniform surface of fresh asphalt. Excessive working of the surface leads to separation of coarse materials, and should be avoided. The mix should never be thrown, scattered or loosely raked. Coarse segregated materials resulting from handwork must be completely removed from the surface along with any other excess material. Attempting to avoid wastage and clean-up of surplus asphalt is false economy if it results in inferior quality in the finished work.

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Asphalt Paving

Handwork should be carried out as quickly as possible to avoid excessive cooling of the asphalt. All surface correction should be completed prior to commencing compaction. The only exception to hand broadcasting or scattering of asphalt material is in correcting minor tearing or isolated areas of open texture in paver spread asphalt. A shovelful of asphalt, skilfully broadcast over the unrolled asphalt, can provide additional material to correct open textured areas. Such hand broadcasting should not be undertaken indiscriminately. If spreading texture deficiencies persist, the source of the problem should be determined and corrected. Possible causes include incorrect paver screed temperature, front and rear settings of paver crown, and segregation of asphalt mixes as a result of deficiencies in mixing, loading or spreading practices.

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Walking on the surface of the uncompacted mix should be avoided at all times.

Figure 8.1: Handwork

8.6 Layer Thickness The thickness of asphalt layer(s) within a pavement should normally be determined by the structural requirements of the pavement. For initial treatments, the determination of thickness is part of the structural design. For retreatments, the thickness will depend on the amount of surface correction and structural strengthening required. If this is considerable, it may be necessary to lay one or more corrective courses before the final course. The thickness and type of course will determine the nominal size of the asphalt mix. The nominal size of an asphalt mix is an indication of the maximum particle size present and is usually expressed as a convenient whole number above the largest sieve size to retain more than 0% and less than 10% of the aggregate material. The selected nominal size of mix will be determined by: ƒ

location of asphalt course in pavement

ƒ

proposed compacted thickness of layer

ƒ

functional requirements of asphalt layer.

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Asphalt Paving

Table 8.1 provides a guide to appropriate mix sizes for ranges of course thickness. Table 8.1: Typical asphalt layer thickness Nominal mix size (mm)

Compacted layer thickness (mm)1

5

15 to 20

7

20 to 30

10

25 to 40

14

35 to 55

20

50 to 80

28

70 to 110

40

100 to 160

Notes: 1

The minimum thickness may need to be increased when placing thin layers in cool conditions or using less workable mixes. Minimum thickness may not apply to some ultra-thin asphalt applications. Maximum thickness may be exceeded provided that asphalt surface shape requirements can be adequately achieved.

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Generally, asphalt should be placed in layers with a compacted thickness of not less than 2.5 times the nominal size of mix in order to: ƒ

prevent the mix tearing during laying

ƒ

assist the compaction process by allowing the aggregate particles to mechanically interlock.

The minimum thickness may need to be increased when placing thin layers in cool conditions or using less workable mixes. Some ultra-thin asphalt surfacing types can be successfully laid with a layer thickness as little as 1.5 times the nominal mix size provided that they are combined with a suitable seal coat or tack coat to effectively bond the asphalt to the underlying pavement. In wearing and intermediate course applications the maximum compacted layer thickness is generally limited to around 4 times the nominal mix size. Greater thicknesses may be used to achieve practical placing thickness; for example a requirement for 50 mm of 10 mm asphalt is better achieved with one 50 mm layer than two 25 mm layers. Where the layer thickness exceeds four times the nominal size, it may be more cost effective to use a larger nominal size mix although larger sized mixes are also more prone to segregation and may not necessarily provide the surface finish required. Base course applications may involve multiple layers with the maximum layer thickness being largely determined by practical placing considerations taking into account the total asphalt thickness and ability to achieve the required finished shape and riding qualities. For most applications, 20 mm is selected as the largest nominal sized mix. The thickness of asphalt layer placement may be specified in a number of ways, including: ƒ

a required average or nominal thickness

ƒ

a minimum compacted depth

ƒ

a rate of kilograms per square metre

ƒ

a rate of square metres per tonne

ƒ

placement to fixed levels.

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Asphalt Paving

The thickness of the uncompacted mat depends on the degree of compaction achieved by tampers and screed. It may be up to 20 to 25% thicker than the required compacted thickness. Field experience will determine the required uncompacted thickness. The thickness of the uncompacted and compacted layers should be checked at 10 to 15 m intervals at the start of each run. The screed can be adjusted to give the desired thickness. The uncompacted layer should be measured directly by probing the mix in the body of the run or by measuring adjacent to the outside edge. The compacted layer thickness may be measured at the outside edge of the run using a straight edge, if the underlying crossfall is uniform. When paving to specified levels, regular checking is required after compaction.

8.7 Joints Construction joints in an asphalt layer are potential planes of weakness and imperfection that can be the first locations to deteriorate. Poorly constructed joints that are more permeable than the rest of the asphalt mat can lead to the ingress of moisture into the pavement, and also cracking and ravelling.

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The number and extent of joints in asphalt layers should be kept to a minimum and the paving pattern should be designed accordingly in advance of the work. Correct jointing technique will ensure that the two mats are joined in such a way as to minimise differences in density, texture, shape and level and also minimise the amount of deterioration at joints caused by traffic.

Longitudinal joint Wearing course Intermediate course Base course Longitudinal joint

150 150 mm mm

Figure 8.2: Overlapping of longitudinal joints in successive courses

8.7.1

Longitudinal joints

Longitudinal joints are construction joints in an asphalt layer parallel to the paving run. The following procedures should be adopted to ensure satisfactory longitudinal joints: ƒ

For wearing courses, longitudinal joints should be continuous and parallel to the road centre line. They must always be located away from traffic wheel paths. Where possible they should coincide with proposed lane markings. If this is not possible, the wearing course joint should be located in the centre of the lane.

ƒ

Good alignment of longitudinal joints is achieved by following a paving line marked on the road, and with pointers attached to the paver as a guide to the operator. Poor or uneven alignment of the first lane makes it difficult to evenly match the next pass of the paver.

ƒ

Joints in successive courses should be offset or staggered by at least 150 mm (Figure 8.2) to minimise the possible occurrence of reflective cracking.

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Asphalt Paving

ƒ

All excess material or segregated particles should be removed and discarded and not incorporated into the mat (as this can increase the roughness of the finished surface).

ƒ

Work should be arranged to avoid longitudinal joint faces being left exposed overnight. Where this is not practical, traffic should be excluded or temporary ramps used and trimmed back the next day.

Compaction of the unconfined edge of the first lane is important. It is critical that the roller make the same number of passes over the edge of the first lane as are made over the rest of the width of the lane. Generally, the edge of the roller drum should extend over the free edge by about 150 mm (Figure 8.3). This ensures vertical compaction of the unconfined edge and reduces any tendency for the asphalt to shove sideways during the compaction operation.

Double drum vibratory roller Overhang 150mm

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Lane 1

Figure 8.3: Overhang of outer edge

Operating the steel roller inside the unsupported edge (Figure 8.4) tends to cause the asphalt to spread. Longitudinal cracks may also open up along the edge of the drum. Placing the edge of the roller directly over the unsupported edge (Figure 8.5) will avoid cracking but will still cause the unsupported edge to move sideways.

Double drum vibratory roller Lane 1

Figure 8.4: Poor roller placement

Double drum vibratory roller

Lane 1

Figure 8.5: Better roller placement

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Asphalt Paving

Hot joints are preferable to cold joints but are usually only possible when using two pavers in echelon, e.g. on large construction works (Figure 8.6). The distance between pavers, operating in echelon, should not exceed 80 m.

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A hot joint is one where both the new mat and the adjacent mat are still workable and have not been compacted. The hot joint should be constructed by leaving an uncompacted strip approximately 150 mm wide along the edge of the first placed lane. Both sides of a hot joint should be rolled simultaneously.

Figure 8.6: Paving in echelon (hot joints)

Figure 8.7: Cutting disc attached to steel roller

When constructing cold longitudinal joints, the first lane should be compacted as described above.

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Asphalt Paving

In some cases, before the longitudinal joint between two adjacent paver lanes is constructed, the edge of the previously placed mix is cut back to remove material that may have a lower density than the main portion of the mat. This is accomplished with a cutting wheel that is usually attached to a roller (Figure 8.7). The cut face should be lightly tack coated before the adjacent lane is placed. Cutting back of the exposed edge is generally not necessary if adequate joint construction procedures are followed as described in this section. Tack coating is generally not required for clean, untrimmed edges. Overlap 25 - 40 mm (max) Lane 2

Lane 1

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Figure 8.8: Correct overlap of longitudinal joint

The key to the construction of good longitudinal joints between adjacent paving lanes is the amount of overlap between the new and previously placed mats. The typical overlap is not more than 25 to 40 mm (Figure 8.8). The height of the new mix above the compacted mix is also important and should be about 6 mm for each 25 mm of compacted mix (¼ of thickness). If the right amount of mix is put in the right place, little, if any, hand raking is required. Overlapping material may be pushed back with a lute to form a bump on the new mat. When rolled, this material is pinched against the vertical face of the first mat to increase density at the joint. If too much material is left over the joint, however, the roller will tend to leave a bump that cannot be removed by further rolling. If there is an excess mix, it should be pulled away from the joint, picked up and discarded. It should not be thrown across the new mat. Excess raking of the joint is highly detrimental to the long term performance of the joint with a potential for lower density and poor appearance from segregated materials. Rolling of longitudinal joints is undertaken in accordance with procedures described in Section 9.8. 8.7.2

Wedge joints

New techniques are being developed to improve the construction of longitudinal joints using devices such as a plate attached to the paver that forms an inclined face at the edge of the mat so that the following adjoining mat forms an overlapping wedge. Such techniques are not commonly used in Australia but continued improvement in this area could result in higher standards of longitudinal joint construction.

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Asphalt Paving

8.7.3

Transverse joints

Transverse joints are construction joints in the asphalt paving, at right angles to the direction of paving, and are formed: ƒ

at the start and finish of each paving run

ƒ

when the work is disrupted causing − −

cooling of the asphalt and/or settlement of the screed when resuming on the next day.

Transverse joints can be a common cause of poor riding qualities in a finished asphalt wearing surface. The importance of correct transverse joint preparation and formation techniques cannot be over-emphasised. Transverse joints should be approximately at right angles to the direction of paving. They should be staggered by at least 1 m between successive layers and between adjacent runs to avoid planes of weakness and possible water entry through the whole asphalt thickness.

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A paving run may be finished against a timber bulkhead to ensure a straight, vertical, well compacted edge, or may be feathered out (ramped) and compacted. For ramped material, the transverse joint is formed by the subsequent trimming back to a line where the minimum layer thickness exists (Figure 8.9). When finishing flush against an existing surface, the paver should maintain sufficient material in front of the screed to pave to the end of the run. It is poor practice to finish machine spreading several metres before the end of the run, lifting the screed, dumping asphalt from the paver, and then hand spreading the remaining material to finish the run. This hand spread material rarely matches the surface finish and uniformity of the machine laid material. Trimming of joints, and edge cutting, may be carried out using: ƒ

a cutting disc attached to a steel wheel roller (Figure 8.7)

ƒ

a concrete saw

ƒ

a jackhammer with spade attachment

ƒ

milling machine.

To facilitate subsequent trimming of a joint, a short section at the end of the run may be removed before rolling, and re-laid on paper or sand to prevent adhesion of the asphalt to the lower layer.

Austroads 2006 — 32 —

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Asphalt Paving

Figure 8.9: Transverse joint construction

When starting a new run from a transverse joint, care should be taken to: ƒ

ensure that the mix is consistent in temperature and blend

ƒ

set up the paver correctly, including heating of the screed

ƒ

set the screed level (on timber blocks) to allow for compaction to match the level of the existing mat

ƒ

ensure there is a sufficient head of material in front of the screed.

It is important to commence paving with the correct amount of material in the auger chamber, otherwise: ƒ

too large a head of material will produce a hump in the new surface

ƒ

too low a head of material will produce a dip.

Any asphalt left on the cold mat should be removed before rolling. The joint area should be checked with a straight edge prior to rolling. The transverse joint is completed by transverse rolling to ensure a pinched joint between the new mix and the existing mat (see also Section 9.7).

8.8 Paving Output To ensure an efficient and economical operation, paving widths and lengths of runs should be as large as conditions permit. The mixing plant and transport capacity will generally dictate the method of operation.

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Asphalt Paving

The paver output, when unrestricted by other limitations such as mix supply, length of run or availability of rollers, is a function of paver speed, screed board width and mat thickness. Proper planning can usually eliminate factors that limit paver output.

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Paver speeds and outputs for various layer thicknesses are shown in Figure 8.10.

Note – Paver speed based on spread 3.7 m wide and a compacted density of 2.40 tonnes per cubic metre

Figure 8.10: Mix deliveries required to match paving machine speeds for various compacted depths

8.9 Automatic Level Control Automatic level sensor equipment should be used, where appropriate, to control the operation of the screed. These controls are used to maintain levels, thickness and crossfall to a greater degree of regularity than that generally obtainable with manual control alone. Automatic controls use sensors to activate the hydraulic mechanisms that control the paver pivot points. Raising and lowering the pivot points changes the angle of attack and thickness of the asphalt mat. Sensor probes may travel: ƒ

on the finished surface of the mix in an adjacent run

ƒ

on a moving level averaging device

ƒ

on a fixed reference line parallel to the desired finished surface.

In addition, level control can be achieved using laser or acoustic technology, or by on-board computer. 8.9.1

Joint matching shoe

This is a small ski, about 300 mm long, sliding along the surface of the adjoining mat or kerb (Figure 8.11). It is a low sensitivity system, which will gradually adjust to variations in the reference surface.

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Asphalt Paving

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As the ski rises or falls it alters the angle of attack of the screed and hence, the thickness of the mat, thereby replicating localised imperfections. Shoes may be used on either one or both sides of the paver, although it is preferable to only use a shoe on the adjacent (freshly laid) mat and slope control to maintain crossfall.

Figure 8.11: Joint matching shoe

8.9.2

Levelling (or averaging) beam

This may be a beam or truss from 9 m to 12 m long mounted on two or more pairs of wheels or multiple small skids (Figure 8.12). The beam moves along on the adjoining pavement with brackets attached to the paver. The sensor detects movement at the centre of the beam and this is transmitted to the electrical switch, hydraulic ram, and screed, as for the joint matching shoe. The levelling beam averages longitudinal deviations in the surface over a greater distance than the wheelbase of the paver. A short rise or fall in grade will be absorbed and have minimal effect on the attitude or elevation of the beam. Longer deflections that affect most of the shoes at once, will change the attitude of the beam and be detected by the grade sensor. The wheel mounted or multiple skid beam gives greater shape correction than the joint matching shoe but some high spots may be scraped bare. The averaging effect of the beam will be a function of the number of independent point contacts with the road and its effective length. Shorter beams, or joint matching shoe only, are generally preferred when matching existing surfaces such as adjacent gutters.

Austroads 2006 — 35 —

Asphalt Paving

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Figure 8.12: Multiple skid beam

8.9.3

Fixed wire

A wire or nylon string line is set by means of special supports to predetermined levels as a reference for a sensor (Figure 8.13). This can be a very efficient means of correcting the shape of irregular surfaces in construction works such as bridge decks. The inherent sag error can be minimised by setting the supports at no more than 5 m intervals. A fixed wire or string line may be used to control the levels of the base or correction course prior to completing subsequent layers with the use of levelling beams. Disadvantages of fixed reference lines include the labour required to install the line and the need for all personnel and equipment, especially trucks, to stay clear so as not to disturb it.

Figure 8.13: Fixed wire

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Asphalt Paving

8.9.4

Crossfall control

Crossfall (or slope) control may be maintained using a pendulum device (that is rigidly attached to a transverse beam on the paver) to transfer levels from one side of the paver to the other. If one side is controlled by a joint matching shoe or levelling beam, a predetermined crossfall can be applied to the other side. For example, in overlaying a two lane rural highway, a levelling beam could be used along the centre line to give the required thickness and improved profile there, with the pre-set pendulum device controlling levels on the shoulder side. This method of control will produce a correct surface shape but may be expensive in material used if existing road shape is poor. Care must be taken to ensure that crossfall controls are kept in adjustment. 8.9.5

Computerised level control

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An on-board computer can be used to control longitudinal profile as well as crossfall. Existing surface level, finished surface level, and chainage data are input before paving commences. Sensors are used to alter paver thickness controls, as paving progresses, in accordance with the predetermined levels. 8.9.6

Laser control

A laser transmitter is used to establish a horizontal reference plane over the site. This plane can be used for grade control or for both grade and slope control. Laser systems can be used in conjunction with existing paver controls for automatic or manual operation. It has been claimed that laser systems enable more uniform depth control and smoother riding quality than conventional automatic controls. However, they are difficult to use when paving with frequent grade changes.

8.10 Service Fittings Care must be taken when paving in the vicinity of service access covers, drainage pits and grates, and other service fittings, to avoid damage to the fittings or the screed. Generally, the procedure should involve: ƒ

covering the service pit cover, grate or other fitting with hessian or paper so that excess material may be removed easily after the paver has passed

ƒ

marking the kerb, or road side, so that the service pit can be located afterwards

ƒ

using a wood block or asphalt pad as a lead-in to the service pit when paving other than the final wearing course

ƒ

handworking the job for a good finish.

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Asphalt Paving

8.11 Safety During Paving Operations Personnel should be instructed in safe working practice and be warned of potentially dangerous situations. Appropriate safety precautions should be observed in the following areas: ƒ

traffic control

ƒ

operation of plant and trucks

ƒ

handling of hot asphalt

ƒ

use of flammable materials.

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There is a need for constant vigilance on construction sites and adherence to safety procedures to ensure the safety of construction workers and the public.

Austroads 2006 — 38 —

Asphalt Paving

9

COMPACTION

9.1 General During the paving operation, compaction is a two-stage process: ƒ

primary compaction by the paver screed

ƒ

secondary compaction by rollers.

This section deals with secondary compaction by rollers and supplementary devices for areas inaccessible to rollers.

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Adequate compaction of asphalt is essential to ensure the design performance of the mix and expected service life are achieved. The compaction should be uniform and achieve a high density. Mix properties that are directly related to the achievement of adequate density include: ƒ

strength, both compressive and shear

ƒ

stability

ƒ

permeability

ƒ

resistance to oxidation and ravelling

ƒ

resistance to rutting

ƒ

surface texture and appearance

ƒ

pavement life.

The main factors influencing the successful compaction of asphalt are: ƒ

rolling procedures and techniques

ƒ

temperature of the mix

ƒ

mix properties

ƒ

soundness and stiffness of the underlying base

ƒ

type and numbers of rollers or other compaction equipment.

The underlying base must be sufficiently sound and rigid to uniformly distribute the stresses from the compaction equipment without distortion. In the event that the underlying base is inadequate for the type of mix and compaction equipment being used, the resultant surface may be uneven and cracked, and the asphalt thickness and density non-uniform.

9.2 Compaction Equipment The compaction equipment for asphalt work includes: ƒ

non-vibratory (‘static’) steel-wheeled rollers

ƒ

vibratory steel-wheeled rollers (Figure 9.1)

ƒ

pneumatic-tyred rollers (Figure 9.2)

ƒ

impact compactors such as vibratory plates (Figure 9.3), hand tampers, etc. (for minor works only).

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Asphalt Paving

The condition of rollers used for asphalt work can have a significant effect on the finished surface.

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To be suitable for asphalt work, the compacting surfaces of the equipment must be true and free from marks.

Figure 9.1: Vibratory steel wheeled roller

It is important that rollers are properly maintained. With pneumatic-tyred rollers the tyre pressures should be even and wheel bearings in good condition to avoid uneven tyre load and resulting differential compaction of the mix. Adequate ballast should also be used.

Figure 9.2: Pneumatic-tyred roller

Rollers should have good brakes and smooth transmission systems to prevent shoving damage to the mix when starting, stopping and changing direction. The steering should also be in proper adjustment.

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Asphalt Paving

To prevent pick-up of the mix, rollers should be fitted with watering systems and fibre mats or scrapers. Care should be taken not to use excessive amounts of water, which may cool the asphalt. Once the tyres of pneumatic-tyred rollers have heated up, the water may not be required. Light applications of diesel oil on this compaction equipment may also be used to prevent pick-up. The use of excessive diesel should be avoided, as it will soften the asphalt.

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Mechanical tampers, vibratory plates, hand guided rollers and hand tampers are used mainly on small jobs or in areas where it is difficult to obtain access for larger rollers, e.g. median nosings.

Figure 9.3: Vibratory plate

9.3 Mix Temperatures for Placing Asphalt must be compacted while it is in a workable state. The major factors in workability or compactability are: ƒ

internal friction, determined by maximum aggregate particle size, particle size distribution and aggregate shape

ƒ

viscosity of the binder, which affects internal cohesion and the shear resistance.

Generally, mixes with coarse gradings, fully crushed aggregates and larger aggregate sizes are more stable and require greater compactive effort. Viscosity of the binder is a function of temperature and binder type. Temperature is by far the most important factor and a key element in the compactability of the asphalt mix. The mix should not, however, be so hot as to result in a low viscosity of binder and insufficient cohesion in the mix to adequately support rollers without excessive displacement. Temperature of asphalt during placing will depend on the initial spreading temperature and rate of cooling.

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Asphalt Paving

The rate of cooling is a function of: ƒ

layer thickness

ƒ

road surface temperature

ƒ

ambient conditions – air temperature, wind, and moisture.

Thicker layers retain the heat longer resulting in more time for compaction and/or a reduced number of passes. Retention of heat in thick layers may also lead to a need to delay placing of subsequent layers until the lower layer has cooled sufficiently to provide a solid working platform. Asphalt will cool more rapidly at low base temperatures. Wind and rain will further increase the rate of cooling. Asphalt mixes containing modified bitumen binders generally require compaction to be completed at temperatures that are typically 5oC to 10oC higher than mixes with conventional binder as the binder stiffens more quickly. Most manufacturers of PMBs provide guidelines for their products.

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All these factors are inter-related and influence: ƒ

conditions under which asphalt work should proceed

ƒ

minimum temperature for delivery and spreading of asphalt

ƒ

time available for compaction and hence choice of rolling equipment and compaction techniques.

Table 9.1 provides a guide to minimum temperatures for spreading dense graded asphalt mixes based on road surface temperature and layer thickness using conventional bitumen binders and conventional compaction techniques. Table 9.1: Asphalt spreading temperatures Range of mix temperature3 (°C)

Minimum mix temperature2 (°C)

Road surface temperature1 (°C)

Thickness of layer, mm < 30

30–40

41–100

> 100

5–10

See note 4

See note 4

145

135–150

10–15

150

145

140

130–145

15–25

150

145

135

125–140

> 25

150

145

130

120–135

Notes: 1.

Surface temperature should be generally that applicable to the coolest area of the pavements, e.g. shade areas, if applicable.

2.

Mix temperatures apply to dense graded mixes with Class 170 and 320 bitumen binder. Use of Class 600, Multigrade, or PMBs may require minimum temperatures 5°C to 10°C higher than those shown. The need to avoid binder drain-off in open graded mixes may result in temperatures of up to 15°C less than dense graded mixes.

3.

The upper limit of temperature of thick layers is applied to avoid excessive displacement under rolling.

4.

Placing asphalt in thin layers under cool conditions may adversely affect the result due to the increased difficulty in achieving proper compaction, effective joints and good surface finish. Additional attention should be paid to issues of mix workability, asphalt temperature, compaction techniques and any influence from additional cooling due to wind or moisture.

5.

Placing of asphalt over a previous layer that has not cooled below about 65°C requires special consideration and mix temperatures should be adjusted accordingly.

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Asphalt Paving

Table 9.2 provides a guide to typical asphalt mix temperatures related to critical binder viscosities for different stages of compaction (Section 9.6). The table is based on the following nominal viscosity levels:

ƒ

Binder drainage in OGA – Binder drainage will be influenced by binder content, filler content and use of fibres. Without fibres, the risk of drainage is increased if the binder viscosity is less than about 0.12 Pa.s.

ƒ

Maximum compaction temperature – The asphalt mix must have sufficient cohesion to support rolling equipment without excessive displacement. This is typically a binder viscosity of around 0.12 Pa.s.

ƒ

Minimum compaction temperature – This is the minimum temperature at which initial rolling by steel rollers remains effective and typically occurs at around 10 Pa.s, although some PMBs may continue to be compacted at slightly higher binder viscosities.

ƒ

Minimum for final rolling – The minimum temperature at which intermediate and final rolling is effective. Typically occurs at around 100 Pa.s.

ƒ

Softening point – Typical values are provided for reference and generally represent a viscosity of around 1,200 Pa.s. Table 9.2: Typical temperatures for placing and compacting of asphalt containing various binders Temperature (°C)1 Softening point (1200 Pa.s)

Minimum for final rolling (100 Pa.s)

Minimum for effective compaction2 (10 Pa.s)

Maximum for compaction (cohesion) (0.25 Pa.s)

Maximum to avoid drainage in OGA4 (0.12 Pa.s)

Optimum mixing2 (0.1 Pa.s)

170

45

65

90

140

150

160

320

48

70

95

150

160

165

600

52

75

100

160

n.a.

170

Bitumen

Binder class

Multigrade

52

75

100

165

165

170

A20E

65

85

105

150

155

1603

A10E

85

100

110

155

160

1653

A35P

60

80

100

150

155

160

A30P

65

85

105

150

160

175

PMBs

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Mixing – a viscosity of around 0.1 Pa.s provides a binder of a suitable consistency for mixing with aggregates, and also tends to be around the maximum temperature for production and delivery.

Notes: 1.

Temperatures are typical of relevant binder classes in about the middle of the classification range.

2.

Polymer modified binders may be mixed and compacted at temperatures that represent a slightly higher viscosity than non modified binders.

3.

Industry Code of Practice for SBS binders permits a maximum of 165°C to avoid fuming.

4.

Fibres can inhibit drainage at higher temperatures.

Placing of thin layers of asphalt at temperatures recommended in Table 9.1 will generally provide around four to five minutes of time before the asphalt mix temperature falls below the minimum required for effective compaction (Table 9.2). For thicker layers the recommended spreading temperatures may provide a period of up to 10 minutes for compaction to remain effective. These times apply to dry conditions and wind speeds less than about 10 km/h. Higher wind speeds or presence of moisture will significantly reduce the time available for effective compaction.

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Asphalt Paving

9.4 Determination and Use of Temperature Profiles In some cases it may be desirable to establish the cooling rate of asphalt as an aid to effective operation of rollers. This may be used to determine the time available for initial rolling and the maximum distance behind the paver that rollers should operate for each compaction phase. A suitable procedure is application of the temperature limits from Table 9.1 to an asphalt temperature profile curve determined from direct measurement of the temperature of the asphalt mat. A thermometer is placed in the centre of the uncompacted layer immediately behind the paver, at the start of the paving operations. Temperature readings are taken as the distance between the paver and the thermometer increases, for 10 m intervals (up to 200 m depending on rate of cooling). The readings are plotted on the Asphalt Temperature Profile Sheet (Figure 9.4), so that the resulting profile curve intersects the lower temperature limits for the three rolling phases. The intersection points define the maximum distance behind the paver within which each phase of the rolling should be completed.

For a particular set of conditions, the asphalt temperature profile only needs to be determined once, unless the asphalt mix temperature, layer thickness or ambient conditions change. 180 Example only Dense-graded asphalt - Class 170 bitumen

160 Temperature of asphalt (C)

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Figure 9.4 is an example only. Although minimum temperatures for completion of rolling phases are specified, temperature profile curves are situation specific and must be drawn up on site as required.

140 120

Initial rolling

100

Intermediate rolling Final rolling

80 60

Maximum distance for completion of each rolling phase

40 0

20

40

60

80

100

120

140

160

180

200

Distance from paver (m)

Figure 9.4: Asphalt temperature profile curve

9.5 Roller Numbers and Speed The minimum number of rollers required to achieve adequate compaction will depend on: ƒ

depth and width of layer

ƒ

laydown temperature and rate of cooling of mix

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Asphalt Paving

ƒ

rate of spreading (output)

ƒ

mix type and binder type

ƒ

compactability of mix.

Table 9.3 provides a guide to the minimum number of rollers required based on rate of spreading, or output. Actual types and numbers of rollers should be determined for each project. Sufficient rollers should be available on the work site to match the desired output of the paver(s). Otherwise, the output of the paver(s) may be restricted. On major projects, stand-by rollers should be available to cover the possibility of plant breakdown. Table 9.3: Number of rollers

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Range of output

Alternative combinations of rollers

Tonnes per hour

Tonnes per day

Steel static 3

Steel vibrating 4

Pneumatic tyred 5

Up to 20

Up to 160

1 −

− 1

1 1

20 to 45

160 to 360

1 −

− 1

1 1

45 to 85

360 to 680

1 − −

− 1 2

2 1 1

85 to 120

680 to 960

1 2 −

− − 2

3 2 1

Notes: 1. 2.

For thin layers in cold weather, additional rollers will be required. Thicknesses assumed for the purpose of this table are up to 60 mm. For greater thicknesses, the numbers of rollers may be reduced, e.g. for rate 45 t/h to 85 t/h and depth of 100 mm, only one steel vibrating or static roller and one pneumatic-tyred roller would be required.

3.

The steel static rollers are assumed to be self-propelled and of 6 t to 12 t mass.

4.

The steel vibrating rollers are assumed to be self-propelled tandems of a minimum of 6 t mass.

5.

The pneumatic-tyred rollers are assumed to be of 10 t to 20 t ballasted mass. Multi tyred rollers are not used on open graded asphalt, ultra thin open graded asphalt, stone mastic asphalt and many minor works.

Rollers should travel at uniform speed, which is sufficiently slow to prevent displacement of the mix. Acceleration and braking should be carried out as smoothly as possible. Roller speeds should be in the following ranges: ƒ

steel wheeled rollers - not exceeding 5 km/h

ƒ

vibratory rollers - 8 to 10 km/h

ƒ

pneumatic-tyred rollers - 6 to 10 km/h.

If the mix is being adversely affected by rolling, the speeds should be reduced to prevent excessive displacement. If transverse cracks appear in the surface, the roller should be removed until the mix is sufficiently stable to support the roller.

Austroads 2006 — 45 —

Asphalt Paving

9.6 Rolling Procedures Rolling should be carried out as soon as possible after placing the mix but should not commence before deficiencies in spreading of the mix are corrected. The rolling of a freshly laid asphalt mix should be carried out in the following order: ƒ

transverse joints

ƒ

longitudinal joints (when adjoining a previous run)

ƒ

outside edge

ƒ

remainder of the mat.

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The sequence of rolling the remainder of the mat is: ƒ

initial rolling

ƒ

intermediate rolling

ƒ

final rolling.

Initial rolling compacts the asphalt to obtain most of the final density. Intermediate rolling increases density and seals the surface. Final rolling removes roller marks and other blemishes left by previous rolling. A typical rolling sequence is shown in Table 9.4. Table 9.4: Typical rolling sequence Rolling phase

Roller type

Initial rolling

Non-vibratory steel roller or vibratory roller in static mode

Intermediate rolling

Vibratory steel roller in vibratory mode plus pneumatic-tyred roller

Final rolling

Steel roller

Vibration is not normally used for very thin lifts, open graded asphalt and stone mastic asphalt. To ensure the required results, the whole rolling procedure must be carefully observed, by both operators and supervisors, so that any adjustments to the rolling technique to adapt to conditions can be made immediately. Heavy equipment including rollers should never stand on recently compacted surface before it has cooled sufficiently to resist deformation under the static load.

9.7 Rolling of Transverse Joints The joint should be rolled transverse to the line of paving with a steel wheel roller initially overlapping about 150 mm onto the fresh mix. This should continue for several passes projecting about 200 mm further onto the mat each pass before commencing normal longitudinal rolling. The joint should be checked with a 3 m straight edge immediately following rolling of each lane, to facilitate correction of excessive deviations.

Austroads 2006 — 46 —

Asphalt Paving

Boards should be used to enable the roller to travel beyond the edge of the layer without causing damage or rounding of the edge of the mat.

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The rolling of a transverse joint is shown in Figure 9.5.

Figure 9.5: Rolling of transverse joint

9.8 Rolling of Longitudinal Joints The most efficient way to compact a longitudinal joint is to place the roller on the hot (new) mat and overlap the joint by a distance of about 150 mm (Figure 9.6). The mix at the joint is compacted into the joint area by the roller as long as the new mix at the joint is at the proper height.

Overlap 150mm

Double drum vibratory roller

Lane 1 (compacted mix)

Lane 2 (uncompacted mix)

Figure 9.6: Compacting longitudinal joint

In the past it was common practice to do the initial rolling of the longitudinal joint from the cold (previously placed) side of the joint. With this method the major portion of the weight is supported by the cold, compacted mat. Only about 150 mm of the width of the roller hangs over the fresh mat, compressing the mix along the joint. Rollers cannot be operated in vibratory mode on the compacted mat and the majority of the compactive force is wasted. While the roller is operating on the cold side of the joint, the rest of the mix is cooling, thereby reducing the ability to obtain the desired density over the entire pavement width.

Austroads 2006 — 47 —

Asphalt Paving

9.9 Initial Rolling Most of the increase in density of the asphalt occurs during initial (sometimes called ‘breakdown’) rolling. It is therefore an important phase of the rolling operation. Initial rolling is normally begun at the lower side of the run with the roller moving and reversing on the same longitudinal track. Rollers with only one driven drum should work with the driving wheels closest to the paver (Figure 9.7) except on very steep grades where the driving wheels are placed down-hill. Rolling should then proceed in parallel passes from the lower to the higher edge of the newly laid mat. Two passes should be completed on each track, with each track over-lapping the preceding track by about 200 mm and finishing beyond the end of the preceding track by at least 1 m.

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Initial compaction is obtained using a steel-wheeled roller, making two to four passes over the material. Each pass of the roller consists of a movement in either direction over the length of uncompacted mix.

Figure 9.7: Operation of rollers

9.9.1

Unsupported edges

When rolling unsupported (free) edges, except those which are to be hot joints, the roller should overhang the edge by no more than about 150 mm. When the layer thickness is 100 mm or more, the following procedure should be applied: ƒ

complete rolling to within 200 mm of edge to minimise displacement of the mix before proceeding to roll the edge

ƒ

the first roller pass of this 200 mm strip should cover about half of the unrolled strip

ƒ

the second roller pass should cover the remainder of the strip, but not overhang the edge by more than 150 mm.

Austroads 2006 — 48 —

Asphalt Paving

9.10 Intermediate Rolling Intermediate rolling usually completes the compaction process to achieve the specified density. It also seals the surface. This phase ensures minimum distortion when the pavement is opened to traffic. Intermediate rolling is achieved with steel rollers followed by a pneumatic-tyred roller. The number of passes will depend on the type of roller used, and is generally between two and seven. Vibratory rollers will generally achieve the required density with fewer passes than static rollers.

9.11 Final Rolling Final rolling is only necessary where roller marks, generally from the intermediate rolling, need to be removed. The marks are removed by making the minimum number of passes necessary, with a steel-wheeled roller.

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9.12 Special Techniques On occasions roller stop depressions may occur that need to be removed by cross rolling in conjunction with intermediate or final rolling.

9.13 Rolling Pattern Best results are obtained only when the rolling is performed in a definite pattern that provides a uniform coverage of the run. A rolling pattern should be developed to suit the particular location, and operators should be required to follow it. The traditional recommended rolling pattern for initial rolling of thin layers with a steel-wheeled roller is shown in Figure 9.8. Rolling patterns should be arranged so that joints are rolled first. The length of a rolling lane should normally be about 30 to 50 m, depending on ambient conditions. Sharp turns should be avoided and any change from forward to reverse should be made smoothly. Vibratory rollers should not be stopped or reversed while in vibratory mode. Lateral changes in the direction of rolling should be made on previously compacted mix. The rolling pattern and number of passes should be selected carefully when using vibratory rollers. Over use of vibration or number of passes can lead to de-compaction and delamination of the mix. This results in a rapid lowering of density, which is extremely difficult to correct with the falling temperature. An alternative technique allows for a sweeping turn at the end of each pass equivalent to the width of an adjacent pass, during the initial rolling. This method results in a diagonal roller stop depression (and ridge of uncompacted material) rather than it being 'square'. When the roller passes over this diagonal stop, the progressive (rather than sudden) transition from the compacted mat to the uncompacted material, results in a better distribution of the material and a reduced depression. Therefore, this technique aims at a better surface finish with less noticeable roller stop depressions.

Austroads 2006 — 49 —

Asphalt Paving

1 2

LAP 1

3

LAP 2

4

7

5

LAP 3

6

Width of lane being paved

Direction of paving

Width of roller

Every pass of the roller should proceed straight into the uncompacted mix and return on the same path After the required passes are completed, the roller should move back across the lane (path 7) and repe the procedure.

Figure 9.8: Typical rolling pattern

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9.14 Hand Compaction Hand compaction is normally restricted to small areas where rollers cannot operate. The mix should be placed in layers of sufficient thickness to allow compaction to adequate density, and should be finished with a texture similar to that of the surrounding surface.

9.15 Compaction of Open-graded Asphalt The rolling procedure for open-graded mixes differs from the above in the following respects: ƒ

compaction is by steel wheeled roller in the static (non-vibratory) mode

ƒ

fewer passes are generally required to achieve the specified density

ƒ

pneumatic-tyred rollers should not be used on open-graded wearing courses because of a kneading and closing effect on the mix.

9.16 Compaction of Deep Lift Asphalt Deeper lifts retain heat longer, have a longer time available for compaction and are generally easier to compact. The larger size aggregate used in deeper lifts tends to make the mix more workable, facilitating compaction. The retained heat also enables a greater coverage with the same rolling equipment before the mix temperature falls below that for optimum compaction. Special care is needed to ensure that good longitudinal and transverse profiles are maintained. Rolling must not be left too long, otherwise the surface may cool off and form a crust, which then causes excessive cracking during rolling.

Austroads 2006 — 50 —

Asphalt Paving

9.17 Compaction of Stone Mastic Asphalt SMA mix is compacted at similar temperatures to dense graded asphalt. Generally, a heavy static roller (minimum 10 t) is preferred. Vibratory rollers can fracture the aggregate particles due to the stone-on-stone contact of this type of mix. Vibratory compaction can also lead to flushing by bringing binder to the surface of the mix. The use of vibration should therefore be kept to a minimum - no more than one or two passes. Pneumatic multi-tyred rollers should not be used because of the tendency to cause flushing. Similarly, opening to traffic before the surface temperature falls to about 40°C can also draw binder to the surface with flushing and loss of surface texture.

9.18 Placing and Compaction of Ultra Thin Open-graded Asphalt Surfacing

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The procedure for placing open-graded asphalt as ultra thin surfacing generally involves a heavy tack coat of polymer modified bituminous emulsion immediately ahead of the spreading of asphalt. This is most effectively achieved in a purpose built paving unit that combines spraying of tack coat and spreading of asphalt in a single machine. The application rate of tack coat is normally 0.9 L/m² but may need to be varied in some circumstances such as use in combination with a SAMI treatment, in which case the tack coat application rate may be substantially reduced. The emulsion is usually applied at a temperature of 60 to 80°C. The process relies on the heat of the OGA mix to break the emulsion. The wearing course may be laid in layers from 12 mm to 25 mm thick for a 10 mm nominal size. Handwork should be avoided as much as practicable. Compaction is similar to conventional open-graded asphalt. Rolling normally uses a minimum of 3 passes of a double drum steel wheeled roller, with a minimum mass of 6 t, before the material temperature has fallen below about 80°C. Due to rapid cooling of the mix, and rate of spreading, two steel-wheeled rollers are usually necessary. Special care should be taken to ensure that joints are properly formed and uniform. The new surface may be opened to traffic immediately upon completion of compaction and when the mix has cooled sufficiently to ensure no damage by traffic.

Austroads 2006 — 51 —

Asphalt Paving

10 FINISHED PAVEMENT PROPERTIES 10.1 General Finished surface properties of asphalt include: ƒ

placing to desired layer thickness and/or finished surface levels

ƒ

placing to required longitudinal and transverse shape

ƒ

smooth longitudinal profile for good riding quality

ƒ

compaction to uniform high standards of density.

Tolerances on thickness, level and shape, minimum standards of ride quality (if relevant) and minimum standards of compacted density are generally included in relevant specifications. Typical values are given in the following paragraphs.

10.2 Thickness and Level

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The average total compacted thickness of the combined asphalt courses should be not less than the specified thickness. The thickness of any individual asphalt course should be not less than the specified thickness by more than 10 mm. Each course should be finished to a plane surface, parallel to the finished surface of the wearing course, so that subsequent courses can be of uniform thickness. The level at the top of each course of asphalt should not differ from the specified level by more than 10 mm, except that where asphalt is placed against kerb and channel, the surface at the edge of the wearing course should be flush with, or not more than 5 mm above the lip of the channel. Open-graded wearing course, which is designed to allow free drainage from the lowest edge, needs to be finished with the entire layer thickness above the lip of the channel unless special edge drainage is provided.

10.3 Shape Surface shape is generally assessed as the deviation measured between any two points under a 3 m straight edge. Commonly specified tolerances for shape are indicated in Table 10.1. Straight edge checking may be carried out at any stage of construction. Checking should be performed at closely spaced intervals and at all joints. Table 10.1: Typical permissible tolerances in shape Deviations from 3 m straight edge (mm) Airports, rural highways where traffic speed >70 km/h

Urban highways, classified roads where traffic speed <70 km/h

Minor roads, residential streets

Parking areas, driveways

Correction course

10

15

20

25

Base course / intermediate course

5

10

12

15

Wearing course

3

5

7

10

Course

Austroads 2006 — 52 —

Asphalt Paving

10.4 Riding Quality The finished surface should have a smooth longitudinal profile for good riding quality. For general asphalt work, the application of shape standards as referred to above, together with the use of good placing practices, should provide adequate surface smoothness and ride quality. Measurement of pavement smoothness may be applicable to high class facilities where the importance of the resulting ride quality justifies both the cost of testing and the additional cost of work procedures that may be required to achieve the required standard, for example, freeways and major arterial roads with posted speed limits of 80 km/h or more. The standard of ride quality that can be achieved will depend on the roughness of the surface on which the asphalt layer is to be placed, and the extent of shape correction and additional asphalt layers that may be applied prior to the final layer.

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Ride quality may be measured in terms of roughness using either a Laser Profiler (Figure 10.1) fitted to a vehicle, or the ARRB Walking Profiler.

Figure 10.1: Laser profiler

Measurement is usually made as the average of three replica runs. Generally, each lane is divided into homogeneous sections 100 m long. Start and finish joints of the project are generally excluded. Typical values are as follows: New Work: A maximum of 40 of 50 NAASRA roughness counts is generally applicable where the contractor has control of the shape of base and wearing course layers. Higher standards are achievable but the cost of obtaining higher ride standards through the use of special techniques and equipment needs to be balanced against the benefit likely to be derived from the attainment of that higher standard of pavement smoothness. Resurfacing: The standard of ride quality achievable will be influenced by the shape of the existing surface and the number of layers, or extent of any regulation and shape correction, prior to placing the wearing course layer. As a general guide, a suitable target for improvement for each layer is to achieve a NAASRA roughness count of 60% of the existing conditions plus 5. For example, for an existing roughness count of 80: Target = 0.60 x 80 + 5 = 53

Austroads 2006 — 53 —

Asphalt Paving

10.5 Compacted Density 10.5.1

General

Testing of density is usually undertaken on a lot-by-lot basis. Lot sizes are generally one shift production of the same mix and layer. Lots must be essentially homogeneous sections of completed pavement. Defective areas of pavement showing cracking, bony or fatty material should be rectified before being tested separately. Density testing is usually not undertaken on lots of less than about 50 t, layers with a nominal thickness less than 30 mm, or layers with a nominal thickness less than 2.5 times the nominal mix size. Density testing should be carried out as soon as practicable after completion of work using either core samples or nuclear gauge testing of in situ materials. Location of test sites should be determined by a suitable method of random sampling.

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Relative compaction is the percentage ratio of the in situ density of the compacted asphalt to the reference density of the asphalt of a particular lot. Characteristic values of relative compaction are calculated using statistical procedures. Typically the characteristic value should represent the lower limit that is exceeded by approximately 80% of the material in the lot. Two methods for determination of reference density are in common use. They are: ƒ

maximum density

ƒ

laboratory compacted bulk density.

Use of maximum density as the reference density allows results to be directly converted to in situ air voids (i.e. 100 – percentage relative compaction). The required value will vary according to mix type and application. Typical minimum characteristic values for air voids for dense graded asphalt work are shown in Table 10.2. Table 10.2: Typical in situ air voids (dense graded asphalt)

Layer thickness/ application

Maximum characteristic value Heavy traffic

Light/ medium traffic

30 to 50 mm

8

9

> 50 mm

7

8

High fatigue base

6

−

Typical values of relative compaction based on laboratory bulk density are shown in Table 10.3. Table 10.3: Typical relative compaction (bulk density) Layer thickness

Minimum characteristic value

30 to 50 mm

95%

> 50 mm

96%

Austroads 2006 — 54 —

Asphalt Paving

10.5.2

Testing of density using nuclear density gauge

Two forms of nuclear gauge used on asphalt work are the standard backscatter gauge and ‘thin lift’ gauge. The thin lift gauge uses a dual detector set that enables the operator to select the measurement depth over a range of 25 to 100 mm whereas the standard gauge is not suitable for layers less than 50 mm in thickness. Gauges are calibrated using blocks of materials of known density. It is also recommended that field density offsets be established by comparison with core samples. Care is required to ensure accurate seating of the meter in order to achieve accurate results.

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Nuclear density testing is quick and non-destructive. A nuclear gauge gives an indirect measure of field density and hence requires calibration in accordance with AS/NZS 2891.14.3 or AS/NZS 2891.14.4 as appropriate. Nuclear density gauges may also be used in the development of rolling patterns, in particular the number of roller passes.

Figure 10.2: Nuclear density testing

Austroads 2006 — 55 —

Asphalt Paving

11 FIELD OPERATIONS CHECK-LIST Planning and preparation

Factor

(a)

Supply of mix

ƒ

Mix type and quantity ordered

(b)

Plant requirements

ƒ

Plant requirements determined and available on site

(c)

Personnel requirements

ƒ

Trained work force available on site

(d)

Traffic control

ƒ ƒ

Traffic management plan developed

Job instructions

ƒ ƒ ƒ

Determine length, width and sequence of runs, layer thickness, etc.

(e)

Equipment and arrangements organised Determine compaction requirements Determine rolling patterns

(f)

Public notification

ƒ ƒ

Affected persons advised of proposed works

(g)

Special requirements

ƒ

Lights for night work, etc.

Advise police of effect on traffic

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Surface preparation (a)

Standard of existing surface

ƒ

Must be sound with acceptable deflection

(b)

Surface preparation

ƒ

Surface defects rectified

(c)

Pre-treatments

ƒ

Prime or primerseal properly cured

(d)

Public utilities

ƒ

Surface fittings located and marked

(e)

Cleaning

ƒ

Sweeping with road broom and all foreign matter and contaminants removed

ƒ ƒ ƒ

Determine number and size of trucks - need to match to paver output

ƒ ƒ

Ensure proper covering of mix (and insulation if required)

Transport (a)

(b)

Truck requirements

Protection of mix

Suitability of trucks for job Reversing alarms Temperature checks

Austroads 2006 — 56 —

Asphalt Paving

FIELD OPERATIONS CHECK-LIST (cont’d)

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Paver capacity and condition (a)

Specification

(b)

Engine

(c)

Slat conveyor and augers

(d)

Tyres

(e)

Screed

(f)

Screed heaters

(g)

Tampers and/or vibrators

(h)

Thickness controls

(i)

Auto controls

(j)

Paver width

Factor

ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ

Appropriate paver capacity Smooth, steady pull on screed Adequate power Steady and adequate supply of material to screed Pressures Traction Surface true and in good condition Check wear of screed plates Uniform heating to prevent material sticking to screed No hot spots or overheating Tampers compact and strike off material so that screed rides smoothly on mat being laid Tamper wear Amplitude and frequency of tampers and vibrators Insignificant backlash in screws Hydraulics must consistently answer electronics Check that controls are switched on and actually in use Hydraulic or manual extensions Extensions vibrating

Paver adjustments

(a)

Paver speed

(b)

Flow control gates

(c)

Slat conveyor speed

(d)

Auger height

(e)

Auger length

(f)

Screed length

(g)

Screed crown

(h)

Automatic level controls: (i) Beam (ii) Joint matcher (iii) Cross slope control (iv) String line (v) Sensors

ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ

Optimal output Match supply rate Effect on mat texture Minimise paver stops Control flow from hopper so that bar feeders can run continuously Should run continuously to maintain head at screed Adjust speed or flow gates 50 to 75 mm above finished mat Inconsistent density and texture Check segregation of mix Auger extensions to ensure an even feed over full length of screed Avoid interruptions to paving tunnelling of mix at centre Extension boxes Hydraulic strike-off Incorrect alignment of screed and extensions causes delineation in mat More crown at leading edge gives smoother flow Too much leading crown gives open texture edges Too little leading crown gives open texture centre Screed follows beam Screed follows matching shoe Where positive control of outside edge is difficult Controls operating correctly Large variations in base levels over short distances Operating correctly

Austroads 2006 — 57 —

Asphalt Paving

FIELD OPERATIONS CHECK-LIST (cont’d) Paving operation

(a)

(b)

(c)

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(d)

Weather conditions

Tack coating

Tipping

Cold or segregated material in hopper

(e)

Material ahead of screed

(f)

Level control

(g)

Variations in material delivered

(h)

Equipment failure

Factor

ƒ

Air temperature

ƒ

Pavement temperature

ƒ

Wind velocity

ƒ

Rain

ƒ

Cationic rapid set

ƒ

Uniform coverage, but not excessive

ƒ

Allow time for breaking of emulsion

ƒ

Protection of services, kerb and gutter, etc.

ƒ

Continuous process

ƒ

Avoid bumping paver

ƒ

Device to attach to truck wheels

ƒ

Will affect mat texture - discard

ƒ

Loosen and feed to screed before hopper is empty

ƒ

Maintain constant height

ƒ

Variations give a ripple in the mat

ƒ

Once set, should not be touched unless material quality varies significantly

ƒ

Check depth of mix regularly

ƒ

Temperature

ƒ

Appearance - see ‘Visual indicators’ below

ƒ

Halt operations until repaired, and suitable alternative equipment is available

ƒ

Form transverse joint

Austroads 2006 — 58 —

Asphalt Paving

FIELD OPERATIONS CHECK-LIST (cont’d) Visual indicators to material variations

Possible cause

(a)

Smoke or fumes from mix

ƒ

Over-heated batch, check temperature

(b)

Stiff appearance

ƒ

Cold batch, check temperature

(c)

Improper coating of larger particles

ƒ

Cold batch, check temperature

(d)

Mix slumped in truck

ƒ

Possibly too much bitumen

(e)

Mix sticking under screed

ƒ

Possibly too much bitumen

(f)

Lean, granular appearance

ƒ

Too little bitumen

(g)

Unusually shiny

ƒ

Possibly lack of fines

(h)

Lean, brown and dull on road

ƒ

Too little bitumen or excess fines. Compare texture

(i)

Rising steam or popping sound

ƒ

Excess moisture, aggregate not dried sufficiently

(j)

Segregation

ƒ

Could occur any time during handling. Correct the procedure

(k)

Contamination

ƒ

Cans, rag, solvents, temporary pavement markers. Correct the procedure

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Transverse joint construction (a)

At start of paving run (i) End of existing mat

(ii)

Paver set-up

(iii) During paving

(iv) Rolling – initial compaction

(v)

ƒ

Surface level with straightedge or string line - if level incorrect, trim back to correct level and crossfall

ƒ ƒ

Face of joint to be compacted, of even texture and near vertical

ƒ ƒ ƒ ƒ

Heat screed

ƒ ƒ ƒ

Sufficient head of mix in front of screed

ƒ ƒ

First pass – 150 to 200 mm overlap on new mat

ƒ

Terminate run at timber bulkhead

ƒ

Ramp down from correct level (to be removed and trimmed back before start of next run)

At end of paving run

Tack coat face of joint Screed level (on timber blocks) Automatic level controls in order Type and temperature of mix Smooth acceleration to required speed Check asphalt levels with straightedge before rolling Subsequent passes - progress on to the new mat with each pass or

Austroads 2006 — 59 —

Asphalt Paving

FIELD OPERATIONS CHECK-LIST (cont’d) Longitudinal joint construction

(a)

(b)

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(c)

Location and alignment of joints

Before commencement of paving run - edge of existing mat

During paving run (i) Paver alignment

(ii)

Screed operation

(iii) Handwork (d)

Rolling (i) Hot joints

(ii)

Cold joints - initial compaction

ƒ

All courses - parallel to centre line

ƒ

Wearing course - under lane edge line or at centre of lane

ƒ

Intermediate course - 150 mm from proposed longitudinal joint in wearing course

ƒ

Corrective course - 150 mm from other side of proposed longitudinal joint in wearing course

ƒ

Alignment

ƒ

Check surface level with straightedge or string line - if level incorrect, trim back to correct level and crossfall

ƒ

Face of joint to be compacted, of even texture and near vertical

ƒ

If cold joint, tack coat face of joint

ƒ

25 to 50 mm overlap on adjacent run

ƒ

Parallel to centre line

ƒ

Precise line and level to avoid handwork

ƒ

Restrict level adjustments to a minimum

ƒ

Joint matcher

ƒ

Confine handwork to pushing back overlapped material

ƒ

No raking or spreading loose material on screeded surface

ƒ

Echelon paving

ƒ

Asphalt temperatures of adjoining runs above 80oC

ƒ

First pass – 150 to 200 mm overlap on new work

ƒ

Second pass - further 200 mm overlap on to new work

ƒ

Final pass - half drum width on new mat

Austroads 2006 — 60 —

Asphalt Paving

FIELD OPERATIONS CHECK-LIST (cont’d) Rollers

Factor

(a)

Specification

ƒ

Capacity to provide specified compaction

(b)

Engine

ƒ

Adequate capacity

(c)

Transmission and brakes

ƒ ƒ

Start, stop, reverse smoothly Check for displacement of mat

(d)

Drums

ƒ ƒ

Smooth and cylindrical Absence of pitting

(e)

Tyres

ƒ ƒ

Correct pressure Smooth surface

(f)

Scrapers, sprinklers and mats

ƒ

Pick up of fines

(g)

Vibrators

ƒ

Amplitude and frequency

(h)

Steering

ƒ ƒ

Maintain proper drive line Safety

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Rolling operations (a)

Compaction equipment

ƒ

Type and number

(b)

General rolling requirements (i) Speed

ƒ ƒ

Slow and uniform Check for ripples and cracking

(ii)

ƒ ƒ

Smooth Abrupt acceleration and braking cause defects in surface

(iii) Change in line and direction

ƒ ƒ

Smooth changes. Abrupt changes cause marks and cracking Change rolling lanes on compacted area

(iv) Travel direction

ƒ

Forwards and backwards in parallel lines

ƒ

30 to 50 m - depends on weather conditions, mix temperature, layer thickness and type of compaction equipment

(vi) Vibration

ƒ ƒ

Check amplitude and frequency Switch vibrators on and off on run

(vii) Standing of rollers

ƒ

On cooled, compacted areas only

(viii) Rolling time

ƒ

Minimum time for thin layers due to rapid cooling, particularly in cold weather

(ix) Water in drums

ƒ ƒ

Sufficient to avoid pick up Excessive water cools the asphalt

(x)

ƒ ƒ

Sufficient to avoid pick up Excessive diesel softens asphalt

(xi) Transverse joints

ƒ

See Transverse joint construction (Section 8.7.3)

(xii) Longitudinal joints

ƒ

See Longitudinal joint construction (Section 8.7.1)

(xiii) Initial rolling

ƒ ƒ ƒ ƒ

Greatest percentage of compaction Rolling pattern - see (c) below Drive wheel towards paver Complete before minimum specified rolling temperature for mix

(xiv) Intermediate rolling

ƒ ƒ ƒ ƒ

Increases surface stability High tyre pressures Same pattern as initial rolling Complete before minimum specified rolling temperature for mix

(v)

Acceleration and braking

Length of rolling run

Diesel on tyres

Austroads 2006 — 61 —

Asphalt Paving

FIELD OPERATIONS CHECK-LIST (cont’d) Rolling operations (Cont.) (xv) Final rolling

(c)

Rolling patterns (i) Lifts up to 100 mm both edges unsupported

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(ii)

Factor

ƒ ƒ ƒ

Improves surface finish

ƒ

Initial rolling − Transverse joint − Outside edge − Rolling from low to high side

ƒ ƒ

Intermediate rolling

ƒ

Initial rolling − Transverse joint − Longitudinal joint − Outside edge − Rolling from low to high side

ƒ ƒ

Intermediate rolling in same pattern

ƒ

Initial rolling − Transverse joint − Rolling from 200 mm inside low edge 200 mm inside to high edge − Outside edges advancing to edge in 100 mm increments

ƒ ƒ

Intermediate rolling from low to high side

ƒ

Initial rolling − Transverse joint − Longitudinal joint − Rolling from longitudinal joint to within 500 mm of outside edge − Outside edges advancing to edge in 100 mm increments

ƒ ƒ

Intermediate rolling from low to high side

Lifts up to 100 mm against supported edge

(iii) Deep lift in excess of 100 mm both edges unsupported

(iv) Deep lift in excess of 100 mm against supported edge

No vibration Complete before minimum specified rolling temperature for mix

Final rolling

Final rolling

Final rolling

Final rolling

Austroads 2006 — 62 —

Asphalt Paving

12 TROUBLE SHOOTING GUIDE Problem Under compaction

Possible cause 1. 2.

Vibrators running too slow Incorrect auger height for material being used Cold or poorly mixed material

1. 2. 3.

Modify production process

1.

Adjust speeds to suit material

2.

Modify mix composition

3. 4.

Incorrect paving, tamper or vibrator speed Unstable mix (aggregate, temperature, etc.) Worn screed plate Cold screed

3. 4.

5.

Cold material in front of screed

5.

6.

Aggregate too large for layer thickness

6.

Replace Check operation of burners, review heating procedures Improve material delivery or, if applicable, raise mix temperature Reduce size of aggregate or increase mat thickness. Aggregate size should not exceed half layer thickness. Replace end plate Clear cold material

3. 1. 2.

Dragging or tearing of mat

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7. 8.

Loose streak down centre of mat

Screed rises at each take off

Auger shadows

Tyre or drum marks parallel to centre line

Bright streak down centre of mat

Increase speed Reset to correct height

End plate not square Cold material build-up at end of augers 9. Augers overloaded 10. Extensions incorrectly installed 11. Excessive auger wear

7. 8.

1. 2. 3.

Insufficient lead crown Flow gates too low Worn augers or worn or missing kickback paddles

1. 2. 3.

Adjust crown Increase opening Repair or replace

1. 2.

1. 2.

Adjust flow gates Improve delivery schedule or adjust paver speed

3.

Overloaded augers Mix in front of augers is cold - waiting too long between loads Varying mix temperature

3.

Improve delivery scheduling

1. 2. 3.

Overloaded augers Flow gates too high Worn augers

1. 2. 3.

Adjust flow gates Adjust flow gates Repair or replace

1.

Asphalt too cold at final rolling

1.

2.

Inadequate final rolling

2.

Improve material delivery, increase mix temperature or increase rolling capacity Increase time of final rolling

1. 2. 3.

Too much lead crown Augers worn out Flow gates too high

1. 2. 3.

Make necessary adjustment Replace augers Adjust as needed

1.

1.

Inform drivers

2.

Improve co-ordination between plant and site

3. 4.

Trucks bumping paver or holding brakes Waiting between loads - running with hopper empty Extensions incorrectly installed Fluctuating head of material

3. 4.

5. 6. 7.

Worn screed plate Worn augers Cold screed

5. 6. 7.

Review installation procedure Check and adjust flow gates; paddle boxes; auger conveyor speed Repair or replace Repair or replace Check burners – review heating procedure

2. Screed marks - poor surface texture

Remedy

Austroads 2006 — 63 —

9. Adjust paver speed and/or flow gates 10. See manual 11. Replace augers

Asphalt Paving

Trouble Shooting Guide (cont’d) Problem

Remedy

1.

Irregular head of material

1.

2. 3. 4. 5. 6. 7. 8.

Incorrect flow gate adjustment Erratic paver operating speeds Loose or worn depth screw assembly Incorrect crown adjustment Trucks holding brakes Unstable mix Worn augers

2. 3. 4. 5. 6. 7. 8.

Check machine adjustment; check material inconsistencies Adjust as needed Maintain constant speed Tighten or replace Refer to manual; review procedure Inform drivers Modify production or mix design Repair or replace

1.

Delay in rolling

1.

Improved co-ordination

2.

Over-correction of depth screws

2. ) Review procedures (see manual)

3.

Overloaded augers

3. ) Review procedures (see manual)

4.

Head of material varying

4. ) Review procedures (see manual)

5.

Too much overlap

5. ) Review procedures (see manual)

1.

Incorrect joint preparation

1.

See manual for correct procedure

2.

Varying head of material

2.

Check machine adjustment, check inconsistencies in material

3.

Incorrect rolling procedure

3. ) Review the recommended procedure in manual

4.

Cold screed

4. ) Review the recommended procedure in manual

5.

Varying mix temperatures

5.

1.

Poor rolling procedures

1. ) Review correct procedures

2.

Fluctuating head of material

2. ) Review correct procedures

3.

Excessive speed of rollers

3. ) Review correct procedures

4.

Unstable mix

4.

Redesign mix or modify production process

Insufficient range of thickness available using either manual or automatic paver screed controls

1.

Screed tow point incorrectly set

1.

2.

Screed depth control incorrectly set

2.

Raise or lower screed tow point to obtain required thickness Adjust as necessary

Insufficient range of thickness available using either manual or automatic paver screed controls

1.

Screed tow point incorrectly set

1.

2.

Screed depth control incorrectly set

2.

Raise or lower screed tow point to obtain required thickness Adjust as necessary

1. 2.

Auger height incorrect Paving, tamping or vibrating speed incorrect Material too cool in front of screed due to low delivery temperature or being allowed to get cold

1. 2.

Re-adjust the height of the auger Re-adjust speeds to suit the type of material

3.

Increase temperature of material and arrange paving speed so that it is in time with deliveries

Ripples/corrugations

Longitudinal joints having open and variable texture or different surface levels on either side of the joint

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Possible cause

Transverse joints having open and variable texture or poorly matched surface levels on either side of joint

Hairline cracks

Inconsistency of density and texture of material on the finished mat

3.

Austroads 2006 — 64 —

Correct at plant

Asphalt Paving

FURTHER READING Australian Asphalt Pavement Association, 1997. HS&E Guide No.3: Guide to safe work clothing for outdoor workers in the asphalt industry, AAPA, Kew, Vic. Austroads 2006a, Guide to Pavement Technology – Part 4B: Asphalt, Austroads, Sydney, NSW Austroads 2006b, Asphalt Manufacture, Austroads, Sydney, NSW Blaw-Knox Construction Equipment Co., 1986, Paving Manual, Rochester England. Department of Main Roads NSW, 1988/1989, Asphalt work - field procedures and supervision, RTA, Sydney, NSW. Pioneer Asphalt Pty Ltd, Asphalt Manual. Pioneer, Sydney, NSW. Transit New Zealand, 2004, Code of practice for temporary traffic management, Transit New Zealand, Wellington, NZ.

Licensed to Mr Giora Rozmarin on 19 Oct 2008. Personal use licence only. Storage, distribution or use on network prohibited.

Standards Australia

AS 1742.3 Manual of uniform traffic control devices, Part 3 – Traffic control devices for works on roads. Handbook 81.6: Field Guide to traffic control on roadworks: Part 6 – Bituminous surfacing works. AS2891.14 Methods of sampling and testing asphalt - Field density tests. AS2891.14.1.1 Determination of field density of compacted asphalt using a nuclear surface moisture-density gauge – Direct transmission mode. AS2891.14.1.2 Backscatter mode. AS2891.14.2 Determination of field density of compacted asphalt using a nuclear thin-layer density gauge. AS2891.14.3 Calibration of nuclear thin-layer density gauge using standard blocks. AS2891.14.4 Calibration of nuclear surface moisture-density gauge – Backscatter mode. AS2891.14.5 Density ratio of compacted asphalt.

Austroads 2006 — 65 —

INFORMATION RETRIEVAL

Austroads, 2006, Asphalt Paving, Sydney, A4, 77pp, AP-T65/06 Keywords: Asphalt, pavements, construction, paving (asphalt), surfacing, compaction, joints, handwork.

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Abstract: This guide has been prepared as part of a series supporting the Austroads Guide to Pavement Technology, Part 4B: Asphalt, to provide details on the process of placing asphalt. The document consists of sections dealing with the planning and preparation necessary to achieve an acceptable outcome, advice on the optimal operating conditions for key plant used in the placement of asphalt and the finished condition of the asphalt layer. Extensive checklists are provided to assist users in ensuring that all aspects related to the placement of asphalt are covered.