Ciria Report 108

'"'

REPORT 108

Concrete pressure on formwork

C.A. CLEAR BSc T.A. HARRISOtf>hD BSc CEng MICE MICT

ISBN: 0 860 17 250 3

© CIRIA1985 REPRINTED 2003

• • ,-·:-:

B

CONSTRUCT ION INDUSTRYRESEARCH AND INFORMATION ASSOCIATION 6 STOREY'S GATE, LONDON SW1BAU TELEPHONE 020 7222 8891 FAX 020 7333 1708 Email [email protected] Website www.ciria.org

The project leading to this Report was ca1Tied out under contract 10 6l!RliA Cement and Concrete Associati on where Mr Clear is a Research Engineer and Dr Harrison is Manager of the Ci vil Engineering Group in the Technical Applications Directorate.

This Report was prepared with the help and guidance of the Project Steering Group. In addi tion to Dr Harrison and Mr Clear, the Group comprised: R.M. Hand BSc(Eng) CEng MICE (Chainnan)

John Mowlem and Company PLC

C.M. Reeves MIC1MlnstSMM

Frodingham Cement Company Limited

J.M. Dransfield BSc

Cement Admixtures Association

P.M. Follett BEng CEng MICE

Pozzolanic Lytag Limited

M.Grant BSc CEng FICE

Kyle Stewart (Contractors) Limited

P.G K. Knight BSc(Eng) CEng MTCE MIS trnctE CEGB, Ash Marketing P.F. Palleu BSc CEng MICE MBIM

Rapid Metal Development Limited

K. Ward BSc CEng MICE

Sir Ro bert McA lpine & Sons Limited

A.R. McAvoy BSc CEng MICE was CIR IA's Research Manager for the project. This project was financial ly supported by the Cement and Concrete Association, C ivi l and Marine Limited, Departm ent of the Env ironment, Frodingham Cement Company Lim ited, and Blue Circle Industries PLC.

Contents page LIST OF ILLUSTRATIONS

4

LIST OF TABLES

4

ABBREVIATIONS USED

5

NOTATION

5

SUMMARY

6

INTRODUCTION

6

1.

DESIGN METHOD

2.

NOTES FOR GUIDANCE 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17

3.

9

Cementitious materials and admixtures Aggregates Mix p:roportions No-fines concrete Workability Concrete temperature at placing Weight density Vertical form height Shape and plan area of the cast section Formwork permeability Formwork stiffness and roughness Slope of the form Placing method Rate of rise Impact of concrete discharge Vibration Underwater concreting

13 13 13 13 13 14 14 14 15 15 16 16 17 17 17 18 18

Bridge abutment Partition wall Lift shaft Mass concrete retaining wall Bridge column 'V' column

20 21 22 23 25 29

EXAMPLES 3.1 3.2 3.3 3.4 3.5 3.6

ACKNOWLEDGEMENTS

31

REFERENCES

31

List of illustrations Figure 1

Comparison of the CIRIAnethods

Figure 2

Comparison between measured and calculated pressures on vertical formwork

Figure 3

Design pressure envelope

Figure 4

Example of formwork pressures and deflection measurements

Figure 5

Height value to be used in the formulae

Figure 6

Plan dimensions, showing sections de/1ned as 'colummr 'walls and bases'

Figure 7

Pressure envelopes on the formwork o/ a wall with a sloping face where the fluid head is fully developed

Figure 8

Relationship between rate of rise and pressure

Figure 9

Pressure measurements on an underwater tremie pour

List of tables Table 1

Value of coefficients t;and

Table 2

Design formwork pressures

4

c;,

r.lR IA RFmn rt 1 OR

Abbreviations used

ore LHPBFC PBFC PPFAC RHPC ggbf.~ pfa

ordinary Portland cement low heat Portland- blast furnace cement Portland-blastfumace cement Portland pulverised-fuel ash cement rapid-hardening Portland cement ground granulated blastfumace slag pulverised-fuel ash

Notation A 8

I(

plan area of mass concrete wall: 111 breadth of mass concrete wall , m coefficient dependent on the size and shape of fonnw~ coefficient dependent on the constituent materials of the conci/iti weigh t density of concrete, kNfn{kgfm3 x 0.0098 1) vertical form height, m vertical form height, m temperature coefficient taken

)II

maximum concrete pressure on fonnwork, kWm volume supply rate, i\ih the rate at which the concrete ri ses vertically up the fonn, m/h concrete temperatu.re at placing, °C width of mass concrete wall, m

c; Ci D H h

CIRIA Report 108

al~)' \T+t 6

5

Summary The Report extends and improves the method used in CIWptort I to cover concretes using adm ixtures and blends o r blended cements. Use of the method is demonstrated in six examples: bridge abutm ent, partition wall, lift shaft, mass concrete retaining wall, bridge column, and \tolumn.

Introduction The method given in CER~ ow CIR IA) Report Cl) for calculating concrete pressures o n formwork has served the construction industr y well since 1965. However, the design charts were limited to plain ordinary Portland cement concretes, and recent trends are towards the wider use o f admixtures and blends or blended cements .• In addition, the com mercial need for faster co nstruction has resulted in a general increase in lift heights and rates of concrete placing. New theories backed up wi th site data were be ing devel-Op~dand it was decided to review the 1965 design method and to extend it to co ver concretes which contain adm ixtures and/or blends or blended cemems. CIR IA instigated a further programme o f site fonn work pressure measurements and the compilatio n of a comprehensive data fi le to contain bo th the new site measurements and previous ly o btained data. Th is file o n fonn work pressures contains over 350 sets of data. Using this data fi le in co11junctio n with recen t advances in the understand ing o f the mechanisms, an improved method of estimating the corncrete pressure on formwork was fomrnlated. Many factors affect the fonn work pressure, some o f which are unknown at the design stage. There are also other random site effects such as impact o n discharge. Because many of the factors are inter-related, different interpretations of the data are possible, and there are several safe po tential methods for estimating the pressure. The revised method is based o n a model using o nly factors which should be known o r which ca n be reasonably estimated at the design stage. T he method is sufficiently conservative to envelo p the spread of results for the factors no t taken into account. Figure 1 shows a comparison of measured pressures against calculated pressures using the calculation methods gi ven in Report I and this Repo rt. This illustration only includes data wh ich are strictly applicable to the Report I method (i.e. the concrete does not contain adm ixtures, pfa o r ggbfs), and where all the data necessary for the calculation were recorded . Figure I (a) and (b) shows that the new method is as safe as the previous method . Fig ure 2 includes the data for concrete containing admixtures, pfa and ggbf.~. Data better handled by hand calculatio n (because of no n-regular section or o ther special conditions) were not included in Figures I and 2. Hand calculatio ns showed that a few tall tapering columns gave measured pressures in excess of the calculated values. The Steering Gro up considered all the data, including t hose not presented in these illustr atio ns, and concluded that it would be uneconomic to recommend a method which enve loped all results irrespective of the exceptional conditions under which some of the results were recorded. The calculated pressures in Figures I and 2 are based on the recorded placing conditions. In practice, the placing conditions are estimated . Assum ing that these are no1mally 'safe 'estimates, this has the effect of introducing an additio nal factor of safety not shown in F ig ures I and 2. • Ablcnd is s cement "tic, c 3 PMl:md ocmcm ha..:; been combined " i1h a bl.Cm hydraulic binder. usually _ggh(.:; or pfa. 31 the 001ching plant. A blended cement i1> 3 oombinatioo. of Ponbnd ocrncni wilh a latent 11:)draulic bindct (uwally ggbfi; or p!S). purcha.t;o..--d d1n.-c1 ftom a cc.mcnt company.

6

CIRIA Reoort 108

150 - - - - - - - - - - - - - - - - - - - - - - - - - - -

.E 120

§

z

,!!,.

@

Ii

90

0

0

:, V,

g E E :, E ·x

CO

60

0~ 0 0 0 00

0

~

o0

30

30

0

0

<:P

0

0~

0

00

0

8

0

90

60

~

150

120

210

180

Coleuoted pressures (kN/m' J (o) 1965 method

150 - - - - - - - - - - - - - - - - - - - - - - - - - - -

.E 120

-:iz

0 0

@

i

90

1i :,

E E :, E ·x

60

:i:

30

0 0

0

0


0~

¥ B Q)

0 0 00

0

0

0

0

0

0

0

0

0

0

0

30

60

90

120

150 7

Coleuloted pressures (kN/m ) {b) 1985 method

Figure 1

Comparison of the C/R!Anethods

180

210

NOTES 1. Tnis illustration incMtes the data lo, internally-vibrated oonetete placed ,n nomnal yvetti<:3.1 fOfm.wtk 2. Data whic:ti contained estimated values of concrete temperature are excluded 3. Oa13 for sloping and irregular shaped fornMOrk are excluded

150

120

0

0

iz

-"'

u, ~ :, u,


90

~Cb

(/)

Q)

a. "' "'"' Q)

0

0

0

60

0

0

0 0

:i

0

0

Q)

E E :, E

0 30

·x

"'

~

0

30

60

90

120

150

180

Calculated pressures (kN/m' )

Figure 2

Companson between measured and calculated pressures on vertical formwork (all mlevant data are included)

210

1. Design method Freshly placed concrete comprises a gradation o f particles from coarse aggregate down to fine cement particles, all o f which are suspended to a greater or lesser extent in water. Th is is not a stable cond ition. T he loss or disp lacement of a mere fraction o f the total mix water (by settlement, leakage or hydration) can change the structure of the fresh concrete from a quasi liquid to a re latively stiff framework of touching particles with the water contained within the voids. This change in structure is important. While the aggregates and cement are suspended in water, the concrete exerts a fl uid pressurll',)(Dn the formwork, but once a stable particle structure has been created, further increments of vertical load have an insign ificant effect on the lateral pressure. T herefore the maximum lateral pressure is generally below the nuid head, and it is controlled by this change of strncture (which can take from a few minutes to a few hours). T he following factors affect this change of state (and hence the maximum fonnwork pressure): Concrete

admixtures aggregate shape, size, grading and density cement itious materials mix pr-Oportions temperature at placing weight density workability

Fom1work

pennea biIity/ watertightness plan a1-e a of the cast section plan shape of the cast section roughness o f the sheeting material slope o f the form sti ffi1es.s of the form vertical fonn height

Placing

impact of concrete discharge in air or under water placing method (e.g. in lifts or continuous vertical rate of rise) vibration

T he complex inter-relationships of these factors are nor descri bed in this Report. A rationalised design equation is p~esented, together with a description of how the variables should be treated under design c ond itions. Using concepts developed at the Cement and Concrete Association and during recent research on the mechanisms creating formwork pressu\l~ the data for OPC concrete were analysed to quantify the relationships between maximum pressure, vertical form height, nue of rise, and concrete temperature at placing. Modifications to these basic relationships were then developed for concrete containing admixtures, ggbfs or pfa. Th is analysis led to the following expression for the maximum concrete pressure on formwork: • The rcscsn.":h by Ck at will he published in a Cemcn1 and C.oncn.'1c A~!',(X'i3Lion RcpM during 1986.

P..~ = [ C, ,.,G + c, where

CJ C2

D

H h

K

J

K fl - C,

,.,G]

or Dh kN/m2 whichever is the smaller.

coefficient dependent on the size and shape of fonn work (see Table I for values), coe fficient dependent on the constituent materials of the concrete (see Table for values), weight density of concrete, kN/m vertical form height, m vertical pour height, m , 6 temperature coefficient taken as ( ) 16

Jmii

r:

the rate at wh ich the concrete rises vert ically up the form, m/h concrete temperature at placing, °C When c,Jn > fl. the tl uid4tJ)e51ho11l<( be taken as the design pressure. R

r

The term c,JH incorporates the effects of vibration and workabi lity, because these factors are largely dependent on size, shape and rate of rise. A ll the effects o f the height of discharge, cement type, admixtures, and concrete temperature at pllacing are incorporated in the te1m :

The design chart, Table 2, quantifies these equations for normal UK conditions where the concrete placing temperature is between 5 and I 5°C. Pressure values shown in bold on the chart are for placing conditions broadly covered by pressure measurements on site, where the highest recorded pressures were 90 kN1rfur walls and I 66 kNihfor columns. Values not in bold are outside recorded experience. They are in accord with the general tr end, but may be somewhat conservative. No change is proposed in the design pressure envelope from that given in the Ref:R,~ I des ign method. The envelope (see Figure 3) comprises tluid pressure to the depth where the maximum pressure obtained from the design equation or chart occurs and then remains at this value. Figure 4 is an example of measurements of fonn work pressure and detlection taken on a site. This illustration shows that once a fonn detlects, it remains in that state unti l the tie bolts are re leased. In theory, a rigid fonn would experience a reduction in pressure after the max imum. In practice, fom1s are not rigid, and some stress remains between the fonn and concrete. For this reason, no reduction in pressure after the maximum is given in the design pressure envelope.

Table 1

Values of coefficients (land

C,,

Walls: Columns: Concrete OPC, RHPC or SRPC without admixtures OPC, RHPC or SRPC with any adm ixture, except a 1·etarder* OPC, RHPC or SRPC with a retarder LHPBFC, PBFC, PPFAC or blends contain ing less tha n 70% ggbfs or 40% pfa without admixtures LHPBFC, PBFC, PPFAC or blends contain ing less tha n 70% ggbfs or 40% pfa with any admixtures, except a retarder* LHPBFC, PBFC, PPFAC or blends contain ing less tha n 70% ggbfs or 40% p fa with a retarder t Blends containing more than 70% ggbfs or 40% pfa t t

Value of <; 0.3 0.3 0.45 0.45 0.45 0.6 0.6

S.."C Sixlion 2 . 1 TI'ICsc combinaiion.,:; of 1iu1c.rial$ are ~ and 111c ...aluci:; i.ncfiC3100 an:: derived from cx~pol:uioo. of lhc d313. togclllcr " ith s cons.idcr.uion of the tb.."Oretical effect,;.

Typlcol envelOpe of p,essure on lorrnwork

/

' ·,.

Height of

'

concrete h(m)

J

'·,.

I___

Figure 3 Design pressure envelope

Design pressure envelope , .,envelope or pressu e tt ' ·, ,,/ concrete ocled os o fluld

'·,

~

. _~--·_ ..______

·,_·,_ .,_ ' ·,_ ·,_,._'....:·';. ·

+--P~

Pressure (kNm')

Section: 0.8 x 5 x 6 m long Concrete: Normal-weight OPC Concrete temperature at placing: 13°C S1ump: 40mm Rate of rise: 3.2 m/h Deflection (mm)

Pressure (kN/m' ) 10.00 0

20

40

60

80

100

120

140

0

2.0

4.0

6.0

10.30 Calculated vertical 11 .00

11.30

0 0

~ 12.00

F 12.30

'

0

Total pressure on the form

Porewater pressure on the fonn 13.00

13.30

No reinforcement in the vicinity of the gauge

Deflection measure on the back of lhe gauge

14.00

Figure 4

Example of formwork pressures and deflection measumments

2. Notes for guidance 2.1 CEMENTITIOUS MATERIALS AND ADMIXTURES Coefficient c; (see Table I) takes into account the effects of different cemen1111ous materials and admixtures. T he term ' adm ixturirl Table I covers the range of products commercially availa ble in 1985. W ithin the grouping ' retard&tfl retarders, retarding water-red ucers, and retarding superplasticisers, also any admixture wh ich is used above the recommended dosage such that it effectively acts as a retarder. A major change from existing practice is the recommendation that superplasticised concrete should be incl uded within the general grouping, and that it does not necessarily req uire desig n pressure equal to the n uid head.

2.2 AGGREGATES Wh ile q uantify ing the design equation, the effects of the aggregate shape and grad ing could not be isolated from the o ther mix parameters, so these factors are not included in the desig n method. W ith the exception of no-fines concrete (see Sectio n 2.4), the formula and tables apply 10 al l graded natural aggregates. T he design eq uations apply 10 co ncrete mixes contam mg maxim um aggregate sizes up to 40 mm. Pressures with larger maxim um sized aggregates are likely to be controlled by the impact on discharge and the heavy vibration required. The design procedures for lig ht- and heavyweig ht aggregate concretes are described in Section 2.7.

2.3 MIX PROPORTIONS T he formula and design tables a pply 10 the whole range of no rmal n11x proportions.

2.4 NO- FINES CONCRETE Because no-fines concrete has a particle structure from the mo ment of placing, it results low formwork pressure. Typical desig n values are of the o rder of 2 10 2.'.!\ k~/that handling stresses are likely to contr ol the design of the fonn.

111

2.5 WORKABILITY Slump is not included as a variable in the design chart for the following reasons: I. The problems with placing low workability concrete around reinforcement lead 10 prolonged vibration and formwork pressures simi lar 10 those o btained wi th more workable concretes. 2. The site data show no consistent difference and high slump concretes.

111

fonn work pressure between low, medium

3. S lump is not a good measure o f the factors which a ffect formwork pressure. Fonn work pressures with nowing concretes are covered in Section 2. I .

CIRIA Report 108

13

a

2.6 CONCRETE TEMPERATURE AT PLACING At low rates of concrete placing, hydration effects become a significant factor in detem1in ing the maximum formwork pressure. Because these effects depend on the concrete temperature at placing, the design equation includes a temperature factor

6 K=(r: 16) 2

Although this only strictly applies to OPC and RHPC concretes, it is sufficiently accurate for al l types of concrete when used in conj unction with coeffici~nfflieK factor represents a ratio of sci ffening effects, which are dependent on temperature at placing. Data for concrete temperatures at placing in excess of 30°C or below 5°C are rare, and it is prudent not to extrapolate the design equation beyond t hese values. (Out of the 352 sets of data recorded over a number of years, on only 17 occasions were the temperatures at placing below 8°C).

2.7 WEIGHT DENSITY The design chart (Table 2) assumes a weight density o f 25 3kN'Ani6 is safe va lue for normal-weight concretes. The procedure for calculating the maximum formwork pressure with light- or heavyweight concretes is to use the appr,opriate weight density in the design equation (see Example 2). The design charts can be used to obtain the pressure with ligh t- and heavyweight concrete by taking the chart value and then adjujttingmtaby weight density.

2.8 VERTICAlFORM HEIGHT The vertical form heigh t is important for two reasons:

1. It limits the potential maximum pressure wh ich can develop (in general, the maxim um des ign pressure is not greater th:iM) . 2. Height of discharge affects the magn itude of the impact forces. Both these factors affect the maximum formwork pressure, and they have been incorporated in the design equation as a function of the fonn heigh·t. Sometimes, the form can be substantially higher than the height of section cast (see Figure 5). In these cases, the limiting pressure might be the nuid pressure (which is obtained from the weight density times the actual pour height). This should be checked with a separate calculation.

o.•

'tJf. .t.o

Q-,·

O· Depth of concrete

Vertical form height

h

H

Figure 5

14

H

H

H

Height value to be used in the fonnulae

CIRIA Report 108

2.9 SHAPE AND PLAN AREflf THE CAST SECTION In a section of small plan area, vibration can be sufficient to mobilise all the concrete in a layer and to tr ansmit a re lati vely high amount of energy to the fom1. This has the effect of increasing the depth over wh ich vibration is effective, and consequently the pressure on the form. In a larger section, all the concrete in a layer is not mobilised at the same time, and less energy is transmitt ed into the fonnwork. The point of concrete discharge and vibration is nonnally moved along the section, wh ich allows concrete a period of rest before the next layer is placed. T he net e ffect is that in ' Vlhlls maximum pressures are lower than in ' columns'. In fundamental tenns, a wall is where the concrete is placed in layers with the point of discharge and vi bration moving along the wall , while, for columns, the po int of discharge and vibration is raised vertically. These conditions can be conservatively de fined using the fo llowing simple defini tions:

wall orbasc - section w here eitheuhe width or breadth exceeds 2 m column - section where botwidth and breadth are 2 m or less. These de finitions are shown diagrammatically in Figure 6. The few site data recorded for small, single-storey columns indicated a nuid pressure distribution. The formula generally pred icts nuid head for small columns. Th is is reasonable, because small columns can be p laced very quick ly and vibrated such that the full nuid head is mobilised. However, an analysis of the forces on column clamps indicates that they would fail if concrete in columns develops full n uid pressure. It is therefore widespread practice 10 design small ply and timber column fonns assum ing less than the fluid head. T he possible explanation of this anomaly has not been experimentally veri fied.

2.10 FORMWORK PERMEABILITY Fonnwork pressure decreases as the fonnwork permeability increases, if all other conditions are equal. This renects the extent to wh ich excess porewater pressure can dissipate through the fonnwork. T he pressures are subs tantially lower with extr emely penneable fonn materials such as expanded metal or fabric. In theory, the design equation should contain a factor fom1 pem1eability. Effects such as reduction of penneability through previous usage and the use of sealers and coatings, throw doubt on the ' practical:ify such a factor. Because the design equation does not include a factor for form pem1eability , the estimated pressures are not applicable to fonn materials such as expanded metal, where they effectively act as free surfaces and prevent the build up of porewater pressure.

Walls and bases

21 - - - - - - - - - - - - ,

1 lo-

Figure 6 Plan dimensions showing sections defined as 'columns' or 'walls and bases'

CIRIA Report 108

Columns

I

I

0

2

3

Breadth (ml

15

2.11 FORMWORK STIFFNESS AND ROUGHNESS S tudy of the data suggests that the use of stiffer forms results in high pressures. Conversely, independent research work shows that the fonnwork pressure decreases substantially if a sti ff form is moved sl ightly outwards. In most practical situations, the sti ffness of a fom1 varies from point to point, and it is difficult to quantify. Formwork stiffoess was not, therefore, included in the design equation. While the concrete is acting as a fl uid, the fonnwork roughness is immaterial, unti l a particle structure fonns and the concrete starts to develop internal friction. Compared with other factors, its infl uence on the maximum pressure is small , and it has not been isolated in the design equatio n.

2.12 SLOPE OF THE FORM The pressure on sloping fonns was not specifically examined in the research, and only a few experimental results were available. However, the C!Ribl:thod described in this Report can be used conservatively with non-parallel sided walls wi th and wi thout a unifom1 rate of rise. If the volume supply rate is varied so that the rate of vertical rise is constant, the equatio n o r tables can be di rectly used. The pressure at any level in the po ur is the same on both faces, and the direction of action is perpendicular to the form (see Fig ure 7). The following method is suggested for calculating the pressure envelope with a constant volume supply rate :

1. Split the pour into horizontal levels with the vertical di stance between each level I m or less. 2. Calculate the plan area at each level.

T

H (m)

1•

Figure 7

16

Pressure envelopes on the fonnwork of a wall with sloping face where the fluid head is fully developed

r. lR IA RFmnrt 1 OR

3. Calculate the instantaneous rat e of rise at each level from unifonn volume supply rate (M h) plan area at the level considered l)n

H"-"'-d = - - - - - - - - - - - - - -

4. Calculate the pressure at each level us ing the full height of thehfomnl,h either the equation or tables. 5. Produce the design pressure e nvelope acting at right angles to the fom1. Th is procedure is illustrated in Examples 4, 5 and 6 (pages 23 to 30).

2.13 PLACING METHOD The difference between placing m lifts and continuous vertical placing has been described m Section 2.9. The design equations do not apply to conditions where the concrete is being pumped from below or where pre-placed large aggregate is grouted from below. In both these cases, the formwork pressures are likely to be higher than those given in this Report. American experience(•) suggests that the fonnwork should be designed to withstand nuid pressure plus 50% for pump surge.

2.14 RATE OF RISE The rate at which the concrete rises vertically up the fonnwork is an important factor, and it is incl uded in the design equation. In practice, this is never constant, but, the use of an average rate of rise is nonnally adequate for vertical fonnwork. The average rate of rise might not be applicable when a considerable l ift is placed rapidly, followed by a long delay before the subsequent lift. As the rate of rise increases, the maximum pressure increases, but the relationsh ip is not linear. At high rates of rise, changes in the rate of rise have less effect on the maximum pressure than changes at lower rates o f rise (see Figure 8).

2.15 IMPACT OF CONCRETE DISCHARGE The effects of impact on discharge are incorporated into the design equation. Attention is drawn to the comments in Section 2.8.

•

-~-----

Fluid pressure

~- - - - -

Q) ~

:::, II) II)

Q)

ct

Figure 8 Relationship between rate of rise and pressure

CIRIA Report 108

Rat e of vertical rise

17

2.16 VIBRATION The design method assumes nonnal internal vi bration. D eep revibration can substantially increase the formwork pressure above the calculated va lue. If this technique is 10 be used, the fonn should be designed to withstand the fluid pressure at the depth o f poker immersion if this is greater than the nonnal design value. This design method does not apply to externally-vi brated concrete. The action of vibrating the form induces str ess addi tional to that created by the concrete pressure.

2.17 UNDERWATER CONCRETING When fonnwork is designed for use underwater, the buoyant weight density (Density of concrete - Density of water = 25 9.81 -. 15 kN/m) is nonn ally used to calculate the e ffective fonnwork pressure. For fully-submerged sections, the fonnwork pressure can be calculated using either l. the design equation (see page 9)

,\!llw

15 kN/ rn

or 2. 0.6 times the value obtained from Table 2 (page 11 ). The procedures are based on the assumption that the static water pressure is equal on both sides of the form work, and that it does not result in stresses in the formwork. T his is a reasonable bas is for des ign when the water leve l is not changing, but it can under-estimate the pressures where there is a rapid drop in water level during concreting. T his latter situation is analogous to an earth dam subjected to ' rapid drawdowmhal\ge in water level produces an instantaneous change in pressure on the outside o f a form, but a much slower change in the water pressure within the concrete, because that depend.s upon the permeability and hydraulic gradient within the concrete. When the water level is falling rapidly, this effect can result in the horizontal pressure exceeding the vertical pressure (see Figure 9). On the other hand, a rising water level reduces the formwork pressure. In these circumstances, the formwork should be designed to resist the effecti ve fonnwork pressure plus a surcharge proportional to the maximum tidal fal l.

Pressure (kN/m 2 ) 0

0800 ___

10

20

30

40

'

'

'

'

50

.

60

70

•

•

80

90

.

100

•

_,_

I -

0900

Tl ,L.

1000

-,

l_

'.

\ \ 'E

1100 -

~

Cl)

E

i=

1200

-

'l

·,

'

.

-

Formwork pressure

_i __ • -----:===•

.-------

\

\

\

\

\ 1300

-

\

Calculated vert ical pressure

:

__ -----

----I I

•--, • I

\

\ 1400

-

Figure 9

'

\. ---4

Pressure reduction on the outside of the 1500 - form as a result of the fall in tide level

• ' I

\

I

I

Press{lre meas{lre.ments on an {lnderwater tremie po{lr

3. Examples The following examples illustrate the procedure for calculating the design pressure d istribution.

3.1 BRIDGE ABUTMENT Section

0.8

X

6

X

5

111

high

Abutment details

c, = 1.0

OPC nonnal-weight concrete

Concrete

n=

Concrete temperature at placia,)°C

D = 25 kN/n'I K = 1.92

2

:. K

h = 5 m

C2 = 0.3

=(~10 + 16 ) =1.92

R = 5 m/ h

Rate of placemen14 nr/h pumped

.

24

Rate o r n s e - 0.8 X 6 From table

From tables P max = 80 kN/ri\

5 m/h P max (al H- 4 mt 75 kN/ri\ P max (al H- 6 mt 85 kN/ri\ P max (al H- 5 mt 80 kN/ri\ 80 kN/ni < Dh = 125 kN/m

From fonnula P max = 80 kN/ri\

From formala P max = 25

~ .o'-'5 + 0 3 x l.?2~5 -

80 kN/m2
Pressure eavelope fcmrmwork desiJ:•

~= 3.2 m

25

5 m

80 kN/ni1

1 0"'5)

3.2 PARTITION WALL

Section

0.2

X

ore

Coacrete

5

X

Wall details c,= 1.0

4 m high

H = ;, =

lightweight concrete

Concrete temperature at placi.ngi°C 36 ) ' K = ( 15 + 16

f? = 10 m/ h

= '-35

Rate of riscO m/ h From tables

Pmo., (al v-

25 kN/riJ

From tables P max = 70 kN/m

= 90 kN/m

: . P max (al o- I'> kN/ riJ

=

19 25

4 m

C2 = 0.3 o = 19 kN/rn /{ = 1.35

X

90

68 kN/m < D!t = 76 kN/r11

=268 kN/m From fonnula P max = 65 kN/m

From formula

P,,,. = 19

~ .0,Jfo + 0.3 X J.35 ~ 4 -

1.0

.fio)

= 67. 1 kN/m < 76 kN/tii Pressure envelope for formwork design

65 _

19

4 m

1~

65 kN/ITT

r. l R IA RP nnrt 1 n R

-

3.4 m

,. I

21

3.3 LIFT SHAFT Section

2m

-

2m

. 2f50nm

Plan wew (8 m high) Contin uous vertical placing and constant vibration unl ike ly. T herefore tr eat as a wall with breadth = 4x 1.75 m

Equivalent scctiod>.25 x 7 x 6 m high

Sltaft details

c,= 1.0 Concrete OPC normal-weight concrete

H=h= 6 m C2 = 0.3 o = 25 kN/ril K = 1.35

Concrete temperature at placingi°C

36

K = ( 15 + 16

)'

I "5

= _.,

J? = 5 m/h

Rate of riso m/h

Pmax = 75 kN/ri\< Oh

From tables

Pressure envelope for formwork design

I

75

25

6m

2

75 kN/m

II

I

I

=

150 kNAn

Pmax

=

75 kN/ri\

3.4 MASS CONCRETE RETAINING WALL Section

Sm

3.Sm

Coacrete OPC nomial-weight concrete with retarder

Wall details 1.0 H= 5 m C 2 = 0.45 D = 25 kN/m

Concrete temperature at placi.rig)°C

K = 1.92

c,=

End elevation (10 m long)

2

:. K= (i/:16) =1.92 Uniform volume supply

~

g=

18 mh

One 6 m truck every 20 mm = I 8l/h1

Rate of riseThe rate of rise increases as the pour progresses because the section narrows. Therefore the pressures are calculated for instantaneous rates of rise at specific levels. The full height of the form work is used in the formula, irrespective of the level for which it is calculated.

R - Un i fonn vol umesupply rate (m3/h) - Plan area at levelconsidered (m'/h) Using formala ~ " = D (C, fa + C, K

Jn-

C, fa )

= 25 (fa+ 0.45 x 1.92 Js - fa ) = 25 (fa + o.864Js - fa )

Area, A = Breadth f/3) x width (w) h (m)

~ o

··4

T h

w

~

0.5

~ 1.0 I 1.5

2.0

I

~ 2.5

3.0

I I

....

3.5

4.0 4.5

I I

5 .0

Calcalatioo of rate of rise and pressure I,

Dl1

(m)

(kN/m2)

w (m)

(m2)

R (m/h)

0 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00

0 12.5 25.0 37.5 50.0 62.5 75.0 87.5 100.0 I 12.5 125.0

1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50

10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0 32.5 35.0

1.80 1.44 1.20 1.03 0.90 0.80 0.72 0.65 0.60 0.55 0.51

A

Pmax (kN/m2)

72 70 68 67 66 65 64 64 63 63

>

Fluid head

Pressure at 11 (kN/m2) 0 12.5 25.0 37.5 50.0 62.5 65.0 64.0 64.0 63.0 63.0

Pressure envelope fcmrmwork dcsig•

Mite: Pressure acts at right angles to face, and the stability of the forms has

5m

631
10

be considered.

3.5 BRIDGE COLUMN Section

-I

l m

I•

I.Sm

I

l~ 5m

5m

5m

5m

2m

End eievonon

2.5m Front view

Sectioa cast Although one plan dimension is greater than 2 111, the section should be cons ide red a column, because the vibratio n and ve rtical p lacing a re likely to be continuous. Coacrete

OPC normal-weight concrete with a ir entraining agent

CIRIA Report 108

Column details 1.5 H = 16 m

c,=

C 2 = 0.3

D = 25 kN/n1

25

Concrete temperatnre at placiift°C ... K= ( 1036 + 16 ) ' = I.,''2

U niform volnme snpply rate

K = 1.92

20 rrr/h

Rate of riseVariable. Has to be calculated at specific depths. Pressure calculated using this rate and the full height of the fo1m . Using formula

P = D (C, /;f + c ,

X

: 25 ~ .5/;f + 0.3 X

J:f) J.92J6 - l.5 J:f)

K x~H - C,

= 25 ~ .5/;f + o.s16J16 - 1.5 J:f) Calculation of rate of rise aad pressure at a uaiform volame sapply rafithof 20 m I,

Dl1

B

(m)

(kN/m2)

(m)

1 2 'J 4 5 6 7 8 9 10 11 12 13 14 15 16

25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400

1.5 1.5 1.5 1.5 1.5 1.7 1.9 2.1 2.3 2.5 2.5 2.5 2.5 2.5 2.5 2.5

?fi

w (m)

A (m2)

R (m/ h)

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

1.5 1.5 1.5 1.5 1.5 1.7 1.9 2. 1 2.3 2.5 3.0 3.5 4.0 4.5 5.0 5.0

13.3 13.3 13.3 13.3 13.3

1.2

1.4 1.6 1.8 2.0 2.5

11.8 10.5 9.5 8.7 8.0 6.7 5.7 5.0 4.4 4.0 4.0

p (kN/1112)

183 183 183 183 183 176 170 164! 160 155 147 140 135 130 127 127

Pressure at ;, (kN/m2)

25 50 > 75 F uid 100 h, ad 125 150 170 164 160 155 147 140 135 130 127 127

Pressure envelope for fom1work design

h(m)

Pressure (l:N/m~

50

j

__I_ 0

I.~ ,,,·> -

__I_ 1

,. ·

__I_ 2

f-

100

__I_ 3 __I_ 4

I

\

'"0 : ~-~~ /

__I_ 5

----

__I_ 6 '1

__I_ 7

)

164 r - -

I

I

__I_ 6

I

I I I

__I_ 9

I

155

147

I--1-

l

\ 140 \

.____

I

t-/

__I_ 10 ~

I

I

I

I

130 I

'

I

I I I

__I_ 13

I I

I I

I

__I_ 14

I

I

127

__I_ 12

I

1

135

__I_ 11

__I_ 15

I

__I_ 16

Front ll1ew

Pressure envelope fcmrmwork dcsiga

h{m)

Pressure (k/.'ffl~

I

0

I

164 160

135

I

2

I

3

I

4

I

5

T

6

..Y..

1

I

8

T

9

I

10

I

11

I

12

I

13

I

14

I

15

I

16

130

127 127

o·C>

127

11•'

IH

End OIOl'Olion

28

CIRIA Reoort 108

3.6 'V'COLUMN 1.2Sm

I•

l.2Sm

•I•

1m

l.2Sm

•I• •I•

1.2Sm

•I•

•I

5m

~--3m

2m

2m

2m

•I End 818""1/on

Front view

Section

Although the breadth exceeds 2111, the possiibility o f continuous vertical placing and vibration cannot be d iscounted

Coaerete

OPC norm al -weight conc re te with superplast iciser

Coaerete temperature at plaeinilf'ery cold win ter cond itions, so taken as 5°C

mtrucks

per hour = 123,lh

Using formala

p=

o(c,JR +c,

X

Jn - c, JR) 2.94J8 - J .5 Ji)

Kx

= 25

~.5/R + 0.3

= 25

~ .5/R + 0.88 J 8 -

X

H= 8 m C2 = 0.3 o = 25 kN/m K = 2.94

K = -36-) ' = 2.94 ( 5 + 16 Uniform volume supply raTevo 6

Column details

c, = 1.5

1.5

Ji)

Calcalation of rate of rise aad pressure at a uniform volume s apply raftthof 12 m

h Dh (m) (kN/m2) 1

2 '~ 4 5 6

7 8

25 50 75 100 125 150 175 200

8 (m)

w (m)

25 2.5 2.5 2.5 2.5 3.0 2.5 2.0

I I

I I I I I I

(1112)

p R (m/ h) (kN/1112)

25 2.5 2.5 2.5 2.5 3.0 2.5 2.0

48 4.8 4.8 4.8 4.8 4.0 4.8 6.0

A

Pressure at h(kN/m2)

25 50 75 100 125 124 130 138

'")

130 > 130 Fluid 130 head 130 124 130 138

Half pressure envelope fisrmwork desig• Pressure (kN/m~

.1 0 .1

I

.1 2 .1 3

.1 5 .1 6

.x.

7

.1 8 fl (m)

FrontvluN

.x.

0

.1

l

.1 3

I

'I .,' \ <

I

4

I

s

I

6

.x.

7

.1 8

Acknowledgements P. A. GAndrews D. A. Biddlecombe P. E. Le Bihan D. P. Burrage A. I. L. Byers R. P. Cannon J. R. C hampion J. Collins A. T. Corn ish J. Dallaway R. M. Ed meads P. J. Egan A. J. Goldsmith P. S. Goodall J. I larrington-Ly nn J. E. llarris J. R. 111ingworth G. S. Kirk F. Lane P. R. Luckett D. Maher W. E. Murphy P. L. Owens K. R. Pook S. M. Rao P. L. Rawlinson B. G Richardson P. Rogerson P. Rowdon B. M. Sadgrove M. F. Taylor C. F. Tum er R. T. Ward P. F. Watson R. V Watson A. S. White P. Williams C. J. Wilshere

Taylor Woodrow Construction Limited G KN Kw ik form Limited Balfour Beatty Construction Limited Steveland Products Limited Balfour Beatty Construction Limited Frod ingham Cement Company Lim ited Sir Alfred McA lpine & Son Limited Mabey ll ire Company Limited Blue Circle Industries PLC/Cement Manufacturers Federation Ove A ntp & Partners Ceme11tatim1 Research Li111 iced/Cement Adm ixtures Association Fosroc Technology Limited/Cement Admixtures Association Wimpey Construction (UK) Limited Pozament Cement Limited Department of the Environment Mabey ll ire Company Limited Wimpey Construction (UK) Limited Blue Circle Industries PLC/Cement Manufacturers Federation S ir Robe11 McAlpine & Sons Limited C hart Formwork Lim ited MB Formwork Limited/National Association of Formwork Constructors Cement and Concrete Association Consultant Property Serv ices Agency Prope11)' Serv ices Agency Stehno Lim ited C IRIA Taylor Woo drow Construction Limited Cementatio:n Construction Limited CIRIA Acrow (Engineers) Limited Rapid Metal Developments Limited Ta rmac Co nstruction Limited Stehno Lim ited Cement and Concrete Associat ion Scaffold ing (Great Britain) Limited John Mowlem & Company PLC John La ing Construction Limited

References I. KINN EAR, R.Get al The pressure of concrete on formwork CERA (now CIRI A) Report I, A pr il 1965

2. I !ARRISON, T.A. The pressure on ve11ical formwork when concrete is placed in wide sections Cement and Concrete Association, Research Report 22, March 1983 3. I IA BGOOD, M.G Site formwork pressure measuremen ts in w ide sections recorded during the period of Ma rch 1980 to J une 198 I Cement and Concrete Association Depa11mental Note 2058, 1982 4. FORD, J.I I. Consolidation of concrete using pla~tic form liners and plastic coated plywoods Conference paper, Second Internatio na l Conference on Forming Economical Concrete Buildings, Chicago, November 1984

., 1

r11:no. R,:,nnrt 1 f'lA