Bre Digest 330 Pt4 Alkali-silica Reaction In Concrete

Alkali–silica reaction in concrete

digest

Simplified guidance for new construction using normal reactivity aggregates

Digest 330 Part 4

2004 Edition

BRE Centre for Concrete Construction

Concrete can deteriorate as a result of an interaction between alkaline pore fluids (principally originating from the Portland cements) and reactive minerals in certain types of aggregates. The mechanism of deterioration is known as alkali–aggregate reaction (AAR); it can occur in a number of forms, the most common being alkali–silica reaction (ASR). This Digest is in four parts Part 1 gives the background to the detailed and simplified guidance contained in Parts 2 and 4. Part 2 gives detailed guidance for minimising the risk of damaging ASR in new construction. Part 3 gives worked examples. Part 4 gives simplified guidance for new construction using aggregates of normal reactivity. ASR map cracking between more substantial but unrelated structural cracks

This Part of the Digest contains simplified, essential recommendations for minimising the risk of damaging ASR. It applies to concrete in which aggregates of normal reactivity are to be used with cementitious materials in mix designs commonly used in the UK. The decision to provide simplified guidance was taken in response to comments received at the public comment stage when the 1997 revision of this Digest was being prepared. This simplified guidance is not a substitute for the full recommendations: readers with doubts about any aspect of this guidance should refer to the full text in Part 2. Because the guidance for aggregate combinations classified as having normal reactivity is more onerous than that for low reactivity aggregates, the guidance here can be used for both.

Guidance The recommendations in this Digest are based on the principle that different limits can be set for alkali contents of concrete; these limits depend on the reactivity classification of the aggregate combination, and on the alkali content of the Portland cement or of the CEM I component of a combination with ground granulated blastfurnace slag (ggbs) or pulverised fuel ash (pfa).

The simplified guidance given in Table 1 applies in the majority of cases of concrete for new construction where aggregates have been classified as having normal reactivity (see Table 2). The qualifying conditions on page 3 which underpin the guidance must also be met. If you are in any doubt, refer to Part 2 of this Digest.

A BRE research project funded by DTI Construction Sector Unit

2 Table 1 Simplified guidance for alkali content of concrete containing aggregate combinations classified as having normal reactivity Alkali content of the CEM I component of the cement or the Recommendation CEM I component of a combination with ggbs or pfa Guaranteed ≤0.60% Na2O eq

No mix alkali calculation is needed as the mix is self-limiting. The conditions set out in ‘Qualifying conditions’ and Note A on the opposite page apply

Declared mean ≤0.75% Na2O eq

A calculation of mix alkali is needed. The calculation takes account only of the alkalis contained in the cement or CEM I component of a combination. The mix alkali content should not exceed 3.5 kg Na2O eq/m3. The conditions set out in ‘Qualifying conditions’ and Note B on the opposite page apply

Table 2 Aggregates classified as having normal reactivity (excluding RCA) Most UK aggregate combinations will be classified as having normal reactivity. Descriptions of aggregates classified as having high reactivity or low reactivity and of recycled concrete aggregate and recommendations are given in Part 2 of this Digest Examples

Comment

Natural aggregate combinations that contain chert or flint,

Flint or chert is found in most of the UK sand and gravel deposits

excluding those which contain more than 10% crushed greywacke

of southern and northern England. This includes most of the sea dredged aggregates Where a flint bearing sand is used in combination with a flint bearing gravel, the risk of damaging ASR is considerably reduced or eliminated. However, in a small number of specific cases, full suppression of ASR has not occurred in laboratory tests where these combinations have been used. For precautionary reasons, combinations of flint bearing sands and gravels are now considered as having normal reactivity

Natural aggregate combinations that contain quartzite, excluding those which contain more than 10% crushed greywacke

Quartzite is a common constituent of many sand and gravel deposits.

3 Qualifying conditions ● The aggregate combination is classified as having normal reactivity and does not include RCA. ● The total cementitious content of the mix does not exceed 550 kg/m3. ● The concrete contains one of the following: a cement which conforms to: BS EN 197-1 CEM I, BS EN 197-1 CEM II/A–L or CEM II/A–LL, BS 1370 or BS 4027 or BS 146 BIIIA or BS EN 197-1 CEM III/A where the proportion of ggbs is≥ 40% by mass, or BS 146 BIIIB or BS EN 197-1 CEM III/B or BS EN 197-1 CEM II/B–V or BS EN 197-1 CEM IV/A (with siliceous pfa only) where the proportion of pfa is≥ 25% by mass, or BS EN 197-1 CEM IV/B (with siliceous pfa only) or BS 6610; or a CEM I cement in a combination where: the proportion of ggbs is≥ 40% by mass of the total combination or the proportion of pfa is≥ 25% by mass of the total combination. If lesser proportions of ggbs or pfa are being considered, refer to Part 2 of this Digest.

● Where used, the ggbs conforms to BS 6699 with an acid-soluble alkali content≤ 1.0% Na2O eq. ● Where used, the pfa conforms to BS 3892-1 or BS EN 450 with a total acid-soluble alkali content≤ 5.0% Na2O eq.

Note A: where the alkali content of the cement is guaranteed≤ 0.60% Na2O eq the alkalis contributed from other sources should not exceed 0.60 kg Na2O eq/m3.

Note B: where the alkali content of the cement has a declared mean≤ 0.75% Na2O eq the alkalis contributed from other sources should not exceed 0.20 kg Na2O eq/m3. If this value of 0.20 kg/m3 for alkalis from other sources is exceeded, it can be accommodated in accordance with Part 2 by: ● reducing the alkali limit by subtracting the total amount contributed; ● adjusting the maximum cement content or combination content in some exceptional cases.

4 References British Standards Institution BS 146:2002 Specification for blastfurnace cements with strength properties outside the scope of BS EN 197-1 BS 1370:1979 Specification for low heat Portland cement BS 3892-1:1997 Pulverized fuel ash. Specification for pulverized fuel ash for use with Portland cement BS 4027:1996 Specification for sulfate-resisting Portland cement BS 6610:1996 Specification for Pozzolanic pulverized fuel ash cement BS 6699:1992 Specification for ground granulated blastfurnace slag for use with Portland cement BS EN 197-1:2000 Cement. Composition, specifications and conformity criteria for common cements BS EN 450:1995 Fly ash for concrete. Definitions, requirements and quality control

Acknowledgement The authors would like to acknowledge the support of the Department of Trade and Industry in the preparation of this Digest.

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