Alkali-Aggregate Reaction in Concrete Structures and Preventive Measures

When the alkali of cement reacts with the reactive constituents of aggregates, the reaction is deleterious though very slow, and the distress is exhibited when the volume inside the concrete is increased due to the formation of alkali-silica gel, causing the concrete to expand and crack.
Dr S.C.Maiti, (Ex) Joint Director, National Council for Cement and Building Materials

Distress due to alkali-aggregate reaction have been noticed in some of our hydro-electric projects. Bridge piers and concrete pavement have also suffered distress due to alkali-aggregate reaction in Poland and U.S.A. OPC containing less than 0.6% alkali (as Na2O equivalent) is considered to be low-alkali cement, as such cement, if used in concrete, the deleterious alkali-aggregate reaction is not likely to occur. PSC with more than 50% slag and having maximum 0.9% alkali is also considered as “low alkali“ cement. Higher slag content (say, 56 to 70%) in PSC, is permitted by IS455. But, such cement will have higher alkali content, and may be suitable against chlorides and sulphates in soil and ground water, but may not be suitable for use in hydro-electric projects, to combat the deleterious alkali-aggregate reaction.

Every cubic meter of aggregate cannot be tested. Therefore, precautions should be taken in construction e.g in hydro-electric projects’ structures, in sea, coastal areas, marshy land etc, where reactive aggregates may be there and moisture is always available for the reaction to take place inside concrete, to use PSC with 50-55% slag and having maximum alkali content of 0.9%(as Na2O equivalent). The International Commission on large dams, ICOLD does not recommend PPC in concrete, to combat the deleterious alkali-aggregate reaction, because of its variable properties.


Introduction
Concrete structures are supposed to provide satisfactory service during their service life. But, sometimes, they suffer distress due to chemical reactions inside the concrete. One such reaction is alkali-aggregate reaction. The alkali of cement reacts with reactive constituents of aggregates. This reaction is deleterious ,but is very slow, and after 15 or 20 years, the distress is exhibited, as the volume inside the concrete is increased due to the formation of alkali-silica gel. The gel imbibes moisture and the volume is increased. The concrete expands and cracks. Fig.1 shows a microscopic photograph of the alkali-silica gel in concrete.

Alkalies of Cement
The alkalies (Na2O and K2O) of OPC vary from plant to plant, as they are dependent on the raw materials used for the manufacture of cement. In case of PPC, the alkali content will depend on the alkali content of OPC, the alkali content of flyash and the percentage of flyash used to produce PPC. In case of PSC, the alkali content will depend on the alkali content of OPC, the alkali content of ground granulated blast furnace slag (g.g.b.s), and the percentage of g.g.b.s used to produce PSC. Thus, the alkali content of cements will vary from plant to plant. Typical alkali content of cements is shown in Table 1. Alkalies of cements are minor components, but they play a very important role for durability of concrete, especially when some of the constituents of aggregates are reactive.

Distress Noticed in Concrete Structures
Two of our dams suffered such distress in concrete. The Rihand dam in U.P and the Hirakud dam in Odissa. The power house of Rihand dam could not be operated for a number of years. Severe cracks were observed in the R.C.C columns. The distress noticed in the power house of the Rihand dam: (I) Cracks were observed in machine foundations in 1972. The depth of cracks in various locations are 7-45cm.(ii) There were operation problems including frequent tripping of generating units,(iii) One of the columns of the penstock gallery opened up at the cracks, and showed 9 of 10 reinforcement bars snapped at the end of the welded joints. Although about 15% flyash was used in concrete, this was not sufficient to combat the deleterious alkali-silica reaction, as the alkali content of OPC was very high(1.2%)[1].

The bridge piers suffered distress due to alkali-silica reaction in Poland (Fig.2)[2]. In U.S.A Portland Cement Association identified cracks in concrete roads due to alkali-silica reaction (Fig.3) [3]. In Korea also, concrete pavement cracked due to alkali- silica reaction[4].


Low Alkali Cement
Various Standards (IS456 and International Standards) define low alkali OPC as having alkali less than 0.6% as Na2O equivalent (i.e Na2O +0.658K2O). The Bulletin No.79 of ICOLD [5] states “Portland cements containing less than 0.6% equivalent sodium oxide are considered to be low alkali cements. North American experience has indicated that deleterious expansion from alkali-aggregate reaction is not likely to occur as long as the alkali content of Portland cement does not exceed the equivalent of 0.6% Na2O”. The reason for considering PSC having higher alkali content (i.e higher than 0.6%) to be “low alkali” cement , as the alkali of slag is not fully reactive.

BS 5328:Part4[6] while describing “Methods of Test for Alkali” for blended cements, stipulates that, “For g.g.b.s, the reactive alkali shall be taken as 50% of the measured value, and for pfa (i.e flyash), the reactive alkali shall be taken as 17% of the measured value”.

For the blend of OPC and at least 50%g.g.b.s, the British Research Establishment Digest 330[7] states “When using a blend, the controlling factor seems to be the greatly reduced ability of hydroxyl ions to diffuse within the cement paste, rather than the alkalies contributed by the cement or slag. These g.g.b.s blends can therefore be regarded as equivalent to low alkali Portland cements. In the recommendations of the German Committeefor reinforced Concrete issued by DIN, a blend of 50% or more g.g.b.s with OPC or a low alkali (less than 0.6% equivalent Na2O) Portland cement are the alternatives for use with a reactive aggregate”.

Recommendations of the Indian Standards on the Alkali Limit of PSC and PPC
Regarding alkali content of PSC or PPC the Indian Standards IS 455[8] and IS1489 (Part1)[9] stipulate, alkali aggregate reactions have been noticed in aggregates in some parts of the region. On large and important jobs where the concrete is likely to be exposed to humid atmosphere or wetting action, it is advisable that the aggregate be tested for alkali aggregate reaction. In the case of reactive aggregates, the use of cement with alkali content below 0.6% expressed as sodium oxide (Na2O +0.658K2O) is recommended. However, in the case of cement having a minimum slag content of 50% (a minimum flyash content of 30% in case of PPC),a maximum alkali content of 0.9%, expressed as sodium oxide(Na2O+0.658K2O) is recommended”.

Combating Alkali-Silica Reaction in Concrete using Portland Slag Cement
The Portland slag cement is manufactured using ground granulated blast furnace slag. The slag is a very useful latent hydraulic material to combat the deleterious alkali-silica reaction in concrete in structures. The granulated slag as per IS:12089[10], a by-product of iron manufacture consists essentially of glass containing silicates and aluminates of lime. The glass content of the granulated slag is about 90%.The Portland slag cement with 50-55% g.g.b.s and maximum alkali content of 0.9% (as Na2O equivalent) is ideal to combat the deleterious alkali-silica reaction in mass concrete structures e.g hydro-electric projects’ structures or any underground foundation structures e.g structures in sea or in coastal areas or in marshy land, as moisture is always available in these places, for reaction to take place inside the concrete. The reaction is also fast, as the fineness of Portland slag cement is high, generally more than 300m2/kg. The reaction product is calcium silicate hydrate, which is continuously increasing inside the concrete, and the resulting microstructure of concrete is dense. Thus, the long-term strength of PSC concrete is higher and higher with time. The impermeability of concrete is so high that, no aggressive agent can enter inside the structures.

In our construction industries specially hydro-electric projects, the above recommendations should be followed. In some project sites, the PSC having slag content of more than 50% (i.e 56 to 70%) is being used, as this is permitted by the Indian Standard IS455. In such cases, the alkali content of PSC is also going to be higher than 0.9%. Such PSC may be useful to combat the chlorides and sulphates in soil and ground water specially in coastal areas, but for hydro-electric projects ,they should not be used. This is because, higher than 0.9% alkali in PSC may not be able to combat the deleterious alkali-silica reaction (if any).

International Recommendations
Pozzonalas like flyash containing alkalies not more than 2% or 3% has been found effective in combating the alkali-silica reaction. Pozzolanas in the concrete mix are beneficial, because they reduce permeability of concrete and therefore reduce the mobility of aggressive ions, present within concrete and which may ingress{11]. G.G.B.S is also effective in mitigating the deleterious alkali-silica reaction in concrete. There is evidence that , when PSC has been used, a maximum alkali content of 0.9% is harmless, when the slag content of the cement is not less than 50%[12].

In the Netherlands, CUR-Recommendation indicates that if cement replacement by at least 25% flyash or 50% g.g.b.s is implemented, then the potential reactivity of the aggregates is of no concern[13]. In the Netherlands, the deleterious expansion in a no. of structures was observed. But it was absent where PSC had been used[14]. Malber et al [15] recommended use of low alkali OPC and cement replacement of 25to40% classF flyash or 40 to 50% g.g.b.s . They discouraged the use of 15% flyash , as it may worsen the ASR expansion, even with classF flyash.

ICOLD Bulletin no.79[4] states that there is now considerable experience of the use of PSC with some types of reactive aggregates, and there is no known instances of deleterious alkali reactions, when the PSC contained more than 50% slag and the slag has less than 0.9% alkali, as Na2O equivalent. The Bulletin does not recommend PPC in minimizing the risk of alkali-aggregate reaction , because of the variability in their properties.

Conclusions and Recommendations
Alkali of cements, although a minor component, plays an important role for durability of concrete. OPC having alkali less than 0.6% (as Na2O equivalent) is considered to be low-alkali cement, because if such cement is used in concrete, the deleterious expansion due to alkali- silica reaction is not likely to occur. The alkalies of PPC and PSC are not fully reactive. Only about 17% of the alkali of flyash and about 50% of the alkali of g.g.b.s are reactive. PSC with more than 50% slag and having less than 0.9% alkali as Na2O equivalent is considered to be “Low alkali” cement by ICOLD.

For combating the deleterious alkali-silica reaction in concrete, IS455 recommends a maximum alkali content of 0.9% as Na2O equivalent, for PSC having minimum slag content of 50%. PSC with higher than 50% slag (i.e with 56 to 70% slag), the alkali content will be higher than 0.9%. Such high-alkali PSC may not be suitable for use in concrete, to combat the deleterious alkali-aggregate reaction (if any). The International Commission on large dams ICOLD does not recommend PPC for combating the deleterious alkali-aggregate reaction, because of its variable properties.

All the aggregates are not reactive. Some Himalyan aggregates are reactive. It is not possible to test every cubic meter of aggregate. Therefore, precautions should be taken in concrete construction i.e in vulnerable cases, e.g in hydro-electric project structures, which are mostly located in hilly regions, underground foundations of concrete structures, structures in sea, in coastal areas, and in marshy land, where reactive aggregates may be there and at the same time, moisture is available for the reactions to take place inside the concrete, by using PSC with 50-55% slag and having alkali content (as Na2O equivalent) of maximum 0.9%, so that the satisfactory service is obtained for the designed service lives of the structures.

References

1. Irrigation Department, Uttar Pradesh. Rihand Dam Expert Committee Report, Vol.1,June 1986,58p.
2. Z.Owslak, J.Zapala-Slaweta, and P.Czapik. Diagnosis of concrete structures distress due to alkali-aggregate reaction. Bulletin of the Polish Academy of Sciences,Vol.63,No.1,2015.
3. J.A.Forney and B.Kerkhoff. Diagnosis and control of alkali-aggregate reaction in concrete. PCA R&D Serial No.20716, Portland Cement Asssociation,USA,2007.
4. Seung-Ho Hong, Seung-Hwan Han and Kyong-Ku Yun. A case study of concrete pavement deterioration by alkali-silica reaction in Korea. International Journal of Concrete Structures and Materials, Vol.1, No.1, December2007, pp.75-81.
5. ICOLD. International Commission on Large Dams. Alkali-Aggregate Reaction in Concrete Dams. Review and Recommendations. Bulletin No. 79, 1992.
6. BS 5328: Part4:1990.Specification for the procedure to be used in Sampling, Testing and Assessing Compliance of Concrete.British Standards Institution,London.
7. BRE. Building Research Establish- ment Digest 330. Alkali aggregate reactions in concrete. Building Research Station, Garston, Watford.
8. IS 455. Indian Standard Specification for Portland slag cement, Bureau of Indian Standards, New Delhi.
9. IS 1489(Part1). Indian Standards Specification for Portland pozzolana cement. Bureau of Indian Standards, New Delhi.
10. IS 12989 Indian Standards Specification for granulated slag for the manufacture of Portland slag cement. Bureau of Indian Standards, New Delhi.
11. A.M. Neville. Properties of Concrete. 5th Edition,2013, Pearson Education Ltd.
12. W.H.Duda. Cement Data Book.2, Berlin, Verlag GmBH , 1984, 456p.
13. V.Jensen and B.Fournier. Influence of different procedures on accelerated mortar bar and concrete prism tests: Assessment of seven Norwegian alkali-reactive aggregatyes. 11th Conference on alkali-aggregate reaction, Quebec, Canada, 2000, pp.345-354.
14. W.M.M. Heunen. Alkali-aggregate reaction in the Netherlands. Proceedings, 9th International Conference on alkali-aggregate reaction in concrete, London, Vol.1, pp.432-437.
15. L.J.Malvar, G.D. Cline, D.F Burke, R.Rollings, T.W.Sherman and J.L.Greene. Alkali-Silica Reaction Mitigation: State –of –the-Art and Recommendations. ACI Materials Journal, September-October 2002, pp.480-489.