Why are you Cracking up?

by tanya_martins | April 15, 2024 10:24 am

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Alkali-Silica Reaction (ASR) is a deleterious chemical reaction that can occur within hardened concrete. It is caused by a reaction between alkali hydroxides from the hydration of Portland cement, certain reactive silica-bearing aggregate constituents, and water. The chemical reaction creates an amorphous, hygroscopic gel (ASR gel), which absorbs water, expands, and exerts pressure on the concrete. This results in cracking, expansion, and significant loss of strength.

Cracks from ASR typically form in a polygonal, map-like pattern with orange or dark staining, often filled with ASR gel. These cracks can allow moisture and deicing salts to enter, leading to further damage such as corrosion of reinforcing steel and freeze-thaw deterioration. In severe cases, ASR deterioration is a structural concern. If ASR is suspected, the concrete should be evaluated by petrographic examination per ASTM C856, Standard Practice for Petrographic Examination of Hardened Concrete.

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One approach to mitigate ASR risk is to specify aggregate that has been tested and shown to be innocuous or specify supplementary cementitious materials (SCMs) such as certain fly ashes or slag cement as partial Portland cement replacements. SCMs can help mitigate ASR by reducing the amount of available alkali hydroxides, which is one of required ingredients for deleterious ASR to occur. In areas known to have reactive aggregates, SCMs are often included in the concrete even if the aggregate is shown to be innocuous. ASTM C1778 and AASHTO PP65 offer guidance on reducing ASR risk.

Water is required for the chemical reaction, with an internal relative humidity of at least 80 percent promoting the reaction. Repeated wetting and drying of concrete can further exacerbate ASR. To manage ASR in existing structures, treatments aim to limit concrete’s exposure to external moisture and minimize wetting and drying cycles. Extensive research by the Texas Department of Transportation (TxDOT) and others on ASR treatment, including the use of silane sealers and breathable coatings, has shown that such treatments can extend the service life of ASR-affected concrete by up to 50 percent compared to untreated concrete.¹

Applying elastomeric coatings is another option to manage ASR-affected concrete. Elastomeric coatings are effective at protecting the concrete from environmental moisture and conceptually should reduce ASR. However, these coatings may trap moisture inside the concrete. Therefore, if this approach is taken, they should be applied when the concrete’s internal relative humidity is below 80 percent and at its driest state.

Notes

¹ Refer to report 4069-3, “Mitigation Techniques for In-Service Structures with Premature Concrete Deterioration: Synthesis Report.” Center for Transportation Research. The University of Texas at Austin

Author

Andrew Lobbestael is a structural engineer and senior associate with Wiss, Janney, Elstner (WJE) Associates Inc., in Detroit, Mich. He can be reached at alobbestael@wje.com.

Kenneth Itle and Hugh (Xiaoqiang) Hou of Wiss, Janney, Elstner (WJE) Associates Inc. also contributed to this column.

The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect that of The Construction Specifier or CSI.

Source URL: https://www.constructionspecifier.com/why-are-you-cracking-up/

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