Alkali-Silica Reactivity Demystified

By Kenneth Kazanis, Technical Manager, Holcim US
 

The demand for extended-life concrete continues to grow as long-term durability becomes increasingly important to project owners and specifiers. While concrete may seem to last forever, there are chemical and physical reactions that can cause even this robust material to deteriorate over time.

Alkali-silica reaction (ASR) is one such distress mechanism that can cause serious concrete durability problems. In the presence of moisture, this reaction between alkali sources in the concrete and reactive silica forms in certain aggregates produces an expansive gel that induces pressure and causes the concrete to crack. As the mature concrete continues to crack, additional moisture and pore solution salts can be introduced, furthering ASR, corrosion, freeze-thaw damage, and loss of structural integrity. 

ASR is a complex technical problem, and specification standards for reducing risk are complicated. Holcim technical services engineers have a deep understanding of deterioration mechanisms, concrete materials chemistry, and high-performance durability requirements. These technical experts are committed to working side-by-side with ready-mix concrete producers in implementing mix-design best practices to mitigate potential ASR issues.

A new approach  

The recommendations for reducing the risk of ASR in concrete have changed. In the past, the industry believed that ASR was a cement issue, and ASTM C150 (Standard Specification for Portland Cement) recommended the use of “low-alkali cements” (maximum 0.60% alkali) with aggregates susceptible to ASR.  This is no longer the case, as low-alkali cements may not be effective in mitigating ASR in the absence of other mitigation measures. As such, the optional requirement for low-alkali cement is no longer referenced in ASTM C150 or the corresponding AASHTO R 80 standard. 

It is now well established that it is the total alkali “loading” in concrete and not just the alkali content of the cement that influences the risk of ASR. Guidance on formulating concrete mixtures, including calculating total alkali loading, to minimize the potential for ASR is provided in ASTM C1778 (Standard Guide for Reducing the Risk of Deleterious Alkali-Aggregate Reaction in Concrete). 
There are no proven measures for effectively preventing damaging expansion with alkali-carbonate reactive (ACR) aggregates and such materials need to be avoided.

Determining aggregate reactivity

If the aggregate has a satisfactory 15-year field-performance record, you can rely on the documented service history and use the aggregate in new construction. If ASR problems have occurred or reactivity of the aggregate is known to vary, analytical evaluations are required. Petrographic analysis (ASTM C295) can be used to classify an aggregate as potentially reactive, but laboratory expansion testing is required to determine the extent of the reactivity and appropriate preventive measures to minimize risk.  

Aggregates are expected to be non-reactive if they yield a 14-day expansion less than 0.10% by the ASTM C1260 accelerated mortar bar method or a 1-year expansion less than 0.04% by the ASTM C1293 concrete prism test. ASTM C1260 is considered a relatively severe test where a mortar bar made with the sample aggregate is exposed to an alkaline solution at an elevated temperature. This quick test identifies many aggregates as potentially reactive that have histories of satisfactory field performance. As such, the ASTM C1260 test should not be used to reject an aggregate and the more reliable ASTM C1293 test should be conducted to verify reactivity. If expansion is greater than 0.04%, the aggregate is considered reactive but can still be used in concrete with appropriate mitigative measures.
 

SCM mitigation options 

If historical experience or laboratory tests demonstrate that ASR is a potential concern, then concrete mixtures must be specifically designed to control ASR. When used in the correct proportions, pozzolans, slag cement, silica fume, and blended cements can effectively prevent excessive expansion due to ASR.  

Class F (low calcium) fly ash at 15% to 40% replacement levels significantly mitigates ASR. The amount required depends on the chemical composition of the ash, reactivity of the aggregate, and the alkali loading of the concrete. Class C fly ash has been shown to aggravate ASR expansion at normal replacement levels. Slag cement offers similar advantages to Class F fly ash but at higher replacement rates, typically 25% to 70%. Slag cement at 40% substitution can provide benefits similar to 25% Class F replacement. Silica fume performs well in controlling ASR at 5% to 12% substitution but can cause ASR if allowed to lump. Class N natural pozzolans—metakaolin, pumice, volcanic tuffs, diatomaceous earth, and zeolites—can provide the same advantages as Class F fly ash.

Blends of SCMs are also effective in controlling expansion due to ASR. Blends of slag cement and silica fume, as well as blends of fly ash and silica fume, have a synergistic effect in mitigating expansion due to ASR, while producing a very workable concrete.

Mix design approaches

The ASTM C1778 guide for reducing the risk of ASR provides recommendations for the safe use of concrete materials using either prescriptive or performance-based alternatives. 

With the performance-based method, the ASTM C1293 concrete prism method discussed earlier or the ASTM C1567 test can be used to establish the required quantity of SCM for minimizing potential ASR. If the ASTM C1293 test is used, the aggregate with the proposed combination of cementitious materials is considered suitable if expansion is less than 0.04% after two years. ASTM C1567 is an accelerated mortar bar test similar to ASTM C1260. If expansion is less than 0.10% after 14 days, the combination is considered acceptable. This test is not appropriate for evaluating SCMs with high levels of alkalies. 
 
The prescriptive approach is a fallback solution to mitigating ASR when data are not available. This starts with determining the level of prevention based on the degree of aggregate reactivity, the criticality of the structure, the nature of the exposure conditions, and the amount of alkalies in the portland cement and SCMs. Once the level of prevention is determined, options for mitigation include limiting the concrete alkali loading or using alternative cementitious materials, such as Class F fly ash or slag cement, to decrease permeability and reduce the amount of alkali in the concrete. In extreme situations, mitigation requires both limiting the maximum alkali loading of the concrete and the use of SCMs.

Your partner in success

Higher alkali cements have multiple options for mitigation. With our highly experienced technical teams, state-of-the-art laboratory capabilities, and vast portfolio of high-quality materials, our technical team has the resources to help you reduce the risk of ASR and other potential concrete durability issues. 

Most project owners will insist on performance-proven solutions and we can conduct the required C1260, C1567, and C1293 tests in our laboratories. We also have a rigorous quality assurance program and strong product standards for all our local material offerings—from aggregates and cement to SCMs (fly ash, slag, and silica fume) and blended cements. In fact, all our aggregates are routinely tested for reactivity following the two-year ASTM C1293 method.