Alkali-Aggregate Reaction

In most concrete, aggregates are more or less chemically inert. However, some aggregates react with the alkali hydroxides in concrete, causing expansion and cracking over a period of many years. This alkali-aggregate reaction has two forms—alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR).

 

Alkali-silica reaction (ASR) is of more concern because aggregates containing reactive silica materials are more common. In ASR, aggregates containing certain forms of silica will react with alkali hydroxide in concrete to form a gel that swells as it adsorbs water from the surrounding cement paste or the environment. These gels can swell and induce enough expansive pressure to damage concrete 


Typical indicators of ASR are random map cracking and, in advanced cases, closed joints and attendant spalled concrete. Cracking due to ASR usually appears in areas with a frequent supply of moisture, such as close to the waterline in piers, near the ground behind retaining walls, near joints and free edges in pavements, or in piers or columns subject to wicking action. Petrographic examination can conclusively identify ASR.

ASR can be controlled using certain supplementary cementitious materials. In proper proportions, silica fume, fly ash, and ground granulated blast-furnace slag have significantly reduced expansion due to alkali-silica reactivity. In addition, lithium compounds have been used to reduce ASR. Although potentially reactive aggregates exist throughout North America, ASR distress in concrete is not that common because of the measures taken to control it. It is also important to note that not all ASR gel reactions produce destructive swelling.

Alkali-carbonate reactions (ACR) are observed with certain dolomitic rocks. Dedolomitization, the breaking down of dolomite, is normally associated with expansion. This reaction and subsequent crystallization of brucite may cause considerable expansion. The deterioration caused by ACR is similar to that caused by ASR; however, ACR is relatively rare because aggregates susceptible to this phenomenon are less common and are usually unsuitable for use in concrete for other reasons. Aggregates susceptible to ACR tend to have a characteristic texture that can be identified by petrographers.

Click here for more on ASR/ACR test methods.

Prevention of Alkali-Silica Reaction in New Concrete

Follow the steps in the flowchart below to determine if potential for ASR exists and to select materials to control ASR. For more information move your mouse over the individual flowchart boxes. (Source: IS413 and IS415).

Effect of Cement Fineness on ASTM C1260 Expansion (SN2963)

The Accelerated Mortar Bar Test, ASTM C1260 or CSA A23.2-25A, is a widely used test to detect alkali-silica reactive aggregates. Mortar bars are cast with the aggregate under investigation and the specimens are stored in 1N NaOH solution at 80oC. The expansion at 16 days after casting is taken as an indication of potential reactivity. ASTM C1260 requires the use of portland cement meeting ASTM C150. In this research, sponsored in part by a PCA Education Foundation Fellowship, the effect of portland cement fineness on ASTM C1260 expansion in conjunction with other potentially influential factors, such as alkali content of clinker, aggregate reactivity, and immersion solution concentration, was studied. The results show that mortar bar expansion increased with higher cement fineness regardless of cement alkali, aggregate reactivity, or soak solution normality