Concrete cracking… could it be Alkali Silica Reaction (ASR)? Well known facts about concrete are that it will harden and crack. Concrete hardens due to chemical reactions known as hydration. Concrete may crack due to shrinkage of the hydration products and temperature changes. Other causes of cracking can be due to structural loading. However, concrete cracking that is often misdiagnosed, misunderstood, and potentially dangerous could be from a reaction known as Alkali Silica Reaction. ASR is found throughout the world and in our desert southwest region.
What are the effects of ASR? Millions of dollars have been spent mitigating the effects of the degradation of concrete due to ASR. In severe cases, complete demolition is necessary due to the compromise of structural integrity. Local examples of ASR caused damage to an airport pavement apron shown here. ASR expansion of concrete can also cause damage to adjacent structures as shown, where the expanding pavement shoved an adjoining column.
What is the science behind ASR? Concrete is a mixture of cement, aggregate (gravel) and water. Locally (Arizona) mined aggregates are often siliceous and found to be reactive. ASR occurs when alkali (generally from the cement) and reactive siliceous materials react with water (surface water, groundwater, vapor emissions) and create a gel like substance. The gel has a high affinity for water that causes swelling and expansion. Internal pressures caused by the expansion exceeds the tensile capacity of the concrete, the concrete cracks, and the cracking leads to degradation.
How do you stop ASR? The best cure is prevention. The reaction between the alkali, silica, and water must be broken by specifying nonreactive concrete mixtures during the project design phase. The project design team, the architect, structural and/or civil engineer are the professionals typically responsible for specifying the concrete mixture.
How can you spot concrete cracking due to ASR? Cracking as a result of ASR is difficult to visually detect even for the professional. However, it can exhibit alligator type cracking and flaking.
Can you test for ASR? There are ASTM test methods for determining if an aggregate source is potentially reactive. In addition, standard test methods are available to help determine remedial measures that can be taken to mitigate the harmful expansion caused by ASR. These tests can be performed on proposed aggregate sources or on job specific mixes. Since most aggregate sources in our region are potentially reactive, it is important to engage qualified materials experts who understand ASR, the challenges of mitigating issues, and the use of applicable test methods.