Alkali-silica reaction

The alkali–silica reaction (ASR), more commonly known as "concrete cancer", is a reaction which occurs over time in concrete between the highly alkaline cement paste and the reactive non-crystalline (amorphous) silica found in many common aggregates, given sufficient moisture.

This reaction causes the expansion of the altered aggregate by the formation of a swelling gel of calcium silicate hydrate (C-S-H). This gel increases in volume with water, and exerts an expansive pressure inside the material, causing spalling and loss of strength of the concrete, finally leading to its failure.

ASR can cause serious cracking in concrete, resulting in critical structural problems that can even force the demolition of a particular structure.

ASR is the same as the pozzolanic reaction, which is a simple acid-base reaction between calcium hydroxide, also known as Portlandite, or (Ca(OH)₂), and silicic acid (H₄SiO₄, or Si(OH)₄). For the sake of simplicity, this reaction can be schematically represented as following:

The mechanism of ASR causing the deterioration of concrete can be described in four steps as follows:

The cracking caused by ASR can have several negative impacts on concrete, including:

ASR can be mitigated in new concrete by several complementary approaches:

In other words, as it is sometimes possible to fight fire with fire, it is also feasible to combat the ASR reaction by itself. A prompt reaction initiated at the early stage of concrete hardening on very fine silica particles will help to suppress a slow and delayed reaction with large siliceous aggregates on the long term. Following the same principle, the fabrication of low-pH cement also implies the addition of finely divided pozzolanic materials rich in silicic acid to the concrete mix to decrease its alkalinity.

As part of a study conducted by the Federal Highway Administration, a variety of methods have been applied to field structures suffering from ASR-affected expansion and cracking. Some methods, such as the application of silanes, have shown significant promise, especially when applied to elements such as small columns and highway barriers, whereas other methods, such as the topical application of lithium compounds, have shown little or no promise in reducing ASR-induced expansion and cracking.

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