Alkali Silica Reaction (ASR) in Concrete: Causes, Effects and Management

Alkali silica reaction (ASR), sometimes called concrete cancer of a different type or simply ASR, is a chemical reaction inside concrete that causes expansion, cracking and progressive deterioration over years or decades. Unlike reinforcement corrosion, which attacks concrete from the reinforcement outwards, ASR damages the concrete matrix itself.

This article explains what ASR is, why it happens, how to identify it, what its structural consequences are, and how affected concrete is investigated and managed.

What is alkali silica reaction?

Alkali silica reaction is a chemical reaction between the alkalis in cement (mainly sodium and potassium oxides) and certain types of reactive silica present in some aggregates used to make the concrete. In the presence of moisture, the reaction produces an expansive gel that absorbs water, swells and generates internal pressure within the concrete. Over time, that pressure cracks the surrounding material from within.

The reaction is slow. It typically takes years and often decades for ASR damage to become visible. Once the reaction is established, however, it is very difficult to stop, because all three of its ingredients (alkalis, reactive silica and moisture) are already present within the structure. Management focuses on limiting further deterioration and addressing its structural consequences rather than reversing the reaction itself.

What causes ASR?

ASR requires three conditions to be present together. First, high alkali content in the cement matrix. This is typically an artefact of the original mix design and cannot be changed after construction. Second, reactive silica in the aggregate. Certain aggregates, particularly some types of chert, opal, strained quartz and volcanic glass, contain silica in forms that react readily with the alkalis. Whether the aggregate used in a particular structure is reactive depends on its source and mineralogy. Third, sufficient moisture within the concrete to sustain the reaction. ASR is uncommon in permanently dry concrete but is a real risk in exposed or wet environments.

Because two of these three ingredients are locked in at construction (cement alkalinity and aggregate reactivity), ASR is fundamentally an inherited condition of the original concrete rather than something the structure develops through normal service. This means ASR damage often only becomes apparent decades after construction, in structures that appeared entirely sound for years.

What are the signs of ASR?

ASR damage shows a recognisable set of visible symptoms:

• Map cracking (also called pattern cracking or crazing) across concrete surfaces, forming a distinctive network of interconnected fine cracks that resembles the pattern of dried mud
• Gel exudation, where the ASR gel itself is visible seeping from cracks or from the concrete surface, sometimes appearing as a whitish or yellowish translucent deposit
• Expansion joint closure, where the concrete has expanded to the point that expansion joints have closed up and the concrete is now bearing against adjacent elements
• Displacement or misalignment of structural elements caused by the volumetric expansion of the affected concrete
• Discoloured “haloes” around aggregate particles visible in cores or in areas of concrete removal, indicating local ASR activity at specific aggregate locations

Map cracking is by far the most common visible indicator. However, map cracking alone is not diagnostic of ASR: it can also be produced by other mechanisms including plastic shrinkage cracking, thermal effects and drying shrinkage. Laboratory analysis of core samples is typically required to confirm whether ASR is actually the underlying cause.

How is ASR confirmed?

Suspected ASR is confirmed through laboratory analysis of core samples taken from the affected structure. Petrographic examination of thin sections cut from the cores can identify the characteristic gel deposits, the reactive aggregate particles and the pattern of reaction sites within the concrete. This is the definitive test for confirming ASR presence and typically distinguishes it from other mechanisms that can produce similar visible symptoms.

Alongside petrographic analysis, moisture testing establishes whether the concrete is wet enough to sustain further reaction. Structural assessment establishes what load capacity remains in the affected element and whether the visible damage has structural consequences. Together, these tests inform the decision on how to manage the affected concrete going forward.

What are the structural consequences of ASR?

ASR reduces the strength, stiffness and durability of the affected concrete. The extent of the reduction depends on how advanced the reaction is and how extensive the cracking has become. In the early stages, structural consequences may be minor and manageable through monitoring. In advanced cases, the affected concrete may have lost significant load capacity and require structural strengthening or replacement.

A particular concern with ASR is that the cracking allows other aggressive agents (water, chlorides, carbon dioxide) to penetrate deeper into the structure, accelerating reinforcement corrosion and other secondary deterioration mechanisms. What starts as ASR often becomes a combined ASR-and-corrosion problem over time, compounding the damage.

Can ASR be repaired?

ASR cannot be reversed. Once the reaction is established in a structure, the chemical processes cannot be undone. However, its progression can be slowed and its structural consequences can be addressed. The typical approach combines three interventions applied together.

First, moisture control. Because the ASR reaction requires moisture to continue, reducing water ingress into the structure slows the ongoing reaction significantly. Waterproofing systems, drainage improvements and protective coatings all limit moisture availability.

Second, structural strengthening. Where ASR damage has reduced the load capacity of affected elements, additional strengthening (typically using confinement systems such as carbon fibre wrap, steel jacketing or concrete overlay) restores load capacity and restrains further expansion.

Third, concrete repair of cracked and damaged areas. This addresses the visible damage, prevents further water ingress through cracks, and integrates with the wider strengthening approach.

The right combination depends on the extent of ASR, the structural function of the affected elements, the intended remaining service life and the client’s asset management strategy.

When to seek professional help

ASR is a specialist diagnosis that should be confirmed through laboratory analysis rather than assumed from visible symptoms alone. Map cracking, expansion joint closure or gel exudation on a concrete structure warrants a proper investigation combining visual inspection, core sampling and laboratory petrographic analysis.

Where ASR is confirmed, the follow-on decisions about monitoring, moisture control, structural strengthening or repair are technical and require coordination with a structural engineer alongside the specialist contractor delivering any works. Early identification and proper investigation typically lead to more manageable and lower-cost outcomes than delayed response.

Closing

Alkali silica reaction is one of the harder concrete deterioration mechanisms to manage because it cannot be reversed and its progression is difficult to stop entirely. But it can be understood, monitored, and its structural consequences addressed through proper investigation and specialist repair. Structures affected by ASR can often continue in service for many years with the right management strategy in place.

If you would like an initial conversation about suspected ASR on your structure, get in touch with the CRS team.