Written and reviewed by the Apolodor LTD concrete repair team

Most concrete damage comes from outside: water, salts, frost, traffic or fire. Alkali-silica reaction is different. It is a chemical reaction that develops inside the concrete itself, slowly causing expansion and cracking that can look alarming and is often misdiagnosed.

Recognising ASR matters because it changes how the structure should be repaired. Standard crack filling and patching on their own rarely solve the problem, and in some cases they simply crack again. This is why, at Apolodor LTD, we never treat unusual cracking as a routine repair until we understand what is driving it.

Quick answer: Alkali-silica reaction (ASR) is identified by a combination of visual signs, mainly random map or pattern cracking, cracks running parallel to the main reinforcement, white or clear gel deposits in cracks, and evidence of expansion, confirmed by laboratory examination of concrete cores. Because ASR needs moisture to progress, managing water is central to any repair strategy.

What Is Alkali-Silica Reaction?

ASR is a reaction between the alkaline pore solution in concrete and certain reactive forms of silica found in some aggregates. The reaction forms an alkali-silica gel. That gel absorbs water and swells, creating internal pressure that cracks the concrete from within.

Three conditions are needed for damaging ASR to occur:

• Reactive silica in the aggregate
• A sufficiently high alkali content in the concrete, mainly from the cement
• Moisture, usually a high internal relative humidity sustained over time

Remove any one of these and the reaction slows or stops. This is why ASR is most commonly found in elements that stay damp, such as foundations, bridge elements, retaining walls, water-retaining structures and external slabs.

In the UK, ASR was first recognised in structures in the 1970s, and guidance such as BRE Digest 330 has since helped specifiers reduce the risk in new concrete. Structures built before these precautions became routine are more likely to be affected.

What Are the Visual Signs of ASR?

Map Cracking

The classic sign is a network of random cracks across the surface, often described as map or crazed cracking. In unrestrained concrete, the cracks have no clear direction. In reinforced or prestressed elements, they often run parallel to the main reinforcement, because the steel restrains expansion in that direction.

Gel Deposits and Staining

A white, clear or yellowish gel may be seen in cracks or around their edges. Dark, damp-looking margins along crack lines are also common. Gel is easily mistaken for efflorescence, so it is worth reading our guide to removing and preventing efflorescence on concrete to understand the difference.

Expansion and Movement

ASR causes concrete to grow. This can show up as closed or crushed expansion joints, misaligned elements, displaced bearings or small pop-outs where reactive particles near the surface have expanded.

Which Problems Can Look Like ASR?

Several other problems produce similar symptoms, which is why visual inspection alone is not enough:

• Plastic or drying shrinkage cracking in new concrete
• Freeze-thaw damage on exposed surfaces
• Sulphate attack in concrete structures, which also causes expansion
• Cracking from reinforcement corrosion

A structured approach to diagnosing building cracks is the first step in ruling these in or out.

How Is ASR Confirmed?

Confirmation requires investigation by experienced specialists, usually including:

• Core sampling from affected and unaffected areas
• Petrographic examination, where a specialist studies prepared sections of the core under a microscope to identify gel, reaction rims and cracking through aggregate particles
• Expansion testing on cores to estimate how much further expansion may occur
• Crack mapping and monitoring over time to measure whether the structure is still moving

Together, these results tell engineers how far the reaction has progressed and whether it is still active. When we suspect ASR on site, we recommend this investigation before any repair is priced, rather than filling cracks that are likely to reopen.

Can ASR-Affected Concrete Be Repaired?

ASR cannot be reversed, but its effects can be managed. The right approach depends on the severity of the damage, whether expansion is ongoing and the structural role of the element.

Typical management strategies include:

• Reducing moisture. Improving drainage, repairing leaks and applying breathable protective treatments can slow the reaction by keeping the concrete drier.
• Sealing cracks appropriately. Flexible sealants or injection can limit water entry, but rigid repairs may crack again if expansion continues. Knowing whether a crack is active or dormant is essential.
• Monitoring. Many affected structures remain serviceable for years with a planned monitoring regime.
• Strengthening or replacement. In severe cases, elements may need confinement, strengthening or replacement, guided by a structural engineer.

How Apolodor LTD Handles Suspected ASR

On site, our role is to recognise the warning signs early and make sure the structure is investigated properly before money is spent on repairs. We record and map the cracking, check for signs of expansion and moisture sources, and work with engineers and testing specialists where laboratory confirmation is needed.

Once the diagnosis is clear, we carry out the practical work: improving drainage, sealing cracks with materials that can tolerate movement, applying breathable protection and repairing any secondary damage. The aim is to slow the reaction and keep the structure in service, not to hide the symptoms.

Areas We Cover

We provide specialized ASR damaged concrete repairs all across UK, including:

• London
• Manchester
• Birmingham
• Liverpool
• Leeds
• Nottingham
• Bristol
• Brighton
• Cardiff
• Plymouth
• Luton
• Reading
• Norwich
• Swindon
• Portsmouth
• Oxford
• Ipswich
• Maidstone
• Cambridge
• Southampton
• Slough
• Warrington
• Sheffield
• Leicester
• Coventry
• Milton Keynes
• Northampton
• Derby
• Stoke-on-Trent
• Wolverhampton
• Hull
• Exeter
• Gloucester
• Sunderland
• York
• Peterborough
• Chelmsford
• Newcastle
• Bradford
• Bournemouth
• Swansea
• Newport
• Middlesbrough
• Preston
• Blackpool
• Bolton
• Stockport
• Huddersfield
• Lincoln
• Worcester
• Colchester
• Canterbury

Key Takeaways

• ASR is an internal reaction between alkalis and reactive silica that causes expansion and cracking
• Map cracking, gel deposits and evidence of expansion are the main visual signs
• Laboratory examination of cores is needed to confirm ASR
• Moisture control is the most important way to slow the reaction
• Repairs must allow for ongoing movement if the reaction is still active

Frequently Asked Questions

Is ASR dangerous?

ASR rarely causes sudden failure, but it can reduce durability and, in severe cases, structural capacity. An engineer should assess affected structural elements.

How long does ASR take to appear?

Damage typically becomes visible several years after construction, and in some structures not for decades, depending on the materials and exposure.

Can a coating stop ASR?

Coatings that reduce water uptake while allowing vapour to escape can help slow the reaction, but they are one part of a wider management strategy rather than a cure.

Speak to a Specialist

If you suspect alkali-silica reaction in a structure, Apolodor LTD can carry out an initial inspection and coordinate further investigation with specialist testing partners and engineers. Call 07808 709670 or contact us here.