How widespread concrete cancer can be effectively managed

This article was first published in the November 2024 issue of the Magazine of the Concrete Society.

Concrete cancer, a highly visible pattern of cracking created by a deleterious reaction within the concrete itself, known as the alkali–silica reaction (ASR), was originally documented in the 1940s. George Korobokis, Senior Materials Consultant at Ian Farmer Associates, shares how his recent work suggests that this reaction and its consequences are far more widespread than previously understood and how they can be best managed.

The ASR has been one of the major concerns regarding concrete durability since it was first diagnosed in the USA back in the 1940s, leading to high maintenance and reconstruction costs. Its occurrence in various structures all over the world indicates the need for more research on detection and mitigation at the initial stage of the reaction in new and existing structures. However, it remains poorly understood.

Within concrete, silica in the aggregates react with the alkaline solution created by the cement reaction when in the presence of water, a vital part of the mixture. This forms a gel that absorbs water, which expands and grows within the hardened concrete. The gel exerts force out from within the concrete, forming microfractures that grow and eventually hit the surface, where they form the distinctive ‘concrete cancer’ map cracking and cause a loss in strength. The problem first appeared in the UK back in the 1980s and resulted in several high-profile demolitions. Studies show that 35% of the concrete’s compressive strength and up to 24% of its tensile strength can be lost once the ASR gel reaches the surface of the concrete.

Initially, the ASR was believed to be related to large reactive aggregates, with coarse non-reactive aggregates considered safe (e.g., basalt, granite and diorite, among others), but what we know now is that almost all concretes will eventually be impacted. Some aggregates will react more slowly than others, with the degree of ASR gel expansion increasing in proportion to the amount of reactive aggregates present, but all concretes will eventually react and produce microfracturing. Often these concretes look unremarkable on the outside, but almost every specimen of concrete that I’ve looked at in the last two years has some degree of ASR-associated microfracturing.

This is obviously a problem given that concrete is the single most widely used material in the world. With that in mind, why has the same high-profile attention not been given to the ASR as has been given to reinforced aerated autoclaved concrete (RAAC)?

The difference is that ASR reactions are not going to cause sudden catastrophic collapse. Until the cracks begin to propagate out onto the surface of concrete, the microcracking and gel formation do not have a huge negative impact on the strength of the concrete. In addition, while RAAC can be addressed only by reinforcing the structure or replacing the concrete, ASR-infected concrete can be reinstated or the reaction can be slowed, potentially by many years.

The first step is to slow the ASR reaction as much as possible from the moment the concrete is laid by preventing ingress of water. Gel and microfractures form because of water within the concrete, but this can be exacerbated by the porous nature of concrete. In dry, arid climates such as the Middle East, as we see from samples in the UAE and Saudi Arabia, ASR-associated microfractures can take an exceptionally long time to form – upwards of 100 years. By contrast, concrete that is within the splash zone of water can have visible cracking from the ASR within a single year of being placed. Therefore, waterproofing membranes will extend the lifespan of the concrete.

In previously laid concrete, we want to prevent cracks from reaching the surface. That means first confirming the presence and extent of ASR gel and microfractures. The best technique for identifying the ASR is the examination of thin concrete sections (30 microns thick) using a petrographic microscope with plane-polarised, cross-polarised and ultraviolet light.

These techniques show the extent of ASR within the concrete, and once this is confirmed, structural engineers get involved. It is possible to use certain sealers such as silane on the surface of concrete that has been affected by typical ASR-associated map cracking. This provides insulation from external relative humidity. A spray of lithium nitrate to seal the ASR surface cracks, or electrochemical penetration of lithium ions on concrete structures, can be used.

For longer-lasting repairs, ASR-afflicted concrete can be reinstated. The upper part of the concrete, where the cracks have reached the surface, must be removed and replaced with a new, fresh layer of concrete. Of course, this is only temporary, and because the ASR is still present in the heart of the concrete, eventually it will penetrate the new cover. However, it can extend the life of the building or structure. Best practice would be to take the layer back past the rebar and reinstate as much as possible, but this isn’t always feasible.

Long term, there are trials using concretes that include extra air voids or hollow plastic fibres with the intention that instead of cracking the concrete, the alkali–silica gel has a place to go inside the concrete structure so that the energy can be dissipated. The use of pozzolans in the concrete mix, such as slag, fly ash, metakaolin, microsilica and others, as a partial cement replacement can reduce the likelihood of the ASR occurring, as they reduce the alkalinity of the pore fluid. Time will tell how successful these are.

In the meantime, as an industry, we need a greater understanding of the ASR and its prevalence in the built environment around us. Because of the slow-forming nature of ASR gel and the fact that it can be reinstated, we can avoid a crisis similar to that of RAAC. Unfortunately, ASR-afflicted concrete is so widespread and there is not enough awareness of it that we are in danger of ending up in a situation where the proper industry knowledge of how to prevent, diagnose and reinstate afflicted concrete is simply not there, leaving us unable to act quickly or effectively.