How Do You Reduce Alkali-Silica Reactions in Concrete?

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How Do You Reduce Alkali-Silica Reactions in Concrete? | Bisley International

It can take years for an alkali-silica reaction (ASR) to show up in your concrete. Once it does, however, it cracks your concrete apart, causing extensive damage. This deleterious swelling reaction happens over time and is fittingly nicknamed “concrete cancer”. It is a form of alkali-aggregate reaction.

It can take up to 10 years after construction for your concrete to show symptoms of ASR. However, concrete that contains highly reactive aggregate and is exposed to a hot and humid climate could show signs of cracking within 2 years of construction.

What Causes ASR?

Three main factors lead to ASR. They are:

  • High pH levels
  • Reactive aggregates
  • Presence of water

How Do You Prevent ASR?

One option is to use non-reactive aggregates. However, it can be impractical and expensive to find non-reactive aggregates and have them shipped to you.

Alkali Loading: Instead, you can target the pH level. by decreasing the alkali loading. Since alkalis come from cement, your best bet is to redesign your mixture by using less cement. You can also use cement with a lower alkali content, though this may not be sufficient depending on how much cement you need to use.

SCMs:  Instead of using cement, you can utilize SCMs, which are Supplementary Cementitious Material. This means less cement and less cement means a lower alkali content. They are also effective at binding alkalis in their hydration products which lower the pH of concrete. The following are examples of SCMs:

  • Fly ash
  • Silica fume
  • Slag cement

It is recommended to use class F fly ash as opposed to class C fly ash. This is because while class F fly ash has been shown to reduce pH by reacting with calcium hydroxide, class C fly ash has been documented to worsen ASR in certain cases.

Decrease permeability: Some methods of decreasing your water’s permeability include: 

  • Lowering the water to cement ratio
  • Good curing
  • Good consolidation
  • Using SCMs.

Another option is to use a lithium-based admixture that renders ASR gel non-expansive.

What to Do When You Already Have ASR in Concrete

Typically, ASR alone won’t damage your concrete past the point of usability. Large cracks will only form at the surface, leaving the rest of the concrete untouched. However, once these cracks form other things start to happen.

Outside chemicals can penetrate the concrete, causing corrosion, freeze-thaw, sulphate attack, a whole host of problems. One solution is to try to prevent water from touching the concrete surface. 

However, the most effective solution is to use a vapour-permeable membrane to reduce internal moisture. This comes in the form of a sealant which you pour over the concrete. This sealant penetrates the surface and reacts in the pores. 

The great thing about vapour permeable sealers is they prevent water from entering the surface while releasing vapour. It turns the concrete into a hydrophobic surface, so if you pour water onto it, it will bead up and roll off instead of penetrating inside.

The combination of the release of vapour and simultaneous prevention of water entering the surface allows the concrete to dry over time. This will stop the ASR reaction once the internal relative humidity reaches below 80%.

Conclusion

The concrete prism test is a commonly used test that identifies the potential for ASR in natural aggregate. In other words, it tests how alkali reactive cement concrete structures are. If you suspect that ASR could develop or has already developed in your concrete, it’s a good idea to get it tested so you can figure out what the follow-up procedure should be. 

At Bisley International, we carry Lithium-based products that reduce ASR, surface sweating, and aid in cleaning and maintenance of your concrete. Contact us today to have your concrete treated!