What Is Alkali-Silica Reaction in Concrete?
What Is Alkali-Silica Reaction in Concrete?
Alkali-silica reaction (ASR), also known as “concrete cancer,” is a chemical reaction that can occur between the alkaline hydroxides in cement and certain types of aggregate. This reaction can cause damage to the concrete in a building and may require extensive repairs or even demolition.
ASR is the most common form of alkali-aggregate reaction (AAR) and is caused by a reaction between the alkaline cement’s hydroxyl ions and reactive forms of silica in the aggregate. This produces a gel that expands when it absorbs water putting pressure on the surrounding concrete and weakening it in a way similar to the effects of freeze-thaw cycles.
ASR is most likely to occur when the cement has a high alkali content, there is a reactive aggregate such as chert present and there is water present in the concrete. It can be detected by examining thin sections of concrete under a microscope to look for the presence of the gel in cracks and within aggregate particles.
To reduce the risk of ASR, low-alkali Portland cement or pozzolans can be used in the concrete mix to lower the alkalinity of the pore fluid.
Causes Of Alkali-Silica Reaction
Alkali-aggregate reaction (AAR) is a chemical reaction that occurs between the alkaline components of concrete and certain types of aggregate. There are two forms of AAR: alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR).
ASR is the most common form and occurs when the hydroxyl ions in the cement pore solution react with reactive forms of silica in the aggregate, such as chert, quartzite, opal and strained quartz crystals.
This reaction produces a gel that expands as it absorbs water, causing the concrete to crack and potentially fail. In concrete without reinforcement, this type of AAR can result in a distinctive pattern of cracks known as “map cracking” or “Isle of Man cracking.
Essential Components Of Alkali-Silica Reaction
ASR, or alkali-silica reaction, is a complex process that occurs in concrete structures and is characterized by the expansion and degradation of the concrete.
Experts believe that the key elements required for ASR to occur are: reactive silica from aggregates, sufficient alkalies from Portland cement and other materials, and sufficient moisture. If any of these three components is absent, ASR will not occur and the concrete will not be damaged.
Alkali-silica reaction (ASR) can damage the structural integrity of concrete if there is enough available moisture for the reaction to occur.
When the temperature increases, the pores in the concrete expand. Similarly, when stress is applied to the concrete, it can cause cracks to form or existing cracks to open. If there is water present, the gel that is produced as a result of the ASR can grow into these newly opened areas.
When the concrete cools or the stress is relieved, it contracts, but the gel, which is incompressible, resists this contraction with an opposing force. This can lead to the development of new cracks or the widening of existing ones, and if the ASR continues, it can ultimately weaken the concrete structure.
Structural Effects Of ASR
Alkali-silica reaction (ASR) can cause damage to concrete in several ways:
Expansion: The swelling nature of ASR gel can cause concrete elements to expand.
Compressive strength: The effect of ASR on compressive strength may be minor for low expansion levels, but can be significant at larger expansions.
Tensile strength and flexural capacity: Research has shown that ASR cracking can significantly reduce the tensile strength of concrete, which can also reduce the flexural capacity of beams. Some studies on bridge structures have even reported a loss of capacity of up to 85% due to ASR.
Modulus of elasticity and ultrasound pulse velocity: The effects of ASR on the elastic properties of concrete and ultrasound pulse velocity (UPV) are similar to its effects on tensile capacity. The modulus of elasticity is more sensitive to ASR than pulse velocity.
Fatigue: ASR can decrease the load bearing capacity and fatigue life of concrete.
Shear strength: ASR can increase the shear capacity of reinforced concrete, with or without shear reinforcement.
What To Do When You Already Have ASR In Concrete
ASR (alkali-silica reaction) typically only damages the surface of concrete and leaves the rest of it undamaged. However, once cracks form on the surface, external substances can enter the concrete and cause problems such as corrosion, freeze-thaw, and sulphate attack.
To prevent these issues, it is important to keep water away from the concrete surface. One solution is to use a vapour-permeable sealant on the surface, which allows the concrete to release vapour and prevents water from entering, creating a hydrophobic surface.
This allows the concrete to dry and stops the ASR reaction when the internal relative humidity falls below 80%.
Prevention Of Alkali-Silica Reaction
There are several ways to prevent or mitigate ASR (alkali-silica reaction) in new concrete:
One approach is to limit the amount of alkali metals in the cement used. Many standards set limits on the “Equivalent Na2O” content of cement to prevent ASR.
Another approach is to use aggregates that are low in reactive silica. Certain types of volcanic rocks, which contain volcanic glass, should be avoided as aggregates because they are prone to ASR. Using calcium carbonate aggregates can help prevent this reaction. However, it’s important to note that using limestone as an aggregate is not a guarantee against ASR.
A third approach is to add very fine siliceous materials, such as pozzolan, silica fume, fly ash, or metakaolin, to the concrete mix. These materials can neutralize the excess alkalinity of the cement through a controlled pozzolanic reaction at the beginning of the cement’s setting process. This helps prevent the formation of expansive pressure caused by the ASR reaction.
Another way to prevent ASR is to limit the external sources of alkalis that come into contact with the concrete system.
One effective way to suppress ASR is to initiate a prompt reaction at the early stages of concrete hardening using very fine silica particles. This can help prevent the slow, delayed reaction that can occur with larger siliceous aggregates over the long term.
To further decrease the alkalinity of the concrete, it is also possible to add finely divided pozzolanic materials that are rich in silicic acid to the mix. This can help lower the pH value of the concrete’s pore water.
The main mechanism by which silica fume works is by consuming portlandite (the source of hydroxyde ions in the solid phase) and decreasing the porosity of the hardened cement paste through the formation of calcium silicate hydrates.
However, it’s important to ensure that the silica fume is very finely dispersed in the concrete mix, as agglomerated flakes of compacted silica fume can themselves cause ASR if the dispersion process is insufficient.
When making large batches of fresh concrete, the presence of coarse and fine aggregates helps ensure that the silica fume is sufficiently dispersed during the mixing process.
