Alkali Silica Reaction (ASR)

Alkali silica reaction (ASR) commonly known as ‘concrete cancer’ is a swelling reaction that happens due to highly soluble alkalies cement paste and non-crystalline silica(siliceous minerals) that are generally found in aggregate.

Why it is called concrete cancer?

As we know concrete is permeable and water percolates in concrete and ASR reaction are happed in the presence of water. In ASR, aggregates containing certain forms of silica will react with alkali hydroxide(NaOH/ KOH) in concrete to form a gel that swells as it absorbs water from the surrounding cement paste or the environment. These Viscous gels can induce enough expansive pressure to damage concrete.

there is sodium silicate (Na2SiO3 • nH2O), also noted Na2H2SiO4 • nH2O or N-S-H (sodium silicate hydrate) form in Gel.

The conditions required for ASR to occur are:

•A sufficiently high alkali content of the cement (or alkali from other sources)

•A reactive aggregate

•Water - ASR will not occur if there is no available water in the concrete since the alkali-silica gel formation requires water.

Effects on Structure:

•Expansion: The swelling nature of ASR gel increases the chance of expansion in concrete elements.

•Compressive Strength: The effect of ASR on compressive strength can be minor for low expansion levels, to relatively higher degrees at larger expansion. points out that the compressive strength is not a very accurate parameter to study the severity of ASR; however, the test is done because of its simplicity.

•Tensile Strength / Flexural Capacity: Researches show that ASR cracking can significantly reduce the tensile strength of concrete; therefore reducing the flexural capacity of beams. Some research on bridge structures indicates about 85% loss of capacity as a result of ASR.

•Modulus of Elasticity/UPV: The effect of ASR on elastic properties of concrete and ultrasound pulse velocity (UPV) is very similar to tensile capacity. The modulus of elasticity is shown to be more sensitive to ASR than pulse velocity.

•Fatigue: ASR reduces the load-bearing capacity and the fatigue life of the concrete.

•Shear: ASR enhances the shear capacity of reinforced concrete with and without shear reinforcement

Mitigation

•Limit the alkali metal content of the cement.

•Limit the reactive silica content of the aggregate: Certain volcanic rocks are particularly susceptible to ASR because they contain volcanic glass and should not be used as aggregate. The use of calcium carbonate aggregates is sometimes envisaged as an ultimate solution to avoid any problem

•Add very fine siliceous materials on very fine silica particles will help to suppress a slow and delayed reaction with larger siliceous aggregates on the long term

•Another method to reduce the ASR is to limit the external alkalis that come in contact with the system.

•There are no treatments in general in affected structures. Repair in damaged sections is possible, but the reaction will continue. In some cases, drying of the structure followed by the installation of a watertight membrane can stop the evolution of the reaction.

•Massive structures such as dams pose particular problems: they cannot be easily replaced, and the swelling can block spillway gates or turbine operations. Cutting slots across the structure can relieve some pressure, and help restore geometry and function.