Alkali–silica reaction

Chemistry

ASR is the same as the pozzolanic reaction, which is a simple acid-base reaction between calcium hydroxide, also known as Portlandite, or (Ca(OH)₂), and silicic acid (H₄SiO₄, or Si(OH)₄). For the sake of simplicity, this reaction can be schematically represented as following:

Mechanism of concrete deterioration

The mechanism of ASR causing the deterioration of concrete can be described in four steps as follows:

1. The alkaline solution attacks the siliceous aggregate, converting it to viscous alkali silicate gel.
2. Consumption of alkali by the reaction induces the dissolution of Ca²⁺ ions into the cement pore water. Calcium ions then react with the gel to convert it to hard C-S-H.
3. The penetrated alkaline solution converts the remaining siliceous minerals into bulky alkali silicate gel. The resultant expansive pressure is stored in the aggregate.
4. The accumulated pressure cracks the aggregate and the surrounding cement paste when the pressure exceeds the tolerance of the aggregate.

Structural effects of ASR

The cracking caused by ASR can have several negative impacts on concrete, including:

1. Expansion: The swelling nature of ASR gel increases the chance of expansion in concrete elements.
2. Compressive Strength: The effect of ASR on compressive strength can be minor for low expansion levels, to relatively higher degrees at larger expansion. (Swamy R.N 1986) points out that the compressive strength is not very accurate paramater to study the severity of ASR; however, the test is done because of its simplicity.
3. 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 indicate about 85% loss of capacity as a result of ASR.
4. 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.
5. Fatigue: ASR reduces the load bearing capacity and the fatigue life of concrete (Ahmed T 2000).
6. Shear: ASR enhances the shear capacity of reinforced concrete with and without shear reinforcement (Ahmed T 2000).

Mitigation

ASR can be mitigated in new concrete by several complementary approaches:

1. Limit the alkali metal content of the cement. Many standards impose limits on the "Equivalent Na₂O" content of cement.
2. Limit the reactive silica content of the aggregate. Certain volcanic rocks are particularly susceptible to ASR because they contain volcanic glass (obsidian) and should not be used as aggregate. The use of calcium carbonate aggregates is sometimes envisaged as an ultimate solution to avoid any problem. However, while it may be considered as a necessary condition, it is not a sufficient one. In principle, limestone (CaCO₃) is not expected to contain a high level of silica, but it actually depends on its purity. Indeed, some siliceous limestones (a.o., Kieselkalk found in Switzerland) may be cemented by amorphous or poorly crystalline silica and can be very sensitive to the ASR reaction, as observed with some Tournaisian siliceous limestones exploited in quarries in the area of Tournai in Belgium. So, the use of limestone as aggregate is not a guarantee against ASR in itself.
3. Add very fine siliceous materials to neutralize the excessive alkalinity of cement with silicic acid by deliberately provoking a controlled pozzolanic reaction at the early stage of the cement setting. Convenient pozzolanic materials to add to the mix may be, e.g., pozzolan, silica fume, fly ash, or metakaolin. These react preferentially with the cement alkalis without formation of an expansive pressure, because siliceous minerals in fine particles convert to alkali silicate and then to calcium silicate without formation of semipermeable reaction rims.
4. Another method to reduce the ASR is to limit the external alkalis that come in contact with the system.

In other words, as it is sometimes possible to fight fire with fire, it is also feasible to combat the ASR reaction by itself. A prompt reaction initiated at the early stage of concrete hardening on very fine silica particles will help to suppress a slow and delayed reaction with large siliceous aggregates on the long term. Following the same principle, the fabrication of low-pH cement also implies the addition of finely divided pozzolanic materials rich in silicic acid to the concrete mix to decrease its alkalinity.

As part of a study conducted by the Federal Highway Administration, a variety of methods have been applied to field structures suffering from ASR-affected expansion and cracking. Some methods, such as the application of silanes, have shown significant promise, especially when applied to elements such as small columns and highway barriers, whereas other methods, such as the topical application of lithium compounds, have shown little or no promise in reducing ASR-induced expansion and cracking.

Treatment

There are no treatments in general in affected structures. Repair in damaged sections are 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.

ASR test

Some ASTM Tests that screen aggregate for the potential of ASR include:

Australia

New Zealand

United Kingdom

United States