Alkali aggregate reactions (AAR) occur when aggregates in concrete react with the alkali hydroxides in concrete producing a hygroscopic gel which, in the presence of moisture, absorbs water and causes expansion and cracking over a period of many years. This alkali-aggregate reaction has two forms, namely: Alkali-silica reaction (ASR) and Alkali-carbonate reaction (ACR).
The former is of higher concern since aggregates containing various forms of silica materials are very common whereas the latter occurs rarely because of the unsuitability of carbonates for use in concrete.
Nonetheless, concrete deterioration caused by each type of alkali-aggregate reaction is similar. It should be known that no structure has ever collapsed due to alkali-aggregate reactions, but there are cases in which structural concrete members demolished due to the effect of alkali-aggregate reactions.
Most of the structures severely cracked by AAR are exposed to the weather or are in contact with damp soil. This is because- for a significant amount of expansion to occur, sufficient presence of moisture is essential. Apart from moisture, high content of alkali in the concrete is also essential.
Types of Alkali Aggregate Reaction
Alkali-silica reaction (ASR)
- Random map cracking and closed joints and attendant spalling concrete are indicators of alkali-silica reactions.
- Petrographic examination can identify alkali-silica reactions.
- It occurs broadly because aggregates containing reactive silica materials are more common.
- Alkali-silica reaction generates enough expansive pressure to damage concrete.
- Cracking initiates in areas with a frequent supply of moisture, such as close to the waterline in piers, near the ground behind retaining walls, or in piers or columns subject to wicking action.
- It can be controlled using proper portions of supplementary cementitious materials like silica fume, fly ash, and ground granulated blast-furnace slag.
- Lithium compounds can be used to decrease alkali-silica reactions.
Alkali-carbonate reaction (ACR)
- It is observed with certain dolomitic rocks.
- It may cause considerable expansion.
- Compare to alkali-silica reactions, ACR is fairly rare because aggregates susceptible to this phenomenon are less common.
- The use of supplementary cementing materials does not prevent deleterious expansion due to ACR.
- So, it is recommended that ACR susceptible aggregates not be used in concrete.
Conditions for AAR Occurrence
- Sufficient moisture supply,
- High content of alkali in concrete
It is shown that when the total alkali content, in terms of equivalent sodium oxide, is less than 3 kg/m3, damage expansion due to AAR is unlikely to happen, provided that known highly alkali-reactive minerals, such as opal and glass, are not present in the concrete.
Sources of Alkalis in Concrete
1. Cement
All ingredients of concrete may contribute to the total alkali content of the concrete, the major source of alkali is from cement.
2. Aggregate
Aggregate containing feldspars, some micas, glassy rock and glass may release alkali in concrete. Sea dredged sand, if not properly washed, may contain sodium chloride which can contribute significant alkali to concrete.
3. Admixtures
Admixture in the context of AAR in concrete means chemical agents added to concrete at the mixing stage. These include accelerators, water reducers (plasticizers), retarders, superplasticizers, air-entraining, etc. Some of the chemicals contain sodium and potassium compounds which may contribute to the alkali content of concrete.
4. Water
Water may contain a certain amount of alkali.
5. Alkalis from Outside Concrete
In the areas of cold weather, de-icing salt containing sodium compounds which may increase the alkali content on the surface layer of concrete. Soils containing alkali may also increase alkali content on the surface of concrete.
Effects of Alkali-Aggregate Reaction
- Loss of strength, stiffness, impermeability
- Affects concrete durability and appearance
- Premature failure of concrete structures
- Consequently, life of concrete structure is declined
- Maintenance cost is increased
Tests for Aggregate Reactivity
- Petrographic Examination (ASTM C 295, BS 812: Part 104)
- Chemical Test (ASTM C289)
- Accelerated Mortar Bar Test (ASTM C 1260, CSA A23.2 25A, DD 249: 1999)
- Concrete Prism Test (ASTM C1293, CSA A23.2 14A, BS 812: Part 123)
- Accelerated Concrete Prism Test
Preventive Measures against AAR
- Use low alkali cement to limit alkali content in concrete
- Use of Cementitious Replacement Materials such as PFA and GGBS in concrete to decrease alkali content in concrete
- Reduce the access of moisture and maintain the concrete in a sufficiently dry state
- Avoid utilization of reactive aggregate otherwise necessary precautions shall be employed to prevent influences of alkali-aggregate reactions.
- Modify the properties of any gel such that it becomes non-expansive, for instance, using lithium salts.