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Alkali Aggregate Reaction in Concrete – Types, Causes, and Effects

Alkali Aggregate Reaction in Concrete

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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) 

Fig. 1: Concrete Failure Due to Alkali Aggregate Reaction

Alkali-carbonate reaction (ACR) 

Conditions for AAR Occurrence

  1. Sufficient moisture supply,
  2. 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

  1. Loss of strength, stiffness, impermeability
  2. Affects concrete durability and appearance
  3. Premature failure of concrete structures
  4. Consequently, life of concrete structure is declined
  5. Maintenance cost is increased
Fig. 2: Affect of Alkali Aggregate Reaction on Railway Bridge
Fig. 3: Treatment Plant Concrete Deteriorated Due to Alkali Aggregate Reaction

Tests for Aggregate Reactivity

  1. Petrographic Examination (ASTM C 295, BS 812: Part 104)
  2. Chemical Test (ASTM C289)
  3. Accelerated Mortar Bar Test (ASTM C 1260, CSA A23.2­ 25A, DD 249: 1999)
  4. Concrete Prism Test (ASTM C1293, CSA A23.2­ 14A, BS 812: Part 123)
  5. Accelerated Concrete Prism Test
Fig. 4: Petrographic Examination

Preventive Measures against AAR

  1. Use low alkali cement to limit alkali content in concrete
  2. Use of Cementitious Replacement Materials such as PFA and GGBS in concrete to decrease alkali content in concrete
  3. Reduce the access of moisture and maintain the concrete in a sufficiently dry state
  4. Avoid utilization of reactive aggregate otherwise necessary precautions shall be employed to prevent influences of alkali-aggregate reactions.
  5. Modify the properties of any gel such that it becomes non-expansive, for instance, using lithium salts.
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