How to prevent the occurrence of Alkali Aggregate Reaction (AAR) in bridge structures?
One of the products of cement hydration is calcium hydroxide which is high alkalinity in nature. This alkalinity environment is ideal to protect the corrosion of rebar in the concrete. However, with the present of silicate, chloride and sulphate in the aggregates, chemical reaction occurred and produces reaction product such as alkali silicate gel. This reaction product is expansive and may cause crack to form.
To prevent such reaction to occurs, testing on the aggregate such as alkalinity test, chloride and sulphate test are necessary to contain these substances below the limit level. Our JKR road spec stipulates these requirements and must comply to the given limits (clauses of testing of aggregates). The alkalinity test can also be referred to relevant ASTM test.
How to solve the problem?
The reaction product gel of alkali-aggregate reaction is expansive and causing the concrete to crack. Once the reaction product has formed, it will then stabilized and the concrete becoming weakened. The treatment of repair will be similar as other concrete attack remedy.
What is best method of repair or solution to overcome the defect?
Repair solution will depend on the deplorable situation of defects ranging from durability rehab to major strengthening.