Alkali- Aggregate Reaction

• Mechanism of deterioration in concrete which may occur when alkaline solutions present in cement react with certain forms of silica in the aggregate to produce an alkali-silicate gel .
• The gel , being hygroscopic absordbs water & this results in an expansion of the volume that creates tensile stresses within the concrete & causes cracking
• The gel , being hygroscopic , absorbs water & this results in an expansion of the volume that creates tensiles stresses within the concrete & causes cracking .
• The main external evidence for deterioration of concrete due to alkali-aggregates reactivity is the development of cracking .
• In unrestrained members , this appears as randomly distributed cracking .
• In concrete where expansive forces are restrained by reinforcement , cracks tends to run parallel to reinforcing bars .
• The main factors influencing the progress of the alkali-aggregates reaction includes the presence of non-evaporable watrer in the paste & the permeability of the paste .
• To prevent the problem it is necessary to reduce water access to the concrete so that athough gelling will still be present , it cannot absorb water & exert an expansive force .