Recently the Canning Dam has been subjected to considerable cracking of the upper parts of the dam and upper gallery. Investigations have shown that cracking was due to strong AAR (alkali aggregate reactivity) in the concrete. AAR results in swelling of the concrete, which may cause secondary compressive stresses, localised map cracks, and, ultimately structural cracks. In addition, the concrete tensile strength and elasticity significantly decreases. Many old concrete dams are known to suffer from AAR, including Fontana Dam in Tennessee and Pian Telessio dam in Italy among others.