Recently the Canning Dam have 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 result 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 decrease . Many old concrete dams are known to suffer from AAR , including Fontana Dam in Tennessee and Pian Telessio dam in Italy among others .