1999 – Investigations on Canning Dam Concrete Gravity Structure and it’s Rehabilitation

Ahmad Shayan, Robert J. Wark and John Waters

The Canning Dam concrete gravity structure located in Western Australia has shown an upward movement of 18.3 mm and lateral upstream movement of 14.2 mm over the past 15 years of monitoring. These movements have been associated with considerable cracking of the upper parts of the dam and the upper gallery. Investigations have shown that the cause of the cracking was a strong alkali-aggregate reaction (AAR) in the concrete, brought about by a deformed granitic rock. Extensive horizontal and vertical cracking in the upper part of the dam wall has necessitated the removal of the section above the floor of the upper gallery level, and construction of a new reinforced concrete section to act as head beam for post-tensioning of the rest of the dam wall.

A set of small diameter cores were taken from the various parts for diagnostic purposes, and a vertical core of 100 mm diameter was taken through the whole thickness of the wall for the determination of the strength properties, alkali content and residual expansion potential. Based on these, a post-tensioning stress of 1.5-2.0 MPa has been calculated for restraining the residual expansion of the concrete. The spillway bridge structure which is part of the dam wall has also shown mild signs of deterioration. The piers and abutment walls and the deck were surveyed for corrosion activity and extent of AAR. This work showed that the spillway bridge structure was sound and only needed maintenance. The performance of a triple blend concrete mix containing a high volume of fly ash (45%) and silica fume (5%) developed for the replacement of the old concrete is also discussed.