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Concrete In Australia : June 2014
24 Concrete in Australia Vol 40 No 2 COVER STORY The rhyolite was ultimately selected as the aggregate source. The alteration and weathering of the rhyolite changed the mineralogy of the rock and this was found to affect the wet/ dry strength variation and the dimensional stability of the aggregate (ie the tendency to expand and contract with changes in moisture content). The primary issue with the use of the aggregate in concrete was that it had a high drying shrinkage potential. It was unclear whether it would be suitable for use on the exposed faces of the dam. Consideration was given to a number of alternatives for facing the dam, including a Carpi membrane and facing the dam with a conventional concrete made from imported aggregate which satisfied all national code requirements. Significant cost savings were available if the site won aggregate could be used, as well as providing a more desirable solution to the project’s sustainability objectives. Following an extensive campaign of technical research, laboratory trials and testing (which included the development of new laboratory testing methodologies), the site won aggregates were ultimately proven to be acceptable for use. Run-of-station flyash was also found to be suitable and this offered significant savings over the commercial fine grade flyash alternative. The mix was refined over time and ultimately only 75kg/m3 of cement was used, with 120kg/m3 of fly ash. Keeping the cement content low meant the magnitude of the thermal loading was kept to a minimum. The findings of the research were presented at an international conference on RCC Dams in Spain (RCC2012) and demonstrates the significance of this technological development. Onsite quarry and crushing operations Approximately one million tonnes of aggregates were sourced from the site quarry (which was developed within the construction footprint for the project), together with material excavated from the dam abutments, with a purpose built crushing plant erected on the site to produce aggregates suitable for RCC. The in situ rock on the site contained varying proportions of weathered and geologically altered material, which required significant testing and development of RCC trial mixes to provide assurance that durability of the dam would not be compromised. The key challenge with the crushing operation was producing a high proportion of fines required for the RCC mix design. This necessitated inclusion of specific elements in the crushing plant design to allow increased fines production without impacting on particle shape or adversely affecting plant production and demonstrated industry best practice for development and implementation of a crushing plant. A key element of the RCC aggregate production is that, due to the large volume of aggregate required well in advance of RCC placement commencing, it is not possible to produce discrete conforming aggregate stockpiles. However, aggregate conformance is still required to ensure a high quality RCC mix is produced throughout the project. A key component of the success of the quarry and crushing regime at Cotter Dam The enlargement of the Cotter Dam was completed in 2013 and exceeded world’s best practice. 22-29 - cover story.indd 24 22-29 - cover story.indd 24 22/05/14 11:45 AM 22/05/14 11:45 AM