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Concrete In Australia : June 2014
Concrete in Australia Vol 40 No 2 25 was the establishment of a defects points system for aggregate particle shape and distribution. Aggregates were tested at the crushing plant using a ‘rapid’ test method to accommodate the high rate of aggregate production. Once stockpiles were conformed at the crushing plant, they were transported to the main aggregate stockpile where they were effectively blended with other aggregate produced previously. This process allowed minor deviations from the nominated grading, dependent on a rolling average grading, to ensure the composite blended stockpile remained within the required grading envelope. While onsite quarry and crushing operations provided an economic benefit to the project, production of fines was driven by a project commitment to sustainability and minimising impacts on the local community by avoiding the need to import the fine aggregates. Aggregate stockpile management Aggregates, from the commencement of crushing placed at the lower levels of the stockpile, remained in place for over 18 months. The presence of some weathered material in these aggregates, which experienced a level of degradation over the time, combined with the migration of fines from rain, resulted in variation of the aggregate particle distribution. This variability was overcome by progressive selective mining of the aggregate stockpile, combined with a forward testing program to further verify aggregate conformance prior to inclusion in the permanent works. This attention to detail ensured very high quality RCC was produced throughout the project. Interestingly, the limited available stockpile area, due to site topography, necessitated an innovative solution to maximise the stockpile volumes. Cubic metre nylon bulker bags were used to create a barrier between the three different aggregate types. A layer of bulker bags, filled with the aggregate from the adjoining stockpile (to expedite later removal), was placed and then aggregate, from both sides was evenly stockpiled against this barrier. This process resulted in a collective stockpile over 20m high and increased the stockpile capacity by over 40%. Aggregate temperature was another key component of the stockpile management. The majority of the aggregate crushing campaign was designed to occur in the cooler months and/or at night to ensure the aggregate temperature was minimised during production and stockpiling. Selective mining of the stockpile during the RCC production phase was required to ensure the benefits of cooler aggregates was not lost. This included working of specific faces in the aggregate stockpile that were orientated away from midday sunlight, localised covers, and hauling at night during warmer months. At the batch plant, a two day aggregate storage facility was constructed to minimise temperature gain during batching. The benefit to the project was significant reduction in RCC batch cooling with ice, which had cost and sustainability benefits for the project. Modified version of GERCC (GERCC-m) Designed and used for construction of the levelling concrete in the foundation of the dam, it was necessary to establish a level surface before RCC placement could commence. Based on the GERCC facing mix, a premixed concrete mix design was developed for construction of the foundation levelling concrete. In total, approximately 15,000m3 of foundation levelling concrete was placed. In addition to the speed and economic benefits of site won versus imported aggregates, the adiabatic temperature rise of the GERCC-m was significantly less than the imported conventional concrete alternative, thereby enabling larger concrete pours and minimising the risk of thermal cracking. It also offered other technical benefits, including far more similar elastic and thermal properties to the bulk of the RCC. This innovation was presented at the ANCOLD conference in 2009. Use of self compacting concrete A 3m diameter diversion pipe was present for the majority of construction to pass normal river flows. In the latter stages of the dam construction this diversion pipe was plugged. The complexity was that, with the upstream blocked by a steel gate structure, the only access for the plug construction was up 70m of pipe from the downstream works. The challenge was to construct an impervious concrete block within a confined space. This solution was the development of a self compacting concrete (SCC) to eliminate the risk of manual vibration, which could have the undesired result in a safety risk for workers and/or a risk to insufficient compaction of the concrete resulting in permeability. A special mix, vastly different from that used in typical contemporary applications had to be developed and proven in multiple trials before incorporation in the permanent works. The learnings from the development of the SCC mix design were incorporated in the secondary concrete for the intake tower. While the main intake tower structure was constructed using a jump form, detailed work for the screen rails, stop log guides and intake pipework bellmouths needed to be done later and it was crucial that this concrete was of a high quality to prevent leakage. The difficulty with this element of the work was the confined concrete pour area combined with a high density of reinforcement and pour heights exceeding 3m, necessitating an innovative approach to ensure a high quality concrete product was achieved. The solution, made possible by the use of the SCC, was to pump the concrete from the bottom of the form. Combined with the SCC, this process ensured a well compacted concrete with excellent surface finish. Concrete paving steps The downstream face of a RCC dam is stepped to aid the construction process and to provide energy dissipation during “There is no known instance of this process having been used on any previous RCC dam in the world.” 22-29 - cover story.indd 25 22-29 - cover story.indd 25 22/05/14 11:45 AM 22/05/14 11:45 AM