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Concrete In Australia : December 2008
PROJECTS Repairing export silos at a coal mine The clean coal silos at the Mt Thorley Warkworth mine in the Hunter Valley were built in 1984. The silos store coal delivered straight from the washery building, ready for filling rail wagons for transport to the shiploader in Newcastle. The silos are therefore a vital infrastructure link in the transport chain. The reinforced concrete silos are about 31.5m tall by 15.5m in diameter, with 250mm thick walls. They have a capacity of 3000t of coal each. Less than 12 months after construction, cracks were noticed on the external faces of the silos and post-tensioned steel cables were installed to prevent further cracking and to provide additional restraint to the walls of the silos from the coal loading within. For some time, the post-tensioned cables were subject to periodic failure owing to corrosion. Following an extensive options study, it was decided that the silos should be remediated using carbon fi bre composites, following major concrete spalling and crack repairs. Investigations were carried out by Izzat Consulting from a dedicated box suspended from a mobile crane. This work included visual inspection, mapping of cracks, drumminess and investigating spalling both inside and outside each of the silos. The investigation checked sound at large cracks (about 0.5mm) and medium cracks (0.25mm to less than 0.5mm). The condition of the underlying reinforcement, including the depth of carbonation, chloride and sulphate profi les and half cell potentials, were checked at a number of locations over the surface of the silo. At other locations concrete cores were extracted to evaluate concrete compressive strengths and cement content. Deterioration of the external post-tension cables was evaluated by using eddy current techniques and visual inspections to determine the cables that presented an immediate safety risk. Using the cable condition data, along with the concrete compressive strengths and reinforcement condition, the residual strength of the silo wall was determined for hoop tension and bending (vertically and horizontally). These capacities were also estimated with the cables being fully redundant. All of these paramount structural design issues were compared to the results of a 3D fi nite element model while considering loads imposed from stored coal, earthquakes, wind and thermal effects. All of these load cases played an important part in various zones of the silo when the strengthening requirements were considered to ensure the structure remained serviceable for at least another 25 years. To provide a serviceable strengthening solution, it 56 Concrete in Australia Vol 34 No 4 Hydro-demlition was used to remove loose concrete and to prepare the surface for the application of carbon fibre strips. was decided that carbon fibre laminates (CFL) should be externally applied (vertically and horizontally) while the cables were removed in a staged fashion to maintain operations. CFL was only to be applied after zones of unacceptable (drummy and or widely cracked) concrete was removed and repaired with a sprayed repair mortar. It was then proposed that the CFL and underlying concrete was to be protected with an anti-carbonation elastomeric coating. The internal cracks were to be repaired with a high tensile strength sprayed repair mortar. Following OH&S, QA and reference assessments of several corporate contractor members of the Australian Concrete Repair Association (ACRA), Buildcorp Asset Solutions (BAS) was awarded the contract to carry out the remedial works following a lump sum tender process. The client, Rio Tinto, suggested that their regular access contractor be engaged to provide full scaffolding access to the inside and out of the silos for the works. However, BAS carried out a cost analysis using a specially designed mast climber system which worked out to be more cost effective. This was