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Hydrodemolition Techniques Used at an Australian Mine
The problem had arisen when an incorrect batch of concrete had been poured for a section of the blade wall measuring 9.3 m high by 9.3 m long and with a thickness of 1 m. Removal of the section was necessary, but had wire sawing techniques been used the deployment of the crane would have had to be scheduled around railway shutdowns. months or more that a crane and wire saw would have required,” says HiTech’s operations manager Damien Turner. The 710V Evolution robot was equipped with an 11 m mast to enable it to comfortably reach the top of the wall. It has been designed for all horizontal, vertical and overhead operations, having a 3-D positioning of the front power head; giving the operator full freedom to reach all areas and to work in confined areas. The HVD6000, which has a similar 3-D feature, was fitted with a standard 5 m mast.
“Apart from the speed and convenience of the removal rate using hydrodemolition techniques, the idea of the 8 mm safety shield helped us to win the contract. It was important that the trains continued working. A full trainload of coal is valued at A$3 million ($3 million), so it was important to ensure there were no delays.” Mr. Turner says that although HiTech was on site for 4 weeks and working or on standby 24/7, the removal of the full 86 m³ of defective concrete was achieved in just 172 blasting hours. “There was a good deal of reorganizing and other work going on at the site, and so much of our time was spent on active standby,” he said. “The actual work presented no unexpected problems for the Aquajet robots.” The concrete being removed was 40 MPa, with the reinforcing bars being 32 mm diameter with generally between 150 and 200 mm spacing, although in some places the spacing was only 50 mm. Basic Principles of Hydrodemolition Material is easily removed as the build up pressure exceeds the tensile strength of the damaged or weakened concrete. This combination of water pressure and flow together with the controlled kinetic and geometric movements of the robotic equipment creates the necessary “effect” criteria for the Hydrodemolition process; leaving sound concrete undamaged. The lower the concrete strength, the larger the material removal rate that can be achieved due to larger penetration and pressurisation of the material. Additionally, the higher the cement matrix strength the higher the concrete resistance. Surface Quality This compares favorably with a surface prepared with hand-held tools which results in a higher probability on interface failures at pull-off testing. With water jet Hydrodemolition, once programmed, the jet moves rapidly and continuously over the selected area for removal. There is no percussive effect on the surface with the water jet penetrating the deteriorated concrete. Extensive investigations have proved that there is no modification of the concrete microstructure during the water jet treatment. Similarly the concrete pore structure is not affected by the water jet. The surface geometry achieved after Hydrodemolition depends on the type and size of the aggregate. With limestone, for example, the surface is comparatively smooth and characterized by a high degree of fractured aggregate gains. |
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“There was a 32,000 V rail line behind the site that is used to transport coal. The line is just 2 m behind the wall,” says Mr. Turner. “We proposed placing an 8 mm thick steel plate behind the wall, which was supported by formwork and props, to prevent flying debris hitting the trains. It also stopped the jet from hitting the power cables.
Use of hydrodemolition ensured that the rebar was not damaged in any way whilst removing the defective concrete.
