Civil Engineering March 2022 | Vol 30 No 2

42 March 2022 Civil Engineering casting. The beam cast on Friday reached 35 MPa on Saturday which meant that all five beams were ready for de-tensioning,” explains Botes. The roofing required 12 083 m of prestressed hollow-core slabs and Cape Concrete used four 150 m casting beds for their production in a strictly controlled quality protocol. They were cast with 12.7 m prestressed strands on four casting beds to yield a strength rating well above the re- quired 50 MPa and hot water was circulated under the casting beds to accelerate the curing process. “The beams, which weighed 12.5 tonnes each, were installed with a 440 tonne crane (one of only two in South Africa) with a 65 m reach operating from outside the reservoir. We also used a 220 tonne crane and a 180 tonne crane for the installation of the hollow-core slabs. During the installation of the first roof section the cranes were positioned 17 m away from the perimeter wall which added to the complexity of the installation process,” Botes adds. The reservoir is being built with a sub-surface drainage system to disperse groundwater. It comprises a layer of no-fines concrete under the slab together with a combination of perforated pipes which follow the grid lines and drain into a concrete gallery which runs under the reservoir. All the pipes are clearly marked so that the source of any possible leakage can be identified. The concrete columns were cast after the main floor slab had been completed and the column reinforcing was tied in with the slab reinforcing to transfer the load through the column bases to the slab. The sloping concrete embankment slabs were cast in sections using a back shutter system to prevent the concrete from slumping towards the bottom of the slope. Rubber water bars, which allow for movement and prevent leakage, are being installed on all the joints in the flooring and walling, and the joints will be covered with a final waterproof bandaging system prior to completion. The external embankments beneath the perimeter walling are being cement-stabilised to improve bearing and shear capacity and the outer walling is being backfilled to a metre below the top of the wall. A service road will be built on top of the embankment. When operational in 2022, the reservoir will be supplied by the Voelvlei Dam and Treatment Works via the existing delivery pipeline which passes close to the site. The reservoir will ultimately feed Blouberg and other areas on Cape Town’s Western Seaboard with pre-treated water. Text and photographs by David Beer on behalf of the Concrete Manufacturers Association (CMA) david.bigsky@gmail.com Rows of in-situ columns prior to the installation of precast beams A 12.5 tonne precast beam during placement by the 440 tonne crane