Civil Engineering March 2022 | Vol 30 No 2

In addition, the design includes three longitudinal expansion joints which run the full length of the structure. They too are filled with a joint former inserted in the gap between the opposing panel sections located above the precast beams. All the expansion joints are waterproofed with a flexible bandage. The completed roof will include a 70 mm mesh-reinforced structural topping cast over the panel sections to ensure proper shear transfer between individual slabs. The topping’s rein- forcing will be tied to the exposed reinforcing in the precast beams which adds continuity to opposing hollow-core bays in the panel sections and increases the overall continuity of the roof structure. The roof includes several galvanised steel manholes to provide light during maintenance work inside the reservoir. Additional manholes are situated above the reservoir discharge and scour outlets for maintenance work. PRECAST VS IN-SITU The reservoir structure was designed by Peter Loubser and GIBB Engineering’s structural team in 2016, acting as specialist structural sub-consultants to WSP which is responsible for the overall contract. Initial site work was begun by a joint venture comprising NMC and Botes & Kennedy in 2017 but came to a halt in 2018 when both parties went into liquida- tion. Construction resumed in early 2021 when CSV Construction was appointed as the main contractor. “We looked at the comparative costing of precast versus in-situ for the roof construction during the design stage, and pre-casting proved to be a significantly better option both in terms of cost and total programme time. In addition, it avoided the disruption that extensive support scaffolding would have caused,” says Loubser. Concrete Manufacturers Association (CMA) member Cape Concrete was awarded the contract to manufacture the beams and 1 358 pretensioned slabs in 2017. “We had already cast 98 beams at the time of the shutdown in 2018, so we decided to cast the remainder and store them in our yard until the project was resumed,” says Cape Concrete Director Walter Botes. CASTING PROCESS “The beams were prestressed and cast on a single 75 m soffit between the stressing heads. This enabled us to cast five beams on one soffit at a rate of one-a-day fromMonday to Friday. We used five sets of end plates and one set of side moulds, the latter being re-used for each beam casting. The beams were steam cured so that the side shutters could be removed the day after Project Team Client: City of Cape Town Main contractor: CSV Construction Civil, electrical and mechanical engineering & overall project responsibility: WSP Specialist structural engineering, environmental and dam safety engineering: GIBB Engineering Health and safety engineering: Frontline Safety Precast concrete elements: Cape Concrete Some of the hollow-core slabs on the partially completed first section of the roof Two rows of precast beams showing exposed beam rebar for tying into the structural topping An in-situ column (foreground) and one of the concrete embankments under construction Civil Engineering March 2022 41