Civil Engineering June 2021 | Vol 29 No 5

Civil Engineering June 2021 39 This is followed by the installation of HCS onto the beams which are then locked into place using tie-steel rein- forcing and in-situ concrete inside their hollow cores. The stirrups protruding from the precast beams serve as a me- chanical interlock to form a composite mechanism with the infill concrete. This is a tried and tested approach that has been used widely in South Africa to construct reservoir roofs on hybrid concrete construction projects where the wall is built using traditional methods. Precast concrete reservoir roof structures are up to five times faster to build than cast-in-place construction processes. A reservoir roof is one of the final as- pects of the works programme when it is constructed using cast-in-place methods, undertaken once the wall and in-situ columns reach their final height. Many tonnes of scaffolding must be installed inside the reservoir to support the forms for the frame of the roof structure. The extensive associated reinforcement and concrete works, including placement and stripping of the shutters, also require close management. This is in addition to the timely coordination of the concrete trucks to site for the continuous concrete pours. A TOTAL PRECAST CONCRETE SOLUTION There is also very little scope for error in the construction of an in-situ reservoir wall. It entails significant steel-fixing, and the installation of the side shutters is an onerous process. Care also needs to be taken when managing processes such as the placement of the concrete to avoid segregation and the displacement of the reinforcement to ensure a watertight structure. Any mistakes in this aspect of the works programme will require rework and result in costly delays. A precast concrete wall system provides absolute control over the entire construction process and is significantly faster than cast-in-place methods. Prefabricated reservoir walls consist of precast concrete panels that have been designed to the required thickness. They have been prestressed during manufac- ture and include cast-in sleeves that have been strategically positioned according to the design requirements for post- tensioning on site. The first panel is placed and sup- ported temporarily and anchored by the subsequent precast concrete ele- ments. This approach requires minimal propping and frees up space. The wall foundation, or “ring footing”, is designed to transfer vertical loads from the wall and roof to the ground. It also translates the shear force in the wall due to water pressure into the ring tension in the foundation. This tension is countered with adequate rebar cast into the ring footing. Unbonded cables are then threaded through the sleeves and joints between the panels. They are temporarily sealed with rubber gaskets to enable the low viscosity grout to flow through all sleeves and joints. This grout is self-healing and achieves strengths of up to 100 MPa in a short pe- riod. It is cooled to ensure flowability and pumped around the circumference of the reservoir in a controlled manner. After the grout has cured to a strength of 80 MPa, the cables are stressed to their specified yield via special buttress panels that have been equally spaced along the perimeter. The wall is then pinned by casting a 200 mm to 250 mm high rein- forced kicker on the wall footing on both sides of the wall panels. Joints between the panels are grouted with a high-flow, high-strength grout. This is followed by post-tensioning that renders the joint in compression to achieve water tightness. The wall has also been designed to a 1 MPa minimum residual compressive stress. Corestruc continues to demonstrate the efficacy of constructing reservoirs in this manner. In addition to ensuring timely and accurate project completion, there are many more benefits of this system. These include decongesting construction sites and enhancing health and safety, which are incorporated during the manufacture of the precast concrete reservoir system.  Precast concrete technologies provide additional quality assurance on reservoir projects