WHO'S WHO IN CONCRETE
SA refines precast reservoir construction Precast concrete has helped to accelerate and mitigate risks on complex civil engineering projects the world over. This includes reservoir projects, where precast construction tends to be more efficient than cast in situ. However, the real innovation lies in the way they are built on-site, says industry specialist Corestruc.
T
he typical precast concrete roof structure comprises hollow-core slabs that are placed on precast concrete beams suppor ted by prefabricated columns. The precast concrete columns are connected to the in situ bases via cast-in components (or column shoes). These are secured to the hold-down bolts in the same manner as a conventional steel structure. Various column base configurations can be implemented to meet project-specific requirements and their designs must consider a minimum safe bearing capacity of 200 kPa at founding level. Precast concrete reser voir roof structures are up to five times faster
The use of precast concrete technologies provides additional levels of quality assurance
to build than cast-in-place construction processes. The process can also be sped up even further by including prefabricated walls. These prefabricated reservoir walls consist of precast concrete panels that have been designed to the required thickness. They have been prestressed during manufacturing and include castin sleeves that have been strategically positioned according to the design requirements for post-tensioning on-site.
Wall installation technique The first panel is placed and supported temporarily and anchored by the following precast concrete elements. 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 period. 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 reinforced kicker on the wall footing on both sides of the wall panels. Joints between the panels are then grouted with a high-flow, high-strength grout. This is followed by post-tensioning to create a watertight structure.
A modular precast approach saves time in constructing reservoirs, mitigating the potential risk of project delays and cost overruns
18
IMIESA May 2021
