Construction World March 2022

by push-pull props and steel brackets at the top of each. Unbonded cables are then pushed through the sleeves which are then grouted monolithically with the joints of the panels. Notably, unlike most conventional post-tensioned reservoir walls which are designed to slide, Infinite Consulting Engineers and Corestruc use a “slide-and-pinned” system. Post-tensioning is undertaken when the wall is not yet fixed to the ring footing and it is, therefore, allowed to slide on a steel bearing or locating plates. The coated post-tensioned cables are not bonded to the grout with the reservoir designed to maintain a residual compression of a minimum of 1 MPa in all directions. Horizontal reactions to the wall base are transferred to the ring foundation through the second phase cast in-situ kicker. This is where the ring tension in the base is also activated to resist the reaction. Additional post-tensioning of the lower part of the wall reduces the amount of rebar required in the cast in-situ ring footing. However, the project was not without its share of challenges. Initially, the municipality was going to build this reservoir using conventional methods. Based on the success that Infinite Consulting Engineers and Corestruc achieved on the other three reservoirs, the client body decided to also build this water-retaining structure in the same manner. Tango’s Consultants, therefore, initially designed an integrated column base-to-floor connection. Infinite Consulting Engineers and Corestruc decided to adopt the system because the principal contractor had already procured the floor and base rebar.There are also other important advantages offered by the system that have been noted by the municipal ity on these projects. This includes the precast-concrete reservoirs’ smaller carbon footprint. The vertically stressed precast-concrete wall panels are thinner reducing the amount of aggregate and concrete used to build the reservoir. The hollowcore slabs, alone, reduce the carbon footprint of the structure by up to 40%, which is augmented by the use of prestressed 665 mm x 460 mm I-shaped beams. The self-compacting concrete used to manufacture the various precast- concrete elements also reduces the volume of the construction material required and conserves energy by el iminating the need for vibration. It is also provides a safer means of constructing reservoirs. This is considering that the various elements are manufactured at ground level and erected on site by a skilled and experienced team. Certainly, one of the project milestones was witnessing all the precast-concrete elements fitting into place and integrating without any discrepancies to complete a structure that is now receiving and retaining water. 

effectively. Trial mixes were again prepared and tested for strength and durability, while also taking into consideration the workability of the concrete. Working in a controlled environment, Coreslab is also able to ensure the perfect dosages of admixtures. This is to avoid overdosing that may delay early strength development, imperative for uninterrupted production. Another significant focus in the factory is to maintain the perfect water-to-cement ratio to ensure concrete strength. Aggregates from suitable suppliers are also graded appropriately to avoid early concrete shrinkage. The prefabricated elements are inspected and approved for dispatch to ensure the accuracy of the placement of the cast-in components. Reinforcement bar size and placement are also verified as part of the quality control procedures at the factory. These steps are complemented by the checks and balances introduced by Corestruc ahead of mobilising to site to commence erecting the structure. Corestruc manages the setting out and installation of the column anchors for the precast concrete roof. By confirming the dimensional accuracies before erection, the company ensures that the beams of the roof structure fit the first-time round. The precast-concrete wall panels are placed and positioned in a similar manner with the assistance of permanent locating plates that are fixed to the ring foundation. Pam Brink reservoir has an inside diameter of 65,2 m and water height of 7,8 m. The roof column grids for the roof slab and beam span are 10,87 m x 10,87 m. The roof structure consists of hollowcore slabs that are placed on precast-concrete beams, which are supported by the prefabricated columns. They are connected to the in-situ bases by components that are cast into the precast-concrete columns, also referred to as “column shoes”. Similar to the construction of a conventional steel structure, the precast- concrete columns are connected to hold-down bolts in the base. Suspended precast-concrete beams are then installed onto the columns. They are secured with dowels that protrude from the columns. The hollowcore slabs are then installed onto the beams and secured into place. This is achieved by tying the steel reinforcing and placing in-situ concrete into the cores of the prefabricated slabs. Stirrups protruding from the precast-concrete beam act as a mechanical interlock to form a composite mechanism with infill concrete. The circular wall comprises 170 mm-thick precast concrete panels that have been prestressed vertically. They feature 32 mm-diameter polyvinyl chloride sleeves that have been positioned according to the post-tensioning design. The panels are placed into position and then supported

From left: Authors of this article – Kobus Kotze, Director of Infinite Consulting Engineers; Tian de Jager, Director of Corestruc; Willie de Jager, Managing director of Corestruc and Kadiebwe Jean Paul Mulunda,Civil Engineer Technologist of Tangos Consultant.

23 CONSTRUCTION WORLD MARCH 2022