The City of Ekurhuleni’s (CEMM) Water and Sanitation Department successfully completed the construction of four prefabricated reservoirs using cutting-edge precast concrete technology.

In addition to these four new reservoirs, the CEMM is building a new water tower which will be completed in late 2024. This tower will be almost entirely prefabricated, excluding only the floor slab and foundation. Upon completion this tower, standing at 36m with a capacity of 2.5 Ml, will be the first pre-cast concrete water tower in Africa.

Why precast

The municipality opted for a precast concrete water tower because it was faster and more cost-effective to build. This while also providing a final structure of exceptionally high quality which, in turn, will provide operating cost savings through reduced maintenance requirements.

The efficacy of prefabrication had proven itself time and again on each of the projects the Water & Sanitation Department completed. Meanwhile, its professional team had also become more proficient in executing the works, refining and honing the sophisticated method from one project to the next.

CEMM opted to use precast concrete because this technology reduces construction time and is more cost-effective. This time and money-saving process produces high-quality structures which in turn save on operating and maintenance costs. The efficacy of prefabrication proves itself with every successful project.

These precast concrete projects require collaboration and the municipality worked with Tangos Consulting Engineers  (the design engineer and project manager), Infinite Consulting Engineers, RSMM Construction (the principal contractor) and Corestruc (a precast concrete turnkey contractor). The partnership between Corestruc and RSMM Construction has earned them the reputation of delivering high-quality water-retaining structures in less time than conventional concrete construction methods. For comparison, this water tower is expected to be completed in 14 months, whereas traditional concrete water towers take between two and three years to complete. As this is the first precast water tower the time of completion going forward is expected to be reduced to 9 months when the process is more familiar.

This tower has a design life of 100 years if maintained correctly. The high-performance concrete used in this precast process is made to last, Corestruc’s quality control technicians calculate the moisture content of aggregates and factor in water for admixtures to modify and maintain the optimum water-to-cement ratio. The concrete used in the water tower is more impermeable than traditional concrete and can withstand damage from chloride, sulphate ions, and other aggressive chemicals.

The water tower design is the result of years of research and development by both Corestruc and Infinite Consulting Engineers. This project serves as the starting point. Another precast water tower is in the works for CEMM and other municipalities are exploring the more effective precast technology to augment their water supply systems.

How it all comes together

The tower consists of 12 columns and spiral beam elements for each of the three sections or “rotations” that comprise the outer portion of the tower. These 30 interconnected semi-circular prefabricated elements wrap around the structure providing the necessary support, while also offering a striking aesthetic effect. The two columns are fixed via the protruding dowels that pass through the underside and topside of the spiral beam which is then filled with concrete. This process was repeated until the columns reached their final height.

Then there are the 15 prefabricated elements that make up the 3,6m-diameter shaft, which is constructed at the same time as the outer portion of the structure. Equipped with precast concrete stairwells with balustrades, it leads to the underside of the prefabricated tank. Thereafter, the shaft connects with stacked precast concrete pipes equipped with a cat ladder. Cast into the reservoir floor slab, this top portion of the shaft will traverse through the water-retaining structure to a manhole on the roof.

The superstructure consists of 12 tapered beams connected to the columns of the last rotation. Resting on bearing pads, they will carry and distribute the load of the precast concrete tank and its contents.

Installed in the triangular-shaped junctions between the tapered beams, cut-to-size and shape hollow-core slabs serve as the shutter for the in-situ foundation for the water-retaining structure. Coping panels have been installed along the perimeter providing an aesthetically pleasing finish.

The floor slab has to be made on-site and is not prefabricated, so Corestuc locally sourced four sub-contractors to handle the job. The slab will be 35cm thick and use a total of 103 m3 of concrete. The slab has to withstand the heavy reinforcement of the beam junctions so the concrete has to be managed carefully to ensure it is compacted correctly, while also being watertight. An articulated boom lift has already been placed on top of the superstructure to assist with the installation, as well as the grouting of the 34 tank wall panels. This is in addition to the two buttress panels for post-tensioning, which will be undertaken in the same way as all of Corestruc’s other reservoirs. All of the wall panels will be propped during their installation. Conventionally, only the first wall panel is propped to free up space.

Meanwhile, the roof structure consists of four columns and beams and 150 hollow-core slabs. The centre portion or “core” will be constructed first, and the outer section completed last.

Meyer van Rooyen, a Corestruc Project Manager, who has supervised all precast-concrete work thus far, says that there is very little scope for error on this project.

“We have achieved tight 15mm tolerances throughout the construction of the three rotations of the substructure, starting with the placement of the first-rotation columns on the in-situ foundation. Among other factors, this is also facilitated by our precise manufacturing processes. Each element undergoes extensive quality assurance before it is dispatched and again once it has arrived on site. Once the elements have been safely transported to the site and as the last component in Corestruc’s precast-concrete value chain, the responsibility rests with my team of riggers to maintain the highest levels of productivity, efficiency and accuracy.”

His team is supported by a 150-ton mobile crane. The 12 tapered beams, for example, each weigh a staggering 18.5 tonnes and the columns for the second and third rotations slightly less.

Precast concrete structures require high-accuracy construction and Corestruc has invested in robotic total stationing to achieve the levels of accuracy needed.

The structure demonstrates both precast and cast-in-place techniques. “Supporting this water tower is a 1m-thick and 25m diameter foundation consisting of 500m2 of 30MPa cast-in-place reinforced concrete. It is underpinned by 200m2 of mass concrete blinding. This took 10 hours to place, starting at 5h00 and using two concrete pumps, while also isolating an entire ready-mix concrete plant just for this purpose,” says Van Rooyen.

Safety is the number one concern on any construction site, and since construction jobs like this are relatively new this project had a height safety specialist on-site to assist with safety protocol as well as provide specialist training to the team.