VHEMBE District Municipality has harnessed the many benefits of precast concrete to deliver critical infrastructure faster than would be possible using conventional cast-in-place methods. The absolute control provided by prefabricated concrete also facilitated a higher quality final build.
The technology was successfully deployed on components of the upgrade of the Vondo Regional Water Scheme, which will provide a reliable and secure source of drinking water to about 500 000 people who reside in Phiphidi.
Located along the road to Sibasa to Nzhelele, the scheme consists of the Vondo Dam and water-treatment works (WTW). From here, water is pumped to two command reservoirs that supply specific areas within the scheme. Water is also pumped and gravitated to the WTW from two other reservoirs where it is distributed to various areas.
The construction of the two reservoirs was undertaken in phases. One of the reservoirs had to be commissioned before an existing water-retaining structure could be demolished to make space for the construction of a new reservoir on the restricted site.
This is where the municipality is successfully exploiting the benefits of prefabricated concrete technology.
The reservoirs were designed by Infraburo Civil and Structural Consulting Engineers and Corestruc, a precast-concrete specialist. Corestruc was also tasked with erecting the system while working alongside principal contractor, Morawa Building and Civils. The various precast-concrete elements that make up the system were manufactured by Coreslab, one of Corestruc’s approved manufacturers.
The first reservoir was erected in record time. It has passed all relevant tests and has now been operating successfully for just under two years. The stellar performance of the reservoir to date further motivated the use of the technology to also build the second water-retaining structure.
Using this precast-concrete system, a 10 ML reservoir can be completed in only four months, therefore, providing significant savings in construction costs for the client.
Corestruc’s system enables the various trades to work simultaneously. The wall and roof are manufactured while earthworks and site terracing, as well as the construction of the roof column bases are under way. This work is undertaken by the principal contractor.
Notably, the system facilitates greater participation of emerging contractors in these projects. Bear in mind their highly specialised nature with only a few contractors having the skills, experience and capacity to build cast-in-place reservoirs.
Corestruc uses 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 1MPa 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.
As part of the final aspects of the build, the concrete floor slab is completed and the hollow-core slabs that make up the outer portion of the roof structure connected to the precast-concrete beams. A grout topping is then placed over the hollow-core slabs to form a single monolithic structure and a precast concrete coping installed around the perimeter of the roof as an aesthetic finish.
