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.
Infraburo Civil and Structural Consulting Engineers, a leading consulting engineering firm, commenced with the preliminary design of the upgraded scheme in 2012. In 2017, the construction work went out to tender and was awarded to Morawa Building and Civils, the principal contractor. The upgrade of the scheme is being financed by the Municipal Infrastructure Grant.
The project entails upgrading the WTW and associated infrastructure. This includes the gravity-feed pipeline from Vondo Dam, as well as a pipeline from the WTW to a command reservoir. Moreover, a new command reservoir was constructed and work on the second water retaining structure was recently completed. This is to supply the 10ML daily water demand with an additional 10ML of water held in reserve. 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.
Vhembe District Municipality is now one of many South African municipalities to use a unique precast concrete system to construct these technically complex structures.
The reservoirs were designed by Infraburo Civil and Structural Consulting Engineers and Corestruc, one of the country’s foremost precast-concrete companies. 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.
Using in-situ construction methods, it can take up to nine months to complete a reservoir of a similar capacity. This is without any errors and the need to redo work. With conventional cast-in-place construction methods, work first commences with the floor slab. This is followed by the wall and columns for the roof structure. The process ends with the completion of the roof which, together with the wall, are the most complex aspects of the build.
However, 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.
Morawa Building and Civils introduced Infraburo Civil and Structural Engineers to the system, noting the role it could potentially play in keeping this aspect of the work scope on track, while also providing two reservoirs of an unrivalled quality.
“We initially designed conventional pre-stressed reservoirs for this project. However, the principal contractor had undertaken extensive research into Corestruc’s system and put forward the idea to use it to construct the two reservoirs. Although initially sceptical, I personally visited one of the company’s approved manufacturers’ factories and spent time with representatives of Corestruc to gain a greater understanding of the system. Needless to say, I was very impressed especially with the quality control processes deployed in the factory to ensure prefabricated concrete elements of the highest possible quality are used to build these structures. I, thus, motivated the use of the system to the client. Vhembe District Municipality was also already aware of the technology considering that it had previously been successfully deployed in several other municipal jurisdictions. The client,
therefore, accepted my proposal,” Rudolph Dippenaar, Infraburo Civil and Structural Consulting Managing Director, says.
Corestruc’s technology consists of a reservoir roof and wall system.
Dispatched to site once the cast-in-situ bases have been completed, the roof columns are the first precast-concrete elements to be erected.
They are connected to the hold-down bolts in the column bases. Suspended precast-concrete beams are then connected to the dowels that protrude from the precast-concrete columns. Thereafter, hollow-core slabs are connected to the stirrups protruding from the precast-concrete beams. Steel reinforcing is placed into the cores of the hollow-core slabs and these voids are then filled with in-situ concrete. By forming a composite mechanism with the infill concrete, the stirrups act as mechanical interlocks.
Once the inner portion of the roof has been erected, the principal contractor is able to commence constructing the in-situ floor slab. The cover provided by the roof structure also facilitates optimal curing conditions.
At this stage, the individual wall panels are ready to be dispatched to site where they are placed on the ring beam that was constructed by the principal contractor.
The first panel is supported by a push and pull prop. For temporary stability, the wall is braced back to the roof structure. The steel brackets assist in holding the panels together and, therefore, eliminating the need for extensive propping to free up space.
Once all of the panels have been placed, unbonded cables are pushed through the polyvinyl sleeves in the panels. They are then grouted monolithically with the joints of the panels.
Hereafter, a grout is poured continuously in between the wall panels and horizontal cable sleeves. It is a high strength and flow grout with an extended pot life so that it does not segregate and set to early. These characteristics are achieved by manipulating the water-to-cement ratio of 0:37 with the use of admixtures. The water temperature is also reduced and controlled to act as a chiller in the mix. In addition, only cement, including an un-hydrated type that reacts with water to seal possible leaks, is used in the concrete mix.
The cables are stressed to 75% when the grout has cured to a strength of 80MPa. This is undertaken via four precast concrete buttress panels that have been spaced along the perimeter of the reservoir.
The wall is then pinned by casting a 200mm to 250mm-high reinforced kicker on the wall footing on both sides of each panel. Joints between the panels are grouted with a high-flow
and strength grout and post-tensioning renders them in compression to achieve water tightness.
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.
Dippenaar says that he was also very impressed with the ease at which the system was erected. “I do believe that Corestruc is at the cutting edge of this technology. It has absolute control over the entire precast-concrete value chain, spanning design and manufacture through to the extensive rigging capabilities required to erect these structures. Based on my experience working with this technology, I will not hesitate to recommend it again. I do believe that it is the future of reservoir construction,” he says.
