CoreSlab’s unique precast-concrete system is assisting South African municipalities significantly accelerate the delivery of drinking water to rapidly-expanding rural and urban areas.
Two 10 Ml reservoirs were constructed in only six weeks using the company’s sophisticated precast-concrete system.
It would have taken four to six months to only construct the reservoir walls using conventional in-situ techniques, and this is without the risk of having to redo the work, considering the technically-complex nature of these construction projects.
CoreSlab is now preparing to manufacture another two 10 Ml precast-concrete reservoirs, and the company is hoping to be appointed to work on a 25 Ml reservoir project by a leading South African civil engineering contractor shortly. The principal contractor will undertake the earthworks, as well as construct the reservoir floor, pipeline and inlet works. CoreSlab, as the specialist sub-contractor, will be tasked with the swift manufacture and installation of the wall and roof of the structure.
Jaco de Bruin, managing director of CoreSlab, notes an increased interest in the company’s reservoir system as more of the country’s municipalities and water authorities start prioritising South Africa’ growing water infrastructure backlog.
“Reservoirs are notoriously complicated and time-consuming structures to build. The construction of the wall demands absolute precision to ensure water-tightness. This slow and meticulous process is followed by the construction of the roof, which entails erecting and installing tons of scaffolding and formwork inside the structure. On most of these projects, work can only take place at one or two faces at any given point in time. Our modular system enables the construction of the floor, walls and roof simultaneously to deliver the infrastructure in a fraction of the time it would using conventional in-situ methods,” De Bruin says.
The roof and wall systems are manufactured at the company’s state-of-the-art factory while the principle contractor completes the earthworks and the reservoir floor.
CoreSlab first installs the centre portion of the roof system, comprising precast-concrete columns, beams and hollow-core slabs.
The process starts with the installation of the columns onto the in-situ bases that have been prepared by the principal contractor while maintaining constant interaction with CoreSlab to ensure high levels of accuracies.
It took the company only four working days to build the centre portion of the roof structures for the two 10 Ml reservoirs in Mpumalanga.
CoreSlab then starts dispatching the wall panels to the construction site on a just-in-time basis once the ring beam has been completed by the main contractor.
They are lifted directly from the truck trailers and placed on top of the ring beam using a mobile crane. The first panel is supported by props that are removed once it has set and the remaining precast-concrete elements are then placed against the other to complete the reservoir wall.
CoreSlab’s installation team uses a Total Station technology to precisely install each panel, maintaining tolerances of about 5 mm.
The walls of the two 10Ml reservoirs comprise as many as 60 panels, each weighing 8t, 9,8 m in length and 1,9 m in width, as well as the four 11,7 ton buttresses that reinforce the structure.
CoreSlab is able to manufacture up to 10 reservoir wall panels of various widths and lengths at a time at its factory using specialised forms that were designed and developed by the company’s own engineering department.
Many man hours were invested into refining the designs of both the wall and buttress panels to ensure the high levels of precision required for their installation.
“The holes at the bottom of the wall panels have to align with the bolts grouted into the ring beam, as well as the steel plates at the top with the voids in adjacent slabs. Just as importantly, the holes that traverse the full width of the panels through which the post-tensioning strands are threaded also need to line up. We also underwent an extensive learning curve during the design of the forms used to manufacture the buttresses. They are extremely complex elements that contain numerous cast-in components that were all manufactured by our engineering department,” De Bruin says.
Notably, the various concrete elements that make up the entire wall and roof system are manufactured in a controlled environment that is far removed from the many variables encountered on conventional construction site.
Every 70 to 80 MPa wall panel has its own technical drawing and documentation, which includes detailed specifications and a thorough account of the pre- and post-inspections.
The extensive quality controls in place at the factory were a major selling point for the civil engineers who promoted the system to municipal officials for use on the two 10 Ml reservoirs.
One of the challenges was the extremely remote locations of the two construction sites. They are, therefore, not serviced by ready-mix producers, and the on-site batching of concrete for the walls and roof of the structure would have required extensive quality controls and careful coordination of the various materials to avoid delays.
De Bruin says that meticulous attention was also paid to the design and development of the grouting and post-tensioning process to overcome the many limitations of other precast-concrete reservoir wall systems in the country.
Perfecting the design of the system took just more five years from conception.
Tian de Jager, technical director of CoreSlab, undertook extensive research into numerous leading international precast-concrete reservoir wall technologies available on the market. This extensive learning and best practice was refined and modified for the unique African environment.
Notably, CoreSlab uses vertical and horizontal tensioning to resist applied forces. This is opposed to conventional construction methods where reinforcing and post-tensioning is used to control applied forces.
About 6,6km of post-tensioning ducts and cables were installed by hand between the joints of the wall panels in preparation for the grouting. This is in addition to the numerous three-dimensional printed components to secure the rubber cast that acts as the temporary shutter.
The grout has been designed to reach a compressive strength of 100 MPa within four days and to further react when the medium comes into contact with water when the reservoir is being filled.
Moreover, the grout has to be extremely flowable so that it can be pumped through all the post-tensioning ducts from a single position using two pumps, and its working time is extended by cooling it down to seven degrees Celsius.
It took up to 40 hours in a continuous process to pump the grout around the entire circumference of the two 10 Ml reservoirs.
The grout underwent extensive testing ahead of its application and CoreSlab even brought its own water from Polokwane to ensure that the medium achieved the desired reaction.
While the cost of the system is comparative to in-situ techniques on smaller structures, it provides a more affordable means of constructing larger reservoirs.
De Bruin says that this is where the real value of the system will be realised, considering the growing backlog in water infrastructure and the pressure municipalities are under to better manage their dwindling budgets.
“The need to innovate to improve infrastructure delivery is also reiterated in an inclusion in the latest Municipal Infrastructure Grant guidelines. Notably, it highlights that the country’s municipalities, especially those in the B and C categories, need to consider innovation in the full spectrum of infrastructure technologies and associated operations and maintenance solutions. CoreSlab is contractually responsible for the performance of all of its systems, and this has allayed any concerns around the deployment of new technology on these municipal projects, especially in light of the poor performance of many precast-concrete structures in the past.”
He adds that public-sector client bodies are also increasingly realising that precast concrete technologies complement their labour-based construction policies.
Precast-concrete factories provide many long-term and secure jobs, as opposed to only temporary employment prospects during the construction phases.
Notably, the main contractors also met all of the municipality’s stringent socio-economic targets during the construction of the two 10 Ml reservoirs.
This focus included ensuring that ample opportunity for employment, as well as skills development and training was created for members of local surrounding communities during the earthworks, in addition to the construction of the floors, inlet and outlet chambers, as well as pipelines.
Piping and construction materials were also procured from local small black-owned businesses in the vicinity.
Notably, there was also not a single safety incident on both sites, considering that only trained CoreSlab teams worked at heights during the installation of the walls and roof of the two reservoir structures.
De Bruin says that the two projects provided an important opportunity to refine and prove the system, and thanks the client body, consulting engineers and contractors for the opportunity to participate in project that has again demonstrated the many benefits of precast-concrete technology.