LOCAL PRECAST INNOVATION HELPS SA MUNICIPALITIES ACCELERATE WATER DELIVERY

There is increasing interest in Corestruc’s reservoir system as more of the country’s municipalities and water authorities prioritise South Africa’s water infrastructure backlog.

Corestruc is preparing to manufacture another two 10-Ml precast concrete reservoirs, and is hoping to be appointed to work on a 25-Ml reservoir project by a leading South African civil engineering contractor. The principal contractor will undertake the earthworks, construct the reservoir floor, pipeline and inlet works. Corestruc, as the specialist sub-contractor, will be tasked with the manufacture and installation of the wall and roof of the structure. 

The company recently constructed two 10-Ml reservoirs in only six weeks using its sophisticated precast system.

Willie de Jager, MD of Corestruc, says that it would have taken four to six months to only construct the reservoir walls using conventional in-situ techniques.

“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 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 methods,” says De Jager.

The roof and wall systems are manufactured at the company’s state-of-the-art factory while the principal contractor completes the earthworks and the reservoir floor.

Corestruc first installs the centre portion of the roof system, comprising precast columns, beams and hollow-core slabs.

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.

Once the ring beam has been completed by the main contractor, Corestruc started dispatching the wall panels to the construction site on a just-in-time basis.

They were 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 elements are then placed against the other to complete the reservoir wall.

Corestruc’s installation team uses a Total Station technology to precisely install each panel, maintaining tolerances of about 5 mm.

The walls of the two 10-Ml 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.

Corestruc is able to manufacture up to 10 reservoir wall panels of various widths and lengths at a time at its factories using specialised forms designed and developed by the company’s own engineering department.

The various concrete elements that make up the entire wall and roof system are manufactured in a controlled environment, far removed from the many variables encountered on a 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.

The extremely remote locations of the two construction sites obviated servicing by readymix producers and on-site batching of concrete for the walls and roof would have required extensive quality controls and careful coordination of the various materials.

De Jager 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 reservoir wall systems in the country.

Tian de Jager, technical director of Corestruc, researched the leading international precast reservoir wall technologies available. This was refined and modified for the African environment.

Notably, Corestruc 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 6,6 km of post-tensioning ducts and cables were installed by hand between the joints of the wall panels in preparation for the grouting.

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.

The grout must 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 7°C.

It took 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 Corestruc even brought its own water to site to ensure that the medium achieved the desired reaction.

While the cost of the system is comparable to in-situ techniques on smaller structures, it provides a more affordable means of constructing larger reservoirs.

De Jager says: “The need to innovate to improve infrastructure delivery is also reiterated in the latest Municipal Infrastructure Grant guidelines. 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. Corestruc is contractually responsible for the performance of all of its systems, allaying any concerns regarding the deployment of new technology on municipal projects.”

He adds that public-sector client bodies are also increasingly realising that precast concrete technologies complement their labour-based construction policies.

Precast 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.

Piping and construction materials were also procured from local small black-owned businesses in the vicinity. 

Both sites were completely free of safety incidents despite teams working at heights.

More information at www.corestruc.co.za