CHRYSO Southern Africa has earned industry recognition for applying its admixtures in a demanding reservoir contract, winning the top award in the specialist suppliers category at the Construction World Best Projects Awards.
Dolomitic geological conditions created demanding requirements for the 30 million litre Khutsong reservoir in Gauteng, and Chryso Southern Africa was there to contribute to the specialised concrete mix.
With the risk of sinkholes in the area, the new reservoir was designed so that it could span a sinkhole up to five metres in diameter, while full to overflowing with water.
This led to Africa’s first round, prestressed reservoir raft floor being constructed. This design option involves applying pre-compression to the raft floor, providing additional resistance to crack-inducing tensile stresses.
The concrete mix design was a critical factor – along with the timing of the stressing and the profile of the cables – in achieving the performance of the 450 mm thick raft floor slab. Several trial mix designs were tested to find the balance between acceptable shrinkage, permeability, workability and early strength gain.
The optimal mix design was found by balancing a suitable water-cement ratio for permeability with Chryso’s shrinkage compensating admixture, Chryso Serenis. Chryso Plast ZA 1559 also proved an effective plasticising admixture and ensured workability after 80 minutes – while also providing adequate early strength gain.
An Afrisam CEM II/A-M 42.5R type cement was selected for both product performance and reliability of delivery. To test the design’s performance, a sample beam measuring 59.25 m long, 0.21 m wide and 0.45 m deep was designed and cast on-site. A single mono strand cable was used to simulate the influence of one cable on the raft floor.
The pouring of the concrete had to be completed within a 24-hour window to ensure the homogeneity of the concrete under early stressing. The site’s location demanded close collaboration between the contractor and readymix partner. The final plan made use of three batch plants, 32 mixer truckers, standby plant and personnel, back-up power and water plants.
