Construction World February 2023

DAMS & RESERVOIRS

The Khutsong reservoir is Africa’s first round prestressed raft floor to deal with dolomitic ground conditions.

The presence of dolomite in the area’s geology meant that sinkholes were common and the design had to accommodate the possibility of a five metre wide sinkhole occurring under the Khutsong reservoir.

meant that sinkholes were common and the design had to accommodate the possibility of a five metre wide sinkhole occurring under the reservoir. 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. Van Vuuren explains that to achieve this innovative result, the concrete mix design was a critical factor and that the timing of the stressing and the profile of the cables were also vital, to achieve the necessary performance from the 450 mm thick raft floor slab. “A careful balance had to be found between acceptable shrinkage, permeability, workability and early strength gain, and CHRYSO’s shrinkage compensating admixture, CHRYSO® Serenis, was used to achieve a suitable water cement ratio and the required permeability,” he says. A CHRYSO water reducing plasticiser ® Plast ZA 1559 also proved an effective plasticising admixture to ensure workability after 80 minutes – while also providing early strength gain.

– creating long narrow crystals that fill and plug the pores, capillaries and hairline cracks of the concrete mass. These crystals prevent water penetration into the concrete, protecting the concrete and reinforcement against corrosive groundwater and chemicals,” Van Vuuren explains. The reservoir design also included a permeable groundwater drainage system comprising three layers of 15 MPa no-fines concrete. To achieve the right consistency in these challenging mixes, CHRYSO® Easydrain was introduced at 140 g/ m 3 of concrete. CHRYSO® Easydrain mechanically and hydraulically binds the aggregates with a uniform cement paste coating. Once dried, the bonded aggregates will not loosen. The additive also fluidises the concrete, which assists with a homogeneous mix and improves curing to reduce the chances of cracking. Khutsong reservoir “While the Khutsong reservoir was much smaller – at just 30 Mℓ capacity – its concrete demands were equally complex,” van Vuuren says. The presence of dolomite in the area’s geology

30 CONSTRUCTION WORLD FEBRUARY 2023