25 January 2023

Supplied by PR

There was no room for error in the construction of an extremely large cylindrical post-tensioned concrete reservoir in Gauteng– with CHRYSO Southern Africa ensuring optimal concrete performance.

Completed at the end of 2022, the reservoir will hold over 210 Mℓ of potable water and is a key part of the Gauteng Department of Water & Sanitation’s service delivery improvements.

The scale of the concrete works on this reservoir project has been impressive, indicated by the 1,800 m3 floor slab layer of 200 mm thickness. This required the 35 MPa concrete to be pumped in 13 sequenced pours of between 250 m3 and 360 m3 each. The reservoir’s internal diameter of 154 m is roughly the length of one and a half football fields. Inside are 272 round reinforced 35 MPa concrete columns, 600 mm in diameter, to support the roof slab. The outer concrete walls are almost 12 m high, reaching about three storeys. They taper from 1,1 m at the base to 300 mm at the top, and comprise post-tensioned 45 MPa concrete.

Included in the construction was a permeable groundwater drainage system comprising three layers of 15 MPa no-fines concrete. The first layer comprised 8,000 m3 of no-fines concrete with a 50 mm thick layer of 9,5 mm stone mix. This was followed by a 245 mm layer of 19 mm stone mix, and then a 125 mm layer of 26 mm size stone mix.

When a good consistency could not initially be achieved with these challenging mixes, CHRYSO® Easydrain was introduced. By adding 140 g of this additive to each cubic metre of concrete, the issue was solved. 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.

In all its concrete mix designs, the project has made use of CHRYSO® Plast Omega 178, a high-performance water reducing plasticiser. This reduces the water content of each mix to 175 ℓ per cubic metre while ensuring workability and strength. This admixture also increases the slump – or flow – without affecting the water content of the mix.

All concrete in the reservoir’s floor slab, wall footing and roof slab was augmented with CHRYSO® Serenis shrinkage inhibitor – to reduce shrinkage of the concrete. CHRYSO® CWA 10 crystalline integral waterproofing agent is also used to reduce the permeability of the concrete in the floor, walls and columns. When moisture is present, this agent creates a reaction – 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.

The largest cylindrical post-tensioned concrete reservoir in the country.
5 mm plaster layer above 145 mm no-fines concrete groundwater drainage layer.
The post-tensioned outer concrete walls are almost three storeys high.

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