Additives ensure quality concrete for mega reservoir

An extremely large 210 Mℓ cylindrical post-tensioned concrete reservoir was recently completed in Gauteng. CHRYSO Southern Africa highlights the use of its admixtures in the concrete used in the construction of the reservoir, which it says helped achieve good concrete consistency, reduced water content and shrinkage, and reduced permeability of the concrete in the floor, walls and columns.

The reservoir project is said to be a key part of the Gauteng Department of Water and Sanitation's service delivery improvements. The 1 800 m3 floor slab layer of 200 mm thickness required 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 notes that its Easydrain was introduced, saying that the issue was resolved by adding 140 g of the additive to each cubic metre of concrete. The company explains that its Easydrain additive 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 made use of CHRYSO Plast Omega 178, described as a high-performance water reducing plasticiser designed to reduce the water content of each mix to 175 ℓ/m3 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, designed to reduce shrinkage of the concrete. CHRYSO CWA 10 crystalline integral waterproofing agent was 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.