best concrete results Admixtures ensure

Construction was completed on a 210-million litre reservoir in Gauteng where good use was made of innovative additives to ensure the best concrete results. As a result, the contractors achieved good concrete consistency, reduced water content and minimal shrinkage. The admixtures used also helped reduce the permeability of the concrete in the floor, walls and columns.

There was no room for error in the construction of an extremely large cylindrical post-tensioned concrete reservoir in Gauteng. Completed at the end of 2022, the reservoir will hold over 210-million litres of potable water and is a key part of the Gauteng Department of Water & Sanitation service delivery improvements. Technology from a specialised concrete admixture supplier helped to ensure optimal concrete performance.

Impressive scale

The scale of the concrete works on this reservoir project was impressive, indicated by the 1 800 m3 floor slab layer of 200mm thickness. This required the 35 MPa concrete to be pumped in 13 sequenced pours of between 250m3 and 360m3 each. The reservoir’s internal diameter of 154m is roughly the length of one and a half football fields. Inside are 272 round reinforced 35 MPa concrete columns, 600mm in diameter, to support the roof slab. The outer concrete walls are almost 12m high, reaching about three storeys. They taper from 1.1m at the base to 300mm at the top, and comprise posttensioned 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 000m3 of no-fines concrete with a 50mm thick layer of 9.5mm stone mix. This was followed by a 245mm layer of 19mm stone mix, and then a 125mm layer of 26mm size stone mix.

Adding to the mix

When a good consistency could not initially be achieved with these challenging mixes, CHRYSO® Easydrain was introduced. By adding 140g of this additive to each cubic metre of concrete, the issue was solved. The 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 has made use of CHRYSO® Plast Omega 178, a high-performance water reducing plasticiser. This reduces the water content of each mix to 175ℓ per cubic metre while ensuring workability and strength. This admixture also increases the slump – or flow – without affecting the water content of the mix.

Dealing with shrinkage and permeability

All concrete in the reservoir’s floor slab, wall footing and roof slab was augmented with the supplier’s shrinkage inhibitor, Serenis – to reduce shrinkage of the concrete. In addition, 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.