Optimal concrete performance FOR GAUTENG RESERVOIR

Construction has been completed on a 210M l reservoir in Gauteng, where good use was made of innovative additives to ensure the best concrete results. CHRYSO Southern Africa assisted the contractors to achieve good concrete consistency, reduced water content and minimal shrinkage. Its leading admixtures also helped reduce the permeability of the concrete in the floor, walls and columns.

There was no room for error in the construction of the extremely large cylindrical post-tensioned concrete reservoir. Completed at the end of 2022, it 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 800m3 floor slab layer of 200mm thickness. This required the 35MPa concrete to be pumped in 13 sequenced pours of 250-360m3 each. The reservoir’s internal diameter of 154m is roughly the length of one-and-a-half football fields. Inside are 272 round, reinforced 35MPa 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 post-tensioned 45MPa concrete.

Included in the construction was a permeable groundwater drainage system comprising three layers of 15MPa no-fines concrete. The first layer comprised 8 000m 3 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 stone 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. 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 helps 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. It 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.

All the 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 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, forming 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 it from corrosive groundwater and chemicals.

Construction of the Msikaba Bridge near Lusikisiki in the Eastern Cape – which started in earnest in late 2019 – is advancing steadily and the project should be complete in the last quarter of 2024.

The bridge, which will span the 198m-deep Msikaba River Gorge, forms part of the N2 Wild Coast project of the SA National Roads Agency Ltd (SANRAL) and is being built by the CME-JV, a partnership between Concor and MECSA, Grade 9CE South African construction companies.

The bridge’s four huge anchor blocks are now at a very advanced stage of construction. Over 50% of the concrete for the bridge’s two 127m-high pylons has been poured and the complex deck construction phase of the project is about to begin.

The project is also maintaining its impeccable safety record. It recorded 3 million lost time injury-free hours on 21 October 2022 – a major achievement, given the challenges that construction presents, including working at both extreme heights and at depths of as much as 20m in excavations.

The project director, Concor’s Laurence Savage, says the cable-stayed bridge is probably the most complex structure of its type ever to be built in SA and, indeed, in Africa.