Placing 1,200m3 of concrete over nine days, through a pipeline that reaches a 133m drop – with a portion of the concrete placed in water – is a feat few would attempt. However, Lafarge Readymix successfully completed this task thanks to a skilled, committed team and a topquality concrete mix containing Chryso admixtures.

A 300mm diameter hole was detected in an underground wall of a mine in Mpumalanga. Behind this wall was a massive reservoir, and water was leaking out of the hole at a rapid rate – creating the risk of flooding the mine.

Roedolf van Wyk, senior engineer at Mantella Trading 310, helped to develop a solution to stop the water from pouring into the mine. “We decided to place twelve, 4m thick and up to 4.5m high and 7m long concrete walls (concrete plugs) at strategic locations in the mine. A total of 1,200m³ of concrete had to be poured over nine days and the concrete had to reach a rapid early strength of 15MPa on the first day, with a final strength of 40MPa after 28 days.

It was established that Lafarge’s Evander Plant in Mpumalanga had the capacity to provide the required volume of concrete and the ability to meet all the technical specifications. Lafarge’s readymix product, Agilia, was used for the project. This self-compacting concrete allows for placing without difficulty,” says Van Wyk.

A Chryso set accelerator helped the mix to reach the required 15MPa strength on the first day, ensuring that the concrete would gain strength within required time. Also incorporated into the mix was a Chryso anti-washout agent specifically designed for concrete to be placed under water – thus preventing the cement and finer particles from leaving the concrete mix.

“The concrete pumps and pipes had to operate while standing in large volumes of water and mud. Concrete was poured into a funnel at ground level and was dropped into receiving kettles 133m underground through a 127mm steel-encased borehole. Once underground, the concrete was placed using a Putzmeister pump. Not all the concrete walls could be placed in a single pour due to the timber shuttering and brattice wall waterproofing system. Thus, the concrete pipes had to be installed or moved up to four times per plug.

“Both the mine and Mantella Trading 310 were very impressed with the quality and volume of work Lafarge produced within the short time frame. The client did not have to wait for a single Lafarge delivery. The concrete mix was a great success and was workable and pumpable for as long as six hours after batching,” explains Van Wyk.

“The concrete had to be a cohesive, sticky self-compacting mix that would not block any of the pipes. This was achieved by using two Chryso superplasticers. The superplasticers were ideal for selfcompacting concrete, and also acted as a set retarder that helped the concrete to retain its workability for an extended period. They made a tricky job possible,” concludes Van Wyk.

Article reproduced with kind permission of the South African Concrete Institute.

Taken from the December 2012 NZRMCA Newsletter