Work is proceeding steadily on one of Africa’s most significant engineering projects, with CHRYSO concrete admixtures ensuring that main contractor Razel-Bec gets optimal results from its concrete mixes. A temporary coffer dam has been constructed and is being dewatered, says Martie Coulson, Director at Mart Solutions Ltd – the Zambian distributor for CHRYSO® Southern Africa.
Contractors are getting closer to the key objective of the Kariba Dam Rehabilitation Project – to reshape the plunge pool below the dam wall and arrest any further erosion.
Concrete is a central element of this world class engineering project, and CHRYSO® Southern Africa’s Zambian distributor, Mart Solutions Ltd has been one of the key suppliers of construction chemicals to main contractor Razel-Bec since the very beginning of the project. This collaborative effort over the past several years has ensured the project’s steady progress. With the aim of stabilising the plunge pool at the foot of the 138 m high dam wall, the project is using vast quantities of concrete in a range of applications.
The total volume of concrete used – which includes the coffer dam and underwater works as well as excavation – is almost 26 500 m3. At the coal face is Mart Solutions Director, Martie Coulson.
“Among the first challenges we faced was in the construction of piers for the temporary coffer dam, where concrete had to be poured underwater to create a foundation,” says Coulson. “Difficulties in finding bedrock for piers 7, 8 and 9 – as well as for the dam’s right bank – meant that 22,000 m3 of concrete had to be placed underwater.”
This arduous process – which took six months and was completed in September last year – involved pumping concrete to 25 m deep, with divers handling the 125 mm diameter pipe to place the concrete. Adding to the complexity was the movement of water from the nearby Zimbabwe’s hydro-electric turbine outlet, which exacerbated the ‘wash’ off the concrete into the surrounding water.
“This raised the risk of the cement being washed out of the mix, leaving the aggregate behind as well as further reducing visibility for the divers,” she explains. “The CHRYSO® Aquabeton product played a critical role as an anti-washout admixture that increases the cohesion of the concrete mix.”
This allowed the placement of a concrete base for the three piers, measuring some 15 m wide and 20 m deep. Another important aspect of the project is repairing a fault in the rock at the foot of the Kariba dam wall. Stitching of the fault will include the use of 40 mm anchors in the rock with CHRYSO NS Grout added to a concrete mix. In addition, 20 mm, 25 mm and 32 mm anchors will be used and will be secured with a.b.e.® dura.grout supplied by Mart Solutions Ltd.
“Some 7 071 m3 of specialised concrete will be used for the large 90 m high, 45 m wide and 2,5 m thick wall which will be built in front of the fault as a protective shield,” she says. “This will be fixed onto the 12 m long, 40 mmdiameter anchors in the wall, which will in turn be secured 10 m into the rock.”
A combination of CHRYSO® Omega 162 and CHRYSO® Fluid Optima 206 will be used in conjunction with densified silica fume and fly ash in the concrete mix for the wall – to assist in preventing the temperature rise in the concrete during placement and to avoid cracking. This is important due to the high ambient temperatures of up to 40 degrees Celsius.
“We have been running laboratory trials for three years to ensure that the concrete mix design will meet the standards specified by the engineers for the wall to ensure that the concrete will remain below 55 degrees Celsius during placement,” she explains.
As part of the preparation of the concrete, therefore, a chiller plant will be cooling the water and aggregate before it is added to the mix. In another aspect of the project, Mart Solutions is providing the anchors to secure the safety nets which are being installed against the risk of rockfalls in the excavation, as well as a.b.e.® dura.grout for securing the anchors. This non-shrink cementitious grout quickly develops placeable consistency and remains cohesive. CHRYSO Jet 30 is also being supplied for shotcreting, mixing with concrete as it is sprayed to ensure adhesion to the rock surface.
“As the coffer dam is dewatered, the contractors are able to start micro-blasting to clear rock from the plunge pool,” she says. “The walls must then be secured to prevent any rocks from falling.”
Mart Solutions has already provided the CHRYSO® Omega 162 superplasticer for building the coffer dam piers. Ranging from 250 tonnes to 700 tonnes in weight, these piers measure 5,5 metres tall.
“The new generation CHRYSO® Omega 162 high range water reducing plasticiser played an important roleto reduce water demand, thereby increasing the concrete’s durability,” she says. “It also improved the concrete’s cohesion and lowered its viscosity, for better homogeneity and off-shutter finish.”
CHRYSO® Omega 162 allows concrete to exhibit extended workability characteristics, while CHRYSO® Fuge B – a pore-blocking permeability reducer for mass concrete – was used in the coffer dampiers to prevent the penetration of water under pressure.
