BENEFICIATION
composite solutions where concrete can be used in the slow, heavily trafficked lanes, with asphalt in the fast lanes; or concrete can be used in areas of steep grades, with asphalt in the flatter sections; or composited vertically using concrete as the main structural layer with an asphalt surfacing, which can be replaced at regular intervals without large user disruptions.
A vertically composite method involves putting down a concrete base, which thereafter does not need to be maintained for as long as 50 years. Asphalt is laid on top for a quieter drive, and which can be laid almost overnight as and when needed.“ The downtime of road construction is a cost which dwarfs all other costs, and most lifecycle costing models fail to incorporate road user delay costs,” explains Perrie.
One example of such composite pavements is on the N3, both north and south of Pietermaritzburg, particularly on Town Hill, where the slow( truck) lane was constructed with concrete and the fast lanes with asphalt.
Concrete has also been successfully used to overlay existing asphalt pavements on a number of freeways in South Africa,“ and this works fine”, says Perrie.
In the eyes of government, the skills shortage compounds the comparative unattractiveness of concrete road building— and even concrete road builders themselves. Perrie explains that even when a concrete pavement tender is issued, several contractors often submit an alternative asphalt bid because they don’ t have, or don’ t want to invest in, the skills necessary to construct the concrete option.“ They hedge their bets both ways,” says Perrie,“ because it is expensive to buy the skills and to buy the equipment for concrete.” If there was a sufficient pipeline of concrete roads, these skills and equipment would be retained, but with one concrete road only every few years or so, the skills wander off to where there is work.
Companies therefore have to walk a thin tightrope.“ Who builds roads? Government. There have never been enough concrete road projects to develop sufficient skills and resources. However, in the US, where there is real competition between asphalt and concrete, it has been shown to bring down the lifecycle cost of roads.”
The South African Pavement Design Programme, an initiative of Sanral, may change all this, says Perrie.“ This requires engineers to look at all options in the preliminary design that may influence decisions regarding use of concrete in the future,” he says.“ For instance, Sanral is looking at upgrading the Durban – Pietermaritzburg N4 from two to four lanes each way, and concrete is under consideration for certain sections, such as steep gradients.” ■
Eamonn Ryan Sarma
Johan van Wyk, director of the Southern African Ready-mix Association.
Bryan Perrie, MD of The Concrete Institute.
28 _ QUARRY SA | SEPTEMBER / OCTOBER 2018