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