The CSIR is formulating new specifications for bitumen. Bitumen is a tar-like substance that is mixed with aggregate particles to pave roads.

CSIR senior researcher and manager of the bituminous binders laboratory, Georges Mturi, says the state of South Africa’s road – and those elsewhere in Africa – has prompted a rethink on bitumen specifications.

“Older empirical methods have not been working well, hence we have seen unprecedented road failures,” says Mturi.

The CSIR embarked on research initiatives about six years ago, looking into changing the current specifications from empirical-based methods to more advanced performance-based methods.

“Choosing bitumen used to be simple,” he says. “For instance, if engineers were constructing a road in a cold region, they would simply use relatively softer bitumen that would be hard enough at such conditions. Alternatively, relatively stiffer bitumen would be used for a hot region where it would retain its hardness while carrying traffic loads.”

A number of factors have contributed to road failures. One major contributing factor, notes Mturi, is the increase in traffic levels and traffic loads.

A paved road in Tanzania.

Mturi says crude oil sources used in the region over the last 20 years varied greatly. Various factors, such as the Iran embargo and bitumen shortages from local refineries contributed to this problem. Bitumen was imported from all over the world, but it was not always of an appropriate performance standard. Current specification tests have been inadequate in differentiating between different types of bitumen.

Moreover, there has been an increased use of bitumen that has been modified with various additives. This has led to an influx of suppliers proposing polymers as additives from different industries, such as crumb rubber from recycled tyres and ethylene-vinyl acetate from shoe soles. These additives claim to improve the performance of bituminous binders, but it is not possible to evaluate these additives using empirical tests. This has prompted CSIR researchers to formulate new test properties that measure the performance of the bitumen, taking climatic conditions and traffic loading into account.

“To consider the impact of the changing climate on bitumen performance, we needed to calculate road surface temperatures throughout the country based on air temperature data from over 120 weather stations over a 20-year period,” he says.

As expected, this exercise made a difference; the bituminous binders used in the Free State are not necessarily the same binders that should be used along the coast, because of the significant differences in road surface temperatures. In a particular temperature zone, engineers can then link the performance of bitumen to the expected traffic loads.

“We identified this gap and we started pushing towards advanced tests for selecting bituminous materials for asphalt roads. The new specifications are based on these advanced tests in South Africa. We are also in the process of developing performance-based specifications for other parts of the continent,” says Mturi.