French asphalt could find application in local roads and airports

14th November 2008 By: Sylvester Haskins

A French asphalt technology, namely high-modulus asphalt (HiMA), is a potentially viable alternative to conventional heavily-trafficked bitumen pavements, which often no longer offer road utilities long-term, and maintenance-free solutions, reports the Council for Scientific and Industrial Research (CSIR).

CSIR Built Environment engineering researcher Benoit Verhaeghe tells Engineering News that roads carrying heavy volumes of traffic, as well as airport pavements, require durable pavement layers, which offer a high-resistance to fatigue cracking and permanent deformation or rutting.

"We think HiMA has great potential for implementation in South Africa, and that it could be ideal as a long-life pavement for use in areas carrying heavy traffic, including airports," he proposes.

The significant increase in traffic loads and volumes has created a need for long-life, low-maintenance solutions for the construction and maintenance of roads, states the CSIR.

"HiMA is designed to combine good mechanical performance with impermeabality and durability, and is designed to yield high-elastic stiffness, high-permanent deformation resistance and high-fatigue resistance, while also offering good moisture-resistance and good workability," reports Verhaeghe.

He advocates the application of HiMA in South Africa, since there is a perception that current local flexible pavement design methodologies and bituminous pavement materials are no longer able to cope with the significant increases in traffic volumes.

"The continual increases in traffic volumes and axle loads, and the high road-user costs associated with lane closures during rehabilitation, necessitate cost-effective alternatives that provide stronger supports which can withstand the increased loading, while also extending the life of pavements," he explains.

A large number of South Africa's high traffic volumes roads have reached, or even exceeded, their design life, mentions Verhaeghe.

These roads now need to be rehabilitated, and upgraded in certain instances, in order to increase capacity, so as to alleviate traffic congestion.

Local Application
In October, the CSIR was awarded a two-year contract by the Southern African Bitumen Association (Sabita) to transfer and implement the HiMA technology in South Africa, reports the CSIR.

The project will include an assessment of the technical viability of HiMA, the adaptation of French testing protocols and HiMA specifications for use in South Africa, the validation of HiMA technology through accelerated pavement testing using the heavy vehicle simulator (HVS), as well as the drafting of technical guidelines and specifications for HiMA.

The key challenge in implementing the French asphalt technology in South Africa was the inability of South African refineries to produce a different grade of bituminous binder, reports the CSIR.

HiMA is a hot-mix asphalt consisting of hard bitumen-blended with a high-binder content with good quality, fullycrushed aggregate, explains Verhaeghe.

He says that the concept of HiMA was not considered viable for South Africa since it required the use of a harder grade of bituminous binder.

"It is only recently that two of our refineries, namely petroleum company Engen, and crude oil refinery Sapref, have indicated that they are now able to produce these harder, high-viscosity binders commercially, which effectively removed one of the major barriers to the introduction of HiMA into local markets," he adds.

Verhaeghe mentions that one of the fastest growing offsets or uses of HiMA has been urban roads, based on its ability to reduce overall layer thickness as a result of the substantially higher stiffness of the material, while still being able to maintain the same level of performance.

"It was initially intended to be used on the most heavily used routes in France, as well as on airport pavements and container terminals, but early successes quickly opened up new avenues for its application," he says.

Research Origin
In 2006, Sabita identified the need to implement flexible pavement solutions that would meet the needs of road owners.

To this end, the French innovation HiMA has been identified by Sabita, as a possible technology solution that could be implemented on South African road and pavement systems.

Verhaeghe says that Sabita and the CSIR have a long-standing agreement, which dates back to the 1980s.

"The CSIR assists Sabita with the production of industry guidelines for generic bituminous products and their applications. These guidelines are often the end result of research activities undertaken by the CSIR for Sabita," states Verhaeghe.

Verhaeghe undertook a study tour of Europe, together with delegates from Sabita, to assess the viability of implementing HiMA locally and of transferring the associated technology.

They visited the transport research laboratory in the UK in order to determine the methods of transferring and implementing HiMA technology from France to another country.

"In order to better understand the product and to assess the viability for the implementation of HiMA in South Africa, a European study tour was undertaken. Following the study tour and a series of seminars held throughout South Africa, it is now accepted that the implementation of HiMA in South Africa would be viable, which led to the initiation of a comprehensive technology transfer programme," reports Verhaeghe.

HiMA can be used in new construction as well as rehabilitation, and achieves direct savings in road construction material usage and construction costs.

Verhaeghe mentions that the better load-spreading ability and fatigue resistance of HiMA result in pavement structures that are less sensitive to loading.

"For the same thickness, HiMA asphalt can accommodate higher tensile strains at the bottom of the layer compared to conventional base courses," he explains.

He says that the use of HiMA has enabled asphalt layer thicknesses to be reduced by between 25% and 35%.

In South Africa, the application of HiMA, likely, will be directed towards the formulation of long-life roads with an expected structural design life of between 30 years and 40 years, rather than towards the reduction of layer thicknesses to conserve materials, envisages Verhaeghe.

"Opting for the long-life concept would result in reduced life-cycle costs, particularly if road-user costs are taken into consideration," he says.

Preliminary desktop studies have indicated that roads with HiMA base courses would extend the design life, when compared with that of typical South African designs, reports Verhaeghe.

He says the implementation of the technology has its constraints which need to be tackled, and these include: the ability of asphalt contractors to manufacture and pave the material; the need to translate French mix design parameters into South African ones; and the ability to market HiMA technology to major decisionmakers.