Much Asphalt, a division of construction company Murray & Roberts, has added additional supply capacity of 300 t/h and 250 t/h to its Benoni and Pamona facilities respectively, to meet asphalt production demand for the Gauteng Freeway Improvement project.

Both are twin drum configurations capable of handling a wide range of different mixes, including efficient use of recycled asphalt (RA), an important requirement in the drive for sustainability of natural resources.

The Benoni facility would now incorporate a plant built by equipment manufacturer Astec in the US. This installation would allow Much Asphalt to employ foam technology for the manufacture of warm-mix asphalt (WMA) for the first time in South Africa.

The company stated that not only would this enable it to substantially decrease its carbon footprint but WMA would provide several additional benefits in both asphalt production and paving.

WMA allows the asphalt manufacturer to reduce the temperature at which the material is mixed and placed on the road through the use of either chemical additives or foam technology.

“We will be the first South African company to produce WMA using foam technology on a big scale. Foam technology as a cold process has been around for some time, but has been used to manufacture bitumen stabilised base course material rather than asphalt,” said Much Asphalt technical director Herman Marais.

All previous technologies for warm mix production relied on chemical additives, special bitumen, special procedures or special bitumen delivery systems to reduce the viscosity of the binder at lower mixing and placing temperatures. The additives also added significant cost for every ton of mix.

The Double Barrel Green System built by Astec would eliminate the need for additives through the use of small quantities of water that come into contact with the hot bitumen, generating steam and in turn, causing foaming of the bitumen.

The volume of the bitumen was drastically increased and the viscosity of the binder was reduced, improving coating on the rock and enhancing its distribution in the mix. The lower viscosity of the binder allowed for lower than normal compaction temperatures.

Temperature reductions of 20 °C to 30 °C were possible, with the obvious benefits of cutting fuel consumption and decreasing the production of greenhouse gases. This would reduce energy costs and the carbon footprint at Much Asphalt’s biggest and busiest plant, Marais pointed out.

In theory, an energy saving of 11% could be achieved with a 10 °C reduction in heat. However, energy savings of between 30% and 35% have been reported owing to the reduced heat losses during the mixing operation. Processes where the aggregate was not heated above boiling point of water had savings as high as between 34% and 47%.

“Research has shown that lowering the production temperature can also drastically reduce emissions, fumes and odours at the plant, improving conditions for workers and paving the way for better relationships with neighbouring industries. Smoke and smells are eliminated because the light oils in bitumen never reach boiling point,” the company said in a statement.

Maintaining a low viscosity at lower temperatures allowed the mix to flow freely through storage, transfer and placement equipment and made it easier to work by hand.

Higher recycled asphalt content could be added to a conventional asphalt mix without excessive emissions or poor workability, contributing to the environmental benefits of WMA. Less aging of the virgin binder in warm-mix production could also allow for higher percentages of RA.

Operational benefits included an increase in plant production with no increase in the cost of the mix. This could be attributed largely to reduced temperature differentials.

“The greater the differential between the asphalt and ambient temperatures, the faster it cools off. With WMA this differential is low and therefore it cools more slowly. This allows a longer period for compaction to take place, improves density, and the asphalt can be stored for longer, hauled longer distances and used in colder temperatures.”