Green is increasingly the new gold standard in road building, and South African roads are set to get “greener” as our road builders and associated organisations recognise the value of more environmentally-friendly asphalt technologies. This is according to Peet Eksteen, product sales manager for asphalt equipment at Osborn Engineered Products. The Johannesburg-based firm, which is part of the Astec Industries group of companies, is the official supplier in southern Africa of the Double Barrel Green System, manufactured in the USA by sister company Astec Inc.

The secret to this system can be summed up in one word, says Eksteen. “Foam!”

While the benefits of warm mix asphalt – such as reduced energy consumption, lowered emissions and elimination of visible smoke – are well-known in the asphalt paving industry, he notes that many warm mix production technologies have drawbacks.

“Many technologies for warm mix production rely on additives, special asphalt cement, special procedures and/or special AC delivery systems to achieve low viscosities at low temperatures. The additives are expensive and add significant cost per ton of mix. Through the use of foam alone – produced by mixing a small amount of water into the AC to create microscopic bubbles – Astec’s Double Barrel Green System allows for the production of warm mix asphalt without the addition of costly commercial additives,” he states.

“The process uses water and there’s also no smoke and no smell because the light oils in liquid asphalt never reach boiling point. This system boasts the ability to run up to 50% of recycle mix with a standard grade of asphalt.” Energy efficiency is a further benefit. Says Eksteen: “Approximately 14% less fuel is used – corresponding to a 14% increase in production due to increased burner efficiency (greater temperature reduction results in bigger fuel savings).”

Explaining the system in more detail, Eksteen says that water is injected into the hot liquid AC and this turns to steam.

“It expands 1 800 times, thereby causing the liquid AC to foam. This foaming action allows the liquid AC to coat the aggregate more effectively at lower temperatures compared to conventional hot mix asphalt (HMA) mixes. Warm mix is typically produced at 28˚C to 42˚C lower than conventional hot mix asphalt.” [Refer to diagram 1]

Eksteen states that a road test undertaken in America by the National Center for Asphalt Technology (NCAT) has shown a significant increase in lifespan and rut resistance with mechanically foamed asphalt and warm mix produced with the Astec Double barrel Green plant.

Other benefits include improved workability because of lower viscosity, and longer pavement life because of less oxidation of the mix and more uniformity of compaction. Oxidation of asphalt (which occurs when the asphalt is combined with oxygen) results in asphalt that is aged, grey in colour and is less flexible and more prone to cracking. Oxidation also reduces the penetration levels of the asphalt.

Eksteen explains that with Astec’s Double Barrel System, water is injected into the liquid AC through a series of all stainless steel injectors located above corresponding foaming chambers. “AC flows through the foaming chambers where water and AC are thoroughly mixed to produce mechanically foamed AC.

“Water is delivered to the system using a positive displacement piston pump capable of accurately metering water into the system. Using feedback controls, pump speed is modulated to maintain the appropriate flow of water based upon the flow of AC.

 

 

 

 

 

 

 

PLC controls provide for smooth and consistent water flow as production rates increase or decrease.

“A skid-mounted, corrosion-free water reservoir is provided with an automatic filling valve,” he adds. “If supply water is lost, a low-water alarm alerts plant personnel so the problem may be resolved quickly without wasting mix.”

Eksteen says that Astec can retrofit warm mix systems to continuous mix and batch plants from any manufacturer, bringing the benefits of the Astec system to all asphalt pavement plant configurations.

There are currently more than 500 Astec Double Barrel Green Systems in operation around the world. Recently a South African company became the first to utilise foam technology for the manufacture of warm mix asphalt when an Astec Double Barrel Green Plant was commissioned at their operation.

As the use of warm mix asphalt technology becomes more widespread in South Africa’s road construction industry, Eksteen anticipates increased demand for Astec’s Double Barrel Green System.

“The future of road building lies in warm mix asphalt, and, as environmental considerations are increasingly at the top of the agenda for companies and governments, green technology like this – that delivers significant, measurable results – is the only way to go,” he maintains.