Inspire : Inspire Winter 2016
62 TECHNICAL ARTICLES BENEFITS The benefits are twofold in case studies around the globe: WMA is better for the environment and better for worker health. A 2010 New Zealand Transport Agency research report shows that the reduced temperature requirements mean reduced heating fuel (and therefore cost savings), as well as reduced greenhouse gases and other volatile emissions. Furthermore, warm mix foaming technology in the US has demonstrated success with reclaimed asphalt pavement content as high as 50% (ie. less going to landfill). Fulton Hogan’s national technical manager in Australia , Bevan Sullivan, says the days of hot asphalt are likely limited. “ When we have more greenhouse gas emissions targets, warm mix will become the main way of producing asphalt in the future,” he says. However, Sullivan sees the biggest immediate benefit in terms of worker safety. Placing asphalt at 120°C is significantly better for workers as bitumen will fume at 160°C – the normal temperature for placing hot mix asphalt. At 120°C , there is virtually no fuming, not to mention less heat and therefore increased safety. For instance, Nor way’s government encouraged the use of warm mix following a 2012 study that showed workers were exposed to 50% less fumes than when working with hot mix. “The other big benefit is the possibility to open earlier to traffic, because that window to cool below 80°C is a lot less than if you produce at 160°C ,” Sullivan says. THE WMA PROCESSES Traditional hot mix asphalt is manufactured by heating aggregates and binders to temperatures that ensure adequate coating and bonding. WMA is achieved mainly through three methods: foaming, additives and surfactants, all of which work to reduce the binder’s viscosity or surface tension. FOAMING BITUMEN A unit is installed at an asphalt plant to inject a small amount of atomised water into a hot bitumen line, prior to mixing with heated aggregates. The foaming reduces the viscosity of the binder and allows the aggregate to be coated at a lower temperature, without sacrifing workability. ADDITIVES A wax or organic additive decreases the viscosity when heated above the wax melting point, allowing for adequate coating. Sasobit technology uses low additions of wax to reduce the viscosity of the bitumen above its 100°C congealing point. SURFACTANTS Chemical processes improve coating by reducing surface tension between the binder and aggregate interface, allowing the mix workability to be improved, plus reducing the likelihood of stripping. ROADBLOCKS AHEAD The boxes that WMA doesn’t seem to tick are in in price and state government encouragement. Blacktown Council’s Acting Director of City Assets Joseph Buttita says his NSW council considered warm mix technologies and then stuck with the traditional hot mix. “The bottom line is we’ve got a proven product in hot mix asphalt and the price difference is negligible,” he explains. “ We know WMA has been used, but at the end of the day we have a tried and proven product . We’ve got no enticement – why move away from a product we know?” Industry contractors say the answer may be in education and attitude, as new technology often needs bedding-down time. At the moment , government guidelines make WMA paving optional. However, it’s looking more likely that safer and more environmentally- friendly technologies will be a government priority in future. Reading the signs, it look s as though the industry should be prepared to see Australia align with other countries in paving advances, and we may soon see requirements for WMA to be supplied instead of HMA where possible.
Inspire Summer 15-16