Saturday, March 22, 2008

Warm Asphalt Mix

Recognizing the impact that construction has on the environment, the construction industry has responded by developing a new approach to reduce their environmental footprint. The traditional approach has been to produce asphalt mix at high temperatures resulting in heavy consumption of fuels and accompanying production of emissions. The new approach addresses this head on by producing the mix in a manner that uses less fuel, and produces fewer emissions.

Currently, asphalt mix is typically Hot Mix Asphalt (HMA). HMA is made by heating asphalt cement (AC) from a semi-solid state until it flows thinly enough to uniformly coat a mixture of aggregates. This process requires high temperatures in excess of 150 C to properly mix the materials and to ensure the mix is still workable when it is being placed and compacted on the road. This mixing process requires large quantities of energy and creates emissions. Warm Asphalt Mix (WMA) is an alternative method with benefits over HMA. WMA can be used with normal HMA materials and normal HMA mix designs using standard paving and compaction construction equipment.

WMA is produced at temperatures about 50 C lower than conventional HMA. Using less energy at lower temperatures during production results in up to a 50% drop in emissions. WMA claims to have the following benefits: its greater ability to be transported over long hauls; a quick opening to traffic; it can be placed in thinner lifts; and potentially could improve the performance of transverse and longitudinal joints. Performance of transverse and longitudinal joints depends on the ability to obtain good compaction along the supported edges. Mix freshly placed against the adjacent previously placed WMA re-heats the mix, allowing construction rollers to further compact the joint and push the mix into the existing joint. This may lead to better joint performance. WMA could also allow construction under cooler weather conditions extending the paving season.

Other benefits of using Warm Asphalt include reduced exposure to fumes for workers during placement and compaction of the WMA. Also lower production temperatures reduce short-term aging of AC which allows for longer hauling distance of the mixture between mixing plant and construction site. Further evaluation is still needed to determine the performance of WMA, impact of the moisture in the mix, potential softness and rutting of the pavement, sustainability of the asphalt as well as the skid resistance, etc. especially for heavily trafficked roads.

Warm Asphalt Mix can be produced through a number of different methods. The process that MTO was first introduced to, and is incorporating in its trials is Mead Westvacos Evotherm technology. Evotherm WMA technology uses a high AC residue emulsion. Evotherm is an innovative chemical additive technology that has been shown to be constructible with mix and compaction temperatures as low as 60 Celsius.
On April 23, MTO staff from both Regional and Head Offices attended a morning presentation of another WMA technology called Sasobit, which incorporates a wax additive blown into the mix. About a month later, a third presentation was given covering the Aspha-Min WMA technology which involves the addition of aspha-min or zeolite which releases moisture in the mix to improve the workability of the mix at the lower temperatures.

An Evotherm WMA Trial was placed in the fall of 2005 on a municipal road near Brechin, Ontario. The trial was observed by MTO to check emission testing and mix temperatures at the Hot Mix Asphalt plant as well as the construction site paving operation. Based on the positive observations made at that time, MTO scheduled a WMA trial to be constructed on a section of Highway 15 from Smiths Falls Northerly up to Franktown. MTO schedules and conducts trials to keep abreast of technology.
With the commitment to reduce the impact of highway construction on the environment and the potential of WMA will assist us in this Endeavour, MTO is likely to schedule more trials in the future using some of the other WMA technologies.

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