Warm Mix Asphalt

Asphalt has long been used as a pavement option on Iowa’s roadway, offering various benefits to local governments. While many cities use Hot Mix Asphalt when using asphalt for road projects, an emerging alternative, called Warm Mix Asphalt, holds promise as a cheaper paving option that could also bring other benefits.

Hot Mix Asphalt

Traditional asphalt, often called Hot Mix Asphalt (HMA), has long been the standard in the industry. The tried and true method involves mixing aggregate and asphalt cement (often called asphalt binder) at temperatures above 300 degrees Fahrenheit, which allows the asphalt binder to be fluid and less sticky while coating the aggregate material. Aggregates are the granular materials, such as crushed stone, gravel and sand, which account for 90-95 percent of the asphalt mixture. Asphalt binder is a dark brown or black substance that is thermoplastic liquid when heated and solid when cooled. The asphalt binder acts as the glue that holds the aggregate together and is usually less than 8 percent of the pavement mixture. The asphalt mixture is then kept at the hot temperature while transported to the work site in order to have workability during placement and compaction. The asphalt then slowly cools to form the pavement.

Warm Mix Asphalt

While HMA has long been in use, a new type of asphalt, called Warm Mix Asphalt (WMA), is gaining traction as an alternative with promise. According to the Federal Highway Administration, WMA uses water, water-bearing mineral chemicals, waxes and organic additives to add to the asphalt binder or mixture. The use of the additives allows the asphalt binder to remain fluid at lower temperatures during mixing, with temperatures 30 to 120 degrees Fahrenheit lower than HMA production. The lower temperatures are one of the key benefits of WMA as fuel consumption during WMA is typically reduced by 20 percent, leading to cost savings. Additional advantages are seen when WMA is used in the field.

One of the advantages of using WMA is the faster cooling rate of the asphalt, leading to streets often being able to re-open to traffic sooner than traditional projects. WMA also creates fewer fumes and odors as a result of its lower temperatures, which makes the work environment safer both at the production facility and construction site and the air a little cleaner for those in the vicinity.

WMA could also reduce costs as it is used more through less fuel needed to heat the asphalt and less labor and time at the work site. The lower temperatures of WMA allows for it to be hauled longer distances, which could also reduce transportation costs. Finally, the paving season could be extended since the asphalt can be worked at lower temperatures and at night.

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