A number of trends in the asphalt pavement industry save money and use less energy.
New asphalt pavement in Washington City, Utah. UDOT's contractor used Hot mix asphalt containing recycled asphalt pavement or RAP.
Sometimes people outside of the transportation industry view pavement as a one-time use of resources. However, many of the products used for pavement can be reused or mixed in ways that use less energy. Asphalt and concrete pavement, for example, can be recycled. This article summarizes some ways asphalt pavement can be re-used or mixed using less energy.
Recycled Asphalt Pavement, old asphalt pavement that has been milled off or otherwise removed from a road or other installation, can be added to new asphalt pavement at a batch plant. In general, state DOTs allow more RAP in base courses than they do in surface courses. For example, the I-15 CORE project uses a multi-layer pavement design. The asphalt portion on the interstate uses thirty percent RAP but the surface streets use a smaller percentage of RAP.
According to Michael Kvatch, Executive Director of the Asphalt Pavement Alliance, tests show that high RAP content pavement on surface streets (10 to 25 percent RAP) can be “just as good, if not better than its virgin counterpart.”
Cold In-Place Recycling is a way to reuse asphalt on site using a long train of equipment. The process involves removing and pulverizing old asphalt, adding binder, spreading, grading and compacting the asphalt. CIR can be used to rejuvenate an old road or as a base course for new pavement. New aggregate can also be added during the process if needed.
CIR offers the advantage of being about one-third of the cost of new asphalt. CIR is a cold process, so the energy used to heat Hot Mix Asphalt is also saved. There are also many CIR processes and uses, which gives contractors and engineers at UDOT options for bidding and designing good solutions for maintaining roads.
However, regular Hot Mix Asphalt made according to a Superpave process is much stronger than CIR, so for high volume roads or cold climates, CIR may not be appropriate.
HMA is also more predictable than CIR, which is subject to many variables, such as original mix design and aggregate size, explains Kevin Van Frank, UDOT’s Engineer for Asphalt Materials. CIR is subject to many variables, including binder and aggregate type and size. Those variables make the characteristics of the final product a challenge to predict, especially when it comes to curing time. Because of the economic advantage of CIR, UDOT Research is funding testing that will establish standards for CIR.
Maintenance of Traffic is also a problem with CIR since the process requires a long train of machines that can stop traffic. UDOT places a premium on keeping traffic moving during construction, so CIR is not used in high traffic urban areas.
Sometimes people outside of the transportation industry view pavement as a one-time use of resources. However, many of the products used for pavement can be reused or mixed in ways that use less energy. Asphalt and concrete pavement, for example, can be recycled.
Warm mix asphalt pavement is produced using less energy. Hot Mix Asphalt is heated to 310 to 350 degrees Fahrenheit; the temperature range for WMA is as much as sixty degrees lower. HMA uses heat to decrease the viscosity of the asphalt in order to be able to place and roll the pavement. WMA uses Zeolite, waxes, surfactants or water to compensate for using cooler temperatures during production.
Zeolite is a “mineral sponge,” that transports water into the asphalt binder. Waxes act as thermosets that increase viscosity above the liquid phase. Surfactants can be added to coat the aggregates to make the binder workable at lower temperatures. Water injected during the mixing process causes the asphalt binder to foam so it achieves the viscosity needed to place and compact the material at lower temperatures.
WMA can be placed in the cooler months of spring or fall, which potentially lengthens the construction season. And, cooler asphalt has a milder odor, which can be good for road users and especially for workers.