Computer Program for Calculating the Rate of Cooling in an Asphalt Pavement
The temperature compaction takes place is an important factor in the construction of asphalt concrete pavements. In cold climates, rapidly cooling mats can contribute to poor compaction. Asphalt cooling then is a very important part of the construction process.
Cooling of asphalt is a combination of loss of heat into the pavement base and into the atmosphere. Major factors affecting the time for a layer of asphalt to cool to the minimum temperature at which compaction remains effective include:
- Laydown temperature: The hotter the asphalt leaves a paver, the longer it will take to fall below the minimum effective compaction temperature. However, too high a temperature can cause binder hardening, mix tenderness and binder fuming.
- Pavement temperature: The colder the pavement, the quicker heat will be drawn out of the asphalt. Heat loss can be particularly high on wet or moist pavements or on concrete and steel surfaces.
- Layer thickness: Thicker layers cool more slowly. Thin layers of asphalt cool quickly.
- Wind speed: The rate of transfer of heat to the atmosphere is increased by higher wind speeds. The cooling rate is further increased by the presence of surface moisture.
Asphalt Pavement
In hot-mix asphalt (HMA) pavement construction, newly placed pavement must be sufficiently cooled before it can be opened to traffic. Estimation of the time required for the pavement to reach the desired temperature level is necessary for highway agencies to make proper specifications and for contractors to plan for daily productions.
A more sophisticated way to determine the time available for compaction is through a computer program called MultiCool. MultiCool is a user-friendly application that calculates the rate of cooling in an HMA mat during construction. This program requires much more detailed input information to accurately determine the rate at which an asphalt concrete mix will cool when placed on the roadway.
The required input information for the MultiCool program is in the following areas:
- Calendar – the date and time
- Environmental – air temperature, wind speed, sky conditions and latitude
- Mixture Specifications – number of pavement layers, mix type, PG binder grade, layer thickness, mix delivery temperature and mix “stop” temperature (usually 175° F)
- Existing Material Surface – type of surface, state of moisture (frozen, wet, etc.), moisture content and temperature of the surface
Based on the value of these inputs, the MultiCool program calculates the time for the mix temperature to be reduced from the delivery temperature to the mix stop temperature.
MultiCool can be accessed online or via a mobile website. It can also be downloaded to run on Windows-based computers, and it is available as an Android app.
While it is no substitute for good judgment and on-the-job observations, MultiCool can help in the planning process and in-field decision making.
The MultiCool temperature program was developed at the University of Minnesota and version V 2.0 was developed by Dr. David Timm and Ben Peters at Auburn University, with funding from the National Asphalt Pavement Association (NAPA). The MultiCool program can be downloaded from the NAPA website at https://www.asphaltpavement.org/multicool.
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