Paving with HMA in Cold Weather
Proper compaction is the single most critical variable in placing quality asphalt pavement with an extended pavement life. In cold weather, proper compaction requires the perfect balance of temperature and time.
Above all else, the temperature of hot mix asphalt (HMA) during the rolling operation determines the success of the compaction operation. The time available for compaction (TAC) defines the total amount of time that roller operators must compact the mix before it becomes too stiff to compact. The primary factors influencing TAC are lift thickness, base temperature, mix delivery temperature, ambient air temperature, and wind speed.
Tips for Maintaining Temperature
Always tarp loads. For short hauls, the crust that develops maintains internal heat and is readily re-mixed with hot asphalt upon unloading and transfer to and through the paver.
Unload trucks in the reverse order. The first couple of truckloads are generally lower in temperature. A hotter mix heats the equipment, and the paver screed faster, which avoids tearing the mat at the start of paving.
Use pickup machines with care. HMA loses approximately 10° to 20°F of its delivery temperature during transfer to the hopper. End dumping results in only about 5°F heat loss. HMA loses another 10° to 20°F when going from the MTV (material transfer vehicle) to behind the screed. Avoid long windrows when using a pickup machine. Do not string out windrows until they are ready to be picked up.
Keep the hopper near full. The mix retains heat better in a large mass and keeps the hopper hot. Communicate with the plant to ensure the proper truck spacing and minimum waiting periods.
Keep handwork and raking to a minimum. Every time the mix is “fluffed,” it loses considerable heat.
Utilize the MultiCool App, available at www.asphaltpavement.org/multicool. MultiCool is used to estimate how rapidly a freshly placed mat cools as a function of the initial mat temperature, ambient conditions, mat thickness, and other properties. The cooling rate prediction can help contractors better plan their rolling operations to more efficiently achieve target mat density.
Tips for Optimal Timing
When paving with HMA in cold weather, the goal is to finish compacting the mix while it is still in the compaction temperature range of 275° to 175°F. To allow adequate time for compaction, take steps to minimize the time of exposure of the mix between mixing and compaction. Specific actions may include working the rollers as close to the paver as possible and using a warm mix as a compaction aid.
Avoid placing a thin HMA course in cold weather. Placing a relatively thick intermediate course, that can be used as the temporary wearing surface until proper conditions return for placing a thin surface course, involves little change to construction procedures and minimal additional risk.