August 30, 2012
FEATURE | Roadbuilding
A winning asphalt road requires diligence and planning
This time of year, we tend to squeeze every last drop out of the limited number of sunny days and warm temperatures left in the calendar. The same holds true for asphalt pavers, who know that the inevitable is around the corner — late season paving. Paving in colder temperatures is less than ideal, and contractors often find themselves coming down to the wire, like a football team, two points behind at the 20-yard line in the fourth quarter.
We do not have to wait too long before we enter the “fourth quarter” — cold weather paving, which is defined at temperatures below 10 C. Because asphalt becomes stiffer as it cools and it cools faster in cold temperatures, paving in cold temperatures can result in less than optimum compaction and poorly performing longitudinal joints.
That said, an ounce of prevention is worth a pound of cure. Why unnecessarily push critical projects to the end of the year? Municipalities (owners) should issue critical contracts early in the year to allow them to be completed in favourable weather. Additionally, as more paving jobs are conducted at night to reduce traffic delays, it is better to call for night work early in the season.
Furthermore, contractors can work with owners to prioritize the schedule and can avoid schedule delays by completing mix designs and obtaining approvals well in advance. Contractors must also ensure sufficient manpower and serviceable equipment is available to complete the project.
However, despite best efforts, late season paving is sometimes unavoidable. There are a variety of strategies that can be used to make paving go smoothly under adverse circumstances. Cold weather paving depends upon having enough time and rollers to obtain proper density before the mat cools. Therefore, contractors should calculate the cooling times of the asphalt to determine the time window left for compaction. The PaveCool software developed by the Minnesota Department of Transportation can provide this calculation. For example, according to PaveCool, in air temperatures of 10 C with the granular temperature at 2 C it takes five minutes for a 40-mm lift to cool to 85 C, making further compactive effort ineffective.
Using finer mixes can produce better compaction results in all temperatures. Also, owners should consider using thicker lifts, which retain heat longer allowing more time for adequate compaction.
Warm mix asphalt is well suited to cold weather paving because it can be compacted at lower temperatures. Because the mix temperature is closer to the air temperature it cools less rapidly, providing a longer window of time.
The next challenge to consider is hauling. Production rates of the paver and plant should be balanced to ensure that full trucks are not left waiting. Tarping also helps to keep the heat in and significantly reduces the formation of a cold crust on the load surface. Cold crust on a load can result in a cold spot on the mat which can produce uneven compaction. The use of MTVs for remixing the load can resolve temperature segregation.
Other considerations to ensure the best compaction and longitudinal joints are using infrared heaters to preheat the rollers and prevent heat loss to the equipment. Using infrared heaters to warm the joints improves joint densities. Paving in echelon, which eliminates joints altogether, is also an option.
When dealing with late season paving municipalities must have experienced inspectors on site to quickly and constructively deal with issues as they arise. Inspectors should also be wary of predicting weather patterns, especially after Thanksgiving. At times it is better to go ahead in marginal conditions instead of waiting for the perfect day.
In the end, sometimes the best planned and played strategy may not be enough, and the “game” may need to be postponed until spring. Otherwise, the quality of the pavement will be jeopardized. Taking the proper precautions, paving either late in the season or postponing to the spring will ensure a winning “touchdown” — a high-performing, quality asphalt road.
Sandy Brown, Technical Director
Ontario Hot Mix Producers Association
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