Cold Weather Asphaltic Concrete Compaction

When compacting pavement, it is important to achieve the highest density possible. This is especially critical late in the paving season, when further compaction by traffic is unlikely before the pavement becomes wet. Cold weather paving means placing and compacting asphalt concrete in temperatures colder than 50°F. When the surfaces of pavements compacted in cold weather are more permeable (i.e. less dense), water infiltration can cause rapid deterioration under traffic conditions.

Variables affecting compaction are: (1) thickness of the lift, (2) base temperature, (3) initial temperature of the mat, (4) ambient temperature, (5) wind velocity, and (6) solar radiant flux.

Thicker lifts (more than two inches) hold temperature longer than thinner lifts (less than two inches). These thicker lifts are easier to compact to a high density, making them less permeable. Unfortunately, these lifts are usually not wearing surfaces, where density is most needed. Thin lifts are placed for wearing surfaces which cool rapidly in colder weather. Higher density is needed for these thinner wearing lifts to make them less permeable during wet months. This lifts insufficiently compacted in cold weather will ravel.

Certain steps insure that compaction is achieved in cold weather. Knowing that the degree of compaction relates directly to the mix temperature during compaction, it is necessary to maintain a high mix temperature during mixing, transporting and placement of the asphalt concrete.

1. Production temperatures can be increased with limitation. Asphalt cement can be damaged if mixed with aggregate that is too hot.
2. Tarp loads for normal hauls. (For normal or short haul times, uncovered asphalt concrete suffers little heat loss. The crust that develops maintains internal heat and is readily mixed with the hot portion of the load through the paving machine.)
3. Avoid placing asphalt concrete on a frozen base.
4. Avoid using a pick-up machine. If you must use one in cold weather, avoid long windrows, and keep a roller close behind the paving machine.
5. Keep handwork and raking to a minimum.
6. All joints need more attention in cold weather. Roll joints within three minutes of placement.
7. If possible, place a thicker surface lift.
8. It is recommended that a rubber-tired roller be used in a breakdown or intermediate position to knead the surface together and reduce permeability.

Through special attention and proper planning, it’s possible to eliminate most of the factors that contribute to poor performing pavements placed in cold weather. Below are tables to guide you to successful cold weather paving.

1. Tarp loads whenever possible. Although research has shown that the difference in the internal temperature of untarped and tarped loads will differ only slightly, the surface of untarped loads will cool much more rapidly than those with tarps. Therefore, untarped loads are more susceptible to developing a crust that may create streaks and holes in the mat behind the paver.
2. Avoid placing asphalt concrete on a frozen base. The asphalt layer will cool more rapidly when placed over a base that has a low temperature. This can make it more difficult to achieve the necessary compaction. Base failure and subsequent pavement failure may also occur if the base is frozen and then thaws after the asphalt layer is placed. A frozen base may give the false impression that it is adequately compacted, however when it thaws, the base may become soft and then yield. It is best to ensure that the base is at the optimum moisture content prior to paving.
3. Ensure the mix has adequate temperature after it is dumped and before it is placed in the paving machine. Avoid using a pick-up machine. If you must use one in cold weather, avoid long windrows, and keep a roller close behind the paving machine. Take temperatures of the mix as it is transferred to the paving machine to ensure that there is adequate heat to allow for placement and compaction. The temperature within the load may vary depending on where the temperature is taken, however, cooler portions will mix with hotter portions as the asphalt travels through the paving machine.
4. Keep handwork and raking to a minimum. Movement of the asphalt by rakes and shovels increases the rate of cooling and thereby makes it more difficult to ensure particle to particle contact in the mix. Colder asphalt will be more difficult to rake and feather and will also increase the likelihood for rock pockets that may later lead to deterioration.
5. All joints need more attention in cold weather. Avoid placing a hot edge against the cold edge of the previous pass. If possible, when placing a paving pass, follow up with the adjoining pass before the pavement cools completely. Roll joints as soon as possible after placement with most of the drum on the hot pass and overlapping the cold pass by about 6 inches. If possible, construct the joints out of the way of the future traffic wheel paths to reduce the stresses placed upon the joints.
6. If possible, place a thicker surface lift. Research shows that thicker lifts will stay hotter for a longer period of time, therefore allowing for more compaction than a thinner lift of the same initial mix temperature. Thinner lifts are more susceptible to raveling and abrading which can be magnified in winter conditions when there is more moisture on the road.
7. Utilize the correct compaction equipment. Use rollers with a wider drum width or use multiple rollers to ensure proper coverage before the temperature of the pavement decreases below that needed for compaction. Rolling faster to keep up with the paver will not result in adequate compaction. A rolling speed of about 3–4 miles per hour is the most efficient.

NOTES

• Air temperature is in the shade.
• Surface is the base upon which the pavement is to be placed.
• Prior to October 1, the minimum temperature when rolling begins shall be 121°C (250°F) and between October 1 and March 1, the minimum shall be 135°C (275°F).

Table 1:Paving cessation requirements for various thickness


Table2:HMA Placement Temperature Limitations

1. Shall not be placed between October 1–March 1 unless otherwise directed.
2. Can be placed only on treated bases, not frozen soils or untreated aggregates.