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Cold Weather Paving - II
The issue of placing hot mix asphalt (HMA) in cold weather comes up every fall, winter and spring. Projects tend to get delayed, as the weather turns cold and damp. Specifications of the mix being used generally set weather and temperature limits beyond which paving must be stopped. But jobs often need to be completed in spite of the specification limits. Everyone starts to wonder whether they should continue to pave. The question is, “Will HMA pavement placed in cold weather perform adequately?”
An industry survey conducted and analyzed by a group of researchers at Auburn University revealed that cold weather paving is more prevalent than previously thought. The responses showed that in the north-central region of the United States up to five percent of all projects get placed outside the normal paving season of April to November, and many of these jobs are placed in adverse weather conditions.
The asphalt industry faces a few challenges during cold weather paving, but nothing that can’t be tackled with the right techniques, use of quality mixes and best practices adopted based on the weather conditions.
1: Hauling and Temperature Segregation
Once the hot mix asphalt is ready, the next challenge is to get the mix into the paver with as much retained heat as possible. To prevent temperature segregation, which is the presence of masses of mix in the mat with temperature differentials preventing uniform compaction. This usually occurs when a load is transported in cold weather without a tarp. This may cause cold crusts to form on the load that could accidentally get placed by the paver creating a cold spot in the mat without adequate compaction. The recommendation is to tightly tarp the loads, especially for long haulsto prevent exposure to precipitation. Tarping loads for short hauls will not save much heat and may take precious time.
The basic objective in cold weather paving is to keep the total time from mixing to compaction as short as possible. Haul trucks should not be kept waiting to unload into the paver. Minimize the handling and exposure of the HMA. Windrow paving and transfer devices that extend the time and further expose the HMA to the environment should probably be avoided. Move the material as a mass directly from the haul truck into the hopper of the paver.
If the HMA course is to be placed on an aggregate base, the base must be solidly compacted at or below optimum moisture and should not be frozen. Frozen or excess moisture rapidly saps the heat out of HMA and may contribute to soft spots in the base. If being placed over an existing paved surface, the surface must be dry and the tack coat material must be set. If obtainable, the use of rapid-curing liquid asphalt for tack should be used. Instances have been reported where contractors have used racetrack jet-dryersor infrared heaters to dry the surface before placement of the HMA.
Areas that require handwork or feathering of the mix can probably not be placed rapidly enough to permit adequate compaction. Work of this type should be avoided during cold weather or be considered as a temporary solution.
Construction of transverse joints must be accomplished with good technique, starting off with the screed at the joint and on starting blocks, so that time is minimized and the need for handwork is eliminated. Paver speed should be regulated to allow the rollers to complete compaction within the time and temperature constraints.
More rollers and higher capacity rollers operating right behind the paver will be necessary to accomplish the compaction in the short time available. The use of rubber-tire rollers may be the answer in obtaining density quickly. However, special care must be used to heat the tires to prevent mixture pick-up. Its recommended that skirts be used around the tires. Contractors have fitted heaters within the skirt enclosures that pre-heat the tires and have ducted the engine exhaust inside the skirt enclosures to keep the tires hot. Silicone-based additives are available for mixing into the water used to prevent mix pick-up on the tires. The provision of additional rollers and operators, heating of tires and special release additives all represent additional costs of cold weather paving.