I do not like cold weather, but I have to concede that it is almost here (at least in Kentucky). The low temperature last weekend was in the low 30s (F). And, I don’t like cold weather paving – but it is a real situation, too. It happens every fall, winter and spring. For a wide variety of reasons, we end up having to pave under less than ideal conditions.
We can construct well-performing asphalt pavements in cold weather, provided we make some adjustments to routine procedures. The key to success is to achieve adequate compaction, and the toughest challenge is to get good density on thin lifts – particularly surface courses. It can be done. It takes more effort and may be more costly, but it can be done.
For hot mix, the problem is having enough time (and, maybe enough rollers) to compact the mat before it cools. A rule-of-thumb for hot mix compaction is that there needs to be at least 10 minutes available for rolling before the mix cools below 175 degrees F.
Raising the mix temperature, but not to the point of damaging the mix, will provide more time for compaction. Be careful not to oxidize (excessively age) the asphalt binder, and be aware that additional burner fuel costs will be incurred and plant production may have to be reduced. (There’s some of the extra costs.)
Unless the haul distance/time is extremely short, the haul trucks should be fully tarped. A tarp may not retain much of the heat but it lessens the formation of a cool crust of mix on the exterior of the mass of material. Materials transfer vehicles (MTVs) and re-mixing pavers also can help with mix-temperature uniformity. Manage the timing of trucks arriving at the paving site; the mix cools while the trucks are waiting.
Two particular concerns with cold weather paving are hand-work and joint construction. Minimize the hand-work – this is usually a thin application and will cool very quickly. It’s hard to get a uniform texture and good compaction with these conditions. Take extra care constructing the joints. Cool mix and poor technique here will soon show up as raveling and open joints, leading to pavement failures.
Rolling is a big deal in cold weather paving. It may take additional rollers and the rollers should work closely behind the paver. A compaction test strip is a good idea to demonstrate that adequate density and decent texture can be achieved.
Another approach to achieving better compaction in cold weather is to use warm mix asphalt (WMA). One of the reported benefits of WMA is that it extends the paving season. WMA does not cool as quickly as HMA and it is still workable/compactable at lower temperatures. Some contractors routinely use a WMA technology as a compaction aid for normal work.
Numerous WMA projects have been placed and documented for cold conditions. I recently read about a New York job that was done on December 8, 2008. The air temperature was below 32 degrees F throughout the day, and icicles were observed forming on the rollers from the water sprayed on the drums. When the paving started, the existing pavement surface was measured at 11.5 degrees F.
That does not sound like ideal paving conditions to me. Yet, it was reported that the Evotherm-modified WMA mix laid well and readily met the (less than 8% in-place air voids) density spec.
My advice is to avoid cold weather paving (and cold weather, too), if possible. But, if it is necessary, make appropriate adjustments and work carefully and quickly. Good results are achievable.
Email Dwight at talkingasphalt@asphaltinstiute.org