This article is from a Technical Bulletin prepared by Flexible Pavements of Ohio
The issue of placing hot mix asphalt (HMA) in cold weather comes up every fall, winter and springs. Projects get delayed and damp. Specifications generally set weather and temperature limits beyond which paving is to 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 the prevalence of cold weather paving. The responses showed that in the north-central region of the country 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 challenge of cold weather HMA paving is to achieve adequate compaction. There is a consensus that if adequate density is obtained the pavement will perform as expected. Thin courses and surface courses are at the greatest risk for low density and poor performance when placed in cold weather. Intermediate and base courses greater than two inches thick generally can be constructed with little change in normal procedures.
Time for Compaction
Cold weather compaction depends upon having enough time and enough rollers to obtain adequate density while the temperature of the HMA mix being placed is still within the compaction temperature range, approximately 275 – 175 degrees Fahrenheit. What factors affect the time it takes for the HMA to cool below 175◦F? All weather factors affect this time – air temperature, wind speed and the presence or absence of sunlight. The type and temperature of the surface on which the HMA is to be placed is a factor also. But the two most important factors are the temperature of the mix and the thickness of the course being placed. It is generally accepted that if conditions do not permit 10 minutes of time for compaction, adequate density probably cannot be achieved.
It is easy to determine this time for any set of conditions. Dickson and Corlew published cooling curves in 1910 showing the time available for compaction for any given set of ambient and mix conditions. Determining the available time became even easier with the development of the PaveCooI software by the Minnesota DOT. (PaveCool can be downloaded from www.mrr.dot.state.mn.us/research/mnroad_project/restools/cooltool.asp)
Let’s look at an example.
30◦F air and base temperature
5 mph wind
Clear and dry
Binder grade, PG 64-22
Single course being placed on an existing asphalt concrete surface.
At a mix temperature of 275◦F and a course thickness of 1.25 inches, the time available for compaction is 7 minutes, too short to realistically achieve density. If the mix temperature is raised to 325◦F and all other factors are the same, the time available for compaction is 12 minutes. Now you have a chance of getting the mat compacted before it cools. If the mix temperature is held at 275◦F but the course thickness is increased to 2 inches, the time available for compaction is 17 minutes.
The other challenge in cold weather construction is economics. Cold weather construction will cost more.
Mix temperature is one of the most influential factors on time available for compaction, so an obvious solution is to produce hotter mix. How much can the mix temperature be raised without causing damage and what is the cost?
Binder suppliers normally recommend a mixing temperature based on viscosity tests. The NAPA publication on cold weather compaction suggests that it is probably safe to mix at a temperature 18◦F above the recommended temperature. However, mixing above that temperature level risks excessively aging the binder or placing too thin a coating on the aggregates. Raising the mix temperature takes extra fuel and lowers the production capacity of the plant. An examination of the plant production tables in the Hot-Mix Asphalt Paving handbook indicates that raising the mixing temperature 25◦F can reduce the production capacity of the plant by 15 percent or more.
Likewise, increased aggregate moisture contents reduce the production capacity even more dramatically. Given the combination of need for a higher mix discharge temperature and the presence of colder aggregates with higher moisture contents, it is easy to see that the plant production rate may be cut in half in cold weather. Stated otherwise, twice as much fuel may be required to produce mix in cold weather.
Hauling and Temperature Segregation
The next challenge is to get the mix into the paver with as much retained heat as possible. The first thought is to tightly tarp truck bets. Research has shown, however, that tarping of loads has little effect on the average temperature of the load for normal haul times. So why bother? This raises the topic of temperature segregation. Temperature segregation is the presence of masses of mix in the mat with temperature differentials that prevent uniform compaction. When a load is transported in cold weather without a tarp, the cold crust that forms on the load may be placed by the paver creating a cold spot in the mat that cannot be adequately compacted. There is little consensus as to how important this phenomenon is. Some believe this may be an important issue in the performance of pavements, and as a result there has been recent proliferation in equipment for re-mixing material as it is fed to the paver. Other point out that we didn’t know about this effect until the advent of the thermal imaging camera – if it wasn’t a problem before, is it now?
Until this is issue is resolved, the recommendation is to tightly tarp the loads, at least for longer hauls, and prevent exposure to precipitation. If tarps are used, they should tightly cover the load and seal over the sides of the truck bed. Loose, flapping tarps may actually increase the heat loss. Tarping loads for short hauls will not save much heat and may take precious time. Tarping loads for longer hauls will not significantly raise the temperature at which the mix is delivered to the paver, but may result in a more uniform temperature mix, thereby minimizing the effect of temperature segregation.
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 devises 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 not 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 and existing paved surface, the surface must be dry and the tack coat material must be set. How do you get that slow-setting emulsion tack coat to break and dry in cold, damp weather? If obtainable, you could use rapid-curing liquid asphalt for tack. Instances have been reported where contractors have used racetrack jet-dryers or infrared heaters to dry the surface before placement of the HMA.
Areas that require handwork or feather 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 block 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.
Understanding that compaction is directly related to the mix temperature, it is necessary to maintain a high mix temperature during production, transportation and placement of the HMA. The main thing to remember in cold weather paving is that the Tim Available for Compaction (TAC) is dramatically reduced.
Specifications and Quality Assurance
Is it worth the extra cost and effort to place HMA in cold weather? Ultimately, only the person paying the bill can answer that question. Research at Washington State has indicated that even a few percentage points less density results in double-digit percentage losses in durability (pavement life). So if you are the owner, it probably makes sense to invest the extra cost to get adequate density if you absolutely have to have the work completed in cold weather.
How do you handle the extra cost and payment for this extra effort? The usual way is by change order, but scarce, suitable working days can be lost while such things are negotiated and processed. If an owner anticipates that such a situation might occur on his project, it may be worthwhile to set up an alternate bid item for the extra cost of cold weather paving in order to establish in advance a price for the extra work needed to adequately place and compact HMA in cold weather. Issues such as changes to course thickness and mix type would have to be addressed and some quality assurance or acceptance measures might have to be altered. If the project were to be a density acceptance project then the effectiveness of the contractor’s compaction procedures would be revealed by the acceptance cores. If density measurements are not available, then some other measure for verifying the effectiveness of the contractor’s placement and compaction procedures would have to be established in the specifications. The owner may require the placing of a control or test strip to ensure minimum acceptable density results from the contractor’s proposed procedures.
Hot mix asphalt paving can be successfully accomplished in cold weather without compromising the performance of the pavement. The mix must be compacted while it is still in the compaction temperature range. Adequate time for compaction can be obtained by:
- Increasing mix temperature
- Increasing the layer thickness
- Minimizing the time/length of haul
- Working the rollers as close to the paver as possible
- Using more and/or higher-capacity rollers