Sometimes when things come down to the wire in football or road construction, you need to make one last push for success. As the winter approaches and the days grow colder, roadway agencies need to start thinking about finishing up the paving season. Though paving in colder temperatures is less than ideal, agencies and contractors often find themselves needing to push the limits more than they might like. In addition, more and more paving jobs are being conducted at night to reduce traffic delays, which also results in cooler temperatures. In this newsletter we’ll take a look at a number of strategies that can be used to make sure that late season or cold weather paving is a success.
Cold weather paving refers to paving at temperatures below 50° Fahrenheit and is generally an issue because of its effect on the ability of rollers to adequately compact an asphalt mat. Asphalt becomes stiffer as it cools, until it reaches a “cessation temperature”. This is defined as the temperature at which additional compaction has no appreciable impact on mat density. Therefore, all compaction must be completed before the pavement cools to this point. There are a variety of strategies that can be used to make compaction “go smoothly” under adverse circumstances. These involve aspects of paving such as asphalt properties, material transportation, and compaction itself.
Paving in cool weather without proper planning can have poor results
Panic in the fourth quarter can be avoided by careful planning and drawing up the right play. First of all, for those in cooler climates, whenever possible, it’s better to finish up your paving for the year during the summer months. However, when it can’t be avoided, there are certain kinds of paving jobs that will work better in a cold weather environment. Thicker lifts and sections are usually slower to cool than thin overlays. If thin overlay projects cannot be completed during the warm season, consider adding thickness to the layer. In addition, any projects that will use warm-mix asphalt are well suited to cold weather paving, which we will discuss later.
Before heading to the jobsite, make sure you’ve done your homework. It is important to know how long you expect to have for compaction once the mix has been placed. There are several methods for determining this number with varying degrees of accuracy. Probably the most modern tool we have available is PaveCool, free software developed by the Minnesota Department of Transportation, which provides a time for compaction for a wide range of project inputs. Once you have an idea of your time window, you can decide whether or not to adjust your production plans. Let’s look at a few adjustments that are easy to make and won’t impact pavement quality.
Using a tarp can prevent formation of a cool crust on the asphalt
Heat is lost while material is being taken from the plant to the paver, and steps must be taken to make sure this is minimized. One important decision to be made is whether to cover asphalt loads with tarps. Tarping has been shown to have a minimal effect on the overall average temperature of the load. However, tarping significantly reduces the formation of a cold crust on the load surface. This crust, if not sufficiently remixed with the rest of the load will lead to low density areas in the finished pavement. The primary determinant in whether or not you should tarp should be the length of the haul. Finally, production rates of the paver and plant should be carefully balanced so that full trucks are not left waiting at the paver.
Avoid full trucks waiting at the jobsite
There are certain considerations that can be taken during the paving process itself to increase the quality of a cool weather pavement. First of all, ensure that you are not paving on a frozen base layer. In addition, while paving, try to minimize the areas that require handwork. Any raking or shoveling of asphalt will increase the rate of cooling, and handworked areas will likely take too long to place to achieve adequate compaction.
A compaction train and rollover patterns should be carefully selected based on the compaction needed and the predicted time that will be available to compact. In cool weather, compaction equipment will likely need to follow close behind the paving train. Rubber tire rollers can be used to accomplish a high degree of compaction over a short period. However, use of these rollers at lower temperatures risks pick-up of the fresh asphalt by the rubber tires. This is more likely to occur when the tires are cool, so special precautions must be taken. This can include fitting skirts around the tires and special heaters to ensure that they are kept warm. In addition, additives can be used in the mix to help limit or prevent pick up.
Warm-mix asphalt, or asphalt that has been modified to be produced and placed at lower temperatures, is well-suited to cool weather paving. This is because warm-mix asphalt can be compacted at lower temperatures than hot-mix. In addition, because the mix temperature is closer to atmospheric temperature, it will also cool less rapidly. These two facts combined mean that you simply have a longer paving window with warm-mix regardless of the specific warm mix production technology you use.
When properly planned and executed, paving can be accomplished successfully in cooler settings such as night time or late in the year. While this can avoid traffic delays or allow completion of larger projects, it should not be undertaken unless there are sufficient resources to produce a quality finished product. Otherwise, the costs of poorly paved roads will outweigh the benefits. Take the proper precautions to make sure you get a touchdown as the clock winds down.