James A. Scherocman, P.E.
The purpose of this article is to discuss the importance of temperature, temperature and temperature on the ability of a contractor to obtain density in an asphalt concrete mixture when placed on the roadway. It is often stated, quite correctly, that density is the most important factor that affects the long-term performance of an asphalt pavement.
Minnesota weather is often variable with either extremely hot temperatures in the summer, rapidly cooling temperatures in the fall season to frigid cold temperatures in the winter. Fall seasons mean for nicer air temperatures for the paving crew to work in, it also means that the asphalt pavement mixture will cool more quickly. This results in more difficulty to compact and to obtain the desired level of density.
Factors Affecting Compaction:
There are several primary factors that affect the cooling rate of an asphalt concrete mixture. That cooling rate is the time it takes for the mix temperature to change from the temperature at which the mix is delivered into the hopper of the paver (laydown temperature) to the temperature at which the mix becomes too stiff to compact. The time between those two temperatures is the Time Available for Compaction.
The first temperature that is important is the temperature of the asphalt concrete mix when it is delivered from the hopper of the paver to the paver screed. The mixing temperature of the material at the plant is not the concern, nor is the loss of temperature that occurs when the mix is in the truck bed during the hauling operation. The temperature of the mix when it passes out from under the screed is the most-important temperature for the compaction operation.
The second temperature that is important is the cessation temperature. The cessation temperature is the temperature at which the mix becomes too cool for a pass of the roller to increase the density of the mix. That lower temperature is typically 175 degrees Fahrenheit. Although it is often possible to take out some roller marks at a temperature below this temperature, no significant increase in density can typically be achieved with additional roller passes.
Two other temperatures are also very important. One is the ambient air temperature. The higher the air temperature, the longer it will take for the mix to cool. The second is the base temperature, or the temperature of the surface on which the asphalt concrete mix is being placed. An asphalt concrete mix, as everyone knows, cools both to the air and to the underlying surface. That existing surface is certainly cooler in the fall of the year than it is in the summer.
Another extremely important factor is the compacted thickness of the asphalt concrete mixture. Obviously, a thicker layer of mix will take longer to cool than a thinner layer of mix. A layer 1 ½ inches thick will take less time to cool than a layer 2 ½ inches thick.
Two other factors that affect the cooling rate to some extent are the wind velocity and the solar flux. An asphalt concrete mix will cool more quickly on a windy day compared to a day when the wind is calm. An asphalt concrete mix will cool more quickly on a cloudy day compared to a sunny day.
Time Available for Compaction Chart:
In the chart shown below, a number of examples are provided in regards to the time that it will typically take for an asphalt concrete mix to cool from the laydown mix temperature to a cessation temperature of 175° F. In this example, no effect of wind velocity or solar flux is included in the estimated time. Twelve examples of the time available for compaction are shown.
Further, for simplicity, it is assumed that the air temperature is the same as the temperature of the surface on which the mix is being placed, so that it does not make any difference whether the primary direction of the mix cooling is upward to the air or downward to the underlying pavement surface. In addition, the pavement thicknesses shown are the compacted thickness of the asphalt concrete layer, not the mix thickness coming out of the back of the paver screed.
From the chart, it can be seen that the most-important factor that affects the cooling rate of the asphalt concrete mix is the temperature of the mix when it comes out from the paver screed. The second-most important factor is the compacted thickness of the mix. A relatively minor effect is seen in regard to the air temperature/base temperature – the change from an air temperature/base temperature from 60° F to 40° F does reduce the time available for compaction, but not nearly as much as the effect of mix temperature and compacted layer thickness.
Examples 5 versus 6:
In example 5, the mix temperature is 300° F, the air/base temperature is 60° F and the compacted layer thickness is 2 inches. The time available for compaction is 18 minutes. In example 6, the mix temperature has decreased to 250° F behind the paver, but the air/base temperature is still the same at 60° F and the layer thickness is still the same at 2 inches. The time available for compaction, however, is only 11 minutes. The reduction of the mix temperature from 300° F to 250° F has reduced the compaction time from 18 to 11 minutes.
Examples 5 versus 7:
In example 5, as discussed above, the time available for compaction is 18 minutes. In example 7, the air/base temperature has been reduced from 60° F, in example 5, to 40° F. The mix temperature remains at 300° F and the compacted layer thickness remains at 2 inches. The time available for compaction has been reduced from 18 minutes to 16 minutes. Thus, the air temperature/base temperature is not nearly as important as the mix temperature.
Examples 5 versus 9:
In example 5, the time available for compaction is shown to be 18 minutes. In example 9, the compacted layer thickness has been reduced from 2 inches to 1 inch. The mix temperature and the air/ base temperature are the same for both examples. The time available for compaction, however, has been reduced from 18 minutes to only 7 minutes. The compacted thickness of the asphalt concrete layer has a very significant effect on the cooling rate of the asphalt concrete mixture.
The two most-important factors that affect the ability of a contractor to achieve the desired level of density in an asphalt concrete mix in cool weather are the temperature of the mix coming out from under the screed on the paver and the compacted thickness of the mixture. The air temperature/base temperature is of relatively lesser importance
In summary, the higher the mix temperature, the more time that is available to properly compact the mix. The greater the thickness of the compacted pavement layer, the more time is available to properly compact the mix.
What the cooling rate chart on page 19 means, in essence, is what is already well known: The rollers need to be directly behind the paver. In order to achieve density, it is necessary to finish the compaction operation before the mix cools to a temperature of 175° F. Attempting to compact the asphalt concrete mixture when it is cold is a waste of time, effort and money.
To quickly compact a 12-foot-wide layer of new asphalt concrete mix, a contractor really needs a 13-foot-wide roller. With that wide a roller, it would only be necessary to make about five passes of the roller up and back over the mix in order to achieve the desired level of density (a pass being defined as once over a point in the pavement surface). The main problem is that we do not have any 13-foot-wide rollers. Thus echelon rolling needs to be considered – two rollers essentially side by side directly behind the paver. Such a roller pattern is shown in Figure 1.
For echelon rolling, the width of the lane being paved needs to be divided transversely into different segments depending on the width of each of the two rollers being used. For example, a 12-foot-wide lane can be compacted with two, 7-foot-wide, double-drum vibratory rollers, one on the left side of the lane and one on the right side of the lane. Each roller will overlap the unsupported edge of the lane – or the longitudinal joint between the lanes – by 6 inches. In addition, each roller will overlap the coverage of the other roller in the center of the lane by 6 inches. The whole width can be compacted by the two rollers, each on their own side of the lane.
If the rollers are not wide enough to compact the whole width (plus overlap) in two passes, then three passes across the width of the paving lane by the two smaller rollers will be necessary. As an example, if the two rollers are only 66 inches (5 ½ feet) wide, each roller will have to compact one edge of the new, 12-foot-wide lane plus additional passes up and down the center of the new lane.
If the two rollers are operated in the vibratory mode, and if the rollers are operated within 300 feet of the back of the paver, the desired level of density should be able to be achieved by making five passes of each roller over their portion of the width of the new pavement lane.
The key to achieving density using the echelon compaction method with two vibratory rollers is to keep the rollers within 300 feet of the back of the paver and make all of the roller passes while the mix temperature is high – well above the 175° F cessation temperature. This roller pattern is shown in Figure 1.
An alternative compaction method that can be used to obtain density in cool weather is a pneumatic tire roller in the breakdown position directly behind the paver and a double drum vibratory roller in the finish roller position – close behind the pneumatic tire roller.
With this combination of two rollers, the new mix is compacted from the bottom up with the pneumatic tire roller in the breakdown position. The pneumatic tire roller should typically make three passes over each point in the pavement surface, staying 6 inches inside the unsupported edge of the lane and traveling directly over the longitudinal joint between lanes.
Immediately behind the pneumatic tire roller is the vibratory roller, operated in the vibratory mode. The vibratory roller is able to easily remove the rubber tire roller marks in the pavement surface because the mix temperature is still high enough to do so. Typically, three passes of the vibratory roller, in the vibratory mode, over each part of the lane width should be adequate to finish the compactive effort and achieve density.
It is important to keep both rollers within 300 feet of the back of the paver and make all of the roller passes while the mix temperature is high – well above the 175° F cessation temperature. This roller pattern is shown in Figure 2.
Multicool Computer Program:
A more sophisticated way to determine the time available for compaction is through a computer program called MultiCool. This program requires much more detailed input information to accurately determine the rate at which an asphalt concrete mix will cool when placed on the roadway.
The required input information for the MultiCool program is in the following areas:
- Calendar – the date and time
- Environmental – air temperature, wind speed, sky conditions and latitude
- Mixture Specifications – number of pavement layers, mix type, PG binder grade, layer thickness, mix delivery temperature and mix “stop” temperature (usually 175° F)
- Existing Material Surface – type of surface, state of moisture (frozen, wet, etc.), moisture content and temperature of the surface
Based on the value of these inputs, the MultiCool program calculates the time for the mix temperature to be reduced from the delivery temperature to the mix stop temperature.
The MultiCool temperature program was developed at the University of Minnesota and version V 2.0 was developed by Dr. David Timm and Ben Peters at Auburn University, with funding from the National Asphalt Pavement Association (NAPA). The MultiCool program can be downloaded from the NAPA website at https://www.asphaltpavement.org/multicool.
Please remember that the three, most-important factors that affect the ability of a contractor to obtain density in an asphalt concrete mixture when placed on the roadway are temperature, temperature and temperature.
Jim Scherocman, P.E., is a consulting engineer based in Cincinnati. For more information about cold weather paving, he is available by email, at email@example.com, or by telephone, at (513) 489-3338.