While snow is already flying in some parts of North America, ambient temperatures are still right for paving and finishing up the season in other parts. There are some mix temperatures you’ll want to attain, and some you’ll want to avoid, to get a bonus-worthy mat. Of course, each temperature you consider depends on a host of variables. In the old days, contractors were advised to lay asphalt when ambient temperatures were 48 degrees Fahrenheit and rising. With the advent of warm-mix asphalt (WMA) designs, cooler ambient temperatures are not as detrimental to success as they once were. So let’s look at the constants.
When you heat the screed at the beginning of the shift, savvy consultants want you to get it to 300 degrees F to match the arriving mix temperature. Use a temperature gun to check many points (at least three) across the full width of the screed plates so you know the screed is heating evenly and none of the elements are malfunctioning.
Here’s a moving constant for you. The temperature of the mix coming from the plant should not vary more than 20 or 25 degrees from truck to truck. Use a stick thermometer to determine exact temperature of the delivered mix. The best hole in the truck bed to use for this is the one that is two-thirds of the way from the back. If the second truck to the job measures 300 degrees F, then you want to see each truckload after that measuring no less than 275 degrees F. (Remember that the first truck on the job might be delivering mix that was waiting in the silo cone, and it may be 10 to 20 degrees lower than the temperatures you’ll expect from the subsequent trucks.)
If there’s a sudden difference beyond 20 to 25 degrees, find out what that specific truck driver did differently during his route. If the truth is “nothing different,” then call the plant operator to find out what’s going on.
Another “constant range” that is no longer set in stone is the temperature of rolling zones. Now that we have WMA designs to consider, the idea of getting off the mat when it has cooled to the 160 to 180 degree F range is not always valid. Depending on stone structure, mix design, and additives/compaction aids, you may be able to roll in static mode when the mat temperature is cooling below 160 degrees F. That’s where creating a test strip and having a QC/QA tech with a density gauge double-check your intelligent compaction numbers is a valuable system to have in place. The tech helps you set up the perfect rolling train, complete with the temperatures to watch, when you place a test strip at headquarters.
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