The Three Musketeers Ride Again

This time they're rolling along with a little box to keep them in line

by John S. Ball III

Three MusketeersNo, this is not your typical bedtime story. Put the kids to bed and I'll tell you how to get compaction using the three different types of rollers - with the assistance of a little magic box better known as the nuclear density gauge.What is compaction? Simply put, it is the act of compressing a given volume of material into a smaller volume. In asphalt paving, compaction is usually accomplished by rolling with steel wheeled, pneumatic tired or vibratory rollers or a combination of those three.

How you achieve compaction depends on a combination of different variables.Some of these variables include mix design, temperature of the mix, speed of the paver, vibration from the screed, roller types being used, weight andwidth of the rollers, and amplitude and frequency of the rollers. How do you check whether you have compaction density or not? You check it with a nuclear density gauge. Nuclear gauges operate through a simple concept known as backscatter gauging. Gamma rays are emitted by a radioisotope source contained in the gauge body. The radiation travels out from the source and penetrates the pavement, where the gamma rays are scattered and absorbed. A counter in the device establishes the number of gamma rays that return, and those returning rays are proportional to the density. Before there were nuclear gauges, pavement core samples were the only source of information a contractor had to tell whether or not he was on the mark as far as density was concerned. The cores were taken from the cooled mat, transported back to the lab, broken down, and analyzed. Cores are still taken today - and the nuclear gauge is by no means the density gospel - but the gauge serves as a tool for both the contractor and the agency to determine whether the mat is in the compaction ballpark or not. The nuclear gauge has made the industry more aware of how to roll and which rolling patterns work best for a specific job. You've got $300,000worth of equipment on the road, and this mighty little yellow box controls the whole project. It controls cost and production. And,it does its job well if it is used properly. Crunching the compaction numbers When you start a job, you must have a good mix design. On your state's sample report form, you'll find the "job aim" category with minimums and maximums listed. The compaction numbers you report must fall into this range to be within density requirements. If your mix is out of tolerance on either the high or low end of the range, your mix design fails. And, you only get two shots at achieving tolerance, so your mix has to be right on the mark. The "slip number" listed on the form corresponds with the truck out of which the sample is taken. With this number, you can track exactly where that sample of mix will wind up on the road. The form will also document the time the sample was taken, the temperature at which it was taken, bulk specific gravity, and the maximum specific gravity. The maximum specific gravity is the number plugged into the nuclear density gauge to calibrate pavement density in the cores.

Calibrating the nuclear density gauge

The gauge is unique in that it has a nuclear power source. It must be calibrated with pavement cores from both the old pavement and the new mat. If the lift to be sampled is 1.5 inches (38 mm) deep, then the gauge should be set for a depth of 1.25 inches (32 mm). Otherwise, the rays will shoot through to the next lift and your reading will be skewed. There are many factors which can cause the gauge to falsify its readings. For instance, if there is a hump in the road, you may in reality have only a 1 inch (25 mm) lift, and this can create a calibration error in the gauge. If it is raining when you take a reading, water will fill the pores in the pavement, and the density gauge reading will be off. If your mix is a three-quarter inch (19 mm) stony mix, which does not have a uniform texture across the mat, the gauge will falsify its readings because the gamma rays will scatter all over the place.When the nuclear density gauge operator starts out in the morning, he or she must make sure the gauge has the power to calibrate correctly. The operator sets down a magnesium block 1 inch (25 mm) thick, and then a retainer, which fits under the gauge, which kicks the gauge 0.75 inches (19 mm) off the plate. A standard count is then taken to check for decay in the nuclear power source within the percent limit allowed. The gauge is calibrated on a regular basis to determine decay, which can also throw off the readings. With an incorrect reading, the mat can be overrolled, underrolled, or deteriorated in such a way that when the core is taken, there is no match with the gauge. And then you're back to square one. You never know where you are as far as density is concerned until you core.

How calibrations and cores are taken

The gauge timer can be set for different reading intervals. Normally, the timer is set for 60 seconds. To take a timed reading, the operator must "seat" the gauge on the pavement. He sets the gauge on the pavement with its handle facing toward the paver. He takes his hands on either side of it and pushes down on the gauge frame to make sure it doesn't rock. He pushes the handle down, pushes 'start' on the control panel, and the machine begins counting 60 seconds. To match the gauge to the core sample from a cooled mat, the timer must be set for 2 minutes. This ensures enough time for the gamma rays to go out and come back into the gauge. The gauge is then turned 180 degrees and another 2-minute reading is taken. The mat is then allowed to cool, and a core is taken in the center of where the gauge had been. Cores can only be taken when the mat is cool. Otherwise, you won't have a true core. The pavement must be well below 100° F (38° C). Cores are usually taken the night after paving or the following day. Sometimes ice is placed on the pavement in order to cool it enough to take a core. To take the core, a hole is drilled and the auger is run the depth of the lift. If the lift is 2 inches (50 mm), the auger should be run 2.5 inches (64 mm) to go into the next lift. The coring process should be done gently, and the core treated like a piece of glass. It may take as long as 5 minutes or so to remove the core from the pavement. Before a job is started, the nuclear density gauge is calibrated and cores are taken on the old pavement. The gauge is seated on the pavement, a 2 minute reading is taken, the gauge is repositioned, and another 2 minute reading is taken. Then the operator gets ready to match his readings to those of the core sample. The process is repeated on the finished mat and at least three cores are taken. Orchestrating the compaction process Once density is determined and the gauge is calibrated to the cores, the pavingand rolling processes are set up. This is the point at which it is decided what the three musketeers will do, and another special tool - a hand-heldn thermometer - comes into play.

Mix temperature is the most important factor in compaction. It affects the temperature of the mat, where the mat will be rolled - rolling zone - and how the mat will be rolled - rolling pattern. The temperature of the mat determines compactibility. If the hot mix leaves the plant at 325° F (165° C), what will its temperature be at the paver? Other compaction considerations include the width of the mat, the thickness of the mat, and the kind of mix being laid.The types and combinations of rollers affect compaction. Static, pneumatic and vibratory rollers are set up in a sequence so that density is achieved. The width of the drum also affects compaction. Will you use a 78 inch (1,950 mm) drum, a 66 inch (1,650 mm) drum, or a 54 inch (1,350 mm) drum? If the mat is 12 feet (3 meters) wide, a 78 inch (1,950 mm) drum means two passes; a 66 inch (1,650 mm), three passes, and a 54 inch (1,350 mm), four passes. The speed of the roller must also be determined. It must be within the confines of the mix. Will a 78 inch (1,950 mm) roller do it for you in breakdown? Where does the second roller come in? This is how you establish your rolling zone and rolling patterns. If you are compacting and you need 92 percent density, how much density are you getting out of the roller? How fast is the paver moving? How much compaction are you getting out of the screed? These numbers must be compared at the highest optimum mix temperature allowed. Roller speed and vibration are critical to the process as well. Typically, you get 74 to 80 percent compaction at the screed. The slower the paver goes production-wise, the more compaction you could pick up - as much as a half percent. And every bit helps when you're trying to make that bonus.

With the first roller, the object is to go for the highest compaction numbers possible. The mat is at 300° to 275° F (150° to 135° C) and the breakdown roller begins its work. The mat cools to 140° to 130° (60 to 55° C), and thesecond or intermediate roller is allowed on the mat. This way, you get another half percent or more in density. Nuclear density readings are taken at certain intervals: The first reading is taken immediately behind the paver and right before the rolling begins. Readings are taken for both the left and right sides of the mat. The first roller then makes its pass and another reading is taken. If you achieve the specified density at that point, your rolling pattern is established as two passes.

The third roller is the finish roller. It polishes up your mat and makes sure there are no cuts or marks on the mat surface. The finish roller usually begins compaction when the mat temperature is under 130° F (55° C), and a density gauge reading is taken after it has passed. The underlying base also affects compaction. You need to know what kind of base you're rolling over. Has a tack coat been applied to it so the mix won't shove? Continuous, consistent paving is the key. If the paver has no feet (meters) per minute gauge but the roller does, have the roller operator work at the same pace as the paver. That way the roller operator can gauge the paver's pace.

A final note: The person designated to operate the nuclear density gaugeshould know not only how to read the calibrations, but how to correct any problems on the mat. This person must be very observant of the paving process, so that he or she can direct the paver or roller operator to adjust their pace or pattern to correct the density readings, if necessary. A good nuclear density gauge operator will know exactly what to do in any given situation, so have faith in the person you've chosen for this position on your crew.

The three musketeers can ride confidently off into the sunset, with the nuclear density gauge watching their backs!

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