“Let’s say that we’re making thousand foot pulls during a shift,” he explains. “One pull will take eight trucks worth of mix to complete.”
Bisnette’s calculation is based on 1,000 feet length, 12.5 feet width, and 2.5 inch loose depth. Each truck hauls roughly 24 tons.
“For that shift, I order 24 trucks,” Bisnette says. “The rule of thumb is to have three times the number of trucks that it takes to complete one pull. Then, we never start a pull until there are three trucks in front of the paver and at least two at the gate waiting to come in. Each truck has GPS so I know where the next three are located.”
Those five trucks will provide about 20 minutes of paving time. If the crew has to wait for more trucks due to traffic conditions or long waits at the security gate, what does the crew do before starting a pull?
“If we have to wait for trucks, we mark grades or we clean the paver,” Bisnette answers. The grid is laid out in 25-foot by 25-foot squares. Since paving width is 12.5 feet, the crew measures and marks the grades to reduce the grid width to match the paving width. Machine cleaning during the shift is important, too, because the polymer-modified mix is very sticky. End gates are constantly cleaned to assure that they’ll float and not hang up.
“We pave 35 feet per minute every pull and we don’t stop until the end of the pull,” Bisnette continues. “We made hundreds of pulls on this project and never stopped to wait for trucks. The only delay we had was the plant shut down during one shift. That was it.”
Constant paving thickness and constant paving speed were under control. What about the last two of Riley’s key elements — rolling patterns and mat temperature?
Planning the compaction process
Based on past experience with the polymer-modified P-401 WMA, the quality control team at Aggregate Industries knows that the mix passes under the screed with about 12% to 13% air voids. Their target, according to Riley, is to compact the mat to 5% air voids (allowable range 3.0 – 7.0%) and to compact the longitudinal joints to 8% air voids (allowable range 3.0 – 10.0%).
Concerning mat temperatures, Riley outlines their targets phase by phase. “We get our best results for a smooth, dense and uniform mat when the WMA passes under the screed at about 260° F. We try to keep our breakdown roller working in the range of 245° to 265° F. The density target is 8% in-place air voids for the breakdown phase.”
For the breakdown phase, Aggregate Industries selected a Cat CB54 asphalt compactor equipped with VersaVibe vibratory system. On the test strip, the quality control team verified that two repeat passes with the vibratory system set on high frequency (3800 vpm / 63.3 Hz) and a medium / low amplitude (0.019” / 0.48 mm) produced in-place air voids in the 7% to 8% range.
The Cat CB54 has 67-inch-wide drums and can cover the 12.5-foot paving width in three overlapping passes. The result was a seven-pass pattern. The working speed was calculated and set at 290 fpm. That working speed and pattern precisely matches the paving speed, assuming a 90% efficiency rate for the compactor to account for non-vibratory stops and reverses. Therefore, the breakdown compactor works in a consistent temperature zone.
“We selected the CB54 for the breakdown roller for two reasons,” adds Riley. “First, we’re familiar with the VersaVibe system and like the dual frequencies with four amplitudes. Second, it’s the newest roller in our fleet and we want to get the best reliability on the project.”
Riley goes on to explain the pattern that works best for this WMA mixture.
“Standard practice for the breakdown roller is to make the first pass (in vibratory mode) approximately 6 - 8” (15-20 cm) off the joint on the hot mat and then come back over the joint on the proceeding pass with about 4” (10 cm) overlap onto the cold mat,” he explains.