Blackstone Measures Moisture Entering the Plant with Troxler’s MMS Model 3630
BY Sandy Lender
The problem that Blackstone Construction LLC, Russellville, Arkansas, has solved is one all asphalt facilities encounter: reading and reacting to moisture content fluctuations. Asphalt Production and Operations Manager Jerry Bowden explained that they’d installed the moisture measurement system (MMS™) model 3630 from Troxler Electronic Laboratories Inc., Research Triangle Park, North Carolina, at two of the company’s asphalt plants within the last three years, and they’ve seen mix design and performance efficiencies from the decision.
The MMS is a set of sensors that use nuclear technology to monitor composite moisture at continuous mix asphalt plants. Troxler installs it on the inclined conveyor/weigh bridge conveyor to receive real time moisture of the aggregates going into the drum through a noncontact “scan” of combined virgin aggregate. Knowing incoming moisture content allows for better binder control, burner temperature during drying, and optimized energy use, and Bowden shared how these efficiencies have come into play for Blackstone Construction.
Even when Arkansas has seen a dry spell, material stockpiles will have different moisture levels in them. “The materials that have gone through the wash plant have inconsistent moisture content,” Bowden offered as an example. When the weigh bridge tells the plant controls how much aggregate is coming in for the mix design, an incorrect reading due to fluctuating moisture contents will result in an incorrect liquid asphalt cement (AC) injection.
To set controls correctly, Bowden explained that the quality control/quality assurance (QC/QA) personnel take AC content readings and adjust its percentage every morning. In the past, that process could be arduous, especially if there had been inclement weather the night before.
Bowden explained: “In the past, it might take the lab up to two hours or more to get the moisture percentages from the stockpiles to the asphalt plant for adjustments and then another hour to get asphalt made to the lab for a check on the AC percent content in the asphalt. At that point, adjustments of moisture content at the plant would have to be made and run again until the AC percent content is right coming out of the plant. That process could take up to four hours to complete and then might need to be adjusted again depending on the moisture content consistency in the aggregate stockpiles.
“After the Troxler MMS system was installed, the plant operator can see in real time what the moisture content of the aggregate is as it goes into the drum, saving the time it took for the lab to get moisture contents from the stockpiles. The lab will still check the AC percent content coming out of the plant, but with the MMS system in place the plant operator usually gets the AC percent content within one tenth of a percent on the first attempt, which is well within the acceptable limits for state highway department jobs. The plant operator also can make adjustments as the aggregate material stockpiles change during the day’s production without any other employees needing to take moisture samples from the stockpiles.”
The value of using the moisture measurement system comes not only from getting consistency in the burner, but mostly from getting consistency in AC contents.
Make Life Easier for the Lab Tech
The MMS measures virgin aggregate on the inclined conveyor belt and provides the continuous moisture content reading to the plant’s control system. This data, when used in conjunction with weigh bridge data, allows the plant to make asphalt closer to the mix design. Blackstone elected to not have the system connect directly to the controls. For them, the data comes in from the MMS and is displayed in the control room for the operator. “We let the operator make the adjustment,” Bowden said.
This has worked out well for the team. Bowden explained that the tonnage of material to be reworked for state jobs has dropped to zero with this QC/QA tool in place.
“If the AC percent content is high or low from the moisture correction not being correct, it can result in a reduction for the amount you will get paid on a 3,000-ton lot of asphalt between 10 to 40 percent.”
For the state of Arkansas, he explained, each 3,000-ton lot has four sub-lots of 750 tons each, which must meet volumetric specs. If the contractor has a failing volumetric, such as AC percent content being too high or a core density being too low as a result of the AC percent content being off, the sub-lot would have to be milled up and replaced at the contractor’s expense.
“If you miss density in three sub-lots, you have to mill up all 3,000 tons of the lot,” he said. “Blackstone has gone from taking up as much as $250,000 in asphalt each year to taking up no asphalt each year after the Troxler MMS system was installed.”
Using the new QC/QA tool has also freed up an employee from the lab for additional tasks at the plant. “I had an employee working in the lab and all he did was test AC content,” Bowden said. “That was his fulltime job. Now that I have this system in place, I could move that employee to another area. That frees up a salary for another task.”
By installing a moisture sensing device at the weigh bridge to communicate real-time data to the control house, Blackstone Construction has given the plant operator a fighting chance at monitoring and regulating AC content throughout the day’s production. QC/QA has improved with tangible cost benefits.