Gives opportunity to correct during laydown
By John Davis
Thermal segregation is defined as temperature differences in the hot mix asphalt (HMA) mat as it is placed and is a potential cause of premature failure in asphalt roads. It can cause a significant reduction in pavement life due to low density from cold spots in the HMA mat.
In the past, problems related to thermal segregation only became evident when pavement distresses developed when the road was already in service – too late to take corrective actions. But researchers at Texas Transportation Institute (TTI) have developed a method of detecting thermal segregation in the asphalt mat while it is being placed. The method is an infrared imaging system that monitors thermal variation in the HMA mat and provides a data collection and processing software package.
Monitoring the overall mat temperature of the mix being placed, and the uniformity of the temperature across the mat, is critical to achieving adequate density. The asphalt mix is compactable only when it is above a certain temperature (which can vary with mix type, placement thickness, etc.)
The system, called PAVE-IR, measures and records surface temperatures in real-time during the asphalt laydown and allows for corrective action as the mat is being placed. Temperature variations are displayed so problems can be immediately detected and corrected before compaction occurs.
The development of PAVE-IR was a result of a partnership between the TTI, the Texas Department of Transportation (TxDOT) and MOBA, a technology development company with headquarters in Germany and Fayetteville, Georgia. These three organizations worked together to create, develop and refine PAVE-IR.
TTI developed the original hardware and software and MOBA developed the prototype field equipment. TxDOT wrote the field specifications for PAVE-IR and provided the projects.
“We all three worked together to get things done,” says Steve Sebesta, Researcher for TTI. Sebesta says that TTI discovered that PAVE-IR could detect temperature changes as they were occurring. “We also discovered that the thermal profile relates to in-place density, and that cold spots affect density.”
TTI brought the concept of the infrared temperature bar to near maturity by 2007, designing it to collect, process and display thermal scans of a paving mat in real time. MOBA started working with TTI in 2008 and in 2009 put out their first version of a working PAVE-IR. The first contractor to use MOBA’s PAVE-IR was APAC in Dallas in December 2009.
The current field-ready PAVE-IR system uses two temperature bars attached to the rear of the paver. Each bar contains six infrared sensors. Two software programs handle PAVE-IR’s functions. PaveApp facilitates calibration functions and real-time data display and analysis. Pave Project Manager provides the tools for post-laydown analysis and report generation.
PAVE-IR can profile the entire pavement width, excluding pavement edges. It measures distance using a distance-measuring instrument (DMI) and is also equipped with a GPS. The system will collect, display, save and analyze temperature readings while in operation. It will determine the low and high temperatures within each profile and calculate levels of segregation according to the specification in use.
The system produces files of pavement temperatures for each day’s placement and a daily summary test report that identifies locations of thermal segregation. It will provide software capable of developing and analyzing thermal profiles for an entire project, and provide an operating system with at least one USB port to save test results.
On a TxDOT Type D mix placed on U.S. 82 in January 2005, a thermal survey revealed significant temperature differentials along the longitudinal profile. The HMA was PG 64-22 with a job mix formula content of 6.4 percent and a target field compaction of 91 to 95 percent density. The contractor was using end-dump trucks and a material transfer device (MTD). Data from 1,700 feet of pavement were collected in the westbound lane.
Based on the thermal survey, TxDOT personnel met with the contractor to talk about improving the quality and uniformity of the project through better control at the plant, use of a different model MTD and altered rolling patterns.
After the project resumed, collected data showed that the recommended changes significantly improved temperature and density uniformity. The recommended changes reduced the range of mat temperatures by over 50 percent. Without using PAVE-IR, the contractor and TxDOT would not have known the extent of temperature variability in the initial paving.
Washington and Texas
On a past project in the state of Washington, WSDOT (Washington DOT) paved 32 miles on Highway 12. Project evaluations concluded that thermal segregation contributed to pavement failure five years before the anticipated 20-year life. The estimated additional rehabilitation cost was $2.4 million. After using PAVE-IR , the density readings on the rehabilitated pavement were all in the 91.8 to 93.8 percentage range.
APAC of Beaumont, Texas, began using PAVE-IR more than two years ago on a one or two mile primary highway project in Jasper.
“TxDOT allowed us to use its PAVE-IR specification as an option,” says APAC’s Cal Kincaid. “We decided to use the specs, so we purchased an infrared imaging bar (IR-bar) from MOBA.
“Using the IR-bar was a big change from using the temperature gun which provided 10 to 20 points of information,” adds Kincaid. “Suddenly, instead of 20 thermal images, we had thousands of data points continuously. Now there was no waiting and wondering, we had an immediate fix on temperature variability.
“We realized we could communicate with one another during laydown and make adjustments in the mix, the haul intervals, or the rolling patterns. Instead of a limited view of temperature differences, we were able to see everything and make immediate adjustments.”
TTI, TxDOT and MOBA think that PAVE-IR has a bright future. Today, Texas is the only state transportation department that has an official PAVE-IR specification, but a number of states are doing demonstration projects with the system and several states are looking at writing a specification.
“As of April 2012,” says Jim Hedderich of MOBA, “there are 18 states that are interested in or doing pilot and demonstration projects prior to implementation of the technology.”
Washington State did three demo PAVE-IR projects last year and plans to do six more in 2012. They just completed the first one in March of this year. Minnesota has done four demo PAVE-IR projects and plans to do three more this year. Two Wisconsin asphalt pavers, Mathy Construction and Payne and Dolan Inc., currently have four PAVE-IR units in use. Wisconsin DOT is still testing the system.
Current users of the system are in accord that PAVE-IR can be a valuable tool in the paver’s toolbox. “Companies that are concerned with high quality will like it and use it,” says APAC’s Cal Kincaid. “It’s light years ahead of the temperature gun and spot checking. It’s a big improvement in one part of the paving process—temperature differences.”