The State of Maine Department of Transportation is conducting pioneering research with regards to improving the quality of road pavements and extending service life. The state department is determined to improve both the material mixes that are used as well as how they are installed in the field.
One innovation is the use of GSSI’s PaveScan RDM which can measure whether the density is uniform throughout the pavement layer with great precision. Studies have revealed that optimal pavement density is a key factor in increasing pavement life.
Pilot tests on about a dozen projects show encouraging results. In fact, researchers are hopeful that this equipment will be an advanced and practical tool to aid contractors to make the necessary adjustments to guarantee excellent pavement quality.
State of Maine Uses a Variety of Methods to Make the Most of Their Road Paving Budgets
Each state is faced with its own road paving funding restrictions. The State of Maine Department of Transportation assigns a major segment of its budget to roadway and pavement improvements and thus needs to make the most of its resources.
The state developed a three-year $2.3 billion work plan; in 2018 the work incorporated 48 miles of highway construction and restoration at an estimated cost of $67.6 million; 356 miles of paving conservation evaluated at $93.8 million, and 600 miles of light capital paving at a cost in the region of $22 million.
Dale Peabody, Director of the Transportation Research Division at Maine DOT, says that there is always a need to find more funds to keep up with all the necessary road pavement work. That’s why he and his team are persistently on the lookout for ways to enhance both the material mixes used and how they are placed in the field.
On the mix side, Maine DOT employs a range of tools to test compacted asphalt mixtures, such as the Hamburg Wheel Tracking Device (HWTD), and the Asphalt Mixture Performance Tester (AMPT), a computer-controlled hydraulic testing machine that subjects samples to cyclic loading across a spectrum of temperatures and frequencies.
Recently, the department also started to use MiST (Moisture Induced Stress Tester), a new technique for testing moisture damage susceptibility of asphalt mixtures.
As for the materials themselves, Maine has carried out research on plant mixed recycled asphalt pavement (PMRAP) construction that utilizes asphalt emulsion and cement; hot in-place recycling (HIPR); and ultra-thin bounded wearing course (UTBWC) surface treatment, which as nationwide research indicates reduces deterioration.
They are also utilizing thin hot-mix asphalt (HMA) overlays to prolong the life of pavement still in serviceable shape. Laboratory performance test equipment is utilized to make sure the mixtures will be more durable and thus last longer.
They are also performing new lab tests that will be improved for the measurement of performance with regard to moisture. On the material lay down side, Maine DOT does as much as it can to ensure the material is properly applied in the field.
For example, the DOT stipulates that the equipment contractors must be in possession of materials transfer vehicles (MTVs), which ensure a more uniform, longer-lasting road surface. They also specify ride smoothness provisions to reduce on the undulations that would ultimately lead to road performance issues.
Also included are prerequisites for joints; thermal profiling with an infrared (IR) scanner; and use of intelligent compaction (IC) rollers, which enable real-time compaction monitoring and rapid modifications to the compaction process by integrating measurement, documentation and control systems.1
We began implementing quality control/quality assurance (QC/QA) practices years ago, and we have trained and certified technicians sampling material on the roadway.
Dale Peabody, Director of Transportation Research Division, Maine DOT
“It’s a continuous process, in which we have gone away from fixing the worst first to aggressive preservation practices. It’s just like how drivers don’t wait until a car breaks down before changing the oil,” he added.
Uniform Density Throughout the Pavement Layer is Critical
According to Maine DOT’s Peabody, optimizing pavement density is the critical component for increasing pavement life: “Optimum density reduces oxidation, reduces moisture damage, and decreases rutting potential. It also offers improved fatigue life and increased load bearing capacity.”
He refers to past studies that relate density to pavement life – the guideline declares that even a 1 percent decrease beneath minimum density creates a 10 percent loss of pavement life.2
Segregation is the enemy of density – and a major cause of premature pavement failure. Image Credit: Geophysical Survey Systems Inc.
Other density measurement methods offer limited sampling. Image Credit: Geophysical Survey Systems Inc.
Peabody has discovered that segregation (separation of the coarse aggregate particles in the mix from the rest of the mass) is the foe of density – and is a primary cause of premature pavement failures.
There are two kinds of segregation: mechanical (physical or gradation) and thermal. Unfortunately, segregation can only be identified visually and is a subjective value that can be problematic to quantify.
It may not even be clear to see at the time of construction, making it extremely complicated to enforce contractually. Previously, Maine has employed nuclear density gauges to quantify density but has backed away from using these gauges.
Contractors now employ a non-nuclear asphalt density gauge for QC purposes and cores are obtained and voids/density established in lab for acceptance. “The down side of these cores is limited sampling and not having real time values.”
National Highway Research Solutions Include Use of GPR
Extensive research and time have been invested into the issue of pavement density as part of the Federal Highway Administration’s Strategic Highway Research Program (SHRP2).3
In the end, SHRP2 produced two non-destructive methods for assessing asphalt pavements during construction: infrared thermal scanning and ground-penetrating radar (GPR), which utilizes electromagnetic wave reflection to “see” through materials.
According to SHRP2 studies, throughout the construction process, GPR can be employed to quantify uniformity and areas of potential defect in asphalt pavements.
Notably, it provides users with real-time testing of potentially 100 percent of the pavement area. This is in stark contrast to current density tests, in which conventional random sampling measures only about 0.003 percent of pavement area.
Widely accepted in numerous applications, GPR is a frequently used tool for utility location, measuring pavement thickness, and bridge deck deterioration. In recent decades, researchers have been evaluating whether it could be employed for effectively measuring pavement density, but they could never accomplish the level of accuracy that would justify the recommendation of its widespread use.
Previously, the use of GPR technique necessitated specialized equipment, a significant amount of data interpretation and numerous manual steps. In 2013, SHRP2 funded work was geared towards developing an efficient and operator-friendly GPR device that would transmit real-time profiling of asphalt mixture uniformity.
Conducted by Texas Transportation Institute (TTI) in collaboration with GSSI, the research expedited the development of the PaveScan RDM asphalt density assessment system technology.
The non-contact PaveScan RDM technology utilizes a sensor that generally outputs a measurement every half-foot along the lane traveled, so a mile’s worth of data incorporates around 10,000 measurements for each sensor used.
This system is perfect for revealing inconsistencies that arise when the paving process is already underway, these include poor uniformity and major variations in density. To determine pavement density, PaveScan RDM measures the dielectric properties of the asphalt surface.
The dielectric constant is a substance’s capacity to store electrical energy in an electric field. For instance, air dielectric is 1.00059; asphalt aggregate is between 3 to 6; while the dielectric of water is 80.
With new pavement, the mixture is uniform; dielectric variation primarily appears due to the percentage of air voids – which instantly relates to density. The measurement is predicated on the ratio of reflection from the asphalt surface to the reflection from a metal plate.
On the Road in Maine
Maine is one of several states that have conducted pilot studies using PaveScan RDM equipment. PaveScan RDM uses one or three 2 gigahertz (GHz) antennas mounted on a portable push cart that can scan a 6-foot width.
PaveScan RDM set up with 3 2 gigahertz (GHz) antennas mounted on a portable push cart. Image Credit: Geophysical Survey Systems Inc.
Each antenna collects a continuous line of dielectric/density. An onboard computer captures dielectric values, which can be correlated to core densities.
Operators scan a pavement section and the device identifies high-, low- and medium-density locations. They take a static reading directly over each location, obtain cores at each location and then test the cores, entering the results in the software. Correlation accuracy depends on obtaining core densities over the entire range of measured dielectric values.
Operators scanning pavement section with readings entered into software. Image Credit: Geophysical Survey Systems Inc.
Typical density profile shows data collected with PaveScan RDM three-sensor system. Image Credit: Geophysical Survey Systems Inc.
The figure below compares the calibration of dielectric to air voids for several pilot study locations.
Calibration of dielectric versus air voids at several pilot study locations. Image Credit: Geophysical Survey Systems Inc.
The first graph below shows the distribution of dielectric values collected at one location, showing curves with a desirable uniformity. By contrast, the second graph shows less uniform curves, which is not what Maine DOT was looking for.
Distribution of dielectric values. Image Credit: Geophysical Survey Systems Inc.
Maine DOT has collected data on about a dozen recent paving projects and found that the data aligns well with what they have seen in the field.
We were looking for a better way to ensure we’re achieving desired density ranges, since studies show that good density leads to improved service life. PaveScan RDM gives a much larger sample of the HMA mat and can be used as a QC/QA tool. The results to date have been really positive.
Dale Peabody, Director of Transportation Research Division, Maine DOT
It helped that Maine DOT had a crew that was already familiar with GPR, making it a relatively easy transition to get up to speed. The crew found the PaveScan RDM equipment easy to set up and use. The only down side they found is that the battery does not last long enough for a full day of data collection.
Other recent adjustments include a laser pointer that can be attached to the equipment to help operators align the scanning equipment, especially for use in night work. Operators found the extra lighting to be very helpful. However, Peabody notes that the technology does have some limitations.
It is affected by surface moisture, does not work as well when temperatures dip below 40 °F and can be affected by mix constituents, which may happen with a change in aggregate source.
Measurement accuracy for layers of less than 1-inch may be affected by the underlying layer, while layers that are 2.5 to 3- inches may be affected by density gradients within the layer. Maine DOT is also looking for further enhancements. For example, some users are adapting the technology for use with vehicle mounts. This issue is one that GSSI is actively pursuing.
Other items on the wish list are improved methods when implementing the technology for longitudinal joints; integration into the Veta intelligent construction software – a map-based tool for analyzing and viewing geospatial data; and enhanced data analysis of intelligent compaction, thermal profile, and GPR density data.
Peabody has shared the findings with New England DOT colleagues and presented the positive results from the pilot studies at a number of SHRP2 workshops.
He also presented the findings recently at the North East Asphalt User/Producer Group (NEAUPG), which targets improving the quality and performance of asphalt pavement applications across the Northeastern United States by promoting communication, knowledge, technology and uniform solutions.
Special Focus on Data Analysis
While not confined to this technology, one of the main factors influencing the decision whether Maine DOT would want to integrate the technology statewide is related to the issue of data analysis.
“Who is going to collect all this data?” asks Peabody. “If there are 20 projects, you can’t have just one crew collecting data. We have thought about asking the contractor to purchase the equipment and use it but we are not ready for that, because for it to work you have to have people who really understand the technology.”
All in all, Peabody believes it to be preferrable if the data acquisition could be performed by a moving vehicle; the vehicle could move between each project and collect data on the go. However, Peabody recognizes that this approach would have its drawbacks – data would not be delivered in real-time, meaning contractors would have to wait to make adjustments.
If they collect data but don’t look at it until the next day, that does not really give the contractor a chance to make adjustments if there are density issues. I’d like a way to share data with the project team in real time, other than only being able to show the display on the Toughpad.
Dale Peabody, Director of Transportation Research Division, Maine DOT
According to Peabody, the big question that needs to be answered is how to manage the data effectively and make near-real time changes during production, and to get reports quickly over to project personnel:
“Currently, there is no good way to get data in the right people’s hands so they can make adjustments on the fly. There is a lot of benefit to having that ability. They could collect data and then go back to specific points on the road. They could select locations of high and low dielectric/density and then cut a core there and send it to the lab to determine the actual density.”
GPR Surveys Exhibit Exceptional Correlation Between Dielectric and Air Voids
The PaveScan RDM surveys demonstrates excellent correlation between the dielectric value and the air void contents.
Maine DOT can use the surveys to rapidly determine and assess low density areas and to check the compaction consistency. While data management is a great effort, they believe the surveys will be extremely useful when combined with other new technologies, including intelligent compaction and pave IR.
- What Is Intelligent Compaction?, http://www.intelligentcompaction.com/learn/intelligentcompaction-fundamentals/what-is-intelligent-compaction/, retrieved 11/20/18.
- HMA Compaction Assessment Using GPR Rolling Density Meter, Rick Bradbury, MaineDOT, Shongtao Dai, MnDOT, Atlantic City, NJ, October 17, 2018.
- Strategic Highway Research Program, Transportation Research Board of the National Academies, Stephen Sebesta and Tom Scullion, Texas A&M Transportation Institute, The Texas pg. 8 A&M University System, College Station, Texas, © 2014 National Academy of Sciences. All rights reserved.
This information has been sourced, reviewed and adapted from materials provided by Geophysical Survey Systems Inc.
For more information on this source, please visit Geophysical Survey Systems Inc.