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Federal Highway Administration > Publications > Public Roads > Vol. 67 · No. 4 > Measuring Pavement Deflection at 55 MPH

Jan/Feb 2004
Vol. 67 · No. 4

Measuring Pavement Deflection at 55 MPH

by Max G. Grogg and Jim W. Hall

A field test shows that the rolling wheel deflectometer enables engineers to evaluate pavement while keeping pace with traffic.

A semitrailer truck equipped with a long beam which was mounted underneath the trailer on the passenger side.
To the casual observer, the rolling wheel deflectometer (RWD) looks like any semitrailer and truck. The long beam mounted underneath the trailer on the passenger side is the giveaway that this is no ordinary truck.

Imagine measuring the loadcarrying capacity of paved highways and detecting possible pavement weaknesses, while traveling with the traffic flow. No lane closures to collect data. No additional congestion. No traffic backups. No work zone safety issues.

How? The answer is one of today's most significant innovations in pavement management systems-the rolling wheel deflectometer (RWD). The benefit of the RWD is that it helps officials prioritize and target funding and projects to those segments of the highway network that need structural improvement and rehabilitation.

Highway engineers already have valuable and reliable tools for gauging surface conditions and structural capacities. A precursor to the rolling wheel deflectometer, the falling weight deflectometer (FWD) is the workhorse for measuring the stiffness of a designated section of a highway. The FWD is designed and used primarily for project-level analysis. Despite the integrity of its data, the FWD's requirement for stationary operation increases delays caused by closed lanes on busy roads, compromises safety of the traveling public and highway workers, and contains information limits for applications at the network level.

The Federal Highway Administration (FHWA) envisioned a process for gathering pavement deflection data at speeds that match the pace of traffic and that would operate continuously for hundreds of miles in a given day. At the end of a week, highway engineers would have a “structural map” of a major portion of a State's network—and be capable of testing the entire system, quickly and accurately.

“Technology is finally catching up with what our highway engineers need to know about the structural integrity of the whole network,” says Associate Administrator King W. Gee, head of FHWA's Office of Infrastructure. “The information we gain from the rolling wheel deflectometer will enable users to determine the remaining life of our highway pavements. It is hoped that it will then translate into proactive programming and strategic management of one of our Nation's greatest investments, our highway transportation infrastructure.”

In July 2003, FHWA tested the most recent version of the RWD on a network of highways in Texas.

Moving smoothly with the stream of traffic at a minimum of 88 kilometers per hour (55 miles per hour), the 16-meter (53-foot) semitrailer and truck collected more than 483 kilometers (300 miles) of network data. The Texas test clearly demonstrated that the RWD can do a job in one afternoon that would have taken days using stationary testing devices.

Frank Botelho, team leader in the FHWA Office of Asset Management, began this success story in 1996. “A part of research and development is conceiving better solutions and tools for existing and future problems,” Botelho says. “In theory, we knew that one day we would need a faster, more mobile solution. However, FHWA needed to develop the technology and a way to apply the technology before it would be useful for the transportation community.” Botelho initiated a research contract under the Small Business Administration's Small Business Innovative Research program.

Research Requirements

The first step in designing the RWD was to investigate whether the technology was already available in the market. Nearly a year's worth of research provided invaluable data that ultimately proved to be the starting point for the engineers. Several other countries-Australia, Denmark, England, and Sweden- and forward-looking States were on the same track, but no agency or company had delivered a fully operational, fast-moving piece of equipment that could collect data without interfering with the traffic flow.

With preliminary research complete, the next step was to design and build an RWD that would meet FHWA's requirements for testing equipment that would be fast, accurate, and safe. The prototype RWD design would need to measure the deflection of asphalt pavement, but not portland cement concrete (PCC) pavements. To measure PCC, additional research would be necessary.

The RWD also needed to supercede the results gained from nondestructive stationary simulations or slow-moving equipment. The RWD would need to measure the overall stiffness (or strength) of a pavement, known as “deflection response,” at the moment it receives the impact of a heavy weight, such as a heavy truck rolling over it. The depth and shape of the deflection provides engineers with information about the pavement's structural capacity. Although the FWD can depict the impact of a heavy truck weight, it must remain stationary on the pavement to take the measurement. The body of the RWD is a moving heavy truck, therefore, it collects data about heavy weight vehicles during impact in real time.

FHWA set the speed requirement for the RWD at approximately 88 km/h (55 mph). Sensor accuracy had to yield data that would correlate to the FWD and similar proven technologies. Vehicle design and software programming would address the remaining issues such as vehicle dynamics, surface texture, and road curvature and crown. The end product must be a working prototype, not simply a contribution to research data at hand.

Small Business Innovative Research Funding

A powerful piece of equipment, the falling weight deflectometer (FWD) provides critical data on a project level but must be moved from point to point to take structural capacity readings. Together, the FWD and RWD are a strong duo to help engineers determine structural capacity. Each year the U.S. Department of Transportation (USDOT) sets aside a percentage of its extramural budget to support an annual, highly competitive process for funding new research and technology undertaken by U.S. entrepreneurs in small businesses. The program, Small Business Innovative Research (SBIR), is run by the Small Business Administration. Since the SBIR began in 1982, thousands of small businesses have profited from an opportunity to participate in the investigative and startup phases of commercially viable products. To learn more, see PUBLIC ROADS November/December 2002 or visit www.sba.gov/sbir/indexsbir-sttr.html#sbir.

Design and Fabrication

Design and fabrication of the working prototype of the RWD did not happen overnight. Guided by early efforts, the engineers finally chose spot lasers to take the deflection measurements. The spot lasers were chosen because of their known accuracy in taking measurements without regard to speed. Using multiple lasers, a spot on the road could be measured before and during its deflection. A semitrailer was to carry a 7.8-meter (25.5-foot) aluminum beam specially designed to house four lasers spaced 2.6 meters (8.5 feet) apart. The beam was mounted on the right side of the semitrailer to follow the wheel path closest to the edge of the roadway, generally the weakest part of the pavement. The semitrailer by itself placed 8,172 kilograms (18,000 pounds) of dead weight over the rear axle, the maximum allowed with the current spacing between the dual tires. A computer system for receiving the data was located inside the trailer.

Information from the lasers downloads, in real time, into the computer. Spot lasers and rapid processing time allowed for a sample every 12.2 millimeters (0.48 inch) at 88 km/h (55 mph).

Data Comparison, Second RWD Road Test
Device RWD Accelerometer FWD
Dates tested July 25-26, 2002 July 25-26, 2002 July 12, 2002
Thick pavement mean deflection, mils 11.1 10.3 8.7
Thin pavement mean deflection, mils 12.8 13.3 14.0

Proof of Concept

The most critical step that followed the design and build phase of the RWD was the “proof of concept.” West of Champaign, IL, Staley Road was selected as the test site for the proof of concept. The road's pavement consisted of both thick and thin asphalt concrete sections. In the summer of 2002, the pavement was in fair condition; the surface had small amounts of cracking, rutting, and the weathering typical of a Midwestern road.

Best results are gained when a road is dry and warm, and the subgrade soil is not frozen- hence the summertime testing. The team prepared by first taking deflection data using the FWD. From those measurements, accelerometers were placed in the pavement at six locations, three in the thick asphalt concrete sections and three in the thin sections.

The proof-of-concept test was designed to correlate the RWD measurements with the corresponding FWD and accelerometers' measurements. An improved computer system allowed for complete data intake. For 3 days, the RWD made multiple passes at 48 to 88 km/h (30 to 55 mph), each time testing two 153-meter (500-foot) sections of the thick and thin pavement. In the end, the RWD data compared favorably with data retrieved from both the FWD and accelerometers.

Calibrating the Lasers

All four lasers must be “adjusted” to a perfectly level reference. In the calibration process used, the reference is obtained using a 7-meter (24-foot)-long water level.

The process is simple in design and effective in results. The researchers cut four short sections of 203- millimeter (8-inch) polyvinyl chloride (PVC) pipe, closed them on one end, and set them up vertically with the open end beneath the laser. These containers are connected by sections of water hose, and each container is filled with water to the same level. A very thin reflective float is placed on top of the water in each container. Each laser is tested to gauge the length of its beam to the reflective tile. Water is added or taken from the containers to ensure that the lasers are providing comparative readings as the depth of the water changes. Remarkably, the differences generally are only slight and are compensated for by using mathematical adjustments in the data processing software.

The falling weight deflectometer and rolling wheel deflector combination help engineers determine structural capacity.
A powerful piece of equipment, the falling weight deflectometer (FWD) provides critical data on a project level but must be moved from point to point to take structural capacity readings. Together, the FWD and RWD are a strong duo to help engineers determine structural capacity.

Ground Truthing

One factor became clear after the analysis of the 2002 data: The rearmost laser would yield better information if it were placed between the dual wheels so it would read the point of maximum deflection. The team had custom-designed rims built with aluminum spacers to provide 25.4 millimeters (1 inch) of clearance between the laser box and the dual wheels. With this enhancement and the continual upgrading of the software, the RWD was ready to hit the road once again in July 2003, this time in Texas.

The Texas Department of Transportation (TXDOT) arranged the field test to include comparisons between the RWD and three other independent data points-the FWD, a multidepth deflectometer (MDD), and a rolling dynamic deflectometer (RDD). The RDD, which was developed by the University of Texas, accurately measures deflection while moving at 2.4 km/h (1.5 mph).

The MDD was used for what engineers call “ground truthing.” Because an MDD sensor is deeply rooted 3.05 meters (10 feet) in the ground, it reads the absolute deflection at the point in the pavement where it is placed. To ensure an accurate comparison between the data from the MDD and the RWD, the lane where the MDD was placed was blocked off for the test. The pavement was visibly marked to assist the truck driver in positioning the lasers directly over the MDD sensors. Cameras were stationed on the roadside to capture the transverse location of the truck (RWD) as it drove over the MDD.

Laser aiming the beam to a reflective disk on the water.
The laser is calibrated by aiming the beam at a reflective disk floating on water. The remaining three lasers are tested simultaneously to identify any differences in the sensors' measurements.

After several passes over the MDD sensors, the Texas RWD test continued on three other types of pavement: weak, intermediate, and strong, such as found on farm-to-market roads and State highways. The Texas team was on the road with the RWD for nearly 7 days to capture quality information in multiple situations.

Initial analyses of the test data are promising and have generated excitement in State DOTs and in transportation agencies in other Nations. “Texas DOT has always been active in equipment development,” says Dr. Michael Murphy, manager of TXDOT's Pavements and Materials System Branch. “After the Texas test, we see the potential for the RWD to be a tool in our inventory that we don't have right now. We'll use it where it works best and expand on it as improvements are made.” Indeed, the team continues to envision future enhancements to the equipment and process, but the prototype has gone the distance and returned results that meet the goal set out in 1996.

Facts and Figures

Lasers: four lasers mounted 2.6 meters (8.5 feet) apart with rearmost laser placed 152 millimeters (6 inches) behind the axle centerline

Trailer: 16 meters (53 feet) to control for pitch and accommodate the long beam Aluminum beam: 51 millimeters (2 inches) wide x 216 millimeters (8.5 inches) high x 7.8 meters (25.5 feet) long

Surfaces tested: various types of asphalt concrete and asphalt surface treatment during summer months

Data collection: every 12.2 millimeters (0.48 inch) at 88 km/h (55 mph), averaged every 30.5 meters (100 feet)

Next Steps

The next step for the RWD technology is to move from research and conceptual testing into real-life deployment. FHWA is seeking State DOTs for further developments in this technology.

the truck wheels sit on a painted grid that indicates the location of the multidepth deflector (MDD) under the laser
Here the truck wheels sit on a painted grid that indicates the location of the multidepth deflector (MDD) under the laser.

Max G. Grogg, P.E., has been FHWA's lead engineer on the RWD project from its inception in 1996 to the present. He is a pavement and materials engineer and team leader for FHWA's Technical Programs Team in the Iowa Division. He has 18 years of service with FHWA in division, region, and headquarters offices, and previously worked for the Illinois DOT and a pavement consulting firm. He has been actively involved in the development of FHWA's RWD, implementation of products developed during the Long-Term Pavement Performance study (such as calibration of falling weight deflectometers and resilient modulus testing), implementing and improving pavement management systems, and measuring and improving ride on highways. He graduated from the University of Missouri-Rolla and holds a master's degree in pavement engineering from the University of Illinois. He is a licensed professional engineer in Virginia and has served on committees and task forces for the American Association of State Highway and Transportation Officials and Transportation Research Board.

Jim W. Hall, Ph.D., is the director of pavement engineering at Applied Research Associates, Inc. He is the principal investigator on the contract to build the RWD for FHWA and has more than 38 years of experience managing large research programs pertaining to highway and airfield pavements. He was a pioneer developer of nondestructive testing and developed early methodologies based on correlations of deflection response measurements to conventional methods for structural assessment for both flexible and rigid pavements. He has directed field, laboratory, and office projects to perform structural and functional evaluations of airfield and roadway pavements. His doctoral dissertation was based on pavement deflection response under moving wheel loads. He holds three degrees in civil engineering: a B.S. from Mississippi State University, an M.S. from Texas A&M University, and a Ph.D. from Auburn University.

Interested States should contact Max Grogg at 515-233-7306 or e-mail him at max.grogg@fhwa.dot.gov. The complete report on the RWD's Texas test, due January 2004, will be available to those who want to learn more. For more information on the Texas field test, contact Dr. Mike Murphy at 512-465-3586.

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