APRIL 2014 | Younger Member Group/Public Outreach 11NEWSMapping the Density of New Asphalt Pavement with GPRby Kenneth R. Maser, President, and Adam Carmichael, Project Engineer, Infrasense, Inc.

Achieving the target density for asphalt pavement during construction is the single most important factor affecting lifetime performance. Over-compacted asphalt mixture can cause rutting, shoving and bleeding, while low asphalt densities allow water and air to penetrate into a pavement, increasing the risk of water damage, oxidation, raveling, and cracking. As a result, in-situ asphalt mixture density or air void content is routinely specified and measured, and often serves as a basis for construction pay factors.

Traditionally, density has been measured using laboratory measurements taken from core samples. However, cores provide data at only a limited number of random locations. Some organizations supplement cores with data from a nuclear density gauge, but this requires special licensing and is also limited in the amount of data provided. Several agencies have evaluated the effectiveness and practicality of using non-nuclear density gauges as a viable substitute to nuclear gauges, primarily for safety concerns and to eliminate costs associated with radioactive material licensing. These are also limited to a small number of test locations.

Extensive research completed in Finland in the late 1990s verified the ability to measure asphalt air void content with Ground Penetrating Radar (GPR). The air void values were obtained from GPR-based dielectric measurements using a small number of cores for calibration. The results showed a very strong correlation between the GPR-based air void measurements and the laboratory measurements (R2 = 0.92). Based on these results, the GPR method is now an accepted standard for asphalt quality control in Finland. The key aspect of the GPR density measurement is that it is continuous, and provides the basis for creating a density “map” of the entire paved area.

Subsequent studies to adapt this technology have been conducted by state DOT’s in Texas, Florida, and Minnesota. In the Florida study, GPR scans were collected on several state highway paving projects. The data was collected while driving down the new pavement at different offsets to that the full width was covered. The GPR survey equipment, manufactured by Geophysical Survey Systems, Inc., and the survey pattern, are shown in Figure 1.

After data collection, the data was processed on site using GPRQA,© and the resulting dielectric/density map was used to locate a small number of cores for laboratory calibration. Laboratory density data was determined at these locations, and correlated with the dielectric readings. A sample correlation is shown in Figure 2. The

correlation is very strong, with an R2 of 0.84, much stronger than measurements made at the same locations with a non-nuclear gauge. In the Minnesota study, two-lane sections were selected for evaluation. Figure 3 shows a sample dielectric/density map of a 300 foot long section of newly constructed pavement. The zero offset represents the seam between the two passes of the paver. As expected, this represents an area of

low density, as seen clearly from the map. The calibrating cores shown have been located to represent a range of densities, and the resulting calibration curve is shown in Figure 4.

In August of 2013, a paving project was carried out on 4 miles of Rt. 125 in Lee, NH. The project length was 4 miles, and the road consisted of 1 lane of traffic in each direction plus shoulders. The project was configured by NHDOT as a test of “intelligent compaction”, a system being promoted by the FHWA to improve the uniformity and quality of new asphalt pavement. The southern half of the project was to be paved using conventional means, and the northern half using intelligent compaction equipment. A GPR density scan was incorporated as part of this project. The scan covered the entire project length and width, and was carried out shortly after the

Figure 1A: FDOT Dual Antenna GPR Van

Figure 1B: Data Collection Pattern

Figure 2: Dielectric/Density Correlation (Florida SR 222)

Figure 4: Dielectric/Density Correlation for Figure 3

Figure 3: Dielectric/Density Map of TH13 (near Minneapolis)

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APRIL 2014 | Younger Member Group/Public Outreach 12NEWSNew Asphalt Pavementcontinued from page 11

paving was completed. The data was collected at normal driving speed in a series of passes, each at a selected offset from the edge of the pavement (Figure 5). A sample of the results of the GPR density scan is shown in Figure 6.

The sample results reveal clear density transitions between the end of day 2 and day 3 paving, and the start of day 5 paving. These transition lines can actually be seen on the pavement surface. What is not visually apparent, however, is that the entirety of day 5 (including area not shown) is, on average, lower in density than that for the first four days. Other observations from the dielectric/density map are the typical low density areas adjacent to the paving seams, and a large low density area in the SB shoulder (coinciding with an area where the material transfer vehicle was not used). Finally, a comparison between the observed density in the intelligent and non-intelligent compaction areas indicated no distinct difference.

The ability to map the density of a new asphalt pavement provides owners and consultants a means to better understand paving processes, not previously achievable with cores and nuclear gauges. Observations such as those made on the Lee project can lead to improved paving processes and ultimately longer lasting pavements. In addition, the GPR density values correlate very well with core densities, and the method shows

Figure 5: GPR Survey Line Layout for Rt. 125

BSCES Legislative Fellow Update from Beacon Hill—Adapting to New Professional Issuesby David Westerling, PE, PLS, PhD, 2013-14 BSCES Legislative Fellow

As professional engineers, we sometimes fall into a soft bunker of life where we are comfortable in staying put as the game goes on above us. Professional development gets

put on hold as perhaps some past accom-plishments and successful projects have provided some breathing room in our daily routine. But recently it became readily apparent that there are some significant issues facing our civil infrastructure that we really need to address.

At a climate change symposium last fall, a nationwide panel presented some city stories on climatic event preparedness. The symposium was an initial venture by the National Institute for Coastal and Harbor Infrastructure (NICHI). This symposium was reported in the December edition of BSCES News. As a follow-up to the symposium a review was made of one of the speakers. Cynthia Rosenzweig is a Senior Research Scientist at the NASA Goddard

Institute for Space Studies, where she heads the Climate Impacts Group. She is also Co-Chair of the New York City Panel on Climate Change, a body of experts convened by the mayor to advise the city on adaptation for its critical infrastructure.

In her paper, “Developing Coastal Adaption to Climate Change in the New York City Infrastructure-shed: Process, Approach, Tools, and Strategies”, she outlines many changes needed in transportation, water and waste infrastructure systems. The city is moving ahead with planning and improvements in these infrastructure systems to provide the City with a better response to climatic events such as Superstorm Sandy.

Recently I met with the Legislative Director in Senator Marc Pacheco’s office about a bill that was filed by the Senator in January. S 2028 is titled, “An Act providing for the establishment of a comprehensive adaptation management plan in response to climate change.”

The bill calls for: …the secretary of energy and environmental affairs and the secretary of public safety and security shall develop and draft and adopt a comprehensive adaptation management action plan.

This bill is a follow-up bill from recommendations in the Massachusetts Climate Change Adap-tation Report, September 2011.

The City of Boston is also gearing up for infrastructure improvements related to new threats from climactic events. The Boston Climate Action Plan (CAP) is moving from just a greenhouse gas emission control plan to a plan that incorporates infrastructure improvements in defense of coastal flooding events.

Feel free to visit these sites and learn about some of these new issues in civil engineering and infrastructure management. Bringing your expertise to the table on these issues may actually lead to professional development in new areas of civil engineering.

promise as a standard tool in the testing arsenal for quality assurance and acceptance testing of new pavement. One practical application would be to

use the GPR-based maps to target core locations to achieve a comprehensive range of density values, rather than using random core locations.

Figure 6: Sample Dielectric/Density Data —NH Route 125 near MP 25