Using a standard van as a breadboard equipment platform, AFRL engineers employed two types of GPR antennas. They mounted air-launched antennas on the front of the van and installed ground-coupled antennas on the back of the van for dragging along the runway; both antenna types sent GPR signals to a display screen in the van. The GPR antennas send a highfrequency signal (900 MHz-2.2 GHz) through the pavement, and a highly accurate clock records the time that elapses between the transmitted and reflected signals. Differences in the dielectric properties of the subsoil materials produce the reflected signals, and signal processors calculate and indicate the depth of an anomaly based on the signal’s two-way travel time (transmitted and reflected). The system displays the results on an operator control unit for evaluation purposes.

The GPR testing conducted at Langley AFB suggested that airfield subsoil(particularly fill material near the storm drainpipe)was not adequately supporting the runway shoulder section. The AFRL team concluded that (1) the cracks in the concrete runway sections were likely due to a loss of underlying support caused by saturated soils, and (2) the saturation was a combined result of a raised water table (from previous rains) and drainpipe leakage. Subsequent ECP testing, which measured the strength and load-bearing capacity of the subsoil, supported the team’s conclusions. The ECP test process requires technicians to gain access to the subsoil. In this case, they did so by removing a 6 in. diameter cylinder of the runway concrete. The ECP equipment then pushes a 1 1/2 in. diameter instrumented rod into the soil and measures the resistance force as the rod penetrates (see figure). The ECP then calculates the California Bearing Ratio (CBR) from this collected data. Specimens from the Langley AFB runway shoulder section measured from 2 to 10 CBR, a range which indicates a weakened subgrade.

The AFRL team made several recommendations to the Langley engineers: (1) replace the damaged concrete shoulder slabs by lifting and replacing (as opposed to rubblizing); (2) examine underlying material (subsoil) for evidence of subgrade disturbance; and (3) if this inspection reveals subgrade disturbance, perform further excavation and visually inspect the storm drainpiping for possible leakage. One additional recommendation suggested that if shoulder slab inspection indicated subsoil deterioration had compromised the runway’s keel section, Langley personnel should begin runway remediation, incorporating foam injection or grouting techniques in the follow-up efforts.

To further advance runway inspection technologies, AFRL is currently collaborating with the University of Florida to couple Global Positioning System technology to the GPR system so that technicians can determine the exact coordinates of detected voids, anomalies, or buried objects. In addition, the team is refining the technologies to help users better distinguish between lime core and concrete, materials that have similar dielectric properties. The group is also exploring ways to simplify inspection data processing through improvements to system software. All of these efforts support the AFRL Airbase Technologies Division mission to provide rapid airfield assessment.

Mr. Gregg Hill, Ms. Melissa A. Lebrón, Mr. Gary Wagner, and Mr. Tim Anderl (Anteon Corporation), of the Air Force Research Laboratory’s Materials and Manufacturing Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp. Reference document ML-H-05-12.