Salvaging an American Icon with Ground-Penetrating Radar

Two Route 66 signs on the road at California Mojave desert highway – By AR Pictures

Imagine being able to look straight into a wood beam and know its structural integrity.

It’s almost like a super power, except its really just amazing science—science that Forest Products Laboratory (FPL) researchers are practicing in order to be at the forefront of investigative timber safety and restoration.

During a recent study, “Ground-Penetrating Radar Investigation of Salvaged Timber Girders from Bridges Along Route 66 in California,” FPL’s Adam Senalik, James Wacker, and Xiping Wang along with colleagues from Jiangnan University School of IoT Engineering, were able to practice this amazing science on two bridges located on Route 66.

San Bernardino county is heavily invested in preserving key cultural and historical resources along Route 66. Approximately six miles east of Amboy, California in the Mojave Desert, there are two aged timber highway bridges in need of restoration. The Dola and Lanzit Ditch bridges were built between 1930 and 1940 using untreated timber from naturally durable species. But exposure to the extreme environment over the decades has caused damage to the structures and posed potential safety risks.

(a) Overview of the bridge structure, (b) underside view, (c) stringers, and (d) concrete deck with asphalt covering.

FPL scientists and colleagues were called in to assess the internal conditions of 18 Douglas fir timbers collected from the bridges. The assessments were done using nondestructive testing (NDT) techniques including acoustic wave, radar wave, ultrasonic, and micro-drilling. However, the focus NDT technique in their study was radar wave test methods.

Using ground-penetrating radar (GPR) on a wooden beam is like taking a picture of that beam’s insides with short bursts of electromagnetic energy from an antenna positioned on the surface of the wood. Scientists gather the two-way travel time and amplitude of reflected waves to create output images. This geophysical technique has two strategic advantages over other testing methods: data acquisition is fast and is easily viewable with high-resolution imaging.

Ground-penetrating radar test equipment: (a) probe and (b) data collection module.

In previous studies, GPR has been used to assess and evaluate bridges, structural beams, and even ancient timbers to find deterioration estimates, hidden defects, decay, foreign objects, and excessive moisture. Out of all the NDT techniques, GPR was found to be the most reliable method to detect and locate internal defects.

But how do scientists know GPR is accurate?

During a study, timber is gathered and tested using NDT techniques as well as visual assessments. Then data is collected using GPR. Finally, the wood is sawed in cross-sections to compare with GPR results. These tests showed that “GPR had high accuracy in detecting subsurface defects such as subsurface metals, knots, and decay in wooden logs.”

GPR was found to be a significant and invaluable testing method on the timbers gathered from the San Bernardino county Route 66 bridges. The GPR tests run on those 18 timbers will help to pave the way for future NDT restoration efforts.

FPL is proud to assist in the preservation of America’s heritage using the latest in scientific and testing methods. To find out more about the amazing advancements our scientists are making, visit the Forest Products Laboratory at: