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Combining Ultrasonic Testing (UT) and Ground Penetrating Radar (GPR) for Anchor Bolt Testing

Streetlights, traffic signal lighting, traffic signage, and support beams are often installed and secured using steel anchor bolts embedded in steel rebar reinforced concrete. Depending on the age of the system, the condition of construction grade bolts and the footer may affect the load bearing capacity of the system. In some cases, damage may be incurred, and a fitness-for-service assessment is required. In many instances the design and fabrication specifications are unavailable and the type of anchor bolt, concrete and rebar system are unknown. The article discusses how ultrasonic non-destructive testing (NDT) and ground penetrating radar (GPR) can be used to test anchor bolts and the concrete – rebar system.




Figure 1: Could anchor bolt non-destructive testing with ultrasonic or concrete footer testing with GRP have prevented this failure?


Anchor Bolt Testing

The goal of anchor bolt testing with ultrasound is to determine if there is any cross-sectional area (CSA) reduction present and/or transverse cracking at or around where the bolt enters the concrete. The anchor bolt is tested with ultrasound and the surrounding concrete is tested with ground penetrating radar (GPR). If the area reduction can be estimated by ultrasonic testing of the bolt, the remaining strength of the system may be estimated. Anchor bolt testing with ultrasound is often complicated because the type of anchor bolt is unknown prior to inspection. Anchor bolts may be straight, angled at 90 degrees, or a J-configuration. Full length coverage with the latter two is challenging with ultrasonic testing due to the change in geometry. Figure 1 shows the catastrophic failure of a parking lot pole light due to shear failure of multiple bolts just underneath the pole baseplate.




Figure 2: Anchor bolt testing using ultrasonic NDT and ground penetrating radar (GRP).


Figure 2 shows the cross-sectional view of a typical anchor bolt embedded in a concrete footer. The anchoring system consists of a series of bolts, washers, and baseplate. In this case, the specification calls for 2x the anchor bolt plus 1” above the base plate. A leveling nut is positioned underneath the baseplate. The spacing between the leveling nut and the concrete must be less than the bolt diameter. The design calls for minimum three inches of thread below grade. Knowledge of these design specifications prior to non-destructive testing will improve the results of ultrasonic testing and ground penetrating radar tests.


In most cases the top of the anchor bolt tested will be eroded, worn or mechanically damaged. Flat topping with disk grinder will be required. Flat topping the bolt head is required prior to ultrasonic testing to permit coupling of the ultrasonic waves into the bolt. Anchor bolt failure is most likely to occur near the concrete surface. This area is prone to corrosion and wear. Combining cross-sectional area reduction wit