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 with shear loading at this area can lead to catastrophic bolt failure as shown in Figure 1. Therefore, the ultrasonic testing should really focus on a smaller testing range a few inches above and below the entry point.


Figure 3 shows some ultrasonic testing data on 48” long construction grade anchor bolts tested using a 2.25 MHz transducer. From left the right, the non-destructive testing data shows a narrow main bang, a reflection from cross-sectional area reduction, and a longitudinal wave reflection at 11” and multiple reflections from the bolt end at 48” . This ultrasonic testing A-scan is typical of a straight bolt embedded in concrete. Angled or J-bolts generally do no provide a measurable backwall from the ultrasonic testing.



Figure 3: Anchor bolt testing A-scan data.


It is usually more common to focus the ultrasonic inspection on the anchor bolt length adjacent to the concrete footer top surface. Example ultrasonic testing A-scan data is shown over condensed 8” range below in Figure 4. The main bang is relatively wide for the selected ultrasonic transducer, pulser voltage and dampening. However, the targeted bolt length was 2-8’ below the top and no significant ultrasonic reflections are observed in this area.



Figure 4: Anchor bolt ultrasonic testing A-scan data.


Ground Penetrating Radar (GPR) Inspection of Concrete Footer


The steel anchor bolt is an important part of the anchoring system but equally important is the concrete footer. Ground penetrating radar (GPR) of concrete footers is an excellent way to screen the structure for cracks, delaminations, and possible air voids, or gaps, between the concrete and anchor bolt. Ultrasonic testing and GPR testing are similar non-destructive testing techniques. The data for both methods is presented in amplitude versus time (A-scans), depth cross-sectional views (B-scans), and 2-D top views (C-scans). Where ultrasonic testing depends on acoustic impedance changes to generate internal reflections from material defects, GPR requires variations in material dielectric constants. The dielectric contrast is a descriptive number that indicates, among other things, how fast radar energy travels through a material. If we know what the dielectric of the concrete is, we can figure out how deep the target is because the dielectric tells us how fast the GPR energy is moving. The higher the dielectric, the slower the radar wave moves through the medium. The values range from 1 (air) to 81 (water). GPR energy moves through air at almost the speed of light. It moves though water at about 1/9 the speed of light. A dielectric of 3 to 12, is typical for construction materials, and corresponds to radar velocities from 7 to 3.5 inches per nanosecond, respectively.




Figure 5: GRP data from concrete and rebar.


GPR reflection polarity provided clues on the embedded target. In most antennas, the transmit pulse has a defined polarity. First a positive leading peak followed by a lagging negative peak. In a grayscale linescan, this looks like a white band followed by a black band.


Every reflection is a copy of the transmit pulse, so most of the GPR reflections start with white followed by black. However, a GPR phase inversion, or GPR polarity change, occurs at a concrete-air interface because of the low dielectric of air. A phase inversion is a mirror image of the normal polarity sequence.


So instead of the positive-negative-positive, or white-black-white peak, the phase inverted sequence is negative-positive-negative, or black-white-black. A concrete-air reflection starts with a negative (black) peak followed by a positive (white) peak – see the concrete bottom reflection in Figure 4. Similarly, the concrete to rebar, or bolt, interface will behave differently if the bond between these two materials deteriorates.


Summary

Ultrasonic testing and ground penetrating radar are two non-destructive testing methods that may be used to inspect the anchor bolts, concrete footers, and the quality of the bond between the two materials.

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