In this article, we will review the application of GPR in bridge deck investigation. Investigating concrete bridge decks can be labor-intensive and time-consuming. Many investigation techniques have been developed over time to study defects in the decks. Corrosion of steel reinforcement is a major challenge in concrete decks, as it remains unnoticed until it is very late. Recent developments in the GPR devices, and post-processing algorithms make it a serious technique for bridge deck investigation.
Conventional Test Methods
Most traditional methods are only useful when the signs of defect and deterioration are visible. For example, cracking in the bridge deck, and delamination are the two famous signs of corrosion in concrete. Traditional test methods are visual inspection, sound test using hammer or chain, which are used to identify the location of delamination.
Corrosion investigation techniques such as Half-Cell potential mapping and corrosion rate monitoring are great techniques for studying the corrosion probability, and kinetics of corrosion; however, their practice often requires removing the asphalt cover, and getting access to the concrete cover, which is labor-intensive and time-consuming.
GPR in Bridge Deck Investigation
Ground penetrating radar (GPR) is a very useful technique for nondestructive evaluation of concrete. GPR uses pulsed electromagnetic radiation to scan concrete. It can be used to locate rebar, voids, and delamination in the depth of concrete deck. When it comes to testing the bridge decks, GPR has a great advantage as it can detect defects from the asphalt overlay. Sneed et al. reported that “GPR can be used to evaluate the condition of a concrete bridge deck with or without an asphalt or concrete overlay. GPR is currently the only non-destructive method that can be used to evaluate a concrete bridge deck with an asphalt overlay. The practice has been standardized by ASTM D6087, 2008.
What is GPR
Ground Penetrating Radar (GPR) was primarily developed and used in geophysics and geology for subsurface scanning and imaging. Since then, the method has been successfully adapted in many other disciplines including civil engineering. The multipurpose nature of GPR and its broad range of applications, make it distinct from existing NDT methods. Got good fortune in civil engineering market, GPR can detect subsurface events and objects using the electromagnetic radar impulse, ranging from 10 MHz to 3,000 MHz. The method has been standardized by ASTM D6432
Application of non-destructive testing (NDT) methods are growing in condition assessment of civil structures. NDT methods are commonly used to evaluate the strength of materials (e.g. strength of concrete, consolidation of soil), to detect/localize embedded objects (e.g. pipe and tube, cables, manhole tunnel), to detect symptoms and side effects of damage mechanisms (e.g. delamination, internal cracking), to map the bed rock or boundaries in multi-layer media (e.g. asphalt/concrete/soil), and to detect the thickness or length of a medium (e.g. pile length, tunnel lining).
How GPR works?
GPR consists of a transmitter antenna and a receiver antenna, and a signal processing unit. GPR emits electromagnetic pulses (radar pulses) with specific central frequency to scan the subsurface medium. The reflected waves from subsurface layers, and objects are captured by the receiver antenna. Depending on impulse frequency, GPR is able to detect the internal events and objects at different depths. When there is a need for high resolution scanning, antenna with high frequency is required; however, the depth of penetration will be limited to the very first few centimeters. Lower frequencies are required when the objective is to detect very deep subsurface events and objects.
Applications of GPR
GPR is used in a wide range of civil engineering projects. The versatility of this NDT method makes it quite unique among other NDT methods. GPR can easily be used in subsurface scanning on soil, bridge decks, or simply locating rebar, live conduits, or other utility ducts. The following briefly presents the most important applications of GPR in civil engineering area:
Concrete Scanning
- Reinforcement (steel rebar, prestressing tendons) locations
- Concrete component thickness
- Location of delamination, voids, and other hidden defects
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