Concrete Imaging and Scanning for Structural Assessment

Structural Assessment is an engineering process where the engineer, or well-trained inspector collects observations and data about the existing condition of the structure through systematic and scientific methods.

The devil is in the detail

Structural components can have physical damages and defects that are visible (sign of corrosion products in reinforced concrete elements, cracks, or delamination), of hidden defects and anomalies (microcracks in concrete affected by Alkali-Silica Reactions, early stage delamination). In this context, the assessment process becomes a combination of art, science, and engineering. Concrete Imagining and Scanning tools can help engineers during the assessment process.

What is Concrete Imaging and Scanning?  

In simple words, any inspection and testing solution that can expose sub-surface events, such as changes in the material properties, or material consistency and continuity can be used as imaging solution. Test method can either be a continuous scan (as in the case of ground penetrating radar), or discrete scan (as in the case of conventional half-cell corrosion potential mapping, or cover thickness measurement).

Concrete Imaging and Scanning help engineers and inspectors with identifying and quantifying sub-surface defects more efficiently. Both terms, identifying and quantifying are crucial in the assessment process, and help engineers evaluate the current condition of the structure. Moreover, as a structural engineer, we need to collect critical information about materials characteristics (mechanical and durability performance), and structural details (reinforcement size, distribution, cover thickness, crack depth, etc). The following is the most critical parameters in RC components:

• Estimate Concrete Strength
• Identify distribution and location of steel rebar, dwells, etc.
• Identify discontinuities in concrete slabs, walls, beams, and columns
• Estimate rebar size, and concrete cover thickness
• Detect and quantify deterioration mechanisms, such as corrosion and ASR

Structural Assessment is similar to solving a jigsaw puzzle. Different non-destructive testing methods have been developed for concrete imaging and scanning. These methods can help find the location of each piece, and get a better understanding of the overall condition of the structural system. In this article, I will review some of the most famous NDT methods for concrete imaging and scanning. Other test methods can easily be used in this context.

1. Ground Penetrating Radar – GPR

Ground penetrating radar (GPR) is a very useful technique for non-destructive concrete imaging and scanning. GPR uses pulsed electromagnetic radiation to scan concrete. 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. 

The main advantage of GPR method is the speed of test. Large areas can be scanned in a relatively short period. GPR can be used for scanning the location and distribution of steel reinforcement. The device can be used over existing coatings. While GPR can not directly detect delaminated areas on the concrete, it can indirectly identify potential delamination where moisture content varies.

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2. Ultrasonic Tomography

The Ultrasonic Tomography is a generally referrred to a group of ultrasonic testing methods that use ultrasonic waves (usually shear wave reflections) using an array of sending/reviving transducers and using mathematical models to reconstruct sub-surface events. Tomography methods have been developed and enhanced over the past few years. The method can now be used to locate flaws and voids in post-tensioning ducts, estimate the thickness of elements with one-side access (tunnel lining, abutment walls), and identify delamination and discontinuity of concrete (bridge decks, parking garage slabs)

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3. Half-Cell Corrosion Potential Mapping

Half Cell corrosion mapping is an effective scanning method for assessing the likelihood of corrosion activity in concrete structures. It is the most well-known procedure to identify the probability of active corrosion. The easy and cost-effective procedure of the test makes it popular among engineers and structure inspectors. The method has been used for bridge deck inspection, parking garages, corrosion monitoring of reinforced concrete beams, girders, and piers.

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4. Seismic Tomography

Seismic Tomography is a special test setup based on the very concept of Ultrasonic Wave Propagation in concrete. In this method, a number of receiving sensors are placed and fixed in the test locations, while the transmitter moves. The pulse generated by transmitter are received by receiving transducers. The pulse velocity at each trajectory is used to obtain a 2D or 3D seismic tomography contour maps from the condition of concrete structures. Seismic tomography is ideal for testing the large scale concrete elements, such as dams, lock structures, and mass foundations of turbines.