Introduction

Ground Penetrating Radar (GPR) is a geophysical inspection method used to scan the surface in many areas of application, including concrete structures such as buildings, dams and bridges. From this article, you will be able to understand through practical examples how GPR works in concrete structures, its main advantages and limitations.

Background

The main causes of the processes that damage the integrity of concrete structures are associated with project design, choice of construction materials, quality of execution (construction) and use during the structure's useful life.

To investigate the current condition of a structure, destructive and non-destructive methods are used. Although destructive methods can extract the necessary information from the structure, they are invasive and time-consuming procedures that can cause damage and influence the deterioration of the concrete. In addition, they often imply a reduction in the asset's availability for operation. In this way, Non-Destructive Testing (NDT) is an alternative that stands out because it allows inspection and evaluation to be carried out efficiently without damaging the structure or requiring it to be shut down.

The main NDTs used to investigate reinforced concrete structures are sonic/ultrasonic methods, infrared thermography, radiography and electromagnetic methods. The latter, which will be dealt with in this article, uses radar technology which is rapidly spreading among NDT methods in the field of structural engineering, including applications in reinforced concrete structures.

The Ground Penetrating Radar (GPR) method makes it possible to locate more than one or more layers of reinforcement embedded in the concrete at depths of up to 80 centimeters. This test is quick to carry out and, unlike pacometry, presents an image result, characteristics that make it a particularly interesting tool for:

• Structures (or areas of them) where design documentation is poor or non-existent (typically old);
• Structures in which it is necessary to check the as-built condition of the reinforcement against the design;
• Assist in carrying out other tests in which the configuration of the reinforcement is relevant.

Video 1, shown below, illustrates portability and speed of execution.

Video 1: Performing the GPR test. SOURCE: Kot Collection.

GPR in Concrete

GPR for concrete is a relatively new non-destructive testing method in Civil Engineering that allows objects inside concrete to be identified, visualized and mapped. It is a technique that has shown great progress in the last decade, especially in locating steel bars and estimating the cover layer of reinforced concrete.

The working principle of GPR is based on the emission of an electromagnetic wave towards the surface of the material by a radar with a fixed central frequency using one or more antennas. The signal then returns to the device and has characteristics determined by the material's properties.

Figure 1 shows the test and the corresponding signal output. In general, the receiver first registers a direct wave that propagates through the air from the transmitter to the receiver on the surface of the material. Next, the electromagnetic wave, which passes through the concrete until it reaches the material with different dielectric properties (reinforcement), is reflected and the receiver records the wave by performing image processing of the signal.

Figure 1: Example of a GPR test and corresponding signal output. SOURCE: Adapted from ACI 228.2R-13

With regard to regulatory standards, more specifically for concrete, ASTM D6087-08 covers in detail a series of GPR procedures for assessing the condition of bridge decks covered in asphalt concrete. ASTM D4748-15 prescribes the determination of sidewalk layer thickness and ASTM D6432-11 the investigation of the subsurface.

Advantages

• Reliability of results;
• Fast data collection compared to destructive tests tests;
• Fast results with high-resolution images;
• Suitable for external surveys;
• Useful information on the main structural elements.

Limitations

• Complexity in interpreting results;
• Limited depth range on the surface.

Figure 2 shows different ways of visualizing the results carried out by Kot Engenharia in the same area of a reinforced concrete structure.

Figure 2: GPR results on a reinforced concrete structure. SOURCE: Kot Collection.

Application example

GPR was used to assess the condition of a reinforced concrete base belonging to a belt conveyor. The aim was to verify and confirm the condition of the structure in relation to the initial design through the use of non-destructive tests.

The pacometry made it possible to locate the reinforcements and also to estimate the diameter of the bars and the concrete cover layer. On the other hand, the number of results (readings) was hampered by the limitations of the equipment and the adverse geometrical conditions found in the tested structure. Therefore, GPR was used as a complementary test in the reinforcement checking process.

The equipment used for the test was a Proceq GP8000 with stepped frequency continuous wave technology, a modulated frequency range of 200 - 4000 MHz and a penetration depth of up to 80 cm.

The results obtained by GPR (Figure 3) showed compliance with the project by identifying the first and second layer reinforcements on the faces where the test was carried out.

Figure 3: GPR test on reinforced concrete structure. SOURCE: Kot Collection.

Conclusion

The GPR non-destructive testing method enables efficient inspection and evaluation. As well as having other applications in Civil Engineering, it demonstrates satisfactory results in locating reinforcement and estimating its cover layer

Its ability to provide quick image results on the main structural elements in a short space of time demonstrates the versatility of this NDT.

Finally, it should be noted that, as it is a relatively new method with great advances, GPR, together with the ability to interpret its results, is a very valuable tool for assessing the condition of reinforced concrete structures. This test contributes to the sustainable development of Structural Integrity management, maintenance and quality control systems in buildings.

Kot Engenharia has a team of professionals qualified in Structural Integrity to develop the best engineering solutions. Contact our team for more information!

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