Ground-Penetrating Radar (GPR) has revolutionized the way we inspect concrete structures. It is a powerful tool for locating potential structural damage and identifying other hazards that may be hidden beneath the surface. GPR can be used in many applications, and its efficiency during winter months is particularly noteworthy. This article will discuss the effectiveness of GPR for concrete inspection during winter months and explore practical ways to ensure maximum accuracy of results.
Ground-Penetrating Radar Technology
Ground-penetrating radar (GPR) is a non-destructive geophysical technique based on the propagation and reflection of electromagnetic waves from 20 MHz to 3 GHz. It is sensitive to changes in the electromagnetic properties of the medium (permittivity, conductivity and magnetic susceptibility). The search is generally carried out by moving the antenna on the surface, with or without contact with the ground. Measurements are taken at regular intervals, allowing a rapid image of the structure of the subsoil.
The GPR records the arrival times of the different waves and adds them up as it progresses along the object studied.
In a last step, the recorded arrival times are converted into distances using different wave propagation speeds. These propagation velocities are calibrated locally using measurements.
Radar technology, originally developed in the 1960s, is now used in geology (detection of bedrock, specific geological formations, cracks, karst phenomena, etc.), archeology (mapping of buried sites), hydrogeology (deep groundwater, detection of contaminated areas), and structures (inspections of concrete structures, roads, railways and other underground structures). During road tests, GPR has proved its worth by evaluating the thickness of the various layers or the quality of the coating (presence of voids, etc.). This technique also makes it possible to visualize the deviations of the structure of the road (identification of homogeneous zones) and thus to orient the installation of boreholes or recognition trenches.
Combined with the deflection and core measurements, it improves the evaluation of the moduli of the different layers by inverse calculation thanks to a better estimation of their thickness and their spatial distribution.
A good command of radar technology is important for the success of measurement campaigns. Thus, careful selection of radar and antenna type (including frequency range or center frequency) can maximize the quality of the data obtained. Likewise, the choice of measurement parameters, then the processing and interpretation of the data, are all factors that require a certain user experience.
How It Works
Most georadars operate over time, emitting very short electromagnetic pulses (of the order of one nanosecond) and recording the reflected signal as a function of time. The corresponding signal has a wide frequency band in the frequency range. Other types of radar (with frequency step) transmit continuous sinusoidal waves for different frequencies, then possibly reconstruct the signal in the time domain using an inverse Fourier transform.
Radars use materials based on the reflection of electromagnetic waves at the interface of materials of different nature. They allow bidirectional positioning with a direct graphical representation in the X-Z plane for a given Y section. They do not determine the diameter of the stiffeners. Depending on the type and the central frequency of the antenna used, as well as the state of the water and the diameter of the internal reinforcements sought in the concrete, the detection depth may reach about 600 mm or even 900 mm if “low frequency” antennas are used, and if the absence of a layer of stiffeners between the siding and the stiffeners required exists.
The more the central frequency of the antenna decreases, the more the investigation depth increases, but also the vertical resolution (minimum distance for detecting two parallel “interfaces” without interlacing the radar signal), and vice versa. For example, a 12 mm diameter armature can be detected by a 900 MHz center frequency antenna, but in the case of a bed close to the cladding, the characteristic signal of the armature will be “encoded” in the surface echo while the “high frequency” antennas offer better separation.
The estimation of the depth requires knowledge of the electromagnetic properties (in particular the electrical conductivity and the dielectric permeability) of the concrete. This requires the calibration of the dielectric constant and, therefore, also of the speed of the radar waves. As such, it is not recommended to use a predefined value of radar wave velocity in concrete to interpret radar charts.
Two calibration methods are generally used. For an element of known thickness, the propagation time of the radar waves allows an average rate of climb. It is strongly recommended to find an element whose thickness can be measured directly. If this does not occur, the drawings provided by the project manager should be used. In case of doubt, it improves the precision of the in-depth calibration on the cores taken from the structure.
Calibration of the radar signal after processing makes it possible to estimate the average speed of the radar waves between the surface and the stiffeners detected, assuming that these stiffeners can be considered “point objects” (in the case of isolated images, when the movement of the radar antenna is perpendicular to these frames). It should be noted that internal water gradients significantly alter the speed of radar waves, and this speed may differ between surface concrete and deeper concrete. It is therefore necessary to attach oneself to “point objects” located at approximately the same depth as the search objects (in the case of the detection of reinforcement, they are generally deposited on the search objects, therefore in practice this is not a problem).
Radar images provide much more information than the pachometric method and at a greater depth (50/60 cm or more depending on the antennas used). Indeed, it is possible to measure thicknesses, locate ducts, cables, voids, separations, heterogeneities, etc.
So if you want direct results via a real-time diagram of your structures without having to interpret the data, you need GPR. This enables real-time location of your targets and X-ray imaging. This acquisition displays high resolution 2D and 3D images for better visualization of structures. The antenna allows inspection of complex areas with vertical and horizontal resolution. The device is equipped with a laser to locate and mark targets with an emergency cursor.