With 24/7 global construction and ageing infrastructure, the need for inspection, maintenance, repair and rebuilding are constantly increasing.
Questions arising can be: Does this structure need maintenance work now or in a year? Can we remove concrete without damaging the rebar? Is there sufficient reinforcement to add another story or increase the load on this floor? Where do we have the main electrical wires or water pipes, where is it safe to core through the concrete?
The standard way for inspecting the interior of a concrete structure and its reinforcement has been, and still is in many places, cutting or drilling. The risk of furthering any potential structural weakness is sometimes huge and could incur a massive cost; a cost which may be more than just financial if a severe failure were to occur.
GPR (Ground Penetrating Radar) is a non-destructive investigation method that uses electromagnetic pulses to produce an image of the subsurface.
This can be achieved without drilling or otherwise impacting upon the structure’s integrity. Whilst also providing better data coverage compared to the more traditional point-by-point invasive tests, GPR is considered a cost–efficient investigation technique, with practically no risk of damaging or destroying the investigated material.
GPR can be compared to a fish-finder (or echolocation device) on a boat. A transmitter antenna sends a pulse into the ground, and that pulse reflects whenever there is a contrast in the dielectric properties of the medium it’s being sent into.
Such a contrast can be due to layers in the concrete construction or discrete features such as rebar, post-tension cables or voids. The reflected pulses are then collected by a receiver antenna, which will measure the time (which can be converted to depth) from when the pulse was transmitted to when a reflected signal was received. The receiver antenna will also record the amplitude of the reflected signals. The transmitter and receiver are often housed in a single antenna box for easy handling in the field but can also be housed in separate units. Different layers, as concrete–isolation interfaces and objects as rebars will give typical features in the resulting radargram.
There are several different MALÅ GPR systems for efficient concrete and rebar investigations, both single and multichannel array options. Depending on the type of application, the measurement technique is adjusted to be as smooth as possible.
Investigation can be done on both horizontal and vertical surfaces, where the antenna is swept over the areas to be investigated in a structured way. Single lines can be measured for layers or several lines for 3D results of more complicated structures. For floor investigations there are ergonomic handles available or the antenna can be placed in a cart.
Frequency is not the only factor that determines the achievable depth and resolution; the concrete through which the signal is sent also determines the quality of the collected data.
The depth and resolution that can be achieved with GPR depends on the frequency of the transmitted electromagnetic pulse. A higher frequency results in higher data resolution, whilst a lower frequency results in better depth penetration. What frequency to use depends on the application, and what resolution and depth is required. For concrete constructions and rebars frequencies around 750 MHz to 2.3 GHz are quite convenient.
|Antenna Frequency (MHz)||Suitable Target Size (m)||Approx. Depth* (m)|
Most often a moist, newly laid concrete is hard to investigate with GPR as the conductivity is too high and the electromagnetic pulses will be attenuated rapidly. If you have several layers of rebar, the top ones will most likely disturb the results from the lower ones.
There are a number of different applications where GPR can be most useful within the application area of concrete structures and rebar:
With MALÅ concrete and rebar solutions, work can be done efficiently, and the results presented directly on site. Rebar particularly are often clearly identifiable on screen in real-time and, with the easy ‘back-track’ option on the GPR system, the location of the rebar can be directly marked.
Direct marking on site. In this project concrete grouting was needed in older concrete walls and, to avoid drilling into the larger load-bearing reinforcement, GPR was used to mark these directly on the wall. Note! In the example raw GPR data are shown, with no processing, as it was only important to see the location of the larger reinforcement below the dense net of smaller bars.
MALÅ concrete and rebar solutions also provide efficient data collection and storage for further processing at site or back at the office. If rebar, and perhaps utilities (such as heating pipes), are organized in nets and possibly at several depths, for example, this is the most convenient way of working: data collection in systematic parallel lines in one or two directions for later analysis and detailed mapping. This can include 3D investigations with single channel or multi-channel array options.
GPR system for structural investigations
MALÅ CX is an easy-to-use GPR system designed for the non-destructive investigation and imaging of concrete and other man-made structures. With its simple user interface and functional software features, the MALÅ CX offers a unique, powerful choice for your high frequency GPR investigations.
The most versatile GPR system
on the market
Depending on the configuration and the antennas used, the MALÅ ProEx can perform various types of measurements, including standard reflection surveys, tomography, or velocity (CMP/WARR) measurements, for virtually any kind of application.
Outstanding data quality, exceptional performance and WiFi controller
MALÅ GX is an integrated GPR solution with a wide range of antenna options and measuring devices. Through unique HDR technology, MALÅ GX offers significantly faster data acquisition rates, with outstanding signal-to-noise ratio and depth penetration.
Efficient 3D GPR system for large scale mapping
MALÅ Imaging Radar Array (MIRA) is a true 3D system providing a cost-effective solution for large scale mapping and subsurface object identification. With a production rate of up to 50,000 m² per day, it is an efficient system that seamlessly integrates acquisition, processing, QA/QC, positioning data and export of interpreted results.