Description
Cost-effective and safe investigations
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.
Methods
Using GPR to investigate conrete and rebars
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.
Locate Targets in Concrete with accuracy
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.
Choosing the best solution
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.
Using the right technique
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.
GPR antenna frequency and estimated resolution and depth
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) |
| 800/750 | 0.03 | 2.5 |
| 1200 | 0.02 | 2 |
| 1600 | 0.015 | 1 |
| 2300 | 0.01 | 0.85 |
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.
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.
Different GPR applications for concrete and associated structures
There are a number of different applications where GPR can be most useful within the application area of concrete structures and rebar:
- Mapping of rebar, reinforcement, tendons etc.
- Detecting conduits such as heating pipes and other utilities within the concrete structure
- Recording concrete thickness
- Mapping of layers in the concrete such as isolation, sandwich elements etc.
- Detection of cracks and voids
- Mapping of water saturation/moisture and corrosion
Projects marked directly on site – “mark out”
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.
Projects analysed after the survey – “post-processing”
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.
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.
Investigating smaller structures
In the example a smaller area has been investigated with parallel lines in two directions. The processing is made in the data collection unit or in a computer. The result is displayed as a 3D volume, showing the investigated area at different depths, in so–called times slices. Note that one layer consists of larger but sparsely situated bars and the other layer of smaller more densely placed bars
Investigating larger structures
When larger concrete structures (for example bridge decks, concrete roads etc.) need to be investigated, a multichannel array solution such as MALÅ MIRA is convenient. In the example the internal rebar of a bridge deck is shown, running along the bridge at 8 cm depth and at 17 cm depth across the bridge.
