CONCRETE STRUCTURES AND REBAR INVESTIGATIONS

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.

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 testsGPR is considered a costefficient 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.


Such a contrast can be due to layers in the concrete construction or discrete features such as rebar, post-tension cables or voidsThe 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 concreteisolation interfaces and objects as rebars will give typical features in the resulting radargram.

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 waySingle 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.

Finding the optimal frequency

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.

GPR antenna frequency and estimated resolution and depth

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)     
800/750 0.03 2.5
1200 0.02 2
1600 0.015 1
2300 0.01 0.85

 

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 solutionswork 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 officeIf 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 mappingThis can include 3D investigations with single channel or multi-channel array options.

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 depthsin socalled 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.

MALÅ Concrete Explorer

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.

 

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MALÅ Professional Explorer

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.

 

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MALÅ Ground Explorer (GX)

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.

 

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MALÅ MIRA

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.

 

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