Infrastructure

Effective use of scanning in structural investigations – understanding the capabilities

Fri 12/11/2021 - 15:28

Using scanning technology for structural investigation surveys has become commonplace these days. However, even the most experienced investigation planners often fail to explain to customers the pros and cons of the different equipment available and, more importantly, what data they will receive and the accuracy of the measurements.

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Joe Cufflin of SOCOTEC UK provides help for end users of the data to understand how and what to specify in terms of getting the information required for their design or repair.

There are two main types of scanner techniques used across the industry: electromagnetic and ground-penetrating radar (GPR). A number of different brands supply equipment that use these different techniques, such as Hilti, Proceq, GSSI, MALA, Radiodetection and many more. The majority of the brands mentioned are used across the various testing departments within SOCOTEC UK.

Both electromagnetic and GPR techniques have pros and cons, and are each more suitable to the different types of scanning required. Nevertheless, between the two scanners, the requirements of a client can be met.

GPR scanning – pros

There are many benefits to using a GPR scanner, the main advantage being the depth of scan. On average, a GPR scanner will allow you to achieve scans of up to two times deeper than other scanning equipment and techniques. GPR scanners can also detect non-metallic objects such as cavities, voids, services and honeycombing, so a more general observation of the material can be obtained.

GPR equipment tend to work using a scanning antenna, accompanied by a tablet, meaning that data caption is much more efficient and straightforward to use compared with other scanning equipment. Another advantage of working from a tablet linked with a GPR kit is that the imagery retrieved from the scan is of a much higher resolution. Figures 1–3 show examples below of scans taken from a survey completed by SOCOTEC UK.

GPR scanning – cons

Despite there being a number of benefits to using GPR equipment, there are also some disadvantages. When it comes to purchasing, GPR equipment is usually more costly. Prices vary within the different providers, but it can be up to 40% more than electromagnetic scanners from the same brand.  

A technical issue with the GPR can occur when using the device to measure the depth of concrete cover to the reinforcement. The accuracy of the equipment is also an issue when working to BS 1881:204(1). The accuracy required within BS 1881:204(1) is ±15% or ± 5mm, whichever is the greater, for reinforcement at covers of less than 100mm. From previous experience of using the GPR equipment, it cannot always provide results to this degree of accuracy, whereas the electromagnetic scanners can.

Although collecting data on the GPR scanners is easier, the interpretation of the raw data requires user knowledge and experience of interpreting GPR data. GPR works on different frequencies that are then returned to the antenna, different interpretations of the images are recorded (see example in Figure 4). To allow for a detailed processing of this data, additional software is required, which tends to come at an additional cost to the equipment itself.

Electromagnetic scanning – pros

This form of concrete scanning allows you to get a more accurate scan and image of reinforcement up to a depth of around 200mm, depending on the equipment being used. Estimates of reinforcement bar diameters can be obtained to depths of around 150mm, which is helpful if intrusive works are not permitted.

When completing non-destructive testing methods on-site, it is essential that the testing is completed to the correct Standard, ie, BS 1881:204 to ensure the accuracy of the results which is ±15% or ± 5mm, whichever is the greater, for reinforcement at covers of less than 100mm.

Electromagnetic equipment is also very easy to use as a general tool in comparison with the GPR equipment. The equipment set-up and user skill required is less onerous.

In comparison with the GPR, this form of equipment is much cheaper (see examples of electro-magnetic scans in Figure 5).

Electromagnetic scanning – cons

Electromagnetic scanning does have its limitations and when scanning into the subsurface of concrete, the equipment will only detect metallic objects. Non-metallic objects will not be shown on the scan image, but may affect the quality and accuracy of the final image shown.

When trying to retrieve the depth location of a single bar, the ease and accuracy of this process can be impacted if the surrounding reinforcement is congested. The scanning equipment may merge two bars that are in close proximity within the concrete into a single, larger bar, making it difficult to confirm the exact reinforcement required within the concrete.

Details of the first layer of the reinforcement can be obtained using this equipment. With this form of scanning, once the result has been returned from the first layer of reinforcement, nothing deeper will be collected. GPR scanning is able to penetrate past the first layer of reinforcement to keep collecting data.

Benefits

Each form of scanning offers its own benefits and can provide the data to ensure that the investigation requested from a client is undertaken and that the required results are confirmed.

In terms of which scanning technique is best for which results, GPR devices can scan more deeply and pick up all materials within the concrete rather than simply providing reinforcement and the benefit of higher resolution images. GPR equipment is therefore a good choice for scanning of concrete when investigating for services, voids and cavities.

Meanwhile, electromagnetic scanners are a more effective choice for generic scanning for reinforcement and reinforcement mapping (especially when the works are to be completed to a British Standard such as BS 1881:204).

Reference:

1. BRITISH STANDARDS INSTITUTION, BS 1881. Testing concrete. Part 204 – Recommendations on the use of electromagnetic covermeters. BSI, London, 1988.

 

[Figure captions]:

[Figure 1]: Front Layer 2D Scan from GPR Scanner

[Figure 2]: Cross Section 2D Scan from GPR Scanner

[Figure 3]: Front Layer 3D Scan from GPR Scanner

[Figure 4]: Cross Section Radargramfrom GPR Scanner

[Figure 5]: Front Layer 2D Scan from Electro-magnetic Scanner

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