The Capabilities of Scanning in Structural Investigations

Scanning technology can be a valuable tool in determining the integrity of a structure. The use of scanning techniques allows for a detailed analysis of the structure and its surroundings, such as underground utilities. This analysis can be used to identify flaws, defects, and other hazards that may not be visible to the naked eye.

Scanning technology is also useful to monitor the condition of a structure over time, allowing for preventative maintenance and repairs to be carried out before serious problems occur. Taking advantage of scanning technology, engineers and architects can ensure the safety and reliability of the structures they design and build.

The use of scanning technology for structural investigation surveys has become increasingly common in recent years. Many investigation planners, even those with extensive experience, don't explain the pros and cons of the different equipment available to their clients. It is also unclear what data will be received and how accurate the measurements will be.

Joe Cufflin, project manager for SOCOTEC UK's Structures & Pavements team, discusses different scanning technologies in structural investigations. This blog aims to provide you with guidance on selecting the most suitable scanning technique for your next project.

What are the main types of scanning techniques?

There are two main types of scanning techniques used across the industry: electromagnetic scanning and ground-penetrating radar (GPR).

A number of different brands supply equipment that uses these different techniques, such as Hilti, Proceq, GSSI, MALA and Radiodetection. The majority of the brands mentioned are used across the various testing departments here at SOCOTEC UK.

Both electromagnetic and GPR techniques have pros and cons. When considering which technique to use, this depends on the different applications of scanning required. Between the two scanners, the requirements of a client can be met.

The benefits of GPS scanning

There are many benefits to using a GPR scanner. The main advantage is the depth of the scan. On average, a GPR scanner allows you to achieve scans up to two times deeper than other scanning equipment and techniques. Moreover, GPR scanners can detect non-metallic objects, such as cavities, voids, services, and honeycombing, which can provide a more general image of the material.

GPR equipment tends to work using a scanning antenna, accompanied by a tablet. This means that data captioning 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.

GPS scanning - what are the cons?

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

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

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

The pros of electromagnetic scanning

This form of concrete scanning allows you to get a more accurate scan and image of reinforcement up to a depth of around 200mm. This is dependent 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 greater, for reinforcement at covers of less than 100mm.

Compared to GPR, electromagnetic equipment is also very simple to use as a general tool. The equipment setup and user skills required are less onerous.

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

What are the limitations of electromagnetic scans?

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 scanned 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. This makes it difficult to determine the exact reinforcement required in the concrete.

Details of the first layer of 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 begin collecting data.

Which type of structural scanning should I use? 

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

In terms of which scanning technique is most appropriate for which results, GPR devices can scan more deeply and pick up all materials within the concrete. This is rather than simply providing reinforcement and the benefit of higher-resolution images. GPR equipment is therefore a suitable choice for scanning concrete when investigating 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).