The use of radar on walls and floors

Engineering and Environmental Applications and Examples

Why use GPR on walls and floors

Archaeologists, architects and engineers often need to understand the make up of a wall or floor either as part of an archaeological or structural appraisal. One of the tools that can be used is high frequency radar. It is non-destructive and easy to deploy. A radar survey can detect voids, changes of fabric, discontinuities, de-lamination of surfacing or facing material as well as assessing structure thickness. The depth of cover and bar spacing of reinforcement steel can also be determined. Typical locations can range from housing, commercial and industrial buildings, civil engineering structures, churches and other historic buildings such as castles and manor houses.

A GPR survey being carried out over the interior walls of the clearstorey of a church to locate areas of water ingress using a 1.5GHz antenna.

How it works and is deployed

A radar system works by transmitting a pulse of radiowaves at a set central frequency into the material being examined. Frequently the transmitter and the receiver are combined for convenience into one antenna. The receiver is set up to scan for returns from the transmitter, normally 40 scans per metre. These scans produce a scrolling picture as the antenna is moved across the search area. The success of GPR relies on the variation in the dielectric constant of different materials in the transmission of radiowaves. These contrasts result in variations in the amplitudes of the radiowaves returning to the receiver as the signal passes from one material to another. These stronger 'bright' returns form anomalies that stand out from the average response and can be extracted onto plans to give an interpretation as to areas of interest. A high frequency antenna is normally used for wall and floor inspection. It has good resolution (20mm) with typically 0.75m to 1m depth of penetration. It is small and so easily manoeuvred. The area to be surveyed is normally gridded orthogonally at 0.2m or 0.25m centres which gives good coverage and allows detailed timeslice plots to be made with little interpolation.

A radargram of a concrete basement slab showing variation in cover to the top layer of reinforcement

CASE STUDY
St Nicholas Church Hurst near Reading

A general view of the east end of the church at Hurst showing the flint facing to the walls. Problems were being experienced with both structural stability and water ingress at the east end.

The east elevation scaffolded out to our specification ready for the radar survey

The east end exterior wall with a timeslice plot superimposed. The brighter areas are likely to be where the flint outer skin has become detached from the core

The analysis of the data includes the inspection of each radargram and abstracting features of interest. These are then correlated with the timeslice plots and the results presented using CAD together with an interpretation.

The interpretation of the radar survey above where areas of concern have been highlighted and recommendations made as to where further investigations should be concentrated.

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