Checking CLT with Ground Penetrating Radar (GPR)

John WIlliams, RSK

November 2025 IRG Newsletter

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Why use Ground Penetrating Radar (GPR) to inspect Cross Laminated Timber (CLT)?

Checking CLT for trapped construction moisture is difficult and time consuming due to its layered structure, where internal layers are concealed by the outer ones. Moisture can penetrate through joints, cracks, or fastener holes, becoming trapped within the layers without visible surface signs: moreover, the joints can act as ‘moisture superhighways’ allowing water to penetrate deep into the panels.
Visual inspections can spot water staining and this is usually at the end grains of panels but does little to determine ‘how wet is wet and how far has it gone?’ Using insulated decay probes fitted to a moisture meter is an effective survey method. However, it can be time consuming and readings taken at different depths need to be taken in order to identify the wet layer. Moreover, the passage of moisture along the ‘moisture superhighways’ further complicates a time consuming process.
GPR is a nondestructive technique based on electromagnetic theory, using radio waves to assess low-loss dielectric materials like concrete, asphalt, and dry wood. Every material has its own dielectric constant and dry timber has a lower dielectric constant than wet wood.
GPR relies on transmitting an electromagnetic pulse through a transmitting antenna which is reflected back through the target material and the response captured by the receiving antenna. The speed of the reflection, displayed as a distortion in the electromagnetic pulse can be related to changes in moisture in materials. Dry wood has a comparatively lower dielectric constant than wetter wood and the wetter the wood then there more energy from the pulse is absorbed by the material.
The greatest potential for using GPR to inspect CLT buildings for trapped construction moisture is the speed at which inspections could be carried out in comparison to conventional moisture content measuring. The antennae are moved across the surface and anomalies may be marked and targeted for more detailed inspection at incremental depths using moisture probes. The ability to scan large areas rapidly is important for buildings with a large footprint as this can reduce potential delay in the construction programme.
Working with Proceq Screening Eagle, the University of Ljubljana Department of Wood Science, with assistance from RSK Limited - a UK based environmental engineering consultancy, has initiated a research programme to develop a GPR survey method that can accurately identify trapped construction moisture in CLT. The objective is to demonstrate that GPR may be used as a ‘first pass’ inspection tool to locate potential problem areas for targeted, more detailed inspection in order to reduce ease labour intensive and time consuming inspections focussed on drilling to measure moisture content.
Previous work has shown that CLT can differentiate between wet and dry wood. However, greater sensitivity is needed to identify timber at or just above fibre saturation point. The ‘problem’ is as follows: A geophysicist may be able to read the site data – but we need Wood Scientists to be able to do this.
We need to work with Proceq to fine tune a scanner so we can pick up changes in MC and have output that is easier to interpret on site. This may require shorter wavelength transmission BUT we risk getting too close to microwave output – not much point cooking the cell tissue of our clients.