Our GPR surveys provide a fast and efficient NDT means of locating tendon ducts, including depth, in pre-stressed or post-tensioned concrete. This is essential before any cutting, coring or drilling works are undertaken in the immediate vicinity.

What is post-tensioned concrete?

Post-tensioned concrete is reinforced with high-strength steel tendons, tensioned at high forces, putting the concrete under compression to improve its performance in service. Tendons may consist of single wires, multi-wire strands or threaded bars. As the name suggests, post-tensioning occurs after the concrete has been cast, and the compression force is primarily transferred to the concrete through the end anchorages. The technique is used in a wide range of buildings and civil structures. Its improved performance allows longer spans, reduced structural thicknesses, and material savings compared to ordinary reinforced concrete. Post-tensioned concrete structures typically deflect less and are less prone to cracking.

Because of the high tension in the tendons, post-tensioned structures should be proactively managed to avoid damage and deterioration of the tendons. Water ingress into a tendon duct can cause the tendon to corrode. No intrusive works, such as breakouts, cutting, coring or drilling, should be undertaken without first locating tendons in the vicinity.

Damage or deterioration of a tendon can result in a sudden and violent failure due to the high stresses involved.

Recommended further reading about post-tensioned and pre-stressed concrete in Wikipedia.

How can you identify if a concrete structure is post-tensioned?

There are three main methods of identifying whether a concrete structure is post-tensioned:

  1. If the design or as-built drawings are available, these will show if post-tensioned tendons are present and their layout.
  2. Visual inspection may reveal grouted end anchor plugs. The best places to look are slab edges; these may be exposed in walls, stairwells or balconies.
  3. A GPR survey offers an NDT method of determining if a concrete slab or other structural element is post-tensioned. It also has the added advantage of being able to identify the position of the post-tensioned tendons.
Post-tensioned concrete with protruding tendons visible in the slab edge.
A post-tensioned tendon protruding from a slab edge. GPR can be used to trace the position of tendons in concrete slab

Using GPR to locate tendons in concrete

Due to the highly stressed nature of tendons, it is important that they are not damaged, either by mechanical means or corrosion. For safety reasons, it is of paramount importance that tendons are accurately located prior to any cutting, coring or drilling works being undertaken in the immediate vicinity.

Ground Penetrating Radar (GPR) can find the location and depth of tendons or tendon ducts in concrete. The survey technique is nondestructive and noninvasive. GPR can also determine the tendon depth, enabling the hyperbolic geometry of the duct along the length of the element to be plotted.

Detection of tendon ducts in post-tensioned concrete can be very difficult. This is mainly due to the following:

  • The depth of the tendon
  • Often located behind at least one layer of reinforcement
  • The close proximity of reinforcing steel

This means that covermeters and the Ferroscan are usually not suitable for locating tendon ducts. GPR, however, offers deeper penetration and better resolution. By selectively scanning and processing GPR data to produce depth slices and pseudo-cross-section views, it is often possible to identify, locate, and map tendon ducts.

Site survey

How the survey is conducted on-site depends on the survey requirements and objectives. If the tendon position is only required locally (e.g. for a proposed floor penetration or for condition or integrity testing), a series of local scans collected locally in the desired location is generally sufficient. If tendons are to be located over a larger area (e.g. for design verification), the required area would be scanned on a regular orthogonal grid. Our concrete GPR floor slab surveys are particularly suitable.

Due to their better resolution, high-frequency antennas are generally best suited for this type of survey. Typically, we would use a GSSI SIR3000/4000 GPR control unit with an antenna in the 2600 MHz to 1600 MHz frequency range.


Tendon survey results can be marked up on-site or plotted in CAD drawings. However, due to possible processing enhancements, better results are obtained with off-site data analysis and interpretation. This is particularly true for deeper tendons or where reinforcement is present.

Need more information?

Contact us

Contact us to find out more about GPR tendon location surveys and how we can help you:

Tel: 0207 565 7056
Email: gpr@sandberg.co.uk

Information sheet

Download our information sheet about locating tendon ducts in post-tensioned concrete.

Are you wondering what else GPR can be used for? GPR surveys can investigate the subsurface and detect unseen features. Learn more about our most popular GPR surveys.