Locating Tendons in Post-Tensioned Concrete

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

Post-tensioned or pre-stressed concrete is concrete, which has been tensioned or compressed in order to improve its performance in service. The compression is produced by the tensioning of “tendons” within the concrete. 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. The technique is used in a wide range of building and civil structures, where its improved performance can allow longer spans, reduced structural thicknesses, and material savings compared to ordinary reinforced concrete.?

Using GPR to locate post-tensioned tendons

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 non-destructive and non-invasive. 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 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 selective scanning and processing of GPR data to produce depth slices and pseudo-cross-section views, it is often possible to identify, locate and map tendon ducts.

Site survey

The way 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.

High-frequency antennas are generally best suited for this type of survey due to the better resolution they offer. Typically, we would use a GSSI SIR3000/4000 GPR control unit with an antenna in the 2.6GHz to 1.5GHz frequency range.

Deliverables

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