Finding grout voids inside PT ducts

The slab was cast, stressed and grouted, and the grouting inside the post-tensioning ducts could not be verified. Handover was held up. An ungrouted length leaves prestressing strand without bond and without alkaline protection.

The duct is the brightest reflector in the slab, so its inside sits in its own shadow. A void is not a new object. It is missing grout in a space the strongest echo already occupies. Brightness alone cannot separate the two, and automatic detection marks the duct either way.

We read the sign of the reflection instead of its brightness. Grout and water invert the wavelet. Air does not. Working in expert mode with the phase kept intact, on a registered grid along each tendon, an air filled length showed a reversed polarity echo at the duct crown, ringing repeats, and reflectors below pulled upward because the pulse crosses air faster.

Void zones marked on the slab with their extent along each duct, in time for the client handover date, with no exploratory breaking of a stressed slab.

The HILTI PS 1000 X-Scan is a three antenna ground penetrating radar system that produces plan view images of concrete, not the single line trace of a covermeter or a rebar scanner. It records a 600 by 600 mm image scan grid in a single set up, extendable to 1200 mm by 1200 mm for larger elements, and detects to a stated maximum of 300 mm in concrete. Default settings assume dry, ordinary concrete, so we acquire in expert mode with the phase of the signal preserved and calibrate the dielectric constant on the element in front of us, processed in PROFIS Detection together with our own MATLAB routines.

Enquiry. Send the element type, thickness, exposure condition and what you need to locate on WhatsApp. Photographs, drawings and any previous scan output help us assess it fast.

Plan. We check access, moisture, exposure and reinforcement congestion against your target, confirm radar is the right test, and schedule the visit.

Scan and mark. Calibrated on the element, scanned in an orthogonal grid, and marked directly on the surface for your coring or cutting crew, same visit.

Report, if needed. Depth slices, plan extents and permissible core diameters, issued as a written report when your project requires formal documentation.