When a concrete element is suspected of honeycombing, cracking, or poor compaction, the obvious way to find out is to drill into it. Ultrasonic pulse velocity (UPV) testing offers another route: it sends a pulse of sound through the concrete and times how long it takes to arrive. Sound travels faster through dense, well-compacted concrete and slower through concrete containing voids, cracks, or honeycombing, so the travel time reveals a great deal about what is inside, without damaging the structure.
The Standards
UPV testing of concrete is covered by BS EN 12504-4, Testing concrete: Determination of ultrasonic pulse velocity, and by ASTM C597, Standard Test Method for Ultrasonic Pulse Velocity Through Concrete. ASTM C597 describes the method as applicable for assessing the uniformity and relative quality of concrete, indicating voids and cracks, evaluating the effectiveness of crack repairs, and estimating the severity of deterioration or cracking in structures.
How the Test Works
- Transducers. A transmitting transducer is held against one face of the concrete and a receiving transducer against another. A coupling gel is used so the pulse passes cleanly into and out of the surface.
- Transit time. The instrument sends a pulse of longitudinal vibrations and measures the time it takes to reach the receiver.
- Velocity. The path length between the transducers is divided by the transit time to give the pulse velocity.
Readings are normally taken on a grid across the element so that velocities can be compared from point to point.
Transducer Arrangements
- Direct transmission: transducers on opposite faces of the element, such as either side of a column or wall. This gives the most reliable readings because the path length is clearly defined.
- Semi-direct transmission: transducers on adjacent faces, for example around a corner.
- Indirect (surface) transmission: both transducers on the same face. Used where only one face is accessible, such as a slab soffit or a retaining wall, but the readings are less precise.
What UPV Is Good For
- Uniformity surveys: mapping velocity across a structure to find areas that are clearly weaker than the rest.
- Locating defects: honeycombing, voids, and cracks show up as zones of low velocity.
- Before and after repair: comparing velocities before and after crack injection or other repairs to judge whether the repair has filled the defect.
- Guiding coring: pointing to the best, worst, and most representative locations for concrete cores, so fewer cores give more useful information.
Limitations
- Not a direct strength test. Pulse velocity is influenced by aggregate type, moisture content, mix proportions, and age, not only by strength. Any conversion to compressive strength needs a correlation built from cores or cubes of the same concrete.
- Reinforcement running parallel to the pulse path can carry the pulse faster than the concrete around it and give misleadingly high readings.
- Surface condition affects the coupling: rough, coated, or damaged surfaces need preparation.
- Moisture: wet concrete generally gives higher velocities than dry concrete of the same quality.
UPV Alongside Other Tests
UPV is most useful in combination. Together with the rebound hammer, which measures surface hardness, it builds a picture of both the surface and the interior of an element. Cores taken at locations chosen from the UPV survey then give actual strength values that can calibrate the non-destructive results.
Note: UPV results should be interpreted by a qualified engineer, taking account of the concrete's mix, age, moisture condition, and reinforcement layout.
