NDT circumstances; ultrasonic pulse velocity test

Circumstances for NDT

• When the cube/cylinder results fail the acceptance criteria (IS 456, Cl. 16) or are doubtful.
• Suspected poor compaction, honeycombing, voids, or cracks in the member.
• Lack of quality-control records, or specimens that were not properly cured.
• Assessment of old structures for change of use, extension, and rehabilitation.
• Damage by fire, earthquake, impact, corrosion, chemical or freeze-thaw attack.
• Checking uniformity of concrete, early stripping time, and cover/rebar location.
• Monitoring strength gain or deterioration over time.

Ultrasonic pulse velocity (UPV) test (IS 13311 Part 1)

Principle: the velocity of an ultrasonic pulse through concrete depends on its density and elastic properties. Sound, dense concrete transmits the pulse faster than porous or cracked concrete.

Apparatus: pulse generator and receiver with two transducers (usually 54 kHz), a timer reading to 0.1 microsecond, a coupling gel, and a reference calibration bar.

Procedure

1. Calibrate the instrument on the reference bar.
2. Clean the surface and apply grease or gel to ensure good acoustic contact.
3. Put the transducers on the concrete, either directly (opposite faces, most reliable), semi-directly (adjacent faces) or indirectly (same face, least reliable).
4. Measure the path length $L$ and the transit time $T$.
5. Calculate the velocity:

$$V = \frac{L}{T}$$

Take several readings and average them. Avoid paths parallel to reinforcement, as steel carries the pulse faster.

Quality of concrete (IS 13311 Part 1)

Strength estimation: a correlation curve between $V$ and the compressive strength of cubes of the same mix (made and tested in the laboratory) is used. The accuracy is only about ±20%, because moisture, aggregate type, age and reinforcement also change $V$. Better accuracy is obtained by combining it with the rebound number (SONREB method).

Uses: uniformity of concrete, locating voids and cracks, depth of surface cracks, deterioration by fire, frost or chemical attack, and dynamic modulus.