Ultrasonic pulse velocity (UPV) test for hardened concrete ?

We use ultra sonic pulse test (UPV) to check the quality of concrete. Concrete construction is generally gives trouble free service throughout its intended design life. However, we require these test , because of structural deficiency, material deterioration. The most common being when we fail to follow the construction specification or when the facility is expose to harsher service environment than the planning and design stages.

UPV test : In this the strength and quality of concrete or rock is assessed by measuring the velocity of an ultrasonic pulse passing through a concrete structure or natural rock formation.

Physical damage can also arise from fire, explosion as well as from restraints, both internal and external against structural movement. Except in extreme cases, most of the structures require restoration to meet its functional requirement by appropriate repair techniques.

Check this post to have an idea about "different tests for hardened concrete"....

Equipment description of UPV

• The test equipment must provide a means of generating a pulse.
• Then should transmit the pulse on to the concrete - Transducers
• Should have a receiver and amplifier for the pulse
• Should measure and display the time taken.

Time measurement is based on detection of the compressive wave pulse.

Initial preparation

• One essential is good acoustical coupling between the concrete surface and the face of the transducer.
• Eliminate the air pockets.
• Smooth the surface using a light medium, such as petroleum jelly or liquid soap.
• If the surface is rough or uneven apply thicker grease or apply a layer of plaster of paris.
• It is also important after each repeatation, we should apply a new layer of smoothing agent.
• Although the measuring equipment is claims to be accurate to ±0_1 microseconds, if we aim to achieve a transit time accuracy of ±1% then it is necessary to obtain a reading to ±0_7 over a 300mm path length.

Note : Special attention should be given while taking the reading. Should avoid any other source of vibration, however slight, during the test.

Transducer arrangement for UPV test:

There are three basic ways in which, we arrange the transducers:

1. Opposite faces (direct transmission)
2. Adjacent faces (semi-direct transmission)
3. Same face (indirect transmission).

Transducers with natural frequencies between 20 kHz and 200 kHz are available, but 50 kHz to 150 kHz transducers are common. This instrument basically is dependent on the Dynamic Young's Modulus, density, Poisson's ratio of the material.

Procedure :

• Place the transducers on the smooth concrete surface to measure the time required for travel.
• then, calculate the velocity as

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

- Where L is the distance between two probes .
- t is the time required to travel the distance between two transducers

• Then we estimate the quality of concrete on the basis of pulse velocity (V)

Pulse velocity (km/sec)	Quality of concrete
Above 4.5	Excellent
3.5 - 4.5	Good
3.0 - 3.5	Medium
Below 3.0	Doubtful

Table 1 : Pulse velocity and Quality of concrete

When the quality of concrete is :

Excellent - Very few or don't require any sampling for rebound hammer test.

Good - Only indicative sampling is done for rebound hammer test.

Medium - Should extend the sampling to entire structure for rebound hammer test.