A pulse of longitudinal vibrations is produced by an electro-acoustical transducer, which is held in contact with one surface of the concrete under test. When the pulse generated is transmitted into the concrete from the transducer using a liquid coupling material such as grease or cellulose paste, it undergoes multiple reflections at the boundaries of the different material phases within the concrete. A complex system of stress waves develops, which include both longitudinal and shear waves, and propagates through the concrete. Measurement of the velocity of ultrasonic pulses of longitudinal vibrations passing through concrete may be used for the following applications:
- Determination of the uniformity of concrete in and between members
- Measurement of changes occurring with time in the properties of concrete
- Correlation of pulse velocity and strength as a measure of concrete quality
- Determination of the modulus of elasticity and dynamic Poisson's ratio of the concrete
It is possible to make measurements of pulse velocity by placing the two transducers on either:
- Opposite faces (direct transmission)
- Adjacent faces (semi-direct transmission)
- The same face (indirect or surface transmission)
Several factors have an effect on the results of this test which include:-
- Moisture content
- Temperature of the concrete
- Path length
- Shape and size of specimen
- Effect of reinforcing bars
- Determination of concrete uniformity
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