Ultrasonic Pulse velocity test
Conducting Ultra-Sonic Pulse velocity (UPV) test at selected locations on the RCC members of the Structures covered under the study by making 300 mm x300mm c/c grids to know about the homogeneity and quality of concrete. The test is performed as per the IS 13311 (Part-1) / ASTM C 597.
The ultrasonic pulse velocity method is used for non-destructive testing of plain, reinforced and prestressed concrete whether it is precast or cast in-situ
Objects: The main objects of the ultrasonic pulse velocity method are to establish
- The Homogeneity of the Concrete
- The Presence of Cracks, Voids and other Imperfections
- Changes in the Structure of the Concrete Caused by the Exposure Condition, Corrosion, Wear etc. which may occur with time,
- The Quality of the Concrete in Relation to the Specified Standard Requirements.
- The Quality of One Element of Concrete in Relation to the Another.
- The Values of the Dynamic Elastic Modulus of the Concrete.
Principle:.This method is based on the principle that the velocity of an ultrasonic pulse though any material depends upon its density, modulus of elasticity and poisson’s ratio. A typical block diagram for the test is shown in fig. 1. In this method, an ultrasonic pulse of longitudinal vibrations is produced by an electro-acoustical transducer which is held in contact with one surface of concrete member under test. After traversing a known path length in the concrete, the vibration pulse is converted in to an electrical signal by a second electro-acoustical transducer held in contract with the other surface of the concrete member and an electronic timing circuit enables the transit time of the pulse to be measured.
The travel time is measured with an accuracy of +/- 0.1 microseconds. Transducers with natural frequencies between 20 kHz and 200 kHz are available, but 50 kHz to 100 kHz transducers are common.
The Pulse velocity is calculated by expression ‘Pulse velocity = pulse travel/ pulse travel time’ and is expressed in m/s.
Higher velocity is obtained when the quality of concrete in terms of homogeneity and uniformity is good. If there is a void, crack or flaw inside the concrete, which comes in the way of transmission of pulses, the pulse strength is lowered and it passes around the discontinuity there by making the path longer and the velocity lower.
Pulse velocity in a material depends on its density and its elastic properties, which in turn are related to the quality and the strength of the material. Following properties of concrete can be found out using U.P.V. test
[1] Uniformity of the concrete
[2] Cavities, cracks, defects due to fire and frost.
[3] Modulus of Elasticity
Pulse Velocity method is a convenient technique for investigating structural concrete. The underlying principle of assessing the quality of concrete is that comparative higher velocities are obtained when the quality of concrete in terms of density, homogeneity and uniformity is good. In case poorer quality of concrete, lower velocities are obtained. If there is a crack, void or flaw inside the concrete which comes in the way of transmission of the pulses, the pulse strength is attenuated and it passes around the discontinuity, thereby making path length longer. Consequently, lower velocities are obtained. The actual pulse velocity obtained depends primarily upon the material and the mix proportion of the concrete. Density and modulus of elasticity of aggregate also significantly affect the pulse velocity.
Transducers: Piezoelectric and magnetostrictive types of transducers are available in the range of 20 kHz to 150 kHz of natural frequency. Generally, high frequency transducers are preferable for short path length and low frequency transducers for long path lengths. Transducers with a frequency of 50 to 60 kHz are useful for most all-round applications. The Following table indicates the natural frequency of transducers for different path lengths
Schematic Diagram of Ultrasonic Pulse Velocity Method
There are three possible ways of measuring pulse velocity through concrete :
- Direct Transmission (Cross Probing) through Concrete : In this method transducers are held on opposite face of the concrete specimen under test as shown in fig. The method is most commonly used and is to be preferred to the other two methods because this results in maximum sensitivity and provides a well defined path length.
- Semi-direct Transmission through Concrete : Sometimes one of the face of the concrete specimen under test is not accessible, in that case we have to apply semi-direct method as shown in fig. In this method, the sensitivity will be smaller than cross probing and the path length is not clearly defined.
- Indirect Transmission (Surface Probing) through Concrete : This method of pulse transmission is used when only one face of concrete is accessible. Surface probing is the least satisfactory of the three methods because the pulse velocity measurements indicate the quality of concrete only near the surface and do not give information about deeper layers of concrete. The weaker concrete that may be below a strong surface can not be detected. Also in this method path length is less well defined. Surface probing in general gives lower pulse velocity than in the case of cross probing and depending on number of parameters.
Different Methods of Propagating Ultrasonic Pulses through Concrete
