Following factors affect the value of pulse velocity: 1. Smoothness of Contact Surface under Test 2. Length of Path of Pulse Velocity 3. Temperature of Concrete 4. Moisture Conditions of Concrete 5. Presence of Reinforcing Steel 6. Material and Mix Proportion of the Concrete.
Factor # 1. Smoothness of Contact Surface under Test:
There should be good acoustical contact between the concrete surface and the face of each transducer. Generally in normal testing, there is sufficient cast surface available for good contact. However in cases as the top surface of cylinder, it is desirable to smoothen the cylinder surface either by grinding or capping and holding the transducer tightly against the concrete surface. In addition a thin film of oil, soap etc. should be used.
Factor # 2. Length of Path of Pulse Velocity:
As we know that concrete inherently is heterogeneous. Thus it is essential that path length of the pulse should be sufficiently long to avoid the introduction of any error due to its heterogeneity. In field work it does not pose any difficulty as the pulse velocity measurements generally are carried on thick structural concrete members, where the path length may be about 300 mm in case of columns and 23 meters in mass concrete as dams. However in the laboratory where small length specimens are used, the path length may affect the pulse velocity readings.
Factor # 3. Temperature of Concrete:
It has been observed that variation in ambient temperature affects the pulse velocity in concrete significantly. The variation between 5°C to 30°C does not affect the pulse velocity significantly, but at temperatures between 30°C and 60°C an reduction of 5% in the pulse velocity has been reported. Probably this might have happened due to the development of micro cracking in the concrete.
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Further it has been observed that below freezing temperature the pulse velocity increases due to the freezing of free water with in the concrete. At 4°C through the water saturated concrete an increase in pulse velocity up to 7.5% has been recorded.
Corrections to be applied to pulse velocity due to change in temperature is shown in Table 19.4 below:
Factor # 4. Moisture Conditions of Concrete:
It has been observed that in general the pulse velocity through concrete increases with the increase in moisture content of concrete. This effect is observed more in low strength concrete than high strength concrete. The difference may be from 10 to 15%. Thus it is considered that pulse velocity in saturated concrete is higher by about 2% than that of similar dry concrete and when water has frozen in concrete the pulse velocity increases by several percent.
Factor # 5. Presence of Reinforcing Steel:
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It has been reported that pulse velocity in steel is 1.2 to 1.9 times that of plain concrete. Thus the pulse velocity measured near the reinforcing steel bars may be high, not representing the true velocity of concrete. Thus the presence of steel bars in concrete affects the pulse velocity to a great extent. Therefore it is desirable to choose the pulse path that avoids the effects of reinforcing steel bars.
When it is not possible to adopt such a path, suitable corrections are applied to the measured values of pulse velocity, taking into account the proximity of the pulse path to the reinforcing steel bars. The value of correction depends upon the quantity and orientation of the steel with respect to the pulse path of propagation, and the pulse velocity in the concrete.
(a) When the axis of reinforcement bars is perpendicular to the direction of propagation and the quantity of steel is small, the effect of reinforcement steel generally is small. The correction factor may be of the order of 1 to 4% depending upon the quality (strength) of surrounding concrete. The higher the quality of concrete lowers the correction factor.
(b) When the reinforcement is parallel to the direction of propagation of the pulse velocity, it is difficult to avoid the effect of reinforcement steel. In this case the measured pulse velocity may be corrected, but the correction factors are not exact, but approximately. In case of two way reinforcement, it is almost impossible to make any reliable corrections.
Factor # 6. Material and Mix Proportion of the Concrete:
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The pulse velocity also depends upon the material and mix proportion of the concrete. The aggregates due to their density and modulus of elasticity affect the pulse velocity to a great extent.