5.1 The ultrasonic pulse velocity, V, of longitudinal ultrasonic stress waves in a concrete mass is related to its elastic properties and density according to the following relationship:

5.2 This test method is applicable to assess the uniformity and relative quality of concrete, to indicate the presence of voids and cracks, and to evaluate the effectiveness of crack repairs. It is also applicable to indicate changes in the properties of concrete, and in the survey of structures, to estimate the severity of deterioration or cracking. If used to monitor changes in condition over time, test locations are to be marked on the structure to ensure that tests are repeated at the same positions.

5.3 The degree of saturation of the concrete affects the ultrasonic pulse velocity, and this factor must be considered when evaluating test results (Note 1). In addition, the ultrasonic pulse velocity in saturated concrete is less sensitive to changes in its relative quality.

Note 1: The ultrasonic pulse velocity in saturated concrete may be up to 5?% higher than in dry concrete.3

5.4 The ultrasonic pulse velocity is independent of the dimensions of the test object provided reflected waves from boundaries do not complicate the determination of the arrival time of the directly transmitted pulse. The least dimension of the test object must exceed the wavelength of the ultrasonic vibrations (Note 2).

Note 2: The wavelength of the vibrations equals the ultrasonic pulse velocity divided by the frequency of vibrations. For example, for a frequency of 54?kHz and a pulse velocity of 3500 m/s, the wavelength is 3500/54000 = 0.065 m.

5.5 The accuracy of the measurement depends upon the ability of the operator to determine precisely the distance between the transducers and of the equipment to measure precisely the ultrasonic pulse transit time. The received signal strength and measured transit time are affected by the coupling of the transducers to the concrete surfaces. Sufficient coupling agent and pressure must be applied to the transducers to ensure stable transit times. The strength of the received signal is also affected by the travel path length and by the presence and degree of cracking or deterioration in the concrete tested.

Note 3: Proper coupling can be verified by viewing the shape and magnitude of the received waveform. The waveform should have a decaying sinusoidal shape. The shape can be viewed by means of outputs to an oscilloscope or digitized display inherent in the device.

5.6 The measured quantity in this test method is transit time, from which an "apparent" ultrasonic pulse velocity is calculated based on the distance between the transducers. Not all forms of deterioration or damage actually change the ultrasonic pulse velocity of the material, but they affect the actual path for the ultrasonic pulse to travel from transmitter to receiver. For example, load-induced cracking will increase the true path length of the ultrasonic pulse and thus increase the measured ultrasonic pulse transit time. The true path length cannot be measured. Because the distance from transmitting to receiving transducer is used in the calculation, the presence of the cracking results in a decrease in the "apparent" pulse velocity even though the actual ultrasonic pulse velocity of the material has not changed. Many forms of cracking and deterioration are directional in nature. Their influence on transit time measurements will be affected by their orientation relative to the pulse travel path.

5.7 The results obtained by the use of this test method are not to be considered as a means of measuring strength nor as an adequate test for establishing compliance of the modulus of elasticity of field concrete with that assumed in the design. The longitudinal resonance method in Test Method C215 is recommended for determining the dynamic modulus of elasticity of test specimens obtained from field concrete because Poisson's ratio does not have to be known.

Note 4: If circumstances warrant, a velocity-strength (or velocity-modulus) relationship may be established by the determination of ultrasonic pulse velocity and compressive strength (or modulus of elasticity) on a number of specimens of a concrete. This relationship may serve as a basis for the estimation of strength (or modulus of elasticity) by further pulse-velocity tests on that concrete. Refer to ACI 228.1R4 for guidance on the procedures for developing and using such a relationship.

5.8 The procedure is applicable in both field and laboratory testing regardless of size or shape of the specimen within the limitations of available pulse-generating sources.

Note 5: Presently available test equipment limits path lengths to approximately 50-mm minimum and 15-m maximum, depending, in part, upon the frequency and intensity of the generated signal. The upper limit of the path length depends partly on surface conditions and partly on the characteristics of the interior concrete under investigation. A preamplifier at the receiving transducer may be used to increase the maximum path length that can be tested. The maximum path length is obtained by using transducers of relatively low resonant frequencies (20 to 30 kHz) to minimize the attenuation of the signal in the concrete. (The resonant frequency of the transducer assembly determines the frequency of vibration in the concrete.) For the shorter path lengths where loss of signal is not the governing factor, it is preferable to use resonant frequencies of 50 kHz or higher to achieve more accurate transit-time measurements and hence greater sensitivity.

5.9 Because the ultrasonic pulse velocity in steel is up to double that in concrete, the ultrasonic pulse velocity measured in the vicinity of the reinforcing steel will be higher than in plain concrete of the same composition. If possible, avoid measurements close to steel parallel to the direction of pulse propagation.