Relative amplitude method

The ultrasonic pulse velocity method is the most widely used ultrasonic non-destructive method for assessing concrete quality. It has been used to estimate concrete strength for over 40 years. However, the relation between strength and pulse velocity is in general not reliable enough for practical purpose. The relative amplitude method is alternative method for estimating strength. The relative amplitude method is basically an attenuation method, which measures the ratio of the wave amplitudes.

Ultrasonic waves are attenuated as they pass through the materials. Basically attenuation is caused by beam divergence (distance effect), absorption (heat dissipation) and scattering.

Only scattering is affected by the characteristics of the materials they pass through. The degree of inhomogeneity and frequency of the transducer affect it. Attenuation caused by scattering is given by:

°¯

D is the average inhomogeneity; in concrete, D may be the void or aggregate size.

The main factor that influences strength of brittle material is porosity. Several models to relate strength to porosity have been proposed, but the most common one is the exponential model:

kP

Ko

K = e^- (30)

where

Ko is the strength at zero porosity, P is the fractional porosity,

k a constant that depends on the system being studied.

For λ >> D, it is found that the concrete strength is related exponentially with the wave attenuation.

11.4.2. Equipment for relative amplitude method

The equipment consists of, as a minimum, a conventional ultrasonic pulse velocity meter (e.g. PUNDIT or James V-meter) and a two channel oscilloscope. Transducers of 50 mm diameter and 54 kHz may be used for generating and receiving ultrasonic waves through samples.

11.4.3. General procedure for relative amplitude method

The best technique is by direct transmission and the semi-direct technique is also possible. However, the surface technique is not possible since the amplitude of the pressure and the torsion waves are difficult to determine.

PROCEDURE:

 Adjust the test range on the oscilloscope (µs/div.) so that the received signal is on the CRT. The time base range should cover at least two times the pressure wave arrival time.

 Measure the amplitude of the pressure wave (i.e. the first amplitude), say Ap.

 Measure the amplitude of the combination of pressure and torsional wave, say Aps. This amplitude should be just after the arrival of the torsional wave. It is about 2 times the pressure wave arrival, Fig. 11.6.

 Measure the attenuation of the ultrasonic waves.

The so-called relative amplitude β is given by:

¸¸

Ap is the amplitude of pressure wave

Aps is the wave amplitude after torsion wave arrival.

Fig. 11.10 shows how Ap and Aps are determined from the oscilloscope display.

0 tp ts

A p A ps

FIG. 11.10. Typical oscilloscope display defining the transit time and wave amplitude.

Ap is the pressure wave amplitude, Aps the amplitude after the arrival of the torsional wave, tp the transit time of the pressure wave and ts the transit time of the torsional wave.

11.4.4. Applications of relative amplitude method

The sole application of the method is in strength estimation. The general relationship between strength, K and relative amplitude, β is an exponential form. For a specific condition, the empirical relationship is, as follows:

β

Above equation is applicable to concrete with a moisture content of 3-4%, an age of 90 days, made from crushed granite aggregate with a maximum size of 20 mm, cured by immersion in water for 28 days, and measured by the direct technique at 150 mm beam path distance without reinforcement bars. The relative amplitude decreases as the strength is increased. The factors, which influence the relationship, are shown in Figure 11.11 and can be described, as follows:

4

FIG. 11.11. Corrections for β^.

1- Crushed granite

MOISTURE CONTENT

Wet samples have lower β than dry samples for the same strength. β varies by ∼2 dB between normal wet and dry samples but it increases by ≥3 dB if the sample is oven dried.

The strength of the oven dried sample increases by 15% as compared to the wet sample and β increases by 50%. Observing β, the drier the sample is, the poorer the quality. The reverse effect is shown on the strength.

CONCRETE AGE

It can observed that β is constant for samples of age 90 days and above, but for samples of age 7 to 90 days, β varies by 4∼5 dB.

AGGREGATE TYPE AND SIZE

Granite aggregate gives the highest β as compared to the other types of aggregate for the same strength. There is a small change in β for aggregate 10 mm and 20mm, but β reduces by

∼2 dB with aggregate of 40 mm.

CURING CONDITION

Concrete cured in air (top surface of the cube not covered) gives the highest β as compared to the cubes immersed in water or covered with plastic bag. The variation is higher for low strength than the high strength concrete.

TESTING TECHNIQUE

Surface technique is not possible for the relative amplitude method. This is due to difficulties in locating the position of the torsion wave arrival on the waveform and might be confused with the surface wave. Similar to the pulse velocity method, the most sensitive technique of the transducer arrangement is also by direct technique. Slight misalignment between the transmitter and the receiver (as long as it does not exceed 20^o) does not significantly change β. With the diagonal technique, the angle between transmitter and receiver is 45^o.

BEAM PATH DISTANCE AND REINFORCEMENT BAR SIZE

There is no significant change in the relative amplitude for the beam path distance

≥150 mm and the bar size of ≤20 mm. The curve for the relative amplitude versus the beam path length and bar size is plotted only for one strength since it is expected that it will be the same slope for other strengths as they are not properties of concrete.

11.4.5. Range and limitations of relative amplitude method

Determination of the amplitude of Aps may be difficult in some concrete. Surface technique is not possible. For a specific condition the error of estimation is ∼4 N/mm2 at 95%

confidence level and the correlation coefficient is about 0.8 ∼ 0.95. Without knowing the history of the sample the estimation of strength is subjected to large error. The main advantage of the relative amplitude over pulse velocity method is that the relative amplitude do not require parameter distance. Low value of β indicates high quality of concrete.

11.5. VELOCITY VERSUS REBOUND NUMBER CURVES