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An Investigation into the Depth in a Cracks in a Reinforced-Concrete Structure using an Ultrasonic Method
Technical University of Szczecin
In many cases cracking of concrete in a concrete construction can indicate the status of its overloading or weakening appeared because of many reasons. In case when the construction is visible only from the side where cracks appeared, situating of the cracks and the width of their opening on the surface of the concrete can be catalogued with the help of easy optical methods. However investigating of the range and orientation of the crack inside of the concrete requires using of penetrating methods. In this situation applying of the ultrasonic method connected with using of the diffraction effect is possible.
The morphology of a single natural crack is characterized by irregularity of its configuration inside of the concrete, linearly variable width of the opening and disappearing zone without clear ending line. The crack filled up with air creates the screen which reflects ultrasonic waves, however the steel reinforcement of the concrete, transversally situated, creates the element of continuity in the reinforced concrete construction. The disappearing zone of the crack is the semi-permeable layer which dissipates waves passing, reflected, and induced in the diffraction field.
The gap, technologically formed in the concrete, or made by an incision with maintaining the continuity of the reinforcement, has got the regular, geometrical shape and the clear ending line. It is the approximate model of the natural crack, but makes better observing of diffraction methods possible.
In case when transducer heads are situated from both sides of the gap, the wave with frequency of 100kHz is emitted as the pulsed wave. The whole pulse, or only its initiating part, including all effects of the direct transmission and diffraction, is recorded in the receiver situated behind the gap. As the first is recorded the pulse of the subsurface wave, transmitted on the opposite side of the gap along of the reinforcement. It results from the shortest way and slightly bigger propagation speed than in the concrete behind the reinforcement. It does not give interesting information about parameters of the gap, moreover disturbs the picture of the farther part of the pulse, including the diffraction effects at the end of the gap (Fig 1).
The diffraction appears on the ending line of the gap or in the zone of disappearing of the natural crack. In the effect of this diffraction the longitudinal, diffracted wave and the transversal wave induced in this place reaches the receiver. Defining of the depth of the gap or the crack is possible on the basis of the geometrical dependence and on the basis of measuring of the transmission time of the longitudinal wave in the direction skew to the transmitter to the place of the diffraction and farther along the ray reaching the receiver. It is possible after filtering off the pulse of the subsurface wave, passing on the opposite side of the gap along the reinforcement. The pulse of the transversal wave induced in the place of the diffraction is recorded with a delay and need not has to be filtered off. In case when it is well seen, it can supply some additional information about the orientation of the crack inside of the concrete. In all situations an independent defining of the speed of the propagation of the longitudinal waves in the concrete, in the non-cracked zone of the same element of the construction, is necessary.
Fig 1: |
The symmetrical configuration of the transducers in relation to the gap (Fig. 2) requires the additional measurement with the full recording of the receiving pulse, in the same line and in the same spacing, but behind of the gap. In both records passing of the wave along the reinforcement gives the similar time and amplitude effects which can be mutually corrected. Comparison of corrected records causes filtering off this interference and makes measuring of the time of receiving of the longitudinal wave (77,6ms) possible, and also less clear signal of receiving signal of receiving of the transversal wave (103,0ms) induced in the diffraction place - on the end of the gap. The depth of the gap, equals in the specified example to 96mm, was defined on the basis of the time of transmission of the longitudinal wave very accurately (95,4mm), and on the basis of the evaluation of the time of the receiving of the transversal wave with the accuracy of 4% (92,1mm).
For two anti-symmetrical measurements AB and CD (Fig. 1) for which the spacing of transducers x1 + x2 will be the same, the time effects of the reception of the wave passing along the reinforcement and the longitudinal wave induced on the end of the gap will be also the same. However amplitudes of both longitudinal and transversal waves in the diffraction field will be different in both records. After correcting of inconsiderable differences of the amplitude in the initial periods of the pulse transmitted along the reinforcement, and after filtering off this effect, remains the difference of amplitudes of pulses received from the diffraction field making possible measurement of the time of receiving of the longitudinal wave diffracted on the end of the gap. The same possibilities of filtration of the pulse are given by the doubled anti-symmetrical measurement, executed in the same points, but with shifting heads into the reverse unit, for example AB and BA. This method is more advantageous, because runs of the wave are recorded on the exactly the same way, but in the reverse directions. Moreover, at small distances between the cracks existing in the construction, it can be the only method of investigating. In case of the non-symmetrical
situating of the heads (Fig. 3), the effect of the investigation of the depth of the same gap is less accurate. After filtering off the picturing demonstrates the time of receiving of the longitudinal wave t=74,2ms, what shows the depth of the gap lowered with 7%.
Similar investigation of the natural crack (Fig. 4) shows the less clear and deformed picture of the receiving pulse filtered off. Deformations result from the spherical, not linear diffraction in the zone of disappearing of the crack. The result of the investigation differs inconsiderably from the real status. Two farther measurements, symmetrical in points 1-3 and 3-1, gave in this case the similar effect, just because of curving of the crack and displacement of its zone of disappearing.
Fig 3: |
Fig 4: |