NONDESTRUCTIVE TESING OF THE BOND BETWEEN CONCRETE FLOOR LAYERS BY IMPULSE RESPONSE AND IMPACT-ECHO METHODS

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1 Czech Society for Nondestructive Testing NDE for Safety / DEFEKTOSKOPIE 0 November -, 0 - Harmony Club Hotel, Ostrava - Czech Republic NONDESTRUCTIVE TESING OF THE BOND BETWEEN CONCRETE FLOOR LAYERS BY IMPULSE RESPONSE AND IMPACT-ECHO METHODS Jerzy HOŁA, Łukasz SADOWSKI Institute of Building Engineering, Wroclaw University of Technology Abstract When making and repairing concrete floors in the civil engineering industry it is crucial to prepare the surface of the structural layer in order to obtain proper bonding. In the following article the impulse response method and the impact-echo method were used in comparative tests of the interlayer bond in a model concrete floor specimen. The aim of the paper was to find out how the results obtained using the above nondestructive methods would differ for the two ways of preparing the bonding surface. Keywords: acoustic methods, adhesion, concrete floors, bond surface. Introduction The durability of concrete floors is to a high degree determined by the pull-off adhesion of the topping to the structural layer []. Both when making and repairing concrete floors it is crucial to prepare the surface of the structural layer in order to obtain proper bonding. The measure of the latter is the adhesion value determined by the pull-off test []. The search for concrete floor areas lacking adhesion at the interface between the layers was the subject of paper [] by Delatte who on the basis of pull-off test results produced a map showing the areas where delamination occurred. Also Garbacz et al. [] proposed to use the pull-off method to obtain a map of delaminations on the surface of laminar concrete elements. However, the pull-off method is much less suitable for evaluating the bond between the layers or locating the places or areas in which there is delamination at the interface between the layers. In the latter case its effectiveness depends on the number of drilled boreholes. The larger the number of boreholes, the larger the number of places in which the floor needs to be repaired after the tests. It seems that this problem can be solved by applying nondestructive methods, i.e. the impulse response method and the impact echo method []. In this study the impulse response method and the impact-echo method were used in comparative tests of the interlayer bond in a model concrete floor specimen. The surface of the specimen s structural layer was prepared in two ways, i.e. half of the surface was mechanically ground in order to increase adhesion, whereas the other half remained in its original post-concreting condition. The aim of the study was to find out how the results obtained using the above nondestructive methods would differ for the two ways of preparing the bonding surface. DEFEKTOSKOPIE 0

2 . Description of tests, their results and analysis The tests were carried out on a 00 x 00 mm model concrete slab consisting of a 0 mm thick structural layer and 0 mm top layer (fig. ). The structural layer was made of grade C0/ concrete with aggregate of mm maximum grading, naturally cured at a temperature of 0 C± C. The structural layer was laid on a 0 mm thick layer of sand. The top layer was made of grade C0/ concrete with aggregate of mm maximum grading. All the tests were carried out after the concrete had cured for 0 days, except for the compression strength tests on the concrete, which were carried out after days of curing. a) b) top layer surface no. II surface no. I bond surface structural layer Fig.. Concrete floor: a) view of structural layer before top layer concreting, b) cross section through concrete floor layers. The following two ways of preparing the structural layer s surface were used: - surface no. I in post-concreting condition, covered with dust (which is often the case in building practice), - surface no. II mechanically ground and dedusted (which should be the standard practice). After the surface of the top layer was marked, a 0 x 0 mm test area was demarcated in its central zone and a grid of measuring points spaced at every 0 mm was marked on this area. The columns were marked with letters from A to H and the rows with numbers from to. In this way measuring points were obtained on each of the surfaces. The impulse response tests consisted in exciting an elastic wave in each point of the measuring grid by means of a calibrated hammer and analyzing elastic wave F, a diagram of wave velocity w and a diagram of mobility N after each excitation. Then using the special software the acquired data were processed and as a result the values of the following parameters in each measuring point were obtained: average mobility N av, stiffness K d, mobility slope M p, mobility times mobility slope N av M p and voids index w. Exemplary results of the impulse response tests for surface no. (no surface preparation) are shown in fig.. DEFEKTOSKOPIE 0

3 A B C D E F G H A B C D E F G H a) b) Fig. Exemplary impulse response test results for surface no. I: a) map of mobility, b) map of stiffness. It appears from the above results that in the demarcated areas of surface I the mobility parameter amounts to 0-0 on most of the surface and stiffness to A local increase in mobility on the mobility map means the susceptibility of the material to deflection, which may indicate delamination. When mobility is high while stiffness is low the presence of delamination is highly probable. Exemplary results of the impulse response tests for surface no. II (no surface preparation) are shown in fig.. DEFEKTOSKOPIE 0

4 A B C D E F G H A B C D E F G H a) b) Fig. Exemplary results of impulse response tests for surface no. : a) map of mobility, b) map of stiffness. It appears from the above results that in the neighbourhood of measuring point F on surface II the mobility parameter ranges from 0 to 0 and stiffness is in a range of As opposed to the results obtained for surface no. I, the results for surface no. do not indicate any delamination. Then in the marked measuring points an elastic wave was generated by means of an exciter placed on the impact-echo tester head. Using the dedicated software and the Fourier transform the signals were processed to obtain an amplitude-frequency spectrum of the recorded elastic wave. The amplitude-frequency spectrum shown in figure was usually obtained from the tests for surface no.. A signal of this type is obtained when the generated ultrasonic wave bounces off a discontinuity. This means that the signal value corresponding to the frequency at the location of the interlayer bond, which amounts to. khz, is analyzed. In the tested concrete floor a discontinuity is present at a depth of about 0 mm, i.e. at the top layer/subfloor interface: 0. 0m/ s T = = 0mm.kHz DEFEKTOSKOPIE 0

5 , khz frequency, khz Fig.. Exemplary amplitude-frequency spectrum obtained from impact-echo tests for point A on surface no. I. The amplitude-frequency spectrum shown in figure was usually obtained from the tests for surface no. II. A signal of this type is obtained when the generated ultrasonic wave bounces off the bottom. This means that the signal value corresponding to the frequency at the place where the wave is reflected from the bottom, which amounts to. khz, is analyzed. Thus the thickness of the specimen amounts to about mm: 0. 0m/ s T = = mm.khz, khz frequency, khz Fig.. Exemplary amplitude-frequency spectrum obtained from impact-echo tests for point E on surface no. II. The results of the nondestructive tests were verified by the semi-nondestructive pull-off test which confirmed the absence of adhesion at the interface between the layers. DEFEKTOSKOPIE 0

6 . CONCLUSION Comparative tests of the interlayer bond in a model test concrete floor specimen were carried out using the nondestructive AE methods: the impulse response method and the impactecho method. A model specimen with its structural layer surface prepared in two different ways, i.e. a half of the surface in post-concreting condition and covered with dust (which is often the case in building practice) and the other half mechanically ground (which should be the standard practice) was subjected to the tests. For the surface covered with dust the area where delamination was present was determined using the impulse-response method. Then the depth at which the delamination occurred was precisely determined using the impact-echo method. In the case of the ground surface, the tests showed the absence of delamination in most of the area. This was confirmed by the impact-echo test results. The results of the nondestructive tests were verified by the semi-nondestructive pulloff method. The study has shown the impulse-response method and the impact-echo method to be suitable for the evaluation of the bond between the layers in concrete floors.. REFERENCES [] Czarnecki L., Mierzwa J., Selected material causes of damage to concrete floors (in Polish) Materiały Budowlane, No. (00). [] ASTM D 0. Standard Test Method for Pull-Off Adhesion Strength of Coatings on Concrete Using Portable Pull-Off Adhesion Testers. [] Delatte N., Fowler D, McCullough F., Full-Scale Test of High Early Strength Bonded Concrete Overlay Design and Construction Methods, Transportation Research Board of the National Academies, (). [] Garbacz A., Krystosiak M., Ultrasonic assessment of adhesion between polymer coating and concrete subfloor (in Polish). st National Conference on Nondestructive Testing, Szczyrk, pp. - (00). [] Hoła J., Sadowski Ł., Schabowicz K., Non-destructive evaluation of the concrete floor quality using Impulse Responses' Mash and Impact-Echo Methods, e-journal of Nondestructive Testing & Ultrasonics, Vol., No. (00). This research was carried out as part of research project no. 0/B/T0/0/0 entitled: New Nondestructive Way of Evaluating the Pull-off Adhesion of the Layers in Concrete Floors, Using Artificial Neural Networks funded by the National Science Centre. 0 DEFEKTOSKOPIE 0