12th International Scientific Conference, April 20-22, 2009 Brno, Czech Republic

APPLICATION OF THE FREQUENCY INSPECTION METHOD TO LIGHTWEIGHT CONCRETE SPECIMEN TESTING

Iveta Plšková*, Barbara Kucharczyková**, Michal Matysík***

Abstract: Current research and development of non-destructive frequency-inspection method show these methods to be very promising for material testing and defectoscopy in the near future. This method is based on the physics of elastic stress wave propagation in bodies. An exciting impulse, being realized, for example, by a mechanical impact on the specimen surface, gives rise to low-frequency stress waves to propagate within the structure and reflect on cracks and the specimen surface. The specimen response to the exciting impulse is picked up on the surface by means of a sensor and transmitted to a computer for frequency analysis. The predominant frequencies may be associated with multiple reflections within the structure, carrying information on the structure integrity and defect localization. The paper presents some results of our experimental study of the application potential of the frequency-inspection method. Our experiments focused on the testing of lightweight concrete specimens

*Mgr. Iveta Plšková,Ph.D., BUT in Brno - Faculty of Civil Engineering,Department of Physics,

**Ing. Barbara Kucharczyková,Ph.D., BUT in Brno - Faculty of Civil Engineering,Department of Building Testing,

***Ing. Michal Matysík, Ph.D., BUT in Brno - Faculty of Civil Engineering,Department of Physics,

1. Introduction

According to the relevant standards, there are three kinds of concrete: lightweight, plain and heavyweight concrete. By definition, the volume mass of lightweight concrete is less than 2000kg/m3. Depending on the intended application, the lightweight concrete group consists of three subgroups: 1) thermal insulating type, 2) structural insulating type for supporting and insulating applications and, 3) structural type for load-bearing capacity.

2. Experiment

Lightweight concrete specimens have been studied in our experiments. Fresh concrete mix consisted of the following: 0-4 mm natural gravel and sand, Liapor CZ4-8/600 lightweight porous aggregates, CEM I – 42,5 R cement, fly-ash, plasticiser and water. Water and lightweight porous aggregates were gauged by volume, all other components were gauged by mass.

Joists of dimensions 100 × 100 × 400 mm were made of the lightweight concrete. To produce the test specimens, moulds were filled progressively in two layers, each of them being vibrated for a period of 30 s. After 24 hrs, the joists were removed from the moulds to be placed into a tank with PE-film-covered wooden slat grids and a constant water level at the bottom. The tank was kept in the laboratory at 20 ± 1°C and relative humidity RH 50 ± 5%. The joist volume mass amounted to 1700 kg/m3 after 28 days.

Hardened-concrete specimens were prepared for compression tests. A notch 8-mm wide and 33-mm deep (one third of the joist height) was cut at the joist centre. Subsequently, first specimen measurements were carried out prior to the compression tests. The joist was placed on 2 supports. Force F was applied to the joist at its centre (see Fig. 1), which is the case of three-point bending of a notched joist. The force F was increased gradually until a crack occurred (it arose at the notch root, Fig. 2). The force was applied slowly in order to prevent the joist breakage, with only the formation of a crack being desirable. Subsequently, the second specimen measurement stage was carried out .

Fig. 1 Three-point bending of a notched joist.
Fig. 2 Post-stress TLB1 specimen – detailed view of the crack.

The application of AE methods to the investigation of structural integrity of elements or structures is based on the finding that AE accompanies not only fracture processes but also any material gradual degradation processes. The AE method has a definite advantage over other methods, i.e., the selectivity – it is able to detect only such defects which are unstable during the structure loading (not in absolute magnitude, but only as magnitude changes).

To test the condition of lightweight concrete specimens, either of the following AE methods may be used: passive AE method (so-called frequency-inspection method), and active AE method, studying the acoustic emission from loaded structures.

If a passive AE method is applied, mechanical waves are generated by means of a hammer stroke on the material under test. The response is picked up to show whether or not the material is homogeneous or whether the structure is disturbed in any way. The inhomogeneity region may be localised in the material if several sensors are used simultaneously to pick up the signal. The homogeneity degree is usually assessed using fast Fourier transform or time-frequency (wavelet) analysis of the AE response.

A metal hammer of a mass of 169 g, which was hinged in a fixture ensuring a constant release level, h=2 cm, was used to hit the specimen. The specimen response to the exciting impulse was detected by means of a piezoelectric sensor of Sedlák S7 type, whose operating frequencies range is from 100 Hz to 50 kHz. The sensor was attached to the specimen surface. The response voltage was fed into the input of a Yokogawa DL1540CL digital oscilloscope and further processed by means of a special signal-analysis software package.

The first measurement stage involved intact specimens. The effect of the material structure inhomogeneities and the joist notch on the signal propagation was studied. In the second stage, post-compression-test specimens were measured. A correlation between the response frequency spectrum and the specimen structure deterioration was examined.

3. Measurement Results

The measurement results are presented in Figs. 3 – 6. An example of specimen No. 1 is shown. Figures 3 and 4 give the time-domain responses for measurements was made before and after a loading test. The frequency spectra from both measurements are shown in Figs. 5 and 6.

Fig. 3 Time-domain response record for specimen No. 1. Measured before a loading test. / Fig. 4 Time-domain response record for specimen No. 1. Measured after a loading test.
Fig. 5 The power spectral density versus frequency plot for specimen No. 1. Measured before a loading test. / Fig. 6 The power spectral density versus frequency plot for specimen No. 1. Measured after a loading test.

Figure 3 shows a waveform recording of specimen. The impulse duration of response was 43 ms. The attenuation ratio was found to equal l = 48 s-1. Figure 5 shows the corresponding power spectral density (in relative units) versus frequency plot for this specimen. A dominant frequency f0 = 3927 Hz is observed. Figure 4 shows a response waveform detected from the specimen after a loading test. The response duration was 24 ms, and the attenuation constant increased to l = 138 s-1. The spectral density vs. frequency plot (Fig. 6) shows that the number of significant frequencies increased in the range from 2.5 kHz to 6 kHz. The dominant frequency appears to have shifted to f1 = 5602 Hz. Table 1 shows dominant frequency before and after test load of tested specimens. Average values of a dominant frequency and the respective variance coefficients are shown in Fig. 7.

Tab.1. Dominant frequency before and after test load of tested specimens
Specimen / Dominant frequency [Hz]
before test load / after test load
No. 1 / 3900 / 5591
No. 2 / 3872 / 5869
No. 3 / 3475 / 5987
No. 4 / 4029 / 5674
No. 5 / 3779 / 6129
No. 6 / 3927 / 5602
Fig.7 Average values of a dominant frequency before and after test load for all six specimens, mean values and variance coefficients.

4. Conclusion

The frequency-inspection analysis was applied to specimens of lightweight concrete. Remarkable resonance frequency shifts were found, and an increase in the number of resonance frequencies occurred in the course of the degradation tests by compressive loading. Average changes of resonance frequency of testing specimens were 2000 Hz. From the results we can see that the frequency-inspection method is a prominent non-destructive testing method.

Acknowledgments

This research has been supported by project of GACR No.103/09/P247 and No. 1M0579.

References

[1]  Matysík M., Kořenská M., Plšková I., Kucharczyková B., Keprt J.: Application of the non-linear ultrasonic spetroscopy method to lightweight concrete specimen testing. In Workshop NDT 2007. Brno, Brno University of Technology. 2007. pp.82-87.

[2]  PLŠKOVÁ, I.; KUCHARCZYKOVÁ, B.; PAZDERA, L.; TOPOLÁŘ, L. Využití akustické emise při testování trámců z lehkého betonu s různými typy vláken. In Proceedings of 6th WORKSHOP NDT 2008. 1. Brno, Brno University of Technology. 2008. p.102-105. ISBN978-80-7204-610-2.

[3]  PLŠKOVÁ, I.; CHOBOLA, Z.; MATYSÍK, M.; MARTINEK, J. Detection of the response parameter change prior before and after realization of the frost-thawing test on ceramic tiles. In EWGAE Krakow 2008. Kanji Ono. Krakow, Cracow University of Technolog. 2008. p.322-327. ISBN978-83-7242-478-5.