International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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IMPACT ACOUSTICS METHODS FOR DEFECT EVALUATION IN CONCRETEMasanori Asano, Toshiro Kamada, Minoru Kunieda and Keitetsu Rokugo
Department of Civil Engineering, Gifu University,
1-1 Yanagido, Gifu-city, Gifu 501-1193, Japan
Development Section, Technological Development Dep., Showa Concrete Industry Co. LDT.,
1-1 Koran, Gifu-city, Gifu 500-8703,Japan
In this study, Impact Acoustics parameters obtained from received sound generated by impact on concrete surface were investigated to develop a evaluation system of defects in concrete. As Impact Acoustics parameters, frequency distribution was employed. In addition to the experiment, 3 dimensional FEM analysis was carried out to understand the theoretical background of parameters. The results of FEM analysis showed good agreement with experimental ones. From analytical and experimental results, it was likely possible to estimate defect sizes using the relation between the resonance frequencies of impact sounds and defects diameter.
Tapping methods that receive sounds with human ears have been used in practice to inspect the concrete linings of railway tunnels . However, such methods completely depend on the detection of "unclear sounds" to identify internal defects. Evaluation by human ears is greatly affected by the experiences and subjectivity of inspectors.
In order to solve these problems, methods using acoustic device such as microphone to receive sounds and to analyze the waveforms have been studied. The name "Impact Acoustics Methods" is introduced as a definition of such type of methods in a committee report  by Japan Concrete Institute.
This study investigated Impact Acoustics Methods that can quantitatively identify the sizes of internal defects and their depths from the surface using the frequency distributions. Experiments were conducted using concrete specimens with an artificial defect inside, which represented voids or delaminations inside concrete. Numerical analyses of the three-dimensional finite element method (FEM) were also conducted to analyze the relationship between frequency distribution and defect information. The frequency distributions were compared between the without defects and with defects concrete sections. The displacements of the concrete surface were monitored using an accelerometer to investigate the phenomena involved in impact acoustics.
OUTLINE OF EXPERIMENTS
Elastic wave measurement
OUTLINE OF FEM ANALYSIS
Authors conducted analytical investigation using a general purpose program code (LS-DYNA). Fig. 2 shows the analytical model used in this study. Concrete part upper side of defect was modeled as a disc shaped plate. Concrete was assumed to be an elastic body (density: 2.3 x 103 kg/m3, elastic modulus: 42 GPa, Poisson's ratio: 0.2), and all degree of freedom of the disc plate base was constrained in the analysis. Load was input as a wave shown in Fig. 3 to the center of the model. The duration of input Tc was derived using the following equation :
where, D is the diameter of the steel ball (m). From Equation (1), the duration of impact to be used for the analysis was determined to be 80µs. The load Fmax applied to concrete by dropping a steel ball was calculated using the following equation :
where, m is the mass (kg) of the steel ball, h is the height (m) from which the ball was dropped, and g is the gravitational acceleration (m/s2). In this analysis, Fmax was 0.88 kN.
RESULTS AND DISCUSSIONS
From Fig.4 and Fig.5, it is obvious that the waveform obtained from sound portion and defects portion show quite different characteristics. In sound portion, duration of the waveform was shorter than that of defects ones. On the other hand, waveforms of defects portion show periodic history curves. With defects case, both defects depth is 3cm and 7cm, waveforms obtained from microphone and accelerometer show same tendency in terms of periodic history and attenuation characteristics. Tendency of waveforms obtained from FEM analysis show good agreement with experimental ones. In both experimental and analytical results, the greater the diameter of defect, the shorter the period.
Where ft : thickness frequency(Hz), V: elastic wave velocity(m/s) and T: concrete thickness(m). In this study, V was 4500m/s, T was 0.2m. By using above values, ft = 11.25kHz is obtained. This is close to experimental value. From Fig.7 and Fig.8, spectral peak obtained from microphone and accelerometer showed same value. This implies that the phenomena of sound generated by impact and surface displacement was equivalent. The larger the depth of the defects, the higher the frequency at the peak. This characteristic seems to show that the defect induced resonance inside the specimen, which caused a spectrum peak of impact sounds to appear. There are good agreements between the experimental and analytical results.
In vibrating disks, the flexural resonance frequency is known to increase as the depth of the disk increases if the disk area is constant . The shift of the spectrum peak described in the above paragraph suggests that the peak component of the spectrum was due to the flexural resonance of the concrete above the defect.
Relation between peak frequency and defects diameters
These figures show lower peak frequencies for larger defects and also show higher peak frequency for deeper defects. This characteristic can be observed both experiment and analysis. On the other hand, in the analytical results, resonance frequency of 10cm depth (equal to plate thickness is 10cm) show lower value than that of 7cm case except for diameter of defects are 10, 30, 50cm. This seems to be attribute to the difference between analytical assumption and actual condition of defects inside the concrete. In analysis, displacement of side in the model was not constrained. However, in experiment, when subjected to impact, the vibration of concrete part upper side of defects was not separated to around concrete. As a result, flexural resonance was not appeared in the analysis.
From Fig. 10 and Fig. 11, under same diameter of defects, the difference of resonance frequency caused by the increase of defects depth seems to be small. Taking into consideration of this relation, resonance frequency is thought to be effective index to evaluate the lateral expansion of the defects inside concrete.
The following conclusions were obtained from this study: