Evaluation of thermal damaged concrete using nonlinear acoustic testing in impact mode Hyo-Gyung Kwak

The Twenty-Fourth KKCNN Symposium on Civil Engineering December 14-16, 2011, Hyogo, Japan. Evaluation of thermal damaged concrete using nonlinear acoustic testing in impact mode Hyo-Gyung Kwak1, *Sun-Jong Park2 and Hong Jae Yim3 Department of Civil and Environmental Engineering, KAIST Daejeon 305-701, Korea khg@kaist.ac.kr, tomems@kaist.ac.kr, yimhongjae@kaist.ac.kr,. ABSTRACT In the context of an increasing need for reliability and safety in concrete structures, nondestructive testing using acoustic effect provides a practical answer. Linear approaches (velocity, attenuation, impact echo and pulse echo, etc.) have a limited sensitivity of the damage assessment, whereas nonlinear approaches have much greater dynamic range. This paper concerns the reliability of nonlinear approaches in the view of thermal induced micro-cracks detection. The three thermal damage cases were considered: two maximum exposure temperature cases varied 300℃ and 600℃, with 2 hours exposure time at the maximum temperature, and one reference. Thermal damage was induced by electric furnace. Nonlinear acoustic interaction was adopted for damage detection of the thermally defected concrete specimens. The nonlinear interaction is generated by a continuous single harmonic ultrasonic wave with a low frequency impact. To check the reliability of nonlinear acoustic effect, ultrasonic pulse velocity and nonlinear indicator were compared. INTRODUCTION In order to prevent disaster due to the damaged concrete structure, assessment of the present state of concrete structures begins to receive attention. To keep the existing structure, various nondestructive testing (NDT) techniques have been proposed, e.g. radiographic testing (RT), ultrasonic testing (UT), magnetic particle testing (MT), and acoustic emission (AE), etc. NDT predicts the present damage of concrete structure by the relationship between damage and experimental values. Among several NDT techniques, ultrasonic testing evaluates specific materials using characteristics of wave propagation, e.g. transmission, reflection, refraction, and attenuation (ACI 228). Traditional ultrasonic NDT techniques are based on linear acoustic theory. According to some measured particular parameters of the wave propagation, the elastic properties of material or some defects are determined. The amplitude and phase in output signals are changed comparing with input signals when the material with defects. However, the frequency is unchanged even though damaged material due to the linear theory. So, conventional ultrasonic NDT techniques are limited to some serious defects and cracks, but detectable discontinuities of those techniques are limited. To enlarge the sensitivity of ultrasonic NDT techniques, Nonlinear Acoustic Effect of ultrasound gives an answer. When acoustic wave propagates through specific material, in output signal, different frequencies compared to the frequency of input signal occurs. This phenomenon, called Nonlinear Acoustic Effect, dues to the new waves of duplicating intense acoustic waves at material discontinuities such as voids, cracks, interfaces. It is very sensitive to micro size discontinuities, comparing to conventional ultrasonic NDT techniques. So it is specialized to early defects. Increasing the amount of discontinuities on material, new wave with different frequencies appears clearly. Especially, ‘Nonlinear Mesoscopic Elastic’ material like rock and concrete, which. 1 2 3. Professor Master’s Candidate Ph. D Candidate. has initially large nonlinearity, load-dependent discrete memory, and hysteresis, exhibits strong amplitudedependent (nonlinear) characteristics and its nonlinearity increases rapidly according to the damage. (Guyer, 1999) This paper concerns the thermal damage of concrete. Concrete consists of mineral aggregates bonded with cement paste, and it is naturally non-flammable. Also, concrete has low thermal conductivity, so it exhibits a good behavior in fire. However, in normal concrete, the strength of concrete is rapidly decreased at high temperature and happens cracking and spalling which cause the serious damage of the concrete structure. Concrete chemo-physically transforms in increasing temperature; water in concrete bonded physically evaporates around 100℃; dehydration of cement paste around 180℃; decomposition of calcium hydroxide most rapidly around 500℃; -quartz transforms to -quartz around 570℃, and this transformation causes microcracking in concrete. Besides, coefficients of thermal expansion are different in each aggregates and cement paste, so micro-cracks are generated among aggregates and cement paste. (Bazant and Kaplan, 1996) The goal of this research is evaluating Nonlinear Acoustic Effect testing to assess thermal damage of concrete. For the reliability of the applied method, linear method, i.e. pulse velocity, is measured to compare with the nonlinear parameter from Nonlinear Acoustic Effect testing. NONLINEAR ACOUSTIC EFFECT Guyer and McCall proposed a 1-dimensional nonlinear model (Guyer, et al. 1999). This model is phenomenological model for rock and concrete. According to the assumption, the relationship between stress and strain is approximated as elastic modulus ( ̇) which is given by (Van Den Abeele, et al. 2001) ∫ (. ̇). (1). In the linear model of strain-stress relationship, ( )̇ would be a constant. However, following the nonlinear approximation, modulus ( ̇) which is dependent of strain rate and strain amplitude is given by ( ( ) ( ̇ )). (. ( ). (. ( ). ( ̇( ))). ). (2). where characterizes the classical nonlinearity, is the non-classical nonlinearity which is concerned about the hysteretic nonlinear behavior, is the instantaneous strain, and ̇ is the strain rate. In the eq. (2), is the local strain amplitude over the previous period of a traversing wave, and it causes the nonlinear behavior to contact condition of grains and friction in micro-cracks. Using Taylor’s expansion to solve stress-strain relationship, classical nonlinear coefficient, is derived. The non-classical nonlinear coefficient represents the criterion of the hysteresis material, like rocks and concrete. Assuming input sound signals as two harmonic signals, whose frequency are and (in the case of ), propagate through a material. According to the phenomenological model, two harmonic signals modulate to each other and generate new sound signals which have related frequency of and . Then, the nonlinear coefficients and are expressed by the ratio of amplitudes at intermodulation of and ( ) ( ) ( ). ( ) ( ) ( ). ( ). In impact mode, a monochromatic acoustic wave of high frequency ( ) is modulated with an impact of low frequency ( ). The whole resonance mode spectrum is generated by an impact. So, there is no clear resonance frequency of impact, and it is complex to determine and in the ratio of amplitude of and . Avoiding the inaccuracy, Van Den Abeele et al. suggest the model using energy ratio (Van Den Abeele, 2000) ( ) where D is a nonlinearity parameter, is energy of modulation (side lobe) energy, is energy of impact vibration, and is propagating (probing) energy of high frequency wave. In this research, nonlinearity parameter is measured by the energy ratio model. EXPERIMENTS Concrete specimens were cast in Φ100 200mm molds. The mix proportion for these specimens is. 0.5:1:1.2:1.8 (Water:Cement:Sand:Gravel) using ASTM Type I Portland Cement. After 28-days water curing, concrete specimens are dried in a thermo-hygrostat at 80℃, relative humidity 5% avoiding thermal spalling of concrete specimens. After 7 days, electrical furnace induces thermal damage to concrete specimens. Three cases of thermal damage are divided by maximum temperature; 300℃, 600℃, and no damage case for reference. Heating rate was 10min/℃. When the temperature of electric furnace reaches at the maximum temperature, electric furnace maintains its temperature to 2 hours. After thermal damaging, specimens are taken out and submerged to water immediately for cooling down and curing. About 3 days later, damaged specimens are cut in half using diamond saw for nonlinear acoustic test.. Fig 1. Measurement setup for nonlinear acoustic testing in impact mode Fig. 1 shows experimental setup of nonlinear acoustic testing in impact mode. First of all, single harmonic continuous signal of frequency 180 kHz transmits through the specimen using a piezoelectric transducer (PANAMERTICS X1019) centered at 180 kHz. Second, impact is generated tapping specimen using impact hammer (PCB 086C03). Simultaneously, the signal of impact hammer triggers data-acquisition systems to record ultrasonic signal of received identical piezoelectric transducer and impact signal of 3-axis accelerometer (PCB 356A33). In order to increase the accuracy of vibration signal, 3-axis accelerometer is used to measure real vibration in specimen caused by impact. In fig. setup, NI PXI 4472B (dynamic signal data acquisition digitizer) and NI PXI 5105 (ultrasonic signal data acquisition digitizer) are used to record signal from accelerometer and piezoelectric transducer. Finally, using FFT frequency analysis is performed. Before FFT, hanning window and zeropadding are adopted to avoid leakage and frequency resolution. In frequency domain, , , and are measured. As shown in Fig. 2, the range of is 180k 200 Hz, and the range of is 170 kHz ~ 190 kHz, excluding the range of , and the range of is 0 ~ 10 kHz.. Fig 2. Impact signal and modulated signal in frequency domain RESULTS The nonlinearity parameters due to different level of thermal damage on concrete are shown at fig. graph. As impact energy increases, sideband energy normalized by probing energy is increases proportionally in all. specimens. Each slope of graphs represents the nonlinearity parameter . Comparing to the intact specimen, the nonlinearity parameter gets bigger as maximum temperature increased. Therefore, nonlinearity parameter and hysteresis of concrete specimen are increased according to the thermal damage.. Fig 3. (a) Nonlinearity parameters of different thermal damaged cases; and (b) ultrasonic velocities of different thermal damaged cases Table 1. Comparison of nonlinear parameter and relative difference in thermal damaged concrete Nonlinear parameter. Difference (%). Pulse velocity (m/s). Difference (%). Ref. 0.0023. -. 4553. -. 300℃. 0.2128. 9152.173913. 3903. 14.27630134. 600℃. 0.5893. 25521.73913. 3231. 29.03580057. Meanwhile, fig. 4 exhibits velocity the experimental result of ultrasonic pulse velocities. Comparison between the nonlinearity parameter and ultrasonic pulse velocity is shown at table 1. In ultrasonic pulse velocities, the maximum difference between reference and damaged specimen is about 30%. However, using nonlinearity parameters the difference is about two hundred times comparing to raw specimen and thermal damaged specimen at 600℃. So, reliability of nonlinear acoustic testing in impact mode on thermal damaged concrete is verified. For the other type of micro-cracking are able to apply the nonlinear acoustic testing in impact mode. ACKNOWLEDGMENTS This work was supported by the Innovations in Nuclear Power Technology (Development of Nuclear Energy Technology) of the Korea Institute of Energy Technology Evaluation and Planning (20101620100050) grant funded by the Korea government Ministry of Knowledge Economy and Specializations of Laboratories of National Defense in Basic Research Area (기초연구분야 국방특화연구실사업 – 고압 충격/복발현상 해석 기법) funded by Agency for Defense Development (ADD). REFERENCES ACI Committee 229 (1998), Nondestructive Test Methods for evaluation of Concrete in Structures, ACI 228.2R98, America Concrete Institute Bazant Z.P., and Kaplan M.F. (1996). Concrete at High Temperature: Material Properties and Mathematical Models. Burnt Mill, England: Longman Group Limited. Guyer R.A., and Johnson P.A. (1999),” Nonlinear mesoscopic elasticity: Evidence for a new class of materials”, Physics Today, 52, 30-35 Van Den Abeele K.E-A., Johnson P.A., and Sutin A. (2000), “Nonlinear elastic wave spectroscopy (NEWS) techniques to discern material damage, part I: nonlinear wave modulation pectroscopy (NMWS)”, Res. Nondestr. Eval, vol.12, pp.17-30, 2000. Van Den Abeele K.E-A., Sutin A., Carmeliet J., and Johnson P.A. (2001), “Micro-damage diagnostics using nonlinear elastic wave spectroscopy (NEWS)”, NDT&E, International 34 (2001), 239-248.