Sun-Jong Park

Nonlinear Ultrasonic Method to Evaluate Residual Mechanical Properties of Thermally Damaged Concrete

An experimental study has been carried out to evaluate the residual mechanical properties of thermally damaged concrete using a nonlinear ultrasonic method. A nonlinearity parameter was measured by the impact-modulation technique, which sensitively reflects the degree of thermal damage in concrete. Key mechanical properties, compressive strength, elastic modulus, and peak strain were obtained from compressive stress-strain curves of cylindrical concrete samples. The correlations between the nonlinearity parameters and the mechanical properties were investigated for various conditions: high-temperature exposure time of 1, 2, and 3 hours and four different mixture proportions. The authors thereupon developed regression equations that can predict the residual mechanical properties from the measurement of a nonlinearity parameter, which indicates the degree of thermal damage.

Evaluation of Microcracks in Thermal Damaged Concrete Using Nonlinear Ultrasonic Modulation Technique

Dept. of Civil and Environmental Engineering, KAIST, Daejeon 305-701, KoreaABSTRACT This paper concentrates on the evaluation of microcracks in thermal damaged concrete on the basis of the nonlinearultrasonic modulation technique. Since concrete structure exposed to high temperature accompanies the development of microcracksdue to the physical and chemical changes from temperature and exposed time, the adoption of nonlinear approach is required.Instead of using the conventional ultrasonic nondestructive methods which have the limitation in evaluating excessive microcracks,accordingly, a nonlinear ultrasonic modulation method which shows better sensitivity in quantifying microcracks is introduced.Upon the analysis for the modulation of ultrasonic wave and low frequency impact to measure the nonlinearity parameter, whichcan be used as an indicator of thermal damage, the verification processes for the introduced technique are followed: SEM inves-tigation and permeable pore space test are performed to characterize thermally induced microcracks in concrete, and ultrasonicpulse velocity tests are performed to confirm the outstanding sensitivity of nonlinear ultrasonic modulation technique. In advance,compressive strength of thermal damaged concrete is measured to represent the effect of microcracks on performance degradation.Correlation studies between experimental data and measured data show that nonlinear ultrasonic modulation technique can effec-tively be used to quantify thermally induced microcracks, and to estimate the compressive strength of thermally damaged concrete.Keywords : nonlinear ultrasonic wave, nondestructive evaluation, thermal damaged concrete, microcracks, impact modulation

Analysis of Factors Influencing Fire Damage to Concrete Using Nonlinear Resonance Vibration Method

Abstract In this study, the effects of different mix proportions and fire scenarios (exposure temperatures and post-fire-curing periods) on fire-damaged concrete were analyzed using a nonlinear resonance vibration method based on nonlinear acoustics. The hysteretic nonlinearity parameter was obtained, which can sensitively reflect the damage level of fire-damaged concrete. In addition, a splitting tensile strength test was performed on each fire-damaged specimen to evaluate the residual property. Using the results, a prediction model for estimating the residual strength of fire-damaged concrete was proposed on the basis of the correlation between the hysteretic nonlinearity parameter and the ratio of splitting tensile strength. Keywords: Fire-Damaged Concrete, Mix Proportions, Nonlinear Acoustics, Post-Fire-Curing, Splitting Tensile Strength[Received: February 9, 2015, Revised: March 19, 2015, Accepted: March 24, 2015] *한국과학기술원 건설 및 환경공학과, **경북대학교 과학기술대학 건설방재공학부, ✝Corresponding Author: Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, KAIST, Daejeon 305-701, Korea (E-mail: kwakhg@kaist.ac.kr)