There has been increasing interest in investigation on safety of concrete structure because the damage in concrete can significantly affect the mechanical properties of concrete such as strength and elastic modulus. This thesis attempt to evaluate the effect of fire damage on concrete using nonlinear resonance vibration method, one of the nondestructive methods. The focus of this thesis is to estimate the residual strength by appraising micro damage of fire-damaged concrete. Traditional nondestructive method cannot sensitively interact with micro damage induced by fire. To improve the sensitivity, nonlinear methods are used to characterize the micro-damaged concrete.
Nonlinear resonance vibration method used in this thesis is that resonance frequency shift occurs when impact applied to concrete specimens. Concrete exhibits not only nonlinear stress-strain relation but also hysteresis and discrete memory effect which are contained in consolidated materials such as rock and cement-based materials. Hysteretic materials show typically the linear resonance frequency shift which is one of the consequences of hysteretic materials. Therefore, the shift of resonance frequency is changed according to the degree of microcracked materials. In case of undamaged concrete, the shift of resonance frequency slightly occurs or remains unchanged. However, resonance frequency shift can evidently be observed when exciting the damaged sample. The more damaged, the more the shift of resonance frequency occurs.
In this study, it was through nonlinear resonance vibration method that assessment of fire-damaged concrete is performed. The five exposure scenarios were considered: 200℃, 400℃, 600℃, 800℃ and reference(20℃) and exposure time of 1hour. Fire damage was applied to concrete specimens by electric furnace. Also, the effect of post-fire-curing on properties of fire-damaged concrete was taken into account. After cooling, concrete samples were cured in ambient conditions for different post-fire-curing periods: 1week, 1month, 6months, 12months. Hysteretic nonlinearity parameter, one of the damage indicators, was obtained by amplitude-dependent resonance frequency shift after specific curing periods. In addition, splitting tensile strength was measured on each sample to characterize the variation of residual strength.
The purpose of this research is to propose a correlation between the hysteretic nonlinearity parameter and residual strength from nonlinear resonance vibration method and splitting tensile strength, respectively. For the verification of the proposed correlation, additional concrete specimens were designed and the correlation was obtained to compare the proposed relation through the same experimental procedures.
화재 발생 시 콘크리트 내부에는 미세 균열이 발생하고 이는 강도 및 탄성계수 등의 물성치에 상당한 영향을 미친다. 본 연구에서는 비파괴 기법 중 하나인 비선형 공진기법을 이용하여 화재 손상을 입은 콘크리트 내부의 미세손상을 평가한다. 비선형 공진기법은 충격의 세기에 따른 공진 주파수의 변화하는 특징을 이용하여 손상 정도를 평가하는 것이다. 손상 정도에 따라 공진 주파수가 변화하는 정도가 달라진다. 콘크리트 내부의 손상 정도를 평가하기 위해 노출온도를 200℃, 400℃, 600℃, 800℃로 설정하였고 화재 손상 후 콘크리트의 내부의 변화를 확인해보기 위해 손상 후 기간을 1주일, 1개월, 6개월, 12개월로 달리하여 실험하였다. 손상 정도의 지표가 될 수 있는 비선형 인자를 비선형 공진기법을 통해 얻은 후 쪼갬 인장강도 실험을 통해 손상 정도에 따른 잔존 강도를 측정하였다. 이를 통해 비선형 인자와 잔존 강도의 관계를 파악하였다. 이 관계의 검증을 위해서 깊이 별 손상 시편을 제작하고 기존 관계와의 비교를 하여 적합성을 확인하였다.
