The non-linear resonance vibration method is a promising non-linear acoustic technique that has been used to evaluate contact-type defects of concrete induced by exposure to fire. In this work, an experimental study was performed on thin fire-damaged concrete discs to evaluate residual tensile strength using the non-linear resonance vibration method. Using correlation analysis, an experimental relationship is proposed between splitting tensile strength and a hysteretic non-linearity parameter (HNP), as measured from some 350 fire-damaged concrete discs of different mix proportions. In addition, the thermal gradient damage of concrete was examined to verify the efficiency of the proposed equation. It was found that the tensile strength of fire-damaged concrete could be effectively estimated from the measured HNP without consideration of the mix proportion, peak temperature, period after the fire or thermal gradient damage. Notation d diameter of sample F maximum applied load recorded at failure f measured resonance frequency at increased input amplitudes f 0 measured linear resonance frequency l length of sample M elastic modulus M 0 linear elastic modulus T s splitting tensile strength T s0 splitting tensile strength of undamaged sample T sn splitting tensile strength of damaged sample t time of one-dimensional (1D) dynamic behaviour of solids AE hysteretic non-linearity parameter (HNP) AE h HNP measured by amplitude-dependent resonance frequency shift AE h0 HNP of undamaged sample AE hn HNP of damaged samplẽ å strain amplitude in proportion to input amplitude å strain of 1D dynamic behaviour of solids _ å strain rate (_ å ¼ då=dt)

Fire damage to concrete causes contact-type defects that degrade its durability through impaired mechanical properties. Various nondestructive tests are used to evaluate defects induced by fire damage. Recently, nonlinear ultrasonic methods such as the nonlinear resonance vibration method and nonlinear modulation method have been introduced. These nonlinear methods are more sensitive to fire-induced contact-type defects than the linear ultrasonic method. This study involved an experimental analysis of the residual material properties of fire-damaged concrete, specifically, compressive strength, splitting tensile strength, and static elastic modulus. The residual material properties of 116 cylindrical concrete samples with various mix proportions and subjected to various heating temperatures were measured by a destructive method, and their nonlinearity parameters were measured by two nonlinear ultrasonic methods. Through regression analysis, correlated relationships that can facilitate the prediction of residual material properties of fire-damaged concrete using measured nonlinearity parameters were identified. In addition, the effect of fire damage on the mechanical strength of concrete was investigated by comparison with the relationships for undamaged concrete, and relationships for the evaluation of fire-damaged concrete were identified through regression analysis.

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This study examined the effect of adding synthetic fibers, that is, polypropylene (PP) and nylon (Ny), on explosive spalling and residual tensile mechanical properties of high-performance fiber-reinforced cementitious composites (HPFRCCs). Three different matrix strengths (100 MPa, 140 MPa, and 180 MPa), four different volume contents of the synthetic fibers (0%, 0.2%, 0.4%, and 0.6%), and three different exposure time (1 h, 2 h, and 3 h) based on the Internatinoal Organization for Standardization (ISO) fire curve were adopted as variables for this experiment. The experimental results revealed that the addition of synthetic fibers improved the resistance to explosive spalling induced by high-temperature, especially when PP and Ny were mixed together. For a higher matrix strength, greater volume content of the synthetic fibers was required to prevent explosive spalling, and higher residual strengths were obtained after the fire tests. An increase in the volume fraction of the synthetic fibers clearly prevented explosive spalling but did not affect the residual tensile strength. In the case of a higher matrix strength, a reduction in the strength ratio was observed with increased exposure time.

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