Impact of High Temperature on the Compressive Strength of ECC

The influence of different cooling regimes (quenching in water and cooling in air) on the residual mechanical properties of ECC (engineered cementitious composites) exposed to high temperature up to 800°C was discussed in this paper. The specimens quenching in water gained better mechanical properties than the ones cooling in air. The strengthening effect of quenching for specimens subjected to 800°C was more significant than for the ones subjected to 400°C. The microstructural characterization is examined before and after exposure to fire deterioration by using scanning electron microscopy. Results from the microtest well explained the mechanical properties variation of postfire specimens

Average Fracture Energy for Crack Propagation in Postfire Concrete

Wedge-splitting tests of postfire concrete specimens were carried out in the present research to obtain the load-displacement curves. Ten temperatures varying from room temperature to 600°C were employed. In order to calculate the accurate fracture energy, the tails of load-displacement curves were best fitted using exponential and power functions. Three fracture energy quantities (fracture energy GF, stable fracture energy GFS, and unstable fracture energy GFU) with their variation tendency and their mutual relationship were determined to predict energy consumption for the complete fracture propagation. Additionally, the stable fracture work WFS was also calculated. All these fracture parameters sustain an increase-decrease tendency which means that the fracture property of postfire concrete shares the same tendency

Crack Extension Resistance of Normal-Strength Concrete Subjected to Elevated Temperatures

Determination of the residual crack extension resistance curves (KR-curves) associated with cohesive force distribution on fictitious crack zone of complete fracture process is implemented in present research. The cohesive force distributes according to bilinear softening traction-separation law proposed by Petersson. Totally ten temperatures varying from 20°C to 600°C and the specimen size of 230×200×200 mm with initial-notch depth ratios 0.4 are considered. The load-crack mouth opening displacement curves (P-CMOD) of postfire specimens are obtained by wedge-splitting method from which the stress intensity factor curves (K-curves) are calculated. In each temperature, with the distribution of cohesive force along the fracture process zone, the residual fracture toughness KR (Δa) increases with increasing crack length Δa, whereas the KR-curves decrease with increasing temperatures Tm for the thermal damage induced. The stability analysis on crack propagation demonstrates that when the residual KR-curve is higher than K-curve, the crack propagates steadily; otherwise, the crack propagates unsteadily

Toughness Calculation of Postfire Normal Concrete

Fracture tests of postfire normal concrete with ten temperatures up to 600°C are implemented. Residual fracture toughness using analytical method is determined. Two situations are divided at critical load when calculating the cohesive fracture toughness. The initial and critical fracture toughness could be calculated from the complete load-crack opening displacement curves. Finally, the validation of double-K fracture model to the postfire concrete specimens is proved

Experimental Study on the Seismic Behaviour of Strengthened Concrete Column-Beam Joints by Simulated Earthquake

AbstractA total of five repaired column-beam joints (C-BJs) strengthened by basalt fiber reinforced polymer (BFRP) had been pre-damaged under cyclic loads in order to study on the seismic behavior. The experimental study including pre-damage, rehabilitation and re-test under cyclic loads concentrated on the effect on different levels of pre-damage and epoxy injection. Through the experimental data, ultimate load-carrying capacity, ultimate displacement, ductility, hysteresis curves and dissipative ability, the following conclusion was drawn: it is indispensable for the load-carrying capacity of frame beams to contribute to slab. The repaired C-BJs can reach or even exceed the level of their ultimate load-carrying capacity and original seismic performance before pre-damage. The load-carrying capacity and bondslip behaviors can be improved by the BFRP in the repaired C-BJs. Concrete columns can be effectively confined to the expansion and deformation. The failure mode of strong beam and weak column was changed into strong column and weak beam after rehabilitations. The study indicated that the method of BFRP-reinforced pre-damaged concrete column-beam joints was reasonable and effective

Reinforced ultra-high performance concrete beam under flexure and shear: Experiment and theoretical model

This paper investigated the influence of different shear span ratios, longitudinal (LR) and stirrup (SR) reinforcement ratios, and material types on the flexural and shear performance of reinforced ultra-high performance concrete (UHPC) beams. A series of bending tests with shear span ratios of 4.0, 1.5, and 1.0 were conducted. Results reveal that UHPC beams with a LR ratio of 2.53% exhibit 1.66, 2.00, and 1.45 times higher load capacity than that of the beams with a LR ratio of 1.34% at shear span ratios of 4.0, 1.5, and 1.0, respectively. A SR ratio of 1.40% can achieve 15% and 35% enhancement in load capacity of UHPC beams at shear span ratios of 1.5 and 1.0, respectively. The UHPC beams made with hybrid polyethylene and straight steel fibers have the highest load capacity compared to the beams with straight steel fibers and hooked steel fibers. The enhancement can achieve 8%, 5%, and 15% at shear span ratios of 4.0, 1.5, and 1.0, respectively. Furthermore, cross-section analysis, a novel truss-arch model, and strut-and-tie model were employed to theoretically calculate the load capacity of the UHPC beams with different failure modes. The relative errors of the cross-section analysis and truss-arch model are smaller than 9% and 25%. The findings of this work could provide guidelines for the structural design of UHPC beams under different loading conditions