Research On Static And Fatigue Damage Of Steel Fiber Reinforced Concrete

Steel Fiber Reinforced Concrete (SFRC) is a new type of composite material made of adding randomly distributed short steel fibers to ordinary concrete. It has excellent properties of ordinary concrete, and steel fibers limit crack propagation, so brittle concrete in nature shows a high crack resistance and can delay the appearance of cracks, and SFRC has larger ductility and toughness and excellent tensile, flexural, impact, abrasion, anti-fatigue properties. In recent years, SFRC has been widely used and deeply studied. According to present research results at home and abroad, static and fatigue damage of SFRC has been studied in this paper, and the main researches and conclusions are as follows:1. The method of how to convert the load-deflection curves of4-point bending beam obtained from tests into the corresponding stress-strain curves has been given. According to the energy equivalent prinple and the weibull statistical distribution theory, the constitutive model and the damage model of SFRC under single direction loading have been derived. As long as accurate determinations of the elastic modulus, the peak stress and peak strain of specimens were given, the constitutive equation and the damage evolution equation of SFRC under single direction loading could be derived.2. The static failure can be seen as a special fatigue failure, which means the static failure is the fatigue failure only bearing one-cycle loading. Damage is also a process of gradual accumulation during loading. By learning from the analysis of fatigue damage, the damage evolution equation describing the relationship between the damage variable and strain under uniaxial loading could be derived based on the damage theory. According to the strain equivalence principle the corresponding constitutive equation could also be obtained.3. Bending fatigue tests of steel fiber reinforced recycled concrete (SFRRC) and steel fiber reinforced pebble concrete (SFRPC) have been carried out. Fatigue lives at different stress levels (S=0.7,0.75,0.8,0.85) were obtained. Analysis results showed that the relationship between the stress level S and the logarithm of fatigue life N was linear, and the correlation coefficient was above0.99; fatigue life of SFRRC was greater than that of SFRPC at any stress level because of different interfacial bonding strength. The fatigue life equation obtained by summarizing present literature could be used to estimate fatigue life under flexural fatigue loading. Fatigue strain displayed the three-stage development law, and fatigue strain of SFRRC developed more slowly than that of SFRPC when increasing the cycle ratio. So it was obvious that SFRRC with the recycled aggregate as the coarse aggregate could not only change construction waste into resource treasure, reduce environmental pollution and realize resource recycling, but also show that its fatigue life and fatigue strain development were superior to those of SFRPC.4. The Lognormal distribution and Weibull distribution were used to test fatigue lives of SFRRC and SFRPC. Results showed that the experimental fatigue lives could preferably obey Lognormal and two parameter Weibull distributions. Linear relationships of the single logarithm fatigue equation and the double logarithm fatigue equation under different survival ratios P and stress levels S were established, and the correlation coefficients were above0.99. Survival ratios P had little effect on the regression coeffients B and b of SFRPC, so average values of these two regression coeffients could be used as universal results. Survival ratios P had little effect on all regression coeffients of SFRRC, so average values could be used as universal results.5. Through fitting the experimental fatigue data of SFRC with two gradings of aggregates, the fatigue equation was recommended, and the regression coefficient was above0.971. It was closer to the experimental fatigue data compared with other fitting equations.6. The fatigue strain evolution equation was proposed to describe the fatigue strain evolution curves of SFRC, and the fitting curves had a good agreement with the experimental curves. It was observed that under constant amplitude fatigue loading, the fatigue modulus was inversely related to the fatigue strain, so by use of symmetry the fatigue modulus evolution equation was obtained according to the fatigue strain evolution equation. Present experimental data of fatigue modulus was used to test the suggested fatigue modulus evolution equation. It was found that the fitting curves expressed by the fatigue modulus evolution equation coincided with the experimental curves very well, and the correlation coefficients were all above0.99. So it showed that the suggested fatigue modulus evolution equation was appropriate for describing fatigue modulus evolution curves of SFRC.7. The damage variable evolution curves obtained by use of the fatigue strain and fatigue modulus defining the damage variable showed that the damage variable evolution curves defined by the maximum fatigue strain and fatigue residual strain were basically consistent and the deviations were quite small, but the damage variable evolution curve defined by the fatigue modulus was significantly larger than that defined by the fatigue strain. When the cycle ratio was0.9, the damage variable value defined by the fatigue strain was about0.35, and the value defined by fatigue modulus was about0.77. When the initial cycle started, the damage variable value defined by the fatigue modulus was about0.34, but the value defined by the fatigue strain was close to0. That was to say, the damage variable value defined by the fatigue modulus was always larger than that defined by the fatigue strain.8、Based on the damage mechanics, the flexural fatigue damage equation of SFRC was derived. Results showed that the fitting curves obtained by the suggested fatigue damage equation had a good agreement with the experimental fatigue evolution curves.9、The residual fatigue life equation and residual flexural strength equation of SFRC were derived by the damage variable according to the essential relation between macroscopic quantity variation of materials and fatigue damage evolution. So the residual fatigue life and residual flexural strength could be got at given fatigue stress level and different damage conditions by these equations This could supply reference for safe estimate and decision on damage structure. It could be known from the relationships between the residual fatigue life and the damage variable and the cycle ratio, the residual fatigue life decreased in curve with increasing the damage variable; when the damage variable was within0.3, the decreasing range of the residual fatigue life was larger; and lower the stress level was, clearer was the decreasing range; when the stress level was0.7, the curve dropped sharply. The residual fatigue life decreased approximately in straight line with increasing the cycle ratio, smaller the stress level was, clearer was the decreasing range. It could be known from the relationships between the residual flexural strength and the damage variable and the cycle ratio, the residual flexural strength decreased approximately in straight line with increasing the cycle ratio; and the residual flexural strength descended sharply close to failure. In addition, variation of the stress level had little influence on the residual flexural strength at the same cycle ratio; the residual flexural strength varied approximately in straight line with the damage variable, and higher the stress level was, larger was the residual flexural strength.