| The use of mineral admixture can offer benefits with respect to the costs of manufacturing of concrete, the overall consumption of energy for unit volume concrete, and to the effects on the environment of cement and concrete products. Moreover, concrete made with mineral admixture can have properties that are desirable for particular purposes, such as lower total heat evolution in mass concrete, better workability, especially improved durability. Fly ash, blast furnace slag are widely used in cement-based materials as typical mineral admixtures. It is well known that lots of concrete structures or elements are subjected to repeated load during their service life such as pavements structures of bridges or airport runway, maritime structures. For the intensive using of mineral admixtures, concrete made with fly ash or blast furnace slag, even at high cement replacement level is inevitable to used in such structures or elements, thus it is of significance to understand their effects on fatigue behavior of concrete and the way how they exert these effects.The investigation reported herein consists of experimental studies and theoretical analyses with respect to the following aspects: effects of fly ash and blast furnace slag on fatigue behaviors and the mechanism, relationships among constitutes, microstructures and fatigue performances of concrete, application principle of mineral admixtures with respect to improve fatigue performance of concrete. A preliminary method is also presented to control the ratio of water to binder in the case of high-level cement replacement by mineral admixtures in order to gain better overall fatigue performance.Effects of fly ash, blast furnace slag on fatigue behavior of concrete The experimental work planed in this investigation consists of 6 mixtures of concrete with cement replacement level as variable. The cement replacement levels are 30%, 50%, 80% by blast furnace slag and 30%, 50% by fly ash respectively (mass fraction), ratio of water to binder=0.35, ratio of fine aggregate to total aggregate=0.38, cement content is 480kg?m-3 for control mixture. 100mm×100mm×400mm beam specimens were test under cyclic flexural load,at different stress levels, ranging from 0.90 to 0.65. The stress ratio was kept constant at 0.10. Constant amplitude sinusoidal loads were applied at frequency of 2~3Hz for S>0.75 and 10Hz for S≤0.75.Fatigue test results show that fatigue life increases with increased content of mineral admixtures when S≤0.80, while decrease when S>0.80. Fatigue strengths of concrete with various cement replacement by fly ash or blast furnace slag corresponding to 2,000,00 cycles ranging from 0.624f to 0.691 f . The fatigue strength of concrete with higher cement replacement by mineral admixture is higher than that of those with lower cement replacement. The curve of stress level (S) vs. logarithm of fatigue life (lgN) well follows a line at lower cement replacement levels, however there appears a"stage"between S=0.85 and S=0.80 with obvious different slope against the whole curve when the cement replacement level reaches 50% by fly ash and 80% by blast furnace slag. At the same stress level, two-parameter weibull distribution can be used to description of fatigue life, thus equations relate stress level S to N numbers of cycles to failure with a survivorship probability of 95% for concrete with various mineral admixtures content obtained.Influencing Mechanism of mineral admixtures on concrete fatigue behavior Based on well established failure mechanisms in different load cycle region, the fatigue damage evolution process in low-cycle load region (equivalent to high stress level) and high-cycle load region(equivalent to lower stresslevel) are analyzed. Accordingly, the load cycle regime is divided into three regions, i.e. matrix cracking dominant region for lgN≤3, wherein matrix cracking is the dominant failure mechanism, bond cracking dominant region for lgN≥4, bond cracking dominates the failure process and transition region for 3 |
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