Investigation Of The Bending Fatigue Performance And Cracking Processes Of Ultra High Performance Concrete

Ultra high performance concrete as a new composite building material,is made of high strength concrete matrix and fibers,with high strength,high toughness and high durability and applied in civil engineering more and more widely.In practice,infrastructures,such as bridges and railways,withstanding repetitive loads frequently suffer from fatigue damages.Therefore,it is of great significance to study the flexural fatigue performance of utra high performance concrete in order to improve the safety and durability of concrete structures.However,by now there are few studies on the flexural fatigue performance of ultra high performance concrete and the extension of cracks in the process of fatigue.Considering these problems,in this study the bending fatigue performance of ultra high performance concrete prepared with the guide of gap-graded particle size distribution model were investigated.The cracking behavior and the extension of cracks were analyzed..In this way,theoretical guidance and technical support for the application of ultra high performance concrete in practical engineering can be provided by this thesis.The main research content and results of this study are summarized as follow:(1)Three kinds of ultra high performance concrete,i.e.steel fiber reinforced ultra high performance concrete,steel fiber and polypropylene fiber reinforced ultra high performance concrete,polypropylene fiber reinforced ultra high performance concrete,were prepared Under the guidance of the gap-graded particle size distribution model the untra high performance concretes were made of river sands of particle sizes from 160?m to 315?m and from 630?m to 1250?m,and cementitious materials including cement,superfine slag and silica fume.Bending toughness test was conducted on these three kinds of ultra high performance concretes.There is almost no difference in bending toughness for steel fiber reinforced ultra high performance concrete and steel and polypropylene fiber reinforced ultra high performance concrete.In comparison,polypropylene fiber reinforced ultra high performance concrete has a much lower bending toughness.(2)Tests of flexural fatigue at different stress level i.e.0.65?0.7?0.75?0.8?0.85 were carried out and the testing results were analyzed.It was found that the flexural fatigue lifes of all ultra high performance concrete specimens at the same stress level could follow the Two Parameter Weibull Distribution.The average S-logN quadratic parabola fatigue equation and S-logN-Pf quadratic parabola fatigue equation considering the fatigue failure probability,from which bending fatigue life could be predicted,were established and comparatively discussed.The bending fatigue limit of steel fiber reinforced ultra high performance concrete,steel fiber and polypropylene fiber reinforced ultra high performance concrete,polypropylene fiber reinforced ultra high performance concrete are 58.5%,59.1%,50.9%,respectively.(3)The fatigue cracking behavior and the evolution of cracks,strains and stiffness in the process of fatigue were studied by means of Three-dimension Digital Image Correction(3D-DIC).Further more,the fatigue cracking mechanism was analyzed.There was brittle fracture phenomena for polypropylene fiber reinforced ultra high performance concrete.However,there were curve cracking paths for steel fiber reinforced ultra high performance concrete and steel fiber and polypropylene fiber reinforced ultra high performance concrete.The evolution of cracks,strains and stiffness for steel fiber reinforced ultra high performance concrete could be divided into three phases,which are 2%-15%,77%-95% and 3%-15%,sequentially.Nevertheless,the evolution of cracks,strains and stiffness for steel fiber and polypropylene fiber reinforced ultra high performance concrete were mostly monitored only the phase two and three.Only three phases accounting for 11%,74% and 15% could be observed under the stress level of 0.8.Ultra high performance concrete shows different cracking behavior in the process of fatigue resulting from the role of steel fiber for macro-crack and polypropylene fiber for micro-crack in preventing cracks development.To be summarized,it is appropriate that ultra high performance concrete designed in this thesis can be conveniently used in practical construction engineering.Investigation on the bending fatigue proprieties and development of crack in the process of fatigue provides theoretical guidance and technical support for the application of ultra high performance concrete in practical engineering.