| Ultra-high performance concrete(Ultra-high Performance Concrete,hereinafter referred to as UHPC)is a new type of cement-based composites with ultra-high strength,toughness and excellent durability.Due to the need to add steel fiber,it is also known as ultra-high performance fiber reinforced concrete(UHPFRC).The distribution of steel fiber in concrete is random and the utilization rate is low.In order to realize the efficient utilization of steel fiber,the orientation of steel fiber in the matrix was improved by hybrid fiber form and flow shear induction,and the mechanical properties tests such as compression,bending and uniaxial tension were carried out.Combined with digital image correlation methods and numerical simulation,the effect of fiber orientation on the strengthening and toughening effect of UHPC was discussed,and the meso-strengthening mechanism of oriented steel fiber was revealed.The main work and conclusions of this paper are as follows:(1)Directional steel fiber UHPC is prepared based on flow shear effect.Combined with multi-crack theory,crack evolution and stress distribution of steel fiber are analyzed by digital image correlation method and numerical simulation,and the influence mechanism of fiber orientation distribution on bending and tensile properties is explored.The following conclusions are drawn: the orientation coefficient of directional steel fiber induced by flow shear is increased from 0.5-0.6 to 0.7-0.75(bending test)and 0.75-0.8(tensile test).The strain concentration of random fiber UHPC first appears at the bottom of the span and runs upward,and a few microcracks appear near the main crack,while the main crack of directional fiber UHPC extends upward,there are several microcracks on both sides of the main crack,and the damage degree of UHPC is effectively characterized by the crack area ratio,which can be used as a quantitative index to describe the damage assessment of UHPC.Under the action of bending load,the bridging effect of directional steel fiber superimposes the stress conduction effect,which makes the tensile stress of the fiber transfer to the same direction fiber,which is characterized by the decrease of fiber proportion in compression zone and high strain zone and the increase of fiber ratio in middle and low stress areas(2)Based on the improved toughness evaluation method and hybrid effect,combined with the wall effect,the effects of mixed fiber combinations on the orientation distribution,bending and tensile properties of UHPC fibers are investigated.The following conclusions are drawn: the wall effect is affected by the hybrid ratio of long and short fibers,and when the content of long fibers is larger than that of short fibers,the wall effect has a significant effect on the fiber orientation of bending specimens.Through the statistics of the buried depth of the tensile noodle fiber and combined with the theory of interface effect,the conditions for determining the hybrid effect of flexural strength and tensile strength are determined.(3)Combined with hybrid effect and fiber reinforcement coefficient,the effects of hybrid combinations and pouring methods on the compressive strength of UHPC are quantitatively evaluated,and the following conclusions are drawn: reasonable fiber hybrid ratio can achieve positive hybrid effect and greatly improve the compressive strength of UHPC.The oriented steel fiber UHPC has anisotropy in compressive strength.When the loading direction is parallel to the fiber orientation,the compressive strength of the test group AP is higher than that of the AC test group whose loading direction is perpendicular to the fiber orientation.(4)the formulas of flexural strength and tensile strength of UHPC are established.On the basis of the existing strength calculation formula of fiber reinforced concrete,the fiber orientation coefficient is taken as the influence factor,and combined with fiber content,fiber length-diameter ratio and hybrid coefficient,the formulas of UHPC flexural strength and tensile strength are established. |
PDF Full Text Request