The Study On The Spalling Performance Of Hybrid Fiber Reinforced High Strength Concrete

When two or more of the hybrid fibers(including a difference in performance, size, both performance and size) are added to the concrete matrix, fiber and matrix are intertwined,constructing a “bridge”, which will significantly improve the strength and toughening of concrete. The spalling performance of hybrid fiber reinforced high strength concrete(HFRHSC) with steel fiber and polypropylene fiber is carried out in the dissertation.It is compared that HFRHSC whose volume rate Vf of steel fibers are 0?2%?4%, volume rate Vp of polypropylene fibers are 0?0.05%?0.1%, and the plain concrete strength grade is C80, the spall fracture performance of concrete is explored and the spalling constitutive relation of HFRHSC is established. The damage strain rate threshold value of spalling peak is proposed, and numerical simulation is carries out. The main research results are as follows:(1)Adding steel fiber and polypropylene fiber into the concrete matrix can increase the compressive strength of HFRHSC. Compared with compressive strength, steel fiber and polypropylene fiber make a more significant impact on the splitting tensile strength of HFRHSC; Steel fiber makes a more contribution to compressive strength and splitting tensile strength than polypropylene fiber and the effect it makes on compressive strength is not obvious.(2)The spalling strength of HFRHSC, with the increase of strain rate, rises first and then decreases(the turning point is damage strain rate threshold value of spalling peak),which reflects the spalling process of concrete is to experience impact compression first and then the tensile wave. The material in the stage of impact compression suffers damage softening. The strain rate is greater, the compression damage is greater. The damage strain rate threshold value of spalling peak ascends with the increase of volume fraction of steel fiber and descends with the increase of that of polypropylene fiber. In the data measured,the damage strain rate threshold value of spalling peak of HFRHSC ranges from 25s-1 to35s-1, under which the spalling strengths of HFRHSC are generally larger than those under the range from 1s-1 to 20s-1. When the spalling strength rises with the increase of strain rate, the gradient remains basically consistent, that is about 1.0MPa·s.Under the same level of strain rate, the spalling strength of plain concrete is lower than that of HFRHSC added with steel fiber and polypropylene fiber, which shows steel fiber and polypropylene fiber can improve the strength and toughness of concrete, and the enhancement effect of polypropylene fiber is as not obvious as that of steel fiber. In the experiment, when Vf and Vp are 2% and 0.05% respectively, the spalling strength of HFRHSC reaches its maximum value.Plain concrete suffers the most serious spalling damage and some fragments fly away from the matrix. HFRHSC mixed with steel fiber and polypropylene fiber can still maintain holistic after the spalling damage. The distance between the spalling location and the free end of the specimen decreases with the increase of strain rate; the spalling frequency rises with the increase of strain rate; the cracking trend is random, basically not parallel to the free end; the spalling section of the specimen is randomly irregular.(3)Based on the maximum tensile stress model and using mechanics of composite materials theory, the spalling constitutive relation of HFRHSC is established. It adequate considers the splitting strength of static plain concrete, the volume fractions of steel fiber and polypropylene fiber, and strain rate make on the spalling strength. The curves obtained are generally in agreement with the experimental results.(4)By using the finite element program LS-DYNA, spalling experiments of specimen C80V0P0?C80V2P0.05?C80V2P0.1?C80V4P0.05?C80V4P0.1 are simulated, the experimental process and failure form are reproduced. The results show that,(1) with the increase of impact velocity, spalling of HFRHSC becomes more and more serious, characterized by spalling frequency increasing, damage accumulating more and more serious and the spalling location gradually moving towards the free end.(2) At the same impact velocity, the spalling strengths are close and the spalling position remains consistent, the ability of C80V0P0?C80V2P0.05?C80V2P0.1?C80V4P0.05?C80V4P0.1 to resist tensile stress is increased in turn.HFRHSC has certain ductility, and its spalling process shows “pseudoplastic” failure behave. In the cross section, the spalling crack of the specimen HFRHSC presents an outside-in extension. Multiple spalling sequence of HFRHSC is related to the impact velocity.When impacting with a low speed(3 m/s, 5 m/s), the spalling sequence develops from the free end to the impacted end; when with a high speed(10m/s), it goes from the middlecompression damage zone to the two ends; when with a medium speed(7m/s), it moves from the impacted end to the free end.