Studies On Mechanical Performance Of Polyvinyl Alcohol-Steel Hybrid Fiber Reinforced Cementitious Composites

Engineered cementitious composites with characteristic of low shrinkage(LSECC)was developed recently in order to overcome the brittleness of concrete and large drying shrinkage of traditional engineered cementitious composites(ECC).However,the characteristic of high ductility no longer exists in high-strength LSECC with mono polyvinyl alcohol fibers,which may limit its applications in structures.The present thesis focuses on the balance study between high-strength and high-ductility of LSECC through adding additional steel fibers.Hence,experimental studies and theoretical simulations are carried out on the mechanical performance of polyvinyl alcohol-steel hybrid fiber reinforced cementitious composites.In addition,critical issues relating two practical applications of LSECC are investigated as well.While keeping a constant volume of polyvinyl alcohol fibers in the composites,the tensile,flexural and compressive behaviors of polyvinyl alcohol-steel hybrid fiber reinforced cementitious composites are evaluated with special focus on the impact of matrix strength(4 water to cement ratios)and additional steel fiber content(from 0 to 1% in volume).The experimental results show that the cracking strength,ultimate strength,ultimate tensile strain are increased with addition of steel fiber.Meanwhile,the individual crack opening is decreased compared with the case without steel fiber.The above enhancement of additional steel fiber is more and more pronounced with increase of matrix strength.Hybrid fiber reinforced composites with compressive strength of 70 MPa,ultimate tensile strain of 10000-18000 micro strain and maximum crack width as tensile strength achieved of 30-60 ?m are obtained.Based on three-point bending tests on pre-notched beam,the bridging stress-crack width relationship of polyvinyl alcohol-steel hybrid fiber reinforced cementitious composites are determined through inverse analyses.Meanwhile,a theoretical model describing fiber bridging behavior is used to simulate stress-crack width relationship with fiber,matrix and fiber/matrix interfacial parameters as model inputs.The effect of geometry of steel fiber on stress-crack width relationship of the composite is investigated with the theoretical model.Fracture mechanics based flexural model for polyvinyl alcohol-steel hybrid fiber reinforced cementitious composites is developed.The flexural performance of the hybrid fiber reinforced cementitious composites is simulated with the model.Through comparing the model and experimental results of the composites under bending load,a good agreement between them is found.In addition,the flexural performance of the hybrid fiber reinforced composites with optimized dimension of steel fiber is investigated with the model and the results show that it is possible to use optimized steel fiber to achieve a better flexural performance of the composite with the same fiber content.The proposed flexural model establishes the relationship between cement matrix,fiber reinforcements and fiber/matrix interfacial parameters and macro mechanical performance of the composite,which lays the foundation of performance-based design of hybrid fiber reinforced cementitious composites.Regarding the applications of LSECC in structures,two practical problems are concerned,one is surface protection layer for concrete structures and another ductile strip for concrete pavements.First,flexural performance of layered LSECC-concrete composite beams is investigated by both model simulation and experimental study.The results show that applying a layer of LSECC beneath a concrete layer could significantly improve the bending properties of the beam.The effects of the strength of LSECC and concrete,thickness of LSECC layer are discussed through model simulations.Second,the application of LSECC in jointless concrete pavements is presented.The design concept is validated from both laboratory tests and in-site construction.The results show that it is possible to localize the tensile cracks into the ductile strip instead of cracking in adjacent concrete slab,that means a jointless concrete pavement is successful.