| Ordinary concrete has serious problems such as low tension to compression strength ratio,high brittleness and easy cracking.Adding fiber to the concrete matrix can consume energy through fiber bridging,pulling out and breaking,and control the generation and development of cracks,so as to improve the above problems.Therefore,fiber reinforced concrete has been more and more applied in the field of bridge engineering in recent years.However,concrete itself is composed of coarse aggregate,fine aggregate,cementitious materials and other substrates of different sizes,which is a kind of composite materials with multi-scale characteristics.It is difficult to achieve multi-scale reinforcement by simply adding a certain kind of fiber.Based on this,the idea of realizing the progressive reinforcement of concrete under different stress and cracking states by hybrid fiber has gradually become a hot spot in the research of hybrid fiber concrete.In this thesis,C50 concrete is used as the matrix material.After a comprehensive comparison of the common fibers used to strengthen concrete at present,steel fiber,polyvinyl alcohol fiber and calcium carbonate whisker are selected as the macro,meso and micro scale fiber reinforcement materials respectively.They are separately mixed,binary and ternary mixed into the concrete substrate.The effects of different fiber content and fiber combination on the basic mechanical properties of the substrate are studied.Through study,the best preparation method and the best fiber combination are obtained.Finally,the research results of new materials are applied to reinforced rectangular beams to study the influence of the new hybrid fiber concrete on the shear behavior of beam members.This research is an interdisciplinary study of materials and structures.The main contents and conclusions are as follows:(1)The mix proportion design and preparation process of fiber reinforced concrete were studied,which provided the basis for the smooth development of the follow-up test.(2)The compressive strength and splitting tensile strength of fiber reinforced concrete were systematically tested,and the tensile to compressive strength ratio was analyzed based on the experimental results.Based on the above data,the effects of fiber type,fiber volume change rate and different fiber combination methods on the compressive strength and splitting tensile strength of concrete were studied,so as to determine the best combination and optimal content of single fiber and hybrid fiber,which provided reference for subsequent structural tests.The results showed that the compressive strength and splitting tensile strength of concrete could be improved by adding monobasic,binary and ternary fibers.Among them,2% steel fiber was the best in the case of single fiber,1.5% steel +0.5% PVA in the case of double fiber,and 1.5% steel +0.5% PVA+1% calcium carbonate whisker in the case of triple fiber.In addition,the three doping was generally better than the double doping,and the double doping was better than the single doping,which indicated that the hybrid fibers with different scales would produce a good positive effect.(3)The micro observation of fiber reinforced concrete was carried out by using scanning electron microscope.Through the analysis of the micro morphology of fiber reinforced concrete,the toughening mechanism of fiber reinforced concrete was revealed.It was found that the end hooks on both sides of steel fiber could form a good anchoring effect with the base material.After adding whiskers,the "friction effect" of whiskers enhanced the fiber bridging effect,so it had a better effect on the reinforcement and toughening of the base material.Because the hydrophilic hydroxyl groups on the surface of PVA could form a good bond with the matrix,PVA could consume energy through bridging,pulling out and breaking at the same time,so as to improve the toughness of concrete.In addition,the micro filling effect and micro bridging effect of whiskers could also improve the initial defects of the substrate to a certain extent.(4)Fiber reinforced concrete with different combinations of fiber types was prepared,and axial compression tests were conducted to obtain the stress-strain curves of the whole process of axial compression including the ascending and descending sections.Mechanical properties parameters such as elastic modulus,axial compressive strength,peak strain and residual stress of the substrate were obtained through the stress-strain curves.The strengthening mechanism of fiber on axial compression mechanical parameters was analyzed,and a full curve constitutive model of fiber concrete under axial compression was obtained based on fitting test data.(5)Through the specially designed test beam,the effects of different forms of fiber combinations on its failure mode transformation,ultimate bearing capacity and corresponding midspan deflection,crack development path,concrete strain,longitudinal reinforcement and stirrup strain were studied.The research showed that after adding fiber,the deformation of concrete beam still conformed to the assumption of plane section,the ultimate load and corresponding midspan deflection of reinforced beam was improved.Among them,the failure mode of ordinary reinforced concrete beam was shear failure,while the failure mode of concrete beams mixed with steel fiber and hybrid fiber was changed to bending failure,which showed that adding fiber had a good effect on improving the shear resistance of beams,and the combination of 1.5% steel +0.5% PVA+1% calcium carbonate whisker had the best effect.Under the same load,the strains of longitudinal bars and stirrups in fiber reinforced concrete beams were smaller than those in ordinary concrete beams,and the cracks showed the characteristics of "thin,dense and many",and this characteristic was more obvious in hybrid fiber reinforced concrete beams,which further indicated that fibers of different sizes showed good positive hybrid effect.By modifying the truss arch model,the theoretical calculation formula of the shear capacity of fiber reinforced concrete beams was established,and the calculated values were compared with the measured ones,which could provide some reference and theoretical guidance for the actual engineering design of fiber reinforced concrete beams. |
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