| Cement-based materials such as concrete and mortar are widely used in the construction of foundation engineering.Compared with other materials,traditional cement-based materials have higher compressive strength and relatively lower tensile strength due to their unique structural characteristics,but at the same time,there are shortcomings such as poor deformation ability,easy cracking and other defects.Under the action of external load and other factors,the internal damage of bridge structure increased and the structural defects continued to develop.And these cracks would continue to expand with the growth of service life,and might eventually lead to the occurrence of concrete bridge accidents.Therefore,it is urgent to find a new cement-based material with high strength performance,good antideformation ability and crack control ability,which can be applied to the reinforcement and maintenance of a large number of small and medium span old Bridges,so as to improve the durability of the whole bridge and extend the service life.Based on the filling effect of calcium sulfate whisker(CSW)on micron scale and the toughening effect of polypropylene fiber(PPF)on macro scale,calcium sulfate whisker reinforced cement matrix composite,polypropylene fiber reinforced cement matrix composite and hybrid fiber reinforced mortar(HyFRM)were designed.The mechanical properties,bending toughness and microstructure of hybrid fibers were studied to determine the optimal mix ratio of hybrid fibers.Combined with basalt fiber reinforced polymer(BFRP)bars,the influence of composite reinforcement materials on the bearing capacity,deflection,strain and crack development of damaged concrete beams is studied,aiming at providing a new idea for the promotion and application of new materials in the field of existing bridge strengthening and maintenance.The main work and achievements of this thesis are as follows:(1)Effects of CSW and PPF content and aspect ratio on flexural strength and compressive strength of HyFRM basic mechanical properties.Firstly,the influence of CSW or PPF on the mechanical properties of cement mortar was analyzed,and the influence of CSW and PPF on the mechanical properties of HyFRM was further analyzed in combination with the enhancement coefficient and hybrid coefficient,so as to explore the optimal dosage of single mixing and mixed fiber.Finally,the microcosmic analysis of HyFRM was carried out by SEM technology,and the action mechanism of hybrid fiber on cement-based composites was explored.(2)The influence of CSW or PPF and mixed fibers on the bending toughness of cement-based materials was studied.The load-deflection curves with different fiber content were obtained by four-point bending test.The flexural toughness of each group of specimens was evaluated by the flexural strength,flexural deflection,peak energy absorption value,initial flexural toughness index ratio and residual flexural toughness ratio.At the same time,the bending failure process of thin slab beam specimens was described,the optimal fiber content was determined,and the toughening mechanism of hybrid fiber on thin slab beam was analyzed.(3)The influence of BFRP bars and fiber composite reinforcement materials on the flexural behavior of damaged beams was studied.Three strengthened beams and one ordinary RC beam as contrast beam were designed and cast,and four point bending tests were carried out on each beam.The main failure modes and crack development process of strengthened beams and ordinary RC beams are analyzed.The load-deflection curves of all beams are studied.The crack control mechanism of damaged beams strengthened by BFRP bars and fiber composite material is discussed.According to three possible failure modes and two boundary failure types of the strengthened beams,the formula for calculating the flexural capacity of the strengthened beams was deduced based on basic assumptions.The theoretical and experimental values were compared to analyze the difference between them and the reasons. |
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