Crack Resistance Mechanism And Multi-scale Experimental Research Of Flexible Fiber Reinforced Concrete Box Girder

Crack is a common disease of concrete box girder,and it is also a technical problem that has not been completely solved for a long time.Flexible fibers have excellent performance but have not been widely used to solve the cracking problem of concrete box girders.The related research and application of flexible fiber concrete concentrate on concrete construction,pavement engineering,bridge deck pavement and so on.At the same time,here is still a lot of work to be carried out in the research object of the anticracking mechanism,the mesoscopic stress and strain environment differences,and the section model and size of the box girder test model.Therefore,it is necessary to study the mechanism of anti-crack enhancement of flexible fiber reinforced concrete box girder.The purpose of this paper is to reveal the anti-cracking mechanism of flexible fiber reinforced concrete in the whole process,and to further improve and enrich the theoretical study of the crack resistance of flexible fiber reinforced concrete;the structure and the anti-cracking mechanism of flexible fiber reinforced concrete are analyzed by multi scale,and the micro and nanoscale structure of flexible fiber concrete is connected with macro and meso mechanical properties to better understand the macroscopic crack resistance performance of flexible fiber reinforced concrete;the effect of flexible fiber on anticracking of reinforced concrete box girder is studied,and the engineering application of flexible fiber concrete is also provided.The main research contents and results of this paper are described as follows:Based on the theory of fracture mechanics energy,from the whole development stage of concrete crack initiation,occurrence,development,criticality,deterioration,and destruction,the evolution of energy transfer is analyzed,and a full-process anti-cracking model of flexible fiber concrete is constructed to reveal the nature of energy dissipation and cracking of flexible fibers.The main conclusions are as follows: the anti-cracking effect of flexible fibers on concrete is essentially the process of energy transfer.During the entire process of crack development,the energy required for the development of cracks will be gradually transformed into fiber breaking energy.Based on the early-age anti-cracking test of polypropylene fiber reinforced concrete,the effect of different fiber content on crack resistance and reasonable fiber content are studied.The internal structural characteristics of polypropylene fiber reinforced concrete with different fiber content are studied through the three-dimensional CT microscopic test of polypropylene fiber reinforced concrete.The multi-scale structural analysis of crack propagation mechanism of flexible fiber reinforced concrete is carried out.The main conclusions are as follows: fiber content is positively correlated with the improvement of crack resistance,and fiber volume content of 0%~0.3% is appropriate;The polypropylene fiber as a whole is a three-dimensional network space structure,and the shape of a single polypropylene fiber is affected by the cement mortar,the average volume of micropores is negatively correlated with the fiber content,and its morphology is mainly spherical and irregular;flexible fiber reinforced concrete materials have different structural features on the macroscopic,microscopic,microscopic and nanoscopic scales.The resistance of flexible fibers is reflected in the macroscopic conduction,microscopic stress,microscopic improvement,and nanometer energy consumption.Taking the typical railway simple box girder as the research object,a similar model design scheme is used to carry out the scaled model test of the flexible fiber reinforced concrete box girder during the multi-test stage.The main conclusions are as follows: the flexible fibers in the elastic test stage,the elastoplastic test stage and the whole process test stage have played a role in enhancing the structural crack resistance,reducing the number of cracks,the maximum width and the total area.And the resistance to cracking of flexible fibers is positively correlated with the amount of fiber.However,the flexible fibers have different anti-cracking ability in each test period,the elastic test phase is optimal,the elasto-plastic test phase is decreased,and the whole process test phase is decreased again.And in the whole process of testing stage,the low-rate fiber anticracking ability tends to be consistent,and the high-content fiber has stronger anticracking ability.The finite element software is used to establish a three-dimensional solid model with nonlinear mechanical properties of flexible fiber reinforced concrete.The numerical simulation of the scale model of flexible fiber reinforced concrete box girder is conducted to study the cracking behavior of flexible fiber reinforced concrete box girder.The main conclusions are as follows: the crack resistance of flexible fiber is optimal in structural elastic stage,and the higher the amount of flexible fiber,the stronger the crack resistance of concrete.The crack first occur at the floor support,then occur in the pure bending section of the floor,and gradually developed into the inner section of the pure bending section.Finally,it developed beyond the pure bending section.The research results have theoretical significance for the study of the mechanism of composite materials to improve the durability of concrete bridges,which is beneficial to the application of high-performance flexible fiber on concrete box girders to better solve the cracking problem of concrete box girders.