Bending Test And Fractal Analysis Of Polypropylene Fiber Reinforced Engineering Cementitious Composites(PP-ECC)

In recent years,with the development of society,the concrete structure is developing towards the direction of higher safety,more stable quality and stronger durability,and the ordinary concrete made of cement composite has been unable to meet the higher engineering quality requirements.The new engineering cementitious composites has attracted more and more attention.Engineering Cementitious Composites(ECC for short)is a kind of functional fiber reinforced cement matrix composites which is usually based on cement,fly ash and fine sand and the short fiber no more than 2.5% by volume.Under tensile load,the hardened material can show the unique strain hardening phenomenon by producing multiple fine cracks.ECC material has high toughness,ductility and good ability of crack control.It is beneficial to solve a series of engineering problems caused by the shortcomings of concrete structures in China,such as high brittleness,easy cracking,uncontrollable crack width,poor durability and ductility.Since the 21 st century,domestic and foreign scholars have spent a lot of time and efforts on studying fiber reinforced cementitious composites.Most researchs are focusd on the theoretical analysis,raw materials,mixture ratio and mechanical properties researchs.As the core reinforcement of ECC,fiber is the research focus of the preparation technology of ECC.At present,foreign scholars mainly prepare ECC materials with special polyvinyl alcohol(PVA)fiber treated on the surface.However,China has not yet mastered the mature PVA fiber surface treatment technology,and this kind of special PVA fiber mostly relies on the import from the United States and Japan,resulting in the high cost of PVA-ECC material,which is vulnerable to foreign technical blockade.Therefore,it is difficult for China to carry out practical engineering application research on ECC material.Domestic polypropylene(PP)fiber which is produced and processed with mature technology in China is cheaper than PVA fiber.If polypropylene fiber reinforced engineering cementitious composites(PP-ECC)materia with stable performance can be prepared,engineering cost can be reduced and engineering application can be promoted.Therefore,different mix proportions of PP-ECC using PP fiber,natural river sand instead of silicon sand are prepared in this paper to study effects of different content of fly ash content,water cement ratio,sand binder ratio and fiber volume content on bending properties of PP-ECC.The load-deflection curves in the four-point bending test are collected and the fractal dimension of the cracks during the stage appearing multiple cracks is studied.Fractal characteristics and variation law of sheet specimens’ surface cracks under different bending load are analyzed.The test results of sheet specimens are verified by the results of beam specimens,and the main content of this paper is summarized as follows:(1)In this paper,the full factor test with four factors and two levels is designed and 16 groups of mixture ratio of PP-ECC sheet specimen are prepared.Collecting the load-deflection curves during four-point bending test and calculating bending strength of the specimens,mechanical parameters such as energy absorption value,toughness factor,initial bending toughness ratio and residual bending toughness ratio are introduced to evaluate the bending toughness of each specimen.At the same time,the cracking stage of PP-ECC and the process of specimen bending failure are described.(2)According to the four-point bending test process and test results,the influence of different fit ratios on the bending properties of PP-ECC is analyzed.The significance of the four influencing factors of fly ash content,water-cement ratio,sand-binder ratio and fiber volume content is calculated by variance analysis,and the major factors of PP-ECC bending performance are screened to conduct multilevel test in the future.(3)On the basis of the fractal theory,this paper explores whether the surface cracks of PP-ECC specimens under bending load have fractal characteristics.Fractal dimension is used to describe the propagation law of surface crack of PP-ECC under bending load,and a simple fractal dimension algorithm is established by Matlab software.The relationship between the fractal dimension of PP-ECC specimen’s surface cracks and the load,deflection is established.The fractal dimension of PP-ECC specimen’s surface cracks is combined with the bending performance to study the crack development of fiber-reinforced cementitious composites from a new point of view.