Study On The Mechanism Of Early Plastic Cracking Resistance Of Fiber Reinforced Concrete And Its Numerical Simulation

Early plastic cracking of concrete seriously affects the durability of concrete structures,which brings a series of engineering problems.In actual engineering,the adding of short-cut synthetic fibers into concrete is usually used to solve the problem of early plastic cracking of concrete.However,the study on mechanism of early plastic cracking resistance of concrete by synthetic fibers is not deep enough,which makes the current domestic technical specification for application of fiber reinforced concrete and corresponding standards have not clearly defined the fiber parameters,and the application guidance for fibers in actual engineering is not detailed enough.As a result,most related fiber parameters are determined according to engineering experience,which limits the promotion and application of fibers to some extent.In addition,the dispersion of fibers has a significant effect on the early plastic cracking of concrete,but there is a lack of relevant research.In this paper,the polypropylene fiber is used as early plastic cracking resistance material.Combining the experimental research and numerical simulation method,firstly the properties of early negative capillary pressure,electric resistance,horizontal shrinkage,plastic settlement,bleeding and cracking of fiber reinforced concrete are studied.Then the geometric model of meso-structure of fiber reinforced concrete is simulated.The main studies are as follows.?1?Air begins to penetrate into concrete when the first micro-crack appears,the corresponding negative capillary pressure value is“air-entry value”,which is closely related to the early cracking of concrete.Therefore,firstly after measuring the development of both negative capillary pressure and electric resistance in early concrete by water potential tensiometer as well as multimeter,the whole process curves of negative capillary pressure and electric resistance developing with time were recorded respectively.By combining the whole development curve of negative capillary pressure and electric resistance,the“air-entry value”of negative capillary pressure denoted as P_aev was obtained.Then the influence of different fiber volume fraction and fiber length on the development of negative capillary pressure in early concrete was studied based on negative capillary pressure theory,meanwhile the relationship between different fiber parameters and“air-entry value”of negative capillary pressure P_aev was discussed subsequently to suggest a comprehensive formula to express the relationship between fiber volume fraction or fiber length and P_aev.In addition,the development process of early horizontal shrinkage deformation of concrete was tested by high-precision laser displacement sensor,obtaining the whole process curves of horizontal shrinkage developing with time,then the influences of different fiber volume fraction and fiber length on the early horizontal shrinkage of concrete were studied.?2?The early plastic settlement and bleeding test of concrete was carried out by high-precision laser displacement sensor,the whole process curves of plastic settlement and bleeding developing with time were recorded respectively.Then the influences of different fiber volume fraction and fiber length on the properties of early plastic settlement and bleeding of concrete were studied based on self-weight consolidation theory.?3?The number of cracks,crack length and crack width of plastic settlement cracking were measured respectively after 24 hours based on the early plastic settlement test of concrete.Firstly,the threshold value of crack image was adjusted by Photoshop software to obtain the binary image of crack.Secondly,the effects of different fiber volume fraction and fiber length on the properties of average crack width,maximum crack width and nominal total crack area of early plastic settlement cracking of concrete were studied.Finally,the relationship between different fiber parameters such as fiber volume fraction,fiber length,fiber spacing,fiber numbers per unit volume and characteristic value of fiber volume fraction?_f and nominal total crack area of early plastic settlement cracking of concrete denoted as A_cr were explored.?4?Based on the MATLAB programming language,the random generation and delivery procedures of polypropylene fibers and coarse aggregates were compiled respectively.As a result,the geometric model of meso-structure of polypropylene fiber reinforced concrete with different gradation,fiber volume fraction and fiber length was obtained,realizing the dispersion simulation of polypropylene fiber in three-dimensional space of concrete,which explained and verified the effect of dispersion of polypropylene fibers on the cracking resistance of early horizontal shrinkage and plastic settlement cracking of concrete well from the point of view of meso-structure.In addition,the MATLAB model was imported into the finite element software ANSYS to establish a finite element model of polypropylene fiber-aggregate concrete.