Study On Geometric Characteristics And Permeability Of Cracks Of Nylon Fiber Reinforced Concrete

In recent years,the durability and permeability of concrete have attracted increasing attention.Concrete often develops cracks in projects,and cracks will affect its permeability,accelerate the aging of materials,and then endanger the safety of engineering,The commonly used methods of permeability of normal concrete specimens are relatively mature,but these methods are obviously not applicable to concrete with cracks.How to accurately evaluate the permeability characteristics of concrete with cracks has become the focus of attention in recent years.Nylon fibers(NF)and nylon fiber fabrics(NFF)recovered from the clothes are used to strengthen plain concrete(PC)in this paper.Firstly,specimens of fiber-reinforced-concrete(FRC)with different crack widths(approximately 50,100,150,200,250 μm)are prepared by the feedback controlled splitting test,and then the variable water pressure permeability test of the specimens with cracks are carried out by using concrete damage detection device to analyze the variation of flow rate of each specimen under different water pressure and investigate the influence of NF and NFF on the permeability of cracked concrete.After water permeability test,the specimens are thoroughly divided into two parts and the basic data of the fracture surface morphology are obtained by the 3D scanner.The roughness coefficient JRC,crack tortuosity τand surface roughness Rs and fractal dimension D of the crack surfaces are quantified by using tools of Geomagic Studio、MATLAB and others and the influence of fiber dosage on those parameters are analyzed.Finally,the functional relationship between the crack surface parameters and the permeability coefficient is established for the Cubic law correction.The main conclusions can be drawn as follows:(1)Incorporation of fiber has little influence on the compressive and tensile strength of concrete,but FRC shows considerable toughness after cracking,and NFF can be well used for concrete reinforcement.The fiber incorporated into concrete can inhibit seepage in cracks and reduce the permeability of cracked concrete.The correction factor ζ of FRC is between 56%and 70%lower than that of PC,and ζ decreases with the increase of fiber dosage.The crack permeability and the correlation coefficient R2 of fitting line gradually decrease with the increase of hydraulic pressure at a fixed fiber dosage.The modified Cubic law can accurately predict the water flow behavior and permeability in small cracks(0-300 μm)of NFRC or NFFRC under the hydraulic pressure no more than 0.1 MPa.NFF can replace NF to reduce the permeability coefficient of cracked concrete.(2)The seepage process in the variable water pressure permeability test accords with the Forchheimer law,the flow rate predicted by Darcy’s law is higher than that of Forchheimer law,and the nonlinear effect increases with the increase of crack width.The linear coefficient A and nonlinear B decrease with the increase of effective crack width,and the empirical formula between A and B is established.The Reynolds number Re increases with the increase of nonlinear factor E,the critical Reynolds number Rec value of the specimens are distributed between 7 and 45,and the Rec decreases with the increase of crack width.(3)The incorporation of fibers in concrete will change the geometric characteristics of the fracture surface and make the original fracture surface rougher.The crack surface parameters such as t JRC,τ,RS and D increase with the increase of fiber dosage,and on the basis of parameter JRC,τ and RS modification of Cubic law,a model to predict the permeability of cracked fiber reinforced concrete is established.(4)MATLAB programming can be well used to quantify the geometry of crack surface,as the fiber dosage increases,the fracture surface becomes more complex and irregular,and the D becomes larger.By introducing the calculated D in the modified Cubic law,the permeability can be predicted well in this paper.During this experiment,the variation range of fractal dimension D of fracture surface is between 2.018 and 2.043,and the fluctuation range is very small.The influence of D on permeability decreases with the increase of fracture width.The geometric characteristics of the crack surface are difficult to interfere with the overall flow state of the seepage when the width of the crack is large to a certain extent.With the increase of the crack width,the seepage becomes closer to the water flow behavior of the smooth parallel plate model.