Influence Of Macro Fibers On Permeability Of Cracked Concrete Under Loading

This paper studies the effect of macro fibers on the permeability of cracked concrete under loading.In the service period,concrete is subject to service load and works with cracks.The existence of cracks can obviously accelerate the degradation of concrete due to the intrusion of water into the concrete with deleterious substances and the durability of concrete are strongly influenced.The effective method to improving the impermeability of cracked concrete is to decrease the crack permeability.Macro fibers can limit the cracks width and increase the crack surface roughness of concrete to increase the water head loss and the impermeability of crack.This work is supported by the National Natural Science Foundation of China(51578109).Firstly,the effect of macro fibers on the permeability of uncracked concrete is studied,and then the effect of macro fibers on the permeability of cracked concrete and the surface topography of cracks under loading before and after freeze-thaw cycles is investigated.The contents and results of this paper are as follows:(1)According to the current domestic and foreign standards,the effect of macro fibers on the permeability of uncracked concrete is investigated.The result shows that the addition of macro fibers increases the permeability of uncracked concrete.With the increasing of macro fibers content,the water penetration of uncracked concrete increases gradually.(2)A vacuum permeability set-up and a laser scanning equipment are designed.The effects of macro fibers on the permeability of cracked concrete and crack surface topography under splitting load are analyzed.The results show that with the increasing of the crack opening,the crack permeability of all samples approaches to that of the value predicted by the Poiseuille flow model.Compared to the modified factor ? of Poiseuille flow model,the permeability parameter a is suitable to quantify the crack permeability of different samples.The presence of both macro fibers and the freeze-thaw damage enhances the roughness of concrete crack surface and reduces the crack permeability of concrete.(3)The effects of macro fiber on mechanical properties and permeability of concrete are analyzed under compressive load.The permeability of cracked concrete under splitting tension and compression load is compared.The results show that under compressive load,the permeability of specimens decreases firstly and then increases,and the tendency of permeability shows a threshold phenomenon.The load level at the threshold point of permeability is approximately equal to that at the threshold point of concrete volume change.The effect of macro fibers on the crack permeability under compressive load is lower than that under splitting tensile load.(4)The correlation among flexural toughness parameters,crack permeability and crack surface roughness of fiber reinforced concrete is revealed.Under flexural load,the vacuum permeability set-up and the laser scanning equipment are applied to evaluate flexural toughness,crack permeability and crack surface roughness of fiber reinforced concrete.Based on the multivariate analysis,different roughness parameters are evaluated and selected.The results show that multivariate analysis suggests the most effective of roughness number(Rn)for evaluating the roughness of crack surface.The crack surface roughness,flexural toughness and crack permeability of the FRC specimen follows the exponential functions.The relationships can be employed to evaluate the crack surface roughness by means of the flexural toughness and then to provide the theoretical support for predicting the crack permeability of FRC.(5)The effects of layer height of FRC and macro fibers on the mechanical property,permeability and crack surface topography of layered fiber reinforced concrete are investigated.The crack roughness of plain concrete layer and fiber reinforced concrete layer is compared.The results show that when the fiber content and layer height of FRC are constant,the axial tensile toughness of concrete is depended on the macro steel fiber content.With the increasing of the proportion of macro steel fibers in hybrid fibers,the axial tensile toughness of specimens increase.For the layered fiber reinforced concrete,the crack permeability of concrete with the hybrid fibers is lower than that of the mono fiber reinforced concretes,and the HFRC specimens show a high impermeability.The crack roughness of plain concrete layer in the layered concrete is influenced by the FRC layer.With the decreasing of layer height of FRC,crack roughness of the fiber concrete layer increases.