The Influence Of Macro Fibers On Toughness And Water Permeability Of Cracked Concrete

The permeability of concrete is one of the important influencing factors on its durability.Due to the increasing problems in durability,the concrete permeability is much concerned.The concrete element usually worked with cracks in serviceability stage.The permeability coefficient increases significantly when the cracks occur,and the durability decreases obviously.The effective method to improving the impermeability of cracked concrete is to limit the crack width.The addition of macro fibers can transform the deep cracks of the concrete members into shallow cracks,and change the macro-cracks into meso-cracks,and the crack geometry(tortuosity,roughness)can also be affected by the macro fibers.Therefore,the addition of the macro fibers demonstrates positive effect on the impermeability of cracked concrete.This work is supported by the National Natural Science Foundation of China(51578109).Firstly,the effect of the macro steel fiber/hybrid fiber on the toughness of the concrete was studies and then the permeability of the concrete was investigated with cracking and under compressive load.The contents and results of this paper are as follows:(1)The bi-axial flexural properties of the steel fiber reinforced concrete at early age was studied using the panel test,the method of the elastic mechanics and the method recommended by the EFNARC were compared,the failure pattern of the specimen was analyzed.the results demonstrated that:the flexural property of the concrete at early age was affected obviously by the macro steel fiber,the flerural toughness was increased with the increasing of the fiber dosage.The textile reinforced concrete(TRC)showed brittle behavior,and the combined use of the steel fiber and the AR glass/basalt fiber mesh were studied in two-way slab.The results showed that:the combined use of the macro steel fiber and the fiber mesh demonstrated positive hybird effect,the combined ues of the steel fiber and the fiber mesh may show positive bi-axial effect on the redistribution ability of inner force.The hybrid use of 50kg/m3 macro steel fiber and the fiber mesh showed better flexural properties than the steel mesh with the reinforcement ratio of 0.2%.(2)According to the ACI Committee 544,the improved device was introduced to analyze the impact resistance of the fiber reonforced concrete,the hybrid effect of the combined use of different macro fibers(macro steel fiber and macro polypropylene fiber)was analyzed,the failure pattern and the penetration depth of the specimen was compared.The results showed that:the combined use of the macro fiber and the steel rebar demonstrated obviously positive hrbird effect and the impact properties of the specimen were improved significantly,the absorbed energy,the penetraton depth and the number of the cracks were increased remarkably.(3)The feedback controlled splitting test was adopted and the specimens with different crack width were made,the permeability of the cracked specimen was measured by a device,the influence of the crack width and the fiber on the permeability were analyzed,the results showed that:the permeability coefficient increased obviously with cracking,the permeability decreased with the addition of the macro fibers,and with the increasing of the fiber dosage the peremability coefficient decreased to some extent.By considering the influence of the crack tortuosity and surface roughness,the modified Poiseuille law can be adopted to evaluate the permeability of the cracked specimen.(4)The self-designed laser scanner was adopted to collect the geometry of the surface,the 3D reconstruction was relaized by the MATLAB,the crack properties(tortuosity and roughness)were evaluated quantitatively,and according the fractal throey,the fractal dimension of the crack surface was also calculated.the results demonstrated that:the crack geometry was influenced by the addition of the macro fiber,the crack tortuosity and surface roughness increased with the increasing of the fiber contents,the projective covering method and the triangular prism surface area method could be used to evaluate the surface roughness,the fractal dimension of the surface also enhanced with the increasing of fiber dosage.(5)The permeability of the fiber reinforced concrete under compressive load was studied,the effect of the load level,fiber types and dosage was evaluated,the ultrasonic pulse velocity of the specimen with different load level was measured.The results showed that:the permeability of the specimen was affected by the load levle,for the plain concrete,when the load level was smaller than 0.6,the permeability coefficient under compressiver load had no obvious increment,while when the load levle was larger than 0.6,the permeability coefficient of the spemcimen increased remarkably.the addition of the steel fiber delayed the appearance of the break point,the permeability of the specimen with the fiber dosage of 60kg/m3 enhanced obviously when the load level reached to 0.8,the ultrasonic pulse velocity decreased with the increasing of the load level and could be used to evaluate the permeability.