Effects Of Fibers On Mechanical Properties And Permeability Of Cracked Concrete

As to the underground structure, such as tunnel engineering, the durability of concrete is more important than others. During the service period of concrete structures, components were basically working with cracks, and it had a great influence on the permeability of concrete. The fiber adding in concrete can avoid from the early age cracking induced by shrinkage of concrete and inhibit crack propagation. So, the fiber can effectively ameliorate the shortcoming that is easy to crack. After adding some fibers, the toughness of the concrete matrix and energy absorption capacity of concrete are also improved. Permeability Test Method that GB recommended for uncracked specimens could not evaluate the impermeability of concrete elements, because they always worked with cracks during service period.So, this paper studied the effects of fibers on mechanical properties and permeability of cracked concrete.Toughness is an important indicator of the mechanical properties of fiber reinforced concrete. It can be used to evaluate energy absorption and ability to resist deformation of the fiber reinforced concrete. According to three-point bending test of notched beam suggested by international standards RILEM TC 162-TDF 2002, this paper investigated the influence of fibers on the toughness and flexural strength of the concrete material. In the meantime, four-point bending test in ASTM was used to analyze the similarities and differences of the two test standards. Results of the two test methods all indicated that the fibers could improve the toughness of concrete.As to the three-point bending test of notched beam in RILEM, fibers also could control crack propagation effectively. And it had a linear relationship between deflection and crack mouth opening displacement (CMOD).The permeability of concrete is very low when it has no crack in the matrix. But, there were cracks when it was in service period, and the permeability could increase to an obviously high level. According to the splitting testing, controllable cracks were brought into the specimen for studying effects of fibers on permeability of cracked concrete. Crack patterns were analyzed by Supereyes digital microscope,IPP software and ultrasonic nondestructive testing method. To study the permeability of cracked specimens, the water permeability experiment was implemented under a varied water head as well as a constant water head. According to the Darcy's law and parallel plate theory, the correcting permeability of fiber reinforced concrete with a single crack was suggested. Research showed that:as to the splitting test, the failure form of plain concrete was brittle and could not control the speed of crack's development. After adding some of fibers, the specimen showed a better toughness and deformation capacity, and that the cracking development could be controlled easily. The fibers could improve the crack tortuosity, reduced the correcting factor and weakened the penetration ability of permeable medium. Compared with the traditional permeability coefficient under the condition of a variable water head, the correcting permeability of concrete with a single crack could sensitively reflect the influence of inner crack pattern under a constant water head, and could also analyze the relationship of them quantitatively.