| The application of recycled aggregate concrete has solved the problem of environmental pollution caused by waste concrete,and it has significant economic and social benefits.However,the basic mechanical properties of RAC such as strength,flexural toughness and fracture properties are lower than ordinary concrete because of the defects of recycled aggregates.How to improve its mechanical properties has become one of the most important issues to be solved urgently.In recent years,research shows that the basic mechanical properties of ordinary concrete can be improved effectively through adding some fibers.At present,a few research results have been obtained for the mechanical properties such as compressive strength and flexural resistance of fiber reinforced concrete.However,the research on the strength,flexural toughness,fracture toughness properties and other aspects of hybrid fiber RAC is relatively rare.The steel fiber with high elastic modulus and polypropylene fiber with low elastic modulus are chosen to add in RAC by single-doped and blended in this paper.The effects of single-doped fiber,hybrid fiber and the amounts of fibers on the fundamental mechanical properties are studied.Then scanning electron microscopy(SEM)is used to observe and analyze the improvement of the transition zone between pores and cracks in the cement paste,aggregate and cement paste,fiber and cement paste.The main work and achievements are as follows:(1)The steel fiber and polypropylene fiber were mixed into RAC with the volume content of 0.5%,1%,1.5% and 0.6%,0.9%,1.2%,respectively.The slump test,cubic compressive strength and splitting tensile strength test of 144 groups of fiber reinforced concrete specimen are conducted.The influences of fiber volume fraction are investigated through variance analysis.Test results show that when the fiber content increases,the slump of the hybrid fiber reinforced concrete is gradually reduced.The results also indicate that the use of fibers with steel fibers and hybrid fibers resulted in an increase in the compressive strength,but the use of polypropylene fibers resulted in a decrease in the compressive strength.And the results show that introducing fibers to concrete resulted in a significantly increase in splitting tensile strength.(2)Based on the flexural toughness test on notched beams with the section of 100mm×100mm×400mm,the effect of fiber volume fraction and hybrid mode upon concrete's flexural toughness are investigated.And the flexural toughness index and residual strength are calculated.In addition,the functional relationship between fiber volumetric content and flexural toughness index is given.Test results show that the flexural toughness of steel fiber,polypropylene fiber and steel-polypropylene hybrid fiber have been improved greatly,and hybrid fiber is better than single fiber.When the volume of steel fiber volume is 1.0% and the volume of polypropylene is 0.9%,hybrid fiber recycled aggregate concrete shows good hybrid effect,and the improvement of flexural performance is the most ideal.(3)Based on the flexural toughness test on notched beams with the section of 150mm×150mm×550mm,the effect of fiber volume fraction and hybrid mode upon concrete's fracture performance were investigated.And the fracture toughnes and fracture energy are calculated.Furthemore,the functional relationship between fiber volumetric content and fracture energy,deflection and CMOD is given.The results indicate that the fracture toughness and fracture energy of steel fiber,polypropylene fiber and steel-polypropylene hybrid fiber have been improved greatly,and hybrid fiber is better than single fiber.When the volume of steel fiber volume is 1.5% and the volume of polypropylene is 0.9%,the improvement of fracture energy is the most ideal.(4)Scanning electron microscopy(SEM)is used to observe and analyze hydration products,pores and cracks in the cement paste,aggregate and cement paste,fiber and cement paste.And the fiber toughening effects are explained from the microscopic level. |
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