Study On The Effects Of Hybrid Fibers On Strengthening And Toughening Of Ultra-high Performance Concrete

The influence of hybrid fibers on strength and toughness of ultra-high performance concrete(UHPC)has been studied in this thesis.UHPC incorporating one of two types of deformed steel fiber(crimped steel fiber and hooked-end steel fiber)was prepared mixed with medium-size straight line steel fibers,respectively.The optimum mixture type was identified via a series of tests.Then,the optimum steel fiber was mixed with each of three types of fiber,i.e.polyethylene fiber,basalt fiber,and medium-size straight line steel fiber,respectively.The optimum fiber combination was identified,which was further mixed with each of polyethylene fiber,basalt fiber,and small-size straight line steel fiber.Finally,partial mortar was replaced with coarse aggregate to study the influence of coarse aggregate on strength and ductility of UHPC.Compressive strength,splitting tensile strength,fracture energy and four point bending toughness have been measured.The test results reveal that hooked-end steel fiber has a better strengthening and toughening effect on UHPC than that of crimped steel fiber when mixed with straight line steel fiber,which may be attributed to the better interface bond behavior between fiber and matrix.As to binary fiber combination,the effect of the steel-polyethylene hybrid fibers is better than that of the steel-basalt hybrid fibers or the steel-steel hybrid fibers.As to ternary fiber combination,the strengthening and toughening of UHPC with steel-steel-polyethylene is more pronouced compared to that with steel-steel-basalt and three different sizes steel fibers.The effect of ternary fiber combination on strengthening and toughening of PRC is more considerable than that of binary fiber combination.Compared with the results with steel-polyethylene fibers,the compressive strength,splitting tensile strength and fracture energy of UHPC with steel-steel-polyethylene fibers increased by 6%,3%and 10%,respectively.The binary fiber combination is also significantly better than that of single fiber,e.g.compressive strength,splitting tensile strength and fracture energy of UHPC with steel-polyethylene fiber were 5%,58%and 2.1 times higher over that with single polyethylene fiber,respectively.With the increase of amount of coarse aggregate,both compressive strength and splitting tensile strength of UHPC decrease.Tensile splitting strength decreases significantly,e g.it decreased by 45%when the dosage of coarse aggregate reached to 30%.