Investigation On Mechanical Behaviors And Design Methods Of Ultra-High Performance Concrete Slabs

Ultra-High Performance Concrete(UHPC)is characterized by high strength,high toughness and high durability.Fiber-Reinforced Polymers(FRP)bar is characterized by the light,high tensile strength and remarkable durability.Combination between FRP bars and UHPC is excepted to fully utilize the excellent properties of two advanced materials,solve the corrosion problem of steel bar and reduce the self-weight of structures.As a result,the FRP reinforced UHPC structures with excellent durability and mechanical properties can be introduced.Reinforced concrete slabs are a type of basic member in the engineering structures.To promote the application of FRP bars and UHPC in slab members,three problems still need to be solved,as following: the first,the punching shear failure may occur for the thin UHPC slabs under a local load.However,the effect of punching shear-span ratio and loading position on the punching shear behavior of UHPC slabs reinforced with steel bars is ignored in the existing researches;the second,the flexural behaviors of UHPC slabs reinforced with FRP bars are different from that of normal concrete slabs reinforced with steel bars due to the steel fibers in UHPC and the different mechanical properties of FRP bars compared with steel bars.However,few,if any study on the mechanism performance of FRP reinforced UHPC slabs has been reported and there is no bending design theory for FRP reinforced UHPC slabs;the third,the connection of FRP bars is necessary when lengthening bars in UHPC structures.Lapping is feasible with the advantages of convenient construction and reliable load transfer.However,few,if any study on the lap-spliced behavior of FRP bars in UHPC has been reported and no stipulation about the lap construction of FRP bar in UHPC exists.Thus,this dissertation,sponsored by the National Natural Science Foundation of China(“Theories and methodologies for structural design and wind-induced catastrophe of super-long span cable-supported bridges with high performance materials”,Grant No.: 51938012)and the National Key Research and Development Program of China(“Key Technology Integration and Demonstration for the New Structural Application of Fiber Reinforced Composites”,Grant No.:2017YFC0703008),was conducted to study the punching behavior of UHPC slabs,flexural behaviors of UHPC slabs reinforced with FRP bars and lap performance of CFRP(Carbon Fiber Reinforced Polymers)bars in UHPC.The main work and conclusions are as follows:(1)Study on punching shear behavior of UHPC two-way slab reinforced with steel bars.Punching shear tests of nine UHPC two-way slabs simply supported on four sides were conducted.The parameters included UHPC strength,slab thickness,reinforcement ratio,local loading area,and loading position.The failure mode of UHPC slabs were obtained.The punching shear failure mechanism and the effect of test parameters on the punching behavior were analyzed.The results show that the punching failure angle and normal punching shear strength decrease with an increase in the punching-span ratio when the punching shear-span ratio is less than 7,while remaining largely the same when the punching shear-span ratio is more than 7.The punching shear capacity increases with the increase of UHPC strength,slab thickness,reinforcement ratio and side length of square loading plate.Based on the results from the present study and other studies,the feasibility of some existing equations for calculating punching shear capacity is discussed.An equation for predicting the punching shear capacity of a UHPC two-way slab is proposed,in which the punching shear-span ratio and the effect of the eccentric load are considered.To avoid the punching shear failure within a steel grid,the bar spacing at the bottom of the slab should not exceed the sum of the side length of the loading plate and 1.15 times the effective slab thickness.(2)Research on the flexural behaviors of UHPC one-way slabs reinforced with FRP bars.Flexural tests of nine UHPC one-way slabs were conducted.The test parameters were the bar type in the tensile zone,reinforcement ratio,confinement condition in compressive zone and mesh size of BFRP gird.The failure modes and crack distribution of UHPC slabs were obtained.The effect of test parameters on flexural performance were analyzed.The results show that the average crack spacing decreased with the increase of young’s modulus and reinforcement ratio of FRP bar,and the decrease of the mesh size of BFRP gird.The failure mode transformed from tensile failure to compressive failure with the increase of reinforcement ratio,leading to an increase of bending capacity and ductility of UHPC slabs.The BFRP gird in compressive zone could improve the ultimate compressive strain of UHPC 150 by 8%,which can enhance the flexural capacity and the ductility of UHPC slabs.For the slab with a tensile failure,the ductility increased with the increase in the tensile strength and ultimate tensile strain of FRP bar.For the slab with a compressive failure,the ductility decreased with increase in reinforcement ratio.Based on the experimental results,the equations for calculating the cracking moment,bending capacity,crack width and deflection of UHPC one-way slab reinforced with FRP bars were proposed.Moreover,the minimum reinforcement ratio,the height of the relative limit compression zone and the maximum reinforcement ratio were given.(3)Research on the mechanical behaviors of UHPC two-way slabs reinforced with FRP bars.Mechanical tests of eight UHPC two-way slabs were conducted.The test parameters were the reinforcement type in the tensile zone,the reinforcement ratio and the confinement condition in the compressive zone.The failure modes and crack distribution of UHPC slabs were obtained.The effect of test parameters on the mechanical behaviors of UHPC two-way slabs were analyzed.The results show that due to the bridging effect of the steel fibers in UHPC,the brittle failure with sudden load drop after peak point did not occur in the specimen without reinforcement.Under the same reinforcement ratio,the failure mode transformed from tensile failure to compressive failure with the increase of the tensile strength of FRP bar,leading to a increase in bending capacity and ductility of UHPC slabs.When the failure modes of UHPC slabs are the same and even the reinforcement ratio of two-way slabs is less than that of one-way slabs,the ductility of UHPC two-way slabs is close to or higher than that of UHPC one-way slabs.The BFRP gird in compressive zone could improve the ultimate compressive strain of UHPC 150 by 8.1%.Based on the plastic hinge line method and the static method,the formulas for calculating the flexural capacity of UHPC two-way slabs reinforced with FRP bars under local load at mid-span were derived and their accuracy was verified by the test results.The impact coefficient of FRP bars was introduced to modify the formula for calculating the punching shear capacity of reinforced UHPC slabs,which was proposed in Chapter 2 in the present study.Then,the equation for computing the punching shear capacity of the FRP reinforced UHPC two-way slabs was proposed.(4)Study on lap behavior of CFRP indented bars in UHPC.Tensile tests were conducted on 27 specimens with lap-spliced CFRP bars in UHPC.The test parameters were the splice length,UHPC strength and stirrup ratio.The failure modes were obtained.The failure mechanism of lap-spliced CFRP bars in UHPC and effect of test parameters on lap performance were analyzed.Test results showed that due to the bridging and confining effects of the steel fibers in UHPC,the development of the splitting cracks was limited and the splitting failure did not occur even in those specimens without any stirrups.The failure modes,pull-out and rupture of CFRP bar,in those lap-spliced specimens were governed only by the splice length of the CFRP bar embedded in UHPC.The pullout failure was caused by the shear failure of indented ribs on the surface of CFRP bars,and the rupture failure was governed by the tensile strength of CFRP bars.For CFRP indented bars with a 2480 MPa tensile strength and a 9.8 mm diameter,the splice length of 30 d in 100 MPa to 150 MPa UHPC can thoroughly bring out the tensile strength of the bars.The bond strength of lap-spliced indented CFRP bars in UHPC is mostly dependent on the shear strength of indented ribs on CFRP bars,little on the UHPC strength and stirrup ratio.A simplified equation for predicting the critical splice length of CFRP bars was proposed and verified well by the test results.Compared with the test results of the same CFRP bars embedded in UHPC,the critical splice length was 1.45 times the anchorage length.