Analysis On Flexural Performance Of Steel Reinforced Ultrahigh Toughness Cementitious Composite Beams

Compared to the traditional concrete materials, UHTCC possesses prominent crack-controll capacity, energy absorption capacity and nonlinear deformation ability, which provide a great potential in the practical applications to improve the seismic performance and durability of the structural members. Investigations of flexural performance of steel reinforced ultra-high toughness cementitious composite beams (abbreviated as RUHTCC beams) are carried out, covering the normal-section load-carrying capacity, flexural displacement, plastic rotation and ductility. The major contents and conclusions are included:1. Based on the equivalent rectangular stress-block method, the equations to calculate the two equivalent parameters α and β, for both bilinear UHTCC uniaxial compression model and nonlinear UHTCC uniaxial compression model, are obtained and the difference between them are discussed. Comparision between the predicted flexural capacity of RUHTCC beams based on nonlinear compression model and the experimental results obtained from the RUHTCC four-point bending tests, verifies the rationality of nonlinear compression model in the design of RUHTCC flexural members.2. In the RUHTCC flexural capacity analysis, a compression zone height enhancement coefficient η is first introduced. Then, the respective contribution of the longitudinal steel and UHTCC to the normal-section flexural capacity during the entire loading are quantitatively calculated. The relationship between flexural contribution of UHTCC and reinforcement ratio is investigated. The results indicate that, in the calculation of normal-section flexural capacity of RUHTCC beam, the tensile contribution of UHTCC may be ignored when the dimensionless reinforment ratio ρ/ρb is gteater than0.45, otherwise the tensile contribution of UHTCC should be considered.3. Based on the force balance principle, the formulas to compute the moment of inertia Icr of the fully cracked transformed section at yielding moment are proposed in terms of the following two cases:the first case is to consider the stiffness contribution of UHTCC in tension zone, named as Icr1, and the second case is to ignore the stiffness contribution of UHTCC in tension zone, named as Icr0. From the comparision of Icr1and Icr0of the RUHTCC beams with different reinforcement ratios, it is concluded that the contribution of UHTCC in tension zone to the moment of inertia Icr may be ignored if the dimensionless reinforcement ratio p/pb is more than or equal to0.459, otherwise the influence of UHTCC in tensile zone on flexural stiffness should be taken into account. 4. The effective moment of inertia Ie of RUHTCC beams with different reinforcement ratios at different load levels are calculated based on the eight calculation equations applicable for the traditional reinforced concrete beams. Comparisons between the predicted effective moment of inertia and the experimental ones show that the expression Ie8accounting for the influence of cracking length of RUHTCC beams is more rational to predict the variation of the moment of inertia until the longitudinal steel yields. The theoretical load-mid-span deflection curves of RUHTCC beams during the stage of normal serviceability are calculated based on the expression Ie8, presenting a satisfying agreement with the experimental curves.5. A simplified bilinear θ-δ model is proposed to illustrate the relationship between mid-span deflection δ and flexural rotation θ of RUHTCC beams. On the basis of this simplified model, the elastic rotation capacity and plastic rotation capacity of RUHTCC beams at ultimate failure moment are analyzed. Results show that with the increase of reinforcement ratios, the plastic rotation capacity decreases in an exponent manner, while the elastic rotation capacity increases in a linear manner. Moreover, the elastic rotation capacity is more than the plastic rotation capacity when the dimensionless reinforcement ratio ρ/pρb is more than0.35. This conclusion implies that there is a maximum limit to reinforcement ratio for the structural design of RUHTCC flexural member, in order to make full use of the nonlinear deformation ability of UHTCC to provide sufficient ductility for the structural member.6. A new dimensionless parameter to evaluate the ductility of RUHTCC beams, denoted as PRC, is proposed based on the simplified bilinear θ-δ model. From the varition of PRC with different reinforcement ratios, it is found that with the increase of reinforcement ratios, the parameter PRC linearly decreses and the ductility of RUHTCC beams reduce correspondingly.