Response of reinforced engineered cementitious composite flexural members subjected to various cyclic deformation histories [electronic resource]

Engineered Cementitious Composite (ECC) is a class of High-Performance Fiber-Reinforced Cement-based Composite (HPFRCC) materials that has been developed and tailored over the last several decades. A composite material made from mortar and short, randomly disbursed fibers, ECC has enhanced tensile and compressive properties, in particular ductility, when compared to typical concrete. These improved material properties lend ECC to multiple uses in the built environment with the potential to increase structural performance and durability, and limit infrastructure maintenance and life-cycle cost. When reinforced with deformed steel bars, reinforced ECC and other reinforced HPFRCC components have enhanced seismic performance of structural components and systems such as coupling beams, steel moment frames with infill panels, joints, columns, and beams. But assessing seismic performance is complex, in part, because the deformations the next earthquake will induce on a structure are unknown. Based on the abundance of recorded ground motions from past earthquakes, it is clear that all ground motions do not induce the same deformation in structures. This dissertation provides the first insights of steel reinforced ECC flexural member response under different deformation histories through a variety of quasi-static laboratory experiments and numerical simulations. Because a large pulse in a deformation history may cause fiber failure within the ECC material and alter the response on a material level, deformation histories that contain initial pulses are of particular interest. Being able to assess the structural response of reinforced ECC components due to a range of possible deformation histories is important if the material is to become widely adopted. The first portion of this dissertation presents the results of six physical experiments of flexural elements subjected to different deformation histories, three cast with reinforced concrete and three cast with reinforced ECC. The steel reinforcement ratio in flexure was 0.95% for all six specimens. Three different cyclic deformation histories were used throughout the experiments in this dissertation: one consisting of monotonically increasing cycles, one containing relatively small initial pulses followed by a series of monotonically increasing cycles, and one containing relatively large initial pulses followed by the same series of monotonically increasing cycles. Between materials, the reinforced ECC specimens maintained more residual stiffness and dissipated more energy than the reinforced concrete specimens. Small initial pulses did not reduce the ultimate drift of either reinforced concrete or reinforced ECC specimens relative to the drift achieved when no initial pulses were applied. Large initial pulses in the deformation history reduced the ultimate drift of the reinforced concrete specimen by 42%. In contrast, the reinforced ECC specimen was able to undergo the same ultimate drift across all three deformation histories tested. Additional experimental tests of reinforced ECC flexural components of different steel reinforcement ratios and bar sizes subjected to the aforementioned three deformation histories were conducted. On average, specimens subjected to the deformation history containing large initial pulses dissipated the least energy per cycle and had the least reloading stiffness of the three deformation histories used in this dissertation. The presence and size of initial deformation pulses had an impact on several response characteristics including cracking, strain development in the steel reinforcement, reloading stiffness, and energy dissipated depending on the steel reinforcement ratio and reinforcing bar size used. The failure mode of 17 of 18 reinforced ECC flexural members was fracture of the steel reinforcement. Specimen ductility was a function of steel reinforcement ratio, and did not vary significantly with the applied deformation history. It was believed bond degradation at the steel-ECC interface led to strain reductions in the steel reinforcement, which facilitated the indifference to deformation history. The ultimate drift achieved by the three reinforced ECC specimens with the lowest steel reinforcement ratio, 0.73%, however, decreased as initial pulse amplitude increased. At the 0.73% steel reinforcement ratio, the two 10 mm diameter reinforcing bars in flexure provided a relatively high bond capacity relative to the bond demand at the steel-ECC interface, which limited bond degradation during cycling. Limited bond degradation in specimens subjected to initial deformation pulses led to reinforcement strain accumulation, and subsequent reinforcing bar fracture at lower drifts than nominally identical specimens subjected to a deformation history consisting of monotonically increasing cycles. Data from strain gages on the steel reinforcing bars, in conjunction with visual observations of splitting crack formation in the ECC during testing, signaled a trend between bond degradation mechanism and deformation history. Splitting cracks and were the dominant bond degradation mechanisms induced by deformation histories containing large initial deformation pulses and interface crushing was the dominant bond degradation in specimens subjected to small or no initial deformation pulses. Two-dimensional models of reinforced ECC flexural members were then simulated to determine the significance of altering a phenomenological bond-slip model based on the applied deformation history. Varying the post-peak bond-slip softening stiffness had little effect on the hysteretic response of the reinforced ECC flexural models tested, which consisted of two different steel reinforcement ratios subjected to two different deformation histories. The post-peak bond-slip softening stiffness did, however, affect the magnitude of strain and the number of reinforcing bar elements that strain-hardened. Overall, a numerical model with constant bond-slip parameters represented well the cracking patterns, hysteretic response, and reinforcement strain across multiple steel reinforcement ratios and deformation histories. Numerical simulations were carried out in three dimensions to explore the formation of splitting cracks and interface crushing in reinforced ECC flexural members. Six beam-end models with varying thickness and material properties of the cementitious interface zone elements near the steel reinforcing bar elements were developed in order to replicate the bond-slip response of an experiment carried out by others. Reducing compressive strength, compressive fracture energy, and tensile fracture energy of the elements within a 2 mm interface zone, due to physical conditions that may result from casting difficulty or a reduced embedded fiber length, altered the bond-slip response and reduced bond strength by 33% relative to a model with homogeneous ECC material properties. A greater number of finite elements in the interface zone experienced compressive strains exceeding crushing strain when a three-dimensional model was subjected to a cyclic deformation history than when the same model was subjected to a monotonic deformation history. The increased crushing under simulated cyclic loading supported the experimental findings of this dissertation by associating a more crushing-dominant response with a deformation history containing monotonically increasing cyclic amplitudes than a deformation history containing large initial deformation pulses. This dissertation concludes with suggested research extensions in the areas of both experimental testing and numerical simulations.

Advisor: Billington Sarah L. (Sarah Longstreth)
Lepech Michael primary advisor. Thesis advisor
Deierlein Gregory G. (Gregory Gerard)