Sarah L. Billington

Simulation of Highly Ductile Fiber-Reinforced Cement-Based Composite Components Under Cyclic Loading

This research investigates ductile fiber-reinforced cement-based composites (DFRCCs) for new design as well as retrofitting of structures in seismic regions. DFRCC is highly ductile and is characterized by strain-hardening in tension to strains over 3% and by unique cycling loading behavior. In order to accurately predict the structural performance of DFRCC components under cyclic and seismic loading, a robust constitutive model is needed for structural-scale simulations. In this paper, a constitutive model based on total strain is proposed and applied to simulate structural component tests. The model in particular captures DFRCCs unique reversed cyclic loading behavior. Simulation results show that the implemented model is robust and reasonably accurate in simulating DFRCC structural components reinforced with steel and fiber-reinforced polymer bars.

CYCLIC RESPONSE OF HIGHLY DUCTILE FIBER-REINFORCED CEMENT-BASED COMPOSITES

The response of highly ductile fiber-reinforced cement-based composites (DFRCC) to uniaxial monotonic and cyclic loading is examined. The DFRCC studied herein is a portland cement-based mortar matrix with a low volume fraction (~2%) of high modulus polymeric fibers. Various cyclic testing schemes were employed to examine the effect of cyclic loading on the materials compressive and tensile stress-strain envelope. The response of different tensile specimen geometries was also examined. A distinctive feature of DFRCC is its pseudo-strain hardening response in tension. Test results identified a unique unloading and reloading response for DFRCC, which is a consequence of fibers debonding and pulling out of the matrix. Reversed cyclic loading was seen to affect the tensile response of the material if the uniaxial compressive strength during loading was exceeded, and not to affect the tensile response if compressive strength was not exceeded. Evaluation of tensile specimen geometries revealed significant variations in strain capacity between the different geometries.

Design Concepts for Controlled Rocking of Self-Centering Steel-Braced Frames

AbstractThe self-centering rocking steel-braced frame is a high-performance system that can prevent major structural damage and minimize residual drifts during large earthquakes. It consists of braced steel frames that are designed to remain elastic and allowed to rock off their foundation. Overturning resistance is provided by elastic post-tensioning, which provides a reliable self-centering restoring force, and replaceable structural fuses that dissipate energy. The design concepts of this system are examined and contrasted with other conventional and self-centering seismic force resisting systems. Equations to predict the load-deformation behavior of the rocking system are developed. Key limit states are investigated including desired sequence of limit states and methods to help ensure reliable performance. Generalized design methods for controlling the limit states are developed. The design concepts are then applied to a six-story prototype structure to illustrate application of the rocking steel fram...

Modeling Residual Displacements of Concrete Bridge Columns under Earthquake Loads Using Fiber Elements

Nonlinear dynamic analysis with fiber-element models is now widely used to assess the seismic response of bridge structures. The ability of such models to accurately simulate response parameters for characterizing the postearthquake condition of bridges, namely residual displacements, is assessed by comparison of analyses of dynamically loaded reinforced concrete bridge columns to experimental data. The models are unable to capture residual displacements, and the cause of the inability to capture residual displacements is investigated through dynamic analysis of fiber-element and single-degree-of-freedom SDOF models. A certain type of pinching present in the numerical hysteretic response shape is found to lead to poor residual displacement simulation both in the SDOF models and in fiber-element models. When eliminating this pinching, improvements to residual displacement simulation are found. A modified concrete constitutive model representing damage accumulation from cyclic loading is implemented for the fiber-element analysis that incorporates changes to reloading behavior when moving from high tensile strain back to compression. Analysis using the modified concrete consti- tutive model leads to improvements in the ability of the fiber-element model to capture residual displacements.

Creep and shrinkage of high-performance fiber-reinforced cementitious composites

The authors studied engineered cementitious composites (ECC), a class of high-performance fiber-reinforced cementitious composites, for its time-dependent properties. A pseudo strain-hardening response was exhibited by the material with multiple fine cracking in uniaxial tension. Information on creep recovery, drying creep, basic creep, and shrinkage of the material was obtained through a series of experiments on ECC specimens and similar fiberless specimens. Established predictive concrete shrinkage and creep models were compared to the ECC data. Greater creep strain development occurred in ECC material than in a similar fiber unreinforced cementitious mixture. Material shrinkage behavior estimates and shrinkage strain measurements were effected by surface cracking. While not developed for such material, a reasonable creep and shrinkage behavior estimate can be obtained through existing predictive models.

Shake-Table Tests of a 3-Story Masonry-Infilled RC Frame Retrofitted with Composite Materials

AbstractThis paper presents a study that investigated the effectiveness of retrofitting unreinforced masonry infill walls with composite materials to enhance the seismic performance of infilled nonductile RC frames. The primary retrofit scheme considered was the use of engineered cementitious composite overlays. Shake-table tests were conducted on a 2/3-scale, 3-story, 2-bay, masonry infilled RC frame that had one bottom-story wall retrofitted with engineered cementitious composites. The influence of this retrofit on the performance of the structure was investigated using the experimental observations and results of nonlinear finite element analyses. Furthermore, after walls in the second story of the structure were damaged, they were repaired by injecting epoxy into cracked mortar joints, and strengthened with a glass-fiber reinforced polymer overlay. It has been shown that both retrofit schemes are effective in enhancing the seismic performance of the structure and preventing diagonal shear failures of ...

Seismic performance of non-ductile RC frames with brick infill

This paper summarizes some of the findings of a research project that investigates the seismic performance of masonry-infilled, non-ductile, RC frames, including the development of reliable analytical methods for performance assessment and effective retrofit techniques. Quasi-static tests were conducted on small and large-scale, single-story, single-bay, RC frames infilled with brick masonry walls with and without openings. Some of the infill walls were strengthened with an engineered cementitious composite material. Furthermore, two 2/3-scale, three-story, two-bay, masonry-infilled, RC frames were tested on a shake table. One was tested with no retrofit measures, and the other had infill walls strengthened with the engineered cementitious composite and fiber reinforced polymeric material in the first and second stories, respectively. The tests have demonstrated the effectiveness of the retrofit measures. Computation models that combine smeared and discrete cracks have been developed and validated by the experimental data. Some of the experimental and numerical results are presented in this paper.

Impact of Reinforcement Ratio and Loading Type on the Deformation Capacity of High-Performance Fiber-Reinforced Cementitious Composites Reinforced with Mild Steel

AbstractHigh-performance fiber-reinforced cement-based composites (HPFRCCs) reinforced with mild steel have been proposed for use in structural elements to enhance component strength and ductility, increase damage tolerance, and reduce reinforcement congestion. Recent research has shown that HPFRCCs have a high resistance to splitting cracks, which causes reinforcement strains to concentrate when a dominant tensile crack forms, leading to early reinforcement strain hardening and reinforcement fracture. This paper presents the impact of longitudinal reinforcement ratio, ranging from 0.54 to 2.0%, and the influence of monotonic and cyclic loading histories on the deformation capacity of reinforced HPFRCC flexural members subject to three-point and four-point bending. Experimental results show that load cycling can decrease deformation capacity of flexural members by up to 67% when compared to monotonic deformation capacity. The impact of load cycling on deformation capacity is shown to be strongly affected ...

Simulation of Unreinforced Masonry Beams Retrofitted with Engineered Cementitious Composites in Flexure

AbstractA two-dimensional non-linear finite-element analysis micro-modeling approach to simulate unreinforced masonry beams in bending is extended to include a retrofit with a thin layer of ductile fiber-reinforced cement-based material referred to as engineered cementitious composite (ECC). The retrofit method is one that has been demonstrated to add significant ductility to unreinforced masonry infill walls under in-plane cyclic loading and is further expected to enhance out-of-plane bending resistance. The objective of the research is to identify and propose a modeling approach for this complex system of four materials and three different types of interface using basic material properties and established model parameters for future analyses of the retrofit system in structural applications. Of the two geometric models investigated, a simplified approach using expanded brick units with zero-thickness mortar elements is recommended and validated. Brick-mortar interface opening, cracking of the ECC layer ...

Cyclic Response of Nonductile Reinforced Concrete Frames with Unreinforced Masonry Infills Retrofitted with Engineered Cementitious Composites

AbstractA new seismic retrofit technique for unreinforced masonry infills in nonductile reinforced concrete frames is presented. The objective was to develop a light and easy to apply retrofit technique for the infills that both delays shear failure at the columns of the nonductile bounding frame when subjected to an earthquake by preventing severe deterioration of the infill wall, and enhances the ductility of the system. The retrofit technique uses sprayable engineered cementitious composites (ECCs), a micromechanically designed cement-based material reinforced with short, randomly distributed polymeric fibers. Four small-scale unreinforced masonry infilled nonductile reinforced concrete frames were tested under in-plane quasi-static cyclic lateral loading. One unretrofitted wall and three different retrofit schemes were evaluated. Continuous steel reinforcement was embedded in the ECC layer and a bonding agent was applied between the masonry wall and the ECC layer. In two retrofits the impact of two di...