Eunsoo Choi

The behavior of concrete cylinders confined by shape memory alloy wires

The purpose of this study was to propose a new method to confine concrete cylinders or reinforced concrete columns using martensitic or austenitic shape memory alloy (SMA) wires. The prestrained martensitic SMA wire was wrapped around a concrete cylinder then heated by a heating jacket. In the process, the confining stress around the cylinder was developed in the SMA wire due to the shape memory effect, which can increase the strength and ductility of the cylinder under axial compressive load. For austenitic shape memory wires, the wires were prestrained as they were wrapped around the concrete cylinders on which post-tensioning stress was generated. In this study, martensitic and austenitic SMA wires of 1.0 mm in diameter were used for the confinement. Recovery tests were conducted on the martensitic wire to assess the recovery stress. Also, a superelastic behavior test was performed for the austenitic wire. The confinement by martensitic SMA wires increased the strength slightly and greatly increased the ductility compared to the strength and ductility of plain concrete cylinders. The austenitic SMA wires showed a similar effect on concrete cylinders to that of the martensitic wires. This study showed the potential of the SMA wire jacketing method to retrofit reinforced concrete columns and protect them from seismic risks.

Design, analysis and application of innovative composite PR connections between steel beams and CFT columns

In this research, three structural design concepts are integrated: the use of composite concrete-filled tube (CFT) columns, the use of partially restrained (PR) connections and the introduction of innovative materials (shape memory alloys) in the connection design. These concepts are used to enhance the robustness and performance of composite-special moment frames. These innovative connections intend to exploit the recentering properties of super-elastic SMA tension bars, the energy dissipation capacity of low-carbon steel bars and the toughness of CFT columns. PR-CFT connection prototypes were designed based on a hierarchy of strength models for each connection component. Simplified user joint elements based on the mechanical modeling approach were formulated in an effort to simulate the realistic behavior of bolted connections. The application of new connections to low-rise PR composite frames is illustrated by designing four buildings in both 2D and 3D for the western US region. The performance of these composite frames was compared with those with conventional welded frames in terms of strength, ductility and recentering behavior. In all three areas, frames with the PR composite connections showed superior performance. This is due primarily to the capability of this system to redistribute inter-story drift more evenly through the height of the frame.

Behavior of Reinforced Concrete Columns Confined by New Steel-Jacketing Method

Jacketing can be an effective way to improve the performance of the lap-spliced region of reinforced concrete (RC) columns. This paper introduces a new steel-jacketing method for RC columns that uses external pressure to attach the steel jackets to the column surface. The advantage of this method is that it does not require the application of grout between the steel jackets and the concrete surface to attach them together. Experimental tests of twelve concrete cylinders and four RC columns are performed to assess the effectiveness of the proposed method. Both single-layered and double-layered jackets are tested. The newly jacketed cylinders show good results in terms of increasing the compressive strength and ductility compared to plain cylinders. The double-layered jackets are estimated to show equal performance to that of a single steel jacket with the same thickness. The proposed steel-jacketing method increased the ductility of lap-spliced RC columns. The double-layered jacket appeared to perform better in terms of increasing ductility and energy dissipation capacity than the single-layered jacket.

Monotonic and cyclic bond behavior of confined concrete using NiTiNb SMA wires

This study conducts bond tests of reinforced concrete confined by shape memory alloy (SMA) wires which provide active and passive confinement of concrete. This study uses NiTiNb SMA which usually shows wide temperature hysteresis; this is a good advantage for the application of shape memory effects. The aims of this study are to investigate the behavior of SMA wire under residual stress and the performance of SMA wire jackets in improving bond behavior through monotonic-loading tests. This study also conducts cyclic bond tests and analyzes cyclic bond behavior. The use of SMA wire jackets transfers the bond failure from splitting to pull-out mode and satisfactorily increases bond strength and ductile behavior. The active confinement provided by the SMA plays a major role in providing external pressure on the concrete because the developed passive confinement is much smaller than the active confinement. For cyclic behavior, slip and circumferential strain are recovered more with larger bond stress. This recovery of slip and circumferential strain are mainly due to the external pressure of the SMA wires since cracked concrete cannot provide any elastic recovery.

Behavior of NiTiNb SMA wires under recovery stress or prestressing.

The recovery stress of martensitic shape-memory alloy [SMA] wires can be used to confine concrete, and the confining effectiveness of the SMA wires was previously proved through experimental tests. However, the behavior of SMA wires under recovery stress has not been seriously investigated. Thus, this study conducted a series of tests of NiTiNb martensitic SMA wires under recovery stress with varying degrees of prestrain on the wires and compared the behavior under recovery stress with that under prestressing of the wires. The remaining stress was reduced by the procedure of additional strain loading and unloading. More additional strains reduced more remaining stresses. When the SMA wires were heated up to the transformation temperature under prestress, the stress on the wires increased due to the state transformation. Furthermore, the stress decreased with a decreasing temperature of the wires down to room temperature. The stress of the NiTiNb wires was higher than the prestress, and the developed stress seemed to depend on the composition of the SMAs. When an additional strain was subsequently loaded and unloaded on the prestressed SMA wires, the remaining stress decreased. Finally, the remaining stress becomes zero when loading and unloading a specific large strain.

Crack-closing of cement mortar beams using NiTi cold-drawn SMA short fibers

In this study, crack-closing tests of mortar beams reinforced by shape memory alloy (SMA) short fibers were performed. For this purpose, NiTi SMA fibers with a diameter of 0.965 mm and a length of 30 mm were made from SMA wires of 1.0 mm diameter by cold drawing. Four types of SMA fibers were prepared, namely, straight and dog-bone-shaped fiber and the two types of fibers with paper wrapping in the middle of the fibers. The paper provides an unbonded length of 15 mm. For bending tests, six types of mortar beams with the dimensions of 40 mm × 40 mm × 160 mm (B×H×L) were prepared. The SMA fibers were placed at the bottom center of the beams along with an artificial crack of 10 mm depth and 1 mm thickness. This study investigated the influence of SMA fibers on the flexural strength of the beams from the measured force- deflection curves. After cracking, the beams were heated at the bottom by fire to activate the SMA fibers. Then, the beams recovered the deflection, and the cracks were closed. This study evaluated crack-closing capacity using the degree of crack recovery and deflection-recovery factor. The first factor is estimated from the crack-width before and after crack-closing, and the second one is obtained from the downward deflection due to loading and the upward deflection due to the closing force of the SMA fibers.

Numerical investigation on the cyclic behavior of smart recentering clip-angle connections with superelastic shape memory alloy fasteners

Superelastic shape memory alloy materials have become increasingly prevalent for recentering devices that have the ability to recover their plastic deformation automatically. For this reason, this study proposed new clip-angle connections incorporating superelastic shape memory alloy bolts. Including component spring models, mechanical joint models of steel bolted connections and shape memory alloy bolted connections are created for numerically simulating their cyclic behavior. The numerical analysis results are then compared to each other in terms of ultimate strength, energy dissipation, and permanent deformation. In particular, over 60% of the total displacement was recovered during unloads in case of shape memory alloy bolted connections, indicating that the proposed smart connections display obvious recentering features in their behaviors.

Pullout resistance of deformed shape memory alloy fibers embedded in cement mortar

In this study, the pullout resistance of deformed shape memory alloy fibers embedded in a mortar matrix is investigated to develop self crack-closing capacity. Three types of deformed shape memory alloy fibers (dog bone–shaped, end-deformed, and crimped) and one type of smooth shape memory alloy fiber, fabricated from two different alloys, NiTi and NiTiNb, were embedded in a mortar matrix with a compressive strength of 55 MPa. The pullout resistance differed considerably depending on the geometry of the fiber and composition of the alloy. The pullout resistance was generally higher for deformed shape memory alloy fibers than for the smooth shape memory alloy fiber. Among the deformed shape memory alloy fibers, dog bone–shaped fibers showed the highest enhancement in bond strength after heat treatment. The pullout resistance was higher for the NiTiNb alloy than the NiTi alloy when the shape memory alloy fiber was deformed, whereas the relationship was reversed when the shape memory alloy fiber was smooth.

Seismic Fragility of Typical Bridges in Moderate Seismic Zone

A set of fragility curves for the bridges commonly found in the Central and Southeastern United States (CSUS) is presented. For this purpose, the typical bridges in the region are determined from a bridge inventory, and detail analytical models of the bridges are developed based on experimental and analytical studies. The developed fragility curves can be used for the economic losses due to earthquakes, and the seismic vulnerability of the bridges estimated from the curves provides the priority of retrofit. Most highway bridges in the CSUS are non-seismic designed, thus it is believed that they are vulnerable to the expected earthquakes. Therefore, it is necessary to assess the vulnerability of the bridges and retrofit measures are required to increase the seismic resistance of the bridges. In this paper, the seismic resistance of the bridges retrofitted by several measures such as elastomeric bearings, lead-rubber bearings, and restrainer cables is also evaluated using fragility analysis and compared with that of the as-built bridges to verify the effect of each retrofit measure quantitatively.

Repairing cracks developed in mortar beams reinforced by cold-drawn NiTi or NiTiNb SMA fibers

In this study, mortar beams reinforced by shape memory alloy (SMA) fibers of NiTi and NiTiNb alloys were prepared to perform crack-repairing tests using three-point bending tests. The SMA fibers had a length of 30 mm, and their types were straight, dog-bone, and dog-bone with paper wrapping. For the bending tests, twelve types of mortar beams with the dimensions of 40 mm × 40 mm × 160 mm (B × H × L) were prepared. Half of them had a top steel reinforcement, and equal numbers of beams were assigned to the NiTi and NiTiNb fibers. Five SMA fibers were located at the bottom center of the beams along with an artificial crack of 10 mm depth and 1 mm thickness. Epoxy was used to fill the cracks to bond the cracked surfaces using injection, and a hot-gun was used to heat the SMA fibers in the cracks. The crack widths were measured before and after the cracks were repaired, and force–displacement curves were obtained to assess the flexural strength recovery ratio of the beams. It does not appear that the crack-closing capacity of SMA fibers is a crucial factor to recover the flexural strength in repaired beams. However, adequate application of epoxy is critical for repairing cracks, and the residual stress of SMA fibers seems to contribute to increase flexural strength of repaired beams. The residual stress of SMA fibers functions as prestress on mortar and delays the initiation of cracking.