Retrofitting of Rehabilitated Bridge Piers

The rehabilitation of severely damaged bridge pier models by reinforced concrete jacketing, incorporates a plastic hinge relocation technique, to restore its lost strength. Subsequently, the relocated plastic hinge was retrofitted using a combination of end-anchored and hoop Fe-SMA strips. Pre-strained end-anchored Fe-SMA strips utilize the recovery stress generated during its thermal activation to apply pre-compression in the relocated plastic hinge region of the rehabilitated specimen.

Meanwhile, the Fe-SMA hoops induces active confinement in the same region. Hybrid simulation of an existing highway bridge incorporating scaled bridge pier test models as experimental elements was conducted to compare the seismic performance of the Fe-SMA retrofitted specimen against the control specimen. Different intensity levels of earthquake excitations were sequentially applied to analyse structural behaviour across a damage spectrum. Cyclic tests were performed after hybrid simulation to determine the ultimate capacity of the specimens. The Fe-SMA retrofitted test specimen exhibited enhanced load carrying capacity as well as improved ultimate displacement, enhanced stiffness, energy dissipation capacity, and minimal damage as compared to the control specimen.

Introduction

Seismic retrofitting of RC bridge piers aims at improving its seismic performance by increasing its strength and ductility. Most common retrofitting techniques like RC jacketing, steel jacketing, FRP jacketing etc. achieve this objective by addition of lateral confinement to concrete sections. Shape memory alloys (SMAs) have lately gained interest among researchers for its ability to exert active confining pressure on concrete members without mechanical prestressing by virtue of its unique thermo-mechanical property - shape memory effect (SME). If the pre-strained SMA is restrained before heating, large recovery stresses are developed in it, which in turn aids in prestressing/confinement of the structural component against which it is fixed. Unlike conventional prestressing techniques, this method doesn’t require heavy machinery in field.

The vast majority of previous studies conducted in the field of application of SMAs to structural elements have focused on Ni-Ti or Ni-Ti-based SMAs (Andrawes & Shin, 2008; Shin & Andrawes, 2011a, 2011b). These studies showcased that SMA-retrofitted columns displayed superior flexural ductility and energy-dissipation capacity compared to their passively confined counterparts, all while being subjected to the same confining pressure.

However, the utilization of Ni-Ti-based SMAs for prestressing applications in large structures is limited due to its high material cost, high activation temperature and a relatively narrow thermal hysteresis. Thus, Fe- based SMAs (Fe-SMAs) have emerged as a viable alternative for prestressing applications owing to its lower cost than NiTi-based alloys, higher elastic modulus and a relatively lower activation temperature (Leinenbach et al., 2012). Few studies have been carried out to test the effectiveness of Fe-SMA for strengthening of RC beams (Michels et al., 2018; Shahverdi et al., 2016). However, there are limited studies on use of Fe-SMA for retrofitting of bridge piers (Sarmah et al., 2023).

The objective of the present study is to carry out rehabilitation of severely damaged bridge pier specimens by RC jacketing to recover its lost strength using a plastic hinge relocation technique. Further retrofitting of a rehabilitated pier using Fe-SMA strips was carried out and seismic performance of an existing bridge with the retrofitted pier was assessed against a control specimen using hybrid simulation. This was followed by cyclic loading tests to determine the ultimate capacities of the test specimens.

Specimen Details

The study focuses on a prototype bridge in Tripura, India, with two spans on roller support and an integrally connected intermediate pier. The pier's vertical load capacity is 54,057 kN, meeting the demands of DL, SIDL, LL, and seismic loads in Seismic Zone-V. One-fifth scaled models of the prototype pier underwent hybrid simulation and cyclic loading test by Kotoky et al. (2018) which resulted in severe damage, including crushed core concrete and ruptured reinforcement. Two damaged pier models were chosen as the test specimen for rehabilitation. The objective of rehabilitation was to repair the original damaged plastic hinge region and to relocate the plastic hinge away from the pier-footing interface, facilitating easier post-seismic repairs. The concept of relocating plastic hinges from the footing, initially proposed by Hose et al. (1997) and later extended by various researchers (Krish et al., 2021; Parks et al., 2016; Rutledge et al., 2014), involves increasing the longitudinal and transverse reinforcement ratio at the damaged plastic hinge region.

The rehabilitation procedure involved removing loose concrete, restoring the vertical position with a chain-pulley system and the cross-section with epoxy grouting and micro-concreting. An additional 16 mm dia. dowel bars were installed next to damaged rebars, anchored 150 mm into the foundation with high-strength mortar. After hacking the concrete surface to ensure proper bond, a formwork (diameter 600 mm, height 450 mm) was used, and M30-grade concrete was poured. After 24 hours, the formwork was removed, and water-curing was done for 28 days. This serves as the control specimen (Specimen C). Details of the rehabilitation design and procedure are furnished in Sarmah et al., (2023).

Another rehabilitated specimen was further retrofitting using Fe-SMA strips and is named Specimen R. The retrofitting scheme adopted in the present study used a combination of end-anchored and hoop reinforcements positioned in the new plastic hinge region of Specimen R. For the retrofitting scheme, 2% prestrained Fe-SMA strips having composition Fe-17Mn-5Si-10Cr-4Ni-1(V,C) (mass %) (procured from reFer AG; Switzerland), width 24 mm and a thickness of 1.5 mm were adopted in the present study. The Fe-SMA strips were fixed on the concrete surface following the procedure shown in Fig. 1 using high strength bolts of 8 mm diameter. These strips were thermally activated at 180°C by resistive heating using a step-down transformer at a current density of 6 A/mm2. Recovery stresses of 300–350 MPa were generated after heating the Fe- SMA to 160 °C.

Experimental Program

To investigate the seismic performance of the Fe-SMA retrofitted RC bridge pier, an advanced testing methodology like hybrid simulation was adopted. The physical test set-up is shown in Fig. 2(a). Hybrid simulation of an existing bridge in Tripura, India (Sarmah et al., 2023) was carried out with the retrofitted piers as the physical test specimens. Hybrid Simulation comprises of two key components, which interact with each other through a controller. The first component is the discrete model of the structure which was computationally analyzed under four scaled segments of the intensity levels, 0.5, 1, 2 and 3MCE of N-S component of El Centro (1940) ground-acceleration history [Fig. 2(b)]. The second component is the physical specimen that is to be tested in the laboratory. Command signal from the controller, in the form of displacement, is imposed on the physical substructure. Corresponding feedback signal, in the form of spring force generated in the test specimen, is passed on to the numerical model for the next analysis time step. Following the completion of the hybrid simulation, cyclic loading tests were performed at a frequency of 0.025 Hz [Fig. 2 c)] to determine the ultimate load- carrying capacity of the test specimens.

Test results

The damage states of Specimens C and R at the end of hybrid simulation and cyclic tests are shown in Fig. 3. It may be observed during hybrid simulation, Specimen C underwent cracking and jacket-pier slippage at the top end of the jacket as well as around 70-100 mm away from it in the original pier [Fig. 3(a)]. However, in Specimen R cracking was observed only at the top end of the jacket [Fig. 3(b)]. Furthermore, at the end of cyclic test, spalling was observed in the jacketed portion of Specimen C [Fig. 3(c)]. In contrast, the propagation of cracks to the jacket was prevented due to the efficient concrete confinement by the Fe-SMA hoops in Specimen R [Fig. 3(d)].

The lateral force-displacement hysteretic responses of the test specimens under seismic excitation of intensity level 3 MCE are presented in Fig. 4. It can be observed that Specimens R showed an increase of 30.03% as compared to the control specimen C. Additionally, Specimen R exhibited a decrease in the drift at the top of the pier corresponding to the peak lateral load by 19.92%. This may be attributed to the increased stiffness and confinement at the relocated plastic hinge region due to the presence of end-anchored and hoop Fe-SMA.


Force-displacement behavior and backbone curves of the test specimens at the end of the cyclic loading test are shown in Fig. 5 and Fig. 6(a) respectively. The peak lateral load carrying capacity of Specimen R showed an increase of 35.6% as compared to that of the control Specimen C. Additionally, percentage improvement in failure displacement of Specimen R was 33.3% as compared to Specimen C. The synergy of vertical prestressing with active hoop confinement, achieved through the combination of Fe-SMA hoops and end-anchored reinforcement, led to improvements in both lateral load carrying capacity and maximum drift at failure.


Fig. 6(b) shows the cumulative energy dissipation capacities of the test specimens during cyclic test conducted after hybrid simulation. The cumulative energy dissipation capacity of Specimen R exhibited an increase of 134% as compared to Specimen C. This increase is due to the increased peak load and gradual slope of the load-displacement curve of Specimen R leading to higher area under the load-displacement curve as compared to Specimen C.

Conclusions

A set of four severely damaged scaled bridge pier models of a prototype bridge located in Tripura, India were first rehabilitated by RC jacketing. This led to shifting of the plastic hinge region from the pier-foundation interface of the original pier model to the top end of the jacket in the rehabilitated pier model. The relocated plastic hinge region of the rehabilitated pier model was further retrofitted using Fe-SMA strips as a combination of end-anchored reinforcement and hoop reinforcement. Hybrid simulation of the bridge with the test specimens as experimental elements was carried out to determine the effectiveness of the Fe-SMA retrofitting scheme under seismic loading followed by cyclic loading to evaluate the ultimate capacities of the test specimens.

Test results demonstrated that the Fe-SMA retrofitted pier specimen exhibited enhanced peak lateral load as well as improved ductility. Additionally, noteworthy improvement was observed in the cumulative energy dissipation capacity of the retrofitted specimen, signifying a substantial enhancement in seismic performance.

References

Andrawes, B., & Shin, M. (2008). Seismic retrofitting of bridge columns using shape memory alloys. Proceedings of Active and Passive Smart Structures and Integrated Systems, edited by M. Ahmadian, Bellingham, WA: SPIE.

Hose, Y. D., Priestley, M., & Seible, F. (1997). Strategic relocation of plastic hinges in bridge columns. Rep. SSRP-97/ 05, Structural Systems Research Project, Dept. of Structural Engineering, Univ. of California, San Diego, La Jolla, CA.

Kotoky, N., Deb, S. K., & Dutta, A. (2018). HyFRC Bridge Piers with Different Detailing at Pier– Foundation Interface. Journal of Bridge Engineering, 23(6), 04018035.

Krish, Z. F., Kowalsky, M. J., & Nau, J. M. (2021). Seismic repair of circular reinforced concrete bridge columns by plastic hinge relocation with grouted annular ring. Journal of Earthquake Engineering, 25(12), 2371-2405.

Leinenbach, C., Kramer, H., Bernhard, C., & Eifler, D. (2012). Thermo-mechanical properties of an Fe–Mn–Si–Cr–Ni–VC shape memory alloy with low transformation temperature. Advanced engineering materials, 14(1-2), 62-67.

Michels, J., Shahverdi, M., & Czaderski, C. (2018). Flexural strengthening of structural concrete with iron-based shape memory alloy strips. Structural Concrete, 19(3), 876-891.

Parks, J. E., Brown, D. N., Ameli, M., & Pantelides, C. P. (2016). Seismic Repair of Severely Damaged Precast Reinforced Concrete Bridge Columns Connected with Grouted Splice Sleeves. ACI Structural Journal, 113(3).

Rutledge, S. T., Kowalsky, M. J., Seracino, R., & Nau, J. M. (2014). Repair of reinforced concrete bridge columns containing buckled and fractured reinforcement by plastic hinge relocation. Journal of Bridge Engineering, 19(8), A4013001.

Sarmah, M., Deb, S. K., & Dutta, A. (2023). Hybrid Simulation for Evaluation of Seismic Performance of Highway Bridge with Pier Retrofitted Using Fe-SMA Strips. Journal of Bridge Engineering, 28(8), 04023050.

Shahverdi, M., Czaderski, C., & Motavalli, M. (2016). Iron-based shape memory alloys for prestressed near-surface mounted strengthening of reinforced concrete beams. Construction and Building Materials, 112, 28-38.

Shin, M., & Andrawes, B. (2011a). Lateral cyclic behavior of reinforced concrete columns retrofitted with shape memory spirals and FRP wraps. Journal of Structural Engineering, 137(11), 1282- 1290.

Shin, M., & Andrawes, B. (2011b). Emergency repair of severely damaged reinforced concrete columns using active confinement with shape memory alloys. Smart Materials and Structures, 20(6), 065018.