Shape Memory Alloys in Civil Engineering

Shape memory alloys (SMAs) are special metallic materials that have witnessed growing interest within the civil engineering community over the last few decades. This chapter presents a basic introduction to SMAs, what they are, including their common types, mechanical properties, and the two distinguished thermomechanical phenomena that make these alloys effective for different structural applications. An overview of the SMA global market and the advantages of using SMAs over other conventional materials is presented primarily for seismic and prestressing applications.

Superelastic properties of shape memory alloys (SMAs) are well-known and have been used in a wide range of commercial, aerospace, and biomedical applications. These materials have recently been assessed for use in earthquake engineering applications. When considering these materials for seismic structural applications, it is crucial to understand the level of complexity needed in the constitutive models describing the behavior of these materials to effectively and efficiently capture the highly nonlinear behavior of SMAs. This chapter first presents an overview of the main types of SMA models. Next, the chapter investigates how using various SMA constitutive models affects the seismic behavior of structural systems that use superelastic SMA devices. Three SMA models with different levels of complexity are used to conduct a sensitivity analysis. The models are evaluated using a simple structural system subjected to three sets of earthquake recordings.

Strong earthquakes can cause excessive displacements at bridge joints, possibly leading to unseating the bridge superstructure. Steel restraining cables are typically used to mitigate this type of failure. However, due to their inelastic behavior, steel restraining cables are not always effective in preventing this damage. The superelastic behavior of shape memory alloys (SMAs) is sought in this chapter to develop bridge-restraining devices with no or negligible inelastic deformations. The chapter first discusses the impact of the hysteretic properties of superelastic SMAs on their efficacy in limiting hinge opening in bridges. Another critical factor discussed is ambient temperature effects. Since the superelastic behavior of SMAs is highly dependent on the alloy’s temperature, a sensitivity study is carried out using a thermomechanical SMA model to assess how temperature variability affects the effectiveness of SMAs as seismic restrainers for bridges. Finally, the efficacy of SMA restrainers compared to other conventional seismic retrofit devices is evaluated.

This chapter introduces a novel passive seismic control mechanism for cable-stayed bridges using superelastic shape memory alloy (SMA) and examines its performance. A 3-dimensional (3-D) bridge model that takes soil-structure interaction into effect is equipped with SMA dampers and analyzed under ground motion excitation. The SMA dampers are installed at the deck-tower and deck-pier connections of the bridge. Three orthogonal components from two historical ground motion recordings are applied to the bridge. The study sheds light on how well the SMA dampers can mitigate deck movements and reduce the amount of shear force and bending moment demands placed on the bridge towers during strong earthquakes.

Fiber-reinforced polymer (FRP) reinforcing bars have been utilized in concrete structures as an alternative to traditional steel reinforcement to combat corrosion issues. However, they are seldom taken into consideration in structures that require ductility and damping features because of the linear behavior of the typically utilized reinforcing fibers. This issue might be resolved by using superelastic shape memory alloy (SMA) fibers as reinforcement in the composite due to their nonlinear-elastic behavior. SMA-FRP composite is created by coupling small-diameter SMA wires with a polymer matrix. In this chapter, we explore the use of SMA-FRP bars to improve the seismic and cyclic performance of reinforced concrete (RC) structures by mitigating residual deformations while preserving the elastic properties associated with traditional FRP. The work presented herein includes discussing the fabrication of SMA-FRP bars, the experimental cyclic behavior of RC components reinforced with SMA-FRP bars, and the seismic behavior of RC moment resisting frames reinforced with SMA-FRP under sequential main shock-aftershock seismic excitations.

Over the past few years, research on the use of shape memory alloys (SMAs) in prestressing concrete buildings has produced encouraging results. One SMA that has shown potential for prestressing applications is NiTiNb. The thermomechanical behavior of thermally prestressed NiTiNb SMA under loading and temperature conditions relevant to civil structural applications is presented in this chapter. The recovery stress of prestrained wires and the monotonic and cyclic behaviors of thermally prestressed wires are investigated in a testing program on NiTiNb wires under varied testing durations and ambient temperatures. A study on the potential impact of severe corrosive environments relevant to civil structures on the mechanical behavior of prestressed NiTiNb SMA is also provided. In order to replicate real-world field conditions, the study subjects NiTiNb specimens to wet-dry cycles in NaCl solution using an accelerated aging testing procedure. Specimens made of carbon steel are also evaluated for comparison. The chapter also sheds light on a new Fe-based alloy (FeNiCoTi) that showed promise for prestressing applications. One advantage of using FeNiCoTi alloy is its cost-effectiveness compared to commonly studied NiTi alloy. Differential Scanning Calorimetry (DSC) tests are carried out to investigate the transformation temperatures of FeNiCoTi alloy under different heat treatment methods and prestrain schemes. In addition, cyclic experiments are performed to investigate the FeNiCoTi alloy’s cyclic behavior following stress recovery. Thermal cycle tests are also performed on the FeNiCoTi alloy to understand better how temperature variation affects recovery stress.

According to recent studies, concrete confinement with shape memory alloy (SMA) spirals is a potentially effective method for seismic retrofitting of reinforced concrete columns that are not flexural ductile. This method involves encircling concrete columns with prestrained SMA wires and heating the wires to activate the confining pressure. This chapter explains the SMA confined concrete’s experimental cyclic behavior and how a 3-D plasticity-based model for the material was created using the test findings. Based on experimental findings, a novel damage parameter, dilation rate function, and hardening/softening function are derived and verified. When compared to experimental findings, the suggested model can accurately replicate the axial and lateral stress-strain behaviors of NiTiNb-SMA confined concrete.

This chapter builds on the discussion that started in Chap. 7 on the application of concrete confinement using shape memory alloys (SMAs). Since columns play a critical role in structures, especially during extreme events like earthquakes, the chapter highlights several confinement applications in reinforced concrete (RC) circular and con-circular columns. SMA transverse reinforcement is placed internally for the newly constructed columns, while for the columns of existing structures that need retrofitting or repair, the SMA is placed externally. The chapter discusses the challenges facing conventional retrofitting of non-circular columns and how SMA confinement could help address this problem. It also highlights the efficacy of the new SMA confinement technique, whether used individually or as a supplement to other confinement techniques (e.g., fiber-reinforced polymer), in mitigating the damage sustained by the columns during strong earthquakes. The efficiency of using SMAs in performing emergency repair of severely damaged columns is demonstrated experimentally.

There is a growing interest in using shape memory alloy’s (SMA) thermal stressing features in various concrete prestressing applications. The primary advantage is the ease of application and time savings due to eliminating the mechanical tensioning process required for prestressing. Thermally stressed SMA systems are easily configured to apply prestressing forces at any direction and at target locations where prestressing force is needed the most (i.e., local prestressing). Studies presented in this chapter show that SMA local prestressing will help with time and material savings and prevent common types of damages associated with conventional prestressing such as end-splitting cracks. This chapter highlights the application of SMA using different configurations in developing internal and external prestressing systems for new construction and strengthening existing structures. Precast concrete railroad crosstie is discussed as a potential candidate component that could benefit from such a versatile prestressing system. The chapter also provides the reader with a fundamental idea of how merging SMA prestressing with computational topology optimization techniques could significantly reduce concrete usage in bridge construction.