The unique characteristics of superelastic NiTi shape memory alloys (SMAs) have attracted the attention of structural engineers worldwide. SMAs are metallic materials that can retrieve their original shape upon exposure to various temperatures or loading/unloading conditions with minimal residual deformation. SMAs have found increasing applications in the building industry because of their high strength, high actuation and damping capacities, good durability, and superior fatigue resistance.
Designing buildings today takes more than just satisfying the requirements of functionality and load-carrying capacity. There is a vital need for designing slender, long-span structures with high adaptability to changes in temperature and loading conditions. Nevertheless, designing structural elements with the highest possible strength-to-weight ratio continues to gain more popularity in construction sector due to economic reasons. Moreover, code-designed reinforced concrete (RC) moment-resisting frames are used as resisting structures against lateral cyclic loads. They withstand the damage resulting from seismic forces, including the collapse and destruction of beam–column joints that are specified as the weakest elements in structural systems. Most of the traditional strengthening techniques used for enhancing the seismic performance of RC beam–column joints subjected to cyclic lateral loads are not able to partially or fully recover the residual displacements after unloading. A promising new way of resolving this problem is to incorporate smart systems and smart materials within the beam–column joint itself. Smart systems are defined as systems that can automatically adjust their structural characteristics with respect to different loading conditions. Smart materials are the core elements in smart systems 
, which can be integrated into smart systems and provide several functions including sensing, actuation, self-adapting, self-healing, and information processes needed for monitoring.
Shape memory alloys (SMAs) can adapt themselves to a wide range of loading conditions and changing environments such as thermal, seismic, and wind loads, and magnetic fields. SMAs can undergo a reversible phase transformation when exposed to a temperature change and magnetic fields, which give them an advantage for changing their shape, and for use in actuation and sensing applications. SMAs can also display superelastic properties by recovering their original shape upon unloading. Moreover, SMAs can exhibit high damping capacity, by absorbing and dissipating energy under mechanical loading. This property makes them suitable for use in vibration control systems, thus improving the seismic performance of structures. SMAs are also among the smart materials that have the capability to recover their pristine shape after a significant deformation of about 8% strain 
. This shape recovery is due to either stress- or temperature-induced phase transformations. Owing to their distinct self-centering capability, SMAs can be used in different civil engineering applications.
2. SMAs and Their Distinct Properties
SMAs are a new group of smart materials characterized by their ability to recover large plastic strains induced while the crystal structure is in the martensitic form. This plastic strain is recovered by raising the temperature and changing the crystal structure to the austenitic form. The alloy returns to its deformed shape once the crystal structure is transformed back to martensitic form. The speed of transformation is dependent on the speed with which the alloy can be heated. The temperature level at which martensitic transformation takes place and the shape of the hysteresis curve are dependent on the alloy composition and processing technique. When electric current is used for heating the alloy, the change can be very fast .
2.1. Superelasticity of SMAs
The maximum superelastic strain is the permanent strain induced by the shape memory effect of SMAs. The SMA is able to recover this strain if the temperature is above the austenite finish temperature. The superelasticity of SMAs (also known as pseudoelasticity) is determined by performing cyclic tensile testing as described in ASTM F2516 
. After loading the SMA specimen to 6% strain, two types of stress are identified: lower plateau stress (LPS) at 2.5% strain, and upper plateau stress (UPS) at 3% strain. The typical cyclic tensile curve presented by Khan et al. 
for superelastic SMAs consisted of different segments, as demonstrated by Wu and Schetky 
. Initially, it can be observed that the austenite phase displays representative elastic deformation from A to B where the UPS is reached, followed by an isostress state from B to C as the cubic austenite structure shears into detwinned SIM, and elastic deformation from C to D. After unloading from D to A, the elastic strain is recovered, and the SIM returns into the previous austenite phase. A typical Nitinol SMA displays superelasticity up to 8% strain before the onset of permanent deformation. However, depending on the SMA type, some percentage of residual deformation is there 
2.2. Shape Memory Effect
The shape memory effect, SME (also known as pseudoplasticity) is the ability of SMAs to return to their predetermined shape upon heating. The shape memory effect of SMAs is mainly induced by thermal phase transformations between martensite and austenite phases. This effect can best be illustrated using the stress–strain–temperature graph and the crystal structure presented by Wu and Schetky 
. Depending on the crystal structure of the SMA, two phases can be identified: the austenite phase, which is strong and stable at high temperature; the martensite phase, which is weak and stable at low temperature.
3. Commonly Used SMAs
The most common type of SMAs used in civil infrastructures is NiTi (nickel–titanium), also called Nitinol alloy. This is due to the superior properties of this alloy compared to carbon steel, e.g., superelasticity, the corrosion resistance, wide range of working temperatures, and high values of strength. To date, several types of SMAs have been developed, including copper (Cu)-, niobium (Nb)-, and iron (Fe)-based SMAs 
. Cu-based SMAs are alloys that contain copper as one of the main alloying elements, in addition to others. They are relatively inexpensive materials with a recoverable strain limited to 2–4% 
. Another potential low-cost SMA type is Fe-based. Fe-based alloys have limited use in building industry applications because they are not available in large-diameter bar or wire forms. 
. However, NiTi SMAs are widely used today, because of their excellent properties including high strength, good corrosion resistance, high fatigue life, good electrical properties, and superior SME and SE properties 
The pie chart shown in Figure 1 indicates that NiTi SMAs have been researched the most among all SMAs, and they have become the most commonly used for commercial applications. Thus, the subsequent sections focus more on Nitinol SMAs, along with their properties, performance, and applications in civil infrastructures.
Distribution of the published research in civil engineering sorted by SMA type.
4. Mechanical Characteristics of NiTi SMAs
4.1. Cycling Loading
The superelasticity of Nitinol SMAs can be utilized effectively under lateral cyclic loads simulating earthquake loads on structures built in active seismic zones.
4.2. Strain Rate Effects
The experimental tests conducted by Azadi et al. 
indicated that the strain rate has an important effect on the mechanical response of NiTi SMAs. Some researchers 
observed that the material liberates energy as heat during the forward phase transformations, whereas it absorbs energy during unloading. They also found that the material may not have sufficient time to transfer heat to the surroundings at high strain rates. A few researchers observed a reduction in the energy dissipated with the increase in strain rate 
, while some other researchers 
noticed a larger dissipation of energy at higher strain rates. Furthermore, Ozbulut and Hurlebaus 
studied the influence of loading frequency within a common range for seismic events on the performance of SE Nitinol SMA wires. They observed up to a 47% decrease in the dissipated energy by increasing the frequency to 2 Hz. The contradiction in the results of the aforementioned studies could be due to variations of strain rates, composition of the material, and testing conditions.
4.3. Shape Memory Effect
Many researchers have investigated the effects of a change in temperature on the phase transformations and superelastic behavior of NiTi wires 
. Their experimental test results revealed a significant effect of the temperature on the superelastic response of SMA. Moreover, they observed that the stress that initiates the phase transformations of SMA increases with temperature. In contrast, they found that the residual deformation and stiffness were not influenced by the temperature change in the superelastic range.
5. Performance of SMAs under Different Environments
5.1. Performance of SMA under Elevated Temperatures
Sadiq et al. 
carried out some experimental tests on the effect of elevated temperatures on the stress–strain curve of NiTi SMA in tension. Unlike reinforcing steel, they found that the Young’s modulus and the yield strength of NiTi SMA increased with the increase in temperature. Moreover, the test results indicated that the SMA specimens exposed to 500 °C exhibited up to a 100% increase in stress with a corresponding increase in strain of about 6% compared to the same specimens exposed to 20 °C.
5.2. Performance of SMA under Corrosive Environment
The experimental investigations carried out by some researchers indicated that SMAs possess corrosion resistance in aggressive solutions comparable to that of carbon steel. Zhao et al. 
found that the NiTiNb SMA was able to maintain adequate mechanical properties even after being subjected to a harsh chemical environment. Joo et al. 
found that the corrosion resistance of an Fe-based shape memory alloy (FSMA), was about 150% more than that of steel, which had a passive coat in an alkaline environment. Furthermore, the passivated FSMA displayed a higher corrosion resistance in concrete because of its high alkalinity.
Shape memory alloys (often referred as smart materials) have great potential for enhancing the performance of civil engineering systems. The distinctive features of nickel-based superelastic shape memory alloys (NiTi SMAs) are extremely beneficial for the design, construction, and retrofit of RC structures. To foster the applications of SMA in the building industry, researchers should make their new discoveries compatible with existing design practices and prepare simple guidelines on SMA use for concrete practitioners.