Materials and structures at cold-region and Arctic low temperatures: A state-of-the-art review

2.1 High-speed railway (HSR) bridges

In the new century, the railways in northern China achieved fast developments, for example, Qinghai–Tibet Railway (see Figure 2A). This railway, connecting Xining and Lasa, China, is about 2000 km long, and crosses the Qinghai–Tibet Plateau that was exposed to the low temperatures of about −50°C. After the new millennium 2000s, the HSRs of China were extensively built, and amounts of them were constructed in cold northern China, for example, Harbin-Dalian HSR in northern China (see Figure 2B), Mudanjiang-Jiamusi HSR in Figure 2C, Beijing-Harbin HSR in Figure 2D, Beijing-Zhangjiakou HSR in Figure 2E, and Lanzhou-Urumqi HSR in Figure 2F. These constructed HSRs in northern China were also exposed to the lowest temperature of about −50°C.

2.2 Bridges

Bridges are important engineering structures for railway and road transportation systems. In the Arctic and cold regions, different kinds of bridges have been built. Figure 3A–D shows the highway–railway combined steel bridges built in China, Canada, and Russia.^16-19 These constructed steel bridges in northern China and Canada were also exposed to the lowest temperature of about −50°C. Therefore, the steel materials and structural members in these bridges at low temperatures are reserved for in-depth investigations.

Including the steel bridges, numerous reinforced concrete (RC) or composite bridges have been built in the countries around the Arctic circle as listed in Figure 4, for example, Sweden, Finland, and Canada.^20-31 These concrete or composite bridges were exposed to the Arctic low temperatures of −50°C to −60°C. Therefore, investigating behaviors of concrete materials and structures at low temperatures becomes necessary.

2.3 Liquid natural gas container

The liquefied natural gas (LNG) provides clean energies for the industry and daily life. The Arctic reserves about 1/3 undiscovered gas of the world, and the explorations in this region have been performed for many years. The Yamal project, locating in the Arctic, Russia, reserves about 1.3 trillion cubic meter LNG, and was started in December 2013. In 2018, the first ship of LNG produced by the Yamal project was transported to China. To assist in the storage and transportation of these LNG, several concrete LNG containers were constructed, as shown in Figure 5, which were exposed to the low temperature below −60°C.

Moreover, to reserve the LNG, amounts of RC containers have been built in different countries. At the end of year 2022, more than 40 LNG containers have been built in China. During the services, these RC containers were under a high risk of LNG leakage, which would expose them to the lowest temperature of about −160°C. Therefore, to ensure the safety of these LNG containers during service life, performing studies on the RC structures at extremely low temperatures above −160°C becomes necessary.

2.4 Platforms and pipelines

As revealed by the U.S. Geological Survey,⁴ the undiscovered oil (or gas) in the Arctic occupies about 13% (or 30%) of the world's reserve. Most of these reservations in the Arctic were distributed along the continental shelf within a seawater depth of 0–500 m. To make explorations of these oil (or gas), different types of platforms have been developed, as shown in Figure 6. Traditional jacket structure (see Figure 6A) has been used for oil exploration in Bohai Gulf, China, after the 1960s. The monopod tower provides another type of platform for oil exploration, as shown in Figure 6B, which has been already used in Southern Alaska. An artificial island, as shown in Figure 6C, provides economic solutions for oil explorations in shallow seawater, which has already been adopted for several projects on the North Alaska coast.³ As the depth of the sea increases, a caisson-retained island, as shown in Figure 6D, is an optional structural form, for example, the Molikpaq caisson in Beaufort Sea, USA. Gravity-based conical platform, as shown in Figure 6E, provides the solutions for oil explorations within a seawater depth of 0–100 m, which has already been used in on-site projects in the North Sea. For deep-sea oil explorations, the floating platform provides solutions, as shown in Figure 6F–I.

After the oil and gas explorations in cold regions, the pipelines act essentially on their transportations, for example, the national project of Natural Gas Transmission from the West–East of China (see Figure 7A), the Yamal–Europe pipelines transporting natural gas from Russia to Austria, as shown in Figure 7B, the Eastern Siberia to Pacific Ocean oil pipeline, as shown in Figure 7C, the Russia–Europe Druzhba pipeline, as shown in Figure 7D, Keystone Pipeline connecting Canada and USA, as shown in Figure 7E, and Mackenzie Valley LNG pipeline connecting Mackenzie and northern Alberta, Canada, as shown in Figure 7F. All these pipelines suffer the lowest temperature in winter within −50°C to −60°C, which critically changed their safety during the service.

The review revealed that amounts of engineering infrastructures, for example, bridges, railways, platforms, pipelines, and containers, have been constructed and put into service in the cold or Arctic regions. During their services, these engineering infrastructures face low temperatures dropping to about −80°C or even lower to about −160°C for LNG containers. The low temperatures challenge the mechanical properties of construction materials, and finally influence the mechanical behaviors of structures.

3.1 Concrete at low temperatures

RC or prestressed concrete (PC) structures act essentially in the cold/Arctic-region infrastructures, LNG containers, and platforms. Mechanical properties of concrete provide the basis for analysis/design on RC/PC structures. Normal weight concrete (NWC), ultrahigh performance concrete (UHPC), and lightweight concrete (LWC) are included in this review.

3.1.1 Compressive behaviors

Amounts of research studies have been reported on mechanical properties of NWC at dropping low temperatures. Lee et al.³⁹ and Yamane et al.⁴⁰ both conducted research on the compressive/tensile strength of NWC at various low temperatures. Lee et al.³⁹ focused on temperatures ranging from −70°C to 20°C and found that as T decreased from 20°C to −70°C, the compressive strength (f_c) and splitting tensile strength (f_t) of NWC increased by approximately 100%, with Poisson's ratio also increasing by 50%. Yamane et al.⁴⁰ extended the low-temperature range from −10°C to −196°C and observed that the f_c, f_t, and modulus of elasticity (E_c) of NWC also increased within this range. Moreover, they found that the increments of f_c, f_t, and E_c were much larger for NWC with higher moisture. Tognon⁴¹ studied the mechanical behaviors of mortars/concretes at T = 20 to −196°C, and found that the concretes with different types of cements exhibited close improvements in f_c as the temperature dropped to −196°C. Marechal⁴² studied the E_c at T = 500 to −160°C, and found that as the T decreased to −50°C, −100°C, and −160°C, the E_c of concrete was increased by about 35%, 20%, and 36%, respectively. Continuous works focused on the applications of NWC in LNG container. Krstulovic-Opara⁴³ reviewed the previous studies on the concrete used in LNG storage. Continuous reviews on the concrete for LNG storage were made by Kogbar et al.,⁴⁴ Jiang et al.,⁴⁵ and Lin et al.⁴⁶ Their works revealed that the W_c, w/c, and aggregate type greatly influenced the increasing rate of NWC f_c, as shown in Figure 8. Further studies were extended to UHPC, Kim et al.^47-49 researched the mechanical properties of UHPC at T = −170°C. They found that both compressive/tensile strengths of UHPC were significantly improved, but the tensile strength of NWC was not affected.

Filiatrault and Holleran⁵⁰ studied the compression behaviors of NWC at −40°C at a strain rate of 0.1/s. Rostásy and Wiedemann⁵¹ researched compression behaviors of NWC at −170°C, which showed large increases in the modulus and strength under compression. Moreover, the water-saturated NWC exhibited larger increments in the elastic modulus/compression strength than those of normally-stored NWC. MacLean and Lloyd⁵² studied compression behaviors of C30 and C50 NWC at T = 20°C, −10°C, −40°C, and −70°C. They found that as T drops to −70°C, the compression strength and E_c of NWC receive their respective maximum increments of about 43% and 59%. Liu et al.⁵³ also researched the low-temperature compressive behaviors of NWC, LWC, and ultralightweight concrete. They also found that the concretes with higher water content and larger w/c ratio received higher improvements in their compressive strength and modulus. Moreover, these improvements were also affected by the concrete type.

Xie and Yan⁵⁴ tested 174 NWC cubes at T = 20°C, 0°C, −40°C, −80°C, −120°C, and −165°C. They found that the f_c of NWC increased with the decreasing T, and the increasing amplitudes of f_c increased with the contents of water. For example, for NWC with w/c = 0.41, as T dropped from 20°C to −165°C, the f_c of NWC with water contents of 0, 1.1%, 3.0%, and 5.3% were increased by 56%, 56%, 102%, and 119%, respectively. Furthermore, they also found that the f_c values decreased with the water content (W_c) at 20°C and 0°C, but significantly increased with the increasing water content at low temperatures of −40°C to −165°C. The change in the microstructure of concrete after low-temperature compressive tests was captured using scanning electron microscopy (SEM), as depicted in Figure 9. It reveals that as T decreased, the microstructure of concrete became increasingly loose, and more pores and cracks were found. This is due to the fact that as the temperature dropped, the capillary water was frozen, which expanded and resulted in microcracks. Finally, Xie and Yan⁵⁴ proposed the prediction equations to estimate the relationships between the increasing index of f_c (I_fc) and w/c, W_c, and T as the following:

$$${I}_{\mathrm{fc}}=9.58{T}^{-0.53}{{\rm{e}}}^{({W}_{{\rm{c}}}/30)}{(w/c)}^{-0.82}$$$()

where T in K, and 108 K ≤ T ≤ 293 K; W_c in %.

Recently, Xie et al.⁵⁵ and Kang et al.⁵⁶ studied behaviors of different grades of NWC under monotonic and cyclic compression loads at T = 20°C to −80°C. They found that as the T dropped from 20°C to −80°C, the NWC peak compression strain was not affected, but their f_cu, f_c, and E_c of C55 NWC were significantly increased by 47.3%, 69.0%, and 45.3%, respectively. Besides, it was revealed that the cyclic compressive stress–strain behaviors exhibit similar patterns to those observed under monotonic loading. In addition to the study on NWC, Kang et al.⁵⁷ also investigated low-temperature monotonic/cyclic compression behaviors of UHPC. Finally, they proposed the monotonic/cyclic compression stress–strain models for NWC and UHPC at T = 20°C to −80°C, as shown in Figure 10.

3.1.2 Tensile behaviors

Previous studies pointed out that low temperatures generally improved the concrete tensile strength, and exhibited their maximum improvements at T = −30°C to −70°C.^{58, 59} Browne and Bamforth⁵⁹ found that below −70°C, the increments in tensile strength due to the low temperature tended to remain unchanged. Meanwhile, Yamane et al.⁴⁰ pointed out that as T dropped from 20°C to −50°C, the NWC tensile strength of wet and dry NWC was linearly increased by about 150% and 50%, respectively, and these increments tended to achieve their maximum values at −30°C to −50°C. Beyond that low-temperature range, the increments in tensile strength tended to be steady with slight reductions. This was mainly related to the frozen ice in pores. As the temperature decreased, the water in gel and capillary pores gradually froze and formed ice mesh inside, which acted similarly to the fibers and could support the load applied to the concrete. Meanwhile, the increased strength of ice at low temperatures improved the tensile strength of concrete. However, due to the different pore structures of various concrete, the ice mesh structure varied at low temperatures. The different freezing processes of pore water resulted in the maximum tensile strength of concrete varying in the range of −30°C to −70°C. Subsequently, as the temperature decreased further, the ice was subjected to increasing temperature stresses. When the ultimate tensile strain of ice was reached, the ice mesh began to collapse, which led to a slight decline in the tensile strength of the concrete. In summary, the increasing rate of tensile strength for NWC was affected by their W_c, aggregate type, and w/c.^{39, 40, 45, 60, 61} And the moisture content and aggregate type exhibited the most significant influences. Figure 11 shows that larger moisture-content concrete received larger strength improvements as T drops, and the tensile strength of LWC exhibits much smaller influences than that on NWC by the dropping T. The increased concrete tensile strength by reducing T due to the ice-net structure.⁶² In this theory,⁶² after the frozen of the capillary water, these frozen ices formed a structural net frame in NWC. Once the concrete is under tensile force, this formed ice-net frame partly shares the tensile force that acts the functions of steel or other types of fibers in concrete, which finally improves the concrete tensile stress. The ice-net tensile strength increased with the dropping T till to a specific value. Beyond this value, the concrete contraction tended to destroy this ice-net structure, and thus reduced its tensile resistances. Including these key influencing factors, Shi et al.⁶³ found that higher-grade concrete showed a smaller increase in low-temperature tensile strength.

Including the splitting tensile strength, the first-mode fracture behavior of concrete was researched. NWC fracture behaviors at T = 20°C, −20°C, −70°C, −120°C, and −170°C were researched by Maturana et al.⁶⁴ and Rocco et al.⁶⁵ Continuous experimental studies on concrete fracture behaviors at 20°C, 0°C, −20°C, and −40°C have been reported by Hu and Fan⁶⁶ and Fan et al.⁶⁷ These studies showed that as T drooped the concrete fracture energy and toughness were improved, but its characteristic length was reduced.

In addition, low-temperature tensile behaviors of UHPC have been experimentally studied. Flexural tensile behaviors of UHPC at T = −20°C have been experimentally studied by Fang et al.⁶⁸ Their studies showed that as the temperature dropped, the UHPC tensile strength was increased, but its ductility was reduced. Moreover, Kim et al.^{47, 48} found that at −165°C, the compressive/tensile strength of UHPC was improved. He et al.⁶⁹ studied the influences of fibers on the flexural behaviors of UHPC after exposure to low temperatures. They found that UHPC adding steel fibers exhibited larger flexural tensile strength than those with PP and PVA fibers.

Xie et al.⁷⁰ experimentally investigated the NWC fracture behaviors at T = 20°C to −80°C through 72 three-point notched NWC beams. They found that the low temperatures improved the cracking and ultimate resistances of NWC beams. As T dropped to −80°C, the NWC fracture toughness (or fracture energy) was significantly improved. This was mainly due to the increased tensile strength by low temperatures. Furthermore, Liu et al.⁷¹ researched fracture and flexural behaviors of UHPC at T = 20°C to −160°C (see Figure 12). Finally, models for fracture energy were developed for NWC and UHPC.

3.2 Steels at low temperatures

Steel materials are widely utilized in engineering constructions. Nevertheless, their mechanical properties are significantly affected by dropping temperatures.

3.2.1 Steel reinforcements

Extensive research works were made to investigate the low-temperature tensile behaviors of reinforcements. Elites et al.⁷² performed tensile tests on hot-rolled/cold-stretched reinforcing steels at 20°C to −180°C. Their tests revealed that the yield/ultimate (f_y/f_u) strength of these reinforcements was improved with the dropping T. The f_y and f_u of hot-rolled/cold-formed steel reinforcements were increased by 88%/42% and 94%/67% at −180°C, respectively. Sloan⁷³ pointed out that the steel-reinforcement f_y and f_u were improved by 26% and 24% at about 40°C. Montejo et al.⁷⁴ also performed tensile tests on reinforcing-steel coupons at T = 20°C to −40°C. They observed that the reinforcing-steel f_y and f_u were improved by about 10%–12% at −25°C to −40°C. Dahmani et al.⁷⁵ reviewed previous studies and pointed out that reinforcing steel received a notable increase in the yield/ultimate strength as the temperature dropped to −196°C, but the elastic modulus was slightly increased. Filiatrault and Holleran⁵⁰ performed tensile tests on steel reinforcements at combined T and varying strain rates. Their test results showed that strain rates below 0.1/s and low temperatures (−40°C) increased the steel-reinforcement f_y (or f_u) by about 20% (or 10%). Levings and Sritharan⁷⁶ also carried out tension tests on ASTM 420 (60) steel reinforcements at T = 20–40°C and strain rates of 0.003–0.3/min. They observed that the yield (or ultimate) strength of ASTM 420 (60) was increased by 5.1% (or 6.3%) as T dropped from 20°C to −40°C.

Yan and Xie⁷⁷ investigated low-temperature mechanical properties of Chinese HRB335 and HRB400 reinforcing steel between 20°C and −165°C. As illustrated in Figure 13, the yield/ultimate strengths of HRB335 and HRB400 were significantly improved, but their ductility was compromised. Decreasing the T from 20°C to −165°C received a respective average increase of 23% (14%) and 17% (14%) for HRB335 and HRB400, but reduced the ultimate tensile strains of HRB335 and HRB400 reinforcements by 33% and 34%, respectively. Moreover, the increasing factors for those reinforcements due to low temperatures were also developed based on these test data.

3.2.2 Steel strands

Previous limited studies have been made on low-temperature mechanical properties of steel strands. Peng et al.⁷⁸ studied the working mechanism of wire rope at T = −5°C, −10°C, and −15°C. Planas et al.⁷⁹ researched low-temperature influence on tendon anchorages. Tensile behaviors of three-wire strands at 20°C to −165°C have been experimentally studied by Xie et al.⁸⁰ They found that compared with strands at 20°C the steel strand's yield/ultimate strength at −165°C was increased by 13%/8% whilst the ultimate tensile strain was reduced by about 34%. Furthermore, Xie et al.⁸¹ conducted 21 tests on steel wire in strands at T = 20°C to −160°C, and obtained their stress–strain behaviors (see Figure 14). As shown in Figure 14, the yield and ultimate strengths of steel wires were increased by approximately 20% as T decreased from 20 to −160. Additionally, the figure further illustrated that as T dropped below −100°C, the failure mode of the steel wire changed from ductile to brittle mode. From these obtained low-temperature mechanical properties of steel wire, the constitutive stress–strain models of different multilayer strands can be obtained by theoretical and empirical models. In addition, the stress relaxation and thermal expansion behaviors of steel strands at T = 20°C to −165°C were also studied by Xie and Yan⁸² and Yan et al.⁸³

3.2.3 Normal mild steel plate

Pisarenko et al.⁸⁴ carried out tension tests on structural steels and alloys at T = −196°C and −270°C, and found that the decreasing T improved the strength of tested steels.

Polyzois et al.⁸⁵ studied mechanical behaviors of cold-formed steel angles at T = 25°C to −45°C, and found that the f_y (or f_u) of steel angles at −40°C was increased by 8% than that at room temperatures. Abdel-Rahim and Polyzois⁸⁶ investigated the low-temperature mechanical properties of A715 (60) and G40.21-300W steel. They pointed out that decreasing T to −50°C improved the strengths of these tested steel materials. Wang et al.⁸⁷ performed extensive investigations on constructional steels at T = 20°C to −60°C, for example, Q235, 16Mn, 15MnV, 16Mnq, and 14MnNbq. They found that reducing T increased the f_y and f_u of constructional steels, but reduced their ductility indexes. Besides, Wang et al.⁸⁸ investigated fracture behaviors of Q345B steels at 20°C to −60°C. They revealed that decreasing T reduced the fracture toughness of concrete. Hu et al.⁸⁹ conducted tension tests on DH36 steel at 20°C, 0°C, −20°C, −40°C, and −60°C. As T reduced from 20°C to −60°C, the f_y and f_u of DH36 were increased by 15.7% and 14.9%, respectively. Moreover, studies^{90, 91} have been performed on fatigue performances of mild steels at decreasing T.

Mechanical behaviors of headed studs at T = 20°C to −80°C were studied by Xie et al.⁹² through 56 tension tests, and they pointed out that as T decreased to −80°C, the headed-stud yield strength was increased by about 10%, and its fracture strain was not reduced but even increased by about 20% (see Figure 15A). Besides, Xie et al.⁹³ studied tensile stress–strain behaviors of China-standard Q235–Q460 steels at T = 20°C to −165°C and developed corresponding low-temperature stress versus strain models (see Figure 15B,C).

3.2.4 High-strength steels (HSSs)

Low-temperature mechanical behaviors and fracture toughness of Q460C HSS and corresponding butt weld have been experimentally studied by Liu et al.⁹⁴ They revealed that reducing T increased the f_y and f_u, but reduced the fracture toughness of Q460C HSS. Tong et al.⁹⁵ studied the toughness of HSS under impact loads with yield strengths of 460–960 MPa at −40°C to 20°C. The test results showed that the impact toughness of HSS with the same nominal yield strength and plate thickness diminished as the temperature decreased. Additionally, the SEM micrographs confirmed that the Q690 and Q960 HSS exhibited ductile fracture at 20°C with more dimple structures, and brittle fracture at −40°C with more significant tongues or river patterns. Rokilan and Mahendran⁹⁶ studied the mechanical behaviors of mild steel and HSS at T = 20°C to −70°C, and found that mechanical behaviors of mild steel were significantly increased by the reducing T (see Figure 16).

Perez-Martin et al.⁹⁷ studied the impact toughness of HSS at T = 20°C to −90°C through Charpy V-notch tests. They found that the HSS impact energy slightly increased with the dropping T. Parkes et al.⁹⁸ performed tensile tests on dual-phase steel and HSS and laser-welded joints at −40°C, 25°C, and 180°C. They pointed out that decreasing the T increased the E_s, f_y, and f_u of HSS and joints. Moreover, Zhang et al.⁹⁹ studied the influences of T = −30°C to 25°C and stress amplitudes (0.5, 0.7, 0.9f_y) on fatigue performances of Q420B steel. They found that decreasing T improved the fatigue behaviors of Q420B steel. These previous reported investigations pointed out that the reducing T has positive effects on the E_s and strength of steels, but reduced their ductility. However, these influences greatly depend on their containing elements and microstructures. Moreover, most of the previous investigations less focused on the tensile stress–strain behaviors of steels at reducing T.

Yan et al.^100-102 studied the influences of T = −165°C to 20°C on mechanical properties of HSS Q690/Q690E/Q960E/S690 and normal steels. They revealed that the f_y and f_u of normal steel or HSS were improved by the decreasing temperature. The full-range stress–strain models were finally developed for normal steel and HSS at low temperatures. These increments in the strength of steels at low temperatures can be attributed to several factors¹⁰³: (1) Dislocation motion inhibition. At low temperatures, the reduced thermal energy within the steel lattice slows down atomic movements, which inhibits dislocation motion within the material. This makes the steel less prone to deformation and, consequently, exhibits higher strength. (2) Lattice structure changes. Low temperatures cause the lattice structure of steel to become more compact and stable, reducing the mobility of lattice defects. This closer packing results in greater resistance to deformation, and, therefore, enhanced strength of the steel. (3) Phase transformation. Certain steels undergo phase transformations at low temperatures, such as martensitic transformation. These transformations result in a more densely packed and complex crystal structure, with higher dislocation densities and grain boundary areas. These microstructural changes contribute significantly to the increased strength of steel at low temperatures. Figure 17A,B⁹⁷ shows the microfracture surfaces of S960 HSS specimens after ambient temperature and −40°C tensile tests, respectively. A classic dimple structure was observed at both temperatures, which indicated ductile failure. Furthermore, these figures reveal that the dimples became shallower as T decreased, which implied a reduction in the ductility of S960 HSS at low temperatures.

The developments of LNG containers and cold-region infrastructures require investigations on corresponding steel at low temperatures. This section introduced five batches of low-temperature tensile tests on reinforcing steels for RC, HSS strands, mild steel, HSS, and headed studs in composite structures. These results revealed that the f_y and f_u of steel at low temperatures were obviously increased, which is mainly caused by the transmissions of steel microstructure at low temperatures, for example, lattice refinement and phase transformation. The dislocation movement is hindered. However, the ductility of steel materials did not necessarily decrease at low temperatures. By these test results, empirical formulae considering the key mechanical indexes of low temperature were proposed. Moreover, the stress–strain models of these steel at low temperatures were also proposed for the analysis/design of engineering structures exposed to low temperatures. However, quite limited studies have been reported on the cyclic stress–strain behaviors of steel at low temperatures, highlighting the need for further research in this area. Potential future research directions could include investigating the effects of low temperatures on the cyclic stress–strain response of different steel grades, and also exploring the potential for developing new materials or treatments to improve the cyclic performance of steel at low temperatures.

4.1 SCIB in RC and PC structures

SCIB determines the stiffness, bending resistance and ductility of RC and PC members, which reserved in-depth investigations. Amounts of research works have been reported on low-temperature SCIBs. Shih et al.^{104, 105} experimentally studied the bar–concrete bonding behavior at T = 20°C to −70°C. They found that with the decreasing T from 20°C to −70°C, the ultimate bar–concrete bonding strength was improved by 28% and 127% for high-strength concrete and NWC, as depicted in Figure 18A. On the basis of their tests, Huang et al.¹⁰⁶ proposed regression models to predict the low-temperature concrete–bar interfacial shear–slip relationships. Vandewalle¹⁰⁷ investigated the steel bar–concrete bonding behaviors at T = 20°C to −165°C by the testing method. It showed that reducing T from 20°C to −120°C increased the bar–concrete bonding strength; however, this bonding strength reduced slightly at T = −120°C to −165°C. Jin et al.¹⁰⁸ studied the low-temperature bar–concrete bonding behavior using numerical simulation methods.

Previous investigations on strand–concrete bonding (SCB) for PCs were mainly performed at room temperatures. Influences of anchorage length, concrete grade, and diameter of strand on the SCB have been reported in Salmons and McCrate¹⁰⁹ and Cousins et al.¹¹⁰ (see Figure 18B). Figure 18 shows that the SCB strength increases linearly with the dropping T within 20°C to −80°C, and is maintained at T = −80°C to −165°C. Gustanvson¹¹¹ carried out tests on the SCB behaviors, and found that the surficial roughness of steel strand significantly influenced interfacial bonding strength. Pozolo and Andrawes¹¹² tested the bonding behaviors between prestressing strands and self-compacting concrete (SCC). They revealed that strands in SCC performed equally to those in NWC.

Xie et al.¹¹³ studied the steel bar–concrete bonding behaviors at T = 20°C to −80°C through 69 pulling-out tests. Their tests showed that this bonding strength increased linearly with the decreasing T. In addition, low-temperature SCB behaviors have also been studied. Xie et al.^{80, 114} performed pull-out tests on SCB behaviors at T = 20°C to −165°C. As shown in Figure 19, the typical load–slip curve associated with these tests could be characterized by three stages: initial elastic, slipping development, and recession stages. Furthermore, two typical failure modes were observed in the tests: pullout failure and splitting failure. Additionally, the figure showed that SCB stress at 1 mm slip and ultimate bond stress increased with reduced low temperatures. However, as T decreased from −80°C to −165°C, the bond stress exhibited limited increase. This is due to the fact that the water content has counteracting effects on concrete strength.

Since the temperature in cold regions tends to fluctuate around frozen point, the bond after freeze–thaw cycles (FTCs) has been investigated. Deng et al.¹¹⁵ studied the BFRP bar–concrete bonding under FTCs, and observed that the FTCs significantly affected the bonding strength. Li et al.¹¹⁶ studied the recycled concrete-steel bar bonding behaviors after −17°C to 5°C cycles through 36 pull-out tests. They found that the bonding strength was reduced as the freeze–thaw (FT) damage and anchorage length increased. Berto et al.¹¹⁷ developed the bonding models of NWC structures after FT damages. Wang et al.¹¹⁸ investigated the bonding behaviors of glass aggregated-concrete after FTCs. They also proposed new bond-slip constitutive models considering different FTCs and the content of glass aggregate.

These studies showed that the decreasing low temperature tended to increase the bonding strength at reinforcement/strand–concrete interface, which depended on the low temperature level and concrete grades. However, the FTCs tended to reduce the reinforcement/SCB strength.

4.2 SCIB in composite structures at low temperatures

At the steel–concrete interface in composite structures, headed studs are common measures to connect the concrete and steel I-beam or plate. The interfacial shear behaviors of studs affect the composite actions of steel–concrete composite structures (SCCSs), and finally influence the structural behaviors of SCCSs at low temperatures.

Amounts of researches have been performed on interfacial shear behaviors of headed studs at room or high temperatures.^119-125 Dalen¹²⁶ performed pioneer push-out tests on studs at −10°C and −20°C. They found that dropping the T to −20°C improved shear resistances (SRs) of studs (diameter = 13 mm), but reduced SR of studs (diameter = 19 mm). Xie¹²⁷ found that the SR and stiffness of studs increased linearly with the dropping T till −40°C. But, the plasticity and ductility deterioration of stud decreased with the reduced low temperature. Hou and Ye¹²⁸ and Hou et al.¹²⁹ studied fatigue behaviors of 14MnNbq headed studs at T = 20°C to −50°C. Research results show that fatigue resistances of headed studs were improved as temperature reduced.

Yan and Xie¹³⁰ made efforts to experimentally investigate the shear behaviors of studs at T = 20°C to −80°C. Figure 20A revealed that reducing the T from 20°C to −80°C increased the headed-stud SR by about 25%, but reduced their slip capacity by about 30%. These effects occurred because the reducing T improved the NWC elastic modulus and strength, and the strength of studs, but reduced the ductility of studs. The increased NWC strength also resulted in the enhanced confinement on the head studs, which in turn deterred the NWC splitting and improved studs' SR. Yan et al.¹³¹ also investigated shear behaviors of studs by finite element (FE) approach (see Figure 20B). This FE model adopted low-temperature mechanical properties of concrete and steel, and made an in-depth parametric study.

In addition, SRs of studs in concrete after FTCs have also been investigated. Li et al.¹³² and Xiao et al.¹³³ investigated the SR prediction model and shear mechanism of studs in SCCSs suffering FTCs and artificial corrosion (AC). They found that shear stiffness and resistances of studs were significantly compromised after the FTCs or AC.

Thus, these studies revealed that the low temperatures improved ultimate and fatigue SRs of studs in SCCSs. However, the FTCs were harmful to the shear capacities of studs.

6.1 CFSTs at low temperatures

Numerous investigations have been performed to explore compression behaviors of CFSTs at low temperatures or after FTCs. Jin et al.¹⁵¹ carried numerical mesoscale numerical simulations on the CFSTs at dropping temperatures. Yang et al.¹⁵² studied compression behaviors of stub CFSTs after FTCs. They found that the compression resistance of those stub CFSTs decreased with the increasing FTCs. Gao et al.¹⁵³ performed compression tests on 24 circular CFSTs after FTCs. They found that the axial compression capacity of CFSTs with different grades of concrete all reduced as FTCs increased, and their deterioration in strength became more serious for CFSTs with lower-strength concrete. Gao et al.¹⁵⁴ also performed further studies on the compression behaviors of CFSTs after combined chloride erosion and FTCs. The ultimate compression capacity of CFSTs decreased almost linearly with the increasing erosion velocity and FTCs. In addition, the erosion velocity, FTCs, and grades of steel tubes exhibited marginal influences on the failure modes of CFSTs. Zhang et al.¹⁵⁵ studied axial-compression-capacity degradation of 40 square CFSTs exposed to FTCs and acid rain erosion. They observed that after exposed to acid rain and FTCs, all the CFSTs failed in steel-tube local buckling and tube-corner vertical fracture. As the FTCs increased to 240, the stiffness and ultimate compression capacity of CFSTs were reduced by 4% and 2%–3%, respectively. Meanwhile, the acid corrosion exhibited much larger reductions than the FTCs on the compression behaviors of CFSTs. Lyu et al.¹⁵⁶ also obtained the same findings as Zhang et al.¹⁵⁵ Continuous works by Song et al.¹⁵⁷ and Zhang et al.^{158, 159} revealed that FTCs compromised the compression or lateral share capacities of CFSTs.

Yan et al.^160-163 performed compression tests on stub CFSTs made of square or circular mild steel tubes and NWC. Figure 23 shows that the steel-tube local buckling and concrete crushing were the major failure modes of those CFSTs, and the only different failure modes from those CFSTs subjected to ambient-temperature compression was the vertical fracture of mild steel tubes due to its brittleness at low temperatures. Moreover, the dropping T increased the stiffness/compression resistances of CFSTs, which were due to the increased steel-tube yield strength, increased compressive strength of concrete, improved tube–concrete bonding strength, and increased confinement at low temperatures. Following this, the low-temperature compression behavior of stub CFSTs adopting Q690 and Q960 HSS tubes was experimentally studied through 30 compression tests.^164-166 These compression tests showed that the CFST with HSS tubes failed in concrete compression, HSS-tube outward buckling, and welding fracture at HSS-tube corners. Further extended experimental studies by Yan et al.^{165, 167, 168} focused on the compression behaviors of CFSTs with UHPC and Q690/Q960 HSS tubes. Their experimental studies revealed that the UHPC crushing, HSS-tube outward local buckling, and weld fracture were the three main failure modes of UFHSSTs (UHPC-filled HSS tubes) after low-temperature compression. Moreover, as shown in Figure 24, the low temperatures actually improve the compressive behaviors of UFHSSTs, for example, increased compression resistance, elastic stiffness, and ductility ratio (DI). More recently, Chen and Yan¹⁶⁹ and Yan et al.¹⁷⁰ studied compressive behaviors of CFST using stainless-steel (SS) tubes. Due to the good ductility of SS S30408, the CFST with circular SS tubes shows a ductile behavior with a long strength hardening in their load-shortening curves.¹⁷⁰ Their main failure modes occurred were concrete crushing and elephant local buckles of SS tube. Different from those CFST with circular SS tube, the CFST with square SS tubes exhibited vertical fracture at the SS-tube corner,¹⁶⁹ which is due to the stress concentration at SS-tube corners.

In summary, these pilot research works studied the compression performances of CFSTs with mild steel, HSS, and SS tubes and different types of cores (NWC and UHPC), but these researches mainly concentrate on stub CFSTs under low-temperature compression or compression after CF cycles. The information on the slender CFSTs is still limited.

6.2 SCCSs at low temperatures

In the 1970s, Long et al.¹⁷¹ studied fatigue behaviors of SCCSs by performing fatigue tests on three continuous SCCSs at room (21°C) and low (−29°C) temperatures. They found that at −29°C the flexural behaviors of continuous SCCSs were not compromised after 500,000 fatigue loading cycles. Zhu et al.¹⁷² performed fatigue tests on SCCBs after FT (the low temperature was −20°C) cycles and chloride erosion. Due to the frost heave damage in concrete due to the FTCs, the strength and stiffness of SCCBs were compromised. Xu et al.¹⁷³ investigated flexural resistances of SCCBs that suffer negative moments after FTCs. They found that after 150 salt FTCs, the flexural bending resistances of SCCB with NWC and UHPC were reduced by 62% and 18%, respectively. These previous studies on the SCCSs confirmed that low temperatures increased the flexural bending resistances of SCC or steel–concrete–steel sandwich composite (SCSSC) beams, but the FTCs reduced the ultimate bending resistance of these SCC beams.

Moreover, the low-temperature compression performances of SCSSC walls were investigated. The reported test results^{159, 171} reflected that as T reduced from 20°C to −30°C, −60°C, and −80°C, the compression resistances of SCSSC walls were significantly increased by 15%, 62%, and 113%, respectively. These improved compression resistances were mainly caused by improved compressive strength of concrete, compressive buckling resistance of faceplates, and improved confinements provided by the faceplate to the concrete core. In addition, Wang et al.¹⁷⁴ developed detailed 3D numerical models to simulate compression behaviors of SCSSC walls by the ABAQUS.

Yan et al.¹⁷⁵ performed bending tests on 3 large-scale SCCBs at T = 20°C, −30°C, and −60°C (see Figure 25). They found that reducing T from 20°C to −60°C improved the ultimate bending resistance of SCCB by 24% and the elastic stiffness of SCCB by 16%. Then, prediction models were developed to calculate the bending resistance of SCCBs. Besides, Yan et al.¹⁷⁶ also performed four-point bending tests on 12 large-scale SCSSC beams at T = −70°C to 20°C. They found that: (1) Flexure failure occurred to SCSSC beams even to those beams with a small shear span of about 3.0; (2) as T reduces from 20°C to −30°C, −50°C, and −70°C, the ultimate bending resistance of SCSSC beams was increased by 8%, 14%, and 16% whilst the elastic stiffness was increased by 5%, 29%, and 33%, respectively. All these increments were mainly due to the improved concrete modulus and compressive strength, steel-plate yield/ultimate strength, and steel–concrete bonding strength. Moreover, comprehensive theoretical and numerical models have also been proposed for simulating low-temperature structural behaviors of SCSSC beams by Yan et al.¹⁷⁷

Including the SCC beams, Yan et al.^{178, 179} also performed compression tests on SCSSC walls at T = 20°C to −80°C. These SCSSC walls exhibited close load-shortening behaviors to those CFSTs that included elastic, nonlinear, and recession stages. The main failure modes that occurred to these SCSSC walls include concrete crushing and faceplate local buckling (see Figure 26).

In summary, these limited experimental and numerical studies confirmed that the low temperatures improved the bending resistance and compression capacity of SCC structural members. However, the FTCs compromised their flexural bending resistances of SCC structures. To further explore the influence mechanism of low temperatures and FTCs on SCC structural members, a comprehensive experimental design along with mesoscopic numerical simulations that encompass a wider range of low temperature and FTC parameters is needed.