With the rapid increase in the scale and number of tunnels in cold regions, the frost damage problems, such as cracking of the lining structure under the action of freeze–thaw cycles are becoming increasingly prominent. This review article collects and sorts out the frost damage phenomena that occur in the lining structure of tunnels in cold regions under the action of freezing and thawing cycles, classifies the frost damage phenomena into structural frost damage and non-structural frost damage, and proposes that the research on the mechanism of lining frost damage and its prevention measures should focus on lining cracking. According to the damage degree of the freeze–thaw cycle to the lining structure and its influence on tunnel operation, the cracking mode and cracking stage of lining are introduced. The analysis focuses on the mechanism of cracking in lining structures subjected to freeze–thaw cycles, considering the external force caused by frost heaving, the alteration of mechanical properties in lining concrete, and the internal changes in the microstructure of the concrete. Additionally, the factors that contribute to the occurrence of lining cracking are summarized. Based on this, corresponding control measures have been organized to provide reference for the development of cracking of the lining structure under the action of freeze–thaw cycle.

Reactive powder concrete (RPC) is a novel high-performance building material widely used in large-scale engineering structures due to its superior mechanical properties and durability. However, structural failure can still occur under dynamic load impacts. Therefore, optimizing the mechanical properties and impact resistance of RPC remains a critical issue for enhancing its engineering applications. In this study, the mechanical properties and impact resistance of RPC were investigated by adjusting the water-cement ratio and incorporating supplementary cementitious materials (SCMs), such as fly ash microspheres (FAM) and silica fume (SF). The effects of these adjustments on water absorption, strength, and impact resistance were assessed. Three water-cement ratios (0.16, 0.18, and 0.20) and various proportions of FAM and SF were selected to evaluate water absorption, compressive strength, bending strength, and impact resistance. The results indicated that reducing the water-cement ratio enhanced the densification of the concrete, reduced water absorption, and improved both compressive and bending strength. Specifically, when the water-cement ratio was 0.16 and FAM and SF were synergistically incorporated, the compressive strength reached 134.4 MPa, the bending strength reached 16.86 MPa, and the impact resistance was 22,838.4 J. Impact test results revealed that combining a low water-cement ratio with an appropriate amount of SCMs effectively increased energy absorption capacity and significantly slowed crack propagation. Analysis based on the Weibull distribution model demonstrated a more pronounced probability distribution of the number of impacts, suggesting that the optimization measures improved the impact resistance of RPC.

At present, the intelligent control technology of highway tunnel lighting mainly includes two types, graded dimming and stepless dimming, both of which have certain energy-saving effects. Tunnel lighting energy saving and consumption reduction, traffic safety, and the security degree are important indicators used to measure the efficient operation of a tunnel. By adding variable correlated color temperature (CCT) control based on stepless dimming, the adjustment of a lamp’s CCT according to changes in the external tunnel environment can be achieved. This not only serves the dual purpose of secondary energy saving and providing comfortable lighting, but also plays a significant role in reducing the reaction time and ensuring tunnel traffic safety. This paper mainly discusses the research achievements and applications of the main intelligent control technologies for highway tunnel lighting. Combining on-site investigations, operating and energy-saving effects achieved are evaluated, and the future development direction of intelligent control technology for highway tunnel lighting is summarized. Furthermore, this paper proposes an optimization model of a stepless dimming control system and intelligent control technology in the tunnel’s variable CCT based on stepless dimming. The results of this review can provide useful technical support for the design, operation and management of intelligent lighting control in highway tunnels.

The construction of pit-in-pit has become increasingly challenging due to the bad geological conditions, particularly in collapsible loess strata. To understand its supporting characteristics and failure mode, it is necessary to study the composite instability mechanism in the loess strata. This research systematically investigates the interacting instability modes of pit-in-pit under a collapsible loess stratum, studies the effects of different reinforcement parameters through physical model tests, analyzes the significance level of each reinforcement factor, and monitors the displacement of the foundation pit during construction in a pit project in Zhengzhou. The result shows that the soil pressure distribution law of the pit in a collapsible loess formation is a complex function of soil parameters, the relative positional relationship between the inner and outer foundation pits, and the time of immersion. The model test shows that the width and depth of reinforced soil have the most significant influence. The reinforcement measures proposed in this paper can effectively control the displacement of each measuring point during the foundation pit excavation, which can provide a reference for similar projects.

Steel slag powder (SS), ground granulated blast-furnace slag (GGBS), and flue gas desulfurization gypsum (FDG) are environmentally friendly and cost-effective substitute materials for ordinary Portland cement (OPC). This study investigated the use of industrial solid wastes, including SS, GGBS, and FDG, as auxiliary materials in OPC to stabilize pretreated recycled concrete aggregate (pretreated RCA). The use of pretreated RCA, mixed cementitious materials, and water at the optimum content created a mixture designated recycled cement-stabilized macadam (RCSM). A series of mechanical tests were conducted to clarify the performance of the RCSM, and microscopic tests were performed to elucidate the microcharacteristics of the mixed cementitious materials. With a curing time from 3 days to 28 days, the unconfined compression strength (UCS) of the mixed cementitious materials (A4) composed of SS, GGBS, FDG, and OPC increased by 5.94–10.79% compared with that of the cementitious material of OPC (A0). The UCS of the mixture composed (C4) of SS, GGBS, FDG, OPC, and pretreated RCA was greater than that of the mixture composed (C0) of OPC and RCA from 7 days to 90 days, increasing by 4.26–8.35%. The total drying shrinkage coefficient of C4 was lower than that of C0, whereas the temperature shrinkage coefficient of C4 was higher than that of C0, indicating that the use of A4 can effectively reduce drying shrinkage cracking in C4. The hydration products of A4 primarily consisted of flocculent calcium silicate hydrate (C-S-H) gel, fibrous calcium aluminate hydrate gel, and needle-like ettringite crystals. The interlocked growth of C-S-H gel and ettringite crystals continued and promoted an increase in the UCS of the cementitious system. The test results provide a reference for the application of similar materials.

Flexural strength is an important mechanical property of steel-fibre-reinforced concrete. By designing three-point bending tests of concrete with five specimen heights, three steel fibre types, and two steel fibre mixing methods, the effects of the specimen height, steel fibre mixing method, and steel fibre type on the peak load, effect of size, section characteristics, strain characteristics, and characteristics of the load–displacement curve of concrete specimens were studied. The results show that the peak load of the control group is basically linear with the height of the specimen. After adding three kinds of steel fibres, the peak load of the specimen is greater than that of the control group in the same case. The peak load of the specimen increases by adding three kinds of steel fibres, and the increase is closely related to the height of the specimen. The residual stage of the load–displacement curve of the milling steel fibre and the end hook steel fibre are relatively flat, while the residual stage of the load–displacement curve of the shear steel fibre is relatively large, and the residual load is also greater than the residual load of the shear steel fibre. The specimens in the control group show brittle failure characteristics. As the height of the specimens increases, the failed section of the specimens is smoother. The development of cracks in the steel fibre specimens is more tortuous than that of the control group, showing ductile failure characteristics. Some tensile failure zones are still present where the fibres are densely distributed, and the failure characteristics of the specimens are further explained and proven by the strain characteristics.