1. Introduction
As economic and technological development progress, the propensity for fire-causing factors escalates, thereby increasing the frequency of building fires and complicating fire prevention and control measures. Recent studies [
1,
2,
3,
4,
5,
6,
7,
8] revealed substantial degradation in the bearing properties, stiffness, and strength of joints following fire exposure. Experimental research by Li et al. [
7,
8] demonstrated a marked decrease in the bearing capacity of joints post-fire. After 75 min of fire exposure, joint bearing capacity can decrease by 13.2–15.8%, and after 120 min, it can decrease by 33.1–34.9%.
Despite these effects, most reinforced concrete structures maintain a certain level of bearing capacity and can continue to be used after appropriate reinforcement. Therefore, reinforcing and repairing fire-damaged buildings not only ensures their usability, but also aligns with China’s sustainable development strategy.
Various repair methods are commonly used in construction engineering [
9,
10,
11,
12] to restore damaged structures, enhance seismic resistance, and improve durability. Methods such as enlarging the cross-section, cladding steel, bonding steel, bonding fiber, base-isolation, and anti-buckling bracing are typically used to strengthen reinforced concrete frame structures. Each of these methods possesses unique advantages and disadvantages, and they were applied and studied extensively in architectural practice. The joint area in frame structures plays a critical role in transmitting and distributing internal forces, ensuring the structure’s integrity. It was found that the joint area is the most vulnerable region of a frame structure to damage. Despite numerous studies on the post-fire residual bearing properties and seismic performance of structures such as reinforced concrete beams, columns, and shear walls [
13,
14,
15], and significant progress in strengthening room-temperature reinforced concrete structures [
16,
17,
18,
19,
20], limitations in testing facilities and methods impeded comprehensive performance studies.
Carbon fiber, a novel fiber composite material with over 95% carbon content, exhibits high strength, high elastic modulus, and an impressive strength-to-weight ratio [
21,
22,
23,
24,
25], making it ideal for joint reinforcement. Therefore, it is crucial from a theoretical standpoint to evaluate the post-fire mechanical performance of reinforced concrete beam-column joints and propose appropriate repair and reinforcement solutions.
Based on the mechanics model theory for reinforced concrete frames, this study employs the finite element method to investigate the post-fire seismic properties of carbon fiber-reinforced joints using numerical simulation. The seismic performance of the joints is assessed using various metrics, including hysteretic curves, skeleton curves, bearing capacity, energy dissipation, ductility, and stress distribution. Our goal is to evaluate the seismic resistance of joints under different reinforcement conditions, providing a comprehensive analysis of the results.
3. Simulation of Temperature Field
The fire exposure duration in the experiment was set at 60 min, with fire applied to all four sides of the specimen. The specific temperature rise and fall curve can be seen in
Figure 2.
In accordance with the fundamental principles of heat transfer, this study utilized non-linear finite element analysis to investigate the beam’s transient temperature field [
26,
27,
28] and constructed a three-dimensional model of the reinforced concrete frame joint. It was determined that the reinforcement had negligible effects on the temperature field and, as such, was not considered in this study. The temperature of the concrete where the reinforcement was located was approximated as the reinforcement temperature.
The thermal parameters of the materials, such as the density, heat transfer coefficient, specific heat capacity, and expansion coefficient of the reinforcement and concrete, were calculated using the corresponding specifications and formulas from Eurocode 3 for simulating the temperature field [
29].
Concerning the interaction, the beams and columns were subjected to fire on all sides, with the ISO-834 standard fire curve, proposed by the International Organization for Standardization, used as the temperature amplitude curve [
30,
31]. The initial temperature of the predefined field was 20 °C, the convective heat transfer coefficient was 25 W/(m
2·°C), the integrated radiation coefficient was 0.7, the absolute zero was −273 °C, and the Boltzmann constant was 5.67 × 10
−8 W/(m
2·k
4).
The division of finite elements impacts the convergence and accuracy of the calculation results. Generally, the finer the mesh division, the more accurate the calculation results will be, and the easier it will be to converge. However, this places high demands on equipment performance and significantly increases calculation time. Bearing these factors in mind, this paper adopted a seed density of 50 mm, a DC3D8 element for concrete, and a DC1D2 element for reinforcement, determined through trial calculations.
Figure 3 presents the maximum temperature at different locations of the post-fire (
t = 60 min) joint. The temperature fields of the beam and column sections exhibit symmetry with respect to the longitudinal and transverse axes of the corresponding cross-section. Heat was transferred layer by layer from the outermost layer of the reinforced concrete to the inside of the cross-section, causing the internal temperature rise to lag behind that of the outer interface. The maximum temperature of the beam and column cross-sections post-fire decreased non-linearly from the surface to the interior, as clearly indicated by the data.
4. Joint Reinforcement after Fire
Carbon fiber composites, composed of over 95% carbon content and exhibiting attributes such as low weight, thinness, exceptional physical and mechanical properties, and robust adhesion, were selected as reinforcement materials for joints post-fire. This study utilized a full bonding method on all four sides for joint reinforcement, with the related methods and dimensions depicted in
Figure 4.
Carbon fiber-reinforced polymer (CFRP) is ideally linear elastic with only ultimate tensile strength and no yield strength; fiber fracture is considered when fiber stress exceeds its tensile strength. This study employed CFRP manufactured by Dezhou Junteng Material Technology Co. Ltd., Dezhou, China, with the primary mechanical property indices presented in
Table 3. In finite element calculations, a membrane element is used for CFRP, and a Tie constraint is set for the contact relationship between CFRP and concrete. The bond-slip between them is disregarded, and it is assumed that the bond between the two is strong.
6. Conclusions
This study presented a numerical simulation of the post-fire seismic performance of reinforced concrete frame joints reinforced with CFRP. This simulation incorporated a fire-exposed temperature field model and a structural mechanics calculation model for the post-fire scenario. References [
32,
33,
34] presented experimental research on the seismic performance of structures post-fire when strengthened with CFRP. These studies consistently demonstrated improvements in the bearing capacity and ductility of joints, a more robust hysteresis curve, enhanced deformation capacity, and slower degradation of strength and stiffness after reaching the ultimate load, following reinforcement. These consistent experimental phenomena strongly support the reliability of our simulated results. Based on our findings, we concluded the following:
When the specimen is subjected to fire on all four sides, the temperature field of the beam and column sections displays symmetry with respect to the longitudinal and transverse axes of the corresponding cross-sections. The heat is transferred layer by layer from the outermost reinforced concrete layer inward, resulting in a delayed rise of the internal temperature compared to the outer interface. After exposure to fire, the maximum temperature of the beam and column cross-sections decreases non-linearly from the surface towards the interior.
In the elastic phase post-fire, the bearing capacity of the reinforced joint remains largely comparable to that of the unreinforced joints. However, the ductility, energy dissipation capacity, and ultimate bearing capacity of the reinforced joint demonstrate significant improvements, though this enhancement does not continue with an increasing number of reinforcement layers. The use of two reinforcement layers results in an energy dissipation capacity and ultimate bearing capacity increase of 26.5% and 30.3%, respectively, showcasing an excellent reinforcement effect. These results suggest that the repaired joints effectively regain their original strength and stiffness.
Analysis of the joint’s stress distribution reveals that as the axial compression ratio increases, the high-stress zones at the joint expand. The failure mode also transitions from plastic damage at the joint’s beam end under low axial compression ratios to the column’s crushing failure under high axial compression ratios.
Our findings indicate that the use of CFRP is a viable method for strengthening reinforced concrete frame joints post-fire. This approach enhances the mechanical properties of the structure and aligns with sustainable practices of continuous use and development. Future research could explore the impact of the bonding method and the dimensions of CFRP on the mechanical properties of the structure. This could further promote the application of the CFRP reinforcement method in the restoration of building structures.