Recent Developments of Advanced Composite Materials for Structural Strength and Resilience Improvements

With the increasing demand for building structures with diversified functions driven by social development, the necessity to cope with more extreme and complex service environments (involving earthquakes, marine corrosion, high temperatures, and droughts) is imposing higher strength and toughness requirements on building structures. Advanced composite materials are characterized by a high strength stiffness, and special functions and represent a key breakthrough for enhancing the performance and resilience of structural systems. With the development of modern automated manufacturing processes, further progress has been made in the mechanics, technology, and analysis of composite materials and structural elements. Composite materials cover a wide array of materials, ranging from the more commonly used forms (such as glass fibers) to more advanced materials (such as carbon fibers and magnetorheological elastomers). New ideas and methods for the application and integration of composite materials have made significant contributions to enhancing the safety, reliability, and durability of structures under multiple scenarios. For instance, magnetorheological fluids, which are used to make magnetorheological dampers, possess the characteristics of adjustability, a fast response speed, small volume, and a light weight. Xu [1] used ultrasonic and mechanical stirring–grafting techniques to coat multiwalled carbon nanotubes (MWNTs) onto carbonyl iron (CI), proposed the single- and double-chain micromechanical model, and compared it with shear yield stress tests to verify the effectiveness and accuracy of this model. In addition, a new piece of equipment that utilizes the synergistic effects of vibration isolation and damping devices, magnetorheological dampers, and viscoelastic dampers was proposed [2,3,4,5,6], adopting a multi-state control concept to regulate the input current of magnetorheological dampers. By comparing the time-domain and frequency-domain responses of damped and undamped structures, it was concluded that these devices can significantly reduce the dynamic response of the platform. However, research on composite materials still faces challenges. On one hand, the preparation techniques and formulation optimization of composite materials have not yet been fully developed. Whether they concern the fiber orientation in fiber-reinforced composites, the deposition sequence in laminated composite materials, or the nanoparticle reinforcement in matrix composite materials, even minor parameter deviations may lead to significant fluctuations in material properties. On the other hand, the integration of composite materials with traditional structures requires practical verification to promote the large-scale application of advanced composite materials in various complex scenarios. These factors have motivated research in the field of advanced composite materials for the purposes of strengthening structures and improving resilience, focusing on areas such as the mechanics of composite materials used to strengthen structures; design methods using composite materials to improve strength and resilience; technologies for the manufacturing of composite materials; experimental and numerical studies on composite materials and devices; experimental and numerical studies on the strengthening of structures using composite materials; and research on and applications of smart composites.

This Special Issue, “Advanced Composite Materials for Structural Strength and Resilience Improvements”, provides an opportunity to share experiences and knowledge regarding the preparation of advanced composite materials and their applications in complex scenarios. It has collected eleven original research papers and one review paper. These papers cover research on the preparation and mechanical properties of composite materials, design methods using composite materials to enhance structural toughness in complex scenarios, and experimental and numerical studies on the application of composite materials to improve structural performance.

Among the studies on the preparation and mechanical properties of composites published in this Special Issue, Li [7] studied the influences of strain amplitude, number of cycles, and heat treatment on the fatigue performance of shape memory alloy (SMA) bars. The experimental and simulation results showed the energy dissipation capacity and small residual strain of the SMA bars under low-cyclic-fatigue loading, implying that SMA bars can be used as self-centering energy-dissipating elements in seismic design.

Xu [8] developed a higher-order fractional derivative model modified with the internal variable theory and temperature–frequency equivalent principle (ITHF) to describe the dynamic behaviors of viscoelastic dampers with varying frequencies and displacement amplitudes. Based on a test on a plate-shear-type viscoelastic damper at room temperature under sinusoidal displacement excitations, the higher-order fractional derivative model and the temperature–frequency equivalent principle were employed to characterize the influence of frequency and temperature, and the internal variable theory considering the evolution of the internal/microscale structure is introduced to capture the displacement affection.

Among the studies on design methods using composite materials to enhance structural toughness in complex scenarios published here, Luo [9] used silicomanganese slag powder to replace grounded blast furnace slag to generate alkali-activated silicomanganese slag concrete (AASSC) with excellent performance. The problem of ferromanganese-silicon slag being difficult to handle and prone to causing environmental pollution can be solved by making use of it in AASSC. The experimental results indicate that the AASSC with a 10% silicomanganese slag substitution and 2% steel fibers retains 80% of its compressive strength, making it suitable for practical needs. The AASSC with a 60% silicomanganese slag substitution also can applied to construction to achieve a massive substitution of GBFS.

Silwal [10] studied the application of unidirectional carbon FRP (CFRP) sheets to steel I-beams. Finite element analysis (FEA) was utilized to investigate the effect of various corrosion levels and the number of circular web openings on the yield and ultimate load capacities of the beams. The simulation results showed that thinning the bottom flange reduced both the yield and ultimate load capacities, with a nearly perfect linear reduction in ultimate load for each 2.5% reduction in thickness. Three configurations that involved applying CFRP sheets to both flanges and the web effectively restored the strength of SBWOs when an adequate number of CFRP layers were used.

Li [11] designed the layout of glass–polypropylene (PP) laminate layers to enhance their stiffness and used them as load-bearing members in civil engineering. A bend model of CSIP thin-wall box-beams under uniform loading and the deflection curve equation under shearing deformation were built. The FE result validated the theoretical method as being applicable for the design of thin-wall box-beams made from composite materials.

Rageh [12] proposed the use of two bonding materials, epoxy (EP) and geopolymer (GPP), with different ratios of short glass fibers (SGFs) to strengthen the bond between glass fiber-reinforced polymer (GFRP) and RC concrete surfaces. Failure load experiments were conducted on seven RC beams. These beams had the same cross-section and length and were bonded with adhesive materials (EP and GPP) mixed with SGF at different proportions. Experiments and simulations demonstrated that the addition of bonding materials plays a significant role in enhancing the failure load-bearing capacity of beams.

Malakopoulos and Salifoglou [13] investigated the performance of mortars composed of Portland limestone cement, calcium carbonate, butyl stearate, and oleic acid, aiming to reduce the carbon footprint and enhance the durability of concrete structures. Measurements of electrical resistivity, chloride migration, porosity, water permeability, drying shrinkage, thermal expansion, and compressive strength were conducted to assess durability. The results indicate that incorporating calcium carbonate, butyl stearate, and oleic acid into mortar mixtures provides enhanced durability.

Among the experimental and numerical studies on the application of composite materials for the purposes of improving structural performance published in this Special Issue, Zhang [14] developed a guided-rail tension device (GR) and a novel guided-rail isolation rubber bearing (GR&RB). The GR is designed to enhance the tensile strength of rubber bearings, while the GR&RB is designed to mitigate the risk of overturning due to tension in rubber isolation bearings. The experimental results suggest that the inclusion of GRs has a limited effect on the horizontal mechanical attributes of rubber isolation bearings but does enhance their tensile strength. The numerical analysis suggests that the utilization of GR&RB can mitigate the tensile stress levels of rubber isolators.

Wu [15] proposed a new type of steel shear-connection horizontal joint to ensure reasonable force transmission and structural integrity. The influence of this new type of joint on mechanical behavior was investigated through simulations, and a bilinear constitutive model based on the M-θ relationship of the new steel shear-connection joint was further advanced and deduced. The results show that the proposed new joint can provide stable shear capacity and superior energy dissipation capacity.

Schmidt [16] researched the response of a novel response-controlled prestressing anchor system used for CFRP NSMR strengthening. An initial assessment of an in situ pre-cast concrete bridge was performed to provide a basis for the most desirable anchor system placement. Five laboratory tests were performed on a novel ductile response-controlled anchor system used for prestressed CFRP NSMR strengthening. These results were used as a basis for further implementation in a pilot project where an in situ cast concrete bridge was strengthened with the developed system.

Li [17] proposed a new nonlinear dynamic model of cylindrical reticulated shells with initial damage to investigate the effect of initial damage accurately. The nonlinear vibration differential equations with damage were built and the nonlinear natural vibration frequency with initial damage was derivatized. After that, a bifurcation problem with initial damage was studied by using the Flouquet Index, and the dynamic stability state at the equilibrium point was analyzed in depth. The theoretical results indicate that as the damage accumulates to 0.618, the nonlinear natural vibration frequency drops to zero, leading to the failure of local stability and even the loss of overall stability in the cylindrical reticulated shell. This study provided a theoretical foundation for the future investigation of overall stability with initial damage.

Huang [18] conducted a review on advanced composite materials. They provided a comprehensive overview of the types and characteristics of advanced composite materials, explored new methods and technologies, and discussed the challenges and opportunities for future work.

Although this Special Issue only includes 12 papers, new advanced composite materials applied in structural strengthening and resilience improvement have been presented as much as possible. Given that this is still a challenging topic, new thoughts, perspectives, ideas, and methods, as well as new advances in materials and devices, are still sought to promote research on advanced composite materials that can strengthen structures and improve resilience.

We are striving to enhance the influence of the journal Buildings and its leading position in the field of advanced composite materials used for structural strengthening and resilience improvement. We are grateful for the contributions of the authors who were vital to making this Special Issue successful.