Materials and Structures最新文献

Foam concrete is characterized by lightweight, self-compacting and high flowability, thereby widely used as a subgrade bed filler. High-speed railway subgrades usually experience inhomogeneous deformation due to the occurrence of freezing-thawing cycles in seasonally frozen soil areas. It is essential to study the deformation behavior of foam concrete under the coupling effect of freezing-thawing cycles and dynamic loading. In this paper, dynamic triaxial tests were performed to study the accumulative deformation of the foam concrete under different numbers of freezing-thawing cycles, freezing temperatures, amplitudes and frequencies of dynamic loading. Based on the scanning electron microscopy (SEM) tests, the characteristics of the pore structure were analyzed quantitatively by introducing the directional distribution frequency and fractal dimension. The research results illustrate that the damage caused by freezing-thawing progress to the pore structure results in more significant deformation of the foam concrete subjected to dynamic loading. There exists an accumulative damage effect induced by the coupling action of long-term dynamic loading and freezing-thawing progress on the microstructure and mechanical properties of foam concrete. The development of the fractal dimension agrees with that of the accumulative strain, indicating a close connection between the microstructure and the dynamic behavior of foam concrete. The findings concluded in this study contribute to a sufficient understanding of the performance of foam concrete used as high-speed railway subgrade fillers subjected to seasonal freezing.

The inherent brittleness of cement hydrates poses a major issue to the mechanical and durable performances of concrete. To conquer this issue, we constructed an organic–inorganic network within the cement matrix by utilizing the synergy of in situ polymerization of monomers and cement hydration, which significantly enhances its flexural strength and toughness while maintaining a comparable compressive strength with ordinary Portland cement (OPC). By tuning the proportions of acrylic acid (AA), methacrylic acid (MAA), and acrylamide (AM), the cement paste experienced an 86% increase in flexural strength with a similar compressive strength to OPC. The in situ formed organic–inorganic (polymer-cement) network provided both flexibility and stiffness, playing a pivotal role in the increased mechanical strength. Cement hydration was retarded with the incorporation of the AMA copolymer, which was supported by the offset of the maximum hydration temperature. In contrast, in situ polymerization of monomers proved more effective than directly adding polymer in improving the fluidity and mechanical strength. We hope this strategy provides a new way to increase the crack resistance of cementitious materials and thereby contributes to their overall durability.

To investigate the impact of relative humidity on the mechanical properties and microscopic pore structure of air lime-stabilized compressed earth, unconfined compression strength (UCS), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) tests were conducted by varying the air lime content and relative humidity (RH) in compressed earth. The results revealed three typical failure modes in unconfined compression strength tests of lime-stabilized compressed earth. Both the unconfined compression strength and characteristic parameters of pore structure in lime-stabilized compressed earth exhibited a trend of initial increase, following by a decrease as the air lime content and relative humidity increased. At the microscopic level, the relative humidity and air lime content interacted with the changes in macro-level unconfined compression strength, and the increase of both could promote the lime hydration reactions, inhibiting the crack development. Considering the influence of relative humidity, mechanical performance, and economic benefit improvement, the recommended air lime content for high humidity and low humidity were 0–28% and 0–26%, respectively, offering valuable insights for the optimization and application of lime-stabilized compressed earth as a modern construction material for structural walls.

Microstructures in asphalt, often resembling bee structures, are pivotal in influencing asphalt performance and, by extension, sustainable fuel production. This study employs deep learning techniques to investigate the impact of different Styrene–Butadiene–Styrene (SBS) modifiers on asphalt microstructures, akin to bee structures. The employed deep learning model was trained on a diverse dataset comprising 200 of images sourced from testing. The dataset was carefully curated to address specific challenges in data labeling precision. This involved individualized labeling sessions and adjustments in the number of targets per image, contributing to enhanced precision and increased dataset size. The research begins with the development of a deep learning model trained on a dataset comprising images featuring bee-like structures within asphalt. The model excels in accurately identifying and segmenting these structures. Subsequently, the deep learning approach is compared with existing methods for bee structure segmentation to establish its precision and superiority. Employing frequency distribution histograms, the distribution patterns of bee structures within various types of SBS-modified asphalt is analyzed, quantitatively assessing the influence of diverse modifier types on these microstructural attributes. The findings in this study underscore the deep learning model's efficacy in recognizing and segmenting bee structures with introduced metrics effectively capturing the distinctive characteristics of various asphalt microstructures. This study paves the way for comprehensive analyses of microstructural metrics, including parameters such as perimeter, area, quantity, and related indicators, thus contributing to the development of fundamental asphalt structural units suitable for processes like molecular simulation and finite element analysis. Moreover, it propels the application of deep learning methodologies in the realm of road materials research, opening new avenues for innovative explorations that can ultimately benefit sustainable fuel production.

This study investigates the behaviour of high-performance alkali-activated slag concrete-filled double steel tubular (HACFDST) columns under axial compression. Utilizing high-performance alkali-activated slag concrete (HPAASC) in concrete-filled double steel tubes (CFDST) results in an innovative composite member combining the benefits of both technological domains. HPAASC, a sustainable substitute for conventional concrete, contributes to improved environmental friendliness, while CFDST enhances structural capabilities. Fourteen HACFDST specimens, comprising six circular and eight square sections, were tested to enhance our understanding of the structural performance of HACFDST members through experimental and numerical methodologies. A numerical model was proposed to predict the behaviour of circular and square HACFDST columns under axial compression. The assessment of experimental and numerical findings demonstrates that the proposed numerical model accurately forecasts the behavior of axially loaded HACFDST columns with circular and square sections. Increasing the L/Do or L/B ratio of the specimen results in decreased axial stiffness but enhances ductility. Additionally, beyond peak strength, square HACFDST columns exhibit a sharper decline in strength compared to their circular counterparts. As the L/Do or L/B ratio of HACFDST specimens increases from 2.88 to 7.95, the compressive strength index decreases from 1.28 to 1.2, emphasizing the need to optimize specimen dimensions to maximize the benefits of steel tube confinement.

To investigate the strain rate effect of basalt fiber (BF)-reinforced ambient-cured lightweight expanded polystyrene (EPS) geopolymer concrete (LEGC), this study conducted uniaxial compression tests on BF-reinforced LEGC with different EPS doping and different strain rates. The test results showed that the final damage mode of the specimen was localized shear damage with increasing strain rate and EPS volume content. The damage pattern of BF-reinforced LEGC under the dynamic strain rate exhibited excellent crack resistance and energy dissipation properties compared to other lightweight concretes. Meanwhile, energy dispersive spectroscopy (EDS) analysis revealed that the Ca/Si ratio was larger in specimens with a higher strain rate and lower EPS volume content. In addition, it also revealed that the modulus of elasticity and compressive strength of the specimens were significantly enhanced with increasing strain rate. The more EPS doped, the more significant the enhancement effect, which showed a significant strain rate sensitivity effect. Subsequently, empirical equations for the variation in the dynamic increase factor (DIF) with strain rate were further developed. Finally, a model of the strength of the effect of the coupling of EPS particles and strain rate on the compressive strength of the specimen was developed based on the theory of fracture mechanics, which considered the effect of the thermal activation mechanism and macroscopic viscous mechanism. The comparison indicated that the strength model was consistent with the variation in the test data, which offered certain theoretical basis for the strain rate effect on BF-reinforced LEGCs.

The expected long-term deformations of concrete structures are calculated using creep models, derived from experiments performed with constant mechanical loads. However, in the majority of real structures, such as bridges, constant creep loads are superimposed with cyclic loads of substantial magnitude. Additionally, such structures are subject to changes in environmental conditions (temperature and humidity). Deformation measurements of existing bridges have shown significant underestimations by established creep models, which might be traced back to the superimposition of cyclic loads and different moisture contents. Therefore, the developments of strains, viscoplastic strains and modulus of elasticity under creep and cyclic loading of a normal strength concrete have been comparatively investigated for two different pore moisture contents (approx. 100 and 75%). The results show that viscous strains due to cyclic loading are significantly higher than those due to creep loading at the mean stress level of cyclic loading. Furthermore, the strains are higher for the higher moisture content. The differences in the development of the modulus of elasticity and viscoplastic strains of both load types give clear indication for load type dependent microstructural deformation mechanisms. The results obtained concerning the influence of the load type and the moisture content need to be considered for the improvement of existing models.

Fine aggregate matrix (FAM), as the matrix phase in asphalt concrete (AC), significantly affects the fatigue behavior of AC. To accurately assess the mechanical properties of FAM, a newly designed experimental strategy for FAM testing was developed, and the viscoelastic continuum damage theory (VECD) theory was applied to analyze FAM’s fatigue cracking characteristics. In this study, a dumbbell-shaped geometry for dynamic shear rheometer testing was designed and verified through the FE-aided method. Subsequently, three AC mixtures’ FAM specimens with this special geometry were fabricated for the frequency sweep and linear amplitude sweep tests. Results showed that the specially designed specimens effectively capture the viscoelastic and fatigue properties of FAM with high replicability. Analyses based on the VECD theory indicated that FAM of porous asphalt (FAM(PA13)), featuring a higher asphalt content, exhibits a significant reduction in pseudo stiffness with increasing damage at the initial stage, but the reduction rate diminishes as damage progresses when compared to the other two FAMs. It was speculated that the higher aggregate content in FAM of dense-graded AC mixture (FAM(AC20) induces stress concentrations in the asphalt mastic near the protrusion areas of aggregates, thereby rendering the sample more susceptible to damage. The proposed methods will be readily extended to characterize other mechanical properties of FAM.

This study analyzed how the self-healing potential of recycled asphalt mixtures with varying levels of RAP (0%, 10%, 30%, and 50%) affects its fatigue resistance. To achieve this objective, the specimens were subjected to controlled tensions corresponding to 25%, 50%, and 75% of the fatigue life determined through Indirect Tensile Fatigue Test (ITFT). Subsequently, self-healing was induced. This process involved heating the confined specimens to 45 °C for 4 h, followed by 24 h of rest at an ambient temperature of 25 °C and then carrying out ITFT tests until rupture. After testing, it was possible to quantify the self-healing index and the percentage of extension on the fatigue life achieved post-self-regeneration. The results indicate that increased RAP content leads to more significant difficulties in mobilizing asphalt binders to seal cracks, resulting in reduced self-healing capacity. However, when self-healing occurs at a lower damage level in RAP-containing mixtures, a substantial increase in fatigue life may occur. As a result, the self-regeneration process plays a subtle yet significant role in extending the lifespan of pavement. This effect is particularly effective in rectifying minor damages caused by traffic-induced stresses.

The performance damage of newly placed concrete caused by vehicle–bridge coupling vibration is an inevitable phenomenon among widening of existing concrete bridge. Calcium sulfoaluminate cement-based engineered cementitious composite (CSA–ECC) was proposed to replace the conventional concrete to address the aforementioned issues. The effects of vehicle–bridge coupled vibration (involve the frequency and the amplitude) on the mechanical properties of CSA–ECC including compressive strength, flexural strength and flexural toughness were investigated. The distribution of air bubbles was analyzed by X-ray micro-computed tomography (X-ray CT) to explore the mechanism of vibration affecting the mechanical properties of CSA–ECC. The results indicate that the volume percentage of coarse air bubbles (>1.0 mm³) decreases from 54.70 to 25.94%, and the volume percentage of micro air bubbles (0–0.2 mm³) increases from 30.89 to 54.19%. As a result, the microstructure of matrix and fiber/matrix interface are densified due to the redistribution of air bubbles caused by the coupling vibration. Therefore, the application of vibration significantly enhances the flexural strength and flexural toughness of CSA–ECC, ascribing to stronger matrix fracture toughness and fiber/matrix interfacial frictional bond. These indicate that the CSA–ECC has a promising application scenario in highway bridge widening projects with exceptional vibration-induced damage resistance ability.