Search Results (879)

The use of recycled aggregate concrete (RAC) enables the valorization of construction waste and supports carbon-reduction strategies. However, long-term service-induced deterioration means that recycled aggregates and their interfacial transition zones inevitably contain defects, which severely restrict the safe application of RAC in load-bearing structures. Notably, although RAC reduces embodied carbon by recycling construction waste, steel tube manufacturing introduces an additional carbon footprint; such carbon trade-offs can be well compensated by the improved structural efficiency and extended service life of steel-confined concrete, achieving superior whole-life carbon benefits. In the present study, a steel-tube-confined recycled aggregate concrete (STCRC) composite system is proposed. Through designed external confinement, the stress state of the internal concrete is altered from uniaxial compression to triaxial compression, thereby enhancing its axial load-bearing capacity. Axial compression tests were performed on 36 short column specimens of steel-tube-confined concrete (STCC) composed of C30 aggregate concrete and Q235 steel tubes with three wall thicknesses (4.5 mm, 6 mm, 8 mm). Further parametric finite element analyses with 16 calculation cases were conducted to quantify the effects of higher concrete strength grades (C40 and C50) and of steel tube strength grades. The evolutionary characteristics of the load-displacement response, the axial stress–lateral strain relation, and the lateral strain–longitudinal strain were systematically investigated across various parameters. Test outcomes indicate that steel tube confinement significantly restrains lateral dilation of RAC and enhances its ductility and ultimate bearing capacity, with higher confinement efficiency observed for RAC than for natural aggregate concrete (NAC). Numerical results further identify the differing sensitivities of NAC and RAC to variations in tube wall thickness, steel yield strength, and concrete strength grade. Using combined experimental and numerical datasets, a peak stress modification factor is proposed, and a tailored stress–strain constitutive model for STCRC is developed and validated. The research findings provide theoretical guidance for the design of axially compressed short columns made of prefabricated recycled concrete. Full article

Aligned recycled carbon fiber (rCF)/polyetherimide (PEI) tapes are a promising material for lightweight aerospace thermoplastic composite structures with reduced environmental impact, provided that their directional properties can be translated into robust components. To assess the viability of rCF/PEI for aircraft interior applications, this study tracks the development of the “Eco Bracket” across four key stages: sustainability screening, coupon characterization, microstructural interpretation, and application-level structural assessment. A project-level life-cycle assessment compared an Additive Fusion Technology (AFT) rCF/PEI bracket with cast and milled aluminum routes and yielded a short-term climate impact of 0.678 kg CO₂-eq for rCF/PEI, representing a significant reduction compared to 5.1 kg CO₂-eq for cast aluminum and 20.9 kg CO₂-eq for milled aluminum. Coupon testing characterized the anisotropic mechanical response of the rCF/PEI tape, including the elastic moduli and strengths under axial and transverse tension, in-plane shear, and axial and transverse compression. Scanning electron microscopy of a representative fracture surface showed a rough, fiber-dominated morphology with exposed fiber bundles, pull-out, and local variations in matrix coverage, supporting the interpretation that consolidation and local wet-out quality govern the transfer from coupon capability to component performance. At the application level, the manufactured rCF/PEI bracket achieved a peak load of 3517.6 N, corresponding to 93% of the strength of an industry-standard polyetherketoneketone (PEKK) reference bracket, and failed in the fastener-hole region predicted as critical by the finite element model. The combined results indicate that aligned rCF/PEI tapes can deliver mechanical properties consistent with the design requirements of aircraft interior brackets, while process robustness, fiber-path fidelity, and local consolidation quality remain the key barriers to repeatable component performance. Full article

Large-wave corrugated steel utility tunnels have been widely used in underground engineering in recent years due to their excellent deformation adaptability and short construction period. However, research on the longitudinal joint bearing capacity is still insufficient and lacks an accurate calculation method for design guidance. In this study, axial compression tests were carried out on four groups of three-wave corrugated steel-plate high-strength bolted connection specimens with different plate thicknesses and bolt strength grades. Combined with finite element models extended to staggered seams, the effects of plate thickness and waveform on failure modes, bearing capacity, and force transmission characteristics were analysed. The test results indicated that the ductility of shear failure connections was obviously lower than that of bearing failure connections, and the brittleness of the shear failure mode became more pronounced as the difference between bolt shear strength and steel plate bearing strength increased. Numerical analysis revealed that although the bearing capacity of the seam improved with increasing waveform, the bearing capacity per unit width decreased. The ultimate displacement was related to the difference in strengths of the bolt and the corrugated steel plate, and the ultimate displacement in shear failure was much smaller than that in seams of bearing failure. The steel plate yielded progressively from both sides inwards along the corrugation direction. The bolt shear force distribution perpendicular to the loading direction exhibited considerable nonuniformity, with bolts at the staggered positions of the longitudinal seams being the first to fail. Based on comparisons of Chinese and American codes, and incorporating waveform and nonuniform shear effects, this study proposed bearing capacity equations for such connections. These equations accurately predict failure modes, with calculated results at 75–85% of theoretical values—a significant accuracy improvement over existing codes. Full article

The structural performance of reinforced concrete (RC) columns can be substantially improved by using advanced confinement systems and sustainable cementitious materials. This study presents an integrated experimental, numerical, and analytical investigation of RC columns strengthened with self-compacting geopolymer concrete (SCGC) and carbon fiber-reinforced polymer (CFRP) under axial compression. Twelve column specimens were tested across four groups: conventional RC columns, unconfined SCGC columns, SCGC-jacketed columns (CONF. SCGC), and CFRP-wrapped columns (CONF. FRP), in three cross-sectional geometries: square (177 × 177 mm), rectangular (265 × 177 mm), and circular (Ø200 mm). Experimental results demonstrated that replacing conventional concrete with SCGC improved deformation capacity, increasing the ultimate axial displacement from 2.07 mm in the reference square RC column (RC C1) to 3.21 mm in the corresponding square SCGC specimen (SCGC C1). On a normalized stress basis, the unconfined SCGC specimens achieved ultimate axial stress values of 56–64 MPa compared to 41–49 MPa for the RC reference columns of the same geometry, representing material-level strength gains of 1.30–1.49×. The application of external confinement further enhanced column behaviour. The CFRP-wrapped specimens achieved the highest material-level strength efficiency, with normalized ultimate axial stress values of 98–108 MPa for the square and rectangular geometries, representing gains of 2.01–2.41× over the corresponding unconfined RC columns of identical cross-section. The SCGC-jacketed specimens achieved the highest absolute load capacities, with CONF. SCGC C2 reaching 8270 kN and a maximum stiffness of 5531 kN/mm and energy absorption of 19,740 kN·mm. However, on a normalized stress basis, the SCGC-jacketed square and rectangular specimens achieved 45–47 MPa, comparable to the RC reference columns, confirming that their absolute load gains are primarily attributable to section enlargement rather than intrinsic material strength enhancement. The circular SCGC-jacketed specimen achieved a normalized stress of 43 MPa, consistent with the same trend. A three-dimensional nonlinear finite element model developed in ABAQUS using the Concrete Damaged Plasticity model and cohesive zone interactions showed close agreement with experimental results, with mean prediction ratios of 1.03 for ultimate load and 0.96 for displacement. An analytical model provided conservative estimates of axial capacity. The findings demonstrate that CFRP wrapping offers superior material-level confinement efficiency, while SCGC jacketing provides the highest absolute load capacity through combined section enlargement and passive confinement, representing a potentially more environmentally friendly strengthening strategy for existing RC columns. Full article

Concrete-filled steel tube (CFT) columns have been widely used in high-rise buildings and bridge structures due to their excellent composite performance. In practical applications, CFT columns inevitably experience eccentric loading due to structural imperfections, asymmetric load distributions, and seismic actions, particularly in corner columns of seismic-resistant structures where biaxial eccentric compression may occur. However, despite extensive studies on the local buckling behavior of CFT columns under axial compression and uniaxial eccentric compression, the behavior under biaxial eccentric compression remains insufficiently understood. In this paper, a theoretical study on the local buckling behavior of rectangular CFT columns subjected to biaxial eccentric compression is presented. Based on classical elastic stability theory and the energy variation method, an analytical model is developed by assuming that both the loaded and unloaded edges of the steel tube are elastically restrained against rotation and selecting an appropriate deflection function satisfying the boundary conditions and compatibility requirements. The relationship of local buckling strength of rectangular CFT columns subjected to biaxial eccentric compression and width-to-thickness ratios under different stress gradient coefficients is obtained. The results indicate that the local buckling strength σ_cr of steel tubes decreases significantly with the increasing width-to-thickness ratios b/t when the stress gradient coefficient α₀₁ and α₀₂ remain unchanged, and the local buckling strength of the broad face is much lower than that of the narrow face. As the stress gradient coefficient increases, the local buckling strength σ_cr of steel tubes increases. When the stress gradient equals 0, the steel tube is subjected to axial compression and the minimum of the local buckling coefficient can be obtained. When the stress gradient equals 2, the steel tube is subjected to pure bending and the maximum of the local buckling coefficient can be obtained. The proposed model provides a rational prediction of local buckling strength under different biaxial eccentric compression conditions. Finally, recommended width-to-thickness ratio limits for steel tube plates with different steel grades and stress gradient coefficients are proposed, which can provide practical guidance for preventing premature local buckling and improving the material utilization efficiency of rectangular concrete-filled steel tube columns. Full article

This study presents a comprehensive analysis and predictive modeling framework for the axial compressive strength (f_cc) and ultimate axial strain (ε_cu) of concrete columns confined within fiber-reinforced polymer (FRP) systems. Large databases comprising 3312 samples for f_cc and 3319 for ε_cu were compiled from the literature, encompassing a wide range of key variables, including unconfined concrete strength from 7 MPa to 204 MPa and diverse FRP confinement configurations. The datasets were subjected to extensive statistical and multivariate analyses to identify the primary factors influencing axial behavior and guide feature selection for predictive modeling. Three groups of machine learning (ML) algorithms were subsequently considered: (i) artificial neural networks (including multilayer perceptrons with one and two hidden layers), (ii) kernel-based models (Gaussian process regression and support vector regression), and (iii) tree-based ensemble models (gradient boosting machine, eXtreme gradient boosting, and light gradient boosting machine). Hyperparameters were optimized using grid search cross-validation, while feature importance analyses were performed to quantify the contribution of each input variable. Among all ML models, eXtreme gradient boosting demonstrated superior predictive performance, effectively capturing the nonlinear and multivariate interactions governing confinement effectiveness. Comparative analysis with the top performing regression-based formulations further highlighted the accuracy, robustness, and generalization capability of the eXtreme gradient boosting model. The findings provide a data-driven and interpretable framework for the design and prediction of FRP-confined concrete columns. Full article

The weak mechanical behavior of recycled aggregate concrete (RAC) stems from microstructural defects in its composition. This study investigates the reinforcement of RAC through steel fiber incorporation. Using RAC replacement ratios and steel fiber contents as variables, this study examined their effects on compressive strength, splitting tensile strength, elastic modulus, and Poisson’s ratio. The axial compressive stress–strain curves of steel-fiber-reinforced RAC specimens were systematically measured. Scanning electron microscopy was employed to elucidate the modification mechanisms of steel fibers in RAC. The findings indicate that although greater replacement ratios weaken the mechanical performance of steel-fiber-reinforced RAC, an increase in fiber dosage enhances its strength. The most significant enhancement occurs when fiber content increases from 0.5% to 1.0%; at a replacement ratio of 0, the splitting tensile strength achieves the highest improvement of 11.14%. By considering the influencing factors, including the replacement ratio and steel fiber content, mechanical performance indices such as cube compressive strength were determined. Furthermore, the quantitative correlations linking the transformed values of various indices and the governing variables, together with the complete stress–strain curve formulations, were developed. The enhancement in RAC performance can be ascribed to the crack-bridging and crack-resisting effects provided by the embedded steel fibers. This research provides crucial experimental evidence supporting the engineering applications of steel-fiber-reinforced RAC. This study offers essential empirical data that underpin the practical implementation of steel-fiber-enhanced RAC. Full article

Small-scale excavations with depths of approximately 1 to 3 m are widely conducted for purposes such as the repair of underground pipelines. In confined construction spaces, earth-retaining systems consisting of lightweight sheet-piles with struts and walers are frequently used. However, comprehensive investigations of the influence of ground conditions on member forces have not yet been conducted. Furthermore, since these temporary structures are generally not designed with seismic considerations, they may suffer damage during earthquakes depending on the soil conditions. Accordingly, this study conducted a comprehensive parametric numerical investigation to evaluate how differences in soil type, such as sandy and cohesive soils, and loading conditions during excavation and earthquake loading affect the internal forces in the supporting members. Excavation analyses using PLAXIS 3D confirmed that as the soil strength parameters (cohesion and internal friction angle) decreased, the demand on the supporting members increased and larger internal forces developed. Dynamic analyses using LIQCA 3D revealed complex behavior in which (i) earth pressure acting on the wall generated compressive forces in the struts, (ii) lateral deformation of the excavation face reduced axial forces in the struts, and (iii) when the ground liquefied, it exhibited a vibration-isolation effect, and the vibration components generated in the structural members became smaller. Full article

To address the engineering issues of local plastic buckling of the chord wall and low ultimate capacity in unreinforced T-joints with square chord and circular brace (CHS-to-SHS) under brace axial compression, this paper proposes a doubler plate-reinforced CHS-to-SHS T-joint configuration. The mechanical behavior of the-reinforced joints is investigated based on a refined finite element (FE) model calibrated against experimental data. An extensive parametric analysis is conducted using FE method to investigate the key factors influencing the failure mode, ultimate capacity, and reinforcement efficiency of doubler plate-reinforced CHS-to-SHS T-joints, including the non-dimensional geometric parameters of the doubler plate (α₂, β₂, τ₂), and the ratio between the brace diameter and chord width (β). The results indicate that the doubler plate thickness has the most significant effect on capacity enhancement, while the width has a limited influence. The ultimate capacity exhibits an initial increase followed by a plateau with increasing doubler plate length, with the optimal length ratio α₂ equal to 1.2. A decreasing trend in the strength ratio is observed with an increase in β. The highest strength ratio of 3.29 is obtained at β = 0.445, and this value declines markedly to 1.4 when β increases to 0.795. However, β ≤ 0.445 tends to induce brace axial yielding, whereas β ≥ 0.795 is prone to cause chord bending failure—both cases constituting over-strengthening. Based on the systematic parametric results, a capacity calculation formula for doubler plate-reinforced CHS-to-SHS T-joints is established and demonstrated to show good agreement with the FE results, providing theoretical basis and design recommendations for the engineering design of doubler plate-reinforced CHS-to-SHS T-joints. Full article

Buried pipelines with corrosion defects crossing reverse faults exhibit complex seismic responses under the combined effects of ground motion and fault displacement. Traditional finite-element analysis is computationally inefficient to comprehensively address such problems. This paper proposes a backpropagation neural network (BPNN)-based method for seismic response prediction and fragility assessment. A three-dimensional finite-element model is first employed to analyze the effects of corrosion depth-to-thickness ratio, diameter-to-thickness ratio, internal pressure, and burial depth on the axial compressive strain of the pipeline. Consequently, a BPNN model is constructed with these parameters, along with fault displacement, as inputs with the peak compressive strain as the output. The BPNN model demonstrated excellent predictive performance, with a maximum prediction error below 15%. The incremental dynamic analysis (IDA) method is then applied to map strength and damage indices of the pipeline, enabling quantitative evaluation of its failure probability and functional integrity under various conditions. It is found that higher diameter-to-thickness ratio (D/t) corresponds to a higher likelihood of the pipeline reaching adverse performance levels; this is also accompanied by a reduction in functional integrity. Specifically, as D/t increases from 72 to 144, the probability of pipe wall damage and the risk of transmission function loss rise significantly, highlighting the pronounced fragility of thin-walled pipelines subjected to fault movement. Moreover, corrosion defects exacerbate pipeline fragility: a corrosion depth equivalent to 10% of the wall thickness substantially amplifies strain responses, resulting in an approximately 80% probability of moderate damage, while a corrosion depth of 40% elevates the probability of severe damage beyond 60%. Full article

This study investigates the use of silica fume as a potential physical medium for carbon nanotube (CNT) incorporation in cementitious mortars, aiming to enhance mechanical performance and improve cement-use efficiency. Four mixtures were produced with a constant water-to-binder ratio of 0.50: a reference mortar (REF), a mortar incorporating 0.2 wt.% CNTs (REFCNT), a mortar with 10 wt.% cement replacement by silica fume (REFSIL), and a hybrid system containing both CNTs and silica fume (SILCNT). CNTs were introduced using a dry pre-mixing approach with silica fume, avoiding the use of surfactants, chemical functionalization, or ultrasonication. The incorporation of CNTs alone resulted in limited mechanical efficiency, leading to a reduction in flexural tensile strength at 7 days and marginal improvements at 28 days. In contrast, the hybrid SILCNT system exhibited the best overall performance, with increases of 6.2% in flexural tensile strength, 13.7% in axial compressive strength, and 16.0% in prismatic compressive strength at 28 days, indicating improved mechanical efficiency of the composite system. Regarding the environmental indicator (EPI), REFSIL and SILCNT showed a reduced value (0.68 kgCO₂/MPa, respectively) compared to REF and REFCNT (~0.86 kgCO₂/MPa). The results suggest that the combined use of silica fume and CNTs improves the mechanical efficiency of cementitious composites, leading to lower cement-based CO₂ emission indicators. The role of silica fume in potentially facilitating CNT distribution is proposed as a plausible hypothesis based on indirect evidence, including mechanical performance trends and microstructural observations. Full article

This methodological paper describes the development and preliminary validation of an experimental procedure for testing rock specimens under volumetric compression using the MDS-450 and MTS-815 servo-hydraulic systems. The apparatus allows the simultaneous application of axial load, confining pressure, pore pressure, and temperature to simulate thermobaric conditions corresponding to depths of about 10–12 km. In this study, the system was used only under room-temperature conditions in order to verify the loading path and data acquisition scheme. The procedure provides stepwise reproduction of reservoir conditions and is designed to record triaxial compressive strength, deformation moduli, dilatancy, and acoustic and ultrasonic responses. Results from preliminary tests on sedimentary rock specimens at confining pressures of 90 and 120 MPa and room temperature demonstrate typical stages of volumetric compression and confirm the stability of the experimental method. To support the verification of the methodology, additional tests were also performed at a confining pressure of 50 MPa, incorporating the temperature factor and acoustic emission monitoring. Full article

To evaluate the durability of desert sand concrete (DSC) in a semi-buried saline-alkali soil environment, this study examined DSC with a 30% desert sand replacement rate, using ordinary concrete (OC) as a control. A 180-day mixed salt attack test was conducted under continuous axial compressive loads (0, 30% f_c, and 50% f_c) and semi-immersion in a mixed salt solution (5% NaCl + 5% Na₂SO₄). Macroscopic and microscopic tests were conducted to reveal the damage evolution patterns of DSC, and a life prediction model was established using a nonlinear Wiener process. The results indicate that after 180 days of semi-immersion, under identical exposure media and load levels, DSC exhibited overall better durability retention compared with OC. A 30% f_c load helped reduce pore connectivity in the DSC, suppressing the development of harmful and highly harmful pores and delaying performance degradation, whereas a 50% f_c load promoted microcrack propagation and pore connectivity, accelerating degradation. Under mixed salt semi-immersion conditions, the “wick effect” caused distinct regional damage in DSC; compared with the corresponding soaking section, the adsorption section showed 3.84–5.17% lower K_n values for compressive strength and a 25.57–42.52% higher proportion of harmful and highly harmful pores. The developed nonlinear Wiener model can reasonably characterize the relative degradation process of DSC under different exposure conditions, and the predicted trends are in good agreement with the results of macro- and micro-scale analyses. Full article

Precast concrete (PC) structures are increasingly adopted in building construction for their sustainable construction advantages. However, theoretical models for seismic design of precast concrete walls with concrete-filled steel tubular (CFST) elements remain limited. The lack of such models hinders the performance-based seismic design and resilience assessment of these hybrid structures. This study investigates the seismic mechanism and develops a restoring force model for precast concrete superposed shear walls with CFST end columns (PCSSWEC). A refined three-dimensional finite element model was established using ABAQUS and validated against quasi-static cyclic test results of three full-scale specimens. The four-stage loading mechanism—elastic, wall cracking, elastoplastic yielding, and ultimate failure—was revealed, with the precast–postcast concrete interface identified as the primary weak link governing post-peak strength degradation. Comprehensive parametric studies examined the influence of shear span ratio (λ = 0.75–3.25), axial compression ratio (n_a = 0.1–0.6), steel tube width-to-thickness ratio (B/t = 20–80), and concrete strength (C30–C60) on seismic performance. Results indicate that intermediate walls (λ = 1.75–2.25) exhibit optimal ductility, and a steel tube with B/t = 40–60 provides a balanced combination of strength and deformation capacity. A tri-linear backbone curve model with explicit formulae for equivalent stiffness and load capacity was developed, along with modified Clough-based hysteretic rules incorporating stiffness degradation through a common yield-point approach. Validation against experimental and numerical results demonstrates reliable model performance for primary structural parameters: lateral load bearing capacity and ultimate drift ratio are predicted within ±10%, while yield load and ductility predictions show larger scatter due to inherent challenges in cyclic behavior characterization. The proposed restoring force model provides a practical tool for performance-based seismic design and resilience assessment of precast concrete buildings. Full article

To overcome the inherent drawbacks of cast-in-place reinforced concrete bracing—such as long construction periods and difficult demolition—as well as the relatively high construction cost of steel structure bracing, while fully incorporating the respective technical advantages of these two traditional support systems, this paper proposes a novel hybrid prefabricated reinforced concrete (RC)–steel structure foundation pit bracing system. In order to investigate the overall bearing capacity variation in the standard components of this structure under complex external forces in foundation pits, a numerical model was established using the finite element software ABAQUS. The study examines the trend of the axial compressive bearing capacity of a single standard beam segment as the steel thickness of its external stiffening sleeve varies, as well as the effects of eccentric loading, oblique loading, and the presence or absence of auxiliary supports on the structural bearing capacity of multi-segment beam assemblies. The numerical analysis results show that the bearing capacity of a single beam segment exhibits a strong correlation with the variation in sleeve thickness, and a fitting curve of compressive strength as a function of thickness was derived. For the multi-segment assembly, an increase of 1 mm in the load eccentricity in the Y and Z directions reduces the ultimate peak load by approximately 20.95 kN and 23.94 kN, respectively; in the XY and XZ planes, an increase of 1° in the eccentric angle of the oblique load reduces the peak ultimate bearing capacity by about 6.02 kN and 9.67 kN, respectively. Auxiliary supports have a relatively minor influence on the structural bearing capacity. This research thoroughly explores the bearing capacity of the prefabricated steel–concrete composite and steel structure foundation pit bracing under complex working loads, providing strong support for engineering design and demonstrating broad application prospects. Full article