Search Results (170)

The use of short cylinders of recycled aggregate concrete (RAC) reinforced with basalt fiber-reinforced polymer (BFRP) circumferential strips and polyvinyl chloride (PVC) tubes has been proven effective in previous studies. However, BFRP circumferential strips are cumbersome to install and do not ensure the integrity of the BFRP strips. Therefore, this study investigates axial compression experiments on RAC short cylinders reinforced with BFRP spiral strips and PVC tubes. A combination of experimental studies, finite element simulations, and theoretical analyses revealed that the winding angle and spacing of BFRP strips significantly affect the load-bearing capacity and ductility of the restrained specimens. Additionally, an improved strength model was developed based on an existing model. When evaluated using both computational and experimental results, the equations generated in this study showed an average error of less than 10%. The findings indicate that the composite structure provides effective reinforcement and offers valuable reference information for practical applications. Full article

To elucidate the effects of iron ore tailings (IOTs) and basalt fiber (BF) on the durability of recycled aggregate concrete (RAC) with different recycled aggregate replacement rates, this study used IOTs to replace natural sand at mass replacement rates of 0%, 20%, 40%, 60%, 80%, and 100% and incorporated BF at volume fractions of 0%, 0.1%, 0.2%, and 0.3%. Carbonation and freeze–thaw cycle tests were conducted on C30 grade RAC. The carbonation depth and compressive strength of RAC at different carbonation ages and the mass loss rate, relative dynamic elastic modulus, and changes in compressive strength of RAC under different freeze–thaw cycle times were determined. Scanning electron microscopy (SEM) was utilized to meticulously observe the micro-morphological alterations of BF-IOT-RAC before and after carbonation. We then investigated the mechanisms by which BF and IOTs enhance the carbonation resistance of RAC. Utilizing the experimental data, we fitted relevant models to establish both a carbonation depth prediction model and a freeze–thaw damage prediction model specific to BF-IOT-RAC. Furthermore, we projected the service life of BF-IOT-RAC under conditions typical of northwest China. The results showed that as the dosages of the two materials increased, the carbonation resistance and frost resistance of RAC initially improved and then declined. Specifically, the optimal volume content of BF was ascertained to be 0.1%, while the optimal replacement rate of IOTs was determined to be 40%. Compared to using BF or IOTs individually, the composite incorporation of both materials significantly improves the durability of RAC while simultaneously enhancing the reuse of construction waste and mining solid waste, thereby contributing to environmental sustainability. Full article

This paper reviews the applications and performance advantages of ultra-high-performance concrete (UHPC), engineered cementitious composite (ECC), and recycled aggregate concrete (RAC) in composite flexural members. UHPC is characterized by its ultra-high strength, high toughness, excellent durability, and microcrack self-healing capability, albeit with high costs and complex production processes. ECC demonstrates superior tensile, flexural, and compressive strength and durability, yet it exhibits a lower elastic modulus and greater drying shrinkage strain. RAC, as an eco-friendly concrete, offers cost-effectiveness and environmental benefits, although it poses certain performance challenges. The focus of this review is on how to enhance the load-bearing capacity of composite beams or slabs by modifying the interface roughness, adjusting the thickness of the ECC or UHPC layer, and altering the cross-sectional form. The integration of diverse concrete materials improves the performance of beam and slab elements while managing costs. For instance, increasing the thickness of the UHPC or ECC layer typically enhances the load-bearing capacity of composite beams or plates by approximately 10% to 40%. Increasing the roughness of the interface can significantly improve the interfacial bond strength and further augment the ultimate load-bearing capacity of composite components. Moreover, the optimized design of material mix proportions and cross-sectional shapes can also contribute to enhancing the load-bearing capacity, crack resistance, and ductility of composite components. Nevertheless, challenges persist in engineering applications, such as the scarcity of long-term monitoring data on durability, fatigue performance, and creep effects. Additionally, existing design codes inadequately address the nonlinear behavior of multi-material composite structures, necessitating further refinement of design theories. Full article

Facing sand and gravel shortages, construction waste accumulation, and the “double carbon” goals, improving the performance of recycled aggregate concrete (RAC) and utilizing mineral waste slag are key to the development of green, low-carbon building materials. To enhance the mechanical performance of RAC and facilitate the sustainable utilization of mineral waste, this study innovatively incorporated copper slag (CS), ground granulated blast furnace slag (GGBS), and basalt fiber (BF) into RAC. The modified RAC’s compressive, split tensile, and flexural strengths were systematically investigated. Experimental results indicated that incorporating appropriate amounts of CS or GGBS as single admixtures could effectively enhance the mechanical properties of RAC, with 20% (w) GGBS showing the most pronounced improvement. Compared with RAC, its 28 d compressive strength, split tensile strength and flexural strength were improved by 21.3%, 9.7% and 8.1%, respectively. As opposed to single admixture, 10% CS + 10% GGBS admixture can further improve the mechanical properties of recycled concrete. Compared with RAC, its 28 d compressive strength, split tensile strength, and flexural strength were improved by 25.6%, 29.7%, and 16.6%. The study also showed that 0.2% BF admixed on top of 10% CS + 10% GGBS could still significantly improve the mechanical properties of recycled concrete, and its 28 d compressive strength, split tensile strength, and flexural strength were improved by 31.3%, 35.9%, and 31.2%, compared with RAC, respectively. By XRF, SEM, and EDS techniques, the underlying mechanisms governing the mechanical behavior of RAC were elucidated from the microscale perspective of basalt fiber and industrial waste residues. These findings provide a solid theoretical foundation and a viable technical pathway for the widespread application of recycled aggregate concrete in civil engineering projects. Full article

Recycled aggregate concrete (RAC) containing recycled rubber gains increasing attention for reinforced concrete structures, owing to its benefits in resource-saving and environmental protection. Bonding between rebars and concrete is critical to ensure the composite action in reinforced concrete members. Nevertheless, previous studies on such concrete mainly focused on material aspects. Bonding behavior for rubber RAC is not clear and needs further research. This study aims to clarify the effects of recycled aggregate and rubber on the monotonic and fatigue bond behavior of deformed steel rebar in concrete and to propose predictive models. Pullout tests under monotonic, fatigue, and post-fatigue monotonic loadings are conducted on a total of 21 monotonic and 30 fatigue specimens, including normal concrete, RAC, and rubber RAC. Four types of failure modes are identified for the tested specimens. Effects of the replacement rate of recycled aggregate, rubber, and load level on the fatigue behavior, such as fatigue life, slip-loading cycle curves, slip development, and residual bond strength, are investigated. With the addition of recycled aggregate and rubber, the monotonic bond strength is increased by 60%. Based on the experimental results, theoretical formulas are proposed to predict the monotonic bond strength, fatigue life, and the slip under fatigue loading. The predictive models are verified by the experimental results, for example, the average and COV of the predicted-to-experimental bond strength ratio are 1.0 and 0.11, which proves the reasonability of the proposed models. Full article

The use of recycled aggregate concrete (RAC) in construction mitigates environmental pollution by repurposing demolition waste, but its lower compressive strength compared to natural aggregate concrete (NAC) limits broader application. Although carbon fiber reinforced polymer (CFRP) composites and polyvinyl chloride (PVC) tubes have individually been shown to improve concrete strength and ductility, existing studies focus on fully wrapped CFRP jackets on NAC columns and do not systematically explore CFRP–PVC hybrid confinement using strips on RAC. To address this research gap, this study investigates the axial compressive behavior of CFRP–PVC–RAC columns by varying CFRP strip width (from 25 to 75 mm), strip spacing (from 31 to 77.5 mm), and the number of CFRP layers (one to nine) over a central PVC tube. Axial compression tests reveal that specimens with a central CFRP strip width equal to or greater than 75 mm achieve peak loads up to 1331 kN and that, after rupture of the central strip, the remaining strips continue to carry load, producing a more gradual stress–strain decline and enhanced ductility compared to fully wrapped controls (peak load 1219 kN). These results show that CFRP–PVC composites enhance the axial compressive strength and ductility of RAC columns. The confinement mechanism increases the ultimate axial strain and redistributes transverse stresses, delaying brittle failure and improving deformation capacity. When two or more CFRP layers are applied, strip width and spacing affect axial stress by no more than three percent. Increasing layers from one to four raises axial strength by approximately 23 percent, whereas adding layers beyond four yields diminishing returns, with less than a six percent increase. Finally, a multilayer lateral confined pressure formula is derived and validated against thirty-two specimens, exhibiting errors no greater than three percent and accurately predicting effective confinement. These findings offer practical guidance for optimizing strip dimensions and layering in CFRP–PVC reinforcement of RAC columns, achieving material savings without compromising performance. Full article

Pores of different sizes and quantities are formed during the molding process of recycled aggregate concrete (RAC). However, few studies have examined the individual and combined effects of porosity and mesoscale pore size (pore size) on the axial compressive mechanical properties of RAC. In this study, the influence of porosity and pore size on the axial compressive mechanical behavior of RAC was examined by incorporating expanded polystyrene (EPS) particles to create prefabrication of pores. Additionally, crack development influenced by pores was analyzed using high-energy X-ray computed tomography (CT). Gray correlation analysis was employed to quantify the influence of pore size and porosity on compressive mechanical parameters. Furthermore, the combined effects of pore characteristics were assessed by introducing damage variables. It was shown that the compressive strength, strength reduction, elastic modulus, and modulus reduction exhibited linear correlations with porosity and exponential correlations with pore size. Cracks within the specimen predominantly propagate through the pores or along their edges. The influence of porosity on both strength and elastic modulus is more substantial than that of pore size. Moreover, the deterioration in mechanical properties is more pronounced when small pore size is coupled with high porosity, compared to the combination of large pore size and low porosity. Full article

This study focuses on improving the recycled coarse aggregate (RCA) replacement ratio in recycled aggregate concrete products. First, the mix design and compressive performance of recycled aggregate concrete (RAC, RCA replacement percentages of 20%, 35%, and 50%) were evaluated using the monofactor analysis method and response surface methodology under three different conditions: single addition of nano-calcium carbonate (NC, dosages of 0.1%, 0.2%, and 0.3%), single addition of basalt fibre (BF, volume content of 0.1%, 0.2%, and 0.3%), and combined addition of both. The results show that the compressive strength of RAC at 7 and 28 days rises as the BF or NC content increases and then falls as the NC content increases. According to the sensitivity analysis, RAC’s compressive strength is significantly impacted by the replacement ratio of RCA, with NC having a more considerable effect on RAC’s 7-day compressive strength than BF, while BF affects the 28-day compressive strength more than NC does. Based on the desirability function, the ideal BF and NC content in RAC was optimised and confirmed by the compressive strength test. It demonstrates that the best compressive performance is achieved by RAC with 1% NC and 0.3% BF. Finally, concrete pavement brick models were created using the ideal mix proportion provided by the compressive strength test. The model compression test results show that RAC pavement bricks (RCA replacement ratio of 60%) with 1% NC and 0.3% BF had a 28d compressive strength of 5.7% and 15.8% higher than NAC and RAC pavement bricks, respectively. Full article

Recycling construction and demolition (C&D) waste into recycled aggregate (RA) and recycled aggregate concrete (RAC) is conducive to natural resource conservation and industry decarbonization, which have been attracting much attention from the community. This paper aims to present a synthesis of recent scientific insights on RA and RAC by conducting a systematic review of the latest advances in their properties, test techniques, modeling, modification and improvement, as well as applications. Over 100 papers published in the past three years were examined, extracting enlightening information and recommendations for engineering. The review shows that consistent conclusions have been drawn about the physical properties in that RA can reduce the workability and the setting time of fresh RAC and increase the porosity of hardened RAC. Its impact on drying and autogenous shrinkage is governed by its size and the strength of the parent concrete. RA generally acts negatively on the durability and mechanical properties of concrete, but such effects remain controversial as many opposite observations have been reported. Apart from the commonly used multiscale test techniques, real-time monitoring also plays an important role in the investigation of deformation and fracture processes. Analytical models for RAC were usually modified from the existing models for NAC or established through regression analysis, while for numerical models, the distribution of attached mortar should be considered to improve their accuracy. Machine learning models are effective in predicting RAC properties. Modification of RA can be implemented by either removing or strengthening the attached mortar, while the modification of RAC is mainly achieved by improving its microstructure. Current exploration of RAC applications mainly focuses on the optimization of concrete design and mix procedures, structural components, as well as multifunctional construction materials, revealing the room for its further exploitation in the industry. Full article

In this study, the mechanical properties and failure characteristics of concrete with 10–20 mm recycled coarse aggregate at 0%, 25%, 50%, 75%, and 100% substitution rates were studied. In addition, the influence of coal gangue powder (CGP) on the mechanical properties of concrete was studied under the dosages of 5%, 10%, 15%, and 20%. The research results show that the peak strength of recycled concrete decreases with the increase in the replacement rate of 10–20 mm recycled coarse aggregate. When the replacement rate is 25%, the decrease in strength is the smallest. When the content of CGP increases from 0 to 20%, the peak strength of recycled concrete increases first and then decreases. When the content of CGP is 15%, the peak strength reaches the maximum value. The peak strength increases slightly. The density of pores and cracks in recycled concrete increases with the increase of 10–20 mm recycled coarse aggregate replacement rate. When the substitution rate exceeds 25%, the proportion of cracks increases by nearly 1.7 times. After adding CGP to recycled concrete, the pore density and crack ratio inside a specimen are significantly reduced. When the CGP content exceeds 15%, the crack ratio tends to be stable. When the CGP content is 15%, the crack ratio is 0.519%, which is 23.5% lower than that of the RAC-25 specimen. When the content exceeds 15%, the crack ratio tends to be stable. Full article

Recycled aggregate concrete (RAC) holds significant promise for reducing the environmental impact of the construction industry. However, the poor mechanical properties of RAC compared to conventional concrete are mainly due to the porous and soft nature of recycled aggregates. While fiber reinforcement has been proposed as a promising method to address this issue, existing studies primarily focus on steel and polypropylene fibers, with limited systematic comparison of alternative fiber types and dosages. In particular, the mechanical enhancement mechanisms of basalt and glass fibers in RAC remain underexplored, and there is a lack of predictive models for strength behavior. This study evaluates the effects of basalt and glass fibers on RAC through uniaxial compression, splitting tensile, and three-point bending tests. Nine mixtures with varying fiber types and volume fractions (1.0–2.5%) were tested, and results were compared to plain RAC. Key properties such as strength, energy absorption, toughness, and flexibility were analyzed using load–displacement curves and advanced toughness indices. Both fiber types improved tensile and flexural properties, with glass fibers showing superior performance, particularly at 1.5% content, where the splitting tensile strength increased by up to 40% and the flexural strength improved by 42.19%. Basalt fibers dispersed more uniformly but were less effective in enhancing toughness and crack resistance. Excessive fiber content reduced matrix homogeneity and mechanical performance. Optimal fiber dosages were identified as 1–1.5% for glass fibers and 1–2% for basalt fibers, depending on the targeted property. Predictive formulas for the flexural strength of fiber-reinforced RAC are also proposed, offering guidance for the design of structural RAC elements. Full article

Recycled aggregate concrete (RAC) has significant potential for sustainable construction; however, concerns regarding its mechanical performance and environmental impact persist. This study evaluates 26 RAC mixtures with varying cement content, water–cement ratios, and recycled aggregate replacement levels (30%, 50%, and 100%) using two distinct recycling processes. The results confirm that while RAC exhibits a decline in mechanical properties compared to natural aggregate concrete (NAC), lower-strength concrete classes maintain acceptable performance. The Life Cycle Assessment (LCA) indicates that fully replacing the natural aggregate with a high-quality recycled aggregate reduces environmental impact by nearly 50%, primarily due to lower resource depletion and transportation emissions. The study demonstrates that RAC can be optimized for structural applications, particularly in foundation structures, without compromising functional integrity. Unlike previous studies, this research provides a systematic evaluation of how a two-stage recycling process enhances aggregate quality, leading to improved RAC performance, and introduces practical strategies for optimizing RAC durability and mix design for real-world foundation applications. Future research should explore alternative mix designs and durability improvements to enhance RAC’s viability for broader construction applications. Full article

To address the growing demand for sustainable construction and efficient recycling of waste concrete resources, this study investigates the interfacial performance and mechanical property prediction of recycled aggregate concrete (RAC) under varying recycled aggregate (RA) replacement ratios (r = 0%, 30%, 60%, 100%). A comprehensive experimental program was implemented, including uniaxial compression tests and microscopic characterization using scanning electron microscopy (SEM), to evaluate the macro- and microscale damage evolution and interfacial transition zone (ITZ) properties of RAC. Based on Weibull’s statistical strength theory, a constitutive model for RAC under compression was developed, and a two-dimensional random aggregate model was implemented in Abaqus to simulate the damage initiation and propagation processes at different replacement ratios. The results demonstrate that the compressive strength of RAC decreases as the RA replacement ratio increases, while the optimal interfacial and mechanical performance is achieved at a 30% replacement ratio. The study reveals that failure in RAC initiates at the ITZ between the recycled aggregates and cement matrix, subsequently propagating to complete structural failure. The proposed constitutive model accurately predicts the stress–strain behavior of RAC across different replacement ratios, showing excellent agreement with experimental data. These findings provide valuable insights into the interfacial performance and failure mechanisms of RAC, offering a theoretical foundation for optimizing the design and application of recycled aggregate concrete in sustainable engineering projects. Full article

This study investigates the effects of incorporating recycled coarse aggregates (RCAs) at different substitution rates on the mechanical and fracture properties of recycled aggregate concrete (RAC). Four concrete mixtures were prepared: a reference mixture using natural coarse aggregates (NCAs) and three RAC mixtures in which 30%, 60%, and 100% of the NCA mass was replaced with RCAs. The RAC mixtures were compared to natural aggregate concrete (NAC) in terms of compressive strength, splitting tensile strength, modulus of elasticity, hydration rate, water-accessible porosity, pore size distribution, post-peak behavior, and fracture energy. The results show that as the RCA substitution rate increases, the RAC porosity rises and its modulus of elasticity decreases. However, the compressive strength and splitting tensile strength remain comparable to, or even exceed, those of NAC, with optimal performance observed at a 60% substitution rate. This optimal behavior can be attributed to a more favorable pore distribution and an increased hydration rate. For an equivalent strength class, RAC with up to 60% RCA substitution exhibits post-peak behavior and fracture energy comparable to NAC. However, at a 100% substitution rate, the behavior becomes more brittle, and fracture energy decreases by 23.20%. Full article

Reusing construction and demolition waste (CDW) as recycled aggregate reduces environmental impact and enhances resource efficiency. While previous research has mainly focused on the use of recycled aggregates (RAs) in concrete, this study evaluates the use of CDW-derived sand in mortars, paving blocks, and structural concrete. Natural and CDW aggregates were characterized, and samples were prepared with two types of Portland cement, replacing up to 100% of the natural limestone sand. Tests were conducted to assess workability, density, strength, and durability. CDW aggregates, primarily composed of limestone and ceramics, reduced sample density as their content increased. Workability improved in the mortars and concrete with higher CDW contents, peaking at 20% CDW in paving blocks. Although the permeability of concrete increased with CDW content, the developed recycled aggregate concrete (RAC) met structural code requirements for all the exposure classes. Despite the decline in strength with higher CDW content, the paving blocks maintained a relative tensile splitting strength above 80%, and the relative compressive strength of the mortars cured for 28 days exceeded 70%. The RAC compressive strength remained within the required range for reinforced concrete (>25–30 MPa). These results validate the feasibility of using CDW-derived sand in various sustainable construction applications with minimal strength loss. Furthermore, they contribute to the development of standardized guidelines for RAs in non-structural applications, fostering broader industry adoption and environmental benefits. Full article