Search Results (346)

The recycling of old concrete from the demolition of concrete structures is necessary for the rational use of natural aggregate resources. Recycled concrete aggregates (RCAs) are the highest quality recycled aggregates as they are the closest to natural aggregates. However, the use of RCAs is always associated with greater fluctuations and usually with a deterioration in workability, mechanical properties and long-term properties. The use of RCA in self-compacting concrete (SCC), where the proportion of aggregate is lower than in conventional concrete, is one way of mitigating the effects of RCAs. In this paper, the effects of coarse and fine RCA are investigated, focusing on dimensional changes due to shrinkage and creep. SCC mixes were developed in which the dolomite aggregates were partially or completely replaced by RCAs and additionally mixes in which 50% of the cement was replaced by fly ash. The average shrinkage strain measured after 180 days increased from 0.34 mm/m for a mix with natural aggregates to 1.04 mm/m for a mix made entirely with RCAs, showing an almost proportional increase in strain with RCA content. At the same age, the creep compliance ranged from 0.07 GPa⁻¹ for the mix with natural aggregates to 0.34 GPa⁻¹ for the mix made entirely with RCAs, and is most strongly correlated with hardened concrete density. Full article

With the rapid advancement of urbanization, the reuse of waste concrete has become more and more important. Recycled aggregate inevitably develops microcracks during the crushing process of waste concrete, resulting in undesirable characteristics such as low density and strong water absorption. This study employed an external calcium source combined with wet carbonation to optimize the performance of recycled fine aggregate (RFA). A series of microscopic analytical techniques, including scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), and the Brunauer–Emmett–Teller (BET) method, were used to elucidate the underlying mechanisms. The results indicate that calcium-rich leachate can be obtained by soaking alkali residue in 0.3 mol/L acetic acid at a solid-to-liquid ratio of 1:6. When this leachate was further used to soak the aggregate at a solid-to-liquid ratio of 1:2, followed by carbonation in a carbonation chamber, the carbonation effect reached its optimum. Under these conditions, the saturated water absorption of the recycled fine aggregate decreased to 16%, the carbon sequestration efficiency increased by 66.8%, and pores smaller than 50 nm accounted for 62.9% of the total pore volume. Furthermore, a Bacillus strain capable of producing carbonic anhydrase was introduced to enhance the carbonation reaction. The results demonstrated that when Bacillus was added to acetic acid-modified recycled fine aggregate, the saturated water absorption further decreased to 14.6%, while the carbon sequestration efficiency significantly increased to 109.04%. Additionally, pores smaller than 50 nm constitute 79.2% of the total pore volume. These findings suggest that utilizing calcium-containing industrial waste as a calcium source for recycled fine aggregate, followed by carbonation modification, is highly effective. This approach not only improves the performance of recycled aggregates but also promotes the reutilization of industrial waste, contributing to sustainable construction practices. Full article

Glass and ceramics have a fundamental and crucial role in our lives due to their properties and aesthetic decoration. However, they create serious environmental problems, mainly due to their high occupation of landfills and harmful emissions. Both wastes could be utilized to reduce the natural resources’ adverse environmental effects and exhaustion. With increasing environmental concerns to reduce solid waste as much as possible, the concrete industry has adopted several methods to achieve this goal. Hence, this study examines the performance of self-compacted concrete (SCC) utilizing various percentages of recycled waste materials such as those deposited from glass and ceramic industries. The idea of utilizing recycled waste materials in concrete manufacturing has gained massive attention due to their impressive results in rheological and mechanical states. Recycled glass (RG) and ceramic waste powder (CWP) were utilized to replace fine aggregate and cement, respectively. Five mixes were designed, including the control mix, and the other four mixes had different dosages of RG and CWP as fine aggregate and cement replacement ranging between 5 and 25%. Mixes were tested for both rheological and mechanical properties to evaluate their compliance with SCC requirements as per codes and guidelines. The results revealed that 20% CWP or less as cement replacement and 10% or less of RG as a fine aggregate replacement would provide suitable rheological properties along with mechanical ones. Utilizing recycled glass and ceramic waste powder provides strength similar to the mix designed with natural resources, which helps us keep structures economically and environmentally friendly. Full article

Geopolymer mortar is an eco-friendly type of mortar that is mainly made of fly ash, slag, and sand as common precursors. Recently, the availability of these materials has become limited due to the huge increase in geopolymer constructions. This is aligned with the recent demand for recycling construction and demolition waste (CDW). In this study, brick waste (BW), ceramic tile waste (CTW), roof tile waste (RTW), and glass waste (GW) extracted from CDW were prepared in the following two sizes: one equivalent to the traditional geopolymer mortar binder (fly ash and slag) size and the other one equivalent to the sand size. The prepared CDW was used to partially replace the binder or sand to produce high-strength geopolymer mortar (HSGM). The replacements were carried out at rates of 25% and 50% by volume. The variety of mechanical and durability characteristics were measured, including workability, compressive strength, freezing/thawing resistance, sulfate attack, water sorptivity, and water absorption. Three curing conditions were applied for the proposed HSGM in this study, namely, water, heat followed by water, and heat followed by air. The results showed that the compressive strength of all HSGM mixes containing CDW ranged from 24 to 104 MPa. HSGM mixes cured in heat followed by water showed the highest 28-day compressive strengths of 104 MPa (when using 25% BW binder), 84.5 MPa (when using 25% BW fine aggregate), 91.3 MPa (when using 50% BW fine aggregate), 84 MPa (when using 25% CTW binder), and 94 MPa (when using 25% CTW fine aggregate). The findings demonstrated that using BW provided good resistance to freezing/thawing and sulfate attack. The water absorption of HSGM increased by 57.8% when using 50% CTW fine aggregate and decreased by 26.5% when using 50% GW fine aggregate. The highest water sorptivity of HSGM was recorded when 50% CTW fine aggregate was used. The use of CDW in HSGM helps reduce the depletion of natural resources and minimizes waste accumulation, enhancing environmental sustainability. These benefits make HSGM an eco-friendly alternative that promotes circular economy practices. Full article

In Mexico, approximately 6.5 million tons of plastic waste is generated, of which 38–58% is improperly managed and has the potential to leak into the environment. Furthermore, producing natural aggregates is associated with the unsustainable use of non-renewable resources. In this sense, this work aimed to evaluate the influence that recycled aggregates from plastic waste have on the behavior of concrete. Coarse aggregates of thermoplastic paint (TP) from paving waste were prepared and incorporated into four mixes, with concentrations of 5 to 20%. In addition, three mixes with fine aggregates from PET were evaluated as one reference mix. The studied properties were slump, compressive strength, flexural strength, rebound number, density, absorption, and porosity. The results indicate that both aggregates have significant potential for use in concrete, including structural use, when replacement percentages of around 5% are considered, with property losses not exceeding 8%. Their use is proposed for active mobility infrastructure, with percentages of up to 20% analyzed in this study. Finally, it is necessary to analyze the influence that the incorporation of plastic waste has on mitigating environmental impacts, as well as the durability properties. Full article

Polyethylene terephthalate (PET) with different grain size after grinding (fine and coarse) was recycled and used as aggregate for non-conventional lightweight cement mortars. The physical and mechanical characteristics were compared to conventional sand-based composites. The workability in the fresh state was evaluated. Accordingly, the composites showed decreases in fluidity with increases in PET percentage weight. Higher thermal insulation and lower mechanical strengths were observed with the increase in plastic dosage due to a density decrease and porosity increase in the composites. Finer grain size PET samples were more resistant (~12–24 MPa) than the coarse-grain samples (~3–23 MPa) due to the higher density and specific surface area of the aggregate. Conversely, higher thermal insulation was obtained with coarse PET addition (~0.6–0.2 W/mK vs. ~0.7–0.35 W/mK). A ductile behavior with discrete cracks after failure was observed after plastic addition to the mixture. Low wettability was observed in PET samples which, although more porous than the sand specimens, showed a hydrophobic behavior which contributed to water repellency. The reported physical, mechanical, thermal, wettability and microstructural features suggest the potential of these composites for both inside and outside applications of non-structural objects. Full article

Recycled aggregate concrete (RAC), which is made by replacing all natural coarse and fine aggregates with recycled aggregate, plays a significant role in improving the recycling rate of construction materials, reducing carbon emissions from construction, and alleviating ecological degradation issues. However, due to its low strength and significant shrinkage and deformation problems, RAC has limited application. The effort of fiber type, fiber admixture, and fiber hybridization on autogenous shrinkage were studied to improve the structural safety of building materials and broaden the application of RAC. Test results indicate that the shrinkage of RAC decreases with an increase in fiber admixture, and steel fiber-reinforced RAC is more resistant to shrinkage deformation than polypropylene fiber-reinforced RAC. The shrinkage deformation of the hybrid fiber group is smaller than that of the single fiber group, and the inhibition of shrinkage deformation is most effective when the volume fraction of steel fiber is 0.5% and the polypropylene fiber content is 1.5 kg/m³. At 120 days, the PF15SF05 mixture showed a 65.3% reduction in shrinkage compared with ordinary RAC. By merging the shrinkage deformation characteristics of fiber-reinforced RAC and introducing the fiber influence coefficient, three theoretical calculation models for autogenous shrinkage applicable to single and hybrid fiber-reinforced RAC were established based on the experimental data. Full article

It is difficult to meet environmental requirements via the coarse treatment methods of landfilling and open-air storage of construction waste. At the same time, the consumption of building materials in highway engineering is enormous. Using construction waste as a filling material for proposed roads has become a research hotspot in recent years. This paper starts with basic performance tests of recycled construction waste materials, and then moves on to laboratory experiments conducted to obtain the road performance of the recycled materials, the testing of key indicators of post-construction filling quality of the roadbed, and analyses of the deformation pattern of roadbed filled with construction waste. Additionally, the ABAQUS finite element software was used to establish a numerical model for roadbed deformation and analyze the roadbed deformation under different compaction levels and vehicle load conditions. The experimental results show that the recycled material has a moisture content of 8.5%, water absorption of 11.73%, and an apparent density of 2.61 g/cm³, while the liquid limit of fine aggregates is 20% and the plasticity index is 5.4. Although the physical properties are slightly inferior to natural aggregates, its bearing ratio (25–55%) and low expansion characteristics meet the requirements for high-grade highway roadbed filling materials. The roadbed layer with a loose compaction of 250 mm, after eight passes of rolling, showed a settlement difference of less than 5 mm, with the loose compaction coefficient stabilizing between 1.15 and 1.20. Finite element simulations indicated that the total settlement of the roadbed stabilizes at 20–30 mm, and increasing the compaction level to 96% can reduce the settlement by 2–4%. Vehicle overload causes a positive correlation between the vertical displacement and shear stress in the base layer, suggesting the need to strengthen vehicle load control. The findings provide theoretical and technical support for the large-scale application of recycled construction waste materials in roadbed engineering. Full article

This research aims to identify an eco-friendly and low-mass substitute for fine aggregate (FA) in self-compacting concrete (SCC). The study specifically examines the potential of waste foundry sand (WFS) as an FA replacement. The primary objective is to explore the impact of processed WFS in SCC, addressing both the WFS disposal issues and enhancing the environmental performance of SCC. After collecting the WFS, it was sieved, segregated, washed thoroughly with water, and then oven dried to remove all clay, carbon, and hazardous content. Treated foundry sand (TFS) is utilized as a substitute for FA in SCC. This study examines the effects of TFS on SCC’s strength, flowability, durability, and microstructural characteristics. Various proportions of TFS are investigated, including replacing 0, 10, 20, 30, 40, and 50% of FA by weight with TFS in the concrete mixture. This research demonstrates that TFS can effectively replace FA in improving the flowability and passing ability of SCC. Furthermore, the findings on SCC’s strength and durability after incorporating TFS suggest that using 30–40% TFS is optimal, as it does not negatively impact the structural performance of SCC. Alternatively, the use of TFS in SCC results in a dense microstructure, improved gel formation, and better bonding of the constituents of ingredients used in SCC. Overall, the results of this study reveal that the use of TFS in SCC can help reduce the amount of waste and improve its sustainability. This also shows that the process can reduce the density of the mix. Full article

Natural fiber composites have gained significant attention in recent years due to their environmental benefits and unique mechanical properties. These materials combine natural fibers with polymer matrices to create sustainable alternatives to traditional synthetic composites. In addition to natural fiber reinforcement, the usage of recycled aggregates in concrete has been proposed as a remedy to combat the rapidly increasing amount of construction and demolition waste in recent years. However, the accurate prediction of the structural performance metrics, such as tensile strength, remains a challenge for concrete composites reinforced with natural fibers and containing recycled aggregates. This study aims to develop predictive models of natural-fiber-reinforced recycled aggregate concrete based on experimental results collected from the literature. The models have been trained on a dataset consisting of 482 data points. Each data point consists of the amounts of cement, fine and coarse aggregate, water-to-binder ratio, percentages of recycled coarse aggregate and natural fiber, and the fiber length. The output feature of the dataset is the splitting tensile strength of the concrete. Extreme gradient boosting (XGBoost), light gradient boosting machine (LightGBM) and extra trees regressor models were trained to predict the tensile strength of the specimens. For optimum performance, the hyperparameters of these models were optimized using the blended search strategy (BlendSearch) and cost-related frugal optimization (CFO). The tensile strength could be predicted with a coefficient of determination greater than 0.95 by the XGBoost model. To make the predictive models accessible, an online graphical user interface was also made available on the Streamlit platform. A feature importance analysis was carried out using the Shapley additive explanations (SHAP) approach. Full article

In recent years, there has been a growing interest in developing sustainable concrete alternatives that reduce reliance on natural aggregates and promote waste recycling. One promising approach involves the utilization of electric arc furnace slag (EAFS) as a fine aggregate replacement. This study aims to investigate the impact of EAFS on the mechanical properties, specifically compressive strength and electrical resistivity, as well as the durability of concrete. Given the importance of accurately estimating concrete performance in the durability domain, this study explores the application of gene expression programming (GEP) to predict the electrical resistivity of concrete containing EAFS. To achieve these objectives, a series of concrete mixes were prepared with EAFS replacement levels ranging from 0% to 100% at water-to-cement ratios of 0.3, 0.4, and 0.5. Experimental results indicated a decrease in compressive strength with increasing EAFS content, particularly at higher water-to-cement ratios. Conversely, electrical resistivity decreased significantly with higher EAFS replacement levels. To enhance durability, it is recommended to incorporate a pozzolanic material alongside EAFS. The GEP models developed in this study exhibited excellent performance in predicting the electrical resistivity of concrete containing EAFS. The high correlation coefficients obtained demonstrate the model’s accuracy and reliability. An accurate outcome is achieved by the model configured with 45 chromosomes, a head size of 15, and a multiplicative linking function. Given the strong correlation between electrical resistivity and other durability properties, such as permeability and corrosion resistance, the GEP model can be a valuable tool for optimizing concrete mixtures and predicting long-term performance in sustainable construction applications. Full article

In recent decades, the construction industry in China has experienced significant growth, leading to substantial consumption of non-renewable natural resources and a large amount of construction and demolition waste (CDW). As a result, the effective utilization of CDW has become critically important in China. This study focuses on the processing of CDW to produce recycled fine aggregates (RFAs) and recycled coarse aggregates (RCAs), which were subsequently used to produce recycled aggregate concrete (RAC). A total of 12 RAC composites incorporating RFAs and RCAs were prepared, and their compressive strength was evaluated in detail. The life cycle assessment (LCA) methodology was employed to assess the environmental impact of 1 m³ of RAC within a “cradle-to-gate” system boundary. Furthermore, the life cycle cost analysis (LCCA) method was applied to evaluate the economic benefits of RAC. The contributions of RCAs and RFAs were analyzed in detail. Lastly, multi-criteria decision analysis (MCDA) using the Data Envelopment Analysis (DEA) approach was proposed to comprehensively compare the environmental and economic impacts of RAC and ordinary concrete. The results of the LCA and LCCA indicate that the inclusion of RCAs and RFAs in composite mixtures leads to significant environmental and economic benefits. The MCDA identified the optimized RAC mixture as one containing 70% RCA and 100% RFA, which demonstrated the best performance in terms of mechanical properties, environmental impact, and economic cost. The composite addition of RCA and RFA in RAC production can significantly reduce both environmental impacts and economic costs, thereby enhancing the sustainability of the concrete industry. Full article

Concrete manufacturing and recycling must evolve to meet sustainability and carbon reduction demands. While the focus is often on reusing coarse aggregates, fine fractions are also produced during recycling. This study explores using ground fine fractions (0/2) as a partial cement substitute. The fines were characterized for their mineralogical, chemical, and physical properties, and experiments were conducted on pastes and mortars with 0% to 30% cement substitution, including isothermal calorimetry and strength tests. Two concrete mixes—a reference mix with natural aggregates and CEM I, and a mix with 10% concrete fines replacing CEM I—using recycled sand and coarse aggregates were tested for compressive strength, carbonation, shrinkage, and freeze–thaw resistance. The results indicated that the recycled concrete had a comparable strength to the reference and a slightly reduced durability in freeze–thaw conditions. In terms of shrinkage, recycled concrete with 10% concrete fines had an increased drying shrinkage and a lower autogenous shrinkage due to the water retention capacity of the recycled aggregates. Full article

Recycled aggregate concrete (RAC) is produced using recycled concrete aggregates (RCAs) obtained from crushed old concrete. Although RCAs offer a sustainable alternative to natural aggregates, the poor durability and mechanical performance of RAC limit its widespread application. This study investigated the enhancement of RAC’s durability and performance through the incorporation of carbon nanofibers (CNFs). A novel processing method was developed to prepare high-slump CNF-modified RAC, and its chemistry, pore structure, and microstructure were analyzed using backscattered scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray powder diffraction (XRD), and mercury intrusion porosimetry (MIP). The results showed that CNFs significantly reduced the porosity and permeability, with a decrease in the porosity by 9.0 wt.% and a decrease in the water permeability by 39.3% at an optimal CNF dosage of 0.5% by weight. Furthermore, CNFs promoted the formation of calcium hydroxide and enhanced the densification of the calcium silicate hydrate (C-S-H) matrix, leading to improved resistance against environmental stressors. These findings provide a critical pathway for designing sustainable, high-durability RAC for structural applications. Full article

Through research that combined green environmental protection with the resource usage of solid waste, we explored more possibilities for mortar using recycled fine aggregate (RFA) as a material. In this work, natural fine aggregate (NFA) with different proportions of RFA in mortar was produced, while maintaining the same particle size distribution. Four types of mortar were produced, with replacement ratios of 25%, 50%, 75%, and 100%, as well as a reference mortar type without RFA. A comprehensive evaluation of the mortar with different proportions was conducted, including its workability, density, capillary water absorption, compressive strength, and flexural strength. The results indicated that the compressive strength and flexural strength of mortar containing 50% RFA improve within 14 days. In addition, with increased RFA usage, the mortar’s mechanical properties decreased. The data obtained from this study will help in the application of RFA in green mortar. Full article