Properties Of Recycled Aggregate Concrete Research Articles

Effect of nano silica-cement paste impregnation-carbonation reinforcement on the properties of low-quality recycled aggregates

The aim of this study was to investigate the impact of a nano silica (NS) -cement paste impregnation-carbonation strengthening technique on both the properties of low-quality recycled aggregates and the recycled aggregate concrete (RAC) they form. First, the changes in the basic properties of recycled concrete aggregates (RCA) after reinforcement were evaluated, followed by pore structure, XRD, thermo-gravimetric analysis, nanoindentation, and SEM tests to explore the microstructure and the mechanism of its influence. The results showed that the crushing value of the recycled aggregate reinforced with NS-cement paste impregnation-carbonation composite were reduced about 35 %, compared to the unreinforced recycled aggregate. The crushing value and water absorption were reduced about 17 % and 28 %, respectively, compared to cement slurry impregnated-carbonated reinforced recycled aggregates. The addition of NS promoted the hydration of cement in the encapsulated slurry, and some unreacted nanoparticles in turn filled the pores of the hardened cement slurry, reducing the porosity of the recycled aggregate. The performance of the ITZ is improved after the aggregate is reinforced. The 28 d compressive strength of recycled concrete formulated with NS-cement paste impregnation-carbonation-reinforced aggregates was increased about 34 % and 52 % for 50 % and 100 % substitution rates, respectively. In conclusion, this study elucidated the mechanism of NS-cement paste impregnation-carbonation strengthening on the properties of recycled aggregates and provided a theoretical basis for the application of NS-cement paste impregnation-strengthening in construction materials.

Mesoscale modelling of recycled aggregate concrete: A parametric analysis of properties of mesoscopic phases of RAC

This paper analyses the relative effects of ten parameters of recycled aggregate concrete (RAC) mix design on its mechanical properties. Research on the effect of the source concrete and of the quality of the interfacial transition zones (ITZ) on the properties of RAC is scarce and additional knowledge would guide RAC mix design towards efficiency and understanding of the effect of recycled aggregates (RA) on concrete. Therefore, this paper presents the outcomes of parametric numerical analyses on this topic. Within the range of parameters commonly found in conventional concrete mix design, it is found that: (1) the quality of both the old and new mortar substantially influences the mechanical behaviour of RAC. High-quality RAC can be produced with RA sourced from, at least, medium quality concrete (C40); (2) a poor-quality ITZ results in a significant decrease in concrete properties, particularly decreasing the tensile strength up to 58%, since micro-cracks that initiate in weak ITZ can readily develop into mortar; (3) the most relevant factors that determine the strength of RAC are the quality of mesoscopic phases, with the relative effects that range from 8% to 53% for compressive strength and from 22% to 59% for tensile strength; (4) the concrete mix design was less relevant, with a maximum influence of 23%; (5) the previous two observations imply that the most efficient way to improve the properties of RAC is to use RA of better quality; (6) a high proportion of coarse aggregates and a lower replacement ratio of natural aggregate with RA can effectively improve the stiffness of RAC by 16% and 15%. This paper contributes to understanding the influence of mix design and raw materials on the properties of RAC and determines the limiting factors that prevent a concrete mix from reaching its intended performance.

Water permeability property of recycled aggregate concrete

Recycled aggregate concrete (RAC) is widely recognized as a promising approach for recycling construction and demolition waste. However, its practical application remains limited. A contributing factor to this limitation is the incomplete understanding of RAC’s durability characteristics, particularly its permeability, which is closely tied to the transport of harmful ions within the concrete matrix. Despite the critical importance of this issue, research in this area is still relatively sparse. This knowledge gap has motivated the current study, which seeks to thoroughly investigate the water permeability properties of RAC. In this comprehensive study, 84 specimens were fabricated for permeability testing. This study explores the influence of various factors, including the sources of coarse and fine recycled aggregates (RAs), the RA replacement ratios, and the water-to-binder ratio. The results indicate that the inclusion of both coarse and fine RAs reduces the impermeability of concrete. However, enhancing the quality of these RAs—specifically by increasing the compressive strength of the source concrete—can mitigate the reduction in impermeability. A statistical relationship is established between the average and maximum water permeation depths measured during permeability testing, leading to the formulation of a correlation between the permeability coefficient and the impermeability grade of RAC. Furthermore, mercury intrusion porosimetry testing was conducted to quantitatively analyze the pore structure of the mortar in RAC, providing a microscopic perspective that explains the macroscopic permeability behavior observed. Finally, a predictive model for estimating the permeability coefficient of RAC is proposed, demonstrating a high level of accuracy.

Study on bonding properties and constitutive model of steel bar and nano-SiO2 reinforced recycled aggregate concrete

Optimizing design and ensuring the structural safety of recycled aggregate concrete (RAC) structures requires a thorough understanding of the bond-slip mechanism between steel bars and RAC. Previous studies have demonstrated the significant enhancement of the microscopic properties of RAC and its bond with steel bars due to nano-SiO2. However, the structural bonding mechanism between the steel bar and nano-SiO2-enhanced RAC remains unclear. Therefore, this study investigates the influence of nano-SiO2 on the bond performance between RAC and steel bars, elucidating the enhancement mechanism through microscopic techniques. Additionally, various factors affecting the bond-slip performance of RAC specimens are analyzed, leading to establishing a multifactorial coupling formula considering thickness-to-diameter ratio, concrete strength, and relative anchorage length. Furthermore, slip index correction formulas and a constitutive model are developed, considering these factors. Acoustic emission (AE) technology monitors the bonding process, clarifying the evolution of AE digital signals and bond damage history. Employing the finite element method, a numerical model of bond-slip behavior is established, revealing specimens' changing bond strength and failure modes. Experimental results indicate that increasing nano-SiO2 content reduces structural porosity, leading to denser and more homogeneous microstructures, thereby enhancing RAC's mechanical properties and bonding performance. Moreover, AE techniques effectively monitor bond damage at different stages. Finally, a bond-slip constitutive model is proposed for steel bar and nano-SiO2 reinforced RAC, offering insights for numerical calculations and engineering designs.

A review of mixture design and preparation methods for high-performance recycled aggregate concrete

Recycled aggregate concrete (RAC) has been widely investigated for recent decades, during which there are a great deal of findings on the modification of RAC to overcome the drawbacks of RA, i.e. high absorption and low strength. In this paper, four methods for designing and preparing RAC are reviewed, including the particle packing method (PPM), equivalent mortar volume (EMV) method, two-stage mixing approach (TSMA) and distributing-filling coarse aggregate (DFCA) process, and their effects on the properties of RAC is compared to reveal the effective approaches to high-performance preparation of RAC. Among these methods, only TSMA presents a positive effect on the workability of RAC. In terms of the properties of hardened RAC, TSMA improves the compressive strength and penetration resistance by densifying the ITZs around RA, and the developed TSMAsc containing silica fume exhibits better efficiency in improving the properties of RAC. The EMV method and DFCA process are beneficial to the compressive strength, elastic modulus and volumetric stability of RAC, and the effectiveness of the DFCA process is superior to the EMV method, indicating that enhancing the coarse aggregate interlocking effect is an effective approach to obtain high-performance RAC. Furthermore, the combination of TSMA and EMV methods can synthesize the enhancements of ITZs and coarse aggregate interlocking effect, contributing to a further improvement in the properties of hardened RAC. It is conceived that the TSMA and DFCA process can be integrated as a promising combination to realize the high performance of RAC.

Effects of wet carbonation with ethanol solution on the mechanical, durability properties, and microstructure of recycled concrete aggregates and derived block products

The carbonation treatment of recycled concrete aggregates (RCA) can significantly enhance their properties and reusability. To improve carbonation efficiency, a wet cyclic carbonation method using ethanol solution was proposed to treat RCA and a wet carbon curing method was applied to its product, recycled blocks (RB). The results showed that wet carbonation with 30 % ethanol solution for 10 min significantly reduced RCA’s water absorption and crushing index and increased its apparent density. This is attributed to ethanol’s high carbon-solubilizing properties, which allow the RCA to generate more calcium carbonate precipitates to fill the voids and optimize the pore structure. When the concentration of ethanol solution was increased above 50 %, the improvement was inefficient and limited. In addition, the second cycle of wet carbonation can further improve the properties of RCA. Optimally processed RCA was used to make recycled masonry blocks. The 10 min wet carbonation curing proved beneficial to the mechanical properties of RB and also improved its resistance to shrinkage and salt freeze-thaw. This rapid method enhances RCA construction materials' performance and durability, improving industrial use in sustainable construction materials.

Investigating the impact of carbonation curing concentrations on the mechanical properties and microstructure of recycled concrete

This investigation examines the impact of different CO2 curing concentrations on the mechanical and microstructural properties of recycled concrete aggregates, aiming to optimize the carbonation process for enhanced practicality. Utilizing 25 recycled concrete samples, the study varied CO2 concentrations (20 %, 40 %, 50 %, 60 %, and 95 %) and pre-conservation durations (24–120 h), conducting tests for carbon sequestration, compressive strength, and splitting tensile strength to determine the effects on recycled concrete's performance. Key findings reveal a direct correlation between increased CO2 curing concentrations and carbon sequestration levels, with an observed pattern of initial increase followed by a plateau or decrease in sequestration with extended pre-conditioning. Interestingly, while compressive strength generally improved with higher CO2 concentrations, splitting tensile strength showed a decrease, suggesting different impacts of CO2 concentration on various mechanical properties. A notable observation was that at 60 % CO2 concentration, compressive strength was slightly lower (by 2.46 %) compared to the 95 % concentration, yet the splitting tensile strength was 2.84 % higher at 60 %, indicating that a 60 % CO2 curing concentration might offer an optimal balance between economic efficiency and material performance enhancement. This balance suggests significant improvements in resource utilization compared to traditional pure CO2 curing methods. In conclusion, this study provides substantial evidence supporting the optimization of CO2 curing for recycled concrete, advocating for a 60 % concentration as the most effective strategy to enhance both the environmental sustainability and the mechanical performance of recycled aggregates. These insights significantly contribute to advancing low-carbon and sustainable practices within the construction industry.

Evaluating external generalizability of machine learning models for recycled aggregate concrete property prediction

Machine learning (ML) models have gained importance in predicting recycled aggregate concrete (RAC) properties, offering supposed benefits over conventional empirical and statistical techniques. However, whether ML models are externally generalizable for predicting RAC properties remains unanswered. This study addresses this gap by developing a systematic experimental framework for evaluating the external generalizability of ML models for predicting the compressive strength of RAC. Using a literature review, the authors created a primary dataset of 414 data points and sourced a secondary dataset comprising 330 data points from a previous paper. In Phase 1, prominent ML models like Random Forest (RF) and Extreme Gradient Boost (XGB) were tested for high-accuracy prediction on both primary and secondary datasets. A coefficient of determination (R2) as high as 0.76 for the primary dataset and 0.82 for the secondary dataset for testing sets was obtained for XGB. However, when the best-performing models of phase 1 were trained and tested with data sourced from different datasets in varying combinations, the ML model's performance significantly deteriorated (R2 < 0.25), demonstrating that ML models are not externally generalizable. The study's results highlight the trade-off between the ML model's prediction accuracy within the given dataset and its external generalizability. The study reveals that complex ML models, like XGB, may over-fit specific data, reducing their generalizability. These findings call for the intelligent usage of ML tools to identify nuanced hypotheses and promote rigorous science rather than unilaterally working on the accuracy of ML models. The study emphasizes the consideration of ML models' generalizability in the future.

Mechanical properties and repairing mechanism of recycled cement stabilized macadam for road base based on microbial induced carbonate precipitation technology

The utilization of recycled coarse aggregate derived from construction and demolition waste (CDW) in the construction of road cement stabilized macadam base could effectively alleviate the shortage of natural sand and aggregate resources. To explore the repairing effect of microbial induced carbonate precipitation (MICP) on recycled cement stabilized macadam (RCSM), the physical, mechanical, and microscopic properties of recycled concrete aggregate under different precipitation precursor concentrations were researched in this paper. The mechanical properties of RCSM with varying contents of recycled concrete aggregate were analyzed before and after repairing. Based on scanning electron microscopy (SEM) and mercury intrusion porosimeter (MIP) tests, the microstructure and pore structure parameters of recycled concrete aggregate were explored, and the repair mechanism of MICP on RCSM was revealed via the performance analysis of recycled concrete aggregate and RCSM. The results show that MICP could effectively reduce the water absorption of recycled concrete aggregates, while simultaneously enhancing its apparent density and crushing resistance. In a certain range, the higher the concentration of the precipitation precursor, the better the repairing effect on the performances of recycled concrete aggregates. However, while the concentration of urea or calcium source concentration exceeding 1.0 mol/L, the activity of urease would start to be inhibited. The unconfined compressive strength of RCSM after MICP repairing could be increased by 22.7 % with a maximum extent compared with that before repair, while the maximum increase of indirect tension strength and flexural tensile strength was 15 % and 8.2 %, respectively. The biological CaCO3 generated by MICP reaction could fill the micro-cracks and pores of the recycled concrete aggregate efficaciously. Besides, it could also optimize the pore parameters and pore size distribution parameters, and enhance the adhesive strength of interfacial transition zone (ITZ) between cement and recycled concrete aggregates, which could improve the mechanical properties of RCSM.

The influence of blast furnace slag on the mechanical properties and mesoscopic damage mechanisms of recycled aggregate concrete compared to natural aggregate concrete at different ages

To improve the mechanical property defects caused by recycled aggregate (RCA) in recycled aggregate concrete (RAC) as well as the effect of blast furnace slag (BFS) addition on the mechanical properties and microscopic damage mechanisms of natural aggregate concrete (NAC) and RAC at long ages. In this paper, the effects of different curing ages (T = 7d, 14d, 28d, 56d, 90d, and 150d) and blast furnace slag contents on the mechanical properties and microscopic damage mechanisms under uniaxial compression of NAC and RAC were investigated, where the cement substitution rate of BFS was 0 % and 35 % (water-cement ratio of 0.46, water-reducing agent dosage of 0.25 %), respectively. Nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) were used to study the internal pore changes and the generation of hydration products in the specimens, and acoustic emission (AE) showed the microcrack development process. The results showed that the peak stress and modulus of elasticity of the specimens gradually increased with increasing age, the peak strain gradually decreased, and the internal porosity gradually decreased. It is worth noting that the addition of BFS caused the mechanical properties of RAC to decrease at 28 days but enhanced the mechanical properties of RAC at 56–120 days. This was attributed to the fact that the properties of BFS were not yet activated in the early stage, whereas it was activated in the later stage under an alkaline environment and reacted with CH to generate more C–S–H hydration products, which enhanced the interfacial transition zone and reduced the porosity. This highlights the potential of BFS to improve the performance of RCA over long periods of time. In contrast, BFS improved the mechanical properties of NAC throughout the age period and more significantly after 56 days. AE results showed that the peak ring counts lagged behind the peak stresses. Finally, combined with the statistical damage principal model, the fitting was performed by Matlab to analyze the microscopic damage mechanism of the specimen during uniaxial compression. It was found that the changes in the four characteristic parameters (εa, εb, εh and H) followed a certain regularity, reflecting the regular changes in fracture damage and yield damage. This revealed the intrinsic linkage between mesoscopic damage mechanisms and macroscopic mechanical properties. It was demonstrated that it was feasible to speculate on the law of the stress-strain curve at the macroscopic level by using the law of change of the four parameters (εa, εb, εh and H) at the microscopic level.

Optimizing mechanical and durability properties of recycled aggregate concrete using pozzolanic slurries and modified mixing approach

A sustainable and reliable methodology to strengthen the mortar adhered to recycled concrete aggregate (RCA) is crucial for optimizing its utilization in construction. This research proposes a mortar strengthening method by soaking RCA in different dosages (weight percentages) of pozzolanic slurries, i.e., fly ash and silica powder (F-S), fly ash and cement (F-C), cement and silica powder (C-S) for various durations. The influence of each slurry dosage and soaking duration on RCA's physical and mechanical properties and surface morphology were examined. Further, a modified two-stage mixing approach (M-TSMA) was employed to produce recycled aggregate concrete (RAC) incorporating 100% untreated and surface-treated RCA. The impact of M-TSMA and treated RCA on concrete was demonstrated regarding its mechanical and durability properties, such as compressive strength, ultrasonic pulse velocity, rebound hammer, sorptivity, and resistance to chloride ions. Results show that soaking RCA in 30% C-S slurry for 6 h enhances RCA's overall properties by increasing the calcium silicate hydrate (C-S-H) content. Moreover, employing M-TSMA and incorporating treated RCA improves the mechanical and durability properties of RAC by reducing microcracks and densifying the microstructure. Overall, the proposed surface treatment and M-TSMA can potentially enhance RAC performance with complete RCA utilization.

High-quality recycled concrete aggregates developed through an integrated thermomechanical approach

This study demonstrates an integrated thermomechanical treatment process to produce high quality recycled concrete aggregate (RCA) for structural concrete. RCA is mechanically treated after 90 min of heating at 250 °C in this process. The material properties of RCA produced through 15 combinations of charges and drum revolutions are compared using a statistical method. The highest ranking of an RCA identifies it as a high-quality class. Concrete composed of 100% RCA (recycled aggregate concrete, RAC) has 1.7% higher compressive strength than natural aggregate concrete (NAC). RAC has tensile and flexural strengths of 0.95 and 1.03 in fraction of NAC. RAC meets durability criteria such as UPV greater than 4.4 km/s, rapid chloride ion penetration test (RCPT) less than 1000 C, electrical resistivity greater than 208 Ohm-m, and sorptivity closer to NAC. Water absorption and abrasion resistance are similar to or superior to those of NAC. Coherent and uniform surface morphology of RCA enhances the ITZ microstructure in RAC.

Experimental Study of Recycled Aggregate, Fly Ash and Steel Fiber on Properties of Concrete

The use of recycled aggregates in concrete opens a whole new range of possibilities in the reuse of materials in the building industry. The utilization of recycled aggregates is a good solution to the problem of an excess of waste material, provided that the desired final product quality is reached.The use of recycled aggregate in concrete can be useful for environmental protection and economical terms recycled aggregates are the materials for the future.The work focuses on the possibilities of the recycled aggregate concrete as structural material with replacement level of 25%,50% and 75% byweight of concrete. Fly ash is also used as a partial replacement of Portland cement with10, 20 &amp; 30 % by weight of cement in order to improve the properties of RAC. The use of fly ash replacement in Recycled Aggregate concrete (RAC) could improve compressive strength to be higher than that of concrete without fly ash at the same W/B ratio.The fly ash is an organic, non combustible residue of coal after burning in power plant. The steel fiber are available imperforated, corrugated or with wide end for better bending.The volume fraction of the hook end steel fiber used in this study varied from 0.25%,0.5%a and 0.75 %. Key words- Fly Ash, Recycled Aggregate, Recycled Concrete Aggregate, Steel Fiber, Water Cement Ratio.

Evaluating Recycled Concrete Aggregate and Sand for Sustainable Construction Performance and Environmental Benefits

This research investigates the potential of utilizing recycled concrete aggregate (RCA) and recycled sand (RS), derived from crushed concrete cubes, as sustainable alternatives in construction materials. The study comprehensively evaluates the properties of RCA and RS, focusing on workability, impact resistance, abrasion resistance, and compressive strength to determine their viability as substitute construction materials. A notable finding is RS’s enhanced fire and heat resistance when used as a fine aggregate in mortar blends, mixed with cement and Sinicon PP in a 3:1 ratio. The experimental analysis included thorough assessments of uniformity, durability, and curing time, alongside Scanning Electron Microscopy (SEM) for structural examination. Results show that RCA has an Aggregate Impact Value (AIV) of 5.76% and a Los Angeles Abrasion Value (LAA) of 21.78%, demonstrating excellent strength of the recycled aggregates. The mortar mix was also prepared using recycled sand, cement, and Sinicon PP, and its stability was confirmed through soundness tests, which resulted in a 0.53 mm expansion and a satisfactory consistency level of 44%. Ultrasonic pulse velocity (UPV) tests also indicated high-quality concrete formation using RCA and RS. SEM imaging corroborated this by revealing a bond between the cement paste and the aggregates. Incorporating RS and RCA in concrete mixtures impressively yielded a compressive strength of 26.22 N/mm2 in M20-grade concrete. The study concludes that using RCA and RS waste materials in the construction sector underlines that sustainable practices can be integrated without compromising material quality. This approach aligns with sustainable development goals and fosters a more environmentally friendly construction industry.

THE STRUCTURAL BEHAVIOR OF RECYCLED AGGREGATE CONCRETE WALLS UNDER FIRE EXPOSURE: A FINITE ELEMENT ANALYSIS

Climate change is raising the demand for change in the construction industry, particularly in the concrete industry. One of the approaches to increase sustainability in the concrete industry is the use of recycled aggregate concrete (RAC). Many studies examined the type and composition of RAC used in construction applications. However, fewer studies examined the structural engineering applications of RAC. Thus, this study investigates the use of RAC in bearing walls. It presents a comprehensive thermal and structural performance assessment of RAC walls exposed to fire using finite element analysis (FEA). The FE model incorporates experimental material models in the literature that consider the degradation of the mechanical properties of RAC at high temperatures. The investigation encompasses a range of critical response measures, including the out-of-plane response of RAC walls under standard fire exposure and the overall stability of the walls during fire scenarios. The walls' out-of-plane and axial deflection and damage patterns were presented. The upper and lower bounds of performance were presented to examine the effect of material models’ variability on the response. The findings of this study reveal the complex interaction between fire scenarios, material properties, and their effect on the structural behavior of RAC walls. This study contributes to the growing knowledge on using RAC in structural engineering applications and promoting environmentally friendly materials in the construction industry while ensuring structural safety.

THE STRUCTURAL BEHAVIOR OF RECYCLED AGGREGATE CONCRETE WALLS UNDER FIRE EXPOSURE: A FINITE ELEMENT ANALYSIS

Climate change is raising the demand for change in the construction industry, particularly in the concrete industry. One of the approaches to increase sustainability in the concrete industry is the use of recycled aggregate concrete (RAC). Many studies examined the type and composition of RAC used in construction applications. However, fewer studies examined the structural engineering applications of RAC. Thus, this study investigates the use of RAC in bearing walls. It presents a comprehensive thermal and structural performance assessment of RAC walls exposed to fire using finite element analysis (FEA). The FE model incorporates experimental material models in the literature that consider the degradation of the mechanical properties of RAC at high temperatures. The investigation encompasses a range of critical response measures, including the out-of-plane response of RAC walls under standard fire exposure and the overall stability of the walls during fire scenarios. The walls' out-of-plane and axial deflection and damage patterns were presented. The upper and lower bounds of performance were presented to examine the effect of material models’ variability on the response. The findings of this study reveal the complex interaction between fire scenarios, material properties, and their effect on the structural behavior of RAC walls. This study contributes to the growing knowledge on using RAC in structural engineering applications and promoting environmentally friendly materials in the construction industry while ensuring structural safety.

Influence of pore property and alkalinity on the bio-deposition treatment efficiency of recycled aggregates

Microbial induced calcium carbonate precipitation (MICP) technology has been successfully used to enhance the properties of recycled concrete aggregates. However, the complex source and varied physical and chemical properties of recycled aggregates may have influence on the modification efficiency since microbes are often sensitive to the surroundings. In this study, two representative types of recycled aggregates, recycled concrete aggregates (RCA) and recycled brick aggregates (RBA), were subjected to two kinds of MICP treatments, basic MICP treatment and sodium alginate (SA) aided MICP treatment. The absorption and desorption of bacteria in/on aggregates during MICP treatments were quantified. The physical and chemical properties of aggregates after the bio-treatments were tested, and the influence of alkalinity and pore structure of aggregates under various MICP treatment methods on treatment efficiency were detailed investigated. Results indicated that, when the aggregates were subjected to the basic MICP treatment, the treatment efficiency was more remarkable in RBA, because of its high porosity and moderate pH (around 8–9), which facilitated the absorption of bacteria in/on aggregates and urease activity respectively. While under SA-aided MICP treatment, the influence of pore structure and alkalinity of aggregates on the treatment efficiency was not significant compared with that under the basic MICP treatment, especially the mass of CaCO3 on the aggregates. The biogenic CaCO3 generated by SA-aided MICP treatment not only plugged micropores, but also distributed all over the entire surface of the aggregates, resulting in a sufficient repair of microcracks. Meanwhile, the surface repair may reduce the influence of pore structure and pH of aggregates on the precipitation process, thereby reducing the impact of varied physical and chemical properties of aggregates on the treatment efficiency, which was conducive to the widely unified application of SA-aided MICP treatment in the modification of recycled aggregate based on construction solid waste.

Effect of additional water content and adding methods on the performance of recycled aggregate concrete

Due to the high porosity of the old cement mortar attached to the surface of the recycled coarse aggregate (RCA), the RCA exhibits a high water absorption property, seriously affecting the workability of recycled aggregate concrete (RAC). The additional water in RCA plays a crucial role in the effective water to binder ratio and various properties of RAC. In order to determine the optimal content and addition method of additional water, this study innovatively investigates the effect of the above two variables on the workability, mechanical properties and durability of RAC. And combined with microscopic tests, the mechanism of the above two variables on the performance of RAC has been revealed. The additional water content includes 100% water absorption (WA) and 75% WA, and the adding methods includes mixing water compensation and pre-wetting aggregates. The results show that the water content of oven dry RCA in water and cement paste reaches 94.7%-95.4% and 71.0%-76.0% WA after 2 h, respectively. The water content of RCA is reduced by 4%-4.9% after immersing RCA into cement paste for the first 5 min under the initial state of 70% WA. No matter the adding methods, the 100% WA additional water has a positive effect on workability but a negative effect on the mechanical properties and durability of RAC. When the additional water content is the same, the adding method has different effects on the mechanical properties and durability of RAC. The microstructure analysis reveals that the interfacial transition zone (ITZ) in RAC prepared with pre-wetted aggregates by 75% WA is the densest of all samples. Compared to the 75% WA water compensation method, the elastic modulus of ITZ in RAC with pre-wetted aggregates by 100% WA decreases by 10.5%, while the thickness of the new ITZ increases by 14.3%. The RAC prepared by pre-wetted aggregates exhibits the lower porosity.

Effect of recycled aggregate treatment methods by pristine graphene on mechanical and durability properties of concrete

Using recycled concrete aggregates (RCAs) in concrete is a promising approach to minimise the environmental impacts associated with construction and demolition waste while simultaneously enhancing the sustainability of concrete. Improving the quality of RCAs is crucial for promoting confidence among materials suppliers and systematic utilisation of RCAs in construction. Pristine graphene (PG) suspensions (0.1% and 0.2% concentrations in water) were investigated for improving the properties of RCAs and thereby improving the mechanical and durability properties of concretes made with 100% RCAs as coarse aggregates. Two PG treatment methods were used – spraying and soaking. Nano-silica (NS) suspensions of the same concentrations were also used for comparison. Tests for axial compressive, splitting tensile and flexural strengths were performed; water absorption and drying shrinkage were also assessed. Microanalysis was conducted using scanning electron microscopy and micro computed tomography. PG was found to be more effective than NS for increasing strength properties and reducing water absorption and drying shrinkage of concrete due to its lower porosity. For a given nanomaterial concentration and type, concretes made with soaked RCA exhibited higher strengths and lower water absorption and drying shrinkage than concretes with sprayed RCA. The concretes made with soaked RCA also had lower porosity and fewer microcracks. The results are promising and indicate that treating RCAs reduces their water absorption, thereby improving concrete performance.

The Mechanical Properties and Microstructure of Tailing Recycled Aggregate Concrete.

The aim of this study was to develop sustainable concrete by recycling concrete aggregates from steel waste and construction waste (iron ore tailings (IOTs) and recycled coarse aggregates (RCAs)) to replace silica sand and natural coarse aggregates. In experimental testing, the compressive strength, peak strain, elastic modulus, energy dissipated under compression, and compressive stress-strain curve were analyzed. Microscopically, scanning electron microscopy and energy-dispersive spectrometry were employed to investigate the microstructural characteristics of the interfacial transition zone (ITZ), and the results were compared with the ITZs of natural aggregate concrete and recycled aggregate concrete (RAC). In addition, the pore structure of concrete was determined by nuclear magnetic resonance. The results revealed that an appropriate IOT content can improve the ITZ and compactness of RAC, as well as optimize the mechanical and deformation properties of RAC. However, due to the presence of a smaller number of microcracks on the surface of IOT particles, excessive IOTs could reduce the integrity of the matrix structure and weaken the strength of concrete. According to the research, replacing silica sand with 30% IOTs led to a reduction in the porosity and microcracking which resulted in a much denser microstructure.