Natural Aggregate Concrete Research Articles

Evaluation on Preparation and Performance of a Low-Carbon Alkali-Activated Recycled Concrete under Different Cementitious Material Systems.

A green, low-carbon concrete is a top way to recycle waste in construction. This study uses industrial solid waste slag powder (S95) and fly ash (FA) as binders to completely replace cement. This study used recycled coarse aggregate (RCA) instead of natural coarse aggregate (NCA). This is to prepare alkali-activated recycled concrete (AARC) with different cementitious material systems. Alkali-activated concrete (AAC) mixtures are modified for strength and performance based on the mechanical qualities and durability of AARC. Also, the time-varying effects of the environment on AARC properties are explored. The results show that with the performance enhancement of RCA, the mechanical performance of AARC is significantly improved. As RCA's quality improves, so does AARC's compressive strength. At a cementitious material content of 550 kg/m3, AARC's 28d compressive strengths using I-, II-, and III-class RCA were reduced by 2.2%, 12.7%, and 21.8%, respectively. I-class AARC has characteristics similar to natural aggregate concrete (NAC) in terms of shrinkage, resistance to chloride penetration, carbonization, and frost resistance. AARC is a new type of green building material that uses industrial solid waste to prepare alkali-activated cementitious materials. It can effectively reduce the amount of cement and alleviate energy consumption. This is conducive to the reuse of resources, environmental protection, and sustainable development.

Evaluation of non-destructive testing and long-term durability of geopolymer aggregate concrete

Recent advancements in concrete technology focus more on increasing strength than durability. Concrete with good durability will withstand adverse conditions like frost, chloride penetration, sulfate assault, alkali-aggregate reaction, steel corrosion, etc., which will lower the strength of the concrete. Strength is vital, but so is durability. The present study examined and discussed the durability parameters of conventional concrete made with geopolymer aggregate (GPA) as a partial substitute for natural aggregate. Strength studies in this research found that the optimal level of substitution for natural coarse aggregate by GPA was 100% replacement to produce the performing concrete. Replacement of natural coarse aggregate by geopolymer aggregate exhibits 9%–15% higher compressive strength than natural aggregate concrete. The findings reveal that concrete with 100% geopolymer aggregate exhibits a compressive strength increase of 9%–15% over that of concrete made with natural aggregates. Ultrasonic pulse velocity measurements for geopolymer aggregate concrete range between 4 km/s and 4.5 km/s, indicating good quality according to IS specifications. Additionally, Rebound Hammer test results further support the enhanced quality of geopolymer aggregate concrete. However, the porosity of geopolymer aggregates results in a sorptivity that is 10%–30% higher than that of natural aggregate concrete. Despite this, the increased resistance to acid and sulfate attacks is noted, attributed to the strong bonding between geopolymer aggregates and the cement matrix.

Experimental investigation and machine learning models for predicting flexural and tensile strengths of recycled concrete: Bridging the gap between sustainable materials and structural design

The sustainability of structural components requires recycled aggregate concrete (RC) to achieve adequate flexural and split tensile strengths for practical use. These strengths depend on mix design and the properties of recycled coarse aggregates (R-CA). The variability in R-CA sources and the inherent heterogeneity of RC complicate strength predictions. No existing model accounts for this variability, leaving a gap between sustainable materials and structural design. This study develops machine learning-based models to predict the flexural and split tensile strengths of RC, regardless of R-CA source and properties. Key factors such as water absorption, effective water-to-cement ratio, coarse aggregate-to-cement ratio, and R-CA replacement ratio are used for predictions. The impact of different R-CA types on RC and natural aggregate concrete (NC) is also experimentally analyzed. A dataset of 353 test results from this study and 33 prior studies is used, and various machine learning algorithms (MLA) are evaluated. Results show a 41 % and 23 % reduction in flexural and split tensile strengths of RC compared to NC, but acid and mechanically treated R-CA can recover up to 94 % and 93 % of NC's strengths, respectively. Among all MLA models, the gradient boost model depicted the highest accuracy in predictions for the flexural and tensile strengths of both RC and NC. This research introduces new equations and a C/C++ tool for predicting RC and NC strengths, contributing to sustainable concrete design and bridging the gap between research and practical application.

Tension stiffening and cracking behaviors in glass fiber reinforced polymer bar enhanced precast recycled aggregate concrete specimen

Glass fiber reinforced polymer (GFRP) bar-enhanced precast recycled aggregate concrete (PRAC) elements combine PRAC’s environmental benefits with GFRP’s corrosion resistance, making them ideal for coastal urban infrastructure. However, in-depth research on the tensile stiffness and cracking behaviors of this composite is lacking. The significant mechanical differences between GFRP bars (low modulus, high strength) and PRAC, as compared to traditional materials, raise safety concerns for widespread use. This study investigates the short-term load capacity, deformation and cracking characteristics of GFRP bar-reinforced PRAC tensile specimens through tests and analyses. The tests examine how variables like water-to-binder ratio, coarse aggregate type, recycled aggregate (RA) content and mixing methods affect the specimens' mechanical responses. Theoretical analyses, using existing equations and a partial interaction model, clarify stress-strain relationships and crack propagation under applied loads, revealing: (a) As RA content increases, PRAC shows a moderate decrease in workability and mechanical performance. The strength variability in PRAC generally falls between that of natural aggregate concrete (NAC) and conventional recycled aggregate concrete (CRAC) using RAs from construction and demolition waste, being closer to NAC. Additionally, the strength conversion relation seen in NAC applies similarly to PRAC. (b) During tension at both ends of the specimen, PRAC's resistance decreases by 24.3 % compared to NAC but is still 20.3 % higher than CRAC. (c) The number of cracks in the PRAC specimen decreases with higher RA content and basalt fibers, while the ratio of stabilized load to cracking load increases. (d) The CEB-FIP standard offers more accurate predictions of the specimen’s composite stress-strain relation, while the Chinese code excels in predicting crack width. The partial interaction model effectively predicts the specimen’s load, deformation and crack characteristics by accounting for the behaviors and bonding of both PRAC and GFRP bars.

Seismic Performance of Recycled-Aggregate-Concrete-Based Shear Walls with Concealed Bracing

Relatively few studies have been conducted on the seismic performance of recycled aggregate concrete (RAC) shear walls with concealed bracing. To promote the development of high-performance green building structures and the application of RAC in structural components, the seismic performance of RAC shear walls under different influencing factors was tested, and low-cycle reversed loading tests were performed on ten RAC shear walls with different shear-to-span ratios. The test parameters included the recycled coarse aggregate (RCA) replacement ratio, the recycled fine aggregate (RFA) replacement ratio, the axial compression ratio, the shear span ratio and whether to set up the concealed bracing. The influence of the above variables on the seismic performance was then assessed. The results revealed that the bearing capacity, ductility, stiffness and energy dissipation capacity of the RAC shear walls decreased in line with an increase in the replacement ratio of the RFA. However, the bearing capacity, energy consumption and stiffness of the RAC shear walls decreased within 10% and the ductility decreased within 15%. The RAC shear walls were able to meet the seismic requirements of the building structure after reasonable design and use. As the axial compression ratio increased, the bearing capacity of the RAC shear walls improved, but their elastic–plastic deformation capacity was reduced. Setting the concealed bracing significantly improved the seismic performance of the RAC shear walls, such that they achieved a seismic performance close to that of the natural aggregate concrete (NAC) shear wall. After setting up the concealed bracing, the load carrying capacity of the RAC shear walls increased by up to 15%, the ductility increased by up to 20% and the energy consumption capacity increased by up to 50%. A mechanical calculation model of the RAC shear wall was then established by considering the effect of recycled aggregate, the calculated results of which were a good match with the test results.

Effect of low-heat cement Portland cement on alkali-silica reaction of aggregates with different alkali-reactivities

This study investigated the impact of cement clinker percentages on the alkali-silica reaction (ASR) mechanisms of aggregates with varying alkali-silica reactivity. Experimental findings indicated that the low-heat Portland cement (LHC) mitigated the development of the ASR process in soda-lime glass aggregate (SLGA) concrete. The ASR in ordinary Portland cement (OPC) mainly initiated within the SLGA, leading to crack propagation through the matrix. The in-situ X-ray micro-computed tomography test indicated that LHC altered the manifestation of ASR in concrete containing SLGA, leading to different destructive forms. Specifically, ASR between the LHC and SLGA primarily occurred at the periphery of the SLGA. The in-situ quantity analysis results illustrated that the LHC mitigated the ASR-induced cracks in the SLGA concrete. For the less alkali-reactive natural aggregate, the effect of LHC cement on ASR was diminished. ASR still only occurred within the natural aggregate concrete, distributing in highly alkali-reactive phases inside the aggregates. The chemical analysis results supposed that the differing ASR pathways could be attributed to the interaction between the calcium content in the LHC and the spatial distribution of alkali-reactive phases within the aggregates. These findings help interpret the mechanism of the slower ASR process in the belite-rich cement concrete.

Machine learning-based prediction method for drying shrinkage of recycled aggregate concrete

The objective of this study is to develop a broadly applicable, high-precision, and robust prediction model for the drying shrinkage of recycled aggregate concrete, a material that exhibits significantly greater shrinkage compared to natural aggregate concrete due to its complex characteristics. To achieve this, the study began by selecting relevant characteristic parameters based on international concrete codes, followed by the application of various machine learning algorithms including Backpropagation Neural Network, Support Vector Machine, Random Forest, eXtreme Gradient Boosting, Gaussian Process Regression, k-Nearest Neighbor, Linear Regression, and Long Short-Term Memory to model and forecast the drying shrinkage of recycled aggregate concrete. Subsequently, the SHapley Additive exPlanations (SHAP) method was employed to identify the crucial factors influencing RAC drying shrinkage, such as drying age, elastic modulus, and water-binder ratio, thereby optimizing the input parameters of the ML model. To further enhance the XGBoost algorithm, the sparrow search algorithm (SSA) and the whale optimization algorithm (WOA) were utilized. The optimized WOA-XGBoost model exhibits superior predictive performance, with a determination coefficient of 0.980 and a mean ratio of predicted to experimental values of 1.025, significantly outperforming traditional specification models. The model's applicability may be limited since the dataset is mainly derived from laboratory conditions, which may differ from actual engineering environments. Future research could consider different types of recycled aggregates and curing conditions and test the model on larger data sets to improve its robustness and applicability.

The performance of waste glass aggregate concrete GFRP-steel double-skin tubular columns under axial compression: Experimental study

In an attempt to alleviate the problem of growing waste glass and fully utilize the excellent material properties of fiber-reinforced polymer (FRP), a novel composite structure, waste glass aggregate concrete glass fiber reinforced polymer (GFRP)-steel double-skin tubular column (WGAC-DSTC), was proposed based on the FRP-concrete-steel double-skin tubular column (DSTC). In the present research, eight WGAC-DSTCs and four waste glass aggregate concrete-filled GFRP tubular columns (WGAC-CFFTs) were investigated to explore their mechanical performance under axial compression. Effects of diverse replacement ratio of waste glass aggregate, GFRP tube thickness, and hollow ratio of column on the mechanical performance of twelve columns were investigated. Meanwhile, this research analyzed and discussed the failure characteristics of columns, load-displacement relationship, ductility coefficient, stiffness coefficient, parameter analysis, ultimate bearing capacity, load-strain relationship and constraint factor. Besides, predicted formulae for estimating the ultimate bearing capacity of WGAC-CFFTs and WGAC-DSTCs were proposed. The research results indicated that (1) Failure mode of the WGAC-DSTCs displayed axial compression failure or shear failure and failure mode of WGAC-CFFTs exhibited mainly axial compression failure; (2) WGAC-DSTCs with 10 % replacement ratio of waste glass aggregate and hollow ratio of 0.4 indicated an increase in the ultimate bearing capacity by 17.24 % and 45.78 %, respectively, as the GFRP tube thickness increased from 1.5 mm to 2 mm and 3 mm. As the column's hollow ratio enhanced from 0.27 to 0.68, the column's ultimate bearing capacity significantly decreased by 7.08 %-60.55 %. The ultimate bearing capacity of WGAC-CFFTs showed a trend of first increasing by 8.33 %-9.41 % but then decreasing by 1.52 % when the replacement ratio of waste glass aggregate rose from 0 % to 30 %; (3) WGAC could replace natural aggregate concrete in traditional DSTC and the bearing capacity could be guaranteed. Besides, it's recommended to utilize 20 % replacement ratio of waste glass aggregate for the WGAC-DSTC whose ultimate bearing capacity enhanced by 15.17 % and possessing optimal mechanical performance; (4) The results calculated utilizing predicted formulae matched well with results of experiment. The application of waste glass aggregate concrete in DSTC can contribute to economic growth and environmental protection and meet the concept of sustainable development.

Experimental study on bond performance of recycled coarse aggregates concrete with circular steel tubes after freeze–thaw cycling

AbstractExtrusion tests were conducted on recycled coarse aggregate concrete with circular steel tubes (RCCST, 100% replacement of recycled coarse aggregate) to investigate the impact of freeze–thaw cycles on bond properties. RCCST samples were subjected to 0, 25, 50, 75, 100, 125 and 150 freeze–thaw cycles and their bond properties were then compared with those of natural coarse aggregate concrete with circular steel tubes (NCCST) after 0, 100 and 150 freeze–thaw cycles. The experimental results indicate that freeze–thaw cycling damages the internal structure of RCCST, causing a significant decrease in interfacial bond strength with an increase in the number of freeze–thaw cycles. Freeze–thaw RCCST exhibits lower peak load and higher peak slip compared to NCCST, with increases of 25.2%, 73.2%, and 70.3% after 0, 100, and 150 freeze–thaw cycles, respectively. Regression analysis was used to derive an exponential equation that describes the relationship between longitudinal strain and strain gauge position. Additionally, a segmented fitting method was employed to obtain an expression for the bond slip of the freeze–thaw RCCST with a circular steel pipe.

Seismic performance of carbonated recycled aggregate concrete shear walls

The carbonated modification method is recognized as an effective method to enhance recycled aggregates (RA), but there is currently a lack of research on carbonated recycled aggregate concrete (CRAC) used for structural engineering. This paper investigated the impact of CRAC on the seismic performance of shear walls, quasi-static tests of one natural aggregate concrete (NAC) shear wall and four concrete shear walls with different replacement ratios (30 %, 50 %, 70 %, and 100 %) of carbonated recycled aggregate (CRA) were conducted. The ductility, energy dissipation, stiffness, strength degradation, and lateral displacement components of CRAC shear walls were evaluated. The results indicate that the seismic behavior of the shear walls degraded as the replacement rate of CRA increased. Based on the comprehensive evaluation of failure patterns and seismic performance, the replacement rate of CRA is suggested to be taken as 70 % in the concrete shear walls. Finally, the strut-and-tie model was validated as an effective method for predicting the shear capacity of CRAC shear walls, and the accurate softening coefficient constant C of the strut-and-tie model for CRAC and RAC shear walls was proposed.

Characterization of heat-processed artificial lightweight aggregates from polyethylene terephthalate plastic waste

Plastic waste is an undeniable source of pollution that threatens the existence of the earth’s flora and fauna. The bulk of plastic waste generated globally does not go through the proper methods of disposal but is carelessly discarded into the aquatic or terrestrial environment. Current recycling efforts are largely inadequate and disposal in landfills is still fraught with environmental and land use challenges. The proper disposal of plastic waste, as well as mitigating the environmental, social, and health impacts of extracting natural aggregates can be achieved by incorporating plastic waste as aggregates in the construction industry. This paper presents a characterization of aggregates manufactured from polyethylene terephthalate plastic waste using thermal/mechanical methods. From the cost analysis, 24,341.67 Ugx (6.09 USD) was spent to produce 1 kg of PET aggregates. Morphological, intrinsic and mechanical characteristics of the produced aggregates were established using standard procedures and equipment. The results of morphological characterization indicate an irregular shaped aggregate with smooth surface, a dense graded aggregate with a fineness modulus of 4.25, flakiness index of 26%, elongation index of 16% and particle index of 13. Intrinsic characterization yielded particle density of 1330 kg/m3, bulk density of coarse aggregates of 715 kg/m3 and water absorption of 0.445%. Mechanical characteristics of aggregates were evaluated, with compressive strength of 50Mpa, Aggregate Crushing Value of 37%, Ten Percent Fines Value of 71KN, Aggregate Impact Value of 24% and Aggregate Abrasion Value of 20%. The characteristics of PET aggregates confirm their suitability for application in structural lightweight concrete and rigid pavement. The produced PET aggregates can be considered in mix design as a total or partial replacement of natural aggregates in concrete.

Experimental study on impact performance of seawater sea-sand concrete with recycled aggregates

On islands distant from the mainland, obtaining raw materials for concrete production is often more challenging. To achieve sustainable development in island reef engineering, using discarded marine concrete and coral waste generated during island construction as recycled aggregates are of considerable significance. The preparation of Recycled Coral Aggregate Concrete (RCAC) for island reef engineering thus holds substantial importance. In this study, RCAC and Natural Aggregate Concrete (NAC), both designed with a compressive strength of C60, were prepared. Initially, the fundamental physical properties of the recycled coarse aggregate, such as apparent density, water absorption, and crushing index, were determined. Subsequently, a comparative analysis of the quasi-static mechanical properties of RCAC with varying proportions of recycled coral coarse aggregate (RCCA) was conducted. Furthermore, the impact compression mechanical properties of different RCAC specimens under various strain rates were examined using the Ф100mm Split Hopkinson Pressure Bar (SHPB) apparatus. The microstructure and long-term drying shrinkage performance of RCAC were also analyzed using Scanning Electron Microscopy (SEM) and a drying shrinkage apparatus. The finding indicated that the 28-day compressive strength of RCAC specimens with 100% coarse aggregate replacement reached a maximum of 62.4 MPa. The quasi-static compressive strength of RCAC specimens with 50% and 100% RCCA replacement was only 11.5% and 14.2% lower than that of NAC, respectively. Under impact loading, the dynamic compressive strength of RCAC specimens increased with the strain rate, with peak stress exhibiting an approximately linear relationship with the strain rate. The energy dissipation of RCAC specimens generally occurred in three stages, with the reflected and absorbed energies of the specimens increasing linearly with strain rate. At the same strain rate, the transmitted energy of RCAC specimens was higher than that of NAC specimens. Microstructural analysis revealed that the morphology of recycled coral aggregate is characterized by its porous and rough surface. The interfacial transition zone between the recycled coral aggregate and the cement mortar was relatively dense. Incorporating recycled coarse aggregate significantly affected the drying shrinkage properties of the concrete, with higher contents of RCCA leading to greater drying shrinkage rates.

Compressive behavior of partially encased recycled aggregate concrete columns after exposure to elevated temperature

To explore the eccentric compressive behaviors of partially encased recycled aggregate concrete (PERAC) columns after exposure to high temperatures, 12 columns were tested in this paper, including 3 columns under axial load as controls. Factors considered in the experiment were replacement levels (r) of recycled coarse aggregate (RCA), temperature (T), and relative eccentricity (e/H). The test results show that the temperature rising of webs is effectively delayed by protection of recycled aggregate concrete (RAC). And the post-heated PREAC column has higher ductility under eccentric load due to the better deformation capacity of RAC than that of natural aggregate concrete (NAC). Similar to the effect of T, when the e/H adds from 0 to 0.2, both ultimate axial strength and initial stiffness of columns decrease significantly. When e/H adds from 0.2 to 0.4, flanges away from the loading axis are gradually changed from compression to tension. All columns eventually fail due to local buckling of flanges near loading axis. Compared with partially encased concrete (PEC) columns, the r of 50% is beneficial for PERAC columns to get better ductility and maintain stiffness. In addition, the modified method based on the code T-CECS719–2020 can be used to predict the ultimate strength of PERAC columns after exposure to high temperatures.

Stress path-based compressive strength model of FRP-confined recycled aggregate concrete

The compressive strength model of fibre-reinforced polymer (FRP) confined recycled aggregate concrete (RAC) is important in practical engineering designs. The existing research has showed that the compressive strength of FRP-confined RAC can be predicted by the models for FRP confined natural aggregate concrete (NAC). However the accuracy of existing compressive strength models for FRP-confined NAC vary greatly because of the unclear understanding of the confinement mechanisms. Previous research has shown that the axial stress of confined concrete tends to be path-dependent. In this study, a confining stress path-based compressive strength model of FRP-confined RAC is proposed based on theoretical analyses of the confining stress path of FRP-confined RAC. In the proposed model, an effect index is determined to express the influence of confining stress path on the compressive strength. The relationship between the effect index and confinement ratio and that between lateral stress dominant index and confinement ratio are obtained. The proposed model is verified by comparison with previous experimental test results and those predicted by existing representative models. It is found that the compressive strength of FRP-confined RAC is path-dependent for small confinement ratios (less than 0.43) and path-independent for large confinement ratios (larger than 0.43).

Stress–strain characteristics of fire‐exposed recycled coarse aggregate concrete

AbstractConcrete sustainability and performance under extreme conditions are of growing interest in construction engineering. This study delves into the influence of recycled coarse aggregate (RCA) content and elevated temperatures on normal‐strength concrete containing RCA. Five different concrete compositions, featuring varying content of RCA (ranging from 0% to 100%), were examined. The heating and subsequent cooling followed the ISO‐834 temperature–time graph up to 800°C. The primary objective was to evaluate residual properties, including the stress–strain behavior, compressive and tensile strength, secant elastic modulus, peak strain, and bond strength of RCA concrete. The findings reveal a consistent decrease in both strength and stiffness parameters of RCA concrete with rising temperatures, while peak strain exhibits a rapid increase at elevated temperatures. Interestingly, RCA content had a negligible impact on the relative deterioration of high‐temperature exposed RCA concrete compared to that at ambient conditions. Moreover, the bond behavior closely resembled that of natural aggregate concrete when used in moderate proportions. Degradation models based on regression analysis of the data were used to quantify the bond strength reduction for RCA‐based concrete and the slip of rebar concerning various temperatures. Importantly, these models demonstrated consistency with those applicable to conventional concrete.

Mechanical properties and fracture behavior of seawater-mixed sea sand recycled aggregate concrete

Despite their undeniable potential for promoting sustainability in construction materials, the characteristics of recycled aggregates (RCA), seawater, and sea sand may vary significantly, thereby affecting their use in structural concrete. This study exploits these materials to investigate their performance in producing sustainable concrete and establishing its mechanical and fracture behavior. The so produced material, referred to as seawater sea sand recycled aggregate concrete (SSRAC) could lend a hand in widening the usage of RCA and sea sand in a variety of applications where the use of ever-depleting natural materials as concrete constituents becomes an issue. Consequently, the variables of the study include the type of water (fresh versus seawater), type of coarse aggregate (RCA versus natural coarse aggregate, NCA), and the type of fine aggregate (dune sand versus sea sand). For each water type, three concrete mixtures are produced using NCA-dune sand, RCA-dune sand, and RCA-sea sand combinations. Analyses of the mechanical characteristics of the concrete, including compressive strength, flexural strength, modulus of elasticity, and fracture toughness, were conducted based on experimental tests. X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) analyses were also conducted on the designed specimens to investigate their microstructure. In terms of fracture characteristics, the use of seawater resulted in improvement in the fracture toughness. The use of recycled aggregate along with seawater resulted in improved compressive strength. However, natural aggregate concrete (NAC) showed higher flexural strengths than recycled aggregate concrete (RAC). The results also revealed that the addition of sea sand had a detrimental impact on all mechanical characteristics. Thus, optimizing the concrete composition in which the investigated materials partially replace the conventional concrete constituents is required in order to consider alternative constituents to generate sustainable concrete for practical applications.

Compressive stress–strain relationship and its variability of basalt fiber reinforced recycled aggregate concrete

Basalt fiber reinforced recycled aggregate concrete (BFRAC) is a high-performance, environmentally friendly material that combines lightweight, high-strength fibers with low-carbon recycled aggregates (RAs), positioned for extensive use in building structures. However, research on its constitutive relationships is currently scarce, which partly restricts component design and analysis. In this context, the current study thoroughly explores the stress–strain relationship and variability of BFRAC under compression, using 240 cylinders for testing to investigate the influence of factors like coarse/fine RA sources, RA replacement rates, and fiber dosage. The study found that the addition of RAs and fibers reduced the workability of the mixture, particularly with the inclusion of fine RAs and short-cut fibers. Using coarse and fine RAs generally reduces the material’s elastic modulus, compressive strength, and post-peak ductility. Adding fibers can slightly improve compressive strength and peak strain, significantly reduce material brittleness, and have a minimal impact on elastic modulus. Importantly, the study noted that the pre-peak segment of the stress–strain curve of BFRAC is most sensitive to the addition of fine RAs, while the post-peak segment is most sensitive to fiber content. Despite this, using high-quality RAs up to 50% replacement and adding 0.4% by volume of fiber can make BFRAC with mechanical properties comparable to natural aggregate concrete. Based on the observed tests, this paper proposes constitutive relationships that incorporate skeleton curves and variability at different points for the compressive stress–strain behavior of BFRAC.

PRODUCTION OF RECYCLED AGGREGATE FOR STRUCTURAL CONCRETE

Recycling demolished concrete is crucial for efficient resource utilization and environmental preservation. The study investigates the use of recycled aggregate in structural concrete production. This study focuses on the mix design of recycled aggregate concrete using the IS mix design procedure with a targeted design strength. Five different concrete proportions were investigated, ranging from 0% ,25% ,50% ,75% and 100% replacement of natural aggregates by recycled aggregates. Results were discussed, with a focus on comparing cube compressive strength to cylindrical compression strength and cylindrical compression strength to split tensile strength. The findings were compared with existing standards and provisions, and regression models were formulated to enhance understanding and prediction of concrete properties. This paper contributes valuable insights into the mix design of recycled aggregate concrete, facilitating sustainable construction practices in line with industry standards and regulations. Key Words: Natural Aggregate Concrete, Recycled Aggregate Concrete, Compressive Strength test, Split Tensile Strength test.

Investigating specimen size and shape effects on compressive mechanical behaviors of recycled aggregate concrete using discrete element mesoscale modeling

In comparison to natural aggregate concrete (NAC), recycled aggregate concrete (RAC) typically exhibits a more diverse composition of materials, heightened variability and greater inherent weaknesses. These characteristics may result in varying effects of specimen size and shape on compressive responses. Within this framework, the present study endeavors to comprehensively investigate these influences on RAC's compressive properties and their underlying mechanisms utilizing discrete element simulation technology. At the mesoscale, RAC is depicted by several components, encompassing both new and old aggregates, mortar and interfacial transition zones. Through integrating contacts for each phase, elucidating mesoscale mechanical responses with a parallel bond model and introducing Weibull random fields to delineate variations in phase properties, this study effectively simulates these mechanical responses. Subsequent extensive calculations scrutinize the repercussions of specimen size and shape on the compressive strength, elastic modulus, peak strain and failure modes of RAC across varying water-to-binder ratios. The study concludes that RAC manifests a size effect, albeit less pronounced than in NAC with an equivalent water-to-cement ratio, yet more significant than in mortar. Moreover, owing to end plate constraints, the reduction in RAC's cubic compressive strength is notably less than its prismatic compressive strength. Based on these findings, it is recommended to establish the strength size reduction coefficient and cube-to-cylinder shape coefficient for RAC at 0.86 and 0.72, respectively. Notably, these values diverge significantly from the respective 0.81 and 0.78 coefficients for NAC.

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.