Recycled Aggregate Concrete Research Articles

Investigating coconut shell biochar as a sustainable solution for removing iron residues in recycled concrete aggregates

Investigating coconut shell biochar as a sustainable solution for removing iron residues in recycled concrete aggregates

Enhancement of Mechanical Properties and Freeze–Thaw Durability of Recycled Aggregate Concrete using Aggregate Pretreatment

Enhancement of Mechanical Properties and Freeze–Thaw Durability of Recycled Aggregate Concrete using Aggregate Pretreatment

Study on Microstructure of Recycled Aggregate Concrete by Scanning Electron Microscopy: A Review

Recycled Aggregate Concrete (RAC) is a new type of concrete made by utilizing waste concrete as aggregate after treatment. With the enhancement of environmental awareness and resource conservation, but the microstructural part of recycled aggregate concrete is not as important as the macro performance enhancement aspect by scholars. And Scanning Electron Microscope (SEM) is a powerful tool for microanalysis, which can provide high-resolution images of samples as well as information on surface morphology and microstructure. In this paper, the micro-structure of RAC will be comprehensively analyzed according to the performance characteristics of RAC materials and the relevant analysis technology of SEM. Based on the microscopic technology of SEM, the research progress of the micro-structure of RAC was carried out, and the morphology and distribution characteristics of recycled aggregate particles, pore structure and porosity, the interface combination between recycled aggregate and cement matrix, and the relationship between micro-structure and RAC properties were summarized, which provided theoretical support for the future understanding of the micro-structure of RAC, increased the understanding of the micro-structure of RAC materials, and more systematically understood the application of microscopic analysis methods in RAC.

Evaluation of Various Treatment Methods for Enhancing the Recycled Aggregate Concrete

Evaluation of Various Treatment Methods for Enhancing the Recycled Aggregate Concrete

Prediction Model-based Multi-objective Optimization for Mix-ratio Design of Recycled Aggregate Concrete

Prediction Model-based Multi-objective Optimization for Mix-ratio Design of Recycled Aggregate Concrete

Prediction Model for the Chloride Ion Permeability Resistance of Recycled Aggregate Concrete Based on Machine Learning

The chloride ion permeability resistance of recycled aggregate concrete (RAC) is influenced by multiple factors, and the prediction model for this resistance based on machine learning is still limited. In the paper, six impact factors (IFs), including the carbonation of recycled coarse aggregates (YN), the replacement ratio of recycled coarse aggregates (r), the bending load level (L), the carbonation time (t) and temperature (T) of RAC, and the replacement ratio of carbonated recycled fine aggregates (f), were considered to conduct a chloride penetration test on RAC. Based on the experimental data, four algorithms, including artificial neural network (ANN), support vector machine (SVM), random forest (RF) and extreme gradient boosting (XGBoost), were adopted to establish the machine learning prediction models and study the relationships between the electric flux of RAC and the IFs. The results showed that the predicted values of all four models were in good agreement with the experimental values, and the XGBoost model had the best prediction performance on the testing set. Based on the XGBoost model, the LIME method was adopted to solve the interpretability problem in the prediction process. The importance ranking of IFs on the electric flux was r > t > f > T > L > YN. A graphical user interface (GUI) was developed based on Python 3.8 software to facilitate the use of machine learning models for the chloride ion permeability resistance of RAC. The research results can provide an accurate prediction of the electric flux of RAC.

A Review of the Shear Design Provisions of ACI Code and Eurocode for Self-Compacting Concrete, Recycled Aggregate Concrete, and Geopolymer Concrete Beams

A Review of the Shear Design Provisions of ACI Code and Eurocode for Self-Compacting Concrete, Recycled Aggregate Concrete, and Geopolymer Concrete Beams

Influence of nano‐silica particles on fracture features of recycled aggregate concrete using boundary effect method: Experiments and prediction models

AbstractThe aim of this investigation is to assess the impact of using nano‐silica (Na) at varying weight percentages of 0%, 1.5%, 3%, 4.5%, and 6% as partial cement substitute on fracture parameters of recycled aggregate concrete (RAC). A servo‐controlled testing system was employed to carry out three‐point flexure tests on 90 notched beams. Boundary effect method was used in order to interpret the fracture features. The results illustrate that adding Na increases the size‐independent fracture energy, fracture toughness and initial fracture energy of RAC by 29%, 32%, and 24% compared to that without Na, respectively. The maximum values of these parameters occur at 4.5% Na. The reference crack length () decreases from 6% to 22% by adding 1.5% to 6% Na. This shows that the RAC gets more brittle by the addition of Na. Moreover, the RAC behavior moves towards linear elastic fracture mechanics criteria by adding Na. Finally, according to the mechanical properties and test variables, multivariable models were suggested for prediction of the fracture parameters of the RAC containing Na. The models predictions were compared with experimental findings of the previous research.

Recycled Aggregate Concrete with Industrially Pre-carbonated Recycled Concrete Aggregates and Low Clinker Content

Recycled Aggregate Concrete with Industrially Pre-carbonated Recycled Concrete Aggregates and Low Clinker Content

Experimental and model evaluation of cyclic compressive performance for RAC-filled BFRP tube composite columns

The mechanical properties of recycled aggregate concrete (RAC) can be significantly enhanced in RAC-filled fiber-reinforced-polymer (FRP) tube composite columns. However, the cyclic compressive behavior of this type of composite columns has been scarcely studied. In this study, a series of cyclic axial compressive tests were conducted on RAC-filled basalt-FRP (BFRP) tube composite columns, and the effects of confinement level (number of BFRP layers) and RA replacement ratio were investigated. The test results indicate that, compared to RAC columns, the compressive capacity and deformability of RAC-filled BFRP tube composite columns increased significantly under cyclic loading. The compressive strength and ultimate strain of BFRP-confined RAC reached 1.42-3.98 times and 2.88-21.53 times those of unconfined RAC, respectively. The compressive strength and ultimate strain under cyclic loading were generally close to those under monotonic loading, with the compressive strength ratios averaging 1.052 and the ultimate strain ratios averaging 1.031. The enhancement efficiency of BFRP confinement increased progressively with increasing confinement level and/or RA replacement ratio. The envelope curves of the cyclic stress-strain curves resembled the stress-strain curves under monotonic loading. The reloading modulus (Ere) and residual modulus (Eun,0) continuously decreased as the unloading strain (εun) increased, while the plastic strain (εpl) increased almost linearly with εun. The values of Ere, Eun,0 and εpl were affected by the confinement level and RA replacement ratio. Finally, typical theoretical models for FRP-confined concrete were used to evaluate the cyclic stress-strain curves of RAC-filled BFRP tube composite columns.

Evaluation of the engineering properties and sustainability of solid masonry blocks produced with recycled concrete aggregates

The rapid expansion of global infrastructure has led to significant natural resource shortages and increasing environmental concerns related to construction and demolition (C&D) waste. Recycling C&D waste presents a viable solution to alleviate resource depletion and promote sustainability. This study explores the use of recycled concrete aggregates (RCAs) derived from C&D waste in producing solid masonry blocks (SMBs). RCAs were employed to replace both natural coarse and fine aggregates in these blocks. Physical and mechanical properties of SMBs were tested, including hardened density, compressive and flexural strengths, and water absorption. The findings showed that decreased strength and increased water absorption for cement-to-aggregate ratio (C: A) increased from 1:6 to 1:24; however, all produced SMBs satisfied the requirements specified by Indian and ASTM standards. The study concluded that using 100% RCAs in SMBs is structurally sound and environmentally beneficial, meeting international standards. Employing recycled aggregate concrete blocks lowers environmental impact and aids in sustainable development by facilitating the life-cycle closure of the building materials.

Enhancement of Recycled Aggregate Concrete Properties Through the Incorporation of Nanosilica and Natural Fibers

This study introduces an innovative approach to enhancing recycled aggregate concrete (RAC) by incorporating nanosilica (NS) and natural fibers (NF), specifically sisal fiber (SF) and palm fiber (PF). This novel combination aims to overcome the inherent limitations of RAC, such as reduced strength and durability, while promoting sustainability in construction. The research focuses on evaluating the mechanical properties of RAC, including compressive and flexural strengths, through the integration of NS and NF. Our findings reveal that NS significantly improves the microstructure of RAC by enhancing the interface transition zone (ITZ) and filling nanovoids, resulting in a denser and more durable concrete matrix. Specifically, the addition of 3% NS increased the compressive strength of RAC by up to 22.5% and the flexural strength by up to 25.6% at a 100% replacement ratio of recycled aggregate. The addition of NF, treated to withstand the alkaline environment of concrete, further strengthens the RAC by providing a bridging effect that enhances flexural strength by up to 46.7%. This work not only advances the performance of recycled concrete but also aligns with the broader goal of environmental sustainability by utilizing waste materials and reducing the carbon footprint of concrete production. The findings have the potential to influence future construction practices, encouraging the adoption of more durable and eco-friendly building materials.

Synergistic effects of SMA fibers and fly ash on the material characterization of recycled aggregate concrete

This study focuses on improving the material characterization of Recycled Aggregate Concrete (RAC) while minimizing its environmental impact. It explores the interaction between Shape Memory Alloy (SMA) fibers and fly ash (FA) in RAC. Comprehensive microstructural and mineralogical analyses were conducted using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD), along with thermal assessments via Thermogravimetric Analysis (TGA) and Differential Thermogravimetry (DTG). SEM images consistently revealed hydration products, including calcium hydroxide and ettringite, across all specimens. The results indicate that incorporating SMA fibers significantly enhances FA-containing RAC by improving mechanical strength, thermal regulation, matrix densification, and crack control, effectively addressing the inherent weaknesses of recycled aggregates.

Effect of CO2 curing on mechanical and physical properties of the recycled aggregates containing silica fume

CO2 curing is a promising method for enhancing the properties of the recycled concrete aggregates (RCAs) due to its economic and environmental benefits. However, the knowledge about its effectiveness in improving the properties of aggregates derived from blended supplementary cementitious materials (SCMs) remains limited. This study explores the impact of CO2 curing on the mechanical and physical properties, mineralogical composition, and microstructural changes of recycled aggregates incorporating varying amounts of silica fume (SF). The results showed that upon the incorporation with 5wt% SF, CO2 curing increased the compressive strength of the aggregate samples by 17.9%. The microhardness improved from 50 to 56 HV with the addition of 10wt% SF. Moreover, CO2 curing modified the physical characteristics of the samples regardless of SF content. However, it caused mechanical degradation in samples containing 20wt% SF, at both marco- and micro-scale. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) revealed that higher SF dosages led to the formation of poorly crystallised CaCO3 during CO2 curing. Additionally, Low-field 1H nuclear magnetic resonance (LF-NMR), N2 adsorption/desorption, scanning electron microscopy (SEM), and thermodynamic models showed that the carbonation products enlarged the capillary pores, and reduced the solid volume of the hydration products. Those findings underscore the importance of calcium hydroxide in protecting against mechanical degradation in SF-blended recycled aggregates during CO2 curing. This study provides an insight of the carbonation of SF-blended composites, potentially accelerating the application of CO2 curing on RCAs.

Influence of steam curing on cyclic triaxial characteristics of recycled aggregate concrete: Experimental analysis and DEM simulation

The engineering application of steam-cured recycled aggregate concrete (often in a cyclic triaxial stress state) can not only improve the recycling efficiency of resources, but also accelerate the construction progress. In this paper, we adopt a combination of laboratory experiments, theoretical analysis and numerical simulation to study the cyclic triaxial characteristics of recycled aggregate concrete (RAC) under different curing regimes (20℃, 40℃, 60℃ and 80℃). The results indicate that as the steam curing temperature increases, the internal damage caused by high temperature continues to intensify, and the cyclic triaxial failure mode of RAC transitions from shear failure to compression failure, and its peak strength, dynamic elastic modulus, and dilatancy angle all show a downward trend. A linear prediction model is established based on the strong correlation between peak strength and steam curing temperature. As the loading cycles increase, the dynamic elastic modulus and dilatancy angle of RAC show exponential and linear downward trends respectively, and the decline rate increases with the increase of steam curing temperature, and the prediction models for dynamic elastic modulus and dilatancy angle are established based on quantitative relationships between variables. On the basis of experimental analysis results, a cyclic triaxial DEM model considering real recycled aggregates is established by introducing steam-cured damage indexes into the mesoscopic parameters, and its applicability in predicting cyclic triaxial mechanical properties of RAC under different curing regimes is verified. The research outcomes can save a lot of test cost and time consumption for developing steam-cured concrete with better performance, and have important theoretical guidance and practical significance for the wide application of steam-cured concrete.

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.

Stiffness degradation and mechanical behavior of microfiber-modified high-toughness recycled aggregate concrete under constant load cycling

The mechanical properties of High-Toughness Recycled Aggregate Concrete (HTRAC) were investigated in this study as an innovative and environmentally friendly construction material, along with its potential applications in structural stability. Small-scale specimens with six levels of micro-steel fiber content were made, and a series of cyclic tests with constant loads were carried out. Using In-Situ 4D CT technology, the damage characteristics of the microstructure of HTRAC and the reinforcing effects of fibers on key mechanical parameters (peak stress, peak strain, ultimate strain, post-peak modulus, and toughness indicators) were analyzed. A comprehensive fiber reinforcing factor calculation model was proposed to assess its contribution to strength, deformability, and toughness, and the correlation between the number of cyclic loadings and stiffness degradation was also quantified. it is confirmed that HTRAC exhibits a significant advantage in toughness compared to traditional recycled aggregate concrete (RAC). The findings of this study provide crucial technical support for the further development and application of HTRAC, indicating its promising prospects in the field of sustainable construction materials

Experimental study on chloride ion permeability of carbonated recycled aggregate concrete considering the effects of multiple factors

The chloride ion permeability is an important index of durability of carbonated recycled aggregate concrete (CRAC), but the resistance to chloride ion permeability of CRAC is affected by many factors and has yet been fully explored, which needs further systematic study. Considering the effects of the replacement ratio of carbonated recycled coarse aggregate (CRCA), the carbonation time of CRAC, the bending load level, the carbonation temperature of CRAC, and the replacement ratio of carbonated recycled fine aggregate (CRFA), the resistance to chloride ion permeability of CRAC was investigated by conducting tests in this paper. The results indicated that, compared with the non-carbonated recycled aggregate concrete (NCRAC), the electric flux of CRAC decreased by 9.14% to 37.34%. An increase in the replacement ratio of CRCA could reduce the resistance to chloride ion permeability of CRAC, while an increase in CRAC carbonation time had a significant effect on improving the resistance to chloride ion permeability. When the replacement ratio of CRCA was 100%, as the carbonation time increased from 0 to 14 days, the maximum reduction in electric flux of CRAC was 31.66%. With the same replacement ratio of CRCA, as the bending load level increased, the decrease in electric flux of CRAC showed an increasing trend compared to that of NCRAC. The resistance to chloride ion permeability of CRAC could be improved with an increase in carbonation temperature. Compared with the CRAC with carbonation temperature of 20 °C, the electric flux of the CRAC with carbonation temperature of 40 °C decreased by 11.03%. When the replacement ratio of CRFA was in the range of 0 to 20%, the resistance to chloride ion permeability of CRAC could be improved by incorporating CRFA, but the improvement effect was not significant. Therefore, compared with the NCRAC, the resistance to chloride ion permeability of CRAC is enhanced, and the research results can provide data support for the engineering application of CRAC.

Durability and Compressive Strength of Composite Polyolefin Fiber-Reinforced Recycled Aggregate Concrete: An Experimental Study

This study investigates of using recycled concrete aggregates along with the reinforcement of polyolefin fibers to augment both the compressive strength and durability of concrete, in alignment with the principles of sustainable development. This study experimentally investigated the compressive strengths and durability of composite polyolefin fiber-reinforced recycled aggregate concrete (PFRRAC) exposed to chloride and acidic environments. For this purpose, 150 cubic samples of 100 × 100 × 100 concrete with various combinations of recycled aggregates and polyolefin fibers were made and subjected to axial compressive loading. The results show that the addition of fibers significantly enhances the compressive strength of concrete, with an increase of up to 34.36% at 5% fiber content. However, increasing the proportion of recycled aggregates reduces the compressive strength, with reductions ranging from 21.12% to 43.85% as the recycled aggregate content rises to 70%. Moreover, the combination of fibers and recycled aggregates demonstrates potential for improving the sustainability and durability of concrete under challenging environmental conditions, particularly in chloride and acidic environments. In acidic environments, the inclusion of fibers significantly enhances the resistance to strength reduction. Furthermore, the study uncovers that a higher concentration of recycled aggregates exacerbates the reduction in strength in chloride-rich settings, emphasizing the imperative nature of meticulous mix design and material selection. The findings for the integration of even minor quantities of polyolefin fibers to amplify the performance and sustainability of concrete mixtures, especially when utilizing recycled aggregates, thus promoting eco-friendly construction practices.

Flexural Fatigue Performance of Hemp Fiber–Reinforced Concrete Using Recycled Concrete Aggregates as a Sustainable Rigid Pavement

Flexural Fatigue Performance of Hemp Fiber–Reinforced Concrete Using Recycled Concrete Aggregates as a Sustainable Rigid Pavement