Recycled Coarse Aggregate Replacement Research Articles

Mechanical characteristics of structural concrete using building rubbles as recycled coarse aggregate

Abstract The aim of this research is to investigate whether construction rubbles may be utilized as coarse aggregates in concrete. Experimentally, the slump, unit weight, compressive, tensile, and flexural strength tests were applied on concrete samples with varying percentages of recycled coarse aggregate (RCA) and compared with reference concrete produced with natural coarse aggregate. This research conducted 96 concrete samples with RCA replacement percentages of 0, 35, 65, and 100%, respectively. The control mixture produced with natural aggregate showed better results than the mixtures containing recycled aggregate; thus, compressive, tensile, and flexural strengths reduced as the amount of the recycled aggregate increased. Using 100% RCA, the compressive, tensile, and flexural strength reduction reached up to 64, 29, and 38%, respectively.

Fracture behaviors of sustainable recycled aggregate concrete under compression-shear loading: Laboratory test, numerical simulation, and environmental safety analysis

A series of compression-shear tests were carried out to investigate the shear failure of sustainable recycled aggregate concrete, and shear failure surfaces were scanned by a 3D laser scanner. The effect of recycled coarse aggregate (RCA) replacement rates and normal stresses on the shear strength, failure mode, and fracture morphology were analyzed. Meanwhile, discrete element method was used to study shear behaviors at a mesoscale. Finally, the carbon emission was calculated to evaluate the effect of RCA on the environment. The result shows that the shear strength, cohesion c, and internal friction angle φ all decrease as the RCA replacement rate increases. The shear damage of RAC can be aggravated by increasing the RCA replacement rate. Fractal dimension and joint roughness coefficient of shear failure surface tend to increase with increasing RCA replacement rate and normal stress. In addition, more microcracks can be found in coarse aggregate with the increase in RCA replacement rate, and the increasing normal stress can cause more microcracks. Given the nonlinear development of stress-displacement, a modified nonlinear constitutive model is proposed. Finally, the total carbon emissions of recycled aggregate concrete are calculated considering production, transportation, and construction, the carbon emission of 100% sustainable recycled aggregate concrete is 10.2% less than that of 100% natural aggregate concrete.

Mechanical behavior of self-compacting recycled concrete reinforced with recycled disposable medical mask fiber

The outbreak of the COVID-19 epidemic has led to a large number of waste disposable medical face masks (DMFMs) worldwide, which seriously pollute the environment and threaten human health. In this paper, waste DMFMs were recycled as mask fiber and mixed into self-compacting recycled aggregates concrete (SCRAC) to form a mask fiber reinforced self-compacting recycled aggregates concrete (FRSCRAC). The effects of recycled coarse aggregate (RCA) replacement ratio, DMFM fiber content, and length on the working and mechanical properties of FRSCRAC were investigated. The results show that the specimens meet the requirements of self-compacting concrete (SCC) in terms of working and mechanical performance, but the RCA and DMFM fiber could reduce the workability of the FRSCRAC, and the higher the amount of RCA and DMFM fiber, the more obvious the reduction. At the same time, the mechanical properties of FRSCRAC increased as the decreasing RCA replacement ratio and growing DMFM fiber content. Compared with specimens without DMFM fiber reinforcement, the compressive strength, split tensile strength, flexural strength, and elastic modulus were increased by 1.3%-9.6%, 1.46%-8.6%, 24.4%-54.69%, and 0.65%-4.73%, respectively. Moreover, the reduction of the DMFM fiber length is favorable to the mechanical properties of FRSCRAC as well. Furthermore, the X-ray CT and scanning electron microscope (SEM-EDS) techniques were applied to analyze the microstructure of typical specimens. The results showed that the DMFM fiber and surrounding mortar could form a multiphase composite, which reduces the porosity and improves interfacial stress transfer efficiency, thus enhancing the mechanical properties of FRSCRAC. In addition, based on the experimental results, the mechanical strength index of FRSCRAC and the calculation formula of its mutual conversion relationship were proposed. The research conclusions of this paper can provide a reference for the application of waste DMFM fibers in FRSCRAC.

Mesoscale-based mechanical parameters determination and compressive properties of fully recycled coarse aggregate concrete

The use of recycled coarse aggregate (RCA) to prepare concrete can reduce the depletion of natural resources, such as sand and stone, and solve the issue of environmental pollution resulting from waste concrete materials. To promote the popularization of the RCA application, a study has been carried out on fully recycled coarse aggregate concrete (FRCAC), which is 100 % RCA replacement. The random convex polygon aggregate model was established using a self-compiled program. Then, A five-phase numerical model of FRCAC was proposed through the finite element software further developed, to realize the numerical simulation of uniaxial compression of FRCAC. The discount factor R between the mechanical parameters of the FRCAC mesoscopic components was determined by comparing the 8 groups of compressive strengths from the physical experiments with the 96 groups of compressive strengths from the numerical simulations. Meanwhile, the effects of RCA volume fraction, water-cement ratio (W/C) and mesoscopic composition on the compression performance of FRCAC were analyzed based on experimental and simulation methods. The result shows: 1) Simulation results exhibited good agreement with experimental results when the interface discount factor RITZ and the old mortar discount factor RM fell within the ranges of 0.45–0.55 and 0.8–0.85, respectively. 2) The compressive strength of FRCAC tends to decrease when W/C and the volume fraction of RCA rise, but the trend of declines is not constant. 3) The compressive properties of FRCAC were found to decrease with increased thickness of the old mortar and the interfacial transition zone, highlighting the necessity for pretreatment of RCA before its use. The findings of this research provide valuable insights and methodological references for the application and promotion of FRCAC and its mesoscopic models.

Mechanical properties of multi-recycled coarse aggregate concrete, with particular emphasis on experimental and numerical assessment of shrinkage at different curing temperatures

This study investigates the behaviour of concrete manufactured with recycled coarse aggregate (RCA) derived from multiple recycling cycles, with a particular emphasis on experimental and numerical assessment of shrinkage at different curing temperatures (20, 40, and 60 °C). Four types of concretes have been produced: a natural aggregate concrete (NAC) with 100 % natural coarse aggregate (NCA) and three generations of recycled aggregate concrete (RAC) where NCA is replaced by RCA with volume percentages of 50 % and 100 % in each generation. Moreover, silica fume is used to improve the mechanical performance of the third-generation recycled concrete with 100 % RCA. In addition, the total shrinkage was predicted by combining the maturity method with finite-element analysis (FEA), using ANSYS© software. The results revealed that the multi-recycling and the replacement ratio of RCA have a detrimental impact on the mechanical properties. The total shrinkage increases considerably with both increasing curing temperature and increasing RCA replacement ratios in each generation. Furthermore, silica fume has a beneficial effect on the mechanical performance of the multi-recycled concrete; nevertheless, it causes an increase in the total shrinkage. Finally, the comparison between the experimental and numerical data shows that the FEA model, using the maturity method and two-phase serial model, can yield an accurate prediction of the total shrinkage of multi-recycled concrete.

Optimization of mechanical properties of pervious recycled concrete using polypropylene fiber and waterborne epoxy resin

Although there are many potential applications for pervious concrete (PC) in alleviating urban waterlogging, noise effect, and heat island effect, its porous nature also means that it has a lower strength than conventional concrete. This study strives to improve the mechanical properties of PC considering conventional PC has mechanical flaws. The optimum mix design for PC using recycled coarse aggregate (RCA), fly ash (FA), and polypropylene fiber (PPF) was first researched and selected based on the orthogonal design technique, and the carbon emission from the production end of raw materials was assessed; Second, the influence of waterborne epoxy resin (WER) modification on the permeability and strength of PC was examined. The results of the orthogonal experimental study demonstrated that adding RCA significantly increased the compressive strength of PC while just marginally affecting the permeability indices. To achieve more than 30 MPa compressive strength, the optimum mix design for PC incorporating RCA, FA, and PPF was established (i.e., 15% target porosity, water-binder ratio 0.31, RCA replacement rate 40%, FA content 5%, and PPF volume content 0.11%). The greenhouse gas analysis revealed that this mixture of 50% RCA and 15% FA emits 41.77% less CO2 than the control mix. The use of WER enhanced the mechanical properties of pervious recycled concrete (PRC) by first improving the pore structure at the microscopic level. The findings of the study may serve as a foundation for producing high-performance green PC.

Behaviour of steel-reinforced recycled aggregate concrete-filled GFRP tubular short columns under eccentric axial compression

Since the mechanical properties of recycled aggregate concrete (RAC) are different from those of natural aggregate concrete (NAC), the mechanical behaviour of reinforced RAC members has attracted great interest. Meanwhile, fibre-reinforced plastics (FRP) have been applied to improve the load capacity of concrete members owing to their good durability. This study primarily focused on the performance of glass FRP (GFRP) confined steel-reinforced RAC short columns (FCSRAC) under eccentric compression. In this study, 29 FCSRAC were used to understand the effects of the recycled coarse aggregate (RCA) replacement rate, strength of RAC, eccentric distance, fibre orientation of the GFRP tube, strength of the GFRP, and longitudinal reinforcement ratio on the eccentric compression behaviour. The results show that the longitudinal steel bar ratio and hoop strength of the GFRP tube play a critical role in the load capacity of the FCSRAC, especially at large eccentric distances. With improved steel bars and GFRP tubes, RAC can be used in core concrete to replace NAC in a column. In addition, a prediction model of the load capacity of the FCSRAC was proposed based on Samaan's model and the static equilibrium principle, introducing the eccentric distance, fibre orientation of the GFRP tube, and RCA replacement rate. The predicted results agree with the experimental results.

A Review of Chloride Penetration of Recycled Concrete with Enhancement Treatment and Service Life Prediction.

The application of recycled coarse aggregate (RA) in structural concrete can save non-renewable resources and reduce land occupation. Developing comprehensive knowledge of chloride penetration and service life modeling of recycled coarse aggregate concrete (RAC) is a prerequisite for practice. However, compared with the natural aggregate concrete (NAC), the inferior durability performance, especially chloride penetration resistance, of RAC hinders its application in structural concrete. Therefore, many RAC performance enhancement methods have been proposed. This paper presents a holistic review focused on the chloride penetration of RAC with/without enhancement methods and service life prediction. The current RAC performance enhancement methods are introduced. The improvement effect of the corresponding enhancement methods on the chloride penetration resistance of RAC are discussed and analyzed in turn. Based on the reviewed data on the chloride diffusion coefficient, the modification efficiencies of assorted enhancement methods are summarized. With the hope of promoting RAC application in structural concrete, the current literature on chloride-ingress-based service life prediction for RAC is also overviewed. In addition, the typical influencing factors on chloride transport properties are also discussed, i.e., RA quality. It can be concluded that enhancement techniques can effectively improve the chloride penetration resistance of RAC. The old mortar enhancement or removal methods can improve the chloride penetration resistance by 15-30%, depending on the specific treatment measures. The modification efficiency of the modifier material depends on the specific type and content of the incorporated substance, which ranges from approximately 5% to 95%. The estimated service life of RAC structures decreases with the increasing RA replacement ratio. Finally, concluding remarks are provided concerning future research on the chloride transport behavior of RAC.

Evaluation of residual bond behaviour between rebar and recycled aggregate concrete after high-temperature exposure followed by water spray cooling

This paper presents an evaluation of the residual bond behaviour between rebar and recycled aggregate concrete (RAC) using granite natural coarse aggregates and different recycled coarse aggregate (RCA) replacement levels (δ = 0%, 30%, 50%, and 100%) for both pull-out and cubic specimens. After high-temperature exposure (T = 200, 400, and 600 ℃) followed by water spray cooling, the results revealed a consistent decline in bond strength and compression strength with increasing temperature. Compared to ambient conditions, bond strength and compression strength decreased by 65.2% ∼ 77.8% and 53.2% ∼ 58.4%, respectively, after exposure to 600 ℃. The reduction rate of residual bond strength changed significantly at about 400 ℃. There was a linear relationship between RAC compression strength and rebar–RAC bond strength after high temperature exposure followed by water spray cooling. With increasing RCA replacement levels, the relative bond strength gradually improved at the same temperature. Based on the regression analysis, empirical formula for the rebar–RAC bond strength and bond–slip relationship models were proposed, all taking into account RCA replacement levels and temperature.

Study on the mechanical properties and deterioration mechanism of recycled aggregate concrete for low-grade highway pavements

This study is motivated by the imperative for sustainable construction practices, specifically focusing on recycling construction waste into low-grade highway pavements to enhance environmental sustainability and resource efficiency. The investigation explores the workability, mechanical properties, and interfacial transition zones (ITZs) of recycled aggregate concrete (RAC), emphasizing the incorporation of recycled coarse aggregate (RCA) from construction waste. The primary objective is to delineate the optimal mixture proportions of RAC that effectively balance resource utilization with the technical requisites for low-grade highway pavements. Through rigorous orthogonal testing and analysis, a mixture was identified that achieves a high RCA replacement rate while preserving workability and mechanical properties. Additionally, nanoindentation tests were employed to probe the nanomechanical properties of the ITZs in RAC, aiming to elucidate the underlying degradation mechanisms. Results reveal that the porous and rough surface of RCA significantly impacts the properties of RAC, with a marked decrease in elastic modulus and an increase in ITZ width identified as primary contributors to mechanical deterioration. Consequently, this study not only provides crucial insights for the application of RAC in low-grade highway pavements but also sets the stage for future research aimed at enhancing the sustainability and performance of recycled construction materials.

Experimental study on dynamic properties of flax fiber reinforced recycled aggregate concrete

Recycling waste concrete is critical to land use and reduction of landfills. It is a good option to recycle the waste coarse aggregates from it and add them to the concrete matrix. Due to the existence of new and old interface transition zones (ITZs), the quasi-static compressive strength of recycled aggregate concrete is lower than that of natural aggregate concrete, which limits the widespread application of recycled aggregate concrete. In order to improve its engineering application of recycled aggregate concrete, the dynamic compressive properties were studied. Thus, based on a Split Hopkinson Pressure Bar (SHPB) device, the effects of strain rate and recycled coarse aggregate (RCA) replacement ratio on the failure mode, dynamic compressive strength, critical strain and toughness were investigated. It was found that increasing the strain rate led to the aggravation of the specimen damage and growth of the dynamic compressive strength, toughness and critical strain. At a high strain rate, the strain rate effect changed the failure mode of recycled aggregate concrete. This reduced the performance difference between recycled and natural aggregate concrete, in terms of the dynamic compressive strength. The recycled aggregate concrete had a similar toughness to the natural aggregate concrete. Although the growth of the RCA replacement ratio decreased the dynamic compressive strength and increased the critical strain, it had a little effect on the toughness. In order to reduce the brittleness of the recycled aggregate concrete and improve its deformability, non-synthetic, green and low-consumption flax fibers were incorporated into the concrete matrix, and its dynamic compressive behaviors were studied experimentally accordingly. Tests verified that the addition of flax fibers significantly reduced the damage and increased the critical strain. Especially when the strain rate was greater than 200 s-1, flax fiber reinforced recycled aggregate concrete exhibited a superior energy consumption ability. Based on the experimental data, empirical equations were proposed to describe the dynamic compressive strength, critical strain and toughness at different strain rates.

Effect of carbonated recycled coarse aggregates on the mechanical properties of 3D printed recycled concrete

A modification method of carbonation combined with presoaking in nano-SiO2 solution was employed to enhance the recycled coarse aggregates (RCA). The water absorption, apparent density and crushing index of RCA before and after carbonation were tested. The water absorption, compressive strength, flexural strength and splitting tensile strength of 3D printed recycled aggregate concrete (3DPRAC) with different RCA replacement rates were further investigated. Microscopic tests including X-ray diffraction and scanning electron microscopy were performed to analyze the changes in phase composition and microstructure of RCA, while X-ray computed tomography was utilized to explore the pore defect evolution of 3DPRAC. The results demonstrated the method of carbonation combined with presoaking in nano-SiO2 solution significantly improved the physical characteristics of RCA and densified the microstructure of 3DPRAC. With an increase in RCA replacement rate, there was a decrease in the mechanical properties in all directions. At the RCA replacement rate of 100 %, the compressive strength, flexural strength and splitting tensile strength increased by a maximum of 24.71 % in the X direction, 12.95 % in the Y direction and 18.27 % in the Z direction after RCA carbonation, respectively. The pore defects of 3DPRAC were effectively optimized by the inclusion of carbonated RCA. The modification mechanism was explained based on the enhanced performance of RCA and pore defect evolution.

Study on Mechanical Properties and Erosion Resistance of Self-Compacting Concrete with Different Replacement Rates of Recycled Coarse Aggregates under Dry and Wet Cycles

Concrete that self-compacts is frequently utilized in engineering construction. Recycled coarse aggregate self-compacting concrete (RCASCC) is made by partially substituting recycled coarse aggregates (RCA) for natural coarse aggregates in order to conserve construction resources. This study examines the impact of linked sulfate erosion, dry and wet cycles, and RCA replacement rates of 0%, 25%, 50%, 75%, and 100% on the mechanical properties and durability of RCASCC. By using the mass loss rate, relative dynamic elastic modulus, corrosion resistance factor, X-ray diffraction (XRD), scanning electron microscope (SEM), and atomic force microscope (AFM) analyses, as well as other macroscopic and microscopic methods, it is possible to examine the deterioration patterns of RCASCC under dry and wet cycles. The results demonstrate that the addition of RCA has a notable impact on concrete’s resistance to sulfate attack during both dry and wet cycles. The erosion products steadily rise, the interfacial transition zone (ITZ) becomes rougher, and the sulfate resistance falls as the replacement rate of RCA rises. According to the findings of SiO2, AFt, and CaCO3, the examination of corrosion products from XRD and microstructure from SEM and EDS is carried out. The old mortar that has adhered to the surface of RCA, as shown by the AFM analysis of ITZ and the SEM analysis of RCA, can significantly affect the roughness of ITZ inside RCASCC.

Machine learning approach for investigating compressive strength of self-compacting concrete containing supplementary cementitious materials and recycled aggregate

Supplementary cementitious materials (SCMs) and recycled coarse aggregate (RCA) have the potential for sustainable development and resource utilization and have been widely applied in self-compacting concrete (SCC). However, traditional experiment approaches for predicting its compressive strength suffer from inefficiency, time-consuming, and high cost. To address these issues, machine learning (ML) models are employed, evaluated, and compared in this paper. First, a database of 337 samples with 10 features is collected and established. Then, the 12 ML models, including SVM, KNN, DT, RF, ANN, GBoost, XGBoost, AdaBoost, CatBoost, LightGBM, HistGBM, and GEP are applied for modeling to predict the compressive strength. Finally, both feature importance, individual conditional expectation, and partial dependence plot are utilized to assess the importance and the effect of each and coupled parameters. The results show that the CatBoost and LightGBM models generally possess superior performance in various aspects, including higher R2 values, lower RMSE, MSE, and MAE values, as well as smaller errors. Among the Boosting models, except for the AdaBoost model, satisfactory predictive ability is observed. The ANN and RF models also demonstrate relatively good performance, and the SVM, DT, KNN, and GEP models present relatively poorer performance. Curing age and W/CMs exhibit the most pivotal features on model prediction. Furthermore, RCA replacement ratio plays a more prominent role than CA, and SCMs contribute somewhat to the compressive strength of concrete, but not as much compared to cement. Also, it is important to acknowledge the significant contribution of SP. This study offers a systematic evaluation of the prediction of compressive strength in SCC containing SCMs and RCA, making a notable contribution to both the existing literature and practical applications.

Optimization on the mix design method of self-compacting concrete with recycled coarse aggregate based on paste rheological threshold theory and material packing characteristics

This study aims to use the mixture design method to optimize self-compacting concrete (SCC) and manufacture SCC with excellent workability using recycled coarse aggregate (RCA) and a ternary cementitious system. The influences of incorporated RCA on the rheological thresholds of SCC with a fixed content of fly ash (FA) and blast furnace slag (BFS) are investigated based on paste rheological threshold theory and material packing characteristics. The original rheological model is improved by introducing the volume of adhesive mortar and modifying the equivalent packing density and mortar film thickness. The improved model is incorporated into the original theoretical system to apply the optimized mix design method to SCC. Moreover, experiments are conducted with different volumes of water, water-powder ratios and RCA replacement levels to verify the applicability of the optimized method. The results indicate that the proposed models can accurately predict the yield strength threshold and plastic viscosity threshold of fresh SCC. Furthermore, compared to the original model, predictions are 53% more accurate with the modified model.

Frost Durability of Self-Compacting Concrete Prepared with Aeolian Sand and Recycled Coarse Aggregate.

Aeolian sand (AS) and recycled coarse aggregate (RCA) can be reasonably utilized as green materials for concrete modification. The paucity of natural sand and gravel in the construction industry is anticipated to be remedied by the use of these two eco-friendly concrete ingredients. This is incredibly important for environmental protection. Study on the damage law of self-compacting concrete with the addition of AS and RCA (ARSCC) under severely cold conditions is of great significance for the promotion and implementation of this material. In this study, 12 groups of ARSCC specimens were prepared for freeze-thaw cycle experiments, with AS substitution rates of 0, 20%, 40%, and 60% as well as RCA replacement rates of 0, 25%, and 50%. Then, the degradation mechanism of ARSCC freeze-thaw damage was discussed from both macroscopic and microscopic perspectives via mass loss rate (Wn), relative dynamic modulus of elasticity (Pn), bubble spacing factor, and SEM analysis. Finally, the response surface method was utilized to determine the damage variable. A freeze-thaw damage model for ARSCC was developed based on the Weibull distribution and Grey theories. The results showed that the Pn could reflect the evolution law of the internal structure of ARSCC. Appropriate addition of AS to fill the large, harmful pores in RCA would inhibit freeze-thaw damage of ARSCC. The optimum substitution rates of AS and RCA were determined to be 20-40% and 25-50%, respectively. In addition, the values obtained from theoretical damage modeling and experiments were in good agreement. The acquired damage model had the potential to predict ARSCC damage under freeze-thaw cycles.

Flexural and bond-slip responses of reinforced concrete beams containing recycled coarse aggregate and polypropylene plastic

Reusing non-biodegradable plastic waste and recycled concrete as a substitute for natural aggregates can help alleviate the pressure on natural resources. Therefore, this study aims to incorporate polypropylene plastic aggregate (PPA) and recycled coarse aggregate (RCA) in concrete mixtures. The PPA is used as a partial replacement of RCA in reinforced concrete (RC) beams. The flexural responses and bond behaviors of RCA based PPA concrete (RPC) beams are investigated. The main variables are PPA percentages (0, 5, and 10%) and shear span to effective depth ratios (a/d = 2.13, 1.52, 1.07). Nine 1100 mm long RC beams are fabricated for bending tests, and nine 350 mm long RC beams are cast for pull-out tests. Results indicate that the ultimate moment capacity is increased by up to 28.6% for 5% PPA content depending on the beam depths, and for 10% PPA content, it is decreased by up to 15%. Ductility and toughness are increased by up to 11.7% and 113.6%, depending on the PPA contents and beam depths. Bond strength is increased by up to 16.6% for 5% PPA content and then decreased for 10% PPA by up to 37.5% depending on different beam depths. It is recommended that 5% PPA be used in concrete to improve the overall performance of the RPC beams.

Simultaneous influence of processed cellulose acetate fiber reinforcement and recycled aggregate replacement on mechanical and durability performances of concrete

The poor performance of virgin cement concrete and its significant carbon footprint have prompted researchers to explore the concurrent integration of recycled coarse aggregates (RCA) or recycled fine aggregates (RFA) along with fibers. This study aims to investigate the mechanical and durability performance of various cement concrete mixes prepared using 53-grade cement with various percentages (0%, 30%, 50%, 70%, and 100%) integration of RCA or RFA and varying volume fractions (0.5%, 1%, 1.5%, 2%, and 2.5%) of processed Cellulose Acetate Fiber (CAF) derived from waste cigarette filters. RCA50CAF1.5 exhibits highest compressive strength and flexural strength among all RCA based concrete mix. RFA30CAF1.5 possess maximum tensile strength by about 3.45% higher compared to reference mix. The increase in CAF with % replacement of RCA or RFA cause decrease in the short term durability performance of the concrete mix. The developed correlations for mechanical and durability performances can serve as a valuable tool for designing concrete mixes with improved mechanical and durability performances for CAF-reinforced concrete.

Cyclic compressive behavior of hook-end steel and macro-polypropylene hybrid fiber reinforced recycled aggregate concrete

One widely accepted strategy is to add fiber into recycled aggregate concrete (RAC) to improve compressive strength. Few existing investigations have focused on the cyclic compressive behavior of hybrid fiber reinforced recycled aggregate concrete (HFRRAC). A total of eighteen groups of prismatic specimens with different fiber combinations and recycled coarse aggregate (RCA) replacement rates were conducted under cyclic compression to investigate the mechanical properties of hooked-end steel and macro-polypropylene hybrid fiber reinforced recycled aggregate concrete. Results indicate that specimens under cyclic loading fail similarly to those subjected to monotonic loading. The envelope curve of the cyclic stress-strain curve for HFRRAC closely resembles the corresponding monotonic stress-strain curve. RCA replacement rates and fiber combinations do not significantly affect plastic strain and stress deterioration. Fibers added to specimens mitigate stiffness degradation and improve hysteretic energy dissipation. The proposed constitutive equations can be used to describe the cyclic compressive behavior of HFRRAC. The fibers improve the density and structure of mortar and aggregate-paste interfaces from the microstructure morphologies. The fracture mechanisms of HFRRAC were also studied using mesoscale numerical models under uniaxial compression loading. The cohesive zone model is suitable for simulating the compressive behavior of HFRRAC. Numerical specimens fail in mixed fracture modes, primarily mode II fractures.

Mechanical properties of recycled aggregate concrete prepared from waste concrete treated at high temperature

The ever-increasing generation of construction and demolition (C&D) waste caused by the frequent occurrence of building fires has posed a threat to environmental protection and waste management over the last few decades. The shortage of natural resources provides an impetus for the utilization of C&D waste after fire. Aiming to provide a cleaner option for waste concrete after fire, the viability of using waste concrete after fire as a substitute for coarse aggregate in the production of recycled aggregate concrete (RAC) was investigated and analyzed in detail. The failure mode, stress–strain behavior, mechanical properties and microstructure of the RAC was evaluated with the variation of four different replacement ratios (0%, 25%, 50%, and 100%) and five different treatment temperatures (20 °C, 200 °C, 400 °C, 600 °C, and 800 °C). The experimental results showed despite the failure mode of RAC being similar to that of natural concrete (NC), an increase in the recycled coarse aggregate (RCA) replacement ratio would decrease the slope of curves and strength, and the fcu and fc with 25% replacement ratio for RAC decrease by about 15% and 10% respectively as compared to those for NC. Nevertheless, treatment of elevated temperature for waste concrete can eliminate the damage of replacement ratio to mechanical properties, and the fcu and fc with 25% replacement ratio after 400 °C increased by approximately 12% and 8% respectively as compared with untreated RAC. Against the test result, a series of calculation formulas is proposed to predict the mechanical properties of the RAC whilst a constitutive model is modified with the existing standard to describe the stress–strain behavior of that with the consideration of replacement ratio and treatment temperature. The microstructure analysis indicated that the increase in the RCA replacement ratio would result in the increasing content of adhered mortar characterized by numerous poles and less bond on RCA, whereas treatment of high temperature for waste concrete can decrease the content of adhered mortar on RCA, which makes the microstructure of the RAC more densified.