Recycled Coarse Aggregate Research Articles

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

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

Effect of recycled coarse aggregate model arrangement mode on the axial compressive performance of MRCAC

The paper presents the results of studies on based the universal morphology of recycled coarse aggregates in reality, an idealized model of recycled coarse aggregates was prepared using stones. Axial compressive strength tests were conducted on prism-shaped specimens of Modeled Recycled Coarse Aggregate Concrete (MRCAC), which were prepared with different old cement mortar coverages and aggregate arrangements. Through comparison with experimental data, the reliability of the finite element analysis model was determined. It was found that the arrangement of MRCA had a significant impact on the failure characteristics and elastic modulus of MRCAC. Thus, different arrangement modes affected the stress distribution within the specimens, resulting in significantly different axial compressive strength.

Experimental investigations on MICP-based autonomous crack-healing recycled aggregate concrete: a sustainable approach for enhancing the strength and durability

ABSTRACT Cracks in concrete, including recycled aggregate concrete (RAC), lead to durability issues from substance penetration. This study introduces microbial precipitation for crack healing in RAC. It substitutes 50% and 100% of natural coarse aggregate with recycled coarse aggregate and replaces cement with blends of hydrated lime and brick powder. Self-healing effectiveness is assessed through compressive strength, RCPT, UPV, and microscopic analysis. Results show complete healing of 0.58 mm cracks over 56 days, with compressive strength regain ranging from 94% to 56%. RCPT variation ratios range from 142% to 173%. Microstructural analysis through FESEM, EDS, and XRD reveals microbial precipitate properties. FTIR and TGA analyses explore CaCO3 through infrared absorbance and mass variation with temperature. The study suggests that microbial-induced self-healing improves strength and durability in recycled aggregate concrete.

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.

Comparative Study of the Crushing Strengths of Recycled Coarse Aggregates Concretes and Natural Aggregates Concretes

Recycled Coarse Aggregate (RCA) is a recycled concrete material from construction sites, which is being reused for fresh concrete production. The use of this material reduces quarrying of Natural Coarse Aggregate (NCA). Properties of RCA are variable depending on the properties of parent concrete. In a fast-growing economy, the generation of construction waste has increased considerably, and social and environmental concerns on recycling the waste have accordingly increased. Construction Engineers have resorted to recycling these demolition wastes to maintain a friendly green environment. This study compared the crushing strengths of Recycled Coarse Aggregate concrete with Natural Aggregate concretes. The RCA was obtained from demolished drains and culverts while the Natural Coarse Granite Aggregate was from Akamkpa, Cross River State, Nigeria. Using RCA and NCA of maximum size 20mm, concrete cubes were prepared for compressive strength tests. Other properties of the aggregates were determined. Three mix proportions of 1:1½:3, 1:2:4 and 1:3:6 at water cement ratio of 0.6 were utilized and the specimens were tested at curing ages of 7, 14, 21 and 28 days. Results indicated that RCA produced concretes with strengths lower than those using NCA. RCA concretes also had slumps lower than those of NCA concretes.

Experimental study on the triaxial compression mechanical performance of basalt fiber-reinforced recycled aggregate concrete after exposure to high temperature

The use of basalt fiber-reinforced recycled aggregate concrete (BFRRC) can effectively treat waste concrete and reduce the consumption of natural resources. BFRRC is often in complex triaxial compression in engineering applications and may exposure to high temperature conditions. To evaluate the conventional triaxial compression mechanical performance of BFRRC after exposure to high temperature, 324 cylindrical specimens were produced. The specimens were designed and fabricated by varying several experimental parameters: the recycled coarse aggregate replacement ratio, basalt fiber dosage, temperature, and lateral confining pressure. The specimens previously exposed to high temperature were subjected to conventional triaxial compression tests and the effects of various experimental parameters on the mechanical performance indexes of BFRRC were analyzed. The results show that the peak stress and elastic modulus of BFRRC all raise with greater fiber dosage and confining pressure but decrease with higher replacement ratio and temperature. The maximum increase of the peak stress and elastic modulus is 352.45% and 224.59%, respectively. The peak strain of BFRRC has two variation trends of gradual increase and gradual decrease with the replacement ratio rising. And the peak strain gradually grows with the temperature and confining pressure increasing. The fiber dosage, however, does not exert the considerable impact on the peak strain. The maximum increase of the peak strain is 132.22%. The research provides a scientific basis for the triaxial mechanical performance, fire resistance design, and post-disaster assessment of BFRRC, and promotes the popularization and application of BFRRC in building structures.

Study on the calcium dissolution of self-compacting recycled concrete

Considering the effects of the replacement rate of recycled coarse aggregate (RCA), content of fly ash (FA), and solution temperature, the accelerated calcium dissolution test of self-compacting recycled concrete (SCRC) was simulated by using ammonium chloride solution. The results show that the physical properties (the mass loss ratio and the dissolution depth) and the strength degradation of SCRC increase with the increase of the replacement rate of RCA and solution temperature. With the increasing content of FA, the physical properties decrease approximately linearly, while the strength degradation first decreases and then increases. For the SCRC with 50% RCA and a dissolution age of 56 days, the dissolution depth of SCRC with the FA contents of 10%, 20%, and 30% decreased by 3.85%, 10.00%, and 15.56% respectively compared with that of SCRC without FA, and the degradation rate of compressive strength also decreased by 11.60%, 25.91%, and 15.01%, respectively. The peak stress of the stress-strain curve of SCRC under axial compression is negatively correlated with the dissolution age, replacement rate of RCA, and solution temperature, while the peak strain is positively correlated with these variables. When the dissolution age is more than 28 days, the calcium silicate hydrate (C-S-H) gels produced by low-volume (10%, 20%) FA after complete hydration lead to an increase in peak stress and a decrease in peak strain of SCRC, while high-volume (30%) FA has the opposite effect due to incomplete hydration. Finally, the performance degradation model of SCRC under calcium dissolution was established.

Effect of surface treatment by mineral admixture-magnesium potassium phosphate cement on the mechanical properties, durability, and microstructure of recycled aggregate concrete

Magnesium potassium phosphate cement (MKPC), a clinker-free cement, was used as the recycled coarse aggregate (RCA) surface strengthening material to enhance the mechanical properties and durability of recycled aggregate concrete (RAC) under two surface treatment methods. The effect of strengthening slurry properties on RAC reinforcement was investigated by adding different types and amounts of mineral admixtures (MA). An interface model was prepared to detect the microscopic characteristics of interfaces. The results indicated that all MA-MKPC slurries improved the RAC macroscopic performances, attributed to the fact that MA-MKPC slurries effectively improved the RCA surface pore structure and promoted a secure connection with RCA or new mortar. MA-MKPC slurry properties significantly affected the performances of modified RAC, and the MA optimal dosage of 10 % was found. Furthermore, the pre-coating without pre-curing method outperformed the pre-coating plus pre-curing method, forming an enhanced new interfacial transition zone within the RAC.

Machine Learning Modeling Integrating Experimental Analysis for Predicting Compressive Strength of Concrete Containing Different Industrial Byproducts

This study aimed to develop accurate models for estimating the compressive strength (CS) of concrete using a combination of experimental testing and different machine learning (ML) approaches: baseline regression models, boosting model, bagging model, tree-based ensemble models, and average voting regression (VR). The research utilized an extensive experimental dataset with 14 input variables, including cement, limestone powder, fly ash, granulated glass blast furnace slag, silica fume, rice husk ash, marble powder, brick powder, coarse aggregate, fine aggregate, recycled coarse aggregate, water, superplasticizer, and voids in mineral aggregate. To evaluate the performance of each ML model, five metrics were used: mean absolute error (MAE), mean squared error (MSE), root mean squared error (RMSE), coefficient of determination (R2-score), and relative root mean squared error (RRMSE). The comparative analysis revealed that the VR model exhibited the highest effectiveness, displaying a strong correlation between actual and estimated outcomes. The boosting, bagging, and VR models achieved impressive R2-scores in the range of 86.69%–92.43%, with MAE ranging from 3.87 to 4.87, MSE from 21.74 to 38.37, RMSE from 4.66 to 4.87, and RRMSE between 8% and 11%. Particularly, the VR model outperformed all other models with the highest R2-score (92.43%) and the lowest error rate. The developed models demonstrated excellent generalization and prediction capabilities, providing valuable tools for practitioners, researchers, and designers to efficiently evaluate the CS of concrete. By mitigating environmental vulnerabilities and associated impacts, this research can significantly contribute to enhancing the quality and sustainability of concrete construction practices.

Effect of Silica Fume on Some Properties of No-Fine Concrete with Recycled Coarse Aggregate

No-fines Concrete is a lightweight porous concrete produced by omitting sand from the traditional concrete mix. It helps replenish the groundwater aquifer by directly allowing precipitation to seep into the earth thanks to its wide pores. This work studied the influence of partially replacing cement with silica fume (SF) with percentages of 5, 8, and 10% on some properties of no-fine concrete made using recycled coarse aggregate. The natural coarse aggregate (NA) was replaced with 10%, 20%, and 30% by crushed reactive powder concrete waste volume as recycled aggregate (RA). The optimum percentage was 10% RA with 10% SF, which indicated a substantial improvement in the no-fine concrete's strength, dry density, and water absorption compared to the design mix. The results exhibit increases in percentages in compressive strength, tensile strength, flexural strength, and dry density by 18%, 14%, 16%, and 1%, respectively, while exhibiting a decrease in water absorption by 5% at 28 days.

Sulphate resistance prediction of RAC considering the coupling effect of RCA and water-binder ratio based on experimental analysis and LSSVM

ABSTRACT Waste concrete removed from old pavement can be crushed and sieved as recycled aggregate to prepare concrete, and the application of recycled aggregate concrete (RAC) can reduce the occupation of land resources by the waste concrete and save natural aggregate. In this paper, the influence of proportion of recycled coarse aggregate (RCA) replacing natural coarse aggregate (NCA) and water-binder ratio on the sulphate resistance of RAC was studied, and the compressive strength degradation rate (Df ) was introduced to evaluate the sulphate resistance of RAC. The experimental results showed that the compressive strength of RAC before and after sulphate attacks linearly decreased with the growth of the replacement rate of RCA, water-binder ratio, and the number of sulphate wet-dry cycles, respectively; while Df of RAC increased with the rise of the replacement rate of RCA, water-binder ratio, and sulphate wet-dry cycles, respectively. The results also indicated that Df of all groups of RAC was less than 25%, which can meet the engineering requirements. In addition, the Df model of RAC under a sulphate attack based on experimental data and that based on Least-Squares Support Vector Machine (LSSVM) was built and compared, and both models predict the sulphate resistance of RAC well. The prediction model based on LSSVM has higher prediction accuracy.

A complete thermo-poromechanical model for freeze-thaw modification of recycled coarse aggregate considering deterioration of each component

Freeze-thaw modification is a novel method that improves recycled aggregates by utilizing the expansion of pore water during phase transitions. However, there is still a lack of theoretical analysis regarding each component in recycled aggregates during freeze-thaw. In this paper, we introduce the parameter of pore radius to transform the traditional 2D phase diagram into a 3D phase diagram of pore water. This transformation makes the phase transition conditions of pore water more clear to understand. It is then combined with the pore size distribution of each component in recycled aggregate to calculate the ice content and freezing pressure. To achieve a more intuitive evaluation of frost susceptibility, this paper establishes a frost vulnerability index (FVI). Based on the FVI curves of different rocks, their frost resistance is categorized into three levels. Finally, according to the FVI of recycled aggregates with diverse material properties, some recommendations are provided for the engineering application of freeze-thaw modification.

Experimental and numerical investigation of preloaded recycled concrete beams strengthened with CFRP

Purpose The use of recycled coarse aggregate in concrete structures promotes environmental sustainability; however, performance of these structures might be negatively impacted when it is used as a replacement to traditional aggregate. This paper aims to simulate recycled concrete beams strengthened with carbon fiber-reinforced polymer (CFRP), to advance the modeling and use of recycled concrete structures. Design/methodology/approach To investigate the performance of beams with recycled coarse aggregate concrete (RCAC), finite element models (FEMs) were developed to simulate 12 preloaded RCAC beams, strengthened with two CFRP strengthening schemes. Details of the modeling are provided including the material models, boundary conditions, applied loads, analysis solver, mesh analysis and computational efficiency. Findings Using FEM, a parametric study was carried out to assess the influence of CFRP thickness on the strengthening efficiency. The FEM provided results in good agreement with those from the experiments with differences and standard deviation not exceeding 11.1% and 3.1%, respectively. It was found that increasing the CFRP laminate thickness improved the load-carrying capacity of the strengthened beams. Originality/value The developed models simulate the preloading and loading up to failure with/without CFRP strengthening for the investigated beams. Moreover, the models were validated against the experimental results of 12 beams in terms of crack pattern as well as load, deflection and strain.

Properties of fiber incorporated concrete blocks manufactured using recycled aggregates

The construction and demolition industry generates a significant quantity of concrete waste, presenting an environmental challenge. The concrete waste generated can be processed to produce Recycled Aggregates (RA) of various sizes. Utilization of Recycled Aggregates (RA) as a substitute to conventional aggregates in concrete has captured considerable attention in the past few years, owing to its promising environmental and economic advantages. However, the combined utilization of recycled fine and coarse aggregate in the production of concrete for low-strength application has not been adequately explored. In this article, an attempt is made to investigate the characteristics of concrete blocks made with RA and polypropylene fiber (PF) are investigated for different cement content. Cement and PF content varied from 8 to 12% and 0% to 2% respectively in production of concrete blocks using Recycled Fine Aggregates (RFA) and Recycled Coarse Aggregates (RCA) at different replacement intervals. Water absorption of blocks manufactured across all replacement intervals of RA was less than 10%. Blocks containing 75% RFA and 25% RCA resulted in improved compressive strength of the order more than 3.8 MPa. Rate of improvement in compressive strength of block was 11% to 20% and 6.5% to 8.2% when the fiber dosage was increased from 0.5% to 1% and 1% to 2% respectively. The optimal fiber dosage was found to be 1%, beyond which no notable improvement in mechanical properties of blocks was observed. Use of RA in concrete blocks reduced embodied energy by 19% to 24% for varying cement content from 8 – 12%. Cost of blocks was found to be reduced by 10 – 15% when made with PF dosage of 0 to 2% with 8% cement content.

Study on fracture characteristics and fracture mechanism of fully recycled aggregate concrete using AE and DIC techniques

Fracture behaviors of manufactured sand-recycled coarse aggregate concrete (MSRCAC), recycled fine aggregate concrete (RFAC), and fully recycled aggregate concrete (FRAC) were investigated by performing three-point bending tests on single-edge notched beams using acoustic emission (AE) and digital image correlation (DIC) techniques. The effects of recycled fine aggregate (RFA), recycled coarse aggregate (RCA), and their interaction on fracture parameters, AE characteristics and fracture process zone (FPZ) of concrete were comparatively analyzed. The results indicated that it was mainly transgranular failure and intergranular failure for RFAC, while transgranular failure for MSRCAC and FRAC. The RFA and RCA reduced the initial fracture toughness, unstable fracture toughness and fracture energy of concrete, and their interaction resulted in poor fracture properties of FRAC. In addition, the RFA and RCA significantly increased the AE activities, the proportion of tensile cracks, and the high-frequency amplitude and peak frequency of concrete. The toughening effect of FPZ was weakened by the RFA and RCA, resulting in an obvious increase in FPZ length and damage propagation of FRAC. Meanwhile, the width of FPZ was reduced, the maximum FPZ width of concrete with different combinations of recycled aggregates was approximately 1.8–3.1 times the maximum aggregate size, which could be explained as the RFA and RCA reduced the aggregate interlocking effect of concrete, while the RCA had a greater effect on the damage mechanism and FPZ of concrete than the RFA.

Experimental Study on Preparation and Characteristics of Concrete Modified by Construction Waste

With the advancement of urbanization construction, the proportion of construction waste to the total urban waste continues to increase, especially waste concrete. The treatment and reuse of waste concrete is a major trend that poses enormous pressure on environmental protection. This article focuses on the problems in the preparation of recycled aggregates from waste concrete, which has important practical value. This article presents a new type of recycled concrete prepared through surface modification of recycled coarse aggregate and design experiments to change the replacement rate of coarse aggregate. The physical properties of recycled coarse aggregate, workability of fresh concrete, and mechanical properties of recycled concrete are analyzed. The research results indicate the following: (1) Through surface modification, recycled concrete can improve the workability of fresh concrete at a fixed water cement ratio, which can meet the requirements of mixing, transportation, and pouring of fresh concrete. (2) By modifying the surface of recycled aggregates, the strong water absorption performance of recycled aggregates caused by old mortar and surface defects has been reduced. And the modification effect of recycled aggregate improves the hydration process of recycled concrete, making the surface structure dense and further enhancing the strength of recycled concrete. (3) The compressive strength of recycled concrete specifications from construction waste shows a decreasing trend with the increase in coat aggregate replacement rate. The final ratio is as follows: modular dose of 12%, modification time of 90 min, and 20% recycled aggregate content.

FEATURES OF USING CRUSHED CONCRETE AS A COARSE AGGREGATE FOR CONCRETE

Problem statement. According to the results of a study by the Kyiv School of Economics as of June 2023 the total amount of direct documented damage to residential and non-residential infrastructure due to destruction and damage as a result of military actions exceeded $150.5 billion. During the post-war reconstruction Ukraine will face the problem of a large number of buildings and structures that will be subject to partial dismantling and demolition. This process will be accompanied by the generation of a significant amount of construction waste, which traditionally in Ukraine is taken to landfills without recycling. On the other hand, there will be a need for a large quantity of construction materials, the share of which in the construction cost may reach 50 %. To reduce the cost of objects, it is advisable to reuse materials from waste generated after dismantling. Such materials are called recycling. The most obvious option is the use of crushed concrete waste as a coarse aggregate for the production of new concrete. The purpose of the article. Identification of the features of recycled coarse aggregates from concrete waste and concrete with their use, determination of the influence of source concrete on the properties of recycled aggregates. Conclusions. The mesoscopic model of concrete with recycled coarse aggregate has been examined. The analysis of the current research results on both recycled concrete aggregates and concrete with their use showed that, in general, with correct calculations of concrete mix compositions that consider the actual physical and mechanical properties of recycled aggregates, the production of structural concrete is possible. At the same time, crushing methods and the grain composition of such aggregates have a significant influence. In order to standardize the approaches to the research of recycled aggregates, it is necessary to create an appropriate methodology or regulatory framework.

Behavior of large-scale LRS FRP-confined steel-reinforced recycled aggregate concrete square columns under axial compression

The combination of environmentally friendly large rupture strain fiber-reinforced polymers (LRS FRPs) and recycled fine and coarse aggregates presents a promising alternative to overcome the scarcity of natural river sand and natural gravels. Recycled aggregate concrete (RAC) can obtain sufficient ductility and strength by the confinement of LRS FRPs, and these two green materials are particularly suited to sustainable construction. However, existing research on LRS FRP-confined RAC is very limited and primarily focused on small-scale specimens. The objective of this paper is to investigate the axial compression performance of large-scale steel-reinforced RAC square columns wrapped with LRS FRP and promote the practical application of this sustainable structural system. The experimental variables include the effective confinement stiffness ratio (ρk,eff) and the type of LRS FRP (i.e., polyethylene naphthalate (PEN) FRP and polyethylene terephthalate (PET) FRP). Test results showed that the application of RAC had a negligible effect on the axial compression behavior of large-scale LRS FRP-confined square columns. The behavior of the test columns was primarily influenced by ρk,eff. Finally, evaluations were conducted on the existing models of LRS FRP-confined concrete with square cross-sections, based on the compilation of available test results from large-scale specimens.

A Brief Review on Recycled Aggregate Concrete

This paper briefly introduces the recycling and regeneration preparation of waste concrete, as well as the crushing and screening process of waste concrete blocks. The properties of crushed aggregate and the physical and mechanical properties of recycled concrete were analyzed, and the research status of compressive strength and splitting tensile strength of waste concrete at home and abroad was summarized and analyzed. Factors such as water-glue ratio, water absorption, apparent density of recycled coarse aggregate, state of recycled coarse aggregate and strength of virgin concrete will affect the strength of recycled aggregate concrete. This paper also summarizes the relationship between the splitting tensile strength of recycled aggregate concrete and the compressive strength of the cube.