Strength Of Recycled Aggregate Concrete Research Articles

Non-destructive evaluation of residual strength distribution of recycled aggregate concrete and bond strength between recycled aggregate concrete and deformed bar with different cover thicknesses after freeze-thaw cycles

The thickness of a cover significantly influences both the bonding performance and the durability of materials against frost resistance. This research investigates the degradation and damage distribution of fundamental mechanical properties in two specimens of recycled aggregate concrete (RAC), each with varying sizes, following freeze-thaw cycles (FTCs). This comparison is facilitated through non-destructive and damage tests. Subsequently, the influence of freeze-thaw damage (FTD) on the bond strength of specimens with differing cover thicknesses is examined via pull-out tests on 14 groups of center pull-out specimens. Lastly, the bond strength is theoretically analyzed. The test results reveal that FTCs diminish the material strength of RAC, and the FTD distribution within RAC specimens is not uniform. The surface concrete sustains a higher degree of damage compared to the internal concrete, resulting in an average FTD of small-size specimens that surpasses that of large-size specimens. The residual strength of RAC can be assessed using a non-destructive test, specifically the ultrasonic wave velocity test. For center pull-out specimens, FTD not only decreases the material strength but also increases the wedge splitting angle while reducing the critical protective layer thickness. This paper proposes a soft sleeve correction theory that comprehensively accounts for the effects of FTD. When combined with non-destructive test results—namely those from the ultrasonic wave velocity test—this theory can effectively evaluate the bond strength of RAC post-FTD. This approach offers valuable insights for the design and assessment of recycled concrete in cold climates.

Strength and durability performance of acid-mechanically treated recycled aggregate concrete with metakaolin, lime powder, and A-fine

This study examines the effectiveness of lime powder (LP) and metakaolin (MK) in concrete containing acid-mechanically treated recycled concrete aggregate (RCA). The acid-mechanically produced recycled fine, i.e., A-Fine (AF), is also evaluated as a supplementary cementitious material (SCM). According to the strength activity index (SAI) for AF, it has a low pozzolanic reactivity in comparison to MK and a higher pozzolanic reactivity in comparison to LP. An optimal dosage of AF (15 %), LP (17 %), and MK (22 %) improves the physical properties of recycled aggregate concrete (RAC). The compressive strength of RAC with AF (RAC-AF) is closer to that of RAC with LP (RAC-LP), and RAC with MK (RAC-MK). As a result of their partial or complete chemical inertness, AF, LP, or MK decrease the self-cementing capability of RCA and reduce the mechanical properties and fracture energy of RAC. While the split tensile and flexural strength of RAC-AF is greater than that of RAC-LP, it is smaller than that of RAC-MK. As a result of their higher elastic modulus, RAC-AF and RAC-MK are more ductile than RAC and RAC-LP. As a result of low ion penetration and high electrical resistance, RAC, RAC-AF, RAC-LP, and RAC-MK are classified as low permeability materials. AF or MK as a cement substitute does not alter the sorptivity of RAC despite improvement. In contrast, RAC has higher initial and final absorption when LP substitutes for cement. According to their pozzolanic reactivity and filler effect, RAC sorptivity varies with these SCMs. There is a strong correlation between the compressive strength and UPV of RAC, RAC-AF, RAC-LP, and RAC-MK with R2 = 0.94. The sorptivity and electrical resistivity are also correlated with R2 = 0.94. Due to the formation of Si-rich C-S-H gels, SEM and EDS show a more uniform and denser microstructure in RAC-AF, RAC-LP, and RAC-MK.

Frost resistance and damage evolution model of basalt fiber-reinforced recycled concrete based on recycled coarse aggregate strengthening and vibration mixing processes

A self-created grinder machine was utilized to clear the wrapped mortar from recycled coarse aggregates (RCA) in order to enhance the performance of basalt fiber-reinforced recycled aggregate concrete (BFRAC). In combination with the RCA strengthening and vibration mixing processes, the mechanical properties and freeze-thaw frost of BFRAC were tested to clarify the mechanism for improving the frost resistance under the synergistic effect of basalt fiber (BF) reinforcement and vibration mixing. Meanwhile, the impact of the production process and BF strengthening technique on RAC performance was explained from a microscopic standpoint using a scanning electron microscope (SEM) test, and aggregate-old mortar interface transition zones were significantly reduced. BF is beneficial in improving the mechanical properties and frost resistance of recycled aggregate concrete (RAC). Among them, the performance improvement effect of RAC is the most significant when using vibration mixing processes, adding BF with a length of 18 mm and a content of 0.2 %. It can increase the compressive strength of RAC by 19 % compared to ordinary mixing, the splitting tensile strength by 37.6 %, and the flexural strength by 34.4 %. A polynomial damage deterioration model was established with relative dynamic elastic modulus and compressive strength as damage variables. The results show that the model can accurately predict the degree of freeze-thaw damage of BFRAC. The research results have significant theoretical value for enhancing the frost resistance performance of RAC.

Multi-objective optimization design of recycled aggregate concrete mixture proportions based on machine learning and NSGA-II algorithm

This paper employs Support Vector Regression (SVR), Random Forest Regression (RF), Gradient Boosting (GB), and Extreme Gradient Boosting (XGB) algorithms to establish the compressive strength prediction models for Recycled Aggregate Concrete (RAC) and analyze the influence of ten inputs on RAC compressive strength. Combined with the best prediction model, the Non-dominated Sorting Genetic Algorithm Ⅱ (NSGA-II) is applied for multi-objective optimization of mixture proportions in RAC addressing cost, carbon emissions, and compressive strength as key objectives. The results demonstrate that the RAC compressive strength prediction model using the GB algorithm exhibits the highest accuracy, with the R2 value of 0.99 for the training set and 0.85 for the testing set. Feature importance and SHAP analysis reveal that cement and water contents are the primary factors affecting the compressive strength of RAC. Among the three mineral admixtures including Silica Fume (SF), Fly Ash (FA), and Ground Granulated Blast Furnace Slag (GGBFS), SF exhibits superior improvement in the compressive strength of RAC compared to FA and GGBFS. Partial dependency plot analysis indicates that concrete strength remains unaffected by recycled coarse aggregate content within the range of 0 to 300 kg/m3. Pareto fronts of a tri-objective mixture optimization problem for RAC are successfully obtained through the GB model combined with the NSGA-II algorithm, which can guide the optimization of RAC preparation.

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.

Effects of steam curing on the pore structure of recycled aggregate concrete

The pores in steam-cured concrete affect its strength and durability, and optimisation of the steaming system can improve the pore structure to some extent. However, the influence mechanism of evaporation on recycled aggregate concrete (RAC) remains unclear. In this paper, the effects of curing temperature (20°C, 40°C, 60°C and 80°C), steam curing time (6 h, 9 h and 12 h) and pozzolans (slag powder and fly ash) on the compressive strength and pore structure of RAC were studied. The microstructure characteristics such as pore size and pore size distribution were analysed by mercury injection porosimetry. The results show that steam curing can significantly improve the compressive strength and reduce the most probable aperture and average pore sizes of RAC. Moreover, the effect of slag powder on RAC pore structure is better than that of fly ash. Also, there is a linear correlation between the compressive strength and the average pore size of steam-cured RAC. This could be an important parameter for constructing future steam-cured RAC strength prediction models.

Durability of Recycled Concrete after Reinforcing the Aggregates with Permeable Crystalline Materials

The utilization of recycled aggregate can significantly mitigate the extraction of natural sand and gravel. However, the practical application of recycled aggregate in engineering is impeded by its inherent characteristics, encompassing high water absorption, high crushing, and low apparent density. This study employed a soaking and air-drying method to enhance the strength of three types of aggregates with varying initial strengths by utilizing permeated crystalline materials. The durability of recycled aggregate concrete (RAC) was studied with three different aggregate replacement rates (0%, 50%, and 100%). The test results demonstrate that the slump, compressive strength, freeze resistance, and carbonation resistance of RAC exhibit a decreasing trend as the aggregate replacement rate increases. The freeze resistance and carbonation resistance of RAC are notably enhanced after incorporating permeated crystalline material. This study contributes to a sustainable and efficient solution for the treatment of construction waste, thereby enhancing the utilization rate of recycled concrete.

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.

Hybrid fibre reinforced recycled aggregate concrete: dynamic mechanical properties and durability

Research into recycled aggregate concrete (RAC) has been on the rise as there is a need to preserve natural resources and promote sustainable infrastructure practices. In our previous study, a new hybrid-fibre reinforced recycled aggregate concrete (FRRAC) with micro basalt fibre (BF) and recycled macro polypropylene fibre (PF) was developed and its physical and mechanical properties were investigated. As a continuous work, this study investigated the dynamic mechanical properties and durability of the developed FRRAC. Dynamic compressive and splitting tensile strength tests were undertaken using a Ø 100 mm split Hopkinson pressure bar (SHPB) system. The failure processes and failure modes of RAC and FRRAC specimens under different strain rates were recorded using a high-speed camera. For dynamic compressive strength, the stress-strain curves, dynamic increase factor for compressive strength (DIFf’c) and energy absorption capacities of RAC and FRRAC were evaluated and compared in the strain rate range from around 27.9 s-1 to 138.0 s-1. The stress-time histories, crack-opening displacement values and dynamic increase factor for splitting tensile strength (DIFfst) at the strain rates of around 3.03 s-1 to 8.17 s-1 of the two materials were obtained and compared. The test results indicated that RAC specimens were slightly more strain rate sensitive than natural aggregate concrete (NAC) under dynamic compression and splitting tension. On the other hand, FRRAC specimens showed similar strain rate sensitivity as their counterparts with no fibres under dynamic compression but showed lower DIFfst values with increasing strain rate under splitting tension. Empirical formulae for DIFf’c, DIFfst and energy absorption capacities against strain rates were proposed. In addition, the durability performance was assessed by examining the water absorption, volume of permeable voids (VPV), water sorptivity and chloride ion penetration resistance. Durability test results indicated that RAC and FRRAC in this study are suitable for structural applications.

Properties evaluation of concrete made with recycled coarse aggregate modified by graphene oxide

Previous application of graphene oxide (GO) in recycled aggregate concrete (RAC) is the direct replacement of cement or binder to achieve performance improvement. To develop a more efficient recycled aggregate (RA) modification technique and improve the properties of the corresponding RAC, this paper performed GO solution pre-spraying and pre-soaking treatment on RA. The modification efficiency of GO was evaluated by compressive strength, interfacial transition zone (ITZ) properties, and chloride ion permeability of RAC. The results show that the pre-spraying and pre-soaking methods improved the compressive strength of RAC at different ages by 20%–28.6 % and 8.3%–30.8 %, respectively. The GO-modified RAC also exhibited lower electric flux in the chloride ion permeability tests. However, no significant effect was observed on the elastic modulus of RAC. The modified RAC showed lower total porosity and harmful pore content. Meanwhile, the GO-modified ITZ between old and new mortar had denser hydration products. As a result, the elastic modulus and hardness of the ITZ increased by 27.8%–65.8 % and 9.8%–107 %, respectively, while the width of the ITZ decreased by 15.4%–30.8 %. The good filling and nucleation effect of GO were credited with the enhancement mechanism of RA by pre-spraying and pre-soaking methods.

3D mesoscale modelling of recycled aggregate concrete

Mesoscale modelling is a promising tool to analyse the phenomenological causes of mechanical failure of recycled aggregate concrete (RAC). Most mesoscale models of RAC are two-dimensional (2D) and 3D models are lacking. This prevents the full understanding of the behaviour of each mesoscopic phase. This paper presents a 3D model of RAC and analyses its mechanical behaviour based on mesoscale phenomena. The analyses concern three types of tests carried out in concrete specimens: uniaxial compressive, uniaxial tensile and splitting tensile strength tests. The mechanical response and fracture behaviour of each mesoscopic phase is analysed in relation to the response of RAC specimen. The comparisons between uniaxial behaviour of 3D and 2D modelling as well as between uniaxial and splitting tensile behaviour of RAC are presented. The main findings of the paper are: (1) the full curve of uniaxial stress-strain response from 3D modelling is comparable to that from 2D modelling for RAC (therefore, 2D models are validated for RAC modelling); (2) the overall behaviour of the concrete elements remains elastic regardless of the fracture development in the ITZ and attached mortar of recycled aggregates until damage propagates to the new mortar matrix; (3) a relationship between modelled uniaxial and splitting strength of RAC is established and validated.

Enhancement of the wet carbonation of artificial recycled concrete aggregates in seawater

This study aimed to improve the carbonation efficiency and reduce processing time of recycled concrete aggregates (RCAs). Different liquid mediums were used for wet carbonation and compared with dry carbonation. Reaction kinetics, phase assemblage, microstructure and performance of RCAs and recycled aggregate concrete (RAC) prepared with carbonated RCAs were evaluated. RCAs carbonated under seawater for 10 min achieved >9 % reduction in water absorption and 3 % increase in density, which were more efficient than dry carbonated samples. Compressive strength of RAC prepared with 1-h seawater carbonated RCAs was significantly improved. Formation of ettringite and calcite under wet carbonation increased density and strength by contributing to solid volume, also leading to improved bond strength in interfacial transition zone in RAC. Seawater presents several advantages as a medium for wet carbonation due to its abundant availability, CO2 capture capacity and accelerated hydration and carbonation, thereby enabling rapid improvement of RCA's and resulting concrete formulations' performance.

Effect of silica fume on frost resistance and recyclability potential of recycled aggregate concrete under freeze–thaw environment

In this study, the effects of silica fume (SF) dosages (0 %, 3 %, 5 %, 8 %, 11 %, 13 %, and 16 %) on the mechanical properties (compressive strength and splitting tensile strength) and frost resistance (relative dynamic elasticity modulus and mass loss rate) of recycled aggregate concrete (RAC) prepared with 100 % recycled concrete aggregate (RCA) under freeze–thaw (F-T) environment were investigated. To assess the recyclability potential of RAC, the apparent density, water absorption, crushing index, and attached mortar content of second-generation RCA after F-T cycles were also studied. The phase composition and microstructure near the interfacial transition zone (ITZ) of RAC were characterized by X-ray diffraction, thermogravimetric, scanning electron microscopy, and microhardness. The results indicate that the incorporation of SF effectively improves the mechanical properties and frost resistance of RAC, with an optimal SF dosage around 13 %. After 400 F-T cycles, the compressive strength of RAC with 13 % SF dosage can still reach 40.1 MPa, which is classified as medium-strength concrete. All second-generation RCAs achieves Class III, but RCA obtained from RAC containing 13 % SF has higher potential to upgrade to Class II and reuse in 40 MPa structural concrete. During the F-T process, the ITZ width of RAC with 13 % SF dosage only increased by 3 μm, and the average hardness of ITZ decreased by 6.7 %. The microscopic analysis showed that an appropriate dosage of SF is conducive to denser ITZ with higher hardness and narrower crack width, thereby hindering crack propagation within RAC during F-T cycles.

Effect of carbonation duration on properties of recycled aggregate concrete

The use of recycled coarse aggregate (RCA) produced from waste concrete in new concrete is an effective way to realize the recycling of construction and demolition waste. The utilization of accelerated carbonation to enhance RCA has attracted a lot attention because it can enhance performance of recycled aggregate concrete (RAC) and has potential for CO2 sequestration. This study systematically investigated the influences of the duration of pressurized carbonation on the properties of RCA, RCA-new mortar interface, and RAC. The results showed that water absorption of carbonated RCA (CRCA) decreased with the increase of carbonation duration. But the beneficial effect of carbonation was less obvious after 1-day carbonation because most of calcium hydroxide and Ca-rich calcium silicate hydrates in RCA had reacted with CO2 within 1 day. After using CRCA, the improvement in different performance indicators of RAC was dependent on different factors. The enhancement in compressive strength and chloride penetration resistance of RAC prepared with CRCA were dependent on the reduced porosity of CRCA. That was why they correlated with the water absorption of CRCA as carbonation duration increased. However, the decrease in initial absorption of water of RAC was mainly dependent on the denser surface layer of CRCA, rather than the reduced porosity of CRCA. This is because the denser surface layer can change water path from “penetrating through RCA” to “passing around RCA”. It is recommended to use 1-day carbonation to enhance RCA, which might have higher cost-effectiveness.

Response of treated recycled aggregate concrete against low-velocity impact loading: Experimental and Weibull statistical analysis

Sustainable and ecologically friendly building practices require increased conservation and recycling of renewable resources. Because of the rise in development and demolition worldwide, using recycled construction materials as aggregate in new concrete can be an essential step towards making the material more environmentally friendly. Removing or fortifying the adherent mortar is the main technique to advance the poor standard of recycled concrete aggregate, which the adherent mortar causes. The study examines the impact performance of recycled aggregate concrete (RAC) comprising different replacement levels (10, 20, 30 and 40 %) of recycled aggregate (RA). Three concentrations (10, 15, and 20 %) of MgSO4 solution were used to pre-treat the RA and subjected to three different immersion times of 5, 10, and 15 days. For this, thirty-seven mixtures were prepared with the combination of RA contents, MgSO4 concentrations and immersion periods. The study was divided into three phases. The physical properties of RA, slump, and compressive strength tests were studied in the first phase. The best thirteen mixtures that exhibited expectational results in the first phase are put forward to the second phase. The effect of RA content, MgSO4 concentration and immersion periods on the impact strength was studied in the second phase. Besides, the differences in the impact test results were studied by Weibull distribution and results were displayed with regard to the probability of survival. The final phase studied the impact response of reinforced concrete slabs under a large mass impact. The findings reveal that the five days of immersion in a 10 % MgSO4 solution has been shown to enhance the impact strength of RAC, whereas a replacement ratio of 20 % is recommended.

Development of chemistry-informed interpretable model for predicting compressive strength of recycled aggregate concrete containing supplementary cementitious materials

Accurate prediction of compressive strength of recycled aggregate concrete (RAC) has great value for promoting the sustainable development of concrete field. The study aims to propose a chemistry-informed interpretable model for predicting compressive strength of RAC containing supplementary cementitious materials. First, this study collected the compressive strength records of RAC. Then, the unsupervised learning algorithm isolation forest was utilized to perform data cleaning, and 1600 samples were obtained to build the dataset. And the topological network constraint was introduced to represent the reactivity of cementitious systems. The five machine learning (ML) algorithms were employed to predict compressive strength of RAC, and the prediction accuracy of above models were compared. In addition, SHapley Additive exPlanations (SHAP) method was integrated with the ML model to shed more light on the outcome of ML model. Results showed that the Categorical Boosting (CatBoost) outperformed other ML models on the test set, and the coefficient of determination (R2), root mean square error (RMSE), and mean absolute error (MAE) were 0.961, 2.650 MPa, and 1.636 MPa. The introduction of chemical information enhanced the predictive accuracy of model, and the R2 increased from 0.935 to 0.961, the RMSE and MAE decreased by 22.7% and 15.1%, respectively. The SHAP values of curing age, natural coarse aggregate replacement ratio and water-binder ratio were higher than those other input variables, indicating that the three features were essential for predictive results of model. And the curing age played a positive role in the strength development of RAC, and the increase of natural coarse aggregate replacement ratio and water-binder ratio caused the loss of compressive strength. The influence trends of features on the outputs were in accord with the engineering experience, which proved the reliability of predictions. The contribution and importance of features on the predictions provide a theoretical guidance to aid the concrete mixture design for engineers to obtain the desired performance.

A study on the strength and durability characteristics of fiber-reinforced recycled aggregate concrete modified with supplementary cementitious material

Recycled aggregate (RA) made from waste concrete is an environmentally friendly alternative to natural aggregate (NA) for concrete manufacturing. However, compared to NA concrete, concrete produced with recycled aggregates has poor characteristics. Supplementary cementitious materials (SCMs) can be used to enhance the poor properties of recycled aggregate concrete (RAC). Silica fume and fly ash are commonly used SCMs in the World, but their high usage led to a shortage of silica fume and fly ash. Still, the deficiency of these materials in large parts of the world is a challenge that requires exploring alternative feedstock materials for the construction industry in the coming years. Wheat straw ash (WSA) is an agricultural waste product that could be used as an alternative SCM due to its pozzolanic behavior to enhance the properties of RAC. In addition, concrete is brittle and needs reinforcement, for which polypropylene fibers (PPFs) can be used. The current research examines the mechanical characteristics of fiber-reinforced RAC, including compressive strength, splitting tensile strength, and ductility performance. Durability indicators, such as chloride diffusion, chloride penetration, acid resistance, and water absorption test, were also assessed. The results showed that concrete samples with 10% WSA, 50% RA and 1.5% PPFs had the highest compressive and splitting tensile strength, 60.2 MPa and 7.25 MPa, respectively, representing increases of 24.75% and 30.65%, as compared to plain samples at 56 days. In these samples, water absorption was reduced by 13% due to the finer WSA particles resulting in the lowest reduction in strength and mass recorded when exposing concrete samples to acidic media. The statistical analysis also validated that irrespective of WSA and PPFs, the concrete with 0% RA had the highest performance in strength and durability behavior. The study showed that WSA and PPFs might be employed in tandem to offset the poor behavior of RA, enhance the bond between fibers and concrete, and improve the mechanical strength and durability performance of RAC, thus demonstrating its suitability as a sustainable and economical construction material.

The Impact of Fractal Gradation of Aggregate on the Mechanical and Durable Characteristics of Recycled Concrete

Properties of recycled aggregate concrete (RAC) are influenced by the composition and particle size distribution of recycled coarse aggregate (RCA). The study herein designed seven distinct groups of RACs with varying aggregate fractal dimensions (D) and one group of natural concrete (NAC). The impact of D on the workability, compressive strength, resistance to chloride ion penetration, and carbonation resistance of RAC was measured. It was found that an increase in the D value led to a decrease in the slump and slump flow, with the compressive strength and chloride ion penetration increasing and then decreasing, and carbonation gradually declined. The optimal fractal dimension was thereby determined to be 2.547 by a strength model accommodating two parameters of D and the curing age. Additionally, the mass percentage of each particle size for the corresponding gradation was presented. The compressive strength and chloride permeation resistance of RAC (D = 1.0) relative to RAC (D = 2.5) was increased by 16.7% and 13.3%, respectively. Furthermore, the carbonation depth of RAC (D = 2.5) was comparable to that of NAC. Additionally, the carbonation resistance of RAC was influenced by both the size distribution and the degree of natural carbonation of RCA, resulting in four distinct features relative to NAC. It is thereby feasible to enhance RAC performance through the manipulation of RCA’s fractal dimensions.

Prediction of Compressive Strength and Elastic Modulus for Recycled Aggregate Concrete Based on AutoGluon

While the civil construction industry brings great convenience to life, the large amount of waste concrete also poses a significant problem of construction waste disposal. As one of the effective ways to utilize waste concrete, recycled aggregate concrete (RAC) can improve the environment while reducing the consumption of construction materials. This study aims to use AutoGluon (AG), an automated machine learning platform, to predict both the compressive strength and elastic modulus of RAC. Then the performance of AG is compared with traditional empirical formulas and multiple linear regression models. The determination coefficient (R2) is chosen as one of the evaluation standards for predicting values. The results demonstrate that the WeightedEnsemble model of AG performed best in predicting both the compressive strength and elastic modulus, which provides a new method for the rapid and accurate prediction of the properties of RAC in engineering construction.

Recycled aggregate concrete-filled steel tube columns with small-diameter circular FRP-confined sea-sand concrete cores: Conceptual and experimental investigations

Recycled aggregate concrete (RAC) prepared from waste concrete can achieve the green recycling of resources; however, its inferior material properties hinder its applications. Sea-sand concrete (SSC) can help alleviate the resource shortage of fresh water and river sands but is prone to steel corrosion. To address these challenges, this study developed a new RAC-filled steel tube column with a small-diameter circular fibre-reinforced polymer (FRP)-confined SSC core. This paper presents and discusses the results of the axial compression tests conducted on RAC-filled steel tube columns, with and without small-diameter circular FRP-confined SSC cores. The study investigated the effects of the configuration of the FRP-confined SSC core, diameter-to-thickness ratio of the FRP tubes, compressive strength of RAC and SSC, and cross-sectional shape of the column. The results revealed that using small-diameter circular FRP-confined SSC cores to partially replace RAC was effective in improving the compressive performance of RAC-filled steel tube columns. The increase in the number of FRP tubes, in-filled concrete strength, and confinement stiffness of the steel and FRP tubes contributed to improvements in the structural strength and ductility, resulting in progressive ladder-shaped declines in the post-peak curve. Finally, a reasonable load–capacity model is presented for the proposed columns.