Development of construction materials using nano-silica and aggregates recycled from construction and demolition waste.

The present work addresses the development of novel construction materials utilising commercial grade nano-silica and recycled aggregates retrieved from construction and demolition waste. For this, experimental work has been carried out to examine the influence of nano-silica and recycled aggregates on compressive strength, modulus of elasticity, water absorption, density and volume of voids of concrete. Fully natural and recycled aggregate concrete mixes are designed by replacing cement with three levels (0.75%, 1.5% and 3%) of nano-silica. The results of the present investigation depict that improvement in early days compressive strength is achieved with the incorporation of nano-silica in addition to the restoration of reduction in compressive strength of recycled aggregate concrete mixes caused owing to the replacement of natural aggregates by recycled aggregates. Moreover, the increase in water absorption and volume of voids with a reduction of bulk density was detected with the incorporation of recycled aggregates in place of natural aggregates. However, enhancement in density and reduction in water absorption and volume of voids of recycled aggregate concrete resulted from the addition of nano-silica. In addition, the results of the study reveal that nano-silica has no significant effect on elastic modulus of concrete.

Creep and Shrinkage Properties of Nano-SiO2-Modified Recycled Aggregate Concrete.

The poor performance of recycled concrete aggregate (RCA) leads to greater creep in recycled aggregate concrete (RAC) compared to natural aggregate concrete (NAC). To enhance the quality of RCA, this paper utilizes a 2% concentration of a nano-SiO2 (NS) solution for pre-soaking RCA. This study aims to replace natural aggregate (NA) with NS-modified recycled aggregate (SRCA) and investigate the creep and shrinkage properties of NS-modified recycled aggregate concrete (SRAC) at various SRCA replacement rates. Subsequently, the creep and shrinkage strains of NAC, SRAC, and RAC are simulated using the finite element method. Finally, a comparative analysis is conducted with the predicted creep and shrinkage strains from CEB-FIP, ACI, B3, and GL2000 models. The experimental results indicate that the creep and shrinkage deformation of SRAC increases with the SRCA replacement rate. Compared to NAC, the creep and shrinkage deformation of SRAC at replacement rates of 30%, 50%, 70%, and 100% increased by 2%, 7%, 13%, and 30%, respectively. However, when 100% of the natural aggregate is replaced with SRCA, the creep and shrinkage deformation decreases by 7% compared to RAC. Moreover, the CEB-FIP and ACI models can predict the creep and shrinkage deformation of concrete reasonably well.

Salt frost damage evolution and transport properties of recycled aggregate concrete under sustained compressive loading.

The frost damage behavior of recycled aggregates concrete (RAC) in a cold region is inherently more complex due to the incorporation of recycled coarse aggregate (RCA). In real-world service environments, the combined effects of mechanical loading and environmental conditions further make RAC's damage mechanism more intricate. This study explores the impact of uniaxial compressive loading (at 0.1fc, 0.3fc, and 0.5fc, respectively), freeze-thaw cycles, and chloride penetration on the relative dynamic elastic modulus (RDEM), mass transport properties, and microstructure of RAC with varying RCA replacement ratios. The results indicate that specimens loaded at 0.3fc exhibit enhanced frost resistance, with reduced water absorption and chloride ion content. Additionally, a damage model is developed to quantify the effects of mechanical loading, freeze-thaw cycles, and chloride penetration on RDEM degradation. The investigation using X-ray computed tomography (X-CT), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) techniques reveals that as compressive stress levels increase, the microstructural density and porosity of RAC initially decrease before increasing. Moreover, the RDEM of RAC decreases with decreasing pore sphericity. Compared to the R100-S55 samples, the pore sphericity of R100-S55-0.5fc samples increased by 60.4 % in the range of 0.4-0.5, resulting in a decrease of approximately 17.72 % in the RDEM. Furthermore, the initial sorptivity of frost-damaged RAC exhibits a significant linear relationship with porosity. Overall, this study elucidates the evolving trends of mass transport properties and microstructure in RAC under loading and freeze-thaw conditions, laying a theoretical groundwork for the widespread application of RCA.

Mechanical Properties and Microscopic Study of Recycled Fibre Concrete Based on Wind Turbine Blades.

In recent years, wind energy has begun to receive a significant amount of attention as clean energy is utilised and demanded in large quantities, resulting in a sharp increase in the use of wind turbines. The demand for wind turbines has gradually risen due to the clean and recyclable nature of wind energy. The current blade life of wind turbines in China is about 20 years, which means that the disposal of obsolete used blades can become a difficult problem in the future. Therefore, this study is of great significance to explore the regeneration performance of the blades after recycling and disposal. In this paper, wind turbine blades were mechanically recycled into recycled macrofibres, which were added to concrete as a reinforcing material to make wind impeller fibre concrete (WIC), and the three proportion ratios of 1%, 1.5%, and 2% were explored to compare the performance. The performance of WIC was also evaluated and its performance was compared to that of glass fibre concrete (GC). In addition, the material physical properties of second-generation recycled aggregate concrete (RAC) based on WIC were explored. The strength and peak strain variations and their causal mechanisms were analysed both macroscopically and microscopically by means of the classical mechanical tests (compression and bending tests), SEM, and XRD. The results show that the compressive strength of WIC was negatively correlated with the fibre content and increased by 6.04-18.12% compared to that of ordinary concrete (OG), with a maximum of 19.25 MPa; the flexural strength was positively correlated with the fibre content, with an increase of 5.37-18.5%. The microstructural analysis confirmed the macroscopic results and the intrinsic model better validated the experimental results.

Investigation of the Time-Dependent Deformation of Recycled Aggregate Concrete in a Water Environment.

The water environment greatly affects the creep deformation of recycled aggregate concrete (RAC). Hence, a humidity-stress-damage coupling numerical model was used for investigating the time-dependent deformation of RAC in the water environment in this study. Firstly, uniaxial compression and water absorption tests were performed to determine the calculation parameters of the creep numerical simulation of RAC in a water environment. Experimental results indicate that the elastic modulus and compressive strength drop as the water content increases. Then, the time-dependent deformation of RAC in a water environment was studied using a numerical simulation test of compressive creep when multiple stress levels were applied, and the critical stress for accelerated creep and the long-term strength of RAC were obtained. Finally, the influence of confining pressures on the long-term deformation of RAC in a water environment was discussed. When there is no confining pressure, the long-term strength of RAC is 23.53 MPa. However, when a confining pressure of 3.921 MPa is loaded onto RAC, the long-term strength of RAC is 47.052 MPa, which increases by 100%. Increasing confining pressures has an obvious effect on ensuring the long-term stable application of RAC in a water environment. Compared with the creep test, the method adopted in this study saves time and money and provides the theoretical basis for evaluating the time-dependent deformation of RAC in a water environment.

Enhancing the Mechanical and Durability Properties of Fully Recycled Aggregate Concrete Using Carbonated Recycled Fine Aggregates.

The global construction industry is increasingly utilizing concrete prepared from recycled aggregate as a substitute for natural aggregate. However, the subpar performance of recycled fine aggregate (RFA) has resulted in its underutilization, particularly in the structural concrete exposed to challenging environments, including those involving chlorine salts and freeze-thaw climates. This study aimed to enhance the performance of RFA as a substitute for river sand in concrete as well as fulfill the present demand for fine aggregates in the construction sector by utilizing accelerated carbonation treatment to create fully recycled aggregate concrete (FRAC) composed of 100% recycled coarse and fine aggregates. The impacts of incorporating carbonated recycled fine aggregate (C-RFA) at various replacement rates (0%, 25%, 50%, 75%, and 100%) on the mechanical and durability properties of FRAC were investigated. The results showed that the physical properties of C-RFA, including apparent density, water absorption, and crushing value, were enhanced compared to that of RFA. The compressive strength of C-RFC100 was 19.8% higher than that of C-RFC0, while the water absorption decreased by 14.6%. In a comparison of C-RFC0 and C-RFC100, the chloride permeability coefficients showed a 50% decrease, and the frost resistance increased by 27.6%. According to the findings, the mechanical and durability properties, the interfacial transition zones (ITZs), and micro-cracks of the C-RFC were considerably enhanced with an increased C-RFA content.

Experimental study on axial tensile properties of basalt fibre reinforced recycled aggregate concrete.

In order to investigate the tensile properties of basalt fibre reinforced recycled aggregate concrete (BFRAC), the axial tensile tests were carried out on BFRAC specimens using the concrete axial tensile testing device. The effects of basalt fibre (BF) content and recycled aggregate replacement rate on the tensile properties of BFRAC were quantitatively investigated, and the tensile damage mechanism of BFRAC was analysed. The following conclusions were drawn: The volume fraction of BF had the most prominent effect on the axial tensile properties of BFRAC. The axial tensile strength and peak tensile strain of BFRAC both showed the change rule of first increasing and then decreasing with the increase of BF volume fraction. The replacement rate of recycled aggregate is negatively correlated with the tensile properties of BFRAC. The larger the replacement rate, the worse the tensile properties of BFRAC. When the replacement rate of recycled aggregate is 30 % and the volume fraction of BF is 0.3 %, the tensile properties of BFRAC are better, as well as its economic and environmental performance. The axial tensile strength and peak tensile strain were 2.08 MPa and 114 × 10-6, respectively. BFRAC exhibits the toughening and crack arresting effect of BF, and the crack development is relatively slow, showing more obvious plastic damage characteristics.

Mechanical Properties of Fully Recycled Aggregate Concrete Reinforced with Steel Fiber and Polypropylene Fiber.

The study and utilization of fully recycled aggregate concrete (FRAC), in which coarse and fine aggregates are completely replaced by recycled aggregates, are of great significance in improving the recycling rate of construction waste, reducing the carbon emission of construction materials, and alleviating the ecological degradation problems currently faced. In this paper, investigations were carried out to study the effects of steel fiber (0.5%, 1.0%, and 1.5%) and polypropylene fiber (0.9 kg/m3, 1.2 kg/m3 and 1.5 kg/m3) on the properties of FRAC, including compressive strength, splitting tensile strength, the splitting tensile load-displacement curve, the tensile toughness index, flexural strength, the load-deflection curve, and the flexural toughness index. The results show that the compressive strength, splitting tensile strength, and flexural strength of fiber-reinforced FRAC were remarkably enhanced compared with those of ordinary FRAC, and the maximum increase was 56.9%, 113.3%, and 217.0%, respectively. Overall, the enhancement effect of hybrid steel-polypropylene fiber is more significant than single-mixed fiber. Moreover, the enhancement of the crack resistance, tensile toughness, and flexural toughness obtained by adding steel fiber to the FRAC is more significant than that obtained by adding polypropylene fiber. Furthermore, adding polypropylene fiber alone and mixing it with steel fiber showed different FRAC splitting tensile and flexural properties.

Cyclic Behavior and Stress-Strain Model of Nano-SiO2-Modified Recycled Aggregate Concrete.

Recycled aggregate concrete (RAC) possesses different mechanical properties than ordinary concrete because of inherent faults in recycled aggregates (RAs), such as the old interfacial transition zone (ITZ). However, the application of nano-SiO2 presents an effective methodology to enhance the quality of RA. In this study, nano-SiO2-modified recycled aggregate (SRA) was used to replace natural aggregate (NA), and the stress-strain relationships and cyclic behavior of nano-SiO2-modified recycled aggregate concrete (SRAC) with different SRA replacement rates were investigated. After evaluating the skeleton curve of SRAC specimens, the existing constitutive models were compared. Additionally, the study also proposed a stress-strain model designed to predict the mechanical behavior of concrete in relation to the SRA replacement rate. The results show that compared with RAC, the axial compressive strength of SRAC specimens showed increases of 40.27%, 29.21%, 26.55%, 16.37%, and 8.41% at specific SRA replacement rates of 0%, 30%, 50%, 70%, and 100%, respectively. Moreover, the study found that the Guo model's calculated results can accurately predict the skeleton curves of SRAC specimens.

Triaxial mechanical behaviours and life cycle assessment of sustainable multi-recycled aggregate concrete.

Multi-recycling of concrete waste presents a promising avenue for carbon-negative development and a circular economy. This study comprehensively assesses the triaxial mechanical performance and environmental impact of multi-recycled concrete (Multi-RAC) through three recycling cycles. The results reveal a triaxial failure mode similar to natural aggregate concrete (NAC). The peak stress and peak strain monotonically increase with confinement stress, showing a significant impact (enlarged by 171.4 % to 280.6 % and 397.4 % to 412.0 %, respectively) from 0 to 20 MPa. All P-values for recycling cycles and confining pressure are less than 0.05, with the confining pressure having a more significant effect. Three best-fit multivariate mixed models predict mechanical properties, and a modified elastoplastic model introduces the recycling cycles factor. Numerical simulations confirm the model's accuracy in predicting the triaxial mechanical properties of Multi-RAC. Comparative analysis reveals that the elastoplastic model-derived non-integral high order failure criterion outperforms the Willam-Warnke failure criterion and other conventional criteria. Regarding environmental impact, all indicators (GWP, POCP, AP, EP, and CED) decrease favourably with the increasing number of recycling cycles, with CED and EP playing the most significant roles. Compared to NAC, the five environmentally favorable indicators for RACIII decrease by 3.24 % to 50.6 %, respectively. These findings provide valuable insights for future research on developing eco-friendlier Multi-RAC for sustainable and green infrastructure.

Unraveling the Interplay of Physical-Chemical Factors Impacting the Carbonation Performance of Recycled Aggregate Concrete.

Recycled aggregate concrete (RAC) includes recycled concrete aggregates (coarse and/or fine) as substitutes for natural aggregates as an approach to achieving a circular economy. Some concerns remain about its performance, including the carbonation resistance. The higher porosity of recycled concrete aggregates is logically a disadvantage, but the analysis must address many other factors. This paper provides an in-depth examination of recent advances in the carbonation performance of RAC. The emphasis is on factors that influence CO2 diffusion and the carbonation rate, e.g., the replacement ratio, source concrete quality, interfacial transition zone features, residual portlandite content, and porosity. The influences of previous treatments, combined action with supplementary cementitious materials, and loading conditions are also discussed. The replacement ratio has a significant impact on the carbonation performance of concrete, but it is also dependent on other factors. During carbonation, the physical effects of the porosity of the aggregate and the physical-chemical effects of the portlandite content in the adhered mortar are particularly important. The residual portlandite is especially significant because it is the primary hydration product responsible for the alkaline reserve for carbonation and the potential pozzolanic reaction, which are per se competing factors that determine the carbonation rate.

Influence of Coal Gangue Powder on the Macroscopic Mechanical Properties and Microstructure of Recycled Aggregate Concrete.

The construction and coal industries generate substantial industrial waste, including coal gangue and construction and demolition (C&D) waste, leading to environmental pollution and high disposal costs. Integrating recycled aggregates (RAs) and coal gangue powder (CGP) into concrete is an effective approach for waste management. However, CGP can affect the performance of traditional recycled concrete. This study primarily aims to optimize the utilization of RAs and CGP while maintaining concrete performance. They utilized orthogonal experimental designs and microscopic characterization techniques, including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Orthogonal experimental analysis indicated that with a water-cement ratio (WCR) of 0.5 and replacement rates of 10% for CGP and 60% for RA, compressive and splitting tensile strengths reached 73.6% and 77.4% of ordinary C30 concrete, respectively. This mix proportion minimizes strength decline in coal gangue powder-recycled aggregate concrete (CGP-RAC) while maximizing recycled material replacement. Microscopic analysis revealed that CGP increased the Ca/Si ratio in cement paste, impeding hydration reactions, resulting in a looser internal structure and reduced concrete strength. These findings are anticipated to provide fresh theoretical insights for solid waste recycling and utilization.

A succinct review on the durability of treated recycled concrete aggregates.

Building materials constitute a considerable portion of all the materials we use and about half the waste (in solid form) generated worldwide. Construction and demolishment (C&D) aggregates can be an invaluable source of construction material. If we measure the quantity of C&D waste in India, it will exceed the amount of all other types of hard solid wastes put together. Therefore, the use of recycled concrete aggregates (RCA) in new construction is being encouraged worldwide. But due to the inferior compressive, mechanical strengths and poor durability performance, it cannot be qualified for structural usage. Hence, there is a need to treat these aggregates and produce better quality aggregates suitable for good structural grade concrete. The present work focuses on the study and comparison of the effects on durability performance due to different treatment techniques of recycled aggregates. Effective treatment techniques can potentially separate or strengthen the weaker portions of the recycled aggregates like the old adhered mortar and the ITZs formed due to them. Effects of different RCA treatment methods along with their combinations such as immersing aggregate in acid solution and silicate solution impregnation, multistage-mixing techniques, biologically induced carbonate precipitation, modifier solution impregnation, ultrasonic cleaning, crushing aggregates at multiple levels, and mechanical grinding are considered for analyzing their effectiveness in improving RCA durability. The durability performance of treated RCA is evaluated based on the improvement in the parameters such as water absorption and resistance to acid attack, permeability, chloride attack, and carbonation.

Effect of Permeable Crystalline Materials on the Mechanical and Porosity Property of Recycled Aggregate and Recycled Aggregate Concrete.

This study investigates the potential of permeable crystalline materials to improve the properties of recycled aggregates and recycled aggregate concrete (RAC). The use of recycled aggregates in concrete production has gained increasing attention due to environmental and economic benefits. However, the lower quality and poorer durability of recycled aggregates limit their wider application. In this study, three types of recycled aggregates were treated with permeable crystalline materials, and their water absorption and crushing index were compared before and after modification. RAC was then produced using modified recycled aggregates with different substitution rates, and their mechanical properties were evaluated. To investigate the mechanism of permeable crystalline materials modification, the microstructure of the modified RAC was observed using nuclear magnetic resonance and scanning electron microscopy. The results demonstrated that the permeable crystalline materials treatment effectively reduced the water absorption and crushing index of the recycled aggregates. The compressive strength of modified RAC also improved, with a higher modification time leading to higher strength. Furthermore, the pore distribution and microstructural denseness of the modified recycled aggregates and RAC were enhanced, as revealed by the microstructural observations. These findings suggest that permeable crystalline materials modification is a promising method for improving the properties of recycled aggregates and RAC, which could contribute to the sustainable development of the construction industry.

Efficiency and Mechanism of Surface Reinforcement for Recycled Coarse Aggregates via Magnesium Phosphate Cement.

Recycled aggregate concrete (RAC) exhibits inferior mechanical and durability properties owing to the deterioration of the recycled coarse aggregate (RCA) surface quality. To improve the surface properties of RCA, the reinforcement efficiency of RAC, and the maneuverability of the surface treatment method, this study used magnesium phosphate cement (MPC), a clinker-free low-carbon cement with excellent bonding properties, to precoat RCA under three-day pre-conditioning. Moreover, variable amounts of fly ash (FA) or granulated blast furnace slag (GBFS) were utilized to partly substitute MPC to enhance the compressive strength and chloride ion penetration resistance. Subsequently, FA-MPC and GBFS-MPC hybrid slurries with the best comprehensive performance were selected to coat the RCA for optimal reinforcement. The crushing value and water absorption of RCA, as well as the mechanical strengths and durability of RAC, were investigated, and microstructures around interfaces were studied via BSE-EDS and microhardness analysis to reveal the strengthening mechanism. The results indicated that the comprehensive property of strengthening paste was enhanced significantly through substituting MPC with 10% FA or GBFS. Surface coating resulted in a maximum reduction of 8.15% in the crushing value, while the water absorption barely changed. In addition, modified RAC outperformed untreated RAC regarding compressive strength, splitting tensile strength, and chloride ion penetration resistance with maximum optimization efficiencies of 31.58%, 49.75%, and 43.11%, respectively. It was also evidenced that the improved MPC paste properties enhanced the performance of modified RAC. Microanalysis revealed that MPC pastes exhibited an excellent bond with RCA or new mortar, and the newly formed interfacial transition zone between MPC and the fresh mortar exhibited a dense microstructure and outstanding micro-mechanical properties supported with an increase in the average microhardness value of 30.2-33.4%. Therefore, MPC pastes incorporating an appropriate mineral admixture have enormous potential to be utilized as effective RCA surface treatment materials and improve the operability of RCA application in practice.

Influence of the ANN Hyperparameters on the Forecast Accuracy of RAC's Compressive Strength.

The artificial neural networks (ANNs)-based model has been used to predict the compressive strength of concrete, assisting in creating recycled aggregate concrete mixtures and reducing the environmental impact of the construction industry. Thus, the present study examines the effects of the training algorithm, topology, and activation function on the predictive accuracy of ANN when determining the compressive strength of recycled aggregate concrete. An experimental database of compressive strength with 721 samples was defined considering the literature. The database was used to train, validate, and test the ANN-based models. Altogether, 240 ANNs were trained, defined by combining three training algorithms, two activation functions, and topologies with a hidden layer containing 1-40 neurons. The ANN with a single hidden layer including 28 neurons, trained with the Levenberg-Marquardt algorithm and the hyperbolic tangent function, achieved the best level of accuracy, with a coefficient of determination equal to 0.909 and a mean absolute percentage error equal to 6.81%. Furthermore, the results show that it is crucial to avoid the use of overly complex models. Excessive neurons can lead to exceptional performance during training but poor predictive ability during testing.

Prediction of Stress-Strain Curves for HFRP Composite Confined Brick Aggregate Concrete under Axial Load.

Recently, hemp-fiber-reinforced polymer (HFRP) composites have been developed to enhance the strength and ductility of normal and lightweight aggregate concrete along with recycled brick aggregate concrete. In addition, both experimental and analytical investigations have been performed to assess the suitability of the existing strength and strain models. However, the theoretical and analytical expressions to predict the stress-strain curves of HFRP-confined concrete were not developed. Therefore, the main objective of this study was to develop analytical expressions to predict the stress-strain curves of HFRP-confined waste brick aggregate concrete. For this purpose, a new experimental framework was conducted to examine the effectiveness of HFRP in improving the mechanical properties of concrete constructed with recycled brick aggregates. Depending on the strength of the concrete, two groups were formed, i.e., Type-1 concrete and Type-2 concrete. A total of sixteen samples were tested. The ultimate compressive strength and strain significantly increased due to HFRP confinement. Improvements of up to 272% and 457% in the ultimate compressive strength and strain were observed due to hemp confinement, respectively. To predict the ultimate compressive strength and strain of HFRP-confined concrete, this study investigated several existing analytical stress-strain models. Some of the strength models resulted in close agreement with experimental results, but none of the models could accurately predict the ultimate confined strain. Nonlinear regression analysis was conducted to propose expressions to predict the ultimate compressive strength and strain of HFRP-confined concrete. The proposed expressions resulted in good agreement with experimental results. An analytical procedure was proposed to predict the stress-strain curves of hemp-confined concrete constructed by partial replacement of natural coarse aggregates by recycled fired-clay brick aggregates. A close agreement was found between the experimental and analytically predicted stress-strain curves.

Mechanical Properties under Compression and Microscopy Analysis of Basalt Fiber Reinforced Recycled Aggregate Concrete.

In this study, the basalt fiber content (0%, 0.075%, and 0.15%) and replacement ratio of recycled coarse aggregate (0%, 50%, and 100%) were used as parameters, and the compressive strength of 15 cubes and 15 prisms was analyzed. The failure morphology of the specimens was characterized, and the cubic compressive strength, axial compressive strength, elastic modulus, Poisson's ratio, and other mechanical property indices of the specimens were measured. Upon increasing the replacement ratio, the degree of damage of the specimens gradually increased, whereas the cubic compressive strength, axial compressive strength, and elastic modulus gradually decreased. As the replacement ratio was increased from 50% to 100%, the cubic compressive strength and elastic modulus were noted to decrease the most by about 9.07% and 9.87%, respectively. On the other hand, the Poisson's ratio first decreased, followed by an increase. Upon increasing the fiber content, the degree of damage of the specimens was gradually reduced, whereas the cubic compressive strength, axial compressive strength, and elastic modulus gradually increased. As the fiber content increased from 0.075% to 0.15%, the axial compressive strength and elastic modulus increased the most by about 6.65% and 10.19%, respectively. On the other hand, the Poisson's ratio gradually decreased. Based on the test data, the functional relationships between the strength indices and different variables, as well as the conversion value of each strength index and different variables were established; after comparison and verification, the formula calculation results were found to be in good agreement with the test results. The microstructural changes in the basalt fiber reinforced recycled aggregate concrete were characterized by scanning electron microscopy (SEM), and the changes in the mechanical properties of the basalt fiber reinforced recycled aggregate concrete as well as the mechanism of fiber modification and reinforcement were explained from a micro perspective.

Effect of Basalt Fiber on Uniaxial Compression-Related Constitutive Relation and Compressive Toughness of Recycled Aggregate Concrete.

The deformation performance of recycled aggregate concrete can be effectively improved when basalt fiber is reasonably added. In this paper, the effects of the basalt fiber volume fraction and the length-diameter ratio on the uniaxial compression-related failure characteristics, feature points of the complete stress-strain curve and the compressive toughness of recycled concrete under different replacement rates of recycled coarse aggregate were studied. The results showed that with the increase in the fiber volume fraction, the peak stress and peak strain of basalt fiber-reinforced recycled aggregate concrete first increased and then decreased. With the increase in the fiber length-diameter ratio, the peak stress and strain of the basalt fiber-reinforced recycled aggregate concrete first increased and then decreased, whereas the effect of the length-diameter ratio on peak stress and strain of the basalt fiber-reinforced recycled aggregate concrete was clearly smaller than that of the fiber volume fraction. Based on the test results, an optimized stress-strain curve model of concrete under uniaxial compression was proposed for the basalt fiber-reinforced recycled aggregate concrete. Furthermore, it was found that the fracture energy is more suitable for evaluating the compressive toughness of the basalt fiber-reinforced recycled aggregate concrete than the tensile-compression ratio.

Testing and Analysis of Ultra-High Toughness Cementitious Composite-Confined Recycled Aggregate Concrete under Axial Compression Loading.

In order to analyze the axial compressive properties of ultra-high-toughness cementitious composite (UHTCC)-confined recycled aggregate concrete (RAC), a batch of UHTCC-confined RAC components was designed and manufactured according to the requirements of GB/T50081-2002 specifications. After analyzing the surface failure phenomenon, load-displacement curves, scanning electron microscope (SEM), and parameter analysis of the specimen, the result shows that UHTCC-confined RAC is an effective confinement method, which can effectively improve the mechanical properties and control the degree of surface failure of RAC structures. Compared with the unconfined specimen, the maximum peak load of the UHTCC confinement layer with a thickness of 10 mm and 20 mm increased by 44.61% and 79.27%, respectively, meeting the requirements of engineering practice. Different fiber mixing amounts have different effects on improving the mechanical performance of RAC structural. The specific rule was steel fiber (SF) > polyvinyl alcohol fiber (PVAF) > polyvinyl alcohol fiber (PEF) > no fiber mixture, and the SF improves the axial compression properties of UHTCC most significantly. When there are strict requirements for improving the mechanical properties of the structure, SF should be added to UHTCC. On the contrary, PVAF should be added to UHTCC.