Recycled Aggregate Replacement Rate Research Articles

Influence of Accelerated Carbonation on the Performance of Recycled Concrete Containing Fly Ash, Recycled Coarse Aggregate, and Fine Aggregate

In order to improve the quality of solid waste utilization, this study simultaneously used recycled coarse aggregate and recycled fine aggregate to prepare recycled aggregate concrete, with fly ash partially replacing cement as a binder. After the particle gradation of recycled aggregate was artificially adjusted into continuous gradation, the effects of accelerated carbonation on the performance and microstructure of recycled concrete were studied. The microstructural change was analyzed using mercury intrusion porosimetry and scanning electron microscopy–energy dispersive spectroscopy. Additionally, the environmental benefits of the recycled concrete were evaluated based on carbon emissions using the life cycle assessment method. The experimental results indicate that accelerated carbonation can increase the compressive strength of recycled concrete by up to 13%, and its microstructure becomes more compact after carbonation. The carbon emissions are reduced by more than 13% after using 20% fly ash, contributing to sustainable development. Additionally, the optimal replacement rate of recycled fine aggregate should be controlled to under 15% when both recycled coarse and fine aggregates are used.

Study on the mechanical property and durability of alkali-activated seawater and sea sand recycled aggregate concrete

This study investigates the potential of Alkali-Activated Seawater Sea Sand Recycled Aggregate Concrete (ASSRAC) as a novel sustainable construction material. By utilizing recycled aggregates from construction and demolition waste, seawater, sea sand, and alkali-activated materials such as fly ash and blast furnace slag, this innovative concrete aims to mitigate carbon emissions from cement production and optimize resource usage. The impact of recycled aggregate replacement rate, water-to-binder ratio, and sand type on the compressive strength of ASSRAC was examined through analysis of variance. Cylindrical axial compression and prism flexural tests were conducted to evaluate the long-term performance of ASSRAC under seawater immersion. The results indicate a decrease in compressive strength with higher recycled aggregate replacement rates and water-to-binder ratios, with recycled aggregates being the primary weak link. Additionally, increasing the water-to-binder ratio hampers hydration by diluting alkali activators, weakening polycondensation reactions. Under seawater immersion, ongoing reactions foster denser C-(A)-S-H structures, enhancing compressive strength and elastic modulus over time. Moreover, higher recycled aggregate replacement rates result in improved ductility, attributed to reactions between the old mortar on recycled aggregate surfaces and slag/fly ash at interfaces. For specimens immersed for 0 and 90 days, most theoretical models accurately predict results, except for the CEB-FIP model, which only accurately reflects the stress-strain curve at 180 days. The seawater-sea sand group's flexural strength initially exceeds that of the freshwater-river sand group due to accelerated early hydration, but this advantage diminishes over time due to high salt content.

Enhancing energy dissipation in mortar utilizing recycled brick fine aggregate impregnated with polymer emulsion under uniaxial cyclic load

Optimizing the damping properties of materials can enhance the energy dissipation capacity of structures and mitigate the negative effects of dynamic loads on building components. This study investigated the effects of the polymer emulsion-impregnated recycled brick fine aggregate (PRBFA) replacement rate, emulsion-impregnation treatment method, and emulsion solid content on the dynamic damping properties of PRBFA-prepared mortar (PBM). Combined with microscopic tests, the enhanced damping mechanism of PBM was discussed. The results show that under the same stress amplitude, the equivalent damping ratio and loss modulus of PBM increase with the PRBFA replacement rate and emulsion solid content. The drying treatment of PRBFA demonstrates superior damping characteristics of PBM, with the equivalent damping ratio and loss modulus increased by 84.9%–129.4 % and 71.0%–117.2 %, respectively, compared to those of PBM with undried PRBFA. The damping enhancement of PBM is attributed to the weak interface between the aggregate and cement matrix, which facilitates slippage and friction. The damping layer formed between the high viscoelastic emulsion and the aggregate, or between the emulsion and the cement matrix, is conducive to energy dissipation. The optimal PRBFA replacement rate for balancing the mechanical and damping properties of PBM is between 38% and 60 %.

Investigation of Axial Tensile Fracture Performance of Recycled Brick Coarse Aggregate Concrete Using a Cohesion Model.

Currently, microscopic research on the tensile fracture properties of recycled brick coarse aggregate concrete has mainly adopted microscopy techniques, which can clearly observe the actual damage situations of each phase material but are unable to individually analyze the effect of a specific material factor on the tensile properties of recycled concrete. This brings much uncertainty to the practical application of recycled concrete. Therefore, this study proposes a cohesive zone model (CZM) for simulating the tensile fracture of recycled brick coarse aggregate (RBCA) concrete. To this end, the study explores the effects of various critical factors on the fracture mode and bearing capacity of recycled brick aggregate concrete, including the replacement rate of recycled brick coarse aggregate, pore structure, interfacial transition zone (ITZ) strength, mortar strength, and volume fraction of brick aggregate. The results indicate that, when the minor to major axis ratio of elliptical pores is 0.5 ≤ K < 1, the following order of influence can be observed: random convex polygonal pores, circular pores, and elliptical pores. Moreover, excessively strengthening the ITZ and mortar does not significantly enhance the tensile performance of RBCA concrete. The distribution location of aggregate has a significant impact on the crack shape of recycled concrete, as does the pore structure, due to their randomness. Therefore, this article also discusses these. These findings contribute to a comprehensive understanding of the tensile properties of recycled brick coarse aggregate and provide insights into optimizing its behavior.

Mechanical properties of recycled aggregate concrete containing brick particles

To investigate the mechanical properties of recycled aggregate concrete containing brick particles (RACB), compressive strength tests were conducted on RACBs with different water/cement ratios and recycled fine aggregate substitution rates. The changes in compressive strength, elastic modulus and peak strain of RACB were analysed as a function of the proportion of substituted recycled fine aggregate, and a stress–strain equation was established. A damage model was established using ultrasonic non-destructive testing technology to reveal the damage mechanism of RACB. Results show that the basic mechanical properties of RACB are best when the replacement proportion of recycled fine aggregate (r) is 30%. The model parameter a of the stress–strain curve equation is consistent with the change of elastic modulus with r. The model parameter b has a quadratic function relationship with r, reflecting the brittleness of RACB. The initial damage of RACB is relatively large, and cracks penetrate the recycled coarse aggregate, leading to specimen failure.

Experimental Study of Mechanical Properties and Theoretical Models for Recycled Fine and Coarse Aggregate Concrete with Steel Fibers.

With diminishing natural aggregate resources and increasing environmental protection efforts, the use of recycled fine aggregate is a more sustainable approach, although challenges persist in achieving comparable mechanical properties. Exploration into the incorporation of steel fibers with recycled aggregate has led to the development of steel-fiber-reinforced recycled aggregate concrete. This study investigates the shrinkage performance and compressive constitutive relationship of steel fiber recycled concrete with different steel fibers and recycled aggregate dosages. Initially, based on different replacement rates of recycled coarse aggregate and different volume contents of steel fiber, experimental results demonstrate that as the replacement rate of recycled coarse aggregate increases, shrinkage also increases, while the addition of steel fiber can mitigate this effect. An empirical shrinkage model for steel fiber recycled concrete under natural curing conditions is also proposed. Subsequently, based on the uniaxial compression test, findings indicate that with an increasing replacement rate of recycled fine aggregate, the peak stress and elastic modulus of concrete decrease, accompanied by an increase in peak strain, and the addition of steel fiber limits concrete crack development and enhances its brittleness while the peak stress and strain of recycled fine aggregate concrete are enhanced. However, the steel fiber volume percentage has a negligible effect on the elastic modulus. A constitutive relationship for concrete considering the effects of recycled fine aggregate and steel fiber is also proposed. This finding provides foundational support for the influence patterns of steel fiber dosage and recycled aggregate ratio on the mechanical properties of steel fiber recycled concrete.

Mechanical properties of steel fiber reinforced recycled concrete under compression-shear stress state

The continuous urbanization of construction has led to an increasing demand for concrete, resulting in large-scale gravel and river sand mining, which seriously affect the sustainable development of the environment. The waste concrete produced by the demolition of old buildings is processed into recycled aggregate to replace natural aggregate, and then the preparation of recycled concrete can effectively alleviate this problem. This paper investigates the mechanical properties of steel fiber reinforced recycled concrete (SFRRAC) under compression shear conditions, a compression shear test with 99 cubic specimens was designed, with the variation parameters being recycled aggregate concrete replacement rate, compressive stress, and steel fiber content. The failure mode of SFRRAC under compression-shear stress was observed, and the shear force-displacement curve was calculated. Furthermore, a thorough examination was performed to assess the mechanical characteristics of SFRRAC specimens under compression-shear stress. The findings of this study suggest that as the compressive stress increases, the failure characteristics of the specimens shift from exhibiting brittle failure to displaying ductile failure. The shear strength is most sensitive to the change in compressive stress. When the compressive stress reaches 6 MPa, the shear strength can reach 3.41 times that of the direct shear specimen. The shear strength of SFRRAC specimens is composed of the bond strength of concrete, shear expansion strength, and friction shear strength. The incorporation of steel fibers can modify the distribution of shear strength among the individual components. The adverse effect of the recycled aggregate replacement rate on the toughness index of the specimen is mainly concentrated after the peak point. Nevertheless, an elevation in compressive stress and the inclusion of steel fibers can counterbalance this detrimental influence. The shear strength calculation formula of SFRRAC in shear state is put forward, which has a reference value. This study provides an experimental reference and a theoretical basis for further examination of SFRRAC.

Compression and Splitting Tensile Strength Model of Recycled Seawater and Sea Sand Concrete after Seawater Freeze–Thaw Cycles

Using seawater, sea sand, and recycled bricks to make concrete for nearshore and marine engineering can save resources and protect the environment. Therefore, this article studies the mechanical degradation law and failure mechanism of recycled seawater and sea sand concrete (SSC) after seawater freeze–thaw (SFT) cycles. Using the replacement rate of recycled brick coarse aggregate as a variable, the mass loss rate, relative dynamic elastic modulus, compression and splitting tensile strength, and microstructure changes of specimens under different SFT cycles were tested, and a mechanical performance degradation model was established. The results indicate that the SFT failure of recycled brick coarse aggregate SSC results from the action of physical SFT and chemical crystallization. Friedel’s salts without cementitious properties and ettringite with expansive properties are generated inside the concrete due to the chloride and sulfate ions in the internal concrete and external seawater reacting with cement hydration products. The formation of harmful crystals leads to the loosening of the concrete, especially in the interface area between brick aggregates and cement. The calculated results of the mechanical model established in this article agree with the test results. The compression and splitting tensile strength decrease linearly with the increase in SFT cycles.

Research on Macroscopic Mechanical Behavior of Recycled Aggregate Concrete Based on Mesoscale.

Recycled concrete is a heterogeneous composite material, and the composition and volume fraction of each phase affect its macroscopic properties. In this paper, ANSYS APDL was used to construct a two-dimensional numerical model of recycled aggregate concrete with different replacement rates of recycled aggregate (0%, 25%, 50%, 75% and 100%), and a uniaxial compression test was carried out to explore the relationship between recycled aggregate content and its macroscopic mechanical behavior. On this basis, the numerical simulation of different strain rates (0.1 s-1, 0.05 s-1, 0.01 s-1, 0.005 s-1 and 0.001 s-1) was carried out. It was found that with the increase in the recycled aggregate replacement rate, the peak stress decreases first and then increases, and the peak strain increases continuously. When the replacement rate of recycled aggregate exceeds 50%, the overall damage area of the material increases rapidly. The strain rate will change the path of the micro-crack initiation and expansion of recycled concrete, as well as the process of damage accumulation and evolution. As a result, the unit area and shape of recycled concrete are different at different strain rates, and the damage degree of each phase material is also different.

Preliminary investigation and life cycle assessment of artificial reefs with recycled brick-concrete aggregates

An economical and environment-friendly artificial reef concrete using fully recycled brick-concrete aggregate as raw material was proposed. This study investigates the mechanical properties, drying shrinkage, chloride ion penetration, and biological adaptability of the reef. A comparative analysis is conducted on the environmental impacts (EI), carbon emission impact and cost efficiency of artificial reefs, considering natural aggregate concrete and recycled brick-concrete aggregate concrete at the material level. The results indicate that the use of recycled brick-concrete aggregates can achieve service performance equivalent to that of natural aggregates. Field investigations involving samples immersed in seawater for six months reveal the positive trapping effects of recycled materials on aquatic organisms. The Life Cycle Assessment (LCA) results reveal that achieving a 100 % replacement rate of recycled brick concrete aggregate (RBCA) creates a positive feedback loop for carbon emissions when appropriately incorporating industrial waste materials like fly ash and silica fume. In comparison to ordinary concrete, the utilization of recycled materials can significantly reduce construction costs, up to 63.7 %, thereby presenting notable economic advantages.

Mechanical Properties of Recycled Concrete with Polypropylene Fiber and Its Bonding Performance with Rebars

This paper explores the influence of polypropylene fiber on the mechanical properties of recycled concrete and the bonding performance between recycled concrete and steel bars. The results indicate that the compressive strength and splitting tensile strength of concrete decrease with the increase in the replacement rate of recycled aggregate. When the replacement rate of recycled aggregate was 100 %, the compressive strength of concrete decreased by 25.0%, while the splitting tensile strength of concrete decreased by 21.7 %. The compressive strength and splitting tensile strength of recycled aggregate concrete show a trend of first increasing and then decreasing with increasing fiber content. When the fiber content is 0.09 %, the compressive strength and tensile strength of recycled aggregate concrete are optimal. The bond strength of recycled concrete pullout specimens with fiber is higher than that of recycled concrete pullout specimens without fiber, while the slip of recycled concrete pullout specimens with fiber is lower than that of recycled concrete pullout specimens without fiber. The maximum improvement in bonding performance between recycled concrete and rebars with a 50 % recycled aggregate replacement rate is 13.7 %, and the maximum improvement in bonding performance between recycled concrete and rebars with a 100 % recycled aggregate replacement rate is 11.8 %. The bond strength shows a trend of first increasing and then decreasing with increasing fiber content, but the degree of decrease was not significant, while the slip shows a trend of first decreasing and then increasing with increasing fiber content. Compared to the compressive strength of concrete, the splitting tensile strength can better reflect the bond strength between concrete and rebar. The nonlinear relationship between the bond strength, compressive strength, and splitting tensile strength of recycled concrete is established. This study can effectively improve the mechanical properties of recycled concrete, expand the application scope of recycled concrete technology, and promote the sustainable development of the construction industry.

Study on the mechanical properties and microstructure of recycled brick aggregate concrete with waste fiber

Abstract Recycled concrete technology can promote the sustainable development of the construction industry, but the insufficient mechanical properties of recycled concrete have become a key constraint on its development. By adding waste fibers, the mechanical properties of recycled concrete can be improved, and the problem of disposing of waste polypropylene fibers can be solved. In this article, the effects of recycled brick aggregate content and waste fiber content on the mechanical properties and microstructures of recycled brick aggregate concrete through macroscopic mechanical experiments and microstructure experiments are investigated. The results show that the addition of recycled brick aggregate reduces the mechanical properties of concrete; when the content of recycled brick aggregate is 100%, the compressive strength and splitting tensile strength decrease by 22.04 and 20.00%, respectively. The addition of waste fibers can improve the mechanical properties of recycled brick aggregate concrete, but it is necessary to control the contents of waste fibers in a certain range. When the content of waste fibers is 0.08%, the best improvement effect on the mechanical properties of concrete is achieved; the compressive strength of concrete with a 50% (100%) recycled aggregate replacement rate increases by 6.06% (8.90%), while the splitting tensile strength of concrete with a 50% (100%) recycled aggregate replacement rate increases by 2.30% (6.16%). Through microstructural analysis, the mechanism by which waste fiber improves the mechanical properties of recycled brick aggregate concrete is revealed. The addition of waste fibers has the effect of strengthening the framework inside the recycled brick aggregate concrete, forming a good structural stress system and allowing the recycled brick aggregate concrete to continue to bear loads after cracking. In this study, waste brick aggregate and waste fiber are effectively utilized, which can not only reduce pollution to the environment but also realize the sustainable utilization of resources.

Deterioration performance analysis of recycled brick concrete subjected to freezing and thawing effect

This study employed experimental and finite element modeling approaches to systematically investigate the mechanical damage mechanisms of recycled brick aggregate concrete (RAC) under freeze-thaw cycling conditions. The investigation encompassed analyses at three hierarchical levels: macroscopic, mesoscopic, and microscopic. The influence of recycled brick aggregate (RA) replacement rate and the number of freeze-thaw cycles on RAC's apparent state and mechanical performance was examined at the macroscopic level. Utilizing equipment such as electron microscopes, a comprehensive observation was conducted from mesoscopic and microscopic perspectives to elucidate the evolving patterns of the cementitious matrix, RA, and the interfacial transition zone (ITZ) within RAC during the freeze-thaw cycling process. On the one hand, more excellent internal space was inherent in RA compared to NA, enabling it to effectively accommodate the volumetric expansion resulting from water freezing. It contributed to mitigating the damage to concrete caused by freeze-thaw cycles. On the other hand, RA's rough and porous characteristics facilitated the infiltration of cement particles into its internal structure, forming a robust ITZ with RA. Concurrently, a numerical simulation of RAC's freeze-thaw cycling process was presented. This simulation generated coupled damage maps and load-deflection curves for RAC under various conditions, encompassing different levels of RA replacement rates, freeze-thaw cycle counts, and loading modes. The research results indicated that the inclusion of RA adversely affects the mechanical properties of concrete while concurrently altering the concrete's failure mode and the propagation pathways of internal cracks. Nevertheless, it was worth noting that the introduction of RA, despite diminishing the mechanical performance of concrete, effectively mitigates the rate of performance degradation during the freeze-thaw cycling process. A favorable equilibrium between freeze-thaw resistance and mechanical performance could be achieved when the replacement rate of RA for natural aggregates (NA) fell within the range of 10–30 %. By employing a finite element model to simulate the flexural performance of RAC after freeze-thaw cycles, the deviation range between the simulated and measured values fell within the range of 5.00–8.23 %. This affirmed the model's fitting solid capability and validated its effectiveness in analyzing the RAC structure's freeze-thaw-load coupled damage and lifespan prediction.

Steel tube filled with recycled concrete incorporating steel-fiber reinforcement and self-stressing effect

To widespread use of construction and demolition waste (CDW), an innovative steel-fiber-reinforced self-stressing recycled aggregate concrete filled steel tube (SFRCFST) is put forward. The mechanical behavior of SFRCFST under compression-bending is explored in this study. 27 specimens are conducted under eccentric compressive load, and the considered parameters are eccentricity ratio, steel tube thickness, concrete strength grade, quality replacement rates of recycled aggregate, steel fibers content and self-stress. Test results reveal that eccentric loaded SFRCFST fails in the same way of ordinary CFST columns. As the eccentric ratio is increased, the adverse impact of alternative recycled aggregates is more distinct. A double-reduction of load (Nu) and moment bearing capacities (Mu) of the specimens is observed when recycled aggregate replacement rate is increased. Introducing self-stress improves Nu and Mu with the average amount of 14.90% and 10.55%, respectively. Moreover, the flexural rigidity of SFRCFST columns is also enhanced. Moderate amount steel fibers with volume content of 1.2% ∼ 1.8% bring the improvement of Nu and Mu, as well as greater plastic development potential of SFRCFST columns. A refined FE model is proposed to simulate the eccentric loaded behavior of SFRCFST columns. Numerical results show that self-stress contributes to the confining effect throughout the loading process.

Calculation Method for the Cracking Resistance and Bearing Performance of SFRAC Beams.

The utilization of recycled aggregate (RCA) in preparing recycled concrete (RAC) is an effective measure to solve the increase in construction waste. Furthermore, applying RAC to flexural members is a viable practice. The addition of steel fiber to RAC to prepare steel fiber recycled concrete (SFRAC) solves the performance deterioration caused by the recycled aggregate, so it is necessary to study the effects of the recycled aggregate replacement rate and fiber-volume ratio on the crack resistance and bending performance of SFRAC beams. In this study, 13 beams were designed and manufactured, with the water-cement ratio, recycled aggregate replacement rate, and fiber-volume ratio as the primary variables, and the cracking moment and ultimate moment of the SFRAC beams were systematically studied. The results show that the cracking and ultimate moments of the SFRAC beams increased with decreases in the water-cement ratio or with increases in the fiber-volume ratio and were unaffected by the replacement rate of recycled aggregates. Based on the experimental results and theoretical analysis, a calculation model and formula for the cracking moment were established. The ultimate bearing capacity of SFRAC beams can be accurately determined using the ACI 318 and ACI 544 standards. The research results serve as a valuable reference for the design of SFRAC beams, effectively address the issue of performance degradation in RAC structural members, and promote the use of green building materials.

An experimental investigation of the bond performance of BFRP bars with recycled-aggregate concrete of different particle sizes

The pull-out test was performed in this study to investigate the bond performance between basalt fiber reinforced polymer (BFRP) bars and recycled concrete. Three factors were considered: aggregate size (5–10 mm, 11–15 mm, and 16–20 mm), bond anchorage length of bars (3D, 5D, and 7D), and the recycled aggregate replacement rate (0%, 30%, 60%, and 100%). The bond–slip mechanism, failure mode, bond stress distribution, and bond–slip curves were all analysed. Based on the results, it was found that situations with a high recycled aggregate replacement rate, large aggregate particle sizes, or long bond lengths have an increased likelihood of experiencing splitting failure. As the aggregate size increases, the bond strength between the BFRP bars and the concrete would also increase, in ordinary concrete, a 22.54% increase was observed, whereas in recycled concrete, the maximum increase reached 48.20%. The effect would be more pronounced on recycled concrete compared to ordinary concrete, and it would also be more significant in specimens with BFRP bars compared to steel bars. Additionally, the optimal bond anchorage length was determined to be 5D. The bond strength exhibited a trend of decrease followed by an increase as the recycled aggregate replacement rate increased. The maximum reduction of 59.7% in bond strength was observed when the replacement rate of recycled aggregate reached 60%. Moreover, when the replacement rate of recycled aggregate increased to 100%, there was a decrease of 19.05% in bond strength compared to the specimen without recycled aggregates. Based on the experimental test data and existing models, an improved two-stage bond–slip constitutive model was proposed.

Mechanical Properties of Steel Fiber-Reinforced, Recycled, Concrete-Filled Intersecting Nodes in an Oblique Grid

The construction of high-rise oblique grid buildings requires a large amount of concrete. To save resources, an oblique grid of intersecting nodes composed of steel outer tubes and steel fiber, recycled concrete inner tubes (OGSFRCIN) has been proposed. ABAQUS is used to study the mechanical properties of the nodes under axial pressure, accounting for the effects of six parameters: the oblique angle, the thickness of the stiffening ring, the thickness of the connecting plate, the concrete strength, the recycled aggregate replacement rate, and the steel fiber content. The results show that the oblique angle, connecting plate thickness, concrete strength, and steel fiber content significantly affect the ultimate bearing capacity of specimens. The reinforcing ring thickness has a significant effect on the evolution of lateral displacement. It is not advisable to use components with a replacement rate of 100% recycled aggregate in engineering practice because of insufficient lateral stiffness and ultimate strength. The specimen’s out-of-plane displacement is tightly restricted when the connecting plate’s thickness is greater than or equal to 10 mm. In practical engineering, the connecting plate and reinforcing ring thickness should not be less than 10 mm, and the recommended steel fiber content is 1.0% to 2.0%. Through the analysis of the mechanical properties of the OGSFRCIN under monotonic axial compression and reciprocating axial tension and compression loads, it can be seen that OGSFRCIN have good mechanical properties and can be applied in engineering practice. Here, the modified formulas for calculating the bearing capacity of OGSFRCIN are put forward.

Experimental investigation on mechanical properties of glass fiber reinforced recycled aggregate concrete under uniaxial cyclic compression

In this investigation, glass fiber (GF) was incorporated into recycled concrete to form a new sustainable material (glass fiber reinforced recycled concrete) to solve the shortcomings of easy cracking and high brittleness of recycled concrete. At present, there are few investigations and experiments on the mechanical properties of the material under uniaxial cyclic compression, and there is a lack of prediction of the behavior of the material in a specific mechanical environment. Thus, this investigation focuses on the stress–strain relationship and mechanical properties of glass fiber reinforced recycled concrete (GFRAC) under uniaxial cyclic compression. In order to achieve this goal, the stress and strain of the specimen under cyclic compression were discussed by designing three variables: recycled aggregate replacement rate, glass fiber content, and loading rate. The failure process of GFRAC under cyclic loading was observed and recorded, and the failure mechanism of GFRAC was analyzed in depth from macro to micro levels. The stress–strain curve law of GFRAC under cyclic action is summarized. The plastic strain and stiffness degradation law of recycled glass fiber reinforced concrete were revealed. The hysteretic energy consumption of each cycle is quantified and compared. The results show that, under the influence of GF content and recycled aggregate replacement percentage, GFRAC specimens appeared oblique shear failure, splitting failure and oblique shear & splitting failure under cyclic action. The stress–strain envelope curve of GFRAC under cyclic loading is similar to the monotonic one. Increasing the replacement ratio of aggregates can increase the peak strain of the specimen under cyclic action, and the maximum improvement value can reach 12.62 percent. The addition of GF can improve the ductility of recycled concrete, increase the hysteresis energy consumption, and finally affect the stiffness degradation rate of the specimen. Among them, under the same recycled aggregate replacement rate and the same cycle numbers, the stiffness degradation rate of the specimen with 0.05 % GF content is twice that of the specimen with 0.15 % GF content. This investigation provides a theoretical basis and reference for further research on GFRAC.

Experimental and theoretical study on bonding performance of FRP bars-Recycled aggregate concrete

Fiber Reinforced Polymer (FRP) bars-Recycled Aggregate Concrete (RAC) can alleviate the environmental damage caused by the massive exploitation of natural aggregates and solve the problem of corrosion of steel bars in concrete structures. However, the bonding performance of FRP bars-RAC determines whether it can replace the original reinforced concrete structure in buildings. In the current study, FRP bars-RAC bond performance factors and bond strength theory have been comprehensively investigated through pull-out tests performed for a large number of FRP bars-RAC. In this context, the effects of Recycled Aggregate (RA) replacement rate (0 %, 25 %, 50 % and 100 %), FRP bar diameter (8 mm, 10 mm and 12 mm) and RAC cover thickness (20 mm, 30 mm, 40 mm and 70 mm) on the bond strength, fracture mode, and bond-slip curve of FRP bars-RAC were investigated. Overall, the findings showed that the inclusion of RA reduces the ductility of concrete. Increasing the recycled aggregate replacement rate resulted in gradual losses in bond strength and ultimate slip of FRP bars. In addition, the increase in the diameter and anchor length of the FRP bars led to a gradual decrease in the bond strength between the FRP bars and the RAC. Nevertheless, through increasing the strength of RAC and the thickness of the concrete protective layer, the bond strength between FRP bars-RAC can be enhanced. Through the analysis of test data, the bond strength model of FRP bars-RAC was established based on the thick-walled cylinder theory. In the model, the diameters of FRP bars, rib inclination angle, rib spacing, RAC strength, and protective layer thickness were taken as variables. While the high degree of agreement between the values calculated from the model and the measured values revealed the success of the established modeling, the ratio of the calculated value of the model to the experimental value was measured as 0.66–1.

Effect of Microstructure on the Mechanical Properties of Steel Fiber-Reinforced Recycled Concretes.

A steel fiber-reinforced recycled concrete (SFRRC) is a porous material, and its macromechanical properties are affected by its microstructure. To elucidate the change rules and internal mechanisms of the mechanical properties of SFRRCs, the mechanical properties and failure modes of SFRRCs were studied at different water–cement ratio, replacement rate of recycled concrete aggregate (RCA), and steel fiber content. Moreover, the microstructures of the interface transition zones (ITZ) of the SFRRC specimens were tested by scanning electron microscopy and mercury intrusion, and the effect of the microscopic pore structure on the macromechanical properties of SFRRC was analyzed. The research results showed that an appropriate amount of steel fibers could reduce the size and number of cracks in the ITZ and improve the pore structure of an SFRRC. Based on the fractal dimension, porosity and other factors, the quantitative relationship between the macromechanical properties and microscopic pore structure parameters of SFRRCs was established.