Recycled Coarse Aggregate Concrete Research Articles

Bond performance of recycled coarse aggregate concrete with rebar under freeze–thaw environment: A review

Abstract In this article, a review focusing on the frost resistance of recycled coarse aggregate concrete (RAC) and the bond performance of RAC with rebar under freeze–thaw environment is carried out. The results show that there are still some controversies about the advantages and disadvantages of the frost resistance of RAC, and many influencing factors have not been considered. The mass of the RAC pullout specimens shows a trend of first increasing and then decreasing after freeze‒thaw cycles. The failure modes of the RAC pullout specimens after freeze‒thaw cycles include pullout failure, pullout-splitting failure, splitting failure, and rebar yielding failure. The bond performance of RAC with rebar shows a degradation trend after freeze‒thaw cycles, mainly in the decrease in bond strength and the increase in bond slip. The bond performance of RAC specimens is inferior to that of natural aggregate concrete specimens after freeze‒thaw cycles. The bond strength prediction models and the bond–slip constitutive relation prediction models of RAC with rebar after freeze‒thaw cycles have been summarized to ensure that engineers can better understand their applicability. The bond stress distribution between the RAC and rebar in the anchoring area is not uniform and constantly changes with the number of freeze‒thaw cycles. The degradation mechanism of the bond performance of RAC with rebar after freeze‒thaw cycles is analyzed.

Research on the mechanism of chlorine corrosion resistance of graphite tailings modified recycled coarse aggregate concrete: Corrosion product transformation and multi-scale mathematical characterization model

This research investigated the impact of substituting sand with graphite tailings (GT) on the chloride ion erosion resistance of recycled aggregate concrete (RAC). The role of GT is to significantly inhibit the penetration of chloride ions into the interior and slow down the corrosion process. However, the durability of graphite tailings recycled aggregate concrete (GTRAC) still depends on the replacement rate of recycled aggregate. At the same time, a multi-scale dynamic model of physical adsorption-chemical binding-free transport and erosion of chloride ions in GTRAC were established based on unsteady viscous fluid force and molecular dynamics.

Durability life evaluation of marine infrastructures built by using carbonated recycled coarse aggregate concrete due to the chloride corrosive environment

Due to the harsh marine environment of chloride ion invasion and corrosion, the issues of long-term chloride transport and durability life evaluation for marine infrastructures constructed/maintained by recycled aggregate concrete (RAC) after enhancement remain poorly understood. For our studies, an accelerated carbonation modification method for recycled coarse aggregate (RCA) was adopted to prepare carbonated recycled coarse aggregate (CRCA) samples, and the macroproperties, i.e., apparent density and water absorption, of CRCA were enhanced by approximately 1.40-3.97% and 16.3-21.8%, respectively, compared with those of RCA. An in-door experiment for chloride transport into concrete specimens subjected to a simulated marine environment of alternating drying-wetting cycles was conducted. The chloride profiles and transport characteristics of carbonated recycled coarse aggregate concrete (CRCAC), recycled coarse aggregate concrete (RCAC), and natural coarse aggregate concrete (NCAC) were analysed and compared. The results indicated that the chloride penetration depths and concentrations of CRCAC were approximately 52.6-96.2% of those of RCAC, which highlighted the better chloride resistance of CRCAC. A chloride transport model for marine concrete structures with various coarse aggregate types in a corrosive marine environment was established. Taking a certain harbour wharf as an example, the durability life of this case considering the application of the CRCAC was evaluated based on the chloride transport model, and the durability life of the CRCAC structure was improved by approximately 28.10% compared with that of the RCAC. The CRCAC developed in this paper has improved mechanical performance and durability than those of RCAC, and it has the potential to replace the NCAC and further support the construction and maintenance of marine infrastructures.

Carbonation of Recycled Concrete Aggregates for New Concrete and Concrete Fines to Make Cement-Free Hollow Blocks

Mineral carbonation provides a way to increase the recycling of concrete waste in added-value products, and contributes to the principles of the circular economy. At present, most concrete waste is still downcycled. The high water absorption of recycled concrete aggregates, among other factors, impedes their recycling in the concrete industry. The quality of coarse recycled concrete aggregates (RCA) can, however, be enhanced by carbonation. Even when starting with high-grade RCA obtained from a selective demolition process, the carbonation process can decrease the water absorption of the RCA to as low as 3.0%. Concrete with a 50% replacement rate of carbonated RCA can be produced without a significant compressive strength reduction. The research further shows that carbonation can be performed at atmospheric pressure and low CO2 concentrations (e.g., 10%). The recycled concrete fines (RCF, 0–4 mm) in combination with 25% stainless steel slag were used to make zero-cement hollow blocks (39 × 19 × 9 cm) by carbonation curing without using any hydraulic binder. The hollow blocks have a compressive strength of 15.4 MPa at the lab scale. Both technologies were demonstrated on a pilot scale. In both processes, CO2 is immobilized in the resulting construction product. The developed production processes use less primary raw materials and cause less greenhouse-gas emissions than the production of traditional concrete products.

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

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

Tensile Mechanical and Stress-Strain Behavior of Recycling Polypropylene Fiber Recycled Coarse Aggregate Concrete

Tensile Mechanical and Stress-Strain Behavior of Recycling Polypropylene Fiber Recycled Coarse Aggregate Concrete

Mechanical and Durability Characterization of Hybrid Recycled Aggregate Concrete.

The recycling of construction and demolition waste (CDW) for the extraction of recycled concrete aggregates (RCAs) to be used to produce recycled aggregate concrete (RAC) is widely acknowledged internationally. However, CDW not only contains concrete debris but may also contain burnt clay bricks. The recycling of such CDW without the segregation of different components would result in recycled aggregates having different proportions of concrete and brick aggregates. The utilization of these aggregates in concrete requires a detailed investigation of their mechanical and durability properties. In this regard, the present study focused on investigating the mechanical and durability properties of hybrid recycled aggregate concrete (HRAC) made by the 100% replacing of natural aggregates with recycled brick (RBAs) and RCA in hybrid form. The partial replacement of cement with fly ash was also considered to reduce the corban footprint of concrete. An extensive experimental program was designed and carried out in two phases. In the first phase, a total of 48 concrete mixes containing coarse RBA and RCA in mono and hybrid forms were prepared and tested for their compressive strength. The test results indicated that the compressive strength of HRAC is greatly affected by the proportion of coarse RBA and RCA. In the second phase, based on the results of the first phase, eight concrete mixes with the most critical proportions of RBA and RCA in hybrid form were selected to evaluate their mechanical and durability performance. In addition, four mixes with natural aggregates were also prepared for comparison purposes. To evaluate the mechanical properties of the concrete mixes, compressive strength and modulus of rupture (MOR) tests were performed, while for the evaluation of durability properties, water absorption and behavior after exposure to aggressive conditions of acidic and brine solutions were studied. The results revealed that a 20% replacement of cement with fly ash resulted in acceptable mechanical and durability properties of HRAC intended to be used for making concrete bricks or pavers.

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

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

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

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

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

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

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

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

Multiscale analysis of recycled coarse aggregate concrete under the synergistic action of KH560 and PVA fibers

Considering the long-term working performance of the waste concrete building elements, this study used 3-Glycidoxypropyltrimethoxysilane (KH560) and Polyvinyl alcohol (PVA) fibers to modify the recycled concrete (RC) with the aim of improving the mechanical and long-term properties of RC. In this study, the mechanical properties of recycled coarse aggregate (RCA) concrete before and after modification were systematically investigated. The macroscale results showed that KH560 compensates for the reduced compressive strength of RCA concrete due to PVA fibers. The synergistic effect of PVA and KH560 substantially increased the flexural and shear strength of RCA concrete, with a maximum increase of 22.6% in flexural strength when the RCA substitution rate was 80%. The results of three microscopic tests (SEM / FTIR/XRD) revealed that KH560 increased the content of cement gel, which then optimized the internal pore structure of RCA concrete. The chemical reaction between KH560 and cement produced SiKH560-OKH560-SiCSH. At the nano-scale, computational analysis methods in atomistic modeling techniques were employed to model PVA fibers, cement and KH560, and the dynamic and static simulation results of the model were combined to discover a consistent SiKH560-OKH560-SiCSH chemical bond between KH560/CSH and both old and new CSH, Additionally, the findings revealed the coexistence hydrogen and ionic bonds. KH560 improves the interaction between PVA fibers and cement by forming numerous hydrogen bonds, and resolves the weak interface between the new and old cement through stable chemical bonds.

Load-bearing capacity of precast recycled aggregate concrete column-to-column connections

Recycled aggregate concrete (RAC) often shows different strength and deformation properties from natural aggregate concrete (NAC) and may therefore have an impact on the connections between precast concrete elements. In this work, three simple connections between precast concrete columns were considered, including a wet connection with bedding mortar, a dry connection without bedding mortar and a monolithic joint with plain concrete. An industrially produced coarse recycled concrete aggregate (RCA) was used to partially or fully replace a coarse natural aggregate in the concrete. A total of twenty-two 200 mm × 200 mm × 500 mm reinforced concrete columns were prepared and tested under axial monotonic loading through displacement control. The results show that regardless of RCA content, the equation specified in ACI 318 (ACI, 2019) and the one proposed by Vambersky (1990) provide safe estimates of the load-bearing capacity of wet connections between reinforced RAC columns.

Correlation between mechanical properties and pore structure deterioration of recycled concrete under sulfate freeze-thaw cycles: an experimental study

Concrete is subject to the combined erosive effects of physical and chemical activities in cold, salty soil regions. In this work, durability tests of recycled concrete (RC) subjected to sulfate freeze-thaw cycles were conducted. The macroscopic performance deterioration law of RC under the influence of the replacement rate (0%, 50%, 100%) and the moisture content of coarse recycled concrete aggregate (CRCA) (0%, 50%, 100%) was investigated by analyzing the change characteristics of apparent damage, mass loss rate, and Relative dynamic modulus of elasticity (RDME) of RC during the erosion process. At the same time, nuclear magnetic resonance (NMR) and microhardness testing equipment were used to examine the multi-parameter evolution features such as porosity, pore distribution, interfacial transition zone (ITZ) width, and strength. The findings indicate that the main cause of the variation in the degree of damage to the RC surface layer is the variance in the effective water-cement (w/c) ratio of the mortar due to replacement rate and moisture content. The strength and area of erosion damage increase when the CRCA replacement rate rises due to the easier inward penetration of the sulfate solution. CRCA with a 50% moisture content could increase the strength of the mortar by decreasing the mortar's effective w/c ratio. The rate and effectiveness of salt solution replenishment inward were simultaneously slowed down by the improved ITZ performance. In erosive situations, the fractal dimension of RC reduces to varying degrees. This is due to the expansion of the pore structure. The porosity/fractal dimension is employed as the comprehensive pore parameter η in this research so as to take into account the integrity of the pore structure and the specificity of the pore distribution. The improved microstructure damage variables can reflect the erosive microstructure deterioration process of RC.

Effect of silica fume on strength of recycled aggregate concrete

The present study deals with investigating the effects of using coarse recycled concrete aggregates (RCA) on compression & split-tensile resilience of M25 grade concrete mixes. Five different mixes of modified recycle aggregate concrete (RAC) were designed and prepared considering replacement proportion of normal coarse aggregates as 0, 25, 50, 75, and 100 % with RCA. The specimens were prepared according to Indian standard codal provisions and experimentally tested for its mechanical characteristics in laboratory conditions. A steep degradation in compressive and tensile strength on increasing RCA proportion was observed, which may result due to improper bonding at interfacial region in RAC mixes. To overcome this strength deterioration, the effect of adding pozzolanic materials like silica fumes is also studied for making strength improvements. The silica fumes as admixture, in percentages of 4%, 8% and 12%, was added in RAC mixes for finding the optimum proportions. Overall, the study provides a sustainable solution for problematic concrete waste dumping while utilizing it in casting new concrete structures of significant strengths.

The durability of spray steel fiber-reinforced recycled coarse aggregate concrete

To boost the application scope of coarse recycled aggregates (RA) with particle size in the range of 5–10 mm, a spray steel fiber reinforced coarse aggregate concrete (SSFRCAC) is developed, and the impact of cube compressive strength, RA replacement ratio, and steel fiber (SF) volume fraction on the durability of SSFRCAC is comprehensively examined through carbonization, sulfate attack, penetration, and freeze-thaw cycle tests. The anti-carbonation test results reveal that as the RA replacement ratio increases, the carbonation resistance of SSFRCAC decreases. The best anti-carbonation performance is achieved when the SF volume fraction is 1%. Additionally, a formula for calculating the carbonation depth is established. Based on the experimental data, the dynamic elastic modulus is recommended as the measurement standard for the freeze-thaw resistance cycle test. The sulfate resistance test results indicate that as the RA replacement ratio increases, the Sulfate Resistance coefficient also increases. SF block the thermal cracks of specimens and corrosion products as well as the formation of expansion cracks, so the corrosion resistance coefficient gradually tends to a certain stable level. According to the anti-permeability test results, the increase in the RA replacement ratio can suppress the anti-permeability performance. When the SF volume fraction is 1%, the concrete has the highest density and the best anti-permeability performance. Overall, the durability of SSFRCAC is found to be much better than that of general recycled SF Recycle aggregates concrete, which makes it more suitable for practical engineering applications.

Assessing the compressive and splitting tensile strength of self-compacting recycled coarse aggregate concrete using machine learning and statistical techniques

The construction industry is adopting high-performance materials due to technological and environmental advances. Researchers worldwide are studying the use of recycled coarse aggregates (RCA) as a partial alternative to natural coarse aggregates in concrete due to their sustainability and environmental benefits. This study compares the predictive abilities of three different machine learning techniques in evaluating the mechanical properties of 28-day-old self-compacting concrete (SCC) incorporating RCA to better understand how design parameters affect the mechanical properties of SCC containing RCA. The study used a range of statistical methodologies and machine learning algorithms, such as ANN, SVM, and M5P trees, to examine the relationship between design elements and accurately forecast the mechanical characteristics of concrete. The ANN model exhibited notable superiority in effectively forecasting compressive strength (CS) and splitting tensile strength (STS) compared to other models, with uncertainty bands of 15.038%− 21.154% for CS and 15.701%− 19.008% for STS. Moreover, all the uncertainties were under the threshold of 35%. Notably, the water-cement ratio emerged as the most crucial parameter in accurately predicting the mechanical properties of SCC. Finally, the parametric evaluation conducted revealed that CS and STS are inversely proportional to water-cement ratio and aggregate-cement ratio, whereas, are directly proportional to the water-binder ratio, and water-solids percentage.

Bond Strength of Steel Bars in Sustainable Concrete Made with Recycled Aggregates

Lately, considerable efforts have been directed towards investigating the practicality of using coarse recycled concrete aggregate as a potential substitute for natural coarse aggregate (NCA) in concrete construction. Research has shown that both the proportion of replacement and the properties of recycled aggregates play a significant role in affecting the tensile strength, compressive strength, and bonding characteristics of concrete. In certain cases, recycled aggregate concrete exhibits lesser mechanical properties when compared to concrete made with normal aggregates. The current study investigated the performance of Sika AnchorFix-1 to enhance the bond strength between steel bars and recycled aggregate concrete. The findings suggest that Sika AnchorFix-1 successfully prevented the concrete rupture in the case of concrete with recycled concrete aggregates, whereas concrete rupture could not be prevented in the case of concrete with recycled brick aggregates. Hence, a lower bond strength was obtained for specimens with recycled bricks aggregates than specimens with recycled concrete aggregates.

Microscopic mechanism analysis and failure criterion study of glass fiber recycled concrete under three‐dimensional compressive stress state

AbstractThis study aims to address the complex stress state commonly observed in concrete structures in practical engineering, as well as the challenges posed by resource scarcity and environmental pollution. To achieve this, a total of 168 cylinder specimens of glass fiber recycled coarse aggregate concrete (GFRAC) were designed for conventional triaxial compression tests. The analysis of the test results reveals that the application of lateral restraint force induces a shift in the failure mode of GFRAC from longitudinal splitting failure to plastic deformation failure, occurring either in the middle or at the end of the specimen. The lateral constraint force can effectively improve the stress and strain at the peak point of GFRAC and the modulus of elasticity of the specimen. The glass fiber increases the peak stress of the specimen, and the maximum growth rate is 20%. On the contrary, the glass fiber reduces the peak strain, and the maximum reduction is 49.66%. Increasing the replacement rate of recycled coarse aggregate will lead to a decrease in peak stress, with a maximum decrease of 19.17%. Glass fiber (GF) limits the appearance and propagation of cracks by bridging, pulling out or breaking. Finally, three different failure criteria are used to analyze the triaxial compression state of GFRAC from a macro perspective.

Performance enhancement of recycled coarse aggregate concrete by incorporating with macro fibers processed from waste GFRP

Reprocessing into coarse aggregates of concrete is a sustainable solution for the disposal of construction and demolition (C&D) waste. The use of recycled coarse aggregates however leads to a degradation in the mechanical properties of the resulting concrete, which then limits the use of recycled aggregates concrete (RAC) in structural elements. The incorporation of discrete fibers shows to be effective in eliminating the mechanical degradation problem of RAC, and this paper has therefore proposed to use macro fibers recycled from waste glass fiber reinforced polymer (GFRP) to reinforce RAC, thus improving the robustness of using RAC in structural elements. The effect of incorporating macro fibers on the mechanical properties of RAC has been first examined through a series of compression and bending tests. The tensile constitutive law of concrete was then derived based on the load-deflection response of the beam specimens using an inverse analysis. The results revealed that the mechanical properties (e.g., compressive toughness, flexural strength, flexural toughness) of RAC can be significantly enhanced by adding macro fibers. For concrete with 50% recycled aggregate (RA), the increase of macro fibers (from 0% to 0.5%, 1%, and 1.5%) leads to the enhancement of flexural strength from 3.13 MPa to 3.54, 4.69, and 5.97 MPa, and the toughness (T150) from 1.4 J to 35.5, 59.2 and 77.6 J, respectively. Such enhancements in the mechanical properties of RAC demonstrate the feasibility of the proposed method.