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As the world's largest producer of construction waste, China's recycling and related policies are of the biggest concern to the world. However, the effective disposal and reuse of this waste has become an important issue since currently China still has a very low recycling ratio compared to developed countries, and most of the waste concrete was only simply broken and used as low-grade recycled aggregates for subgrade cushion, cement stabilized crushed stone, and filler wall. In this paper, a concrete cycle model focusing on how to effectively recycle and utilize waste concrete is put forward to prepare high quality recycled concrete, especially through a series of technical means, such as effective separation, carbon sequestration, and reactivation. Producing high quality recycled concrete can not only replace traditional concrete but also effectively reduce the consumption and waste of raw materials. What's more, the calculation results show a potential of significantly carbon sink; for every ton of recycled cement produced, the CO2 emission could be reduced by 0.35–0.77 tons compared to ordinary Portland cement, corresponding to a reduction of 47%–94%; and for every ton of recycled concrete produced, the CO2 emission could be reduced by 0.186 tons compared to normal concrete. A yearly CO2 sequestration of 1.4–3.08 gigatonnes could happen if the ordinary Portland cement could be replaced by the recycled cement around the world. Taking the currently accumulated construction and demolition (C&D) wastes globally, the production of recycled cement, recycled aggregates, and recycled concrete could induce a significant carbon sink in the world. [ABSTRACT FROM AUTHOR]

If recycled concrete is to be widely, reasonably, and safely applied in practical engineering, the study of bond‐slip performance between steel bars and concrete is crucial. In this paper, the central pull‐out test of recycled concrete under different loading rates was conducted. The characteristics of failure mode and crack directions of the specimens were observed and analyzed. The ultimate bond strength and slip displacement between rebar and concrete were analyzed. The effects of three parameters, namely concrete strength grades, replacement percentage of recycled coarse aggregate, and loading rates on the failure modes and bond‐slip mechanical properties of recycled concrete, were analyzed and summarized. The experimental results showed that there were significant differences in the influence trend and amplitude of the above three parameters on the ultimate bonding strength, slip amount, and slip curve. Based on the above experimental and theoretical analysis, a reasonable bond‐slip constitutive model was established in this paper and was in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Translation abstract Durable and sustainable bridge caps made of non‐metallic reinforcement and recycled concreteDue to chloride‐induced corrosion, bridge caps made of steel‐reinforced concrete need to be replaced regularly or more often than planned. As part of a ZIM project funded by the Federal Ministry for Economic Affairs and Climate Action, bridge caps made of non‐metallic basalt fibre reinforcement and recycled concrete were developed to mitigate the weakness of corrosion and to better utilize the materials in terms of the material cycle. To achieve this, bend reinforcement elements made of chloride‐resistant basalt fibre reinforcement and concrete with high frost‐thaw resistance were developed, consisting of up to 100 % recycled aggregate. All sub‐developments were integrated into a holistic approach, and a prototype of a resource‐efficient and durable bridge cap was successfully produced and tested. [ABSTRACT FROM AUTHOR]

To study the bond properties between basalt fiber reinforced polymer (BFRP) bars and hybrid fibers which were basalt fiber (BF) and polypropylene fiber (PF) reinforced recycled concrete under freeze-thaw cycles, conducting center pull-out tests to study the effects of three factors on the bond properties: the number of freeze-thaw cycles, the volume fractions of single fiber, and the volume fractions of hybrid fibers. Based on the data obtained from the test, establishing a four-stage bond-slip constitutive relationship model. The results showed that the failure modes were pull-out failure and splitting failure. The bond strength decreased when adding the single fiber, with a maximum reduction of 13.18%, but the peak slip increased, with a maximum increase of 69.92%. When the volume fraction of BF was 0.3%, it achieved the optimal effect. The bond strength and peak slip increased when adding hybrid fibers, with maximum increases of 13.47 and 130.08%, respectively. However, excessive fiber content will reduce the increase of bond strength. The bond strength between BFRP bars and hybrid fiber-reinforced recycled aggregate concrete (HFRAC) increased when the number of freeze-thaw cycles increased but decreased when the number of freeze-thaw cycles exceeded 50. The four-stage bond-slip constitutive relationship model fitted well with the bond-slip curves. Compared with other fiber-reinforced recycled concrete specimens, this model fitted better with the curves of HFRAC specimens after freeze-thaw cycles and had the best fitting effect for the internal crack slip stage of the curves. [ABSTRACT FROM AUTHOR]

Use of 100 % recycled aggregates in concrete – report on practical applications This article reports on eight practical examples of applications for concrete with 100 % recycled aggregates. The applications include reinforced/unreinforced bored piles, foundations, load‐bearing internal/external walls, columns and floor slabs as well as balcony elements. Some components were realised as architectural concrete components. The respective project is summarised together with the results of the concrete development and concrete testing as well as the experience gained from obtaining special approvals in individual cases. Based on the findings from the various practical projects, it can be concluded that concrete with 100 % RC‐GK can be used for a large number of applications, so far limited to up to XC4 and XF3. Compressive strength for ready‐mix concrete up to C35/45 with good workability even with pumped concrete (here: pumping distance 45 m) using 100 % RC‐GK type 1 were possible without any problems. A concrete with sufficient early strength and a final strength of C40/50 was developed for precast elements. In two examples, RC‐GK type 2 was used for the coarse grain fraction and RC‐GK type 1 for the sand. All the projects presented have already been completed or are nearing completion. The feasibility of 100 % RC concrete in practice has thus been successfully demonstrated in several cases. [ABSTRACT FROM AUTHOR]

Geopolymer concrete, a cement-free concrete with recycled concrete aggregate (RCA), offers an eco-friendly solution for reducing carbon emissions from cement production and reusing a significant amount of old concrete from construction and demolition waste. This research on self-compacted, ambient-cured, and low-carbon concrete demonstrates the superior performance of one-part geopolymer concrete made from recycled materials. It is achieved by optimally replacing treated RCA with a unique method that involves coating the recycled aggregates with a one-part geopolymer slurry composed of fly ash, micro fly ash, slag, and anhydrous sodium metasilicate. The research presented in this paper introduces predictive models to assist researchers in optimising concrete mix designs based on RCA rates and treatment methods, including the incorporation of coated recycled concrete aggregates and basalt fibres. This study addresses the knowledge gap regarding geopolymer concrete based on recycled aggregate, various RCA rates, and novel RCA treatments. The novelty of the paper also lies in presenting the effectiveness of Artificial Neural Network (ANN) models in accurately predicting the compressive strength, splitting tensile strength, and modulus of elasticity for self-compacting geopolymer concrete with various rates of RCA replacement. This addresses a knowledge gap in existing research on ANN models for the prediction of geopolymer concrete properties based on RCA rate and treatment. The ANN models developed in this research predict results that are more comparable to experimental outcomes, showcasing superior accuracy compared to linear regression models. [ABSTRACT FROM AUTHOR]

As a green ecological concrete, recycled concrete is widely used in ecological water conservancy projects. In order to improve its mechanical properties and durability, this paper pretreated the recycled aggregate to dry, semi-saturated and saturated states by internal curing method, replaced ordinary concrete with 50% and 100% substitution rates, respectively, and set up ordinary concrete in dry state as a control group, analyzed the influence of water content on the macro and mesoscopic properties of recycled concrete under different substitution rates, and analyzed the correlation of various parameters, and interpreted the mechanism of internal curing from the mesoscale by establishing a water absorption and desorption model of recycled aggregate. The results show that under the same substitution rate, the recycled concrete group with 50% moisture content has the better compressive strength after curing for 90 days. The porosity of R50P and R100P decreased by 14.2% and 10.3% respectively after 90 days of curing compared with that at 7 days, indicating that the recycled aggregate with 50% moisture content could improve the porosity and interface transition zone structure to a certain extent. The internal curing mechanism of aggregate water absorption and desorption is the main reason for the continuous growth of the strength of recycled concrete in the later stage. [ABSTRACT FROM AUTHOR]

The energy dissipation and stress-strain characteristics, and characteristic stresses, namely the crack initiation σci , dilatancy σcd , and peak σf stresses, of polypropylene fiber-reinforced recycled concrete under uniaxial compression were studied. According to the research results, the crack initiation and peak stresses of the specimen with single-blend polypropylene coarse fiber (No. 3) and the specimens with mixed-blend coarse and fine polypropylene fibers (No. 4 and No. 5) are higher than those of the specimen with single-blend fine fiber and plain concrete. The analysis of the energy characteristics and failure mechanism of polypropylene fiber-reinforced recycled concrete during loading based on the principle of energy conservation showed that the total strain energy, elastic strain energy, and dissipation energy absorbed per unit volume increase with the blending of polypropylene fiber. The strain energy and elastic strain energy of coarse aggregates with a 5-10 to 10-20 mm coarse aggregate size ratios of 5:5 are higher than those of 4:6 and 6:4. It was found that the continuous blending of polypropylene fiber increases the elastic strain energy, causing the point at which the dissipation energy exceeds the elastic strain energy move further and further back. The position where the dissipation energy exceeds the elastic strain energy can be used to evaluate the blending effect of polypropylene fiber. The further back the position, the better the blending effect. [ABSTRACT FROM AUTHOR]

To investigate the mechanical properties and a compressive constitutive model of recycled concrete under sulfate attack considering the effects of multiple factors, two waste concrete strengths (i.e., C30 and C40), four replacement ratios of recycled coarse aggregates (i.e., 0, 30%, 50% and 100%), and two water–cement ratios (i.e., 0.50 and 0.60) were considered in this study, and a total of 32 recycled concrete specimens were designed and tested. The results indicated that the failure processes and patterns of recycled concrete were not significantly influenced by the replacement ratio of recycled coarse aggregates, the waste concrete strength, the water–cement ratio, or sulfate attack. The higher the replacement ratio of recycled coarse aggregates and the water–cement ratio and the lower the waste concrete strength, the more obvious the reduction in cubic compressive strength, with a maximum reduction of 38.48%. A prediction model for the cubic compressive strength of recycled concrete under sulfate attack was proposed. The higher the replacement ratio of recycled coarse aggregates and the water–cement ratio and the lower the waste concrete strength, the more significant the reduction in axial compressive strength, with a maximum reduction of 37.82%. A prediction model for the axial compressive strength of recycled concrete under sulfate attack was established. A compressive constitutive model of recycled concrete under sulfate attack considering the effects of the replacement ratio of recycled coarse aggregates, the waste concrete strength, and the water–cement ratio was established. The pore structure of recycled concrete was significantly destroyed by the expansion stress generated by Na2SO4 crystals: a large number of Na2SO4 crystals were attached to the surface of concrete matrix, and the concrete matrix became loose. The research results can provide a theoretical basis and data support for engineering applications of recycled concrete. [ABSTRACT FROM AUTHOR]

To mitigate the impact of vehicular emissions on air quality and reduce the concentrations of atmospheric pollutants, this study developed pavement materials with purification functions using TiO2 as a photocatalyst and construction waste as an aggregate. Polymethyl methacrylate (PMMA) acrylic particles were added to enhance the light transmittance of the concrete, and the volume replacement percentages of PMMA replacing aggregate (sand) were 20%, 40%, 60%, and 80%. The physical, mechanical, and photocatalytic properties of the photocatalytic recycled concrete were characterized through testing the water absorption, drying shrinkage, compressive strength, flexural strength, splitting tensile strength, and degradation efficiency. The results showed that the addition of PMMA increased the water absorption, reduced the drying shrinkage, and decreased the mechanical strength (compressive strength, flexural strength, and splitting tensile strength) of the photocatalytic recycled concrete. Notably, PMMA significantly improved the photocatalytic efficiency by increasing the light transmittance, but the promotion effect was weakened when the PMMA replacement percentages exceeded 60%. When the PMMA replacement percentages ranged from 20% to 40%, the photocatalytic recycled concrete maintained good mechanical strength and photocatalytic efficiency. This study presents an effective approach for improving the catalytic efficiency of photocatalytic recycled concrete while concurrently enhancing the utilization rate of construction waste. Practical Applications: The photocatalytic recycled concrete developed in this study has significant potential in practical applications, especially in urban environments. It can serve as a material for roads, sidewalks, and building surfaces. Due to its air purification function, it can break down harmful pollutants in the atmosphere, such as nitrogen oxides and volatile organic compounds, thereby aiding in the improvement of urban air quality. In addition, the use of construction waste as aggregate promotes the recycling of construction waste and reduces dependence on natural resources. [ABSTRACT FROM AUTHOR]

This article presents the results of research evaluating the compressive strength of concrete using recycled coarse aggregate, in the case of using fly ash (CP1) and without using fly ash at a content of 5% (CP2). The experimental samples were compressed to test the compressive strength value at different times, including 7 days old, 14 days old and 28 days old. The study designed a recycled concrete composition with a 50-50 ratio of regular aggregate and recycled coarse aggregate, and the concrete has a design compressive strength of 15 Mpa. The compressive strength of concrete was studied and compared in cases with and without fly ash, and compared with another research result on concrete using recycled coarse aggregate.

Recycled concrete is a new and environmentally friendly material for future construction. When applying recycled concrete to cold and severe regions, it is necessary to consider the freeze–thaw resistance of recycled concrete. Using an indoor freeze–thaw cycle test system, the rapid freezing method was used to conduct rapid freeze–thaw tests on recycled concrete specimens under set freeze–thaw cycle conditions. Relevant parameters (such as compressive strength, quality loss rate, rebound value) were tested on recycled concrete specimens that completed the set number of freeze–thaw cycles. The influence of various factors (freeze–thaw cycle number, replacement rate of recycled aggregates) on the compressive strength, quality loss rate, and rebound value of recycled concrete was analyzed. The results indicate that with the increase of freeze–thaw cycles and recycled aggregate content, the workability, rebound value, and compressive strength of the specimens decrease, and the quality loss rate increases. The changes in workability of concrete are sharp, with a slump difference of 21 mm between concrete with 100% recycled coarse aggregate and concrete using natural coarse aggregate. The rebound value of the specimen shows a decreasing trend overall, but the rebound value varies for different measuring points on the same specimen. The attenuation of compressive strength is significant. When fully using recycled aggregates to prepare concrete, the compressive strength after 30 freeze–thaw cycles decreases by 49.42% compared to the compressive strength after 0 freeze–thaw cycles. The overall quality loss rate shows a decreasing trend, but the quality loss is not severe. [ABSTRACT FROM AUTHOR]

The focus on building energy efficiency using alternative materials in structures, especially concrete, and the main technical and environmental challenges therein, aligns with Sustainable Development Goals (SDG). This study proposes a review that analyzes structures made with recycled concrete, relating to the energy efficiency of buildings. A classification structure was proposed, addressing the following questions: (i) What are the various dimensions in which research into energy-efficient recycled concrete is concentrated? (ii) What are the themes and classes of research associated with these dimensions? (iii) What are the main shortcomings of current approaches, and what would be a good research agenda for the future development of energy-efficient recycled concrete? A bibliometric analysis was carried out, presenting geographical and cluster maps to understand different research trends and refine future research. This was followed by a bibliographic analysis, reviewing the most relevant studies from the last five years (2019–2024). The results showed some residual alternative materials (around 45 types from five different industries) used in the production of energy-efficient concrete. And, as a negative effect, as substitution rates increase, porosity is the property with the greatest impact on energy efficiency. The greater the number of pores and the greater their interconnection, the lower the material's thermal insulation. [ABSTRACT FROM AUTHOR]

Recycled concrete aggregates are potentially interesting for the precast concrete industry as they provide a new use for high-quality waste from its products' life cycle. In precast concrete structures, it is common to use headed bars in several connection types between structural members. This paper presents the results of experimental tests to investigate the impact of replacing coarse natural aggregates with coarse recycled concrete aggregates in the concrete breakout strength of cast-in headed bars embedded in slender structural elements. Results of 12 tests on 16 mm headed bars embedded in 500 × 200 × 900 mm concrete members with an effective embedment depth of 110 mm are presented. The percentage of replacement of natural aggregates by recycled concrete aggregates was 0%, 30%, and 100%, and the flexural reinforcement ratio of the structural elements varied from 0.5% to 3.5%. The behavior and strength of the tested specimens are discussed, and comparisons with theoretical strength estimates are presented. The results showed that the concrete breakout strength of the headed bars was not affected by the use of recycled concrete aggregates and that the flexural reinforcement ratio significantly impacts the load-carrying capacity of the headed bars as they control the crack widths before failure. [ABSTRACT FROM AUTHOR]

To promote resource reuse and the green, low-carbon transformation of the construction industry, this study uses recycled aggregate from crushed waste concrete and natural bamboo fibers to formulate bamboo fiber-reinforced recycled-aggregate concrete. This study investigates the effects of natural bamboo fiber (NBF) content, NBF length, and the water-to-cement ratio on the performance of concrete through an orthogonal experiment to determine the optimal mixing proportions of NBF-reinforced concrete. Additionally, recycled aggregate completely replaced natural aggregate. The mechanism by which NBF influences concrete was also analyzed. The results demonstrate that the NBF-reinforced specimens exhibited good integrity during compression failure, with NBFs effectively tying the concrete together. The optimized parameters for NBF-reinforced concrete were an NBF length of 20 mm, an NBF content of 0.4v%, and a water-to-cement ratio of 0.55. Almost no flaky Ca(OH)2 crystals were observed in the NBF-hardened cement–paste transition zone, indicating effective bonding at the interface. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of recycled concrete‐filled steel tube (CFST) columns with reinforced concrete (RC) beam frames using recycled aggregate under cyclic loading. A 1/3‐scale model of a CFST column‐RC beam frame with 100% recycled coarse aggregate was tested, the failure process and mode were monitored, and the hysteresis and skeleton curves were obtained. The seismic performance indexes were analyzed based on the test data, including the seismic force, plastic hinge sequence, ductility coefficient, inter‐story drift, dissipation capacity, and stiffness degeneration. The general behavior of the CFST column‐RC beam frame is discussed and compared with that of a normal aggregate concrete‐RC frame structure. The test results demonstrate that CFST column‐RC beam frames with 100% recycled coarse aggregate exhibit remarkable seismic performance and can be applied and popularized for construction in aseismic regions. [ABSTRACT FROM AUTHOR]

The inferior mechanical performance and freeze–thaw (FT) resistance of recycled concrete are mostly due to the significant water absorption and porosity of recycled coarse particles. In this study, different dosages of zeolite powder were used in recycled concrete. A series of macroscopic tests were used to evaluate the workability and FT durability of zeolite powder-modified recycled concrete (ZPRC). X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to reveal the micro-mechanisms of FT resistance in ZPRC. The results show that the increase in zeolite powder content leads to a decrease in the slump and water absorption of ZPRC. Additionally, ZPRC with 10% zeolite powder has superior mechanical characteristics and tolerance to FT conditions. The higher strength and FT resistance of the ZPRC can be attributed to the particle-filling effect, water storage function, and pozzolanic reaction of zeolite powder, which results in a denser microstructure. The particle-filling effect of zeolite powder promotes the reduction of surface pores in recycled coarse aggregates (RCAs). The water storage function of zeolite powder can provide water for the secondary hydration of cement particles while reducing the free water content in ZPRC. The pozzolanic reaction of zeolite powder can also promote the generation of hydrated calcium silicate and anorthite, thereby making the microstructure of ZPRC more compact. These results provide theoretical guidance for the engineering application of recycled concrete in cold regions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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With the increasing implementation of sustainable development strategies, recycled concrete (RC) has garnered attention in research circles due to its substantial environmental and economic advantages. The presence and properties of various interface transition zones (ITZs) in RC play a vital role in its mechanical properties. This research uses a combination of multiphase inclusion theory and finite element numerical simulation to investigate and compare the impact of ITZs on concrete's mechanical properties. The multiphase inclusion theory offers a theoretical framework for understanding ITZ behavior in concrete, categorizing it into new mortar, old mortar, new ITZ, old ITZ, and natural aggregate based on meso-structure. With simplified RC at the mesoscale, the study accurately predicts the mechanical properties of RC by adjusting the elastic modulus, Poisson's ratio, and thickness of new and old ITZ models. Through finite element simulation and theoretical validation, the study achieves a minimal error of 6.24% in predicting the elastic modulus and 1.75% in predicting Poisson's ratio. These results highlight the effectiveness of multiphase inclusion theory in capturing the meso-structure characteristics of RC and forecasting its macroscopic mechanical behavior while comprehensively considering the complexity of ITZs. [ABSTRACT FROM AUTHOR]

Aqueous carbonation of end-of-life concrete fines is a promising method to alleviate greenhouse gas emissions by CO2 sequestration from point-source emitters. This produces a valuable material that can be utilized in new cement formulations. This study investigates effects of the composition of cement pastes and of the carbonation conditions on the reactivity and phase assemblage for aqueous carbonated Portland cement pastes incorporating silica fume, fly ash, and blast furnace slag. Results from 27Al and 29Si NMR show that hydration of the carbonated pastes under reactivity test conditions lead to phase assemblages dominated by a C-(A)-S-H phase, with reduced Al/Si ratio, as well as by ettringite and hemi/monocarbonate AFm phases. The results from the reactivity tests demonstrate that the carbonated blended cement pastes exhibit superior reactivity compared to carbonated neat Portland cement paste because of their increased fraction of reactive alumina and silica species. The variations in carbonation conditions (i.e., temperature, CO2 gas concentration, and solution composition) do not alter significantly the reactivity of the carbonated pastes. These findings demonstrate the robustness of aqueous carbonation of concrete fines and support its wider application as a mean to reduce CO2 emissions and enhance circularity of cement-based materials.

In the pursuit of sustainable construction practices, the utilization of recycled concrete has emerged as a pivotal strategy, distinguished by its commitment to resource conservation and environmental stewardship. Nevertheless, the inherent micro-porosity and micro-cracking within the old mortar of recycled concrete may lead to weak bonding performance at the interfacial transition zone, culminating in diminished strength, reduced density, and elevated water absorption rates compared to conventional concrete, which critically impairs its performance in cold climates subjected to freeze-thaw cycles. Consequently, this paper provides a structured examination of the frost resistance properties of recycled concrete subjected to freeze-thaw cycling. Initially, the study delineates the mechanisms of frost-induced damage in recycled concrete by synthesizing the degradation pathways observed in both conventional and recycled concrete during freeze-thaw exposure. Subsequently, a detailed analysis is conducted to identify the pivotal factors affecting frost resistance, encompassing the proportion and moisture affinity of recycled aggregates, the addition of silica fume and fly ash, the water-to-cement ratio, and the degree of water saturation. In the final segment, the study compiles and reviews the strategies for bolstering the frost resistance of recycled concrete, including the incorporation of air-entraining admixtures, fiber reinforcement, and aggregate modification approaches. The objective of this research is to offer a thorough comprehension of recycled concrete, with a concentration on the mechanisms of frost damage, the critical determinants of frost resistance, and interventions to augment its resilience against freezing conditions. On this basis, the present paper, in conjunction with the characteristics and current research status of recycled concrete, proposes recommendations for the application of recycled concrete in cold regions. This review is anticipated to facilitate researchers in gaining a comprehensive understanding of the freeze-thaw characteristics of recycled concrete and the measures to enhance its frost resistance. Furthermore, it aims to assist engineering and technical personnel in selecting appropriate treatment methods to improve the frost resistance of recycled concrete in cold regions, thereby promoting the practical engineering application of recycled concrete in such areas.

Abstract Research on concrete recycling has led to the discovery of many processes for recycled concrete aggregate (RCA) treatment, each with its pros and cons on quality improvement, resource requirements, or scalability. This study aimed to propose a technique that combines two methods (mechanical grinding and acidic soaking) for mortar removal and improves the quality of RCA so as to realize a zero-waste, highly efficient, few-hours-long single-step process which we call acid milling. A quantitative comparison of aggregate and final concrete quality has been performed between each separate method. The proposed method shows about 57% relative improvement of aggregate quality for water absorption and density, while the concrete experiment shows up to 10% relative improvement for the compressive and flexural strengths compared with the untreated RCA concrete. This study paves the way for an efficient and sustainable concrete recycling process to be applied on a large scale.

Abstract In order to promote the engineering application of recycled concrete, the effects of PPF and nano-TiO2 dioxide on the mechanical properties and durability of recycled concrete were studied. Polypropylene fiber recycled concrete(PRAC) and nano-TiO2 recycled concrete(TRAC) were prepared by adding different volume contents of PPF and nano-TiO2. The experimental findings demonstrated that the PPF and nano-TiO2 improved the splitting tensile strength of RAC better than the compressive strength. When the volume content of nano-TiO2. and PPF is 0.8% and 1.0%, respectively, the corresponding splitting tensile strength of concrete reaches the maximum value(3.4 and 3.7 MPa). The contribution rates of nano-TiO2 and PPF with different volume contents to the mechanical properties of RAC have optimal values, which are 0.4 and 1.0%, respectively. The incorporation of nano-TiO2 and PPF can effectively inhibit the loss of RAC mass and the generation of pores under freeze–thaw conditions, and slow down the decrease of dynamic elastic modulus. When the volume content of PPF is 1.0% and the volume content of nano-TiO2 is 0.4%, the protection effect on the internal structure of RAC is better, and its carbon resistance is better. The results of RSM model analysis and prediction show that both PPF and nano-TiO2 can be used as admixture materials to improve the mechanical properties and durability of RAC, and the comprehensive improvement effect of PPF on RAC performance is better than that of nano-TiO2.

The effects of basalt fibers (BF) and nano-silica (NS) on the mechanical properties and microstructure of recycled concrete (RC) in early low-temperature environments were investigated by placing the BF and NS modified RC specimens in the environments of −20, −10, 0, and 25°C for curing for 6 h, followed by standardized maintenance. The damage morphology and mechanical properties of modified RC were analyzed in such environments. The formulae for the compressive strength of RC, which was affected by BF and NS, were fitted using statistical product and service solutions, and a micromorphological analysis of the modified RC was conducted using scanning electron microscope. The mechanical properties of RC decreased owing to the influence of early low temperatures, among which 0°C caused the largest damage crack and the most serious effects. In the early low-temperature environments, the physical properties of RC generally increased and then decreased with the increase in BF dosage; however, increasing NS dosages improved its mechanical properties. The composite doping of BF and NS was more obvious than the single doping of BF or NS to enhance the performance of RC, and the internal pore structure was considerably improved. The preferred doping amounts were 3 kg m−3 of BF and 2% NS.

Recycled aggregate concrete is a promising low carbon material, which need further analysis on the durability. To investigate the influence of sulfate erosion on recycled concrete made with high-belite sulfoaluminate cement (HBCSA), this study utilized compressive strength and microhardness test to evaluate the effect of sulfate on ITZ. Then by using backscattered electron imaging, scanning electron microscope and energy spectrometry, the mechanism for the microstructural changes that occurred before and after sulfate erosion is revealed. Results showed that after 60 days of sulfate erosion, both aggregate-new mortar and new–old mortar interfaces display reduced microhardness and widened transition zones. Specifically, in the C30 group post-erosion, the microhardness values for the aggregate-new mortar interface ranged from 50.3 to 66.9 MPa, while the new–old mortar interface exhibited a range of 47–77 MPa. Besides, the new–old mortar interface exhibit marginally superior sulfate resistance to the aggregate–new mortar interface at the same concrete strength grade. Moreover, sulfate resistance improvs across all interfaces with increasing strength class. For instance, in the aggregate-new mortar interface, the maximum microhardness post-erosion for C40 showed a 51.6 % increase compared to C30. Through microstructure analysis, it is found that after sulfate erosion, the aggregate–new mortar and new–old mortar interfaces of HBCSA recycled concrete are both enriched with abundant ettringite and gypsum, which causes considerable swelling erosive product formation and even cracking. The less formed cracks in new–old mortar interfaces results in the better sulfate resistance. The finding is helpful for the future research on the high sulfate resistant recycled aggregate concrete.

This study investigates and assesses the bond performance between steel reinforcement and recycled concrete, subject to the confining effect of transverse stirrups post high-temperature exposure. A series of 24 specimen groups was examined, with variations in parameters including transverse stirrup ratios (0 %, 0.24 %, 0.67 %, 1.34 %), recycled concrete aggregate (RCA) replacement ratios (0 %, 50 %, 100 %), and temperatures (300 °C, 500 °C, 700 °C). Central pull tests were administered to discern the impact of each parameter on the bond integrity of steel-reinforced recycled concrete. The results revealed that high temperatures reduced the thickness of the critical cover, altering the failure mode of the specimen. With rising temperatures, the bond strength of steel-reinforced recycled concrete weakened. Nonetheless, an increase in stirrup ratio served to counteract the detrimental effects of high temperatures. In contrast, a higher RCA replacement ratio exacerbated these effects. For example, specimens with a 1.34 % stirrup ratio showed a 30 % decrease in the adverse impacts of high temperature on bond strength compared to those with no stirrups. A comprehensive computational model, devised from experimental data and theoretical analysis, predicts the bond-slip behavior of steel-reinforced recycled concrete, incorporating the constraint coefficient k, temperature T, and replacement ratio r as pivotal parameters. The model’s predictive curve corresponds closely with the experimental findings, effectively capturing the influence of high temperature, lateral restraint, and RCA replacement ratio on the bond performance of steel-reinforced recycled concrete.

Research on concrete recycling has led to the discovery of many processes for recycled concrete aggregate (RCA) treatment, each with its pros and cons on quality improvement, resource requirements, or scalability. This study aimed to propose a technique that combines two methods (mechanical grinding and acidic soaking) for mortar removal and improves the quality of RCA so as to realize a zero-waste, highly efficient, few-hours-long single-step process which we call acid milling. A quantitative comparison of aggregate and final concrete quality has been performed between each separate method. The proposed method shows about 57% relative improvement of aggregate quality for water absorption and density, while the concrete experiment shows up to 10% relative improvement for the compressive and flexural strengths compared with the untreated RCA concrete. This study paves the way for an efficient and sustainable concrete recycling process to be applied on a large scale. [ABSTRACT FROM AUTHOR]

In order to promote the engineering application of recycled concrete, the effects of PPF and nano-TiO2 dioxide on the mechanical properties and durability of recycled concrete were studied. Polypropylene fiber recycled concrete(PRAC) and nano-TiO2 recycled concrete(TRAC) were prepared by adding different volume contents of PPF and nano-TiO2. The experimental findings demonstrated that the PPF and nano-TiO2 improved the splitting tensile strength of RAC better than the compressive strength. When the volume content of nano-TiO2. and PPF is 0.8% and 1.0%, respectively, the corresponding splitting tensile strength of concrete reaches the maximum value(3.4 and 3.7 MPa). The contribution rates of nano-TiO2 and PPF with different volume contents to the mechanical properties of RAC have optimal values, which are 0.4 and 1.0%, respectively. The incorporation of nano-TiO2 and PPF can effectively inhibit the loss of RAC mass and the generation of pores under freeze–thaw conditions, and slow down the decrease of dynamic elastic modulus. When the volume content of PPF is 1.0% and the volume content of nano-TiO2 is 0.4%, the protection effect on the internal structure of RAC is better, and its carbon resistance is better. The results of RSM model analysis and prediction show that both PPF and nano-TiO2 can be used as admixture materials to improve the mechanical properties and durability of RAC, and the comprehensive improvement effect of PPF on RAC performance is better than that of nano-TiO2. [ABSTRACT FROM AUTHOR]

Using recycled aggregates has many positive environmental impacts because of the conservation of natural resources and minimization of waste. The use of recycled aggregates in downcycling processes is already common in Germany, whereas utilizing them to produce high-quality recycled concrete is rarely applied in practice. The reasons behind this lag have been investigated based on surveys and interviews with stakeholders. Miscommunication and missing information were identified in all stakeholder groups. Therefore, establishing a robust network and facilitating knowledge transfer by specifying the demand for recycled aggregates in the case study region have been considered as prerequisites. Therefore, the paper presents a novel concept of a stakeholder network for an integrated construction and demolition waste center. The conceptualization integrates the recycling companies and construction product manufacturers in one venue with research, service, and educational divisions. The design of the facilities is based on calculations regarding future construction activities and the demand for concrete production. The proposed concept aims to supply the region in the west of Germany with high-quality recycled products while also establishing a robust network that offers benefits in terms of logistical optimization and knowledge transfer. [ABSTRACT FROM AUTHOR]

Methods for quantifying the CO2 storage potential of recycled aggregates The necessity of utilizing CO2 sinks is becoming increasingly relevant for the construction industry, as the manufacturing of certain industrial goods will process‐related continue to produce emissions. An innovative approach to this is the utilization of waste concrete respectively recycled aggregates produced from it for CO2 sequestration, which expands the recycling of construction waste in the context of resource conservation (Urban Mining) to include climate compatibility. Initial implementations in practice have already occurred, and an increasing future application is emerging. One challenge lies in the uncertainty regarding the CO2 uptake potential of the recycled aggregates. Accurate assessment is essentially possible only after carbonation has taken place by determining the masses and monitoring the experimental conditions. The primary influencing parameters are the grain size and the time available for carbonation, as well as the concrete properties and exposure conditions of the available waste concrete. This article presents a practical method by which the respective carbonation potential can be determined prior to demolition of a structure providing a simple, manageable means to evaluate the effectiveness of the additional required processing step. [ABSTRACT FROM AUTHOR]

Sensitivity analysis of influencing factors on frost resistance is carried out in this paper, and a two-stage neural network model based on grey theory and Back Propagation Neural Networks (BPNNs) is established for the sake of predicting the frost resistance of active-admixture recycled concrete quickly and accurately. Firstly, the influence degree of cement, water, sand, natural aggregate, recycled aggregate, mineral powder, fly ash, fiber and air-entraining agent on the frost resistance of active-admixture recycled-aggregate concrete was analyzed based on the grey system theory, and the primary and secondary relationships of various factors were effectively distinguished. Then, the input layer of the model was determined as cement, water, sand, recycled aggregate and air-entraining agent, and the output layer was the relative dynamic elastic modulus. A total of 120 datasets were collected from the experimental data of another author, and the relative dynamic elastic modulus was predicted using the two-stage BPNN prediction model proposed in this paper and compared with the BPNN prediction results. The results show that the proposed two-stage BPNN model, after removing less-sensitive parameters from the input layer, has better prediction accuracy and shorter run time than the BPNN model. [ABSTRACT FROM AUTHOR]

The utilization of reclaimed asphalt pavement (RAP) and recycled concrete aggregate (RCA) in road infrastructure construction projects contributes significantly to the sustainable reduction of solid wastes being landfilled. This research aims to evaluate the mechanistic performance of asphalt concrete with RAP and RCA modified with natural hemp fiber (HF) reinforcement (HF-RAP-RCA-AC). The effect of influence factors, such as RAP/RCA ratio, HF length, and HF content, on the static and dynamic-mechanistic performance was evaluated. The static performance of HF-RAP-RCA-AC was assessed through the Marshall stability (MS), strength index (SI), and indirect tensile strength (ITS), while the dynamic performance was evaluated through the indirect tensile resilient modulus (IT Mr), indirect tensile fatigue life (ITFL), and rutting resistance tests. The HF-RAP-RCA-AC sample was found to have comparable static properties (MS and SI) to conventional asphalt concrete. The dynamic-mechanistic performance of HF-RAP-RCA-AC was however found to be significantly lower and hence HF-RAP-RCA-AC was found to be suitable for low-traffic roads. The improved ITS due to the addition of RCA and HF was found to influence the IT Mr improvement in the linear relationship. The improved resilient properties contribute to the ITFL and rutting resistance improvement. The optimum mix was found to be at 0.05%HF content, for all HF lengths and RAP/RCA ratios. The potential of HF reinforcement on the material properties was positively impacted by increasing the RCA content. The proposed distress model based on the critical analysis of the cyclic test data of HF-RAP-RCA-AC was developed and was found to have the same logarithm relationship. The outcome of this research will encourage the application of HF-RAP-RCA-AC as a sustainable pavement material for low-volume roads. [ABSTRACT FROM AUTHOR]

The transportation sector is responsible for about 25% of greenhouse gases and plays a pivotal role in countering global warming and emissions to the air. Railway is the most efficient transport system due to its low emissions per transport unit for freights and passengers. However, it is meaningful to assess the impacts of the infrastructure itself at the Product Stage, Construction Process Stage, and End of Life Stage. This study assesses the environmental impact of the production, installation, and dismantling of one 230 VN prestressed concrete railway sleeper with fastening system. The methodology used is the Life Cycle Assessment - LCA according to the standard UNI EN 15804:2012 + A2:2019. The examined life cycle involves the emission of 83 kg of CO2 eq. whose 10% is from the Construction Process Stage. Raw materials extraction, electricity production, and road transportation contribute 92%, 3%, and 5% to climate-altering impacts of the Product stage. The results highlight the variables to be addressed to increase the sustainability of the rail transportation system. [ABSTRACT FROM AUTHOR]

Using waste concrete as recycled aggregate, Grade I fly ash was treated with wet grinding to prepare recycled concrete with different replacement rates of fly ash. The effects of different substitution rates of Grade I fly ash on the mechanical and durability properties of recycled concrete were studied through XRD, SEM, mechanical performance testing, and dry shrinkage testing. The results show that the glass body on the surface of fly ash after wet grinding treatment is destroyed, and the volcanic ash effect is enhanced, thus improving the hydration rate of recycled concrete, increasing the number of hydration products C—S—H and AFt, and increasing the compactness of concrete. The compressive strength of fly ash recycled concrete first increases and then decreases with the increase of fly ash replacement rate. The maximum compressive strength of concrete with a 45% fly ash replacement rate is 42.05 MPa, and the content of its hydration product Ca(OH)2 is up to 11.742%. The deflection test shows that the yield stage of concrete with a 45% replacement rate of fly ash corresponds to a load of 217 N, with a maximum deflection of 1.7 mm. The minimum dry shrinkage of concrete with a 45% replacement rate of fly ash at 28 d is 1.021×10-4. Overall, it can be concluded that concrete with a 45% replacement rate of fly ash has the best performance. [ABSTRACT FROM AUTHOR]

Copyright of Bulletin of the Chinese Ceramic Society is the property of Bulletin of the Chinese Ceramic Society Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The present work investigated the manufacture of elements such as water tanks from recycled concrete for applications where industries require water heating. This proposal leverages precast rejects for recycled concrete and incorporates colouring pigments. It is expected to contribute to the circularity of construction materials (due to the total replacement of natural aggregates by recycled aggregates) as well as to energy and emissions savings, which are attributed to improved thermal performance driven by the thermal behaviour that the coloration pigment gives to the manufactured concrete elements. To assess the efficacy of the proposed solution, on the one hand, mechanical tests were carried out in tensile, compression and modulus of elasticity, which showed a suitable concrete dosage for HA-30 structural concrete. Simultaneously, in search for a material that would increase the internal temperature of the tanks, thermal tests were carried out in a controlled laboratory environment on samples with different percentages of pigment, and an optimum concentration of 1% was obtained. It was also found that the thermal conductivity remained almost unaffected. Finally, two water tank prototypes were manufactured and tested under real environmental conditions: one with the optimised pigment concentration solution and other (the reference tank) without pigment. The results revealed that the colourised tank with the optimal concentration resulted in an average water temperature increase of 2 °C with respect to the reference tank. Finally, the economic and environmental benefits of this temperature increase were studied for industrial processes requiring water heating with a potential saving of 8625 kWh per month. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Architecture & Civil Engineering is the property of Chang'an Daxue Zazhishe and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

High-strength cement-free geopolymer concrete with 62.9 MPa (after 28 curing days) was experimentally produced from blast furnace slag, recycled concrete, and metakaolin as alumina-silicate binders as well as silica fume as silica-rich ultra-fine powder. Combining slag with metakaolin and silica fume allowed to increase its proportion up to 58.1 wt. %, without affecting shrinkage and hardness. Residual concrete from demolition was also used for making the geopolymer concrete as coarse aggregate as well as alumina-silicate binder together with fly ash, but not with granulated slag, metakaolin, and silica fume, this constituting the paper originality. [ABSTRACT FROM AUTHOR]

With the development of the circular economy and low-carbon society, the large-scale application of construction solid waste in buildings, such as recycled concrete, is becoming imperative. Accurately predicting the carbonation depth of recycled concrete is of great significance. Quantitatively analyzing the impact of each parameter on carbonation and elucidating the relationships between these parameters present challenges in predicting the carbonation of recycled concrete. In this study, different machine learning models and prediction equation models were applied and compared to predict the carbonation depth of 576 datasets associated with recycled concrete. The machine learning models used include Automation Machine Learning (AutoML), LightGBM, CatBoost, Neural Networks, Extra Trees, Random Forest, XGBoost, KNN (K-Nearest Neighbor). The results indicate that the machine learning method shows higher accuracy than the traditional equation, the AutoML model exhibits the best prediction accuracy among the investigated machine learning models, and carbonation test results further verified the favorable carbonation depth prediction effects of AutoML model. Furthermore, SHAP (Shapley Additive Explanations) was utilized to quantitatively analyze and explain the prediction results. The results demonstrate that carbonation time and the water to cement (W/C) ratio of recycled concrete have the most significant impact on the carbonation depth of recycled concrete buildings.

Combining waste with building materials can effectively improve resource utilization and protect the environment.The inclusion of waste rubber particles in geopolymer recycled concrete can enhance its brittleness, but it also significantly reduces its mechanical properties This study aims to address this issue by modifying the waste rubber particles and incorporating agricultural waste rice husk ash. The experimental study focused on investigating the impact of Span-40 on the surface of waste rubber particles. Various properties such as compressive strength, flexural strength, and splitting tensile strength of geopolymer recycled concrete were evaluated using different dosages of modified waste rubber particles (0 %, 5 %, 10 %, and 15 %) along with varying single dosages of RHA/silica fume (SF) (0 %, 5 %, 10 %, and 15 %). The microstructure of the reclaimed concrete was analyzed using XRD and SEM. The findings indicate that the improvement in compressive strength of Span-40 modified rubber particle polymer recycled concrete increases with higher rubber particle incorporation ratio,curing 28d increased by about 8 %. Furthermore, the addition of RHA effectively enhances the mechanical properties of the reclaimed concrete, with the optimal effect observed at a 5 % RHA content,curing 28d increased by about 5 %.

Basalt fiber (BF) and nano-silica (NS) are used to improve the properties of recycled aggregate concrete (RAC) and produce modified RAC (MRAC). The fatigue failure patterns of the MRAC were observed through a uniaxial compression fatigue test, and the reasons for the different patterns were explained. The effects of the stress level, replacement rate, and loading frequency on the fatigue life were investigated, and the effects of BF and NS on the fatigue strain, damage evolution, and energy dissipation evolution of MRAC were analyzed. The enhancement mechanism of BF and NS on RAC was revealed using microstructural analysis techniques, such as scanning electron microscopy (SEM) and computed tomography (CT). The results showed that the fatigue failure modes of the MRAC could be divided into split, shear, and mixed failures. The fatigue life and fatigue strain of RAC exhibited a pattern of initially increasing and subsequently decreasing with an increase in the BF content (from 0 kg m−3 to 3 kg m−3 and then to 6 kg m−3). At stress level of 0.80, the average fatigue life of the specimens increased by 76.15 % compared to RAC when the BF doping was 3 kg m−3; however, when the BF doping was increased to 6 kg m−3, the average fatigue life of the specimens increased by only 47.68 % compared to RAC. The mixed BF and NS improved the fatigue life of RAC more than the single mixed BF or NS. Fatigue equations and unified single logarithmic fatigue equations, including fiber parameters, were established to accurately describe the relationship between the fatigue stress level, fatigue life, and failure probability. The fatigue strain gradually decreases with increasing NS content (0–2 %); with increasing loading frequency (from 1 Hz to 10 Hz and then to 15 Hz), the fatigue strain gradually decreases. The fatigue life of the MRAC decreased dramatically at low loading frequencies (1 Hz) and then steadily increased as the loading frequency increased. BF and NS have an improving effect on the porosity of microstructure, improve the fatigue life of RAC, and BF exhibits a more significant improvement than NS. A composite fatigue damage model that comprehensively considers the contribution rates of NS and BF was established, and the fatigue damage evolution of the MRAC was predicted.

In this paper, the structural performance of exterior fly-ash based concrete beam-column joints reinforced with hybrid glass fiber reinforced polymer (GFRP) and steel bars was studied, to inspect the hybrid concept efficiency under lateral loads. This paper studied two mixes of concrete, normal strength concrete (NC) and fly-ash based concrete (FC), a 20% replacement ratio of the cement weight with superplasticizer is considered in FC mix, which is considered as an environmental-friendly and cost-effective alternative to ordinary Portland cement. Ten joints were tested under the effect of monotonic lateral load experimentally. The studied joints were divided into two groups based on their concrete type, in each group, five joints with different GFRP to steel ratios “from 0% to 100% with an increment of 25%” were tested. The obtained results showed that the fly ash presence in concrete resulted in an average increase in both ductility and load capacity of 18% and 28.3%, respectively. Also, the average total energy absorption increased by 35%, and the average initial and post-yield stiffness increased by 34% and 41%, respectively. Moreover, the ductility index for each model was obtained, the FC model with 75% GFRP to steel ratio achieved the maximum ductility index, so it was considered as the optimum one. After that, the finite element analysis FEA was utilized to study the performance of the tested beam-column joints. The FEA showed identical results compared to the experimental ones concerning energy absorption, load capacity, and model stiffness. Finally, using FEA, a deep parametric study on FC models reinforced with GFRP to steel bars ranging from 60% to 90% was performed to obtain the optimum GFRP to steel reinforcement ratio. Through the performed parametric study, it was found that the optimum range of the reinforcement ratio was 68% to 82%. According to this research, the usage of fly ash in concrete increased RC beam-column joints' efficiency especially when provided with hybrid GFRP and steel reinforcement, in addition to the important environmental impact achieved through the recycling process. It was recommended to utilize the composite reinforcement ranging between 60% to 75% GFRP to steel bars in the presence of fly ash-based concrete for the beam-column joints.