To alleviate the excessive extraction from natural resources and to properly manage construction waste, recycled concrete technology is globally recognized as an eco-friendly way to address these escalating challenges. This study explores the influence of three particle size distributions (PSD) (upper, median, and lower limits) and two curing conditions (normal: 19–25 °C, humidity 48–56 %; lab standard: 20 ± 2 °C, humidity ≥ 95 %) on the compressive strength, tensile splitting strength, and strength development of recycled concrete through a series of experiments. The detailed data make up the research gap in this aspect and reveal that the influence of the PSD on the compressive strength and tensile splitting strength is limited. However, a favourable curing condition benefits the mechanical properties of recycled concrete, especially in resisting tension. In terms of compressive strength, this study indicates that recycled concrete has the potential to replace natural aggregates totally and is feasible to be applied in almost all practical engineering applications, which provides a solid foundation for the future of sustainable construction.

@en

The authors regret that the original version of this article contained numerical errors in Figure 2 within the main text and Table A1 of Appendix A. Supplementary data. Corrections that need to be made are presented as follows: • For Figure 2 in the main text, the label “0∼4mm SS (42.5%)” of a flow should be corrected to “0∼4mm SS (44.5%)”, as shown in Fig. C1 below. [Figure presented]• In Table A1 of Appendix A, the electricity usage for the “Wet processing” under the S1 BAU WP scenario should be corrected to “400 kWh” instead of “60,000 kWh”.The authors would like to apologise for any inconvenience caused and state that the changes reported do not affect the scientific results and conclusions of the manuscript.

@en

Recycled coarse aggregates (RCA) from End-of-Life (EoL) concrete face resistance due to inconsistent quality. To address this, a mobile, containerized sensor-based inspection system is developed, capable of processing over 100 tons of RCA per hour. Using advanced 3D scanning and laser-induced breakdown spectroscopy (LIBS), the system ensures reliable real-time analysis of particle size distribution (PSD) (Root Mean Square Error: <5.5%) and contaminant detection (Accuracy: 0.94). Incremental learning techniques dynamically update chi-square distribution parameters as new spectral data becomes available, refining models continuously without full retraining and sustaining high classification performance. Monitoring data are recorded on radio frequency identification (RFID) tags, enhancing traceability. This innovation improves efficiency compared to traditional methods, supporting sustainable practices in the construction industry. Its applications also extend to related fields such as mining, waste management, and resource recovery, contributing to the circular economy, reducing reliance on natural aggregates, and promoting environmentally friendly infrastructure development.@en

The construction industry urgently requires a resilient information system for effective coordination of data transmission among various stakeholders, including both the public and private sectors. Such an advanced digital solution would not only enhance transparency along the value chain but also improve both the quality of and confidence in recycled materials. Achieving circularity and reducing environmental impact are closely tied to the efficient management of material flows and life cycles. Within this context, Material Passports (MPs) are posited as a foundational element, particularly when integrated with a digital database. This integration is particularly beneficial for increasing the circularity of concrete, beginning with end-of-life concrete, a major contributor to global construction and demolition waste. MPs effectively transmit crucial information about the quality of recycled aggregates, thereby enabling their use in future construction projects. This study explores the feasibility of employing Radio Frequency Identification (RFID) technology as an MP, aiming to enhance sustainability in the concrete industry by improving transparency, traceability, and data reliability in the recycled concrete supply chain. Extensive laboratory tests carried out in three distinct experimental phases revealed that RFID tags exhibit remarkable resilience to mechanical stress typical in the supply chain and consistently maintain readability when embedded in concrete. The water content in concrete samples was identified as a significant factor influencing initial tag readability, although readability improved over time. Other factors, such as the type of aggregates, particle size distribution, and proximity to steel rebar, had minimal to modest impacts on tag performance. Additionally, the study confirmed that the readability of RFID tags remains robust at typical transport speeds, which highlights the potential of an RFID-based system in advancing supply chain management. This study provides a solid foundation for future research in this evolving area.

@en

This study addresses a critical gap in circular construction practices by assessing the use of high-quality Recycled Coarse Aggregates (RCA) from end-of-life concrete on an industrial scale. Unlike previous studies, which predominantly relied on theoretical mix designs or laboratory-level experiments, this research focuses on real-world applicability, employing commercially produced RCA and conventional production methods in industrial settings to identify upscaling challenges. Advanced Dry Recovery technology is utilized to produce high-quality RCA for both ready-mix and prefab concrete production. To ensure practical relevance, the research examines three water-to-cement ratios for ready-mix concrete and three strength classes for prefab concrete, all prepared and cast in a commercial setting using standard industrial practices. The results show that by selecting the appropriate application for RCA, there is potential for concrete companies to produce mixes using 100% RCA that meet standard requirements in terms of fresh, mechanical, and durability properties without the need for extra treatments or specific mixing methods, particularly when the water absorption of RCA is less than 4%. Achieving optimal performance requires adjustments in the mix design, specifically by considering the effective water-to-cement ratio. Additionally, the study underscores the impact of the parent concrete's properties on the RCA quality. This research not only demonstrates the feasibility of employing RCA in industrial-scale concrete production along with its associated challenges but also highlights the potential for enhancing circularity in the construction industry through large-scale adoption of RCA, thereby contributing to sustainable and circular construction practices.

@en

Recycling coarse aggregates from construction and demolition waste is essential for sustainable construction practices. However, the quality of recycled coarse aggregates (RCA) often fluctuates significantly, in contrast to the more stable quality of natural aggregates. Contaminants in RCA notably compromise its quality and usability. Therefore, automating the quality control of RCA is necessary for the recycling industry. This study introduces an industry-focused, innovative, and rapid quality control system that combines Laser-Induced Breakdown Spectroscopy (LIBS) with 3D scanning technologies to enhance the detection of contaminants in RCA streams. The system involves a synchronized application of LIBS for spectral analysis and 3D scanning for the physical characterization of different materials. Results reveal that the dependability of single-shot LIBS analysis has been enhanced, thus elevating the precision of contaminant detection. This improvement is achieved by accounting for the laser shot's angle of incidence and focal length adjustments. The introduced technology holds potential for application in the real-time examination of substantial volumes of RCA, facilitating a rapid and reliable quality control method. This rapid assessment technique delivers online data about the concentration of contaminants in RCA, including recycled fine aggregates, cement paste, bricks, foam, glass, gypsum, mineral fibers, plastics, and wood. This data is both essential and sufficient for choosing a cost-effective mortar recipe and guaranteeing the performance of the final concrete product in terms of strength and durability in construction projects. The system can monitor the quality of RCA flows at throughputs of 50 tons per hour per conveyor, characterizing approximately 4000 particles in every ton of RCA, in this way signaling the most critical contaminants at levels of less than 50 parts per million. With these characteristics, the system could also become relevant for other applications, such as characterizing mining waste or solid biofuels for power plants.@en

To upcycle End-of-Life (EoL) concrete from demolished buildings, it is essential to efficiently identify the different materials that may contaminate it. The precise identification and classification of materials and contaminants are vital processes for in-line quality inspection of recycled concrete aggregates transported on a conveyor belt. In this study, a total of eight potential contaminants are considered as target contaminant materials in the streams made of coarse and fine aggregates resulting from the upcycling of EoL concrete. These contaminants degrade the quality of the aggregates even at low concentrations, so it is essential to identify the presence of such contaminants along with the main products of recycling which are recycled coarse aggregates (RCA) and recycled fine aggregates (RFA). An efficient method is proposed to identify and classify EoL concrete waste along with RCA and RFA in motion on conveyor belts via laser-induced breakdown spectroscopy (LIBS) coupled with a cluster-based identification algorithm. The model is verified with an accuracy of 0.97, a precision (weighted average) of 0.98, a recall (weighted average) of 0.97, and an F1-score (weighted average) of 0.98 for the validation set, under the optimal conditions. This study suggests that LIBS may be well suited for fast and in-line analysis of recycled concrete aggregates in industrial applications. This approach presents an innovative approach for the quality characterization of secondary materials produced from EoL concrete being transported on conveyor belts, and therefore can be of great value for the processing and high-end utilization of EoL concrete.

@en

This study focuses on formulating the most sustainable concrete by incorporating recycled concrete aggregates and other products retrieved from construction and demolition (C&D) activities. Both recycled coarse aggregates (RCA) and recycled fine aggregates (RFA) are firstly used to fully replace the natural coarse and fine aggregates in the concrete mix design. Later, the cement rich ultrafine particles, recycled glass powder and mineral fibres recovered from construction and demolition wastes (CDW) are further incorporated at a smaller rate either as cement substituent or as supplementary additives. Remarkable properties are noticed when the RCA (4–12 mm) and RFA (0.25–4 mm) are fully used to replace the natural aggregates in a new concrete mix. The addition of recycled cement rich ultrafines (RCU), Recycled glass ultrafines (RGU) and recycled mineral fibres (RMF) into recycled concrete improves the modulus of elasticity. The final concrete, which comprises more than 75% (wt.) of recycled components/materials, is believed to be the most sustainable and green concrete mix. Mechanical properties and durability of this concrete have been studied and found to be within acceptable limits, indicating the potential of recycled aggregates and other CDW components in shaping sustainable and circular construction practices.

@en

Currently, natural resources are consumed at an alarming rate than their production (United Nations Environment Programme, 2014). This imposes unprecedented pressure on the environment. The problem seems to get worse with the present increase in population and demand for infrastructures. To alleviate these and other related problems, a circular approach to construction material flow is crucial. Recycling construction and demolition wastes (C&DW) to generate high-quality materials is essential to ensure materials circularity in the construction sector. This article, therefore, describes the development of two large-scale technologies called Advanced Dry Recovery (ADR) and Heating Air classification System (HAS). ADR and HAS are industrial-scale technologies aimed to recycle End-of-Life (EoL) concrete into coarse, fine and ultrafine particles. While ADR is used to sort out clean coarse aggregates, HAS is used to produce clean fine aggregates by heating and separating the ultrafine hydrated cement components. The process parameters and the quality of aggregates are briefly described. Accordingly, concrete made of recycled coarse and fine aggregates displays comparable mechanical properties as the reference concrete which is made of natural aggregates. Both technologies are designed to be mobile so that EoL concrete wastes are processed at the site of demolition or close to ready-mix concrete plants, reducing the heavy traffic related to construction activities. These technologies, in general, have a potential in increasing sustainability and thereby greening the construction sector. This is by far a convenient contribution towards sustainable development and a big step towards closing the recycling loop in the construction sector.@en

A stronger commitment towards Green Building and circular economy, in response to environmental concerns and economic trends, is evident in modern industrial cement and concrete production processes. The critical demand for an overall reduction in the environmental impact of the construction sector can be met through the consumption of high-grade supplementary raw materials. Advanced solutions are under development in current research activities that will be capable of up-cycling larger quantities of valuable raw materials from the fine fractions of End-of-Life (EoL) concrete waste. New technology, in particular the Heating-Air classification System (HAS), simultaneously applies a combination of heating and separation processes within a fluidized bed-like chamber under controlled temperatures (±600 °C) and treatment times (25–40 s). In that process, moisture and contaminants are removed from the EoL fine concrete aggregates (0–4 mm), yielding improved fine fractions, and ultrafine recycled concrete particles (<0.125 mm), consisting mainly of hydrated cement, thereby adding value to finer EoL concrete fractions. In this study, two types of ultrafine recycled concrete (either siliceous or limestone EoL concrete waste) are treated in a pilot HAS technology for their conversion into Supplementary Cementitious Material (SCM). The physico-chemical effect of the ultrafine recycled concrete particles and their potential use as SCM in new cement-based products is assessed by employing substitutions of up to 10% of the conventional binder. The environmental viability of their use as SCM is then evaluated in a Life Cycle Assessment (LCA). The results demonstrated accelerated hydration kinetics of the mortars that incorporated these SCMs at early ages and higher mechanical strengths at all curing ages. Optimal substitutions were established at 5%. The results suggested that the overall environmental impact could be reduced by up to 5% when employing the ultrafine recycled concrete particles as SCM in circular cement-based products, reducing greenhouse gas emissions by as much as 41 kg CO2 eq./ton of cement (i.e. 80 million tons CO2 eq./year). Finally, the environmental impacts were reduced even further by running the HAS on biofuel rather than fossil fuel.

@en

The increasing volume of Construction and demolition waste (CDW) associated with economic growth is posing challenges to the sustainable management of the built environment. The largest fraction of all the CDW generated in the member states of the European Union (EU) is End-of-life (EOL) concrete. The most widely applied method for EOL concrete recovery in Europe is road base backfilling, which is considered low-grade recovery. The common practice for high-grade recycling is wet process that processes and washes EOL concrete into clean coarse aggregate for concrete manufacturing. It is costly. As a result, a series of EU projects have been launched to advance the technologies for high value-added concrete recycling. A critical environmental and economic evaluation of such technological innovations is important to inform decision making, while there has been a lack of studies in this field. Hence the present study aimed to assess the efficiency of the technical innovations in high-grade concrete recycling, using an improved eco-efficiency analytical approach by integrating life cycle assessment (LCA) and life cycle costing (LCC). Four systems of high-grade concrete recycling were analyzed for comparison: (i) business-as-usual (BAU) stationary wet processing; (ii) stationary advanced dry recovery (ADR); (iii) mobile ADR; (iv) mobile ADR and Heating Air Classification (A&H). An overarching framework was proposed for LCA/LCC-type eco-efficiency assessment conforming to ISO standards. The study found that technological routes that recycle on-site and produce high-value secondary products are most advantageous. Accordingly, policy recommendations are proposed to support the technological innovations of CDW management.@en

Circular concrete is one of the main focuses of current research in the construction industry. This is due to the fact that the construction sector is facing several pressures from society and policymakers. In an attempt to assure the sustainability of the sector, different research options have been put in place. Among them, the use of recycled aggregates and recycled cement is a subject of current research.

The objective of this study is to optimize the particle size distribution of aggregates and understand the compaction of concrete made of recycled aggregates and recycled cement, so as to reduce the amount of cement used in the new concrete mix. Particle size optimization is aimed at improving the density of concrete made of regularly packed aggregates and the distribution of voids, thereby improving its sustainability and strength. This could be achieved by optimizing the amount of water, the size of aggregates and fillers, and adjusting the amount of cement in the mix design.

In this specific study, recycled aggregates (<12mm) are used along with natural aggregates of different sizes (<12mm and <16 mm). The role of maximum aggregate size and the effect of the complete substitution of natural aggregates with recycled aggregates have been studied. Furthermore, the effects of compaction, the role of internal pores, and aggregate size distribution on the mechanical properties of hardened recycled concrete are also investigated. Three recipes have been designed to employ different types of aggregates. The water-to-cement ratio of the recipes was adjusted based on the magnitude of the slump. The recipes were cast in various procedures associated with different durations of vibration to understand the role of vibration on their compressive strength after 7, 28, and 91 days.

After optimizing the duration of vibration and casting concrete samples, the selected specimens with natural aggregates and recycled aggregates were scanned by a macro-CT scanner to comprehend the spatial distribution of aggregates and air bubbles inside the hardened concrete. Results indicate that the duration of vibration was observed to be critical depending on the type and size of aggregate. Despite the direct relationship between vibration and internal pores, the impact seems to be negative beyond a certain time. The compaction and mechanical properties of concrete made of recycled aggregates need more vibration compared to natural aggregates.@en