Review on physical performance, modification mechanisms, carbon emissions and economic costs of recycled aggregates modified with physical enhancement technologies

With the continuous advancement of urban renewal, the application of recycled aggregates (RA) is a win-win measure to solve the treatment of construction waste and provide the required building materials. However, the existence of a large amount of old adhesive mortar (OAM) makes it difficult for RA to equivalently replace natural aggregates (NA) due to their higher water absorption and crushing index, as well as a lower apparent density. From the published literature on enhancing RA, the most mature and easiest method for construction is physical enhancement technology. Therefore, through a review of recent related researches, this article summarizes and compares the modification effects of mechanical grinding technology, traditional heating and grinding technology, and microwave heating technology on the physical properties of RA, including water absorption, apparent density, and crushing value. The related modification mechanisms were discussed. Additionally, the impacts of different physical enhancement technologies on the environment and economy effects are assessed from the perspectives of carbon emissions and cost required during processing. Based on multi-criteria analysis, microwave heating technology is more efficient and cleaner, which is the most recommended in the future.

A study on the strength and durability characteristics of fiber-reinforced recycled aggregate concrete modified with supplementary cementitious material

Recycled aggregate (RA) made from waste concrete is an environmentally friendly alternative to natural aggregate (NA) for concrete manufacturing. However, compared to NA concrete, concrete produced with recycled aggregates has poor characteristics. Supplementary cementitious materials (SCMs) can be used to enhance the poor properties of recycled aggregate concrete (RAC). Silica fume and fly ash are commonly used SCMs in the World, but their high usage led to a shortage of silica fume and fly ash. Still, the deficiency of these materials in large parts of the world is a challenge that requires exploring alternative feedstock materials for the construction industry in the coming years. Wheat straw ash (WSA) is an agricultural waste product that could be used as an alternative SCM due to its pozzolanic behavior to enhance the properties of RAC. In addition, concrete is brittle and needs reinforcement, for which polypropylene fibers (PPFs) can be used. The current research examines the mechanical characteristics of fiber-reinforced RAC, including compressive strength, splitting tensile strength, and ductility performance. Durability indicators, such as chloride diffusion, chloride penetration, acid resistance, and water absorption test, were also assessed. The results showed that concrete samples with 10% WSA, 50% RA and 1.5% PPFs had the highest compressive and splitting tensile strength, 60.2 MPa and 7.25 MPa, respectively, representing increases of 24.75% and 30.65%, as compared to plain samples at 56 days. In these samples, water absorption was reduced by 13% due to the finer WSA particles resulting in the lowest reduction in strength and mass recorded when exposing concrete samples to acidic media. The statistical analysis also validated that irrespective of WSA and PPFs, the concrete with 0% RA had the highest performance in strength and durability behavior. The study showed that WSA and PPFs might be employed in tandem to offset the poor behavior of RA, enhance the bond between fibers and concrete, and improve the mechanical strength and durability performance of RAC, thus demonstrating its suitability as a sustainable and economical construction material.

Construction and demolition waste as recycled aggregate for environmentally friendly concrete paving

Recycled aggregates (RA) from construction and demolition waste (CDW) instead of natural aggregates (NA) were analysed in the manufacture of new eco-friendly concrete. Fine (FRA) and coarse (CRA) recycled aggregates were used in different percentages as substitutes of natural sand and gravel, respectively. The results revealed that the use of RA in percentages of up to 50 wt.% is feasible. Additionally, RA were used to produce paving blocks in accordance with industrial requirements. Thus, values of water absorption lesser than 6.0% and tensile strength upper than 3.6 MPa were obtained, which are similar to those of a reference sample and within the limit values established by the regulations. These results were achieved by reducing the incorporation of cement, thereby saving production costs and minimizing environmental impact.

Environment-friendly recycled asphalt pavement design for road maintenance applications

Potholes are one of the most common road distresses in Kuwait especially after winter season in 2018. Pavement deterioration rate significantly increases as the pavement exposes to moisture. Paving road requires using high-quality materials. This paper aims to investigate the feasibility of using recycled asphalt pavement in hot mix asphalt (HMA) design. Questionnaires and a series of laboratory tests were conducted to analyze the effects of pores on the society and test the recycled and the regular mixtures performance to choose the best option regarding it. Marshal test portrayed that the recycled mixture has a high stability and flow. According to the tensile strength ratio test requirements which set a minimum ratio of 75%, it is recommended to reduce the proportion of recycled aggregate. The Hamburg wheel tracker (HWTD) test proved that the recycled asphalt mixture has a low rutting depth for wet and dry samples compared to the general one. On the other hand, the wet recycled mixture requires a reduction by of 20% to satisfy the specifications. The physical properties for both mixtures were compared, and the voids filled with asphalt (VFA) and voids in mineral aggregate (VMA) of the recycled mixture are lower than those in the regular mixture. The recycled mixture reveals a higher efficiency in saving costs and improving mixtures used for potholes maintenance activities. All the results proved that the mixtures consisting of recycled asphalt are most preferred since they are affordable and perform reasonably well compared to mixtures made of regular asphalt. As a future work, smaller percentages of recycled aggregate should be tested to check the robustness and sustainability of the designed recycled asphalt mixture using different tests such as Hamburg wheel tracker test (wet samples) and indirect tensile strength test. Furthermore, more experiments can be run to test other mix parameters and properties such as durability.

Comparative environmental evaluation of recycled aggregates from construction and demolition wastes in Italy

Ensure sustainable consumption and production patterns requires urgent actions to combat climate change and its impacts as established by Sustainable Development Goals (SDGs). In this context, this study demonstrates the feasibility to produce structural concrete using recycled aggregates from construction and demolition waste in Italy. More specifically, the present research aims to analyze the environmental impacts caused by five mixtures of concrete, with similar mechanical properties and workability, but with a different amount of recycled coarse aggregate and natural coarse aggregate (0% - 30% - 50% - 70% - 100%). Fixed plant and a mobile plant treatments are investigated as two different modes of production of recycled aggregates. Life Cycle Assessment (LCA) methodology is applied to achieve this goal. The main results demonstrate that mixtures formed by recycled coarse aggregates have a better environmental impacts than the only one formed exclusively by natural coarse aggregates and results improve when the amount of recycled coarse aggregate is higher.

Transition to circular economy in the construction industry: Environmental aspects of waste brick recycling scenarios

The extensive exploitation of natural resources, together with an inefficient use of end-of-life materials, results in the generation of vast amounts of waste. The current material streams are to be reconsidered to mitigate the environmental burdens and achieve the sustainability goals. However, these intentions usually lead to material downcycling, which does not provide significant environmental benefits. In this paper, the potential of waste brick recycling is assessed from the environmental point of view as the recycling options of waste bricks attract an eminent attention due to rationalization and optimization of material streams, including transformation to the circular economy model according to the EU commitments. Three different scenarios are taken into account in that respect: replacement of natural aggregate, partial replacement of cement binder, and alkaline activation. The life cycle methodology is used at the assessment and the obtained results are presented on both midpoint and endpoint levels. The analysis of environmental impacts shows only minor improvements resulting from the replacement of natural aggregates by recycled waste bricks. The partial replacement of cement by waste bricks in powdered form can provide the most substantial benefits including decarbonization of the construction sector. The application of alkaline activators can harm the potential of alkali-activated materials considerably due to their negative effects on human health. A complex assessment of recycling scenarios is found to preferable to one-sided analyses aimed at carbon dioxide emission reduction only if a real sustainability without any hidden risks is to be achieved.

Construction and demolition waste applications and maximum daily output in Spanish recycling plants

The pursuit of construction sustainability has driven the use of partially or wholly waste-based eco-materials. New applications are being sought for recycled aggregate (RA) to further the use of this material and ensure the survival of the construction and demolition waste (C&DW) industry. RA, currently used in the construction of pavements, fills and embankments and only incipiently to manufacture structural or non-structural concrete, is a mere 8.99% of the total aggregate extracted in EU countries where RA is produced. Against this backdrop, the utility of this study lies in the overview afforded of RA typology, the application of the product in bound or unbound materials, the pursuit of new applications, structural or otherwise, and the assessment of daily maximum output by C&DW recycling facilities in Spain. The findings show that irrespective of its origin, RA most commonly adopts the form of wet mix macadam, gravel or sand used primarily in unbound applications with only dubious quality standards. Plant managers contend that RA from clean waste can feasibly be used in bound applications that require higher-performance materials. Maximum daily output varies widely, with capacity under 200 t d^-1 in 30% of the plants. One of the conclusions drawn is that the current business model is in need of revision, with an emphasis on aggregate quality control (certification) and the adoption of technology for separating out impurities, pollutants and undesirable materials to improve the quality of RA.

Sustainability evaluation of concretes with mixed recycled aggregate based on holistic approach: Technical, economic and environmental analysis

This research combines technical, economic and environmental analysis of the use of recycled aggregates from construction and demolition wastes in concrete production. Firstly, an experimental campaign to evaluate the effect of recycled aggregate on mechanical properties was developed. The mixture designs studied were: a reference concrete (without recycled aggregate), two concretes with 20% and 100% replacement of coarse natural aggregate by recycled concrete aggregate; and three concretes with 20, 50 and 100% replacement of coarse natural aggregate by mixed recycled aggregate. To analyze their technical feasibility, these concretes were made in the laboratory and in a concrete plant. The economic viability was also studied indicating the additional costs incurred due to the utilization of recycled aggregate in different economic scenarios. Finally, the differences in environmental impacts were analyzed for each concrete. For this purpose, energy consumption, global warming, eutrophication, acidification, photochemical ozone creation, waste generation, and abiotic depletion were accounted. 20% replacement of recycled concrete aggregates does not cause practically variations in the cost or the environmental loads, only a reduction of waste generation and abiotic depletion of 8% and 10.6% respectively. In contrast, the use of 100% replacement by mixed aggregates may increase the global warming indicator an 11% when double transport distance is assumed. But in exchange, the waste generation decreases 35% and the abiotic depletion 50%. Aggregate transport distance is a key factor that will determine the cost, energy consumption, and global warming of the mixed recycled aggregate.

Long-term leaching and mechanical behaviour at recycled aggregate with different gypsum contents

Construction and demolition recycling is regarded as an essential subject in the EU, as the target established by its policies to 2020 ratio is far from being achieved. The use of materials recycled from such waste has been widely deemed a contribution to the sustainability of the construction sector. Gypsum is one the limiting components of recycled aggregates used as a base layer in road construction. The aim of this research was to analyse the effect on mechanical properties, leaching behaviour and dimensional changes at long term in recycled aggregates with different gypsum contents. Load bearing capacity was conducted by California bearing ratio on prepared samples. Moreover, the compressive strength was conducted on samples prepared with a 3% cement addition. Both tests were studied long term. Dimensional changes were studied through swelling in California bearing ratio test mould under the modified Proctor conditions for 1 year and using an oedometer device for 5 months. Furthermore, environmental risk assessment was performed, classifying the material with gypsum addition as non-hazardous, given that sulphate anion was above the inert limit. Good mechanical behaviour in the long term and no significant dimensional changes were found regardless of gypsum content.

Properties of high-calcium and low-calcium fly ash combination geopolymer mortar containing recycled aggregate

This study presents the properties of a recycled aggregate geopolymer mortar made with a blend of high-calcium fly ash (HCF) and low-calcium fly ash (LCF). An experimental study was divided into two series. In series I, an effort was made to produce a more durable HCF geopolymer by partially replacing a portion of the HCF with LCF. A mortar with a 50:50 weight blend of HCF and LCF provided a high early strength and showed excellent potential in an acidic environment. In series II, recycled aggregate was used in the LCF-blended HCF geopolymer mortar. The results showed that the compressive strength of the geopolymer mortar decreased with an increase in the recycled aggregate content. The results also indicated that application of the mortar made with recycled aggregate under aggressive conditions should be avoided. However, a mixture with 25% recycled aggregate showed a compressive strength similar to that of the control mixture containing 100% natural aggregate.

A bi-level environmental impact assessment framework for comparing construction and demolition waste management strategies

Several pioneering life cycle assessment (LCA) studies have been conducted in the past to assess the environmental impact of specific methods for managing mineral construction and demolition waste (MCDW), such as recycling the waste for use in concrete. Those studies focus on comparing the use of recycled MCDW and that of virgin components to produce materials or systems that serve specified functions. Often, the approaches adopted by the studies do not account for the potential environmental consequence of avoiding the existing or alternative waste management practices. The present work focuses on how product systems need to be defined in recycling LCA studies and what processes need to be within the system boundaries. A bi-level LCA framework is presented for modelling alternative waste management approaches in which the impacts are measured and compared at two scales of strategy and decision-making. Different functional units are defined for each level, all of which correspond to the same flow of MCDW in a cascade of product systems. For the sole purpose of demonstrating how the framework is implemented an illustrative example is presented, based on real data and a number of simplifying assumptions, which compares the impacts of a number of potential MCDW management strategies in New York City.

Characterizing the environmental impact of metals in construction and demolition waste

Large quantities of construction and demolition (C&D) waste are generated in China every year, but their potential environmental impacts on the surrounding areas are rarely assessed. This study focuses on metals contained in C&D waste, characterizing the metal concentrations and their related environmental risks. C&D waste samples were collected in Shenzhen City, China, from building demolition sites, renovation areas undergoing refurbishment, landfill sites, and recycling companies (all located in Shenzhen city) that produce recycled aggregate, in order to identify pollution levels of the metals As, Cd, Cr, Cu, Pb, Ni, and Zn. The results showed that (1) the metal concentrations in most demolition and renovation waste samples were below the soil environmental quality standard for agricultural purposes (SQ-Agr.) in China; (2) Cd, Cu, and Zn led to relatively higher environmental risks than other metals, especially for Zn (DM5 tile sample, 360 mg/kg; R4 tile sample, 281 mg/kg); (3) non-inert C&D waste such as wall insulation and foamed plastic had high concentrations of As and Cd, so that these materials required special attention for sound waste management; and (4) C&D waste collected from landfill sites had higher concentrations of Cd and Cu than did waste collected from demolition and refurbishment sites.

Environmental evaluation of green concretes versus conventional concrete by means of LCA

A number of green concrete mixes having similar basic properties were evaluated from the environmental point of view by means of the Life Cycle Assessment method, and compared with a corresponding conventional concrete mix. The investigated green concrete mixes were prepared from three different types of industrial by-products, i.e. (1) foundry sand, and (2) steel slag, both of which were used as manufactured aggregates, and (3) fly ash, which was used as a mineral admixture. Some green concrete mixes were also prepared from a recycled aggregate, which was obtained from reinforced concrete waste. In some of the green concrete mixes the recycled aggregate was used in combination with the above-mentioned types of manufactured aggregate and fly ash. All of these materials are able, to some extent, to replace natural aggregate or Portland cement in concrete mixes, thus providing an environmental benefit from the point of view of the saving of natural resources. Taking into account consequential modelling, the credit related to the avoidance of the need to dispose of the waste materials is considered as a benefit. In case of the recycling of waste concrete into aggregate, credit is attributed to the recovery of scrap iron from the steel reinforcement. In the case of the use of steel slag, credit is attributed to the recovery of metals, which are extracted from the slag before being used as an alternative material. The disadvantage of using alternative materials and recycled aggregates can sometimes be their relatively long delivery distance. For this reason, a transport sensitivity analysis was carried out. The results indicate that the use of the discussed alternative and recycled materials is beneficial in the concrete production industry. Preference is given to the fly ash and foundry sand scenarios, and especially to those scenarios which are based on the combined use of recycled aggregate with these two alternative materials. It was found that longer delivery distances of the alternative materials do not necessarily affect the results significantly. However, variable delivery distances may have a greater effect when choosing between different alternative scenarios.