Environmental assessment, mechanical behavior and new leaching impact proposal of mixed recycled aggregates to be used in road construction
Graphical abstract
Introduction
In Europe in 2016, approximately 925 million tons of construction and demolition waste were generated (European Union, 2019), representing 36.4% of the total waste that was generated by all member countries. In order to comply with the indications of the Waste Framework Directive of the European Parliament on waste for the 2020 horizon, member States have to increase their rate of use of non-hazardous materials from C&DW to a minimum of 70% by weight. Accordingly, characterization studies of recycled aggregates (RA) from C&DW have been carried out in recent years in Spain (Agrela et al., 2011; Martin-Morales et al., 2011), and numerous international studies have shown the possibilities of recycling these materials in road construction (Jiménez et al., 2012; Vegas et al., 2011; Agrela, 2012; Poon et chan, 2006; Pasandín et al., 2014; Martinez, 2013; Bravo, 2015), recycled concrete (Gonzalez-Fonteboa et al., 2011; Pacheco et al., 2019; Del Bosque et al., 2020) and other application in mortar manufacturing (Cuenca-Moyano et al., 2020; Goncalves et al., 2020) etc.
There are three main types of RA obtained from C&DW: recycled concrete aggregate (RCA), composed of crushed concrete particles, recycled masonry aggregate (RMA) composed of crushed ceramic particles and mixed recycled aggregate (MRA) composed of a mixture of the different materials of C&DW. (Silva et al., 2014). The RCA are the highest quality RA and several countries, and even EU (EN-12620, 2002) have developed specifications which include their definition and have been widely studied their application for road construction, mortar and concrete manufacturing and recycled concrete structures construction (Lu et al., 2019; Duarte et al., 2019; Thomas et al., 2019).
On the other hand, mixed recycled aggregates have less development level than RCA and are not yet recognized as building material in various countries, such as Spain. Some studies about the application of MRA in road construction have been developed in last years. An experimental road section was performed in Malaga, Spain in 2012 (Agrela et al., 2012) and the study concluded that RMA can be used in the execution of subbase layers of roads as cement-treated materials in an amount of 3% by dry mass. Del Rey et al. (2016) developed a laboratory study of cement-treated MRA in a size fraction 0/8 mm in order to probe the use as subbase layers for low-traffic obtain positives results. Other studies have analyzed the properties of MRA in different applications, such as the analysis of the functional and structural parameters of a paved roads constructed with mixed recycled aggregates (Tavira et al., 2018) or analysis of the most important physical properties and mechanical behavior of MRA in geotechnical applications and construction of unpaved roads (Cardoso et al., 2016).
From an environmental point of view, it is very important to characterize the leachate components of RA, to ensure safety in the use of these recycled materials (Van der Sloot et al., 2002). Leaching tests are focused on determining pollutants concentrations that are limited by European directive 2003/33/EEC of landfill admission. Different uses of MRA involve different problems derived from leaching, thus, manufacture of concrete is limited by sulphates concentration (Martin-Morales et al., 2011) or road layers execution that is limited by heavy metals concentration (Barbudo et al., 2012a; Cabrera et al., 2016). Galvín et al., 2014a, Galvín et al., 2014b analyzed four MRA for use on roads, concluding that all MRA can be classified as non-hazardous waste that is caused by high sulfate concentrations due to a composition rich in plaster and ceramic materials.
Several studies have shown that the use of RA from C&DW as replacement of NA reduced the environmental impact of emissions generated during concrete manufacturing (Yazdanbakhsh et al., 2018) and masonry mortars (Cuenca-Moyano et al., 2019). Similarly, Mroueh et al. (2000) analyzed alternative road sections using different wastes such as crushed concrete waste, and the results showed the impact reduction due to the use of RA instead of NA. Butera et al. (2015) evaluated the use of RA from C&DW in road sub-base, and concluded that the impacts were less than those generated by depositing the landfill. Finally, Shi et al. (2019, 2018) have recently shown that the use of RA from reclaimed asphalt pavement in Portland cement concrete as an aggregate replacement for pavements applications has economic, social and environmental benefits during materials production and construction phase. In this sense, the use of life cycle assessment (LCA) in the construction sector can provide the development of strategies within the framework of the circular economy, which encourage the efficient use of resources by reducing the high environmental impact associated with the sector (Mercante et al., 2012; European Commission, 2014a, b; European Parliament and Council, 2008).
In this study, physicochemical characteristics of mixed recycled aggregates were studies in order to establish an international classification of the different MRA types that exist. This study also analyzed the mechanical behavior of the MRA, which determine their potential uses in roads construction application. Finally, the environmental aspects were analyzed by the leaching analysis of the materials ensuring compliance with European landfill regulations, in addition, the environmental impact of the MRA and natural aggregate were determinate through LCA, that will allow establish the ecological potential of the use of recycled materials. The new proposed leaching limits established in this study make it possible to increase the use of MRA by increasing the rate of substitution of MRA by natural aggregates. It was concluded that the use of MRA is an ecological and sustainable solution for road construction.
A large number of countries have now published regulations in order to increase the amount of RA from C&DW in building and road construction (Goncalves et al., 2010). These regulations are adapted to the typical construction materials in each area, resulting in an AR with significant differences between its characteristics (de Brito et al., 2019a).
De Brito, Agrela and Silva (2019 presented a proposal for the international classification of six types of RA that can be used for road construction in order to simply their use. In this classification proposal, RA are classified according to their main composition and a series of physicochemical properties (composition, minimum density, water absorption, Los Angeles abrasion test and water-soluble sulfate) are determined through international standardized tests.
The classification (Table 1) shows six types of RA. The first type (RCA) is the highest quality that could be obtained, composed mainly of concrete particles and natural aggregates, 90-85% by mass minimum (RCA-I and RCA-II respectively). The MRA is divided into three types (I, II and III) depending on the amounts of concrete particles, natural aggregate and rubble masonry in it. This is the most common type of RA obtained from C&DW recycling plants, since these are all types of waste with varied and difficult steps to separate their composition (de Brito et al., 2019b). Finally, The RAA (Recycled Asphalt Aggregate) type is obtained from the demolition of road surface layers and is composed mainly of bituminous materials, 50% by mass minimum.
Rc: concrete and natural aggregates with adhered mortar; Ru: particles of natural materials (rocks, gravels, etc.); Rb: particles of ceramic bricks, tiles, calcium silicate masonry units, etc.; Ra: bituminous mixture particles; Others: wood, plastic, plaster, aluminum, etc.
The main composition with the properties shown in Table 1 classifies the type of aggregate recycled after C&DW treatment in the plant. SSD density and water absorption determine the compaction properties of RA. Finally, the organic matter content and acid-soluble sulfates determine both the use of RA in the manufacture of cement-based materials and they present environmental limitations due to leaching problems.
Section snippets
Materials
In this section, the production processes and physicochemical properties of MRA and artificial gravel (AG), that is a control material to establish a comparative between recycled and natural aggregates, are studied.
Mechanical behavior
This section shows the results of the compaction study carried out on the materials using the modified Proctor test and bearing capacity results measured by the CBR index are shown.
Discussion of the results of mechanical behavior
Curves shape and values obtained in Modified Proctor test differ considerably between MRA and AG and the values also vary between the four MRA. The AG had the highest dry density value and the lowest optimum moisture content value. The different maximum dry density and optimum moisture content between both types of materials is mainly due to the physical properties, since MRA have compositions with percentages of ceramic and plaster material, materials with a low density and a high porosity (
Conclusion
This work shows an exhaustive study of the feasible use of mixed recycled aggregates applied in road layers compared to natural aggregates. The study covers the analysis of physical-chemical properties, mechanical behavior, environmental impact of the leachates and life cycle analysis of the natural and recycled materials analyzed.
CRediT authorship contribution statement
Agrela F: Conceptualization, Methodology, Supervision, and, Resources. J.L. Díaz-López: Writing - original draft, Writing - review & editing, writing — original draft preparation, writing — review and editing and, D. Rosales J: Writing - original draft, Writing - review & editing, writing — original draft preparation, writing — review and editing and, Data curation. G.M. Cuenca-Moyano: Writing - review & editing, writing — review and editing and, D. Herminia Cano: Supervision, and, Resources.
Declaration of competing interest
The authors agree with the presentation of this work to the Journal of Cleaner production as they have mentioned the organizations that have financed the research and express the Declaration of Interest Statement of this publication.
Acknowledgment
The author would like to thanks:
To the geotechnical laboratory of CEDEX (Minister of Civil Works, Government of Spain), for making available the financial support needed for this investigation, and special thanks to José Estaire and María Santana.
To the Spanish treatment plants for construction and demolition waste: Gecorsa company in Córdoba, and Arecosur company in Málaga, for their selfless contribution of materials.
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