Recycled Concrete Aggregates Applications in Asphalt Paving: Comparison
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The sustainability of road pavements is highly influenced by the proper selection of the materials used in the production and construction. The material circularity principle aims to completely or partially replace traditional aggregates and fillers with wastes or by-products. Adapting asphalt mixtures to incorporate waste materials reduces the consumption of naturally mined materials, minimizing the carbon footprint and pavement industry’s impact on the environment. Furthermore, the use of waste materials minimizes landfill volumes and limits the disposal of waste produced from various sources. Thus, this document dpresents a liscussesterature review on the use of recycled concrete aggregates (RCA) from construction and demolition waste (CDW) as alternative aggregates for application in asphalt paving.

  • steel slag aggregate
  • recycled concrete aggregates
  • waste products
  • asphalt mixture
  • sustainable materials

1. Characteristics of Construction and Demolition Waste

The construction sector is associated with almost 36% of waste production volume in Europe and close to 67% in the United States [1], with the corresponding potential environmental impacts. This reality represented the production of around 924 million tons of construction and demolition waste (CDW) in the European Union in 2016 and 2.36 billion tons only in China in 2018 [2].
In addition to the very significant quantities of CDW produced, these materials have other characteristics that make their management and recycling difficult, as typified in Figure 1, primarily due to:
  • A heterogeneous constitution with fractions of several size gradings and levels of hazard;
  • Scattered origins in terms of geography;
  • Occasional or temporary production at each place of origin considering the construction works’ temporary nature.
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Figure 1.
Some of the difficulties that CDW recycling has faced.
These problems lead to problematic quantification, frequent uncontrolled deposition, and systems supported by end-of-line treatments promoted by the low exploitation of mineral resources and landfill costs. This situation is challenging to revert and has very high environmental, social, and economic impacts, such as the environmental and visual impacts of illegal disposal and the costs to eliminate them [3].

2. Properties and Treatments of Recycled Concrete Aggregates Applied in Road Pavements

2.1. Properties of Recycled Concrete Aggregates Applied in Road Pavements

Concrete is globally the second most widely used material, after water, with an annual global production estimated at 25 billion tons in 2009 [3][4]. This situation generates significant impacts on the environment and society in general. One of the problems is cement production, which accounts for 4.4% of global annual industry emissions. Another problem is the end-of-life of concrete materials and structures since these materials are landfilled after demolition, generating large amounts of CDW—more than 850 million tons/year in the European Union [5]. Therefore, one way to minimize this problem is to incorporate recycled concrete aggregates (RCA) into paving materials to produce sustainable asphalt mixtures [6].
Several researchers have been pointing out some ways to process and incorporate recycled concrete aggregates (Figure 2) in different types of asphalt mixtures during the last decade, namely in hot mix asphalt [7][8][9][10], warm mix asphalt [11][12], stone mastic asphalt [7], foamed asphalt mixtures [13], and emulsified asphalt mixtures [14].
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Figure 2.
Recycled concrete aggregate (RCA) resulting from CDW processing.
Nowadays, the main concepts involved in the comprehensive waste management process should prioritize strategies favoring implementing the most recent 7 R’s rule: Rethink, Redesign, Reuse, Repair, Remanufacturing, Recycling and Recover [15]. These principles may also explain why incorporating recycled concrete aggregates in asphalt mixtures or other layers for flexible pavements, or even in other types of work, has also been recently described in several articles and technical documents [16].
A suitable way to accomplish the goals mentioned above is to reuse RCA in asphalt mixtures. However, recycled concrete aggregates usually have a high porosity and water absorption, which has been reported as the main reason for increasing bitumen content in asphalt mixtures. Meanwhile, some recycled aggregate fractions usually have many angular and rough-textured particles. Thus Marshall stability tends to increase, and Marshall flow tends to decrease [17].
Considering the general requirements listed in the relevant standards, the adequacy of these recycled concrete aggregates resulting from construction and demolition waste processing shall be evaluated to be used in asphalt mixtures. The selection and assessment of RCA quality must be made for each type of asphalt mixture and layer to offer the best mechanical and functional performances; namely, regarding the quality of asphalt, the resistance to raveling and stripping, the resistance to weathering, the resistance to freezing and thawing, the bitumen absorption, the skid resistance, the compaction, the strength and stability, and the resistance to fragmentation [18]. In order to ensure adequate performance levels in these tests, Martinho et al. [19] reported that different natural, artificial and recycled aggregates could be combined and used in more sustainable asphalt mixtures.
Đokić et al. [20] analyzed the mineralogical–petrographic and physical–mechanical properties of RCA, the natural aggregate (dolerite), and their combination (RCA rates between 15% and 60%). They confirmed that the granular mixtures presented good wearing and fragmentation resistance (MDE 14–15%, LA 22–27%) and an acceptable Polishing Stone Value (PSV) of 55–57. They concluded that RCA could be used in asphalt mixtures or flexible pavement layers and several traffic loads.
Moreover, as Galan et al. [8] reported, among other differences in using RCA in HMA, a 50 % higher duration on the mixing time is required until a 100% bitumen coating is reached, and the HMA compaction is more difficult due to greater internal friction on the aggregate. However, these authors also concluded that using RCA rates up to 60% improves the mechanical performance of the HMA.
The recycled concrete aggregates also tend to improve the rutting resistance of asphalt mixtures, but some works analyzed by Pasandín and Pérez [21] mention that this parameter can decrease when high percentages of RCA are used.
Regarding the resistance to fatigue cracking, using these recycled concrete aggregates usually slightly reduces the performance of HMA mixtures [22], even though this tendency was not observed in some studies mentioned by Pasandín and Pérez [21].

2.2. Treatments of Recycled Concrete Aggregates for Road Pavement Application

The RCA is included in the European Waste Catalogue (EWC) under code number 17 01 07 and, in general, has lower quality when compared with other natural aggregates. Therefore, many treatments [5][10][21][23] have been considered to avoid the consequences of using RCA material with inferior properties on asphalt mixtures.

3. Legislation on the Use of Recycled Aggregates from CDW

The first European Directive (2008/98/EC) regarding the use of construction and demolition waste already defined a minimum threshold increase to 70%, by 2020, in respect of preparation for reuse, recycling, and other forms of material valorization for non-hazardous CDW (except natural materials as defined in the category 17 05 04 of the EWC).
In this regard, the situation in Europe remains very heterogeneous, with the CDW recovery rate ranging from less than 10% to more than 90% [24]. Therefore, these authors claim that it is essential to create conditions for secondary materials generated from the recycling of CDW to be effectively integrated into the market and used in high added value applications, dynamizing this market and promoting a circular economy in the construction sector.
Thus, the European Commission prepared a protocol in 2016 [25] to strengthen the confidence in the CDW management and the quality of recycled materials obtained. The Commission thought that this objective would be achieved as follows:
  • Improving the identification, separation of the origin, and collection of waste;
  • The improvement of waste logistics;
  • The advance in waste processing;
  • Quality management;
  • The appropriate policy and framework conditions.
In particular, the importance of improving the identification, separation of the origin, and collection of CDW have been highlighted by several authors as one of the most relevant features affecting the general acceptance and use of these alternative materials [26][27][28][29][30].
In 2018, the European Directive 2018/851 amended the Waste Framework Directive 2008/98/EC. Among other changes, this new document replaced the concept of a “European recycling society” with that of a “European circular economy”, and a new disposition was introduced to promote selective demolition. Consequently, hazardous substances must be safely removed, and selective reuse and high-quality recycling be facilitated by establishing a sorting system for CDW, including mineral fractions (e.g., concrete, bricks, tiles, ceramics, and stones). This objective’s fulfillment is expected to contribute to obtaining RCA with better quality in the future.
Nowadays, recycled aggregates obtained from the CDW can also have the CE mark under the Regulation EU No-305/2011 of the European Parliament and Council because they can be used in different civil construction activities, replacing natural aggregates extracted from quarries.
Nonetheless, there is still an urgent necessity to implement measures to promote new applications of these recycled materials by all private and public owners that should contemplate the mandatory incorporation of a minimum percentage of recycled aggregates in the works’ technical specifications. Thus, national environmental agencies have prepared guides that allow municipalities and companies to improve CDW streams’ management and effectively fulfill their legal obligations [31][32][33].

4. Environmental Evaluation of Recycled Concrete Aggregates from CDW

Sun et al. [34] studied the workability and fracture toughness of natural aggregate and recycled concrete aggregate combined with a blowing agent through an environmental study. The acceptability of these materials was investigated and discussed based on their mechanical, micro-mechanical, and ecological performance evaluation through compressive strength, flexural strength, X-ray diffraction (XRD), scanning electron microscope (SEM), and CO2 emission tests. The results indicated that natural aggregates (NA) have better compressive strength performance, while recycled concrete aggregates have improved flexural strength. Finally, the CO2 emission per unit of NA was higher than RCA, indicating that using recycled concrete aggregate over other conventional resources will reduce energy consumption and meet the goal of being environmentally friendly.
Another approach to evaluate the viability of using recycled aggregates in asphalt mixtures consists of their LCA [35][36]. The work of Nwakaire et al. [36] on the use of RCA for sustainable highway pavement applications specified the five life cycle phases to be considered for their use in road pavements (material production, construction, use, maintenance, and end of life). The authors concluded that RCA could be fully used in pavement lower layers and is a sustainable substitute for natural aggregates for highway pavements.
Another example considered a cradle-to-laid LCA method to evaluate the prospective environmental impacts related to the use of RCA in an HMA for a binder course [35]. This researchtudy identifies the main processes and the system boundaries, taking into account the following four life cycle phases:
  • The aggregates production and transportation to the asphalt plant;
  • The production at the asphalt plant;
  • The asphalt mixture transportation to the construction site;
  • The pavement construction.
These authors defined three percentages of RCA replacements, ranging from 15% to 45%, and used data collected from Colombian contractors to model the foreground system. Next, they used the SimaPro software for modeling the processes, and all the life cycle inputs and outputs related to the functional unit were considered for potential impacts studied with the TRACI v.2.1 impact assessment methodology. According to their conclusions, the HMA with 15% and 30% RCA can be considered eco-friendly alternatives to the HMA with NA. However, the HMA with 45% RCA presented a lower environmental performance. Nevertheless, this work demonstrated the advantages of using recycled concrete aggregates processed from CDW in new asphalt mixtures [35].

5. Case studies of Recycled Concrete Aggregates Incorporation in Asphalt Mixtures or Road Pavements

5.1. Incorporation of Recycled Concrete Aggregate in Conventional HMA

As already well recognized, one possible approach for using recycled concrete aggregates processed from CDW is to use them as an alternative to natural aggregates, particularly in hot asphalt mixtures.
As concluded in the final report of the IRCOW project [37], the main reasons that have led many European public authorities to pay more attention to the CDW problem were the following:
  • CDW is an essential source of waste in the EU and can be reused or recycled;
  • Directive 2008/98/EC, from 2008, amended by Directive (UE) 2018/851, already indicated a target of 70% for the reuse of these materials by 2020;
  • Recycling and reusing CDW saves natural resources and energy and can be cheaper than natural aggregates.
Another worthy effort to encourage the reuse of recycled aggregates was given by the DIRECT-MAT project, developed between 2009 and 2011 and involved twenty partners from fifteen European countries [38].
Many projects and research studies have followed this CDW recycling or reuse principle, resulting in numerous reports and technical articles over the last two decades. Since one of the materials being addressed in this researchview paper is recycled concrete aggregate, the main conclusions of some of the most recently released studies conducted by different authors on diverse RCA applications are presented below.
Sánchez-Cotte et al. [39][40] evaluated the incorporation of RCA of two different origins (CDW of a building; CDW of a rigid pavement) on HMA mixtures, assessing the RCA’s physical and chemical properties and comparing the performance of the mixtures produced with both RCA and NA aggregates. They used different techniques to evaluate these aggregates and their influence on HMA: X-ray fluorescence, XRD, UV spectroscopy, and atomic absorption spectrometry. They replaced NA with RCA at 15%, 30%, and 45% for each coarse fraction. The mineralogical tests showed that the potential reactions in asphalt mixtures by nitration, sulfonation, amination of organic compounds, and reactions by alkaline activation in the aggregates could be neglected. These authors concluded that coarse RCA could be used in asphalt mixtures without affecting their chemical stability.
Furthermore, the laboratory test results showed that the HMA with and without RCA offered a similar behavior. However, RCA promotes more significant environmental benefits and potential cost savings. Nevertheless, the authors also stated that the HMA performance is strongly associated with the RCA source and dosage. Finally, similarly to other authors’ conclusions, they note that HMA with RCA induces a higher optimum binder content (OBC) than NA.
Pasandín and Pérez [21] published an extensive and comprehensive review of the properties of HMA with RCA. These authors found that the laboratory results had significant variations, probably due to RCA’s different origins. These aggregates include cement mortar, promoting a higher water/bitumen absorption, and a lower mixture quality. Most of the analyzed studies also reported a high tendency for stripping on mixtures with RCA. Some treatments were also identified which minimize this problematic characteristic and increase the moisture resistance of mixtures. However, other aspects need to be analyzed, including their economic and environmental impacts and practical feasibility at the asphalt mixing plant. These authors also documented the costs of using RCA in asphalt mixtures (e.g., with a higher bitumen consumption, lower density, and lower environmental impacts) and the lack of technical specifications as two critical aspects to take into account.
Regarding the specifications, the Marshall method can underestimate HMA properties because compaction can break some RCA particles and some countries use the same limits for natural aggregates in this test. Pasandín and Pérez [21] also mentioned that some studies point out that RCA can be used on the pavements of low-traffic roads and favor sustainable growth. Thus, they have concluded that new specifications must frame RCA’s use in asphalt mixtures to increase the application success in trial sections, defining the type of roads and heavy traffic categories suitable to each use.
Tahmoorian and Samali [18] highlighted that asphalt mixtures include more than 65% coarse aggregates, and the large quantities of RCA available in different construction sites turn their use in asphalt mixtures to almost mandatory. In their laboratory work, they assessed the suitability of RCA and basalt to be incorporated in asphalt mixtures as coarse aggregates. In the tests performed, these authors [18] confirmed that, compared to basalt, the RCA provides better workability, compaction, and permanent deformation resistance to the asphalt mixtures. The test results also revealed that the RCA exhibits more absorption and wet/dry resistance variation than conventional aggregates and that the RCA still complies with aggregate requirements for asphalt mixtures in the remaining tests.
Gopalam et al. [41] also developed an experimental study to evaluate the influence of the binder type on the performance of dense graded asphalt mixtures that included RCA replacing NA in the HMA coarse fraction. They produced different Marshall samples of dense asphalt macadam (DBM) mixtures under the Indian technical specifications, using three binders, conventional VG 30 and VG 40 bitumen’s, and a crumb rubber modified binder (CRMB), all of them with RCA or NA. Then, they evaluated the Marshall characteristics, indirect tensile strength, moisture sensitivity, resilient modulus, and permanent deformation resistance. Based on the results, they confirmed that, in general, all the mixtures fulfilled the requirements for the Marshall and moisture susceptibility characteristics. The CRMB and the VG40 binders offered the best performance with RCA or NA, although the latter showed slightly better results.
The need for recycling CDW (as RCA) for use in road pavement construction was also studied by Kanoungo et al. [42]. They compared some of RCA’s available treatment methods; namely, acid treatment, thermal treatment, and asphalt emulsion, to define what would be the best method. Using these treatments, they produced DBM mixtures with RCA and analyzed the Marshall characteristics and moisture sensitivity. The results pointed to the asphalt emulsion treatment of RCA as the most suitable method.
The possible use of fine and coarse RCA as NA substitutes in nine asphalt mixtures for base course layers was also studied by Radević et al. [43]. These researchers assessed the influence of the RCA rate (up to 45 wt.%) on the physical and mechanical properties of an AC 22 base 50/70 mixture and compared the results with those obtained in a reference mixture produced with NA. They found that RCA’s addition needs a higher binder content (up to 1%), which resulted in lower stiffness and higher fatigue cracking resistance of the asphalt mixtures, while their low-temperature resistance was slightly inferior. In conclusion, it is possible to use up to 45% RCA in asphalt mixtures without significantly reducing the mechanical performance.
Nwakaire et al. [36] claimed that more unanimous standard guidelines should be developed to guarantee excellence and sustainability when using RCA in HMA. These authors stated that more studies are needed on the use of RCA for porous and SMA mixtures and rigid pavements. Moreover, other essential research areas are assessing the actual field performance of in-situ RCA pavements, surveying the professional’s perspectives on the challenges of using RCA, and identifying all feasible utilization potentials for RCA based on different circumstances and scenarios using LCA methods. Regarding the mix design of RCA mixtures, it is necessary to harmonize the binder requirement for RCA mixtures, develop an innovative asphalt binder with an improved affinity with RCA, adjust the particle size requirements of the conventional HMA for RCA mixtures, and adjust the solution to the sources and nature of RCA used.

5.2. Incorporation of Recycled Concrete Aggregate in WMA Mixtures

The use of RCA in WMA was evaluated by other authors [11][12][44][45] to assess the influence of these alternative aggregates (processed from CDW) on the performance of WMAs. Martinho et al. [12] selected three WMA mixtures with RCA considering different laboratory results, optimizing them before being produced in an asphalt plant and then compacted under real conditions in several pavement trial sections. A dense-graded AC 20 base mixture was used as a reference mixture. The RCA characteristics were also evaluated according to EN 933-11, mainly comprising concrete (84%), unbound natural aggregates (9%), masonry elements of clay materials (5%), and traces of asphalt material and other CDWs (respectively, 0.7% and 0.6%).
The results obtained in that study, supported by the literature (up to that date), led to the conclusion that the general performance of WMA with 60% of RCA was good; namely, when using an organic wax as the WMA additive. The use of a chemical additive reduced the rutting resistance compared to the wax. The alternative aggregates slightly reduced the stiffness modulus and water sensitivity results of the WMA mixtures, although the results are still satisfactory for a conventional base/binder layer. The fatigue life of WMA with RCA was adequate and close to the performance observed for the HMA and WMA mixtures used as references. Although conventional equipment was used to build the experimental pavement sections, no specific problems were observed in the field for the WMA mixtures with RCA in any of the necessary operations: mixing, transport, laying, and compaction.
Abass and Albayati [46] investigated the possibility of using RCA in WMA, testing five replacement rates (ranging from 0% to 100%) for the coarse fraction of natural aggregates, using untreated RCA and RCA treated with hydrated lime slurry or hydrochloric acid. The binder contents were obtained with the Marshall mix design method, and the WMA performance was assessed by moisture damage, resilient modulus, and permanent deformation tests. The untreated RCA mixes presented a higher binder content than treated RCA mixes. The moisture susceptibility of treated RCA was improved by nearly 10% compared to untreated RCA. However, the resilient modulus and rutting resistance for mixtures with 100% RCA were lower than those obtained with natural aggregates.
Another innovative paving technique toward a circular economy model was promoted by Neves et al. [47], studying the properties of an AC 20 WMA mixture composed of 60% RCA and a modified binder (4.5%). After assessing the aggregate-bitumen affinity, Marshall properties, moisture sensitivity, stiffness, rutting, and fatigue resistance, the authors concluded that the WMA with RCA shows adequate performance for base course layer use, comparable to equivalent WMA or HMA mixtures with natural aggregates.
In several case studies, Polo-Mendoza et al. [45] have examined WMA mixtures with various RCA contents. This researchtudy developed a trade-off methodology based on the LCA technique and statistical analysis to determine the maximum incorporation rate of RCA in WMA without generating increasing environmental impacts due to the higher binder content of those mixtures.
In a different type of WMA mixture, Zou et al. [13] reported that using RCA in foamed asphalt mixtures (FAM) does not affect the binder curing time, but the mixture performance is influenced when the processed aggregates include redbrick. These authors also concluded that FAM fulfills technical requirements for the strength and moisture resistance of base and subbase course with 100% RCA. The inclusion of redbrick in RCA weakens the FAM performance (a redbrick content limit of 5% is suggested in FAM with RCA), but the addition of 0.5% to 1.5% cement content increases the indirect tensile strength (ITS) and wet/dry indirect tensile strength ratio (ITSR) of FAM. These authors also recommended using harder bitumen’s in foaming, as they include more asphaltenes and offer a higher viscosity.
Monu et al. [48] investigated the optimum proportion of hydrophobic RAP and hydrophilic RCA (separately and combined) for the production of FAM mixtures. The incorporation of RCA affected FAM performance but delayed the hydration of residual cement grains, which could enhance the performance of FAM. FAM mixtures with 20% RCA, 40% RAP, or a blend of 30% RCA and 10% RAP satisfied the specified requirements for pavement applications.

5.3. Evaluation of Different Incorporation Ratios of Recycled Concrete Aggregate

Different amounts of fine and coarse RCA (from 20% to 60%) were integrated by Daquan et al. [49] in an AC 20 HMA to evaluate their effect on mechanical performance. The authors concluded that the OBC increased, and the bulk density of mixtures decreased for higher amounts of RCA. Besides, they observed that fine RCA had higher OBC than coarse RCA, and mixtures with 50% coarse RCA or more had reduced moisture resistance. Furthermore, all mixtures presented good low-temperature properties, but the rutting resistance of asphalt mixtures with RCA was lower than that of HMA with natural aggregates. The decrease in RCA content led to an increased resilient modulus and extended fatigue life (particularly when reducing coarse RCA). The mixtures with 20–40% fine and coarse RCA generally showed the best performance.
Galan et al. [50] studied the influence of RCA percentage on binder content, curing time, and temperature by studying the stiffness of HMA samples. The asphalt mixtures were produced with different percentages of RCA (0%, 5%, 10%, 20%, and 30%) and bitumen (3.5%, 4.0%, and 4.5%). The samples were cured for 0 h, 2 h, or 4 h before being tested at different temperatures (0 °C, 10 °C, and 20 °C). This researchtudy concluded that temperature was the most influential factor in decreasing the stiffness modulus. Conversely, the percentage of RCA was not very relevant to changing the stiffness modulus.
Zhang et al. [51] produced a dense-gradated AC 16 HMA incorporating RCA processed from low strength concrete [52] as a partial substitute for NA at different rates: 30%, 50%, and 75% by weight of NA. Compared with NA, this RCA presented higher wearing and fragmentation values, a lower apparent relative density, a higher water absorption, and a more inadequate bitumen adhesion. The water sensitivity of the mixture decreased up to 50% with the increase in RCA content. Nevertheless, the authors concluded that HMA incorporating up to 50% RCA could be satisfactorily used in road construction.

5.4. Combined Use of Recycled Concrete Aggregate with Other Waste or By-Products

Most studies combining RCA with other waste or by-products to produce new road paving materials include RAP [53][54][55][56]. There are also some examples of simultaneous use of RCA and steel slag aggregates (SSA) as presented at the end of this section.
Abedalqader et al. [54] studied the temperature influence on the performance of asphalt mixtures with RAP and coarse RCA, concluding that the mechanical properties of these asphalt mixtures decreased as RAP and RCA incorporation levels increased for the same test temperature.
Another work was developed to obtain a 5–10 mm RCA and RAP aggregate fraction to substitute natural aggregates in hot mix asphalt [55]. The results show the possible combination of both wastes in RCA/RAP ratios equal to 25/75 or 50/50 to obtain a coarse aggregate fraction meeting the specifications and great environmental benefit due to the reduced use of natural resources.
Coban et al. [56] investigated recycled aggregate base layers for road pavements with two different RCA materials with different gradations and a blend of RCA and RAP materials, compared to the conventional solution with natural aggregates. After performing lab tests (resilient modulus) and field tests (falling weight deflectometer), they concluded that all the recycled aggregate base layers had satisfactory performance.
The fatigue cracking and moisture resistance of HMA mixtures produced with 0%, 35%, and 42% RCA and 10% waste tire rubber modified bitumen were evaluated by other authors [57][58]. This investigation showed the beneficial effect of simultaneously using RCA and crumb rubber on fatigue life, although crumb rubber increased the water sensitivity of the RCA mixture. Nevertheless, these solutions have adequate properties for medium-traffic roads.
Giri et al. [59] explored combining waste materials such as coarse RCA and waste polyethylene in asphalt paving mixtures. They observed that all the developed mixtures satisfy the Marshall and moisture susceptibility specified requirements. The use of waste PE chiefly improves the engineering properties at higher temperatures.
Various studies evaluated the potential use of SSA and recycled concrete aggregate in asphalt mixtures separately. However, only a few studies assessed asphalt mixtures with the simultaneous incorporation of SSA and RCA. Martinho et al. [12] compared the mechanical performance of several warm asphalt mixtures with recycled concrete aggregate, EAF steel slag, or both by-products as partial substitutes for the natural aggregates. Initially developed in the laboratory, this researchtudy selected asphalt mixtures later applied in road pavement trials. Conventional HMA and WMA mixtures without by-products were used as references. The research evaluated aggregate substitution rates of 60% RCA, 30% EAF SSA, or a blend of 40% RCA and 35% EAF SSA. The authors concluded that using EAF SSA or RCA in WMA mixtures increases Marshall stability and could increase or decrease the rutting resistance. The results also showed that the water sensitivity and the stiffness modulus are slightly reduced, and the fatigue resistance does not change significantly. The overall performance of WMA mixtures with RCA or SSA was satisfactory, and the best results were obtained with 60% RCA. Arabani and Azarhoosh [60] developed a study to determine the mechanical properties of asphalt mixtures produced simultaneously with recycled concrete and SSA. Six different asphalt mixtures containing three types of aggregate (dacite, recycled concrete, and steel slag) were produced to obtain Marshall specimens and determine the optimum binder content. Marshall stability, indirect tensile resilient modulus, dynamic creep, and indirect tensile fatigue tests evaluated the mechanical characteristics of the asphalt mixtures. The results indicated that the asphalt mixture with the best performance contains steel slag coarse aggregates and recycled concrete fine aggregates. Roque et al. [61] evaluated the concurrent incorporation of RCA processed from construction and demolition waste and a steel slag aggregate in granular drainage layers of road pavements. Considering the high durability and permeability of the granular materials prepared with these by-products, the authors concluded that these materials could be used together in the drainage base or sub-base layers of transport infrastructures.

5.5. Incorporation of Recycled Concrete Aggregate in Other Types of Asphalt Mixtures

Nwakaire et al. [62] studied the performance of an SMA 14 mixture after replacing 20% to 100% of natural coarse aggregates with RCA. They also used a control SMA with 100% granite. The effect of RCA replacement on SMA quality was assessed through volumetric properties, Marshal stability, indirect tensile strength (ITS), moisture susceptibility, resilient modulus, fatigue and rutting performance, abrasion, and skid resistance. The SMA with RCA performed worse than the control SMA in the ITS and resilient modulus tests, contrary to the remaining tests. Nevertheless, the authors recommended an optimum replacement of 40% RCA because the SMA with RCA requires higher binder contents for optimum performance. The skid resistance of all SMA mixtures was satisfactory, and the rutting resistance of the SMA with RCA was lower than the control SMA at the initial cycles but was better at the end of the test.
The incorporation of RCA in cold asphalt mixtures was studied by Zou et al. [14] by investigating the feasibility of using RCA from CDW to replace natural aggregates in emulsified asphalt mixtures (EAM). Their work was based on the laboratory’s assessment of the optimum moisture and emulsified asphalt contents of some EAM samples that included different RCA rates. They also evaluated the RCA EAM in-service performance after adding cement, including the moisture sensitivity and high and low-temperature performance. This work found that RCA increased the high-temperature performance and reduced the low-temperature performance and moisture damage resistance of EAM mixtures. Moreover, the addition of cement enhances the in-service performance of EAM so that RCA can substitute NA in EAM mixtures when combined with cement.
Chen and Wong [63] evaluated mechanically and functionally PA mixtures made of 100% RCA. Drain down, Cantabro, Marshall, permeability, and aging tests were used to assess the performance of three PA designs: 100% RCA; 100% RCA with enhanced asphalt binder; and control PA with granite aggregates. The results for PA made of 100% RCA assured the essential drainage function, although it is necessary to use enhanced binders to fulfill the Marshall criteria for regular highway applications. The results suggest the possible application of 100% RCA in PA.
Another study incorporated several RCA fractions in semi-dense asphalt (SDA) mixtures. Mikhailenko et al. [64] replaced 100% and 50% of the natural aggregates with three fractions of RCA (coarse, sand, and filler) and evaluated the mixtures’ volumetric properties, water sensitivity, ITS, fracture energy, and rutting resistance. The results confirmed that coarse RCA absorbed higher amounts of binder and reduced the workability. The RCA mixtures presented increased ITS results and brittleness, reducing crack resistance. Higher aggregate replacements significantly affected the moisture susceptibility of the mixtures and decreased the fracture energy (mainly when replacing sand fraction). The incorporation of recycled concrete aggregates enhanced the SDA’s rutting resistance, especially when replacing the coarse fraction. In general, RCA’s use in SDA can be incorporated in limited amounts, and replacement by volume is recommended.

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