A Critical Overview of Using Reclaimed Asphalt Pavement (RAP) in Road Pavement Construction

Abstract

In view of the climate crisis, green technologies should be used to ensure sustainable structures in the construction industry. Road construction could also contribute to the creation of a circular economy, as it is partly responsible for several current environmental phenomena, such as greenhouse gas (GHG) emissions and depletion of natural aggregates and landfills. The use of Reclaimed Asphalt Pavement (RAP) is considered one of these recycling solutions, as it can be reused in road construction projects. Implementing it in practice is already a topic that should be included in the technical guidelines for road construction and maintenance. Therefore, this study is a critical overview of the worldwide experiences with the installation of RAP in road pavements published by different road authorities worldwide, aiming to prove the inconsistency in using these materials in pavement courses. The results of this review are analyzed to identify possible knowledge gaps regarding RAP content. It was shown that the use of RAP is different on all five continents. The main findings were that the RAP content in asphalt layers is still at a low level of about 30% and that the use of RAP materials in unbound layers in road pavement construction is not yet fully accepted. It is expected that the results of this study will help to improve further research on the performance of RAP and motivate more countries to develop appropriate guidelines for the use of RAP materials in road pavement construction.

1. Introduction

According to the United Nations, infrastructure needs to be reliable, sustainable, of high quality, easily accessible, and, in general, promote human well-being [1]. Especially for road infrastructure, nearly covering 21 million kilometers worldwide [2], it comes to be an important part of human’s everyday life. However, the construction and maintenance of road pavements are still energy- and resource-extensive activities responsible for multiple impact categories that are harmful to the environment. To begin with, the maintenance and construction of road pavements are responsible for around 30% of energy consumption and carbon emissions [3]. In addition, a large quantity of natural aggregates (NA) is required, considering that NA is a major component of both bound and unbound pavement courses, while it is estimated that 20% of natural aggregates used in the construction industry, refers to transport infrastructure in a total amount of 3 billion tons consumed every year [4]. At the end of their life, potential waste materials end up in landfills and contribute to construction waste, which accounts for more than 30% of global waste generation [5].

Consequently, regarding the phenomenon of climate change, the depletion of a non-renewable source, namely NA, and the landfilling due to waste materials, any small recyclability actions may be assumed to be significant in order to reduce the environmental impact of the road infrastructure sector [6]. Therefore, these actions include the employment of several types of recycled aggregates for pavement sustainability. The most used recycled aggregates are recycled concrete, steel slag, waste glass, crushed brick, and reclaimed asphalt pavement (RAP) [2]. Focusing on RAP is a waste material derived from the milling or removal of the asphalt layers of flexible pavements, which are undergoing either rehabilitation or reconstruction [7].

Research has shown that incorporating RAP into pavement layers can help to reduce the environmental footprint of road construction. First and foremost, the use of RAP significantly reduces the amount of raw materials required. In addition, the addition of RAP in asphalt layers can lead to a percentage reduction in the overall environmental impact of 15 to 30% [8,9,10,11,12]. For example, by replacing virgin aggregates by up to 50%, greenhouse gas emissions can be reduced by up to 17% [12]. When used in unbound layers, even a proportionate amount of RAP can reduce environmental impacts such as greenhouse gas emissions, energy and water consumption, eutrophication, acidification, and human health by up to 20% compared to the use of NA [13]. On the other hand, regarding the global production and use of RAP, studies [14,15] report that Europe and the United States of America (USA) combined produced over 100 million tons of RAP in 2020, more than 95% of which was reused. In Asia, the use of RAP is also apparent, as multiple countries, such as Indonesia, Japan, Korea, Malaysia, the Philippines, Singapore, and Thailand, have adopted recycling practices: in-plant or in situ [16]. In Oceania, ARRB reports the usage of RAP in multiple pavement applications in both Australia and New Zealand [17]. Finally, in Africa, reports show that Egypt and South Africa contribute to the global production of RAP [18,19].

The widespread use of RAP and its importance in road construction projects is thus confirmed by numerous reports worldwide. This study is a critical overview of the worldwide experiences with the use of RAP in road pavements published by various road authorities, aiming to prove and demonstrate the inconsistency in the selection of RAP quantity for incorporation in bound and unbound pavement layers on a global scale and not to present the way of processing RAP materials or using related technology. Subsequently, based on the results of this review, a comparative analysis of RAP applications with respect to the principles of pavement sustainability is conducted to identify possible knowledge gaps regarding the behavior of RAP. Finally, based on the analysis of the review results, conclusions are drawn, and future directions are identified.

2. Methodology

In this study, the research methodology includes three parts: (i) the literature review of the standard specifications for the use of RAP, (ii) the comparative analysis of the results of the literature review, and (iii) the conclusions of this study. First, a literature review is conducted on the installation of RAP in pavement layers worldwide, considering geographical elements. In particular, more than 90 papers and reports were found and analyzed during the search in the five continents of America, Europe, Asia, Africa, and Oceania to identify the countries with RAP applications in pavement layers. The criteria for including certain countries in this study are (a) whether they are based on official data (specifications) and (b) whether the published version of the report or paper is the most recent. The identified countries are further elaborated on regarding the availability of standard specifications or technical guidelines. Therefore, the main steps of the literature review are (i) searching the worldwide literature for countries with RAP applications in pavement layers, (ii) identifying countries with standard specifications for RAP applications, (iii) reviewing the latest available standard specifications or technical guidelines for the countries from step (ii), and (iv) comparatively analyzing the review results (see also Figure 1).

In the following sections, each step of the research methodology is described in detail. Section 3 presents the approach and results of the literature review, while Section 4 and Section 5 present the comparative analysis and conclusions of this study.

3. Worldwide Experience

3.1. Searching in International Literature

A literature survey showed the availability and wide utilization of RAP in different countries worldwide. In the continent of America the United States of America (USA), Canada, and Brazil report the reuse of RAP in road pavement layers. The USA produced almost 85 million tons of RAP in 2020. Most of the reused RAP—nearly 93%—was recycled in asphalt layers, while only 7% was reused in unbound layers or other civil engineering applications. As a consequence, the quantity of RAP landfilled was nearly zero, reaching up to 0.016% [15]. Those quantities equated to an average RAP content of 21%. This is similar to estimates in Canada, where about 15–20% of the RAP is reused [20]. In Brazil, nowadays, RAP has been mainly used on rural roads and as a granular material in the base course without any special treatment [21,22].

In Europe, according to EAPA, seventeen countries of Europe produce nearly 36 million tons of RAP, which is available for all uses. Specifically, 76% of the reclaimed asphalt is reused in asphalt mixes, 22% is used as granular materials in unbound layers or other civil engineering applications, and only 2% ends up in landfills [14]. The most important quantities are produced in Austria, Belgium, the Czech Republic, Denmark, France, Germany, Great Britain, Italy, Spain, and Turkey, as shown in

Table 1. Amount of RAP available for use in European countries [14].

Country	Available RAP Amount (Million Tons)	Country	Available RAP Amount (Million Tons)
Austria	0.8	Hungary	0.18
Belgium	1.03	Ireland	0.005
Croatia	0.24	Italy	9.2
Czech Republic	2.5	Norway	1.6
Denmark	1.12	Romania	0.034
France	7.34	Slovenia	0.051
Germany	12.9	Spain	2.0
Great Britain	5.2	Turkey	1.7

. The Netherlands, Ireland, and Malta also present a strategy for RAP management [23,24,25].

Australia, Indonesia, Japan, Korea, Malaysia, New Zealand, the Philippines, Singapore, and Thailand use either plant-based recycling or in-situ recycling, while Brunei uses cold mixing. Plant-based recycling is mainly used on federal and municipal roads, while New Zealand, Singapore, and Thailand accept RAP on expressways. In addition, more than half of these countries use RAP, where existing asphalt waste is mixed with virgin materials at the recycling plant and then used in new road projects, while some countries such as Japan, Korea, New Zealand, Singapore, and Thailand use RAP in unbound layers. Japan, in particular, reports that 99% of the RAP produced is reused for new road pavements, while, in Australia, almost 50% of reclaimed asphalt is used in asphalt applications, and the remaining quantity is used in cold recycling in small amounts or as a fill-in base and subbase materials [16,17]. It is also reported that the annual production of RAP materials may reach up to 220 million tons in China, while 30% of the produced material is reused [26]. In Gulf countries, the usage of RAP has been reported in different projects, such as the Kadra-Shawka Road in the United Arab Emirates (UAE) [27].

Finally, in Africa, Egypt produces about 4 million ton of RAP annually, which led towards the direction of RAP utilization, while South Africa produces 3.5 million ton of RAP annually, 10% of which is estimated to be reused [18,19].

3.2. National Specifications around the World

Following the international literature review mentioned above, it was investigated whether national technical guidelines exist in each of the countries mentioned. According to this analysis, the most important countries in America with available standard specifications are the United States of America (USA), Canada, and Brazil. For Europe, the technical guidelines of eleven countries were included in this study, based on the information contained in Table 1 and the availability of standard specifications from other European countries. These countries are Belgium, the Czech Republic, Denmark, Germany, Ireland, Italy, Malta, Spain, Switzerland, the Netherlands and the United Kingdom. For Oceania, Australia and New Zealand have standard specifications. In addition, Asian Eastern countries have no suitable standard specifications or guidelines for the determination of RAP in pavement layers [27]. However, India, Japan, China, Malaysia, and Saudi Arabia already have recycling strategies in place. Other countries such as South Korea, Singapore, and Thailand allow the use of RAP in asphalt layers defined by national technical specifications, while the use in unbound pavement layers is based on previous and worldwide experience without legislation [16]. Therefore, the specifications of a total of twenty-six countries are cited and analyzed for this study. These countries are shown in Figure 2.

Section 3.3 discusses in detail the strategies of the countries listed in Figure 2 for incorporating RAP into road pavements.

3.3. Review of Available Standard Specifications

3.3.1. America

As mentioned in Section 3.2, there are standard specifications in the Americas, such as the USA, Canada, and Brazil, which were included in this study. It is also worth noting that there is a wide range of information on the use of RAP in the USA due to its division into fifty states. Therefore, in the USA, there is no uniform strategy for the installation of RAP in road pavement layers. Each state has its own specifications that define the possible placement of RAP in bound and/or unbound layers, the RAP content, and special requirements. A search of the fifty United States and the Federal District of Columbia revealed that the standard specifications of four states are not accessible, and only one state, Arizona, does not include a RAP management strategy in its standard specifications [28]. The remaining states and the federal district provide guidelines for the use of RAP in pavement layers.

In particular, as for asphalt layers, Table 2 summarizes the maximum allowable RAP content incorporated in different types of asphalt layers—surface, binder and base courses (Figure 3)—for corresponding United States. If a single number of RAP content is mentioned, it is referred to all asphalt layer types, while if any RAP content is mentioned in the respective standard specifications, the different strategy for RAP usage is presented.

In detail, Alabama reports that the maximum allowable proportion of RAP depends on the mix type. For a plant mix bituminous base, this percentage is up to 25%, while RAP is not permitted to be used for a permeable asphalt-treated base or an open-graded surface course. On the other hand, for a Superpave or a Stone Matrix Asphalt (SMA) mix, surface layers may contain up to 20% RAP of total mix, and all the other asphalt layers may contain up to 35% RAP of total mix [29]. Alaska reports that the selection of RAP content depends on the mix type and its quality according to Marshall’s design procedure [30]. Arkansas allows a maximum proportion of RAP up to 30% for all asphalt layer types with regard to the design requirements of conventional asphalt mixtures, including their volumetric properties and performance [31]. Similar requirements are set by Georgia’s standard specifications. In this case, the maximum allowable RAP content is up to 40% with respect to mix plant type, while this rate is limited to 15% for SMA mixes [32]. Finally, the rates of the permitted RAP content in Ohio may vary regarding the used mix plant—flow drum or mini-drum batch plant, in the second case of which the maximum rates are increased by 5%, while a specific binder grade may be required regarding the RAP percentage in any asphalt layer type [33]. The last state is also for the states of Kentucky, Oklahoma, and Rhode Island [34,35,36].

Table 2. Maximum allowable RAP content (%) or respective RAP strategy for each state.

State	Maximum Allowable RAP Content (%)
	Surface Course	Binder Course	Base Course
Alabama [29]	20	35	35
Alaska [30]	Not mentioned—quality requirements
Arkansas [31]	30
California [37]	25
Colorado [38]	Not mentioned to be used in asphalt layers
Connecticut [39]	20
Delaware [40]	Not mentioned—quality requirements
District of Columbia [41]	15		25
Florida [42]	20	30	30
Georgia [32]	40
Hawaii [43]	15	15	40
Idaho [44]	30
Illinois [45]	Based on virgin binder replacement (%)
Indiana [46]	Based on virgin binder replacement (%)
Kansas [47]	Not mentioned—quality requirements
Kentucky [34]	10	20	30
Louisiana [48]	15	20	30
Maine [49]	30
Maryland [50]	Not mentioned—quality requirements
Massachusetts [51]	15	40	40
Michigan [52]	No upper limit mentioned
Minnesota [53]	Not mentioned—quality requirements
Mississippi [54]	20	30	30
Missouri [55]	Based on virgin binder replacement (%)
Nevada [56]	15
New Hampshire [57]	Based on virgin binder replacement (%)
New Jersey [58]	15	25	25
New Mexico [59]	35
New York [60]	20	20	30
North Carolina [61]	Based on virgin binder replacement (%)
North Dakota [62]	35
Ohio [33]	25	40	55
Oklahoma [35]	0	25	25
Oregon [63]	20	30	30
Pennsylvania [64]	No upper limit mentioned
Rhode Island [36]	0	25	25
South Dakota [65]	Not mentioned—quality requirements
Tennessee [66]	20
Texas [67]	20	30	40
Utah [68]	25
Vermont [69]	20	50	50
Virginia [70]	30	30	35
Washington [71]	40
West Virginia [72]	No upper limit mentioned
Wisconsin [73]	Based on virgin binder replacement (%)
Wyoming [74]	Not mentioned—quality requirements

Table 2. Maximum allowable RAP content (%) or respective RAP strategy for each state.

State	Maximum Allowable RAP Content (%)
	Surface Course	Binder Course	Base Course
Alabama [29]	20	35	35
Alaska [30]	Not mentioned—quality requirements
Arkansas [31]	30
California [37]	25
Colorado [38]	Not mentioned to be used in asphalt layers
Connecticut [39]	20
Delaware [40]	Not mentioned—quality requirements
District of Columbia [41]	15		25
Florida [42]	20	30	30
Georgia [32]	40
Hawaii [43]	15	15	40
Idaho [44]	30
Illinois [45]	Based on virgin binder replacement (%)
Indiana [46]	Based on virgin binder replacement (%)
Kansas [47]	Not mentioned—quality requirements
Kentucky [34]	10	20	30
Louisiana [48]	15	20	30
Maine [49]	30
Maryland [50]	Not mentioned—quality requirements
Massachusetts [51]	15	40	40
Michigan [52]	No upper limit mentioned
Minnesota [53]	Not mentioned—quality requirements
Mississippi [54]	20	30	30
Missouri [55]	Based on virgin binder replacement (%)
Nevada [56]	15
New Hampshire [57]	Based on virgin binder replacement (%)
New Jersey [58]	15	25	25
New Mexico [59]	35
New York [60]	20	20	30
North Carolina [61]	Based on virgin binder replacement (%)
North Dakota [62]	35
Ohio [33]	25	40	55
Oklahoma [35]	0	25	25
Oregon [63]	20	30	30
Pennsylvania [64]	No upper limit mentioned
Rhode Island [36]	0	25	25
South Dakota [65]	Not mentioned—quality requirements
Tennessee [66]	20
Texas [67]	20	30	40
Utah [68]	25
Vermont [69]	20	50	50
Virginia [70]	30	30	35
Washington [71]	40
West Virginia [72]	No upper limit mentioned
Wisconsin [73]	Based on virgin binder replacement (%)
Wyoming [74]	Not mentioned—quality requirements

Moving forward, the design of asphalt mixtures containing RAP in states such as Connecticut, Florida, Idaho, Massachusetts, Pennsylvania, Utah, and West Virginia depends on the binder grade and/or blending charts, which determine the RAP content based on the desired performance grade of the final mixture. Idaho’s specifications consider two types of RAP, based on its quality and source, for usage in the asphalt mixtures. For RAP material Type 1, the maximum permitted percentage is 30%, and for RAP material Type 2, this proportion is limited to 10% for upper layers. In addition, for RAP content higher than 17%, a binder grade adjustment is required [44]. In the state of Connecticut, the selection of RAP proportion is based on performance tests of asphalt mixtures, while blending charts are used to relate the binder grade of recycled mixture with the proportion of RAP. However, a total of 20% RAP is allowed in all asphalt layer types [39]. A similar strategy is followed in the state of Pennsylvania. Although a mixture containing up to 15% RAP is considered a virgin mix and no further testing or binder grade adjustment is needed, a maximum allowable RAP content is not mentioned, implying that the recycled asphalt mixture can contain any RAP proportion so that it has equal performance to a conventional mixture [64]. In the states of Vermont and Virginia, RAP content varies regarding the type of asphalt mixture and layer, but for mixes containing more than 25% RAP, binder grade adjustment is required [69,70]. This rate is limited to 15% for the states of Florida and West Virginia, while, for the second state, a blending chart may be needed when more than 25% RAP is incorporated into asphalt courses [42,72]. The last observation is also for the state of Massachusetts, despite the upper limits of RAP content presented in Table 2 [51].

In other states, such as California, Illinois, Indiana, New Hampshire, and Wisconsin, the ratio of virgin binder replacement to the recycled binder is considered as a factor regulating the usage of RAP. In Indiana, in all asphalt courses, the virgin binder can be replaced up to 25% by the binder contained in RAP. Additionally, asphalt surface mixtures used in high-volume roads should contain fractionated RAP, while coarse RAP may be used in a content of 20% [46]. In New Hampshire, RAP materials are permitted in asphalt layers, as long as the total reused binder may replace virgin binder only up to 1.5%, while, in Wisconsin, the standard specifications report that the virgin binder replacement may be up to 40% and 25% for lower and upper layers, respectively [57,73]. In the state of California, despite the fact that a proportion of RAP up to 25% is allowed, the recycled binder may replace up to 25% of virgin binder in the surface course, while this percentage increases up to 40% for the lower courses. In addition, if unfractionated RAP is used in asphalt layers, the maximum permitted RAP content is limited to 15%. It should also be noted that, for mixes with RAP content higher than 15%, a grade asphalt binder adjustment is required, and the temperature classification should be lowered [37]. Similarly, in Illinois, the selection of RAP proportion is based on virgin binder replacement by recycled binder contained in RAP with respect to traffic volume and possible RAP fractionation. Therefore, when unfractionated RAP is used in asphalt mixes, the maximum allowable percentage of virgin binder replacement in surface and binder courses is up to 30%. On the contrary, when fractionated RAP is used in asphalt mixes, this percentage is 45 and 55% for surface and binder courses, respectively. In both cases, those rates are limited as traffic volume rises. Finally, when the rate of virgin binder replacement exceeds 20%, the binder grade should be adjusted [45].

Texas allows the incorporation of RAP materials into multiple asphalt mixture types, such as dense graded asphalt (DGA) mix, Superpave mix, SMA mix, and permeable and thin bonded surface course mixes. For the selection of the maximum allowable RAP content, the virgin binder replacement and the possible fractionation of RAP are taken into account for each mix type, as it is presented in

Table 3. Maximum allowable RAP content (%) in different asphalt mix types for the state of Texas.

Mix Type		Maximum Virgin Binder Replacement (%)	Maximum Allowable RAP Content (%)
			Unfractionated	Fractionated
Non-surface pavement		40	20	30
Dense graded asphalt mix	surface	30	10	20
	binder	35	10	30
	base	40	10	40
Permeable asphalt surface course mix		15	-	10
Superpave mix	surface	30	10	20
	binder	35	10	25
	base	40	10	30
SMA mix	surface	15	-	15
	Binder/base	20	-	20
Thin bonded surface course mix		15	-	10

. It should also be noted that the rates of virgin binder replacement and RAP content for the dense-graded asphalt and Superpave mixes are chosen based on the performance binder grade [67].

In the states of North Dakota and Tennessee, there is an additional factor for the selection of RAP content in asphalt mixtures: if RAP materials are used in mainline or shoulder pavement layers. In this case, the maximum allowable proportion of RAP is 20 and 25% for mainline pavements in North Dakota and Tennessee, respectively, while, for both states, a total of 35% RAP can be incorporated in shoulder pavements [62,66]. A similar strategy is seen in Oklahoma’s technical guidelines, as the maximum allowable RAP for mainline binder and base course content may be up to 25%, which may be increased up to 35% for temporary detours [35]. Furthermore, in the state of New Jersey, higher rates of RAP content up to 100% may be allowed in the asphalt layers, considering that the rutting and cracking performance of the final asphalt mixture meets the requirements of the respective state specifications [58]. Finally, the guidelines for five states, Delaware, Maryland, Minnesota, South Dakota, and Wyoming, do not specify the maximum allowable RAP content, but they mention the required quality in order to be incorporated into asphalt pavement layers [40,50,53,65,74]. Those properties may refer to aggregates’ quality, grain size distribution, amount and grade of binder, the required number of testing samples, stockpile conditions, and RAP source. In any case, a requiring issue in all states is to ensure that the asphalt mixtures with RAP materials have equal performance to those mixtures with virgin materials.

Concerning the unbound pavement layers, seventeen states report the incorporation of RAP into base, subbase or/and subgrade layer. In

Table 4. State policy for RAP incorporation in unbound layers.

State	State Policy
Colorado [38]	base or/and subbase layers
Florida [42]	as an unbound material only on non-limited access paved shoulders or other non-traffic bearing applications an embankment material and stabilized material for the surface of subgrade layer
Idaho [44]	unbound layers of mainline pavements, up to 50%
Illinois [45]	unbound layers in low volume traffic roads
Indiana [46]	a reclaimed base layer, containing of RAP, existed base and subgrade materials
Maine [49]	base or/and subbase layers
Minnesota [53]	base or/and subbase layers, backfills and structural bedding
Missouri [55]	base or/and subbase layers
New Hampshire [57]	unbound layers of shoulder pavements, up to 75%
New Jersey [58]	unbound layers of mainline pavements, up to 50%
New Mexico [59]	unbound layers of mainline pavements, up to 50%
New York [60]	RAP as unbound material in road applications with low volume traffic
Ohio [33]	In lower subgrade layers
Vermont [69]	subbase layer of mainline and shoulder pavement—in equal proportion with virgin aggregates, up to 50%
Virginia [70]	only in asphalt stabilized base layer in a proportion up to 20%
West Virginia [72]	shoulder base and subgrade layers
Wisconsin [73]	unbound layers of mainline pavements, up to 75%

, the policy for each of those states is presented. Most of them do not specify a maximum allowable RAP content, but they refer to quality and grading requirements of the final aggregate mixture, so that it performs equally to virgin aggregate mixture.

Canada has ten provinces and three territories. Searching over these areas, the standard specifications for two out of ten provinces and two out of three territories are not available for recording RAP usage, while Northwest Territories do not include any RAP management strategy in their standard specifications [75]. The remaining areas report the inclusion of RAP in both asphalt and unbound layers. Most areas do not specify the maximum allowable proportion of RAP, and the technical guidelines include the quality and grading properties of RAP materials, which may be incorporated in pavement layers. This condition is also the same for the provinces of Nova Scotia and Manitoba for RAP incorporation in asphalt layers [76,77]. In the province of Prince Edward Island, the maximum amount of RAP permitted is 15%, and a specific binder grade should be used for asphalt mixtures, while, in the province of Saskatchewan, the recycled binder contained in RAP materials may contribute to the total amount of binder content up to 20% regardless of mixture type [78,79]. In the province of Alberta, the maximum allowable proportion is up to 30%, while a binder grade adjustment is required for mixes containing more than 10% RAP [80].

Moreover, in the province of New Brunswick, surface and base courses may contain RAP materials up to 15 ± 5% and 30 ± 5%, respectively [81]. In the province of British Columbia, the selection of RAP proportion is based on the classification of the road—highway or other roads—and the replacement of asphalt concrete. For highways, the virgin binder replacement may be up to 15% for upper asphalt courses and 30% for lower courses, while, for the class of other roads, the virgin binder replacement may be up to 30% for all courses [82]. In the province of Ontario, the selection of RAP content contained in asphalt layers depends on the selected design procedure, the road traffic volume, and the type of asphalt layer, while the binder grade should be adjusted for mixes containing more than 15–20% RAP. Totally the maximum allowable RAP content in surface and binder courses is 15 and 50%, respectively. In addition, RAP materials are permitted to be used up to 30%, mostly in lower courses of base and subbase layers of mainline pavements, in subbase layers of shoulder pavements, in backfills, and on surfaces of unpaved roads [83]. Lastly, in the province of Newfoundland and Labrador, RAP materials may be incorporated into asphalt base courses in a proportion up to 20%. Granular base layers may contain up to 30% in the mainline pavement and up to 50% in granular shoulders, while RAP content up to 100% is permitted in base layers if only an asphalt surface course is applied above granular layers, as an overlay [84].

Finally, the Brazilian specifications allow the use of RAP in asphalt courses if the minimum requirements for the mechanical properties of the mixture are fulfilled [85]. There is no RAP quantity limit and limited utilization of recycling methods. However, the recycling process is supported by the local community through research projects to increase the recyclability of road pavements [86].

3.3.2. Europe

In Europe, the standard specification of EN13108-8 specifies the requirements for the evaluation of reclaimed asphalt as a constituent material for asphalt mixtures, and it actually declares caution before using RAP materials regarding its source, its homogeneity, and the required number of testing samples, as it is advisable to be tested as many times as possible to ensure the required quality of the final asphalt mixture [87]. It is also available for use in all European countries to validate RAP materials for use in asphalt pavement layers.

As for the European countries, the Netherlands, Belgium, Germany, and Denmark have the longest periods of experience utilizing RAP in all pavement layers [88]. In The Netherlands, asphalt binder and base courses may contain RAP of up to 50%, while the maximum allowable RAP content for asphalt surface courses is up to 30% [23]. In particular, in The Netherlands, the higher humidity of RAP materials, the increased stiffness of aged RAP binder, and the presence of finer particle aggregates limit the rate of RAP content in asphalt layer mixtures. Therefore, several asphalt-producing technologies, such as rejuvenators, parallel drums in asphalt plants, and warm-mix asphalt, are tested to be used to increase RAP content [89]. Moreover, most of the related literature references about Belgium are related to the region of Flanders in Belgium. Therefore, according to Flemish Regulation, the use of RAP in asphalt surface layers is not permitted. For asphalt base courses, the selection of RAP content depends on the way of pre-heating and on the type of mixture, implying that RAP proportion may be unlimited, while, for mixtures with high modulus asphalt and mixtures with cold-added RAP, it is limited to 20% [90]. In addition, it is reported that the current average RAP content in new asphalt mixture applications is 40% [91].

In Germany, there are technical specifications that regulate the composition of mixes containing conventional and RAP materials, as well as the quality requirements for their incorporation in asphalt layers. Therefore, RAP content may be up to 50% for surface and binder courses, reaching up to 100% for base courses, while, for other uses (unbound or hydraulically bound base layers), the proportion of RAP may be up to 30% [92]. However, the dimensions of a pavement layer and their role in pavement serviceability limit RAP proportion, as an increased RAP content affects pavement performance [93]. Additionally, Danish guidelines report that RAP can be used in all bituminous base courses, while surface layers may contain RAP up to 30% according to mixture type. For binder courses, the RAP proportion is based on the traffic volume, meaning that RAP is not allowed in binder courses for high-trafficked roads, while RAP content in binder courses for low-trafficked roads can be up to 30% [94]. In the Czech Republic, the maximum percentages of RAP are 15%, 25%, and 50% for surface, binder, and base courses, respectively [95]. In Switzerland, the maximum amount of RAP depends on the road traffic volume and the asphalt layer type. The current standards do not allow the use of RAP in the surface course of high-volume roads, implying that RAP can be only used in pavement layers without direct contact with the vehicle loads (see Figure 3) [86].

Moving forward, in Spain, RAP utilization is applicable only in case of pavement rehabilitation, following any recycling method: in-situ or in-plant recycling methods. In this case, the respective technical guidelines consider a range from 15 to 80% of RAP in roads regarding the selected recycling method and intended use, but mixtures containing RAP have to meet the requirements of the standards about granulometry, binder contents, and binder infiltration, among others. For example, the Spanish road authority restricts the maximum allowable RAP content for hot in-plant recycling of asphalt mixtures for its use in surface and binder courses of low-traffic volume roads. In this case, the long-term mechanical performance of recycled mixtures, the interaction between aged RAP binder and virgin binder, and the properties of RAP materials from multiple sources raise higher concerns about the utilization of RAP in asphalt layers [96]. In Italy, the legislation permits the incorporation of RAP up to 20, 25, and 30% for surface, binder, and base courses, respectively [97].

In addition to the countries mentioned above, the United Kingdom (UK), Ireland, and Malta also make an important contribution to the use of RAP. According to the UK standard specifications, RAP material can be used in all asphalt layers: Surface course, binder course, and base course. There is no restriction on the permitted RAP content, and quality testing is similar to that of conventional materials. However, if the RAP proportion exceeds 25%, additional quality testing is required [98]. In Ireland, RAP materials are also only permitted in asphalt layers, except surface courses, while the maximum permitted RAP content for binder and base courses is up to 30%. Furthermore, if the RAP content exceeds 10%, additional requirements are needed in the design process, with the exception of conventional quality testing [24]. Finally, a similar strategy is followed in Malta. The maximum allowable RAP content in surface, binder, and base courses is 10%, 30%, and 50%, respectively, while mixtures with a RAP content of more than 10% require additional quality testing conforming to EN13108-8 standard [25].

3.3.3. Oceania

All states of Australia and the Northern Territory (NT) allow the incorporation of RAP in pavement layers, bound or/and unbound [17]. Therefore, the maximum allowable RAP content in surface courses, binder courses, base courses, and unbound layers for each Australian state and the Northern Territory is presented in Figure 4a,b and Figure 5a,b, respectively.

Thoroughly, considering the asphalt layers, in New South Wales (NSW), the maximum allowable RAP content for wearing courses varies between 20 and 25% regarding the volume traffic, while, for binder and base courses, it is up to 40%. Similar rates are reported in the standard specifications of the state of Queensland. In Western Australia (WA), the inclusion of RAP is only allowed in binder courses in proportions up to 10%. In South Australia, RAP content in surface courses may be up to 10% or 20% for coarse and fine mix asphalt, respectively, while, in binder and base courses, the maximum allowable RAP percentage is 50%. The states of Victoria and Tasmania follow the same strategy as for RAP management. In their specifications, it is reported that up to 40% RAP content is allowed for dense graded asphalt, depending on traffic volume. In NT, the usage of RAP is allowed for surface and base courses, and the RAP content may be up to 10% and 15%, respectively.

Furthermore, for all regions, there are extra limitations regarding mix type. Generally, RAP is not allowed in SMA, Open Graded Asphalt (OGA) mixes, and mixes containing Polymer Modified Asphalt (PMA), Crumb Rubber Asphalt (CRA), or High Modulus Asphalt. However, Queensland allows RAP up to 15% in surface courses containing PMA or high-modulus asphalt. This rate is limited to 10% for the state of NSW and increases up to 20% for the state of SA. Finally, in New Zealand, RAP is also allowed to be incorporated into asphalt layers. For RAP content up to 15%, there are no restrictions regarding quality testing and applications, while for RAP content between 15% and 30%, there are additional design requirements to be fulfilled. A RAP proportion of more than 30% is permissible if the contractor can demonstrate suitable production facilities and quality control procedures that ensure uniform asphalt production [99].

As for the unbound layers, in NSW, RAP content may be up to 40% for unbound, modified, and bound base and subbase layers. Queensland allows the incorporation of RAP into base and subbase layers for proportions up to 20%, which increases up to 45% for the lower subbase and subgrade layers. In SA, RAP content is allowed to be up to 20% in granular pavement materials. Base pavement layers in the states of Victoria and Tasmania may contain RAP in a proportion up to 15%, which may increase up to 20% for lower trafficked roads. Accordingly, unbound and bound subbase layers may contain RAP up to 40%, which increases up to 50% for roads with low traffic volume. Additionally, New Zealand does not allow the usage of RAP in unbound layers [17]. Finally, all states of Australia and New Zealand require continuous testing during RAP processing (i.e., fractioning, storage and stockpiling, quality and homogeneity testing) before its incorporation in pavement layers.

3.3.4. Africa and Asia

According to SABITA, South Africa has developed common recycling methods for reusing RAP materials, namely Hot In-place Recycling (HIR), Hot In-plant Recycling (HIP), Cold In-place Recycling (CIR) and Cold In-plant Recycling (CIP). Cold recycling processes are mainly used for stabilized asphalt courses, while the HIR method is mostly appropriate for thick asphalt layers, and its use is limited in South Africa. At last, the HIP recycling method is mostly used in this country, where a blend of recycled and new materials is prepared according to project specifications [18]. In addition, SABITA reports that RAP may be used in asphalt pavement layers, considering the asphalt mix type. For SMA, RAP is not allowed to be incorporated in asphalt layers. For porous asphalt and generous asphalt, RAP content may be up to 20% and 50%, respectively. However, when more than 20% RAP is used in asphalt, extensive testing of RAP materials and their aged binder is required. SABITA refers to the factors that influence the quality of RAP materials, including the milling and crushing process of asphalt layers, stockpiling conditions, the quality of aggregates contained in asphalt mixes, and possible contaminants in recycled binder [18].

In India, MoRTH reports that RAP materials could be used in embankments, subgrades or backfills, and asphalt layers. Especially for asphalt layers, the Indian specifications allow the incorporation of RAP materials up to 60% by using In-plant Recycling, while the evaluation of the materials is based on the same testing procedures as those conducted on conventional materials. Stockpiling and RAP processing before RAP incorporation in pavement layers are set to be significant issues for the quality of RAP materials [100]. In accordance with this, IRC recommends several practices for the utilization of RAP materials, such as HIR, HIP, CIR, and CIP. Using the HIR method, 100% of reclaimed material can be used, while, in the rest recycling methods, RAP content should be limited to 30–50%. Furthermore, additional tests on the extracted and recovered binder for RAP content higher than 25% are required [101]. IRC also reports that, for asphalt-treated base layers, the inclusion of RAP is based on quality requirements, such as layer thickness, resilient modulus value, and indirect tensile strength [102].

China has systematically developed asphalt recycling technologies, including in-situ and in-place recycling methods, such as HIR, Hot Central Plant Recycling (HCPR), CIR, Cold Central Plant Recycling (CCPR), Full Depth Reclamation (FDR), as well as warm mix technologies in terms of road pavement maintenance. Hot-mix technology exhibits the widest application in China. In accordance with this, cold recycling technologies are applied to the construction of base courses and surface courses in medium to low-traffic volume roads. FDR is not applicable in urban areas, but it is more suitable for road sections suffering from severe structural deterioration. However, due to the unpredictable gradation and binder content of RAP materials from different sources, a high proportion of RAP materials is unacceptable by Chinese road authorities, leading engineers and constructors to select conventional mixtures or mixtures with low RAP proportions [26]. In particular, Chinese technical specifications report that the content of RAP in asphalt layers should generally be up to 30%, and mixtures with higher RAP proportions should be demonstrated before they can be used, especially in surface courses [103].

In Japan, RAP materials can be used in asphalt layers to meet quality requirements, and the selection of RAP content is based on blending charts, in which RAP content is related to those quality requirements, such as binder penetration and Indirect Tensile Coefficient. Additionally, these guidelines report that if RAP materials fail to meet the mentioned requirements, they can be used for other purposes, such as unbound base layers [16]. According to LTA, RAP materials are permitted for utilization in asphalt layers of pavements in Singapore. In particular, RAP content may be up to 20% and 30% for surface and base courses, respectively. Regarding the evaluation of RAP materials, the requirements for recycled binder should comply with those for virgin bitumen content, while there are special requirements for the gradation of aggregates contained in RAP. Additionally, while stockpiling, RAP materials should be derived from one single source [104]. In South Korea, RAP can be used in bound and unbound layers. According to the Guidance in Disposal and Reuse of Construction Wastes, at least 40% RAP on the total amount of aggregate is required to be used in new road construction projects. However, a maximum limit of 20% and 30% RAP is set in asphalt layers of highway and local roads, respectively, leaving the remaining required amount of RAP up to 40% to be incorporated in unbound layers [105]. Other practices in South Korea for recycling existing asphalt pavement materials include also the rejuvenation of RAP materials to ensure the desired pavement performance [106].

In Malaysia, the latest standard specifications refer only to rehabilitation processes regarding the utilization of RAP materials [107]. Specifically, it is reported that RAP can be incorporated in asphalt layers with respect to quality and grading requirements similar to those for virgin materials, such as the Los Angeles index, flakiness index, and binder penetration. In Thailand, RAP can be recycled by using in-place and in-plant recycling methods in road applications. Despite the fact that RAP can be recycled directly in new hot-mix asphalt concrete, its proportion is generally limited to 25% (or less) of the new material according to the standards [108]. In Turkey, RAP materials are permitted in HMA layers in a proportion of up to 25%, while they can also be used as a subbase material [109]. Finally, Saudi Arabia allows the incorporation of RAP into asphalt layers except surface courses. It is reported that RAP could be added to the asphalt mix so that the recycled binder replaces no more than 15% of the mix design binder content by mass. Generally, a RAP proportion of up to 25% is allowed to be blended with virgin materials with respect to the performance grade of the binder, while the binder grade should be adjusted for mixes containing more than 15% RAP. Finally, it is reported that testing on RAP materials is similar to testing on conventional materials, while stockpiling conditions and RAP source are crucial factors in the acceptance of RAP materials for use in new asphalt layers [110].

4. Discussion

The systematic review revealed that the use of RAP materials in pavement layers is a common practice worldwide. In the United States, forty-five states, including the Federal District, allow the use of RAP in asphalt layers, while only eighteen states allow the use of RAP material in unbound layers, i.e., base, subbase, or subgrade. Each state has a different strategy for dealing with RAP, particularly in choosing the maximum percentage that can be incorporated into pavement layers. For asphalt pavements, twenty-eight states specify the maximum allowable percentage of RAP in their standard specifications. The states of Georgia, Hawaii, Massachusetts, Texas (40%), Vermont (50%), and Ohio (55%) have the highest allowable percentage of RAP. In contrast, Nevada is the only state in the USA with stricter limits for the RAP content, as the maximum permissible RAP proportion is 15% for all asphalt layer types. Overall, according to Table 2, an average allowable RAP content of 15, 30, and 35% of the total mixture is achieved for surface, binder, and base courses, respectively. For the other six states, the allowable RAP content is not specified, but the quality requirements for RAP materials, as well as the requirements for storage prior to use, are specified, which means that the RAP content must be determined by the contractor. In the remaining six states, the RAP proportion is based on the replacement of virgin binders with recycled binders. Figure 6 shows the strategy for RAP usage available in each state on the map of the USA.

A similar pattern in the use of RAP can also be observed in Canada, where the average maximum permissible RAP content for surface and binder or base courses is 15 and 30%, respectively. For unbound layers, RAP materials are only permitted in the provinces of Ontario, New Brunswick, Prince Edward Island, and Newfoundland and Labrador. In the provinces of Ontario and Newfoundland and Labrador, in particular, unbound layers in low-volume road pavements may contain up to 30 and 100% RAP. The technical guidelines for the other provinces mentioned only requirements for quality and classification. In Brazil, too, no maximum permissible RAP content is specified; the guidelines only refer to asphalt layers.

In Europe, most of the countries mentioned allow the use of RAP in asphalt layers in high proportions, even up to 100%. On the other hand, the use of RAP material in unbound layers is limited and refers only to unbound applications. In Oceania, most Australian states allow the use of RAP material in both bound and unbound pavement layers, and only WA and NT restrict the use of RAP in asphalt layers. In addition, New Zealand does not appear to limit the amount of RAP in asphalt layers as long as several requirements for RAP content greater than 15% are met, while RAP materials are not permitted in unbound layers. Finally, in Africa and Asia, the standard specifications of most of the countries mentioned generally provide for the addition of RAP in asphalt layers.

Comparing the strategies of all countries and continents on RAP incorporation into asphalt layers, the standard specifications for twenty-two out of twenty-six cited countries report a limit for RAP content in asphalt layers. In

Table 5. Maximum allowable RAP content (%) in asphalt layers regardless of layer type.

Country	Max RAP Content (%)	Country	Max RAP Content (%)
USA	30	Ireland	30
Canada	30	Malta	50
The Netherlands	50	Australia	35
Belgium	100	New Zealand	30
Germany	100	South Africa	50
Denmark	30	India	100
Czech Republic	50	China	30
Spain	80	Singapore	30
Italy	30	Thailand	25
Turkey	25	South Korea	30
UK	100	Saudi Arabia	25

, those countries and the corresponding limits for RAP content in asphalt courses are presented. For the USA, Canada, and Australia, an average value of RAP content limit from each state was considered in order to be incorporated in the Table below.

Based on the data in Table 5, Figure 7 shows the frequency of RAP content in asphalt layers, which varies between 25 and 100%. In other words, this Figure describes the trend in the use of RAP material in asphalt layers around the world.

According to Figure 7, the rate of RAP content selected to be incorporated in asphalt layers is accumulated around 30%, as most of the countries permit maximum rates of RAP content varying from 25 to 35%. Subsequently, the depicted highest rates, 80 and 100%, correspond to countries such as Spain and India, where RAP materials are used following an in-plant or in-place recycling method, while the countries of Belgium, Germany, and the UK may allow the usage of 100% RAP in newly constructed road pavements with respect to its quality properties.

Furthermore, for most of the investigated countries, the type of asphalt layer is considered to be the main factor regulating the selection of RAP content. Therefore, the standard specifications of the fourteen cited countries provide sufficient data regarding the maximum permitted limit for RAP content in each type of asphalt layer—surface, binder, and base course, as presented in

Table 6. Maximum allowable RAP content (%) in asphalt layers based on layer type.

Country	Maximum Allowable RAP Content (%)
	Surface Course	Binder Course	Base Course
Australia	20	35	40
Belgium	0	100	100
Canada	15	30	30
China	0	30	30
Czech Republic	15	25	50
Germany	50	50	100
Ireland	0	30	30
Italy	20	25	30
Malta	10	30	50
Saudi Arabia	0	25	25
Singapore	20	30	30
South Africa	20	50	50
The Netherlands	30	50	50
USA	15	30	35

. Similarly to Table 5, for the USA, Canada, and Australia, an average value of RAP content limit from each state was considered in order to be included in the Table below.

Based on the data in Table 6, Figure 8 is constructed showing a different trend of using RAP materials in surface, binder and base course, respectively.

This can be validated by Figure 9, in which a statistical analysis of the maximum rates of RAP content permitted in each course type is presented. According to this figure, the values of RAP content are accumulated around 15, 35, and 45% for surface, binder, and base courses, respectively. Thus, higher proportions of RAP are allowed in lower courses of asphalt layers, while, for surface courses, RAP materials may be restricted, as is noticed in areas such as the states of Oklahoma and Rhode Island in the USA, Belgium, China, Ireland and the state of WA in Australia. The last observation seems reasonable, considering that the upper pavement layers consist of high-quality materials, so the deterioration of the foundation layers is to be avoided.

Other important factors influencing the RAP content in asphalt layers are the asphalt mix design method chosen, the type of asphalt mixture, the traffic volume, the proportion of RAP binder in the total binder of the mixture, the possible need for RAP treatment, the possible presence of contaminants, the RAP source and the conditions on the stockpile. In particular, the design method, i.e., Marshall or Superpave, has a standard effect on the evaluation criteria for the performance of asphalt mixtures, even when RAP materials are incorporated into these mixtures. In addition, more stringent RAP content limits can be set for different types of asphalt mixtures, such as SMA, DGA, OGA, or high-modulus asphalt mixtures. For SMA mixtures, for example, the proportion of RAP material can be up to 15%. This also applies to asphalt mixtures with a high modulus, which are usually laid in asphalt layers of roads with high traffic volumes. In addition, both the proportion of recycled binder in the overall binder of the mix and any necessary fractionation of the recycled material can influence the grain size distribution of the aggregate mixture. The higher the proportion of recycled binder in the mixture, the finer the final aggregate mixture will be. On the other hand, fractionation of the RAP material can help to achieve a uniform grain size distribution of the final aggregate mix for paving in asphalt layers. Finally, the possible presence of contaminants, the origin of the excavated material, i.e., the RAP—single, multiple, or random—and the conditions of the stockpile can influence the degree of homogeneity of the RAP in the form of unbound material, which in turn determines its maximum quantity in the asphalt layers.

As for the unbound layers, the use of RAP is limited globally, as only seven out of twenty-six cited countries, including Australia, Canada, Germany, Japan, South Korea, Turkey, and the USA, report that RAP materials are allowed to be incorporated in base, subbase and/or subgrade layers of mainline pavements of low volume roads or shoulder pavements, which means that design traffic volume determines the possible RAP usage as an unbound material. In any case, a common acceptable rate of RAP content is around 50%. At last, similar to asphalt layers, the possible presence of deleterious materials, the RAP source, and the stockpile conditions are decisive factors governing RAP usage in unbound layers.

All in all, according to the standard specifications of several countries worldwide, both asphalt layers (bound) and granular layers (unbound) in road pavements could be produced with RAP materials. The evaluation of their performance seems to be a common strategy found in all technical guidelines worldwide and follows the same testing procedure used for virgin materials. Although most countries are trying to reach acceptable levels for the use of RAP materials in road pavements, the different RAP content limits presented show that they are not yet reliable enough to be used more intensively. This could be due to the limited data on the long-term behavior of RAP materials. For unbound layers that cannot be easily rehabilitated and/or are costly to rehabilitate, most countries do not allow or restrict the use of RAP materials on low-volume roads. Furthermore, although most countries allow the use of RAP materials in asphalt layers in higher proportions, up to 50 or 100%, additional testing may be required if the RAP content exceeds 15%. In this context and since it is a high-traffic road, countries such as Belgium, China, Ireland, Saudi Arabia, and some states in Australia and the USA do not allow the addition of RAP in asphalt surface and/or binder courses. In addition, differences or similarities between countries’ approaches may be due to the road design process, which considers aspects such as speed limits and weather conditions that vary from country to country. Another factor may be the geology of the country, which affects the quality of the natural aggregate and, therefore, the quality of the reclaimed material –RAP. Finally, the approach to the use of RAP in road pavements in each country may depend on policy, considering resource and financial management issues and the technological knowledge available.

5. Conclusions

This study provided an overview of global practices for the incorporation of RAP in road pavements. The overall assessment of these practices leads to the following conclusions:

• The use of RAP materials is accepted worldwide and is common practice on all five continents.
• In several countries, the proportion of RAP in asphalt layers can be as low as 50%, while in India and Spain, where in-situ recycling is practiced, the proportion of RAP can be as high as 100%.
• According to the analysis, the average RAP content in the different asphalt layers—surface course, binder course and base course—is 15, 35 and 45%, respectively, while the average acceptable RAP content in asphalt layers worldwide is generally around 30%.
• In deeper asphalt layers, higher RAP contents are permitted worldwide, while in surface courses the maximum permissible RAP content varies between 10 and 25% or RAP materials are restricted.
• Although most countries set a maximum value for the RAP content in asphalt layers even at higher rates, it has been shown that a threshold value of 15 to 25% is decisive for the requirement for additional quality tests.
• The incorporation of RAP material into unbound layers is not fully accepted worldwide and most of the practices mentioned relate to low traffic volume roads or shoulder pavements where an average RAP content of 50% is acceptable.
• The quality of mixtures containing RAP material is assessed according to the test procedure for mixtures with conventional materials.

Overall, it is clear from the findings of this study that efforts are being made globally to incorporate sustainable materials and practices into road pavements, as most of the world’s largest economies have their own strategy for the use of RAP. However, it is highlighted that there is a knowledge gap about the behavior of RAP in road pavements after construction. The inconsistent global strategy for the use of RAP confirms this statement and shows that the behavior of RAP is not completely incomprehensible, especially in unbound pavement layers. In particular, factors such as the high degree of oxidation of the RAP binder during stockpiling, the high proportion of fine RAP due to the milling process, the variability of the aggregate, the nature of the binder content and its aging, and the nature of the RAP content in relation to its geological origin influence the quality and behavior of RAP materials and lead to limitations in terms of RAP content, especially in the upper pavement layers. Furthermore, future research could include exploring how these properties of RAP influence the strength of each road pavement layer. In addition, technologies for the RAP process, such as asphalt aggregate separation technology, could be considered to eliminate these limitations in favor of using high-quality RAP material. The economic benefits of using RAP materials should also be considered by conducting Life Cycle Cost Assessment (LCCA) studies to achieve cost-effective incorporation of these materials in pavement layers. Hence, the significance of this research is not related to a simple recording of standard specifications worldwide, but it is about highlighting the concern about RAP quantity included in both bound and unbound pavement layers. Therefore, further research in this direction is needed to improve the use of RAP on a larger scale and in all pavement layers to enable the construction and maintenance of sustainable pavements. It is considered that this study could motivate more countries to develop standard specifications for the use of RAP on pavements, which could lead to a greater number of RAP applications and a better understanding of the long-term performance of RAP.