1. Introduction
Asphalt, as a binder, is one of the oldest building materials. The appearance of polymers for asphalt modification had great impact on development of road pavement technology. The first worldwide modifications undertaken with the use of the elastomer SBS (thermoplastic elastomer styrene–butadiene–styrene) occurred the 1950s. Modifying asphalt pavements with polymers extends their durability, whereas pavements of ordinary asphalt types do not survive long service and require milling. If a multi-layer road structure is not of proper capacity or does not meet the requirements of a given traffic load category or, if resistance of top layers to rutting or durable deformations is not enough, it is also necessary to exchange the pavement due to progressive degradation [
1,
2,
3].
At present there is a worldwide tendency to search for solutions in construction investment, including roadwork, which make possible the reduction of negative impacts on the natural environment. One such solution is the recycling of asphalt pavements. Technologies taking into consideration aspects of sustainable development are currently highly appreciated in road construction. Within this scope, the effective recycling of existing but destroyed during operation asphalt pavements allows for a complex approach to construction in accordance with the rules of reclamation and reuse of building materials [
4,
5,
6]. Moreover, in cases of material shortage (aggregate, asphalt, cement), and when supplies “pile-up” as is the case at the end of a working season, using reclaimed asphalt pavement (RAP) is a highly desirable and valuable way to increase the efficiency of roadwork investment [
7,
8].
Across the global asphalt pavement industry, a successive increase in the amount of RAP content used within courses can be observed, exceeding 25% in wearing courses and up to 55% in other courses [
9]. In general, contemporary road pavements are 100% recyclable provided they do not contain tar. The most technologically advanced form of pavement recycling uses RAP as an ingredient of asphalt mixtures (AMs) produced in a mixing plant equipped with a special, so-called “black” drum, which serves to heat the RAP. The required quality of the RAP can be reached through the control of its homogeneity. The moisture content of RAP is an important issue [
10,
11,
12,
13]. The higher the percentage content of RAP and its moisture content, the higher the temperature required for the aggregate. To reach a proper resultant temperature of the mixture, the aggregate needs to be heated to between 250–300 °C.
West and Copeland (2015) [
5] indicate that a high degree of recycling has been reached in Japan, where the average content of RAP in AM amounts at 47%. In addition, the following studies [
14,
15,
16] point at the preferred tendency to increase RAP content in both hot-mix asphalt (HMA) and warm-mix asphalt (WMA). Based on the research works [
4,
5,
6,
13], RAP originating from existing roads of classes A, S, GP (the highest road classes in Poland corresponding with highways, express roads, and main motor roads according to the International European road standard) is a high-value ingredient of AM. It has been proved that presence of RAP in AM does not lower the functional parameters of the mixture. For instance, stone mastic asphalt (SMA) with content of 30% RAP constitutes a mixture of a quality matching SMA without RAP [
13].
Valdes et al. (2011) [
17] present experimental research aimed at characterizing the mechanical properties of AM with a high content of RAP. Two half-dense mixtures of maximum aggregate size 12 and 20 mm underwent assessment, with RAP contents of 40% and 60%, respectively. The results show that large amounts of reclaimed material can be included in AM thanks to proper characteristics and specific technological production regime of mixtures with RAP content.
The study [
6] describes AM types containing 20%, 40%, and 50% of RAP. The tests undertaken by Marshall and Duriez were performed on different mixtures with the use of road asphalt and asphalt modified with an SBS polymer. It was observed that parameters of recipe containing recycled asphalt with 20% of RAP are much closer to parameters of the mixtures with the original unmodified road asphalt. Moreover, the addition of the SBS polymer improved the properties of recycled AM even at a high RAP content. Huang et al. (2005) [
18] present lab tests with mixing components of mixtures containing RAP. A comparative analysis of a 20% RAP-based pavement created using unmodified AM was performed. The results of this experiment showed that only a small part of the old asphalt in RAP had in fact taken part in the new amalgamation, activation, and progression of the technological process, meaning that other parts created a rigid shell around the RAP. Therefore, there is a real problem of effectively combining RAP with both asphalt binder and virgin aggregate.
The majority of procedures concerning the recycling of HMA use up to a 40% RAP content. Recently, technologies have been developed which allow this content to be increased to 100%, e.g., the All-RAP process, Ammann RAH 100, Rapmaster, Astec RAP King, HyRAP, Alex-Sin Manufacturing, RATech, HERA System, Bagela, RSL, and dense-graded high-RAP [
19,
20,
21,
22]. The main barrier against the common usage of 100% RAP is a lack of durability tests of such pavements, as well as the lack of a homogenous and rational system of selection and designing of such mixtures.
On the basis of complex lab tests [
23], it has been found that the presence of RAP in AM generated increased stiffness and decreased shear deformation. Stretching resistance was also increased for samples of AM with a higher RAP content. German technical documents such as TL Asphalt-StB 07/13 [
24], TL AG-StB 09 [
25], and M WA 2009 [
26] contain good examples of possible RAP usage.
At present, research and implementation works are being held across the world to replace conventional asphalt liquefiers with solvers originating from plants in order to lower the technological temperature of AM. On the basis of previous experience, it was claimed that the production of ecologically fluxed asphalts and AM in warm technology with an RAP component is possible thanks to the application of liquefiers of vegetal origins.
Zaumanis et al. (2014) [
27] include an analysis of lab samples from 100% recycled HMA modified with use of five successively added additives of different types (waste plant oil, vegetable waste, smear, organic oil, and distilled tall oil) and a proprietary rejuvenator (aromatic extract) at levels of 12%. The properties of the binder and the mixture were tested. All the samples showed perfect resistance to rutting, ensuring, at the same time, a larger endurance limit in comparison with the original characteristics of the mixture and the most lowered critical temperature of cracking. Similar tests were conducted by Guduru et al. (2021) [
28], where five different rejuvenators (waste engine oil, waste lubrication engine oil (commercial product), tallow, waste vegetable oil, and crude tall oil) were used. The authors suggested that the rutting of AM due to the excessive softening of the rejuvenated bitumen should not be a big problem if the target dosage of the rejuvenator is determined based on the softening point value or viscosity (at 60 °C) test results. Meanwhile, Baqersad and Ali (2021) [
29] have investigated the use of a nanomodified asphalt binder in the recycling of RAP material to increase the incorporation of RAP percentages in recycled asphalt. In general, there are many different studies on the use of rejuvenators for the homogenization of asphalt mixtures with RAP [
30,
31,
32,
33,
34,
35,
36,
37,
38,
39,
40,
41,
42,
43].
Meroni et al. (2020) [
44] show that AM containing up to a 30% content RAP can be designed without a rejuvenator. However, a 45% RAP content in a mixture would not be possible without a proper rejuvenator which eases the adhesion of recycled aggregates to asphalt and aggregates.
It can be concluded that using rejuvenators in mixtures of high RAP content will become a common practice. They are being added to RAP-containing mixtures in order to decrease the stiffness of RAP binders and to improve their low-temperature characteristics, making possible to include into the mixture larger amounts of RAP [
45].
In light of the studied literature, the subject of the homogenization of RAP with an asphalt binder and virgin aggregate with use of a rejuvenator is important for determining homogenization mechanisms and co-existing phenomena. Thus far, they have not been recognized or described to a satisfactory degree. Proving that the original rejuvenator generates an improvement in homogenization process definitely adds value on the subject of AM containing RAP. Moreover, the literature lacks a comprehensive description of any research methodology of new types of organic rejuvenators. There are no procedures that allow for the evaluation of specific adhesive properties of oil substances that rejuvenate and revitalize asphalt with an RAP content. Additionally, the subject area surrounding the use of rejuvenators in mixtures of increased stiffness is of pioneering character. Research works covering topics such as water and frost resistance and resistance to durable deformations were executed for the designed AM types with addition of RAP and a rejuvenator. On the basis of the lab and field tests on test road sections, an evaluation was undertaken regarding the possibility of to applying an AM with an increased RAP content in lowered technological temperatures through the usage of an authorial rejuvenator. Tests checked to see if the rejuvenator used was responsible for the better homogenization of RAP containing asphalt binder and virgin aggregate. Lab test results were confirmed on test road sections.
2. Materials Used in Lab Tests
2.1. Characteristics of the Rejuvenator
The new generation of binders constitute mixtures of asphalt and vegetable oil or its derivatives in the form of methyl esters [
10]. During the execution of the process, it is possible that their activation determines the usability of these vegetal resources as components of asphalts.
Generally, vegetal components of this type can be used to liquidate both unmodified asphalts and ones modified with polymers. Enriched binders can be used in the following technologies:
during warm-mix pavement production with lowered AM production temperatures, below 150 °C,
for AM types produced with emulsions or fluxed asphalt (such a binder ensures the workability of AM within a certain time period).
Previous experience in rejuvenator use for warm-mix asphalt types made it possible to formulate the following advantages of the binders:
the elimination of vapors causing discomfort in the process of AM paving,
the better homogenization of RAP with the asphalt binder and aggregate,
the easier paving and compacting of AM in the process of construction,
the use of renewable resources.
Considering the above, an original rejuvenator ®Asfix Alfa has been created, which can be used as additive to AM type road asphalts, e.g., SMA, porous asphalt, asphalt concrete, etc. It is to be added immediately to the AM production process. It mixes easily with asphalt with a flash point exceeding 210 °C, and its viscosity in 30 °C is about 31 mPas. This rejuvenator is a liquid of light-yellow color with main active components of natural origins. It is intended for all types of hot paved AM types produced with use of ordinary and modified (with thermoplastic elastomers) road asphalts. It can be used as a component of AM types containing RAP. Such actions cause the revitalization of recycled asphalt contained within the RAP through the completion of the old oil fraction in the asphalt. The use of the rejuvenator aims at majorly improving the adhesion quality of the asphalt and aggregate and various degree of small particle pollution together with considerable improvement in workability. This agent improves adhesion in the production process of asphalt mixtures based on RAP, making it possible to increase the content of reclaimed materials in AM. It unifies RAP parts of similar parameters, allowing for the full control of void content, the optimization of content, and the considerable improvement in water and frost resistance of AM types, especially ones with an RAP content. The discussed rejuvenator causes an effective decrease in arduous smell at production, transport, and paving of AM in comparison to other agents which smell bad. Other existing adhesive and liquidating agents are based on hazardous waste and are dangerous substances themselves which generate toxic waste. The discussed rejuvenator is defined as safe and fully biodegradable and made of renewable resources. Asfix Alfa® is a patented component based on a plant recipe with multidirectional advantages facilitating the final benefits of the target mixture through the improvement of the homogenization process. Homogenization is especially important when combining RAP with virgin aggregates and asphalt.
A high flash point makes it possible for the discussed rejuvenator to be used in mixtures of high production temperatures, i.e., with the technology of cold RAP dosing. It does not cause a loss of its own characteristics in comparison with traditional chemical agents which easily undergo destruction and are unstable. The specific physical and chemical properties of the original Asfix Alfa rejuvenator are presented in
Table 1. The Asfix Alfa properties are compared with the traditional chemical agents (Wetfix BE and Wetfix AP17, Akzo Nobel, Stenungsund, Sweden).
The research carried out so far shows that adding Asfix Alfa does not significantly change the basic rheological properties of the asphalt when added in contents of between 0.3% and 0.8%. The change in asphalt penetration after adding the rejuvenator, compared to the starting asphalt, is in the range of −10 to +6 (dimensionless unit). Moreover, both the softening point and the temperature of asphalt brittleness with the addition of Asfix Alfa do not change significantly.
2.2. Description of AM Content with the Addition of RAP and a Rejuvenator
Research was held on an AM of asphalt concrete of continuous type of grain size 0/22 mm intended for base course under traffic load categories KR3–4 [
48]. It corresponds with the designed traffic category related to the classification of design traffic as regards the total number of equivalent standard axes 100 kN in all the design period N100 (expressed in millions of axes falling on a conventional lane). In addition, it amounts at 0.5 < N
100 ≤ 2.50 for KR3 (i.e., medium traffic load on the calculated road lane) and 2.5 < N
100 ≤ 7.30 for KR4 (i.e., heavy traffic). The AM content falls within the scope described in design guidelines for road structures [
48], which directly match German guidelines [
24,
25,
26]. The examined mixture contained: road asphalt 50/70, limestone filler, and broken dolomitic of various grain sizes. The mixture was enriched in RAP of two fractions, 0/8 mm and 8/16 mm, originating from the compound milling of wearing and binding courses. The determination of the grain size of the AM was carried out in accordance with PN-EN 12697-2 + A1:2019-12 [
49]. The test consisted in determining the percentage content of individual fractions of the AM after extracting the soluble binder by sieving on a set of laboratory sieves and weighing the content remaining on each sieve. A set of laboratory sieves with square meshes with an automatic Multiserw Morek (Multiserw Morek, Marcyporeba, Poland) shaker, a forced air dryer (Multiserw Morek, Marcyporeba, Poland), and an RADWAG (Radom, Poland) electronic non-automatic scale with a weighing range of up to 6000 g were used for the tests.
Figure 1 presents the grain size curves of virgin aggregates (dolomitic) and RAP as recycled pavement aggregates used in the examined mixtures.
The examined mixture contained asphalt, with the addition of asphalt originating from RAP (5.8% of 0/8 mm grain size and 4.3% of 8/16 mm grain size) and fresh asphalt 35/50.
Designs of two asphalt concrete mixtures were prepared for base course Asphalt Concrete 22P (AC 22P) with 20% and 40% RAP contents, called AM-20% and AM-40%, respectively. The total amount of asphalt in both mixtures was constant and it amounted to 3.9%. The main binder properties are presented in
Table 2. Both the mixtures had similar designed grain size curves (
Figure 2) in order to compare them, despite containing different amounts of destruction material.
Table 3 presents the composition of both of the designed mixtures.
Rejuvenator content was selected in relation to the total content of binder in the AM (
Table 4) at a level of 1.3%.
6. Discussion
Control and acceptance checks of AMs containing RAP are the same as in the case of mixtures made from virgin materials only. In the design of AM with RAP, the lab recipe does not differ from in the design of AMs from new components. One needs to consider all the material components of reclaimed asphalt. Additionally, the parameters of the resultant product need to be checked.
The effective usage of RAP coming from recycled materials was possible thanks to application of a rejuvenator which rejuvenated the asphalt contained in the RAP. Designs of authorial AMs concerned both base courses for traffic categories KR3 and KR4 and wearing and binder courses for KR1–2. The rejuvenator improved the homogenization of RAP with asphalt binder and virgin aggregate in each mixture type.
RAP dosed in cold-mix technology directly to the mixer required more aggregate heating in accordance with [
48]. Using high contents of RAP from 20% to 40% together with the rejuvenator requires RAP of a maximum humidity of 2%. The rejuvenator helps towards the effective mixing of the virgin asphalt binder and RAP. However, it is important to follow the recommendations concerning moisture contents in RAP due to the fact that the rejuvenator does not eliminate the negative impact of humidity coming from the RAP and does not neutralize it.
At present, according to [
48] it is acceptable to use RAP quantities which do not exceed 20% of AM mass. However, as the research shows, if a rejuvenator is used in cold-mix technology, the RAP content can be increased to up to 40% of the AM mass. Using RAP in quantities exceeding Polish requirements for AM does not contradict European norms, where this practice is fairly acceptable.
Based on the literature, e.g., Meroni et al. (2020) [
44], AM containing up to a 30% content of RAP can be designed without a rejuvenator. This study also states that a 45% content of RAP requires the use of a rejuvenator which eases the adhesion of recycled aggregates to asphalt and virgin aggregates. However, the use of Asfix Alfa in each configuration of the RAP content (even 100%) makes it possible to reduce the technological temperature of AM production.
The analysis of the obtained test results for various compaction temperatures showed that lowering the temperature by 20 °C in relation to the standard temperature used in hot-mix technology with the use of fluxed asphalt does not cause a decrease in water or frost resistance. However, lowering the compaction temperature by 40 °C does cause a decrease in the ITSR index by about 10% in comparison to the mixture compacted at 150 °C. These results correspond with voids content in tested mixtures, hence the lack of considerable ITSR change in 130 °C and a drop of ITSR for the compaction temperature of 110 °C, which corresponds with an increased content of voids in this mixture.
Due to protection of limited resources of RAP and binders, as well as the necessity to manage destructed asphalt, it is purposeful to implement solutions effectively using reclaimed materials. At the same time, using recycled materials cannot worsen the properties of AMs produced in this manner. Research results show that producing AMs containing 100% RAP is possible with use of a rejuvenator that helps towards the homogenization of mixture components. It has been proven that, as proportion of rejuvenator in AM-100%, changes in the following properties take place:
the bulk density of the samples increases—which proves the increased compatibility of the mixture,
the density of the AM decreases,
the voids content considerably decreases in comparison to stable increase of the compaction index.
Moreover, it was observed that the increased use of a rejuvenator causes a decrease in voids in comparison to samples where it was not used.
The AM with no additional rejuvenator had an increased number of voids (9.8%) at a compaction index of 99.7%. Using the rejuvenator allowed for the repeated optimal use of the RAP and attained the proper voids content in the pavement, in compliance with requirements [
24,
25,
26,
48,
61].
Technology using asphalt and the rejuvenator allows for the production and implementation of AMs with RAP which have properties comparable to the ones without granulate. On the basis of the lab tests results it should be stated that AM-40% meets the requirements determined in [
11] for mixtures intended for pavements under traffic loads KR1–4 (from light to heavy traffic). The rejuvenator used makes it possible to undertake effective compaction of layers of AM containing RAP in a final compaction temperatures that is lowered by about 20–30 °C. The research works have proved that the original, bio-rejuvenator was responsible for the effective homogenization of the RAP with asphalt binder and virgin aggregate.
Generally, chemical rejuvenators are applied in high amounts (even up to 12%) to rejuvenate AMs with high contents of RAP [
27,
30,
33]. The presented results indicate that it is possible to effectively use an organic rejuvenator. The application of 100% RAP together with the rejuvenator Asfix Alfa allows for the creation of AMs with similar physical and mechanical parameters (in lowered technological temperatures) to those obtained using other technologies with high contents of RAP [
19,
20,
21,
22].
The initial fatigue tests for AMs with 20%, 40%, and 100% RAP contents using the organic rejuvenator indicate that incorporating a high content of RAP into AM did not lead to a significant difference in the fatigue resistance, which may suggest that the blending of the RAP and the virgin binder is working as well as with the virgin binder alone.