Research on the Possibilities of Reusing Mixed Reclaimed Asphalt Materials with a Focus on the Circular Economy

Abstract

This article presents the results of a 10-year research study on the possibilities of implementing circular economy principles into the recovery of construction waste in road construction and paving traffic areas. According to Eurostat, construction waste accounts for approximately 25–30% of the total amount of waste produced in Europe. New legislative policies strongly support selective demolition and recycling with the aim of recycling at least 70% of construction waste. The subject of this research was mixed reclaimed asphalt material (MRAM) composed of 70% asphalt mixture, 10% aggregate, 10% concrete, and 10% soil. Isomorphic models and experimental sections made of MRAM showed that the required characteristics cannot be achieved when using MRAM without heating and compaction. When laying MRAM using a light dynamic plate and additional heating due to solar radiation, the LDD 100 device detected a 53% increase in the deformation modulus. On isomorphic MRAM models, the CBR test showed a 4-, 5-, and 14-times increase in the CBR value when the temperature was increased from 20 °C to 40, 50, and 70 °C. The laboratory results were confirmed by monitoring the surface condition of a local road rehabilitated between 2017 and 2025 using MRAM, where some sections showed the properties of semi-bound layers after eight years. The road surface was improved with a 20 cm layer of MRAM at an air temperature of 30 °C and compacted with a 10-ton smooth roller. The research results presented made it possible to create a proposal for a systematic approach to the evaluation of materials obtained from asphalt roads, optimized on the principles of the circular economy.

1. Introduction

Following the publication of Our Common Future [1], the global political and environmental situation has changed significantly; the concept and practice of sustainable development as a guiding institutional principle and specific political goal remain important in addressing many of the challenges of the new global order, as the authors in [2] note. However, the way in which sustainable development is conceptualized and practiced depends crucially on the willingness of scientists and practitioners to accept a plurality of epistemological and normative perspectives on sustainability. In [3], on sustainable construction, it is stated that Kibert in 1994 [4] laid the foundations for the practice of sustainable construction, minimization, and reuse of natural resources; use of renewable and recyclable resources; and minimization of the carbon footprint. Vanegas and Pearce in [5] introduced the concept of sustainable construction based on resource depletion and degradation, impact on the built environment, and human health. Pulaski, in [6], introduced a comprehensive approach to sustainability in building operations. Sustainable construction tools and standards were first developed for buildings. The first set of concepts and tools for assessing/evaluating the sustainability of buildings in the U.S. was provided by the Green Building System (The U.S. Green Building Council—USGBC LEED), based on the UK’s Building Research Establishment (BRE) environmental assessment method (BREEAM). The U.S. Green Building Council is committed to a sustainable and prosperous future through LEED (Leadership in Energy and Environmental Design), the leading program for green buildings and communities worldwide. The council’s vision is for buildings and communities to regenerate and sustain the health and vitality of all life within a single generation; to change the way buildings are designed, constructed, and operated; and to enable environmentally and socially responsible, healthy, and prosperous environments that improve the quality of life.

The World Green Building Council has issued a bold vision for how buildings and infrastructure around the world can reach 40% less embodied carbon emissions by 2030 and achieve 100% net zero emissions, building to net zero operational carbon goals by 2050. Research reports that global material use has tripled over the past four decades, with annual global extraction of materials growing from 22 billion tons in 1970 to 70 billion tons in 2010. Building and construction are responsible for 39% of all carbon emissions in the world, with operational emissions (from energy used to heat, cool, and light buildings) accounting for 28%. The remaining 11% comes from embodied carbon emissions or “upfront” carbon that is associated with materials and construction processes throughout the whole building lifecycle [7].

The waste sector is a major anthropogenic source of global greenhouse gas (GHG) emissions, ranking as the fourth highest contributor to global GHG emissions [8,9]. GHG emissions from specific waste types and treatment methods have been assessed across various regions and countries worldwide [10,11]. A comprehensive evaluation of life cycle GHG emissions from the municipal solid waste management sector remains limited at the global level due to variations in waste composition, management practices, infrastructure, and economic conditions across countries, as well as the scarcity of representative data. Solid waste management is a cross-cutting issue that affects various areas of sustainable development around the world. It is also a critical component for achieving the Sustainable Development Goals and their related targets, all of which have a direct link to solid waste management [12].

The Global Carbon Budget report by the Global Carbon Project revealed that global CO₂ emissions from fossil fuels and industry rose to 37.4 Gt in 2024 [13], an increase of 0.4 Gt from 37.01 Gt in 2023. Additionally, atmospheric CO₂ concentrations reached 422.3 ppm in 2024, a 0.7% rise from 419.3 ppm in 2023, and a staggering 52% increase from the preindustrial level of 280 ppm in 1750 [14].

Sustainable development principles are becoming increasingly important in the paradigm shift from a linear to a circular system of road construction, especially in terms of sustainable road pavements. In this article, the authors address the issue of sustainable road pavements, which, according to [15], are road pavements that minimize environmental impact by reducing energy consumption, natural resources, and related emissions while meeting all performance requirements and standards.

The most commonly used materials for bonded structural layers of road pavements and improvements to their subgrade include asphalt, cement, and lime. However, their production is associated with significant environmental impacts, particularly greenhouse gas emissions [16]. It is estimated that the production of one ton of cement releases approximately 900 kg [17] or one ton of CO₂ [16], with cement accounting for 10% of global CO₂ emissions [18]. Lime production emits approximately 860 kg of CO₂ per ton produced [19].

The absence of a systematic approach to the recovery of construction waste often leads to the creation of open dumps that pose significant environmental threats [20,21]. According to Kaza et al. [22], cities around the world produce nearly 2 billion tons of solid waste annually, with more than a third of it lacking proper and safe environmental management. The increase in the amount and variety of solid waste, limited availability of landfill space, and fewer natural resources are encouraging the world to seek innovative ways to recycle and reuse waste materials [23,24]. This premise of incorporating recycled waste into road construction is, according to [25,26,27], the main challenge of road construction. Road infrastructure is important for accessibility and plays a key role in national growth and economic prosperity. At the same time, road infrastructure leaves a significant environmental footprint throughout its life cycle, including material extraction and preparation, construction and maintenance, use, and end of life [28]. Sustainable development faces environmental challenges such as resource depletion, global warming, pollution, and biodiversity loss. Quantitative measurements are essential for assessing these impacts, which help in analyzing environmental consequences [29]. Many recent life cycle analysis (LCA) studies and research have quantitatively assessed the environmental footprints of these recycled materials throughout the entire life cycle of a road [27,30,31,32].

Sustainability and environmental aspects in road construction must be consistent with technical requirements for road pavements, in particular ensuring effective mechanical resistance to traffic loads and climatic and weather influences. Recycling, as a process of reusing previously used materials, is successfully applied in the case of materials from asphalt roads. Asphalt mixture is an example of a material for which it is ecologically and economically irresponsible to dispose of or place in landfills. When using 10% or 20% reclaimed asphalt (RA), the necessary strength and deformation characteristics of asphalt mixtures and layers are reliably achieved, with a demonstrable impact on reducing the consumption of natural resources. Life cycle analyses [33] show that 15% recycled content in WMA mixes can reduce CO₂ emissions by 13% and fossil fuel consumption by 14%. Modern hot and cold recycling technologies allow for the incorporation of high proportions of recycled materials, but with the necessary addition of new binders such as bitumen, bituminous emulsions, cements, or additives. Research work within the Recypma project [34] verified the use of higher proportions of 40% RA in new asphalt mixtures using both paving-grade bitumen and polymer-modified bitumen, and the mixtures produced in this way achieved satisfactory values for quality properties such as resistance to permanent deformation (wheel-tracking slope, WTS_AIR, and proportional rut depth, PRD_AIR), sensitivity to water, stiffness (indirect tension strength, ITS, determined by IT-CY procedure), and fatigue resistance. If milled or excavated material from road pavements contains material bonded with various types of binders, together with unbound layers, such material cannot be used in the production of new asphalt mixtures, and it is also undesirable to store it in landfills. It is effective to recycle such material into road base layers as mixed RA or non-asphalt RA, using cold recycling technology. A study [35] in the Journal of Materials Science confirms that adding Portland cement and bituminous emulsion to recycled asphalt mixture significantly increases density, compressive strength, and water resistance. In China [28], hot-in-place recycling with high RA content (over 60%) was investigated, which resulted in an improvement in stability at high temperatures of up to 257%, although elasticity decreased compared to mixtures with 0% RA content, indicating significant brittleness that could affect long-term durability in cold climates.

In some cases, it is possible to use mixed reclaimed asphalt as a surfacing layer on roads with low traffic loads, e.g., local, special-purpose, forest roads, and parking or other paved areas, provided that the requirements of the relevant applicable technical standards are met. The aim of this research is to demonstrate the suitability of the use of reclaimed asphalt and mixed reclaimed asphalt in pavement structures for the conditions in Slovakia.

2. Circular Economy in the Slovak Construction Industry

According to EUROSTAT statistics [36], construction waste accounted for 38% of the total waste volume in 2022 (Figure 1a). The construction industry has become one of the critical areas where a transition from the current linear economic model to a circular economy model is essential. According to the European Parliament [37], the circular economy (CE) is a model of production and consumption that involves sharing, leasing, reusing, repairing, refurbishing, and recycling existing materials and products as long as possible. In this way, the life cycle of products is extended, and the graphic interpretation of the circular economy is shown in Figure 1b. In practice, it implies reducing waste to a minimum. When a product reaches the end of its life, its materials are kept within the economy wherever possible, thanks to recycling. These can be productively used again and again, thereby creating further value. This is a departure from the traditional, linear economic model, which is based on a take–make–consume–throw away pattern. This model relies on large quantities of cheap, easily accessible materials and energy.

The Slovak Institute of Circular Economy, in [39], provides the following definition or explanation for the term. The circular economy, also called circularity, is an economic model based on the (repeated) return of materials, components, and products back into the production process. By circulating them, waste, energy consumption, and otherwise required for the production of new inputs and overall production costs are radically minimized.

The authors of [40] summarized 114 definitions of circular economy, through the analysis of which they found that the CE is most frequently depicted as a combination of reduce, reuse, and recycle activities, whereas it is oftentimes not highlighted that CE necessitates a systemic shift. The main aim of the CE is economic prosperity, followed by environmental quality; its impact on social equity and future generations a link to sustainable developments is barely mentioned. Notable concepts that are also supposed to operationalize sustainable development for businesses are the green economy and green growth concepts [41], whereas the CE concept is argued to be the one with the most traction these days [42,43]. The authors of [44] integrate legislative, cognitive–behavioral, and construction approaches into green infrastructure in the context of sustainable urban engineering involving green space as a fundamental attribute of quality of life. They perceive green infrastructure as an institute with a multi-beneficial meaning within the framework of administrative science, understood as a public interest incorporating a range of partial issues from the primary interest of environmental protection, economic development of settlements, promotion of public and mental health, social issues, and sports.

Closing material flows and maximizing resource efficiency in transportation construction is a key aspect of the circular economy, where the outputs of production processes are constantly transformed into inputs. The main goal of the circular economy is to achieve the highest possible usability and value of products and components, while minimizing negative impacts on the environment. Due to its ecological, technical, economic, and social potential, CE is becoming increasingly popular and represents a real alternative to the traditional linear economy [45]. The circular economy is a tool for achieving sustainability in transportation construction (STC), within which, in the USA, it applies to every phase of decision-making: planning, design, project and infrastructure implementation, daily operation, and maintenance. In [3], it is stated that Kibert [4] laid the foundation for the practice of STC, minimization and reuse of natural resources, use of renewable and recyclable resources, and minimization of the carbon footprint in the construction industry. Miyatake, in [46], stated that one way to strive for STC is to change from a conventional linear process to a cyclical one. In principle, this means a significant increase in recycled, recovered, and reused materials, leading to a significant reduction in energy and resource consumption. Vanegas and Pearce, in [5], introduced the concept of STC based on resource depletion and degradation, impacts on the built environment, and human health. Pulaski, in 2004 [47], introduced a comprehensive approach to sustainability in construction operations.

EU initiatives propose to deal with the issue of construction and demolition waste (CDW) according to the principles of a circular economy: the 3Rs (Reduce, Reuse, and Recycle). CDW is generated during the whole life cycle of construction. The lack of information about the quantity of CDW produced during the design phase of buildings is solved by quantifying it during the construction design phase in a circular economy.

The authors of [48] processed the statistics of total waste production in the Slovak Republic between the years 2005 and 2023 (Figure 2), as well as the amount of construction and demolition waste produced between 2005 and 2023 (Figure 3). The figures show that the share of CDW in the total production of registered waste in Slovakia in 2005 was 10.1%, 33.7% in 2015, and 44.0% in 2023. The presented percentage increases were, in our opinion, caused by better separation of municipal waste and a significant increase in new construction and reconstructions of civil and transport infrastructure in Slovakia.

As early as the construction design phase, there is the possibility of choosing technologies, construction processes, and materials that have a higher degree of circularity in the economy [49]. Sustainable design and building information modeling have introduced challenges and opportunities to improve the efficiency of construction project management [50].

3. Recycling of Road Pavements

According to [51], recycling is the process of reusing previously used materials and products, enabling the conservation of renewable and non-renewable resources and thus reducing the load on the environment (utilizing waste instead of landfilling it). Materials from road pavement structures, such as crushed aggregate, hydraulically bonded mixtures, and asphalt mixtures, can be recycled and reused, which extends their service life. With modern technologies, you can effectively restore road pavement layers without reducing their performance properties. The basic options for using road pavement materials are shown in Figure 4.

A prerequisite for asphalt material recycling is that the properties of the asphalt mixture/asphalt course containing reclaimed asphalt material meet the requirements of the relevant regulations. The following options for using milled and excavated asphalt material [52] are generally available:

• Asphalt disposal or landfilling: In most cases, it is environmentally and economically irresponsible and is prohibited in some countries;
• Base courses: Friendly to raw material resources, economic recovery, and feasible to use relatively simple technologies;
• Pavement surface construction: Friendly to raw material resources, economic recovery, technically demanding technologies, and the highest added value.

Over the years, asphalt mixtures have become a high-quality technical product that must meet a number of requirements for compliance with quality parameters according to the purpose of the use and functionality of individual layers of the road pavement structure. Research conducted over the past 30 years has shown that asphalt mixtures produced with the addition of reclaimed asphalt material have properties that are just as good as conventional mixtures produced from only original raw materials.

A premise for the recycling of asphalt material from road pavement layers is the preservation of the properties of their material components (aggregate and bituminous binder), especially the ability of bitumen, as a binder, to coat and bind the aggregate after reheating. Asphalt mixture obtained by milling asphalt road layers, crushing excavated asphalt layers (by separating rather than breaking the grains), asphalt mixture residues, or unused asphalt mixtures is referred to as R-material (reclaimed asphalt, or “RAP” in the USA: reclaimed asphalt pavement). Obtaining R-material by milling asphalt layers ensures the necessary homogeneity of the material. In the case of milling wearing courses, it is possible to obtain material containing high-quality aggregate and binders (aggregates with high resistance to polishing and wear and polymer-modified bitumen). In this way, it is possible to obtain asphalt R-material (in accordance with the requirements of [53]) from road pavements, which can be used as input material for the production of asphalt mixtures mixed with R-material bonded to various types of bituminous binders, together with unbound material (the content of asphalt-bonded grains in the mixture ranges from 20 to 80%, and binder content ranges 0.7 to 4.0% by weight of the mixture) and non-asphalt R-material with asphalt-bound grain content of less than 20% by weight and bitumen content less than 0.7% by weight. By increasing the proportion of recycled asphalt material in newly produced mixtures, the amount of work required, e.g., in terms of quality control, also increases.

European countries produce 250 million tons of asphalt mixtures annually [54] (with a 269 million tons produced in 2023) and, at the same time, approximately 50 million tons of recycled mixtures (

Table 1. Percentage shares of reuse and recycling of asphalt materials in Europe [54].

Year	Total Amount of Site-Won Asphalt Generated in Year in Million Tons	Amount of RA Available to be Used by the Asphalt Industry in Million Tons	Total Reuse in %	Total Recycling in %	Total Landfill in %
2021	56.009	32.166	72.0	25.0	3.0
2022	45.484	40.681	71.3	22.6	3.9
2023	52.755	41.441	76.0	20.0	4.0

). The percentage of available reclaimed asphalt, which was reused for the production of new mixes (including HMA, WMA, Half-WMA, and CMA) and recycled as unbound road layers and other civil engineering applications, is increasing year by year.

4. Research on Properties and Applicability of MRAM in Pavement Construction Layers—Materials and Methods

The reclaimed material obtained by breaking up road pavement layers can be classified as follows, depending on the content of bitumen-bound grains in the mixture as used in [55] and related to the subject of the article:

• Reclaimed asphalt (RA material)—processed asphalt material obtained from a construction site (pavement) and, after testing, assessment, and classification according to the EN 13108-8 standard [53], usable as an input material for an asphalt mixture. Processing may include one or more of the following methods: milling, crushing, sorting, mixing, etc.
• Mixed reclaimed asphalt material (MRAM)—material obtained by breaking up materials from road structures bound to various types of binder, together with unbounded layers; the content of bitumen-bound grains in the material ranges from (20 to 80) %, and the bituminous binder content is (0.7 to 4.0) % by weight of the mixture.
• Non-asphalt reclaimed material (NARM)—material obtained by breaking up materials from road layers; the content of asphalt grains in the mixture is less than 20% by weight, and the bitumen binder content is less than 0.7% by weight.

In our research activities, the main subject was reclaimed asphalt material (MRAM) obtained by milling asphalt pavements, which are unusable in terms of their direct use in the production of asphalt mixes in plants, mainly in terms of the proportion of contaminants. The reclaimed asphalt contained 70% asphalt mixture, 10% aggregate, 10% concrete, and 10% soil. The above composition does not preclude the reuse of such material for the construction of paved surfaces for roads with low traffic loads, as well as for the prevention of surface erosion.

In addition to determining the basic composition of MRAM, the bitumen binder content in the mixture was determined by performing material extraction using the automatic method and a continuous flow centrifuge. Binder content determined by extraction was 3.6%; particle size distribution of the MRAM is shown in

Table 2. Reclaimed asphalt material (MRAM) grading.

Sieve size [mm]	22.4	16	11.2	8	4	2	1	0.5	0.25	0.125	0.063
Passing [%]	100	99.6	94.1	78.2	51.3	27.9	22.1	16.3	13.1	10.9	9.3

.

The aim was to demonstrate the suitability of using mixed recycled asphalt in road construction for conditions in Slovakia.

The applicability of the MRAM material was verified based on four main factors:

• Experimental field for research of unbound pavement construction layers (EFRUPCL), unbound MRAM: Verification of the mechanical efficiency of MRAM that was cold-laid at an air temperature of 10 °C, without the use of compaction equipment (Figure 5).

Figure 5. EFRUPCL construction process in 2015 in the village of Dolný Hričov.

Figure 5. EFRUPCL construction process in 2015 in the village of Dolný Hričov.

• Scientific Research Workplace of the Faculty of Civil Engineering of the University of Žilina (SRW of FCE UNIZA): A compacted unbound MRAM layer was cold-laid at a temperature of 20 °C using a compaction plate (Figure 6). This verifies the hypothesis that MRAM, after compaction with a light vibrating plate in combination with solar heating, will demonstrate significantly higher deformation characteristics than when laid without compaction. Measurements were performed at different temperatures (e.g., 6 °C, −4 °C, and 18 °C).

Figure 6. Construction of the SRW of FCE UNIZA and laying of mixed RA material.

Figure 6. Construction of the SRW of FCE UNIZA and laying of mixed RA material.

• Rehabilitation of the local road pavement: Application of a 20 cm thick, semi-bound MRAM layer (Figure 7) at an air temperature of 30 °C and compaction with a heavy 10-ton roller. Subsequently, surface condition monitoring was carried out from 2017 to 2025.

Figure 7. Views of the rehabilitated pavement one day after the layer was laid (16.8.2017).

Figure 7. Views of the rehabilitated pavement one day after the layer was laid (16.8.2017).

• Laboratory determination of MRAM properties: The California Bearing Ratio (CBR) and Immediate Bearing Index (IBI) tests of mixed recycled asphalt were performed at temperatures from 20 to 70 °C (Figure 8).

Figure 8. Views of measurements of input values needed for evaluation of CBR [56] and IBI parameters [57].

Figure 8. Views of measurements of input values needed for evaluation of CBR [56] and IBI parameters [57].

To assess the practical applications of the MRAM in construction layers, tests were performed to determine the deformation characteristics (such as static modulus of elasticity, static modulus of deformation, modulus of transformation, reaction modulus, and impact deformation modulus):

• Static plate load test (SPTL) according to [58]; test performed with circular plates of various dimensions for load distribution.
• Impact deformation modulus test using the LDD 100 device (ECM, Brno, Czech Republic); 11 series of 9 measurements evenly distributed over the area of the MRAM layer and performed during the years 2022 to 2023.

To determine the possibilities of using MRAM for the construction of paved road surfaces for low-traffic loads, further tests of the following properties were carried out under laboratory conditions:

• The Proctor test is used to determine the maximum dry density and optimum moisture content by compacting it with a standardized amount of energy;
• The California Bearing Ratio (CBR) test is used to characterize compactability and strength at different loads (Proctor standard PS and Proctor modified PM methods) and temperature conditions (standard laboratory temperature conditions, ±20 °C, and a temperature range of 40 to 70 °C);
• The Immediate Bearing Index (IBI) is used to determine the strength characteristics of compacted samples without surcharge at different moisture values of the material.

The CBR and IBI tests were performed under standard laboratory conditions at ±20 °C. The MRSM was pre-dried. To determine the effect of temperature on the MRAM material during compaction, a temperature range of 40 to 70 °C was selected. The temperature range was determined based on experience with temperatures achieved on hot summer days on the road surface, mainly in the wearing course of roads, in which asphalt mixtures are applied. The MRAM, as well as the mold and the hammer, were heated to a steady constant temperature (set temperature) before compaction.

The specimens for the IBI test were prepared from pre-dried MRAM, to which various amounts of water were added (for a moisture content of 3 to 10%). After mixing, the thus-prepared mixture was cured under laboratory conditions while preventing water evaporation. The curing time was 24 h, which achieved wetting of the material and homogeneity of the mixture. Subsequently, the MRAM mixture was compacted by a PM compaction energy, and the IBI test was immediately performed. The CBR and IBI measurements were performed on both the upper and lower surfaces of the samples, and the result was an average value.

5. Results and Discussion of Practical Applications and Experimental Measurements

5.1. Properties of MRAM as an Unbound Pavement Layer Without Compaction

Verification of the mechanical efficiency of MRAM was carried out at the experimental field EFRUPCL, which was built as a research field of the Department of Road and Environmental Engineering of the Faculty of Civil Engineering of the University of Zilina (Figure 5). The original part of the local road in the village of Dolný Hričov was widened with loamy soils and reinforced by laying MRAM (milling asphalt mixtures containing stony particles) in a thickness of 20 cm.

The load-bearing capacity was measured using a special device for SPLT (ECM, Brno, Czech Republic) of building constructions according to [58], with a loaded truck as the counterweight (Figure 9).

The device generates the required load, p, in MPa and measures the deformations, y, in m. The required load, p, on the subgrade surface is achieved by at least five loading stages. After the deformation has stabilized at each stage, the deformation is read, the pressure is increased to the next stage, and the cycle is repeated. After reaching the last stage of loading, the loading plate is relieved, and the deformation value at a contact pressure of p = 0 MPa is recorded. The dependence, y = f(p), is plotted from the measurements (Figure 10). The essence of the test lies in determining the magnitude of the deformations generated by pressing a circular plate in separate loading stages on the subgrade surface or other underlying layers of pavements. The most commonly used method is a series of circular plates (Figure 9):

• Small plate with an area of 0.100 m² (diameter d = 357 mm, radius r = 178.5 mm);
• Middle plate with an area of 0.200 m² (d = 505 mm, r = 252.5 mm);
• Middle plate with an area of 0.283 m² (d = 600 mm, r = 300 mm);
• Large plate with an area of 0.500 m² (d = 798 mm, r = 399 mm).

Table 3. Recapitulation results on EFRUPCL.

Date	Assessed Deformation Characteristics	Assessed Deformation Characteristics of SPLT for 1. and 2. Load Cycles (LCs) and the Area of the Load Plate
		A = 0.100 m2			A = 0.200 m2			A = 0.283 m2
		1. LC	2. LC	Ratio 2. LC/1. LC	1. LC	2. LC	Ratio 2. LC/1. LC	1. LC	2. LC	Ratio 2. LC/1. LC
23 November 2017	Static modulus of elasticity, Ei [MPa]	27.1	25.4	0.93	23.1	22.6	0.98	24.8	24.5	0.99
	Static modulus of deformation, E0,i [MPa]	14.5	23.7	1.63	10.4	20.0	1.92	15.6	22.0	1.41
	Modulus of transformation, Edef,i [MPa]	13.8	25.1	1.82	11.9	21.8	1.83	16.8	22.0	1.31
	Reaction modulus, kpi,1.27 mm [MN/m3]	37.0	41.3	1.12	27.8	32.6	1.52	43.5	40.4	0.93
	Recalculated reaction modulus, kpi,0.07 mm [MN/m3]	38.8	41.0	1.06	19.5	32.8	1.68	44.0	40.4	0.92

and Figure 11 show the following deformation characteristics evaluated on the MRAM surface of the EFRUPCL according to the requirements of [58]:

• The static modulus of deformation, E₀, in MPa, is determined by a static load test performed with a small or middle plate so that at the last loading stage, the plate pressure exerted is greater than that used for calculating the modulus according to Equation (1):

$$E_0={\displaystyle \frac{\pi }{2}}\cdot \left(1-{\mathsf{\nu }}^2\right)\cdot {\displaystyle \frac{p\cdot r}{f_{tot}}},$$

(1)

• The static modulus of elasticity, E, in MPa, is determined in the same way as the deformation modulus, E₀, and is calculated according to Equation (2), in which f_e is the average elastic deformation of the material in m:

$$E={\displaystyle \frac{\pi }{2}}\cdot \left(1-{\mathsf{\nu }}^2\right)\cdot {\displaystyle \frac{p\cdot r}{f_e}},$$

(2)

where ν is the Poisson number (-), p is the plate pressure exerting the total average plate compression f_tot (MPa), r is the plate radius (m), and f_tot is the total average plate compression (m).

• The reaction modulus, k, in MN/m³ is determined according to [58] using a static load test with a large plate, performed so that the total average plate deformation is 1.27 mm, calculated according to Equation (3):

$$k={\displaystyle \frac{F}{A\cdot f_{tot}}}={\displaystyle \frac{p}{0.00127}},$$

(3)

where F is the plate load exerting a total average plate compression of f_tot = 1.27 mm (MN); A is the load plate area (m²); f_tot is the total average plate compression, which is equal to 0.00127 m; p is the plate pressure exerting a total average plate compression of f_tot = 1.27 mm (MPa).

On soils and unbound materials with k ≤ 60 MN/m³, an accelerated load test procedure can be used. A pressure of p = 0.07 MPa is applied to the soil, which causes the total average plate compression, f_tot. The reaction modulus, k, in MN.m⁻³ obtained by the accelerated load test procedure, which is calculated by Equation (4):

$$k={\displaystyle \frac{p}{f_{tot-0.07}}}={\displaystyle \frac{0.07}{f_{tot-0.07}}},$$

(4)

When the reaction modulus, k_d, is evaluated from measurements using a plate with a small or medium diameter, d (mm), the values need to be converted to a large plate according to Equation (5):

$$E_{def,i}={\displaystyle \frac{\pi }{2}}·(1-{\mathsf{\nu }}^2)·r·{\displaystyle \frac{\mathrm{\Delta }p}{\mathrm{\Delta }y}},$$

(5)

where E_def,i is the modulus of transformation according to [59] (MPa), Δp is the change in contact stress (MPa), and Δy is the change in plate compression (deformation of the test environment) (m).

As shown, the results of the deformation characteristics (modulus of elasticity, modulus of deformation, modulus of transformation, and reaction) in Figure 11 for the quasi-elastic semi-half space are roughly the same, so a homogeneous elastic half-space can be presumed. The area of the plate does not significantly influence the calculation of the deformation characteristics, which is evident from the values of the modules with a 0.505 m plate, which are lower than when using plates with diameters of 0.357 m and 0.600 m. The static modulus of deformation (Figure 11b) and modulus of deformation (Figure 11c) increase significantly after the second measurement cycle for all load plate sizes. Both graphs clearly show that repeated loading (second cycle) leads to higher values for both modules and confirm the positive effect of the consolidation of the structural layer on the mechanical properties of the pavement material. The upward trend in values in the second cycle is consistent regardless of the size of the plate, which indicates an improvement in the structure of the material after repeated stress.

5.2. Properties of MRAM on the SRW of FCE UNIZA as an Unbound Pavement Layer Without Compaction

As part of our research activities at the SRW of FCE UNIZA, the following hypothesis, that an MRAM layer compacted by a light vibrating plate (Figure 6, bottom right) in combination with the long-term effects of solar energy will exhibit significantly higher deformation characteristics than when laid without compaction, was verified. The standard vibrating plate, with a weight of 119 kg and a frequency of 90 Hz, generates a dynamic pressure of 32 N.cm⁻². For this purpose, measurements were taken on the surface of the MRAM (Figure 12) under the conditions specified in

Table 4. Average air temperatures and humidity, MRAM.

Test Date	Air Temperature [°C]	Average Moisture MRAM w [%]
3 November 2022	6	3.78
10 February 2023	−4	4.17
10 August 2023	18	2.42

, and the results are in Figure 13.

In the instructions for the use of the LDD 100 device, the relation for calculating the impact deformation modulus, E_vd, in MPa is stated as follows:

$$E_{vd}={\displaystyle \frac{F}{d\cdot y_{e1}}}\cdot \left(1-{\nu }^2\right)$$

(6)

where d is the diameter of the loading plate (m), F is the impact force (N), and y_e1 is the amplitude of deflection at the center of the loading plate (m).

Figure 13 presents the results of impact deformation modulus, E_vd, measurements performed on the MRAM surface on 3 November 2022, 10 February 2023, and 10 August 2023 in the form of 3D graphs. The graphs present a three-dimensional representation of the impact deformation modulus measured in 11 series of nine measurements. The histograms (Figure 13) show the distribution of the impact deformation modulus values on different dates. The first histogram (3 November 2022) shows that the highest occurrence is in impact modulus intervals of 5–10 MPa and 10–15 MPa. On 10 February 2023, a shift toward higher E_vd values can be observed, with values in the ranges of 15–20 MPa and 20–25 MPa occurring most frequently. On 10 August 2023, even higher ranges dominate—the most frequent occurrences are between 20–25 MPa and 25–30 MPa, confirming a slight increase in the average values of the impact deformation modulus for the period under review.

Within the 99 measurements performed, the following average values were determined for the dates mentioned: E_vd = 12.1 MPa, E_vd = 16.6 MPa, and E_vd = 18.5 MPa. For the mentioned average values, 99 measurements were always carried out, and standard deviations were evaluated at the level of 4.14, 4.77, and 4.53 MPa. Based on the statistical rule of 3 sigma, also known as the 68–95–99.7 rule, for the above statistical values of E_vd, we can evaluate the following interval estimates. In the measurements made on 3 November 2022, 68% of the E_vd values were in the interval (8.0, 16.2) and 95% in the interval (3.8, 20.4); for measurements on 10 February 2023, the intervals were (11.9, 21.4) and (7.1, 26.2), and for 10 August 2023, they were (14.0, 23.1) and (9.5, 27.6).

A comparison of all three dates shows that there has been a gradual improvement in the stiffness of the material over time. Analysis of possible internal and external factors on the MRAM layer showed that this slight increase (from 12.1 MPa to 18.5 MPa) in the deformation modulus was, in our opinion, caused by the consolidation of the layer under the influence of solar energy. It should be noted that the MRAM layer was not loaded by traffic. In addition, considering the fact that according to the parameters of the LDD 100 plate and the theory of impact, the pressure derived during the measurement was 0.1 MPa, and the MRAM layer compaction was performed using a pressure of 0.32 MPa, we do not assume that the cause of the increase in the modules was the additional compaction during the measurements with the LDD 100 plate itself. It is clear from Figure 13 that the hypothesis of a significant increase in deformation characteristics due to solar radiation has not been confirmed.

5.3. Rehabilitation of the Local Road Pavement, Semi-Bound Layer

The authors have long been engaged in research on the possibilities of recycling mixed RA materials from asphalt road construction: MRAM. In 2017, they laid 10 cm of MRAM in Dolný Hričov in Peklina. The laying was carried out on a summer day with a temperature of 30 °C. The surface condition of the rehabilitated local road was monitored over a long period of time through visual observation and photographic documentation in 2017, 2020, 2023, and 2025. Examples from this photographic monitoring are shown for the years 2017 and 2025 in Figure 14.

In addition to the overall views, the figure shows detailed views of the condition of the MRAM used for the rehabilitation of the road of interest. It is clear from Figure 14 that after 8 years of operation, the surface shows a significant difference in quality. In some sections with longitudinal slopes of 3 to 7 percent without transverse drainage from old guardrails, the surface was destroyed by the impact of rain. In flat, cross-drained sections, the influence of solar energy and the repeated passage of passenger and truck vehicles led to local consolidation of the MRAM, and we called the resulting material semi-bound MRAM.

5.4. Laboratory Determination of Mixed Reclaimed Asphalt Properties

The MRAM material used to construct the pavement surface of the local road was tested under laboratory conditions. The MRAM compaction properties, which are critical for ensuring a stable and strong course, were determined by the Proctor laboratory test according to [60]. These properties are generally expressed in terms of maximum dry density and optimum moisture content. The tested MRAM is characterized by the following values: dry bulk density of 1865 kg/m³, water content of 4.2%, and wet bulk density of 1943 kg/m³.

The CBR values according to [56] were determined under laboratory conditions for different compaction temperatures of the MRAM. The Proctor test and modified Proctor test were used for compaction of the MRAM. The objective was to determine the potential for increasing the bearing capacity of the MRAM material at a moderate increase in temperature that can be achieved during high summer temperatures. Thus, we determined if it would be possible to compact the RA material layer to be able to carry future loads specified for this road category.

In the next procedure, the effect of moisture on the compactability of the MRAM was investigated (Figure 15). Water was added to the MRAM in varying proportions (3 to 10%), mixed, and allowed to rest. The moistened MRAM was compacted by the work of PS and PM, and the immediate California Bearing Ratio and parameter IBI [57] value were determined. This refers to a specific measurement used to assess the load-bearing capacity of the base or subgrade (unbound materials or soils). An IBI value indicates how well the material can withstand pressure or weight, with higher values generally indicating better load-bearing capacity.

The observed values in

Table 5. Results of CBR and IBI tests of MRAM.

Parameter CBR				Parameter CBR
Compaction Method
Proctor Standard		Proctor Modified		Proctor Modified
Temperature [°C]	CBR [%]	Temperature [°C]	CBR [%]	Moisture [%]	IBI [%]
40	6	40	4	3.2	28
50	13	50	11	3.5	27
55	16	60	22	5.5	15
60	24	65	32	7.5	14
70	38	70	45	10.5	3

show the relationship between MRAM material compaction temperature, degree of compaction, and material strength expressed by the CBR and IBI parameters. The increasing CBR values (Figure 16) show the positive effect of increasing material temperature for both compaction methods, from 6% to 38% for Proctor standard compaction and from 4% to 45% for Proctor modified compaction. Conversely, increasing moisture content decreases the instantaneous bearing capacity of the CBR material, as shown by the IBI value (Figure 17). At the same time, it appears that the Proctor modified method provides higher CBR values than the Proctor standard, indicating more effective compaction of the MRAM material. Higher CBR values are the result of the higher compaction energy of the Proctor modified method, which uses a heavier hammer dropped from a greater height. The CBR values achieved under standard conditions correspond to the results of the studies carried out by [61,62]. However, these studies primarily focused on testing different MRAM configurations (pure recycled asphalt; recycled asphalt with crushed stone or with cement stabilization). The CBR values decrease with an increasing percentage of recycled asphalt in the mixture, especially with high RA content without stabilizers.

For the observed compaction temperatures, the increase in bearing capacity evaluated by the CBR parameter indicates a dependence according to Figure 18. The trend curve is described by the power function with a very high coefficient of determination (R² = 0.93), which indicates a strong correlation between the compaction temperature and the CBR value. Based on the above facts, it can be stated that the correlation dependence of interest shows a “very strong” dependence according to Spearman and a “very high positive” according to Pearson. The rising trend curve shows that as the MRAM compaction temperature increases, its bearing capacity in the form of the CBR parameter increases significantly. The trend equation makes it possible to predict the CBR value for any temperature in the examined range, which is very useful in the design and optimization of compaction technology.

Overall, the data show that increasing the temperature and using a modified compaction method significantly improve the mechanical properties of the reclaimed asphalt sample, and increasing the temperature during the construction of layers with MRAM can lead to a significant increase in bearing capacity.

5.5. Summary of Results

As part of this long-term research, the main focus was on the possibility of using MRAM in road construction layers and the reinforcement of trafficked areas. As part of the research at the EFRUCPL (Figure 5 and Figure 9), the deformation characteristics of MRAM mixtures were evaluated according to [58,59].

The objectively determined research results presented in this paper, supplemented by further results in [63,64,65], led to the proposal of a systematic approach (Figure 19) to the evaluation of the re-use and recycling of materials from the construction of road pavements, including mixed reclaimed asphalt material, with low traffic loads. The proposed system is considered a contribution to improving the sustainability of roadways.

The current trend in the world is to recover suitable industrial wastes for recycling asphalt pavements, such as plastics, used tires, kitchen waste, glass, fly ash, wood waste, and ash industrial textiles [66,67,68]. For example, through the appropriate use of waste cooking oil, eggshell powder, and other biodegradable waste in asphalt mixture, we can obtain hybrid materials, serving as rejuvenators and modifiers of RA materials [69,70,71,72]. Although the recycling of RA material can reduce costs, there are still some concerns about new asphalts containing RA, particularly in the base layers [73]. The National Asphalt Paving Association estimates that the use of recycled pavements in infrastructure could save nearly $2 billion annually [74]. It is generally accepted by the scientific community that RA has a negative impact on long-term cracking performance but performs similarly or better than asphalts without RA in terms of rutting resistance, raveling, and ride quality [75].

6. Conclusions

This article concerns the issue of the circular economy in road engineering. Road engineering at the beginning of the 21st century must imply interdisciplinary, globally established, holistic approaches to the preparation, construction, management, recycling, rehabilitation, and liquidation of roads, with a significant emphasis on the circular economy, and their implementation in the sustainable preparation, construction, and management of the EU’s integrated transport infrastructure.

This article clearly demonstrates that the recycling of asphalt and concrete pavements is an integral part of CE, ensuring the sustainability of pavements. The focus on MRAM material supports the minimization of construction and demolition waste landfilling, which accounted for up to 44% of total waste production in Slovakia in 2023.

As part of the research activities carried out on possibilities for implementing the principles of the circular economy in the recovery of construction waste in road structures, the authors examined MRAM obtained by breaking up asphalt road layers together with non-asphalt layers (with the content of bitumen-bound grains of 70%), and the following conclusions were made:

• Using isomorphic models and test sections of rehabilitated roadways using MRAM, it was found that when using MRAM without heating and compaction, it is not possible to achieve the required characteristics (specified by relevant standards and technical regulations). The achieved low values of the static modulus of deformation from the second loading cycle in the range of 20.0 to 23.7 MPa indicate the necessity of intervention.
• The influence of the size of the load plate on the load distribution was not evident. The deformation characteristics (modulus of elasticity, modulus of deformation, and modulus of reaction) were approximately the same, confirming the assumption of a homogeneous elastic half-space.
• A comparison of the deformation modules of the MRAM layer on the SRW of the FCE UNIZA, measured by an LDD 100 device during the years 2022 and 2023, shows only a slight improvement in the stiffness of the MRAM (an increase in the impact deformation modulus from 12.1 MPa to 18.5 MPa), which, after excluding mechanical effects on the layer, was caused by the consolidation under effect of solar energy. However, the hypothesis of a significant increase in deformation characteristics meeting the required values due to solar radiation was not confirmed.
• The hypothesis that the long-term effect of solar energy alone would lead to a significant increase in the deformation characteristics of compacted MRAM was not confirmed. Measurements of the non-traffic-loaded test section (SRW FCE UNIZA) showed only a slight improvement in stiffness over time, increasing the impact deformation modulus from an average of 12.1 MPa to 18.5 MPa due to consolidation under the effect of solar energy. This indicates that solar heating without traffic loading is insufficient to activate the required bearing capacity.
• Laboratory tests demonstrated that temperature is the critical factor for enhancing MRAM bearing capacity. On isomorphic MRAM models, the CBR test showed a 4-, 5-, and 14-times increase in CBR values when the temperature was increased from 20 °C to 40, 50, and 70 °C. At the highest tested compaction temperature, it is possible to reach a CBR value of 45 MPa. Conversely, increasing moisture reduces the immediate bearing capacity of the MRAM, expressed by the IBI value.
• The laboratory results were confirmed by monitoring the surface conditions of a rehabilitated local road between 2017 and 2025 using MRAM, where some sections showed the properties of semi-bound layers after eight years.

The research results showed that MRAM becomes a valuable building material if it is processed using thermal, e.g., solar, energy and an appropriate compaction method, thereby “activating” its load-bearing capacity, similar to how heat is required to bond the particles together to form a cohesive, load-bearing structure.

The results of the research performed in the laboratory and in practical applications have led to the design of a systematic approach to the evaluation of recycled road pavement materials. Future studies should focus on optimizing this approach to ensure that all CE activities (including the use of MRAM) are performed cost-effectively and meet required quality standards.