Mechanical Performance of Sustainable Asphalt Mixtures Incorporating RAP and Panasqueira Mine Waste

Abstract

The increasing demand for sustainable practices in road construction has prompted the search for environmentally friendly and cost-effective materials. This study explores the incorporation of reclaimed asphalt pavement (RAP) and Panasqueira mine waste (greywacke aggregates) as full replacements for virgin aggregates in hot mix asphalt (HMA), aligning with the objectives of UN Sustainable Development Goal 9. Three asphalt mixtures were prepared: a reference mixture (MR) with granite aggregates, and two modified mixtures (M15 and M20) with 15% and 20% RAP, respectively. All mixtures were evaluated through Marshall stability, stiffness modulus, water sensitivity, and wheel tracking tests. The results demonstrated that mixtures containing RAP and mine waste met Portuguese specifications for surface courses. Specifically, the M20 mixture showed the highest stiffness modulus, improved moisture resistance, and the best performance against permanent deformation. These improvements are attributed to the presence of stiff aged binder in RAP and the mechanical characteristics of the greywacke aggregates. Overall, the findings confirm that the combined use of RAP and mining waste provides a technically viable and sustainable alternative for asphalt pavement construction, contributing to resource efficiency and circular economy goals.

1. Introduction

Industrial waste is a by-product generated from a variety of processes, including manufacturing, mining, energy production, and construction [1,2]. It can exist in solid, liquid, or gaseous forms, examples include ash, dust, slag, sludge, and various residual chemicals [3,4,5]. Recycled industrial materials have shown considerable potential as components in pavement construction, where they can improve the physic-mechanical properties of asphalt mixtures. However, due to transformations during processing and usage, these recovered materials often differ significantly from natural aggregates in terms of performance characteristics [6].

The use of industrial waste in asphalt mixtures offers a sustainable alternative to conventional raw materials such as virgin aggregates, bituminous binders, Portland cement, and hydraulic lime [7,8]. Numerous studies have demonstrated that these materials can enhance the mechanical behavior of bituminous mixtures, functioning as effective modifiers of performance-related properties [3,9,10].

Pavement construction and maintenance require vast quantities of natural resources, leading to environmental degradation and resource depletion. In response, many countries—including Canada—have promoted the adoption of recyclable materials in road infrastructure to reduce environmental impact [11]. Integrating recycled materials throughout the pavement lifecycle—from design and production to maintenance—supports the principles of the circular economy and aligns with the United Nations Sustainable Development Goal 9: “Build resilient infrastructure, promote inclusive and sustainable industrialization, and foster innovation” [12].

In recent decades, global efforts have increasingly embraced sustainable technologies in road construction. Recycling materials into asphalt mixtures helps reduce waste generation, reliance on landfills, energy consumption, and greenhouse gas emissions, directly contributing to climate change mitigation. For instance, Ref. [13] documented extensive adoption of pavement recycling practices across Europe (Belgium, Finland, Italy, the Netherlands, Portugal), Africa (Nigeria), Asia (Malaysia, Saudi Arabia), Oceania (Australia), and South America (Brazil, Colombia).

Reclaimed asphalt pavement (RAP) consists of previously used bituminous materials collected during road maintenance or resurfacing activities. While the binder and aggregates in RAP undergo aging, they still retain properties that allow for their reuse in new asphalt mixtures [14]. Recycling RAP not only offers a cost-effective and environmentally responsible alternative to virgin materials but also reduces the volume of construction waste and promotes sustainable material management [15,16,17,18].

The proportion of RAP incorporated into bituminous mixtures significantly affects their mechanical performance [19,20,21]. The Superpave Expert Task Group (1997) recommended RAP contents of up to 25% without modifying mixture characteristics. For RAP contents between 25% and 30%, a reduction by 6 °C in production temperature is advised, and for contents above 30%, further assessments are necessary to ensure the quality of the final mixture [22,23,24].

In the European context, the European Asphalt Pavement Association (EAPA) reported that 40.6 million tons of RAP were available across sixteen countries, with Germany leading, followed by Italy, France, the Czech Republic, and Spain [25]. This distribution is illustrated in Figure 1, which underscores the potential and urgency of incorporating RAP and other industrial by-products into pavement design as part of broader sustainability efforts.

Figure 2 shows the percentage of reclaimed asphalt available that was reused in the production of new asphalt mixes (including HMA, WMA, Half-WMA, and CMA), recycled into unbound road layers, AC14 surf, and other civil engineering applications, or used in unidentified applications or sent to landfills.

It is important to distinguish between the “reuse” and “recycling” of reclaimed asphalt pavement (RAP). Reuse refers to the direct incorporation of RAP into new asphalt mixtures without altering its original function, whereas recycling typically involves applying RAP in secondary uses where its original role is changed [25].

The first step in RAP utilization involves recovering the material from pavement milling or demolition. The RAP is then processed to remove impurities and sorted according to particle size and quality specifications. Crushing and fractionation reduce the aggregate size, promote homogeneity, and facilitate uniform distribution in the new asphalt mixture [26]. After processing, granulometric analysis is performed to ensure compliance with mixture design requirements and to guarantee the structural integrity of the final product [27,28].

Mining waste also represents a promising sustainable alternative for pavement construction. Generated during the extraction of resources such as coal, copper, iron, and tungsten, these materials, especially coarse-grained waste rock, can be reused with minimal physical or chemical treatment [29]. However, their application in road infrastructure requires prior characterization of mineralogical and mechanical properties, since these vary depending on the geological origin [2].

The Panasqueira mine, located in central Portugal, is one of Europe’s largest tungsten producers. It is situated within the Hesperian Massif, specifically in the “Schist–Greywacke Complex” of the Central Iberian Zone. The mine produces large quantities of greywacke waste rock with high angularity and siliceous mineral content—characteristics that may offer mechanical benefits when incorporated into asphalt mixtures [30,31,32].

This study seeks to contribute to sustainable pavement design by evaluating the combined use of RAP and Panasqueira mine waste as full replacements for natural aggregates in hot mix asphalt. Three asphalt mixtures were analyzed: a reference mixture (MR) using conventional granite aggregates, and two recycled mixtures (M15 and M20) incorporating greywacke from Panasqueira and 15% or 20% RAP, respectively. Laboratory tests evaluated density, air voids, Marshall stability, water sensitivity, stiffness modulus, and rutting resistance. The goal is to determine the feasibility and optimal design of a recycled mixture that meets technical standards while promoting environmental sustainability in road construction.

2. Materials and Methods

The experimental methodology followed a systematic approach to the characterization, design, and performance evaluation of asphalt mixtures incorporating recycled and mining waste materials. Initially, material properties were characterized in accordance with EN 933-1 [33]. Subsequently, asphalt mixture designs were developed, integrating varying percentages of Panasqueira greywacke aggregates, RAP, and hydraulic lime. The mechanical performance of the produced mixtures was assessed through a series of laboratory tests, including Marshall stability, water sensitivity, stiffness modulus, and permanent deformation resistance, ensuring compliance with Portuguese road specifications. This rigorous methodology aimed to determine the most effective design that guarantees long-term durability, strength, and environmental sustainability in pavement applications.

2.1. Materials

This study evaluated three asphalt mixtures: a reference mixture (MR) composed of conventional granite aggregates and two modified mixtures (M15 and M20) containing Panasqueira mine waste (greywacke aggregates) and RAP.

2.1.1. Bitumen

A 35/50 penetration grade bitumen (Cepsa Petróleos, S.A., Madrid, Spain) was used. n supplied by Cepsa Petróleos, S.A. was used. Suitable for course surface applications, this bitumen requires mixing temperatures between 162 °C and 166 °C and compaction temperatures between 152 °C and 156 °C. The bitumen was characterized through the penetration test (according EN 1426) [34] and the softening point test according to EN 1427 [35], yielding results of 46 × 10⁻¹ mm and 50.8 °C, respectively.

2.1.2. Reclaimed Asphalt Pavement (RAP)

The RAP used in this study was sourced from the surface course of the A23 highway located in Castelo Branco, Portugal, which had been in service for approximately 15 years under medium to heavy traffic conditions (national highway with consistent freight transport). The recovered material was milled from the upper asphalt layer during scheduled maintenance activities and consisted primarily of dense-graded hot mix asphalt (HMA) originally produced with 35/50 penetration grade bitumen.

The composition of the RAP includes natural siliceous and granite aggregates consistent with the original materials used during road construction. Laboratory characterization revealed an aged bitumen content of 5.6%, determined following the EN 12697-1 [36] standard. To evaluate the level of aging, penetration and softening point tests were conducted in accordance with EN 1426 [34] and EN 1427 [35] standards, yielding values of 11 × 10⁻¹ mm and 77.8 °C, respectively. These results confirm significant hardening of the binder due to long-term oxidative aging and exposure to environmental stressors such as UV radiation and thermal cycling. The particle size distribution of the processed RAP, after screening and crushing to eliminate oversized particles and contaminants, is presented in

Table 1. Gradation of natural aggregates, greywacke aggregates, and RAP.

Sieve Size [mm]	Cumulative Passing [%]
	Granite Aggregates		Greywacke Aggregates			RAP	Hydraulic Lime
	Stone Dust	Gravel 8/16	Stone Dust	Grave l2/10	Gravel 8/14
20	100	100	100	100	100	100	100
14	100	96	100	100	96	98	100
10	100	61	100	96	37	91	100
4	100	7	93	17	1	74	100
2	85	5	41	3	1	56	100
0.5	44	1	6	2	1	21	100
0.125	14	0	1	1	0	5	100
0.063	5	0	0	0	0	0	100

.

2.1.3. Natural and Waste Aggregates

Granite aggregates (stone dust and gravel 8/16) were used for the MR mixture. Greywacke aggregates from the Panasqueira Mine (stone dust, gravel 2/10, and gravel 8/14), as shown in Figure 3, were employed in M15 and M20 mixtures. The addition of hydraulic lime in all mixtures improved the adhesion between bitumen and aggregates, enhancing pavement durability and resistance to permanent deformation [37]. The gradation values represent cumulative percentages passing through each sieve size, determined according to EN 933-1 [33], and are presented in Table 1. The composition of the asphalt mixtures (% by weight in the asphalt mixtures) is presented in

Table 2. Asphalt mixture compositions [%].

Aggregate	MR	M15	M20
Natural Stone Dust	35	-	-
Natural Gravel 8/16	62	-	-
Greywacke stone dust	-	28	20
Greywacke Gravel 2/10	-	16	17
Greywacke Gravel 8/14	-	35	35
Hydraulic lime	3	6	8
RAP	-	15	20

and was defined considering the limits for a surface layer (AC14 Surf) specified by the Portuguese Road Administration [38].

Grading curves of different mixtures (MR, M15, and M20) and grading limits are shown in Figure 4.

2.2. Methods

This section describes an initial preliminary study to determine the optimal bitumen content using the Marshall test. Subsequently, several tests were conducted to analyze the physical and mechanical characteristics of the asphalt mixtures, including the stiffness test by indirect tension, water sensitivity test, and permanent deformation test.

2.2.1. Optimum Bitumen Content

The initial bitumen content (Pb), expressed as a percentage of the total aggregate weights, was calculated using the Asphalt Institute (1986) design method [39] (Equation (1)):

$$P_b=0.035 \times A+0.045 \times B+K \times C+F$$

(1)

where P_b is the initial bitumen content, relative to the total weight of the mixture (%); “A” represents the aggregates retained on the 2.36 mm sieve (%); “B” represents the aggregates passing through the 2.36 mm sieve and retained on the 0.075 mm sieve (%); “C” represents the aggregates passing through the 0.075 mm sieve (%); K is a constant depending on the amount of material passing through the 0.075 mm sieve, with the following values: K = 0.15 for 11–15%; K = 0.18 for 6–10%; K = 0.20 for ≤5%; F is the absorption factor of the aggregates, ranging from 0–2%. In the absence of specific data, F = 0.7% is assumed. This approach considers the gradation and absorption properties of the aggregates. The constant K = 0.18 was adopted, based on the percentage of fine particles (6–10%) present in all mixtures.

For mixtures incorporating RAP (M15 and M20), the aged binder content was also taken into account, and the amount of new binder (PbN) was adjusted accordingly using Equation (2) [40].

$$P{b}_N=Pb-{\displaystyle \frac{P{b}_{RAP}\times TR}{100}}$$

(2)

where Pb_N is the new bitumen content (%); Pb is the original bitumen content (%); Pb_RAP is the RAP bitumen content (aged bitumen) (%); TR is the recycling rate (%).

Following these calculations, M15 and M20 mixtures were produced with three different bitumen contents: 3.4%, 3.9%, and 4.4%. The optimum content was identified based on Marshall stability and porosity values, ensuring compliance with Portuguese specifications [38]. The reference mixture (MR) used a standard bitumen content of 5.2% for traditional surface courses.

All aggregates and RAP materials were pre-dried at 110 °C ± 5 °C for at least 24 h. Mixtures were prepared at 165 °C and compacted at 155 °C. A total of 28 cylindrical specimens (100 mm diameter) were produced, with four replicates for each bitumen content. The specimens were tested 36 h post-production after immersion in a 60 °C water bath for 50 min. Marshall stability and flow were measured following EN 12697-34 [41]. According to national road requirements [38], AC14 surf BB mixtures intended for surface courses must achieve Marshall stability values between 7.5 and 15 kN and flow values between 2- and 4-mm.

Table 3. Mechanical and volumetric properties of the different mixtures.

Bituminous Mixtures	Bitumen [%]	Bulk Density [kg/m3]	Marshall Stability [kN]	Marshall Flo [mm]	Marshall Quotient [kN/mm]	VMA [%]	Porosity [%]
MR	5.2	2340	17.1	3.1	5.6	16.9	5.1
M15	4.4	2448	13.7	4.3	3.2	15.3	4.8
	3.9	2430	14.2	3.0	4.8	16.6	6.4
	3.4	2393	12.1	2.9	4.2	18.6	8.4
M20	4.4	2464	12.6	2.7	4.6	14.2	3.7
	3.9	2450	13.1	3.3	4.0	15.5	5.0
	4.9	2475	11.7	3.7	3.2	13.1	2.5
Portuguese road requirements	-	-	7.5–15	2–4	>3	>14	3–5

summarizes the mechanical and volumetric properties of the MR, M15, and M20 mixtures that were evaluated according to EN 12697-34 [41], in line with Portuguese specifications for surface course mixtures (AC14 Surf BB). Bulk density was determined according to EN 12697-6 [42]. Procedure B: Bulk density—saturated surface dry (SSD). Based on the results, the optimal bitumen content for M15 and M20 was deter-mined to be 4.4%.

2.2.2. Stiffness Modulus

The stiffness modulus was determined in accordance with EN 12697-26 [43] (Annex C), using indirect tensile testing on cylindrical specimens with optimal bitumen content. Five specimens per mixture were tested along two perpendicular diameters using the Nottingham Asphalt Tester (NAT). The results are shown in Table 3.

Tests were conducted at 20 °C with a Poisson’s ratio of 0.35, a load rise time of 124 milliseconds, and a maximum horizontal deformation of 5 μm. A series of preloading cycles preceded the main loading. The modulus was calculated using Equation (3):

$$S_m={\displaystyle \frac{F\times (\vartheta +0.27)}{(z\times h)}}$$

(3)

where $S_m$ is the measured stiffness modulus (MPa); F is the peak value of the applied vertical load (N); z is the amplitude of the horizontal deformation obtained during the load cycle (mm); h is the mean thickness of the specimen (mm), and ϑ is the Poisson’s ratio.

2.2.3. Water Sensitivity

Water sensitivity was evaluated using the indirect tensile strength ratio (ITSR), following EN 12697-12 [44]. Thirty specimens were produced: ten for each mixture (MR, M15, M20), all compacted with their respective optimum bitumen contents.

Specimens were divided into two groups: one stored dry at 20 °C and another subjected to vacuum saturation and immersion at 40 °C for 72 h, followed by conditioning at 15 °C. Dimensional stability was confirmed to ensure no specimen exceeded a 2% volume increase. The ITS was then measured using diametral compression, according to EN 12697-23 [45], at a loading rate of 50 mm/min. ITSR was calculated by comparing the wet and dry ITS values using Equation (4):

$$ITSR={\displaystyle \frac{{ITS}_W}{{ITS}_D}}\times 100$$

(4)

where ITSR is the indirect tensile strength ratio (%); ITS_w is the average indirect tensile strength of the wet group (kPa) and ITS_d is the average indirect tensile strength of the dry group (kPa).

2.2.4. Resistance to Permanent Deformation

The resistance to permanent deformation was assessed using the wheel tracking test according to EN 12697-22 [46] procedure B; prismatic slabs (30 × 30 × 4 cm) were produced for each mixture and tested at 60 °C under a wheel pressure of 600 ± 30 kPa.

The test concluded after 10,000 load cycles or upon reaching a 20 mm rut depth. Two parameters were evaluated: rut depth (RD) and wheel-tracking slope (WTS), the latter calculated between 5000 and 10,000 load cycles using Equation (5):

$$WTS={\displaystyle \frac{(d_{10000}-d_{5000})}{5}}$$

(5)

where WTS is the wheel-tracking slope (mm/10³ load cycles); d₅₀₀₀ and d₁₀₀₀₀ is the rut depth after 5000 load cycles and 10,000 load cycles (mm).

These indicators provide insight into the mixtures’ deformation behavior under repeated traffic loading and elevated temperatures.

3. Results

3.1. Stiffness Modulus

Table 4. Stiffness modulus (average of 5 specimens).

Asphalt Mixtures	Bitumen [%]	Stiffness Modulus [MPa]
MR	5.2	7195
M15	4.4	11,343
M20	4.4	11,739

presents the average stiffness modulus for the three asphalt mixtures evaluated, calculated from Equation (3). The reference mixture (MR) exhibited the lowest stiffness modulus (7195 MPa), indicating greater flexibility but reduced resistance to permanent deformation. In contrast, mixtures M15 and M20 demonstrated significantly higher stiffness values (11,343 MPa and 11,739 MPa, respectively), attributed to the presence of aged RAP binder, which exhibits lower penetration and elasticity. This increase in stiffness contributes to enhanced load-bearing capacity, particularly under repeated traffic loads.

3.2. Water Sensitivity

The water sensitivity test results are shown in Figure 5, which shows the indirect tensile strength (ITS) in dry and wet conditions, along with the corresponding indirect tensile strength rating (ITSR), calculated from Equation (4). As expected, dry specimens yielded higher ITS values due to the absence of moisture-induced damage. However, both M15 and M20 mixtures outperformed the reference mixture in ITS under all conditions. Specifically, M15 showed an 11.9% increase in ITS (dry) and a 17.6% increase (wet), while M20 recorded gains of 9.7% (dry) and 24.0% (wet). These improvements indicate superior adhesion and cohesive strength in the presence of RAP, enhancing the mixtures’ resistance to moisture damage and contributing to long-term durability.

3.3. Resistance to Permanent Deformation

The wheel-tracking test results are summarized in

Table 5. Wheel-tracking test results (average of 2 specimens).

Asphalt Mixtures	Bitumen [%]	RD [mm]	WTS [mm/103 Cycles]
MR	5.2	5.9	0.31
M15	4.4	9.6	0.80
M20	4.4	3.3	0.19

, WTS was calculated from Equation (5). The M20 mixture achieved the best performance, with the lowest rut depth (3.3 mm) and the smallest wheel tracking slope (0.19 mm/10³ cycles). Compared to MR (5.9 mm rut depth) and M15 (9.6 mm), M20 exhibited 44% and 65% improvements, respectively. This enhanced rutting resistance is attributed to the increased RAP content, which introduces a higher amount of stiff aged binder, effectively improving resistance to plastic deformation. Additionally, the wheel tracking slope values indicate that M20 sustains less deformation over repeated cycles, confirming its robustness under high-temperature and heavy-load conditions.

Collectively, these findings demonstrate that incorporating Panasqueira mine waste and RAP not only maintains compliance with national specifications but also improves key mechanical properties. In particular, the M20 mixture presents a highly durable, moisture-resistant, and deformation-tolerant solution suitable for sustainable road surface applications.

4. Discussion

The experimental findings of this study indicate that asphalt mixtures incorporating RAP and Panasqueira mine waste can perform as well as—or even outperform—conventional mixtures in key mechanical parameters. These results are consistent with previous studies highlighting the advantages of integrating industrial by-products and reclaimed materials in pavement construction to enhance both performance and sustainability [16,23,29,47,48].

The significant increase in stiffness modulus observed in the M15 and M20 mixtures is primarily attributed to the presence of aged binder within the RAP, which undergoes oxidative hardening, resulting in higher stiffness and reduced ductility. Similar behavior was reported by Khan et al. [17], who observed that aged binders enhance rutting resistance in asphalt mixtures by contributing to the structural rigidity of the pavement. Hashim et al. [49] reported that high RAP mixtures rejuvenated with waste oils showed improved rutting resistance due to the stiff binder, matching our results. In our study, the M20 mixture achieved the highest modulus, indicating an optimal balance between rigidity and workability suitable for medium to heavy traffic loads.

Water sensitivity, assessed through the indirect tensile strength ratio (ITSR), improved in the RAP-modified mixtures. This improvement is likely due to the combined effects of hydraulic lime and aged binder coatings, which promote stronger adhesion between aggregates and bitumen. Ziari et al. [50] reported that adding RAP to surface treatments improves rutting and moisture resistance, aligning with our findings.

Wheel-tracking tests showed that the M20 mix offered superior rutting resistance. Similarly, Zhuang et al. [51] reported that mixtures with RAP levels ≥ 25% exhibited reduced rut depth (up to 60%) due to binder stiffness and increased aggregate interlock.

Mechanistically, these improvements result from the following synergistic interactions: (i) structural stiffness from RAP, (ii) moisture resistance from hydraulic lime, and (iii) excellent aggregate interlock from Panasqueira greywacke. Together, they form a dense, cohesive matrix, as confirmed by reduced porosity in M20 mixtures.

The results comply with Portuguese national standards and also corroborate findings from previous works using 100% RAP [16] and mining waste [2]. Nevertheless, caution is advised, as studies note decreased moisture and low-temperature resistance at very high RAP contents [52].

Further studies should investigate the long-term aging performance and fatigue resistance of these mixtures under field conditions. Evaluating environmental impact through life cycle assessment (LCA) and exploring the use of rejuvenators for high RAP contents could also expand the practical applicability of these sustainable formulations.

5. Conclusions and Recommendations

This study assessed the performance of asphalt mixtures in which Panasqueira mine waste and reclaimed asphalt pavement (RAP) completely replaced natural aggregates. The mixtures were evaluated against Portuguese national standards for surface layers (AC14 surf BB) and EN 12,697 series test methods, including stiffness modulus, water sensitivity, and rutting resistance. The experimental findings confirmed that all modified mixtures—particularly M20 with 20% RAP—met or exceeded these specifications.

The incorporation of RAP significantly increased the stiffness modulus, attributed to the aged bitumen’s lower penetration and greater rigidity. M20 achieved the highest stiffness value (11,739 MPa), indicating its suitability for medium to heavy traffic loads. This is consistent with findings from Hashim et al. [49] and Khan et al. [17], who reported improved structural stiffness in mixtures with moderate RAP content.

In terms of water sensitivity, RAP-modified mixtures outperformed the reference mixture. M20 showed excellent indirect tensile strength under both dry and wet conditions, with ITSR values exceeding the 80% threshold required by specifications. The literature also supports this behavior, as Zhang et al. [10] observed similar enhancements in moisture resistance in mixtures combining RAP and industrial by-products.

The wheel-tracking test demonstrated that the M20 mixture had the lowest rut depth and slope, suggesting better permanent deformation resistance than the reference mix. This is in line with studies such as Zhuang et al. [51], who reported significant rutting resistance in RAP-rich mixtures due to increased binder stiffness and aggregate interlock.

Overall, the positive elements of this research include technical compliance with national and European standards, improved mechanical performance, and the environmental benefit of incorporating recycled and local industrial materials. The synergy between RAP, Panasqueira greywacke aggregates, and hydraulic lime resulted in dense, cohesive, and high-performance asphalt mixtures that align with sustainable construction goals.

However, one limitation is that the study was restricted to a maximum RAP content of 20%. Although no adverse effects were observed at this level, existing literature warns of potential brittleness and reduced durability at higher RAP contents. Therefore, further studies are recommended to assess fatigue performance, long-term aging, and environmental impact using life cycle assessment (LCA). Future research should also consider the inclusion of rejuvenators to enable the safer use of higher RAP percentages.

In conclusion, the use of RAP and Panasqueira mine waste in asphalt mixtures is a technically viable, environmentally responsible, and standards-compliant approach that contributes to more sustainable and resource-efficient pavement construction.