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Article

Reclaimed Asphalt Pavement from BBTM Mixes as a Component in Soil–Cement: Preliminary Effects on Density, Moisture and Unconfined Compressive Strength

by
Alaitz Linares-Unamunzaga
1,
Hernán Gonzalo-Orden
1,*,
Ángel Aragón-Torre
1 and
Heriberto Pérez-Acebo
2
1
Department of Civil Engineering, University of Burgos, c/Villadiego, s/n, 09001 Burgos, Spain
2
Mechanical Engineering Department, University of the Basque Country UPV/EHU, Paseo Rafael Moreno Pitxitxi, 2, 48013 Bilbao, Spain
*
Author to whom correspondence should be addressed.
Appl. Sci. 2026, 16(18), 9289; https://doi.org/10.3390/app16189289
Submission received: 22 July 2026 / Revised: 8 September 2026 / Accepted: 16 September 2026 / Published: 19 September 2026
(This article belongs to the Special Issue Recent Advances in Sustainable Construction Materials and Structures, 2nd Edition)
Browse Figures
Versions Notes

Abstract

Current Spanish road specifications, in accordance with circular economy strategies, allow reclaimed asphalt pavement (RAP) to be used in hot-mix asphalt production. RAP is produced by milling asphalt layers during pavement rehabilitation activities. The percentage of permitted RAP depends on the pavement layer and expected traffic conditions. However, current regulations do not authorize its use in the base or sub-base layers of flexible pavements with unbound materials or in semi-rigid pavements, such as soil–cement or gravel–cement layers. This research evaluates the influence of replacing natural aggregates with RAP obtained from a BBTM surface mixture (gap-graded asphalt mixture) in the manufacture of soil–cement for semi-rigid pavements. This study focuses on the effects of RAP incorporation on the maximum dry density and optimum moisture content determined through the modified Proctor test, as well as on the unconfined compressive strength (UCS). Four mixtures were analyzed: a reference mixture containing only natural aggregates and three additional mixtures in which 25%, 50%, and 75% of the aggregate was substituted with BBTM-RAP. In total, 3.5% of cement was used in each mixture. Modified Proctor and 7-day UCS tests were subsequently performed to assess the mechanical behavior of the mixtures. The results showed that increasing the RAP content reduced the maximum dry density while increasing the water demand required for compaction. Higher RAP percentages also led to lower compressive strength values. Nevertheless, the average unconfined compressive strength (UCS) at 7 days for all mixture formulations exceeded 2.5 MPa. These findings support the ongoing investigation into the potential application of BBTM-RAP as a more sustainable alternative material for semi-rigid pavement base layers.

1. Introduction

The management of bituminous material derived from milling, specifically reclaimed asphalt pavement (RAP), poses a considerable environmental challenge owing to its incomplete reuse, which leads to its accumulation at authorized waste disposal sites. For example, in the Basque Country (Spain), with an area of 7234 km2 and a population of 2.2 million inhabitants, more than 1.8 million tons of construction and demolition waste were produced, of which 234.690 metric tonnes were related to bituminous mixtures [1]. Milled pavements are generally marked by significant cracking and have nearly reached the end of their service lives. This condition often results from enduring substantial heavy traffic over extended periods or complications related to the drainage and plasticity of subgrade soils.
In recent years, the increased awareness of the need to conserve natural resources has driven researchers to seek alternative solutions, such as incorporating RAP from milling into pavement as a partial replacement for natural aggregate. This strategy not only alleviates the excessive extraction of materials from quarries but also addresses the issue of construction and demolition waste by repurposing it, thereby fostering sustainable development in the construction industry.
The density of bituminous waste produced during the milling of a deteriorated road surface is generally lower than that of natural aggregates. The mineral composition of this residue is affected by the grain size used in the manufacture of the asphalt concrete on the road surface, which is currently being milled. It also depends on the milling process, including speed and depth.
RAP waste is commonly used in the production of hot and warm mix asphalt, as well as cold mixtures [2,3,4,5,6,7,8,9,10,11]. It also serves as a substitute for natural aggregates in untreated layers of flexible pavements, along with aggregates obtained, for example, from construction and demolition waste (C&DW), quarry waste, and electric furnace slag, and as a substitute for natural aggregates in full-depth reclamation (FDR) processes [12,13,14,15,16,17,18,19].
In FDR, the inclusion of RAP in the mixture is influenced by the thickness of the bituminous layer, generally comprising less than 50% of the total aggregate content [20]. The Spanish Instruction 6.1-IC for road pavement design [21], from 2003, explicitly forbade the use of bituminous material obtained from pavement milling for any material type. The corresponding Spanish Ministry, in a bid to advance the adoption of recycled road pavement materials, published FOM/2523/2014 in 2015, a legislative update to Article 542 of PG-3 2014 [22] that authorizes the use of RAP in designated scenarios and specified proportions for constructing base and binder layers in bituminous mixtures.
In 2023, Order 1/2023 [23] was promulgated to amend the specifications concerning the thicknesses and varieties of hot and semi-hot bituminous mixtures, as detailed in Instruction 6.1. IC [21]. This order expands the permissible applications of RAP by specifying its use in projects such as road layout conditioning, widening, platform enhancement, and lane expansion using the FDR technique, making it comparable to soil–cement or gravel–cement techniques. Although FDR is a recycling technique that combines, in situ, all the asphalt pavement sections and a previously established amount of the underlying untreated base material with a stabilizer, generally Portland cement [20,24,25], it is important to note that this order [23] does not explicitly refer to the reuse of RAP in soil–cement or other cement-treated base materials manufactured in plants [26,27].
Given the current capability to incorporate RAP waste material into both bituminous mixtures and materials with hydraulic binders, such as in FDR, it is of significant interest to investigate the feasibility of utilizing RAP in cement-treated base layers, particularly in soil–cement applications produced in fixed plants.
In the production of a cement-treated layer where natural aggregates are replaced with reclaimed asphalt pavement (RAP), numerous studies have emphasized the necessity of controlling key variables to maintain quality. These variables include the maximum dry density (MDD), optimum moisture content (OMC), and unconfined compressive strength (UCS) assessed at seven days. These parameters fluctuate based on the proportions of cement and RAP used. Focusing on the effects of increasing the percentage of added cement, Ghanizadeh et al. [14], Yuan et al. [28], and Pratikso et al. [29] indicated that it increases the maximum dry density (MDD). Similarly, Ghanizadeh et al. [14], and Pratikso et al. [29] reported that the optimum moisture content (OMC) decreases with an increase in the cement content. Furthermore, there is a consensus regarding the unconfined compressive strength (UCS) at 7 days, which is observed to increase with a higher cement percentage, as evidenced by Ghanizadeh et al. [14], Yuan et al. [28], Pratikso et al. [29], Kasu et al. [30], Mohanty et al. [31], Arshad [32], Fedrigo et al. [33], and Goldoni et al. [34].
Regarding the effects of increasing the percentage of RAP added as a substitute for natural aggregate, Yuan et al. [28], Mohanty et al. [31], Euch Khay et al. [35], and Adresi et al. [36] showed that it reduces the maximum dry density (MDD). In terms of optimum moisture content (OMC), Adresi et al. [36] reported that a higher percentage of RAP leads to an increase in the OMC. Furthermore, studies by Yuan et al. [28], Euch Khay et al. [35], Kasu et al. [30], Mohanty et al. [31], Arshad [32], Fedrigo et al. [33], Goldoni et al. [34], and Memon et al. [37] indicate that an increased percentage of RAP results in a decrease in unconfined compressive strength (UCS) at 7 days.
The studies analyzed mainly focused on the use of reclaimed asphalt pavement (RAP) from conventional dense-graded hot-mix asphalt (HMA) or did not indicate it directly [14,28,29,30,31,32,33,34,35,36]. No specific studies focused on RAP obtained from gap-graded asphalt mixture have been found.
The company Asfaltos Uribe, S.A., located in Bilbao (Spain), aiming to evaluate the feasibility of incorporating RAP from a BBTM surface layer (a bituminous mixture with discontinuous granulometry) in soil–cement produced in concrete plants, proposed to the University of the Basque Country (UPV/EHU) and to the University of Burgos (UBU) to conduct the project ARASC (Analysis about employment of Reclaimed Asphalt in Soil-Cement), granted by the Provincial Council of Bizkaia (Spain).
As shown in Figure 1, the continuous gradation of a conventional wearing course HMA, such as an AC 16 S (asphalt concrete with a maximum aggregate size of 16 mm), defined in article 542.3 of the PG-3 [22], differs from the gradation of a BBTM 11A (gap-graded asphalt with a maximum aggregate size of 11 mm), defined in article 543.3 of the PG-3 [22].
Based on the results of the literature review, where the reclaimed asphalt pavement (RAP) primarily came from conventional hot-mix asphalt (HMA) with dense grading, a laboratory study was conducted to assess the impact of incorporating different amounts of BBTM-RAP on the maximum dry density, water demand (optimal moisture content), and compressive strength. BBTM mixtures are relatively recent compared to HMA mixtures and their use has been expanding since the late twentieth century in parallel with the development of polymer-modified bitumens. BBTM mixtures address the need to restore wearing courses with minimal thickness, providing excellent surface macrotexture and skid resistance without overloading the structure or excessively raising the grades. Recognizing the established correlation between density and compressive strength in cement-treated materials, this study investigated the influence of BBTM-RAP added to the mixture on the UCS at 7 days. This period is crucial as it corresponds to the regulatory standards in Spain and in the Basque Country for the validation of soil–cement layer construction [38,39].
To achieve these objectives, the maximum dry density (MDD), optimum moisture content (OMC), and unconfined compressive strength (UCS) measured after seven days were analyzed on four distinct samples: a control sample consisting entirely of natural aggregate, and three additional samples in which 25%, 50%, and 75% of the natural aggregate was substituted with BBTM-RAP. A total of 3.5% cement was added to each sample.
The overall objective is to advance the circular economy by incorporating the maximum feasible amount of waste into the construction of new road infrastructure.

2. Materials and Methods

The following materials provided by Asfaltos Uribe’s (Bilbao, Spain) were used to manufacture the cylindrical specimens required for both the control sample and the three proposed mixtures.

2.1. Materials

2.1.1. Natural Aggregate (NA)

Figure 2a illustrates the NA, which is composed of gray limestone extracted from Amantegi quarries in Mañaria (Bizkaia, Spain). This sample was classified as type AGT0-40C, falling within the SC-40 (soil–cement with a maximum grain size of 40 mm) granulometric range and with sand equivalent values [40] of 45.
Based on the material characterization tests conducted, including particle size distribution [41], Atterberg limits [42], sulfates [43] and organic material assessments [44], it can be concluded that the natural aggregates are classified as sandy gravel with no plasticity, containing non-plastic fines devoid of organic material or sulfates. Prior to utilization, the material with a maximum particle size of 40 mm was distributed to expedite moisture evaporation. Once desiccated, it was mixed, homogenized, and subsequently stored in one-cubic-meter bags.

2.1.2. Reclaimed Asphalt Pavement (RAP)

Figure 2b illustrates the BBTM-RAP, which was sourced from the milling of the surface layer, with a thickness of 3 cm, of the A-8 freeway in the Malmasín tunnel, in the direction of Donostia-San Sebastián, near Bilbao (Bizkaia, Spain). This surface layer comprised a gap-graded ultra-thin asphalt wearing course (BBTM 11A) with a modified bitumen (PMB 45/85-65), characterized by a density of 2.35 g/cm3, a minimum bitumen content of 5.2% and sand equivalent values [40] of 88. The BBTM-RAP lacks organic material [44] and plasticity [42], and it has been observed that the bitumen coating on the aggregates appears to render them waterproof and imparts a degree of water repellency.
Particle density and water absorption tests were conducted in accordance with the UNE-EN 1097-6 standard [45], yielding a particle density of 2.562 Mg/m3 and a water absorption value of 0.7%. Although no specific testing was performed to evaluate the current condition of the asphalt mixture, it appeared visually sound with no apparent signs of distress. Regarding the morphology of the material, Figure 3 illustrates that following the recycling process, two distinct aggregate clusters can be identified: on one hand, the constituent crushed ophitic aggregate coated with bitumen, which exhibits high sphericity, and on the other hand, larger elements (strictly below 40 mm) resulting from the agglomeration of the aforementioned particles. These latter elements display a more elongated shape. In both cases, the aggregates exhibit an angular morphology with sharp edges and high surface roughness.
Additionally, given the characteristics of the recycled pavement, the presence of fines is notably limited. The BBTM has a maximum nominal aggregate size of 11 mm, with a percentage of voids in the mixture between 4 and 18%. These mixtures are used as a wearing course with a thickness of 2 or 3 cm. Prior to its application, the material was spread out to expedite moisture evaporation.
Once adequately dried, it was mixed, homogenized, and subsequently stored in one-cubic-meter bags. To mitigate the effects of material variability arising from its inherent heterogeneity, a quartering procedure was performed prior to its utilization.

2.1.3. Cement

The CEM II B-M (V-L) 42.5 R cement utilized in this study is sourced from the Heidelberg Materials plant in Arrigorriaga, Spain. The material’s environmental profile is documented through an International EPD System declaration (EPD-IES-0015645) [46], ensuring full transparency in compliance with the ISO 14025 standard [47].
According to the manufacturer, this cement is classified as a composite Portland cement, characterized by its high final mechanical strength, rapid hardening properties, and reduced CO2 emissions, which are 30% lower than those of standard cement. Additionally, it incorporates a minimum of 20% recycled material.
The cement utilized complies with all the specifications outlined in standard EN-197-1:2011 [48], considering parameters such as initial compressive strength, setting time, stability, and chloride content.

2.2. Methods

Chapter 513 of PG-3 [38], which focuses on layers of cement-treated materials, delineates the required characteristics for soil–cement concerning grain size, contingent upon traffic type, as well as cement content and compressive strength. This chapter further specifies the utilization of two types—SC-40 and SC-20—with maximum sizes of 40 mm and 20 mm, respectively. SC-40 can be employed in any heavy traffic category, including the highest one, so it was selected as the material to test. Additionally, the chapter stipulates a minimum cement content of 3% to ensure the homogeneity of the mixture. Furthermore, the reference density must be determined based on the modified Proctor test [49]. Additionally, it is essential to achieve a UCS at 7 days value between 2.5 and 4.5 MPa for the mixture to be deemed suitable in Spain [38] and values over 4.5 MPa (for the type of cement used in this research) if the roads are built using the Basque Country regulations [39]. Such high resistance values mean that it is necessary to pre-crack the soil–cement every 2–3 m. This distance will depend on the heavy traffic category, climatic zone, and thickness of the layers placed above [39].
The aim of this study is to examine how the integration of a BBTM-RAP affects the maximum modified Proctor density and water content required to achieve this density and the UCS values of cylindrical specimens at 7 days considering the impact of bitumen present in the aggregates. To this end, modified Proctor compaction tests [49] and UCS tests at 7 days [50] were conducted on a control sample consisting entirely of natural aggregate, as well as on three additional samples in which the natural aggregate (NA) was substituted with 25%, 50%, and 75% BBTM-RAP, respectively. The formulation of mixtures requires the incorporation of a specified quantity of cement. According to Spanish regulations, the minimum cement dosage for soil–cement is 3% (Article 513 of PG-3 [38]). To prevent potential issues and ensure consistent results, the authors selected a cement content of 3.5% throughout the study to evaluate the influence of bitumen within the mixture.
To achieve the desired objective, a series of tests were conducted to ascertain the suitability of the material for SC-40 [38]. Granulometric analyses were performed in accordance with UNE-EN ISO 17892-4 [41]. The Atterberg limits were determined according to UNE-EN ISO 17892-12 [42]. The organic material content was assessed in accordance with UNE-103204 [44]. Soluble sulfate content was evaluated in accordance with UNE-103201 [43]. Upon confirming the validity of the base materials, compaction tests were executed on four distinct formulations: 100% NA, 75% NA–25% RAP, 50% NA–50% RAP, and 25% NA–75% RAP. For these tests, the dry material was systematically divided into five uniform portions, each of which was supplemented with a moisture content ranging from 3% to 7%. Finally, for each mixture, six UCS specimens were tested at 7 days [50] to evaluate the impact of RAP incorporation on its strength.
The specimens were prepared in accordance with UNE-EN 13286-41 [50]. Given that the maximum particle size of the aggregates was 40 mm, cylindrical specimens with a diameter of 150 mm and a height of 180 mm were utilized.
For each mixture, two independent batches consisting of three specimens were prepared, using an initial dry mass of 28 kg and incorporating the respective BBTM-RAP replacement ratios (0%, 25%, 50%, and 75%).
For each batch, the oven-dried constituents were introduced into a concrete mixer and blended for 2 min. Subsequently, 3.5% cement was added, and the mixing process continued for an additional 2 min. Finally, the optimum water content determined from the modified Proctor test for the corresponding mixture was introduced, followed by a final 4 min mixing cycle. The resulting wet mixture was immediately used for specimen fabrication.
The three specimens comprising each batch were manufactured within 1 h to prevent the initiation of cement setting. Each specimen was compacted in three successive layers. Each layer was compacted for 10, 15, or 20 s using a Kango electric hammer operating at 1950 rpm. The target density was established at 100% of the modified Proctor (MP) maximum dry density, with a minimum acceptable threshold of 98% [38]. The compaction time for each layer was adjusted to achieve the specified target density.
Upon compaction of the three layers, the upper surface of each specimen was capped with a cement–fines mixture formulated using fines extracted from the corresponding material. This procedure was implemented to ensure a sufficiently plane and level surface, thereby guaranteeing uniform contact with the loading platens of the testing machine.
Following fabrication, the specimens were stored in a curing room maintained at 20 ± 2 °C and a relative humidity (RH) of ≥95% [51].
The specimens were demolded after 24 h and remained inside the curing room. Based on prior experimental evidence and the strictly controlled environmental conditions of the curing room, no additional sealing treatment was deemed necessary. The specimens were subsequently kept in the curing room until testing. The UCS tests were performed at a constant loading rate of 1470 N/s [50]. To prevent excessive disintegration of specimens post-peak strength, the test was programmed to terminate automatically upon detecting a 5% reduction from the maximum recorded stress.

3. Results

As shown in Figure 4 and Table 1, granulometric testing (100% NA, 75% NA–25% RAP, 50% NA–50% RAP, and 25% NA–75% RAP) verified that the four proposed mixtures were aligned with the SC-40 range [38]. The materials utilized in the four proposed mixtures were devoid of plasticity and plastic fines [40,42]. The examined samples were also free of organic matter and sulfates [43,44].
To achieve a minimum compaction level of 98% of the modified Proctor density [38], it is essential to ascertain the maximum dry density (MDD) obtained from laboratory testing [49] and the optimal moisture content (OMC) necessary to attain this density. Consequently, the four proposed mixtures were analyzed separately.
Figure 4 also illustrates that the control sample, composed entirely of natural aggregate (100% NA) and devoid of any BBTM-RAP, was more centrally positioned within the SC-40 granulometric range than the other mixtures.

3.1. Determination of Maximum Dry Density and Optimal Moisture Content

To calculate the maximum dry density and optimal moisture content in the four proposed mixtures (100% NA, 75% NA–25% RAP, 50% NA–50% RAP, and 25% NA–75% RAP), compaction tests were conducted using the modified Proctor method [49]. The results are presented in Figure 5.

3.1.1. Determination of Maximum Dry Density and Optimal Moisture Content for a Mixture Consisting Entirely of Natural Aggregate (NA)

Compaction tests for a 100% NA mixture were conducted using the modified Proctor method in accordance with the standard UNE-EN 13286-2 [49], employing five moisture levels of 3%, 4%, 5%, 5.5%, and 6%. This test was performed on the control mixture, which consisted entirely of natural aggregates, without the incorporation of reclaimed asphalt pavement, as a normal soil–cement. Figure 5 presents a graphical representation of the results, indicating a maximum dry density of 2.36 g/cm3 and an optimum moisture content of 5.0%.

3.1.2. Determination of Maximum Density and Optimal Moisture Content for a 75% Natural Aggregate (NA)–25% Reclaimed Asphalt Pavement (RAP) Mixture

In light of the observations made in Section 2 and according to Adresi et al. [36], higher water percentages (4.5%, 5%, 5.5%, 6.5%, and 7%) were proposed to calculate the optimum moisture content for the 75% NA–25% RAP mixture. As illustrated in Figure 5, using the modified Proctor Compaction Test [49], the maximum dry density for this scenario attained a value of 2.30 g/cm3, with an optimum moisture content of 5.3%.

3.1.3. Determination of Maximum Density and Optimal Moisture Content for a 50% Natural Aggregate (NA)–50% Reclaimed Asphalt Pavement (RAP) Mixture

Based on the results in Section 3.1.2 and considering the findings of Adresi et al. [36], the last two water percentages were reduced. In this case, the percentages studied were 4.5%, 5%, 5.5%, 6%, and 6.5%. As shown in Figure 5, the maximum dry density for this scenario reached 2.23 g/cm3, with an optimum water content of 5.5%.

3.1.4. Determination of Maximum Density and Optimal Moisture Content for a 25% Natural Aggregate (NA)–75% Reclaimed Asphalt Pavement (RAP) Mixture

Drawing from the results outlined in the preceding sections, this analysis focuses on moisture levels of 5%, 5.5%, 6%, 6.5%, and 7%. Figure 5 illustrates that a maximum density of 2.13 g/cm3 and an optimal moisture content of 6.1% were achieved in this scenario.
As seen in Figure 5, an increase in the amount of BBTM-RAP results in a decrease in the maximum dry density, whereas the optimum moisture content increases with the addition of larger quantities of BBTM-RAP to the soil–cement.
Additionally, Table 2 details the moisture content and dry density for each of the five calculated points for each mixture, utilizing the modified Proctor method [49]. Furthermore, the OMC and MDD values for each of the four mixtures are specified. The optimum moisture content increased from 5.0% without BBTM-RAP to 6.1% with the incorporation of 75% BBTTM-RAP. Conversely, the maximum dry density decreased from 2.36 g/cm3 at 0% BBTM-RAP to 2.13 g/cm3 with 75% BBTM-RAP and 25% natural aggregate.

3.2. Evaluation of the Unconfined Compressive Strength of the Four Mixtures Under Investigation

To evaluate the influence of different proportions of BBTM-RAP on compressive strength, unconfined compression tests were conducted on cylindrical specimens measuring 15 cm in diameter and 18 cm in height, in accordance with UNE-EN 13286-41, which specifies the test method for determining the compressive strength of hydraulically bound mixtures [50]. These tests were performed after 7 days.
Table 3 displays the unconfined compressive strength (UCS) values at 7 days for the six specimens prepared for each mixture configuration. It includes the mean UCS for the reference mixture (100% natural aggregate, NA) and for the other three mixtures: 75% NA–25% RAP, 50% NA–50% RAP, and 25% NA–75% RAP. The standard deviations of these values are also provided.
As shown, a significant decline in the UCS appears as the proportion of BBTM-RAP increased. The UCS of the reference mixture was 10.05 MPa. Substituting 25% of the natural aggregate with BBTM-RAP resulted in a 22% reduction in UCS, yielding a value of 7.85 MPa. A 50% replacement led to an approximate 52% decrease in strength, resulting in a UCS of 4.87 MPa. In the most extreme scenario studied, where 75% of the natural aggregate was replaced by BBTM-RAP, the UCS decreased by 72%, reaching 2.80 MPa.

4. Discussion

As illustrated in Figure 4, the materials employed conform to the specified particle size distribution, indicating that almost any combination thereof is appropriate for this study. Table 4 presents the changes in the variable values under examination, which were influenced by the percentage of asphalt recycling included in the mixtures. These changes are illustrated in Figure 6, Figure 7, Figure 8 and Figure 9.
By employing modeling techniques based on the obtained results and the percentage of RAP present in each mixture, it is feasible to derive the following relationships for each of the determining variables, specifically, the maximum dry density (MDD) and optimum moisture content (OMC). The models are illustrated in Figure 6:
MDD = −0.003 · BBTM–RAP + 2.369        R2 = 0.9857
OMC = 0.014 · BBTM–RAP + 4.95         R2 = 0.9459
where MDD is the maximum dry density (g/cm3), OMC is the optimum moisture content, expressed as the percentage of water to the dry material (in %), and BBTM-RAP is the percentage of BBTM-RAP in relation to the aggregates of the mixture, not considering the cement (in %).
The results with BBTM-RAP are consistent with the findings reported by Yuan et al. [28], Mohanty et al. [31], Euch Khay et al. [35], and Adresi et al. [36] that showed that the increased presence of RAP reduces the maximum dry density (MDD). Kasu et al. [30] indicated that the relation is not significant.
Adresi et al. [36] formulated an equation correlating MDD with the percentages of RAP and cement. To facilitate a comparison with Equation (1), we substituted the cement percentage with 3.5% and adjusted the units to ensure that both equations yielded the MDD in g/cm3. The adapted Adresi equation is expressed as follows:
MDD = −0.00224 · RAP + 2.470
Compared to Equation (1), this equation features an intercept that is one-tenth higher and a lower coefficient for the RAP percentage than BBTM-RAP, as shown in Figure 7. Although there is a small difference, the trend is similar in both cases: as the percentage of RAP increases, which contains binder that is less dense than the aggregates and less angular particles, the MDD decreases.
Furthermore, Adresi et al. [36] proposed an equation linking the OMC with the percentages of RAP and cement. For comparative purposes, the cement percentage in Adresi’s equation is set to 3.5 (as in this research), resulting in the following formula:
OMC = 0.015 · RAP + 5.99
Comparing Formula (4) with Formula (2), it is evident that the slope of the line is similar to the one (0.014) proposed in this study for BBTM-RAP. The primary distinction between the two equations is that the OMC line for the RAP used by Adresi et al. [36] is shifted one point higher than the results for the BBTM-RAP, as shown in Figure 8.
Regarding the effect of incorporating BBTM-RAP into the UCS, Figure 9 illustrates the mean 7-day UCS values for each of the six specimens tested within each analyzed mixture. Additionally, it presents the extreme values and standard deviation for each group.
As illustrated in Figure 9, the highest values, along with the greatest standard deviation, were observed in the control sample composed solely of natural aggregates. The incorporation of bitumen milling resulted in a notable reduction in the 7-day UCS values, contingent upon the percentage of addition. Nevertheless, these values exhibited greater stability, as indicated by the standard deviation measurement.
The results from BBTM-RAP align with the trends in strength loss reported Yuan et al. [28], Kasu et al. [30], Mohanty et al. [31], Arshad [32], Fedrigo et al. [33], Goldoni et al. [34] and Euch Khay et al. [35]. The relationship between the UCS at 7 days and the BBTM-RAP content is presented in Figure 10, and its mathematical representation can be found in Equation (5).
UCS = −0.0989 · BBTM–RAP + 10.102        R2 = 0.9952
Here, UCS is the unconfined compressive strength at 7 days (in MPa) and BBTM-RAP is the percentage of BBTM-RAP in relation to the aggregates of the mixture, not considering the cement (in %).
As illustrated in Figure 10, similar to the trend observed with density, the UCS diminishes as the proportion of RAP in the mixture increases. This reduction in strength appears to be solely attributable to the presence of RAP in the mixture and can be accurately predicted by the linear relationship described in Equation (5).
Consequently, when incorporating BBTM-RAP into the mixture, the inclusion of 25% and 50% presents preliminary favorable results under the Basque Country regulations (>4.5 Mpa) [39] although pre-crack will be necessary. Furthermore, even BBTM-RAP levels of approximately 70% may be deemed acceptable under Spanish regulatory standards (2.5–4.5 MPa), as they surpass the threshold of 2.5 MPa [38].
These results support the preliminary laboratory assessments. Subsequent research will advance in evaluating, among others, stiffness, tensile or flexural performance, shrinkage, cracking susceptibility, erosion resistance, fatigue, wet–dry durability, temperature sensitivity, moisture susceptibility, leaching, and scanning electron microscope (SEM) microscopic observation, to analyze the morphology of the interface transition zone, field compaction, nuclear magnetic resonance (NMR) observation, aggregate gradation, environmental impacts, or economic performance [16,17,18,19,20,24,25,26,27,28,29,30,31,32,33,34,35,36,37,52,53].

5. Conclusions

The management of reclaimed asphalt pavement (RAP) derived from milling processes is increasingly presenting an environmental challenge, as it is not being fully recycled. In response to this concern, the University of Burgos, the University of the Basque Country (UPV/EHU), and the asphalt company Asfaltos Uribe, S.A., conducted the project ARASC (Analysis of the Employment of Reclaimed Asphalt in Soil-Cement). The project aimed to examine the impact of incorporating bituminous material (BBTM-RAP), derived from the milling of the surface layers, into the base of semi-rigid pavements, with a particular focus on the soil–cement layer. To achieve this objective, modified Proctor compaction tests and unconfined compressive strength (UCS) tests were conducted on four distinct blend compositions: the reference mixture (100% natural aggregate, NA) and the other three mixtures (75% NA–25% RAP, 50% NA–50% RAP, and 25% NA–75% RAP), deemed suitable for forming a soil–cement (SC-40) with the addition of 3.5% cement.
In this study, aspects such as the following can be highlighted:
  • Large amounts of BBTM-RAP can be incorporated and used to replace quarry aggregate in the production of soil–cement without its unconfined compressive strength falling below the minimum established by the regulations.
  • The incorporation of this waste (RAP from a gap-graded asphalt mixture) provides economic value to the recycled material.
  • As the proportion of BBTM-RAP in the mixture increased, a greater volume of water was required to achieve the optimal compaction density, whereas the maximum density value decreased linearly. Two equations have been proposed to calculate the values if other percentages of BBTM-RAP are added.
  • The UCS for 7 days decreased linearly as the proportion of BBTM-RAP in the mixture increased. If other percentages of BBTM-RAP were added, an equation to calculate the UCS was provided.
  • The findings indicate that incorporating large amounts of BBTM-RAP content is feasible.
  • Consequently, these preliminary proposed solutions illustrate that the integration of BBTM-RAP into road infrastructure might be a viable strategy for advancing the circular economy. This approach facilitates the reuse of waste materials, reduces the consumption of virgin resources, and mitigates the overexploitation of quarries, thereby aligning with the objectives of sustainable development.

Author Contributions

Methodology, formal analysis and evaluation, A.L.-U. and H.G.-O.; investigation, A.L.-U., H.G.-O., Á.A.-T. and H.P.-A.; conceptualization, A.L.-U. and H.G.-O.; writing—original draft preparation A.L.-U. and H.G.-O.; supervision H.P.-A.; data curation, A.L.-U. and H.G.-O.; validation Á.A.-T. and H.P.-A.; funding acquisition, H.P.-A.; writing—review and editing A.L.-U., H.G.-O., Á.A.-T. and H.P.-A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Provincial Council of Biscay (Bizkaiko Foru Aldundia/Diputación Foral de Bizkaia), under grant number 5/12/V/2023/00005, ARASC project, of the grant program aimed at promoting innovation in road infrastructure 2023, and under grant 5/12/IV/2024/00005, ARASC 2 project, of the grant program aimed at promoting innovation in road infrastructure 2024. Additionally, financial support was provided through research contracts with Asfaltos Uribe, S.A., under contract numbers 2024.0037, 2024.0819, W58A06, W40B06, W25C06 and W57B06.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
NANatural aggregate from quarry
BBTMGap-graded ultra-thin asphalt wearing course
RAPReclaimed asphalt pavement from milling used as aggregate
BBTM-RAPRAP of a bituminous mixture with a discontinuous granulometry
MDDMaximum dry density (g/cm3)
OMCOptimum moisture content (%)
SCSoil–cement
UCSUnconfined compressive strength (MPa)
RHRelative humidity

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Applsci 16 09289 g001
Figure 1. Granulometric range of conventional AC 16 S dense-graded hot-mix asphalt (HMA) (article 542.2 of PG-3 [22]) and of BBTM 11A gap-graded asphalt mixture (article 543.2 of PG-3 [22]).
Figure 1. Granulometric range of conventional AC 16 S dense-graded hot-mix asphalt (HMA) (article 542.2 of PG-3 [22]) and of BBTM 11A gap-graded asphalt mixture (article 543.2 of PG-3 [22]).
Applsci 16 09289 g001
Applsci 16 09289 g002
Figure 2. Materials used as aggregates in the manufacture of the mixtures under study: (a) natural aggregate; (b) reclaimed asphalt pavement.
Figure 2. Materials used as aggregates in the manufacture of the mixtures under study: (a) natural aggregate; (b) reclaimed asphalt pavement.
Applsci 16 09289 g002
Applsci 16 09289 g003
Figure 3. Magnification of the material for morphological observation.
Figure 3. Magnification of the material for morphological observation.
Applsci 16 09289 g003
Applsci 16 09289 g004
Figure 4. Grain size curves of selected BBTM-RAP, SC-40 granulometric range, and different aggregate mixtures.
Figure 4. Grain size curves of selected BBTM-RAP, SC-40 granulometric range, and different aggregate mixtures.
Applsci 16 09289 g004
Applsci 16 09289 g005
Figure 5. Maximum dry density and optimum moisture content for 100% NA, 75% NA–25% RAP, 50% NA–50% RAP, and 25% NA–75% RAP.
Figure 5. Maximum dry density and optimum moisture content for 100% NA, 75% NA–25% RAP, 50% NA–50% RAP, and 25% NA–75% RAP.
Applsci 16 09289 g005
Applsci 16 09289 g006
Figure 6. Maximum dry density and optimum moisture content as a function of % RAP in the mixture.
Figure 6. Maximum dry density and optimum moisture content as a function of % RAP in the mixture.
Applsci 16 09289 g006
Applsci 16 09289 g007
Figure 7. Maximum dry density as a function of % BBTM-RAP and as a function of % RAP, as proposed by Adresi et al. [36] (considering 3.5% cement).
Figure 7. Maximum dry density as a function of % BBTM-RAP and as a function of % RAP, as proposed by Adresi et al. [36] (considering 3.5% cement).
Applsci 16 09289 g007
Applsci 16 09289 g008
Figure 8. Optimum moisture content as a function of % BBTM-RAP and as a function of % RAP, as proposed by Adresi et al. [36] (considering 3.5% cement).
Figure 8. Optimum moisture content as a function of % BBTM-RAP and as a function of % RAP, as proposed by Adresi et al. [36] (considering 3.5% cement).
Applsci 16 09289 g008
Applsci 16 09289 g009
Figure 9. Mean values and standard deviation for the UCS for 7 days as a function of % RAP in the mixture.
Figure 9. Mean values and standard deviation for the UCS for 7 days as a function of % RAP in the mixture.
Applsci 16 09289 g009
Applsci 16 09289 g010
Figure 10. UCS for 7 days as a function of % RAP in the mixture.
Figure 10. UCS for 7 days as a function of % RAP in the mixture.
Applsci 16 09289 g010
Table 1. Granulometric testing results for the analyzed mixtures and the SC-40 grading envelope.
Table 1. Granulometric testing results for the analyzed mixtures and the SC-40 grading envelope.
Sieve Size
(mm)		100% NA75% NA–25% RAP50% NA–50% RAP25% NA–75% RAP100% RAPSC-40 Upper LimitSC-40 Lower Limit
Passing Percent %
50100100100100100100100
401009910010010010080
25899194969910067
20848690929710062
12.5747680788510053
864616055578945
448433930286530
237322720185217
0.517151397375
0.06386532202
Table 2. Summary of MC, DD, MDD and OMC values for the four mixtures analyzed.
Table 2. Summary of MC, DD, MDD and OMC values for the four mixtures analyzed.
MixturesMoisture Content (MC) %Dry Density (DD
(g/cm3)		Optimum Moisture Content (OMC) %Maximum Dry Density (MDD)
(g/cm3)
100% Natural Aggregate3.68
4.19
4.43
5.14
5.67		2.245.02.36
2.30
2.35
2.34
2.33
75% Natural Aggregate–25% RAP4.36
4.88
5.51
6.11
6.80		2.215.32.30
2.29
2.27
2.23
2.21
50% Natural Aggregate–50% RAP3.56
4.1
4.67
5.49
6.14		2.055.52.23
2.1
2.22
2.23
2.21
25% Natural Aggregate–75% RAP4.65
5.16
5.39
6.22
6.84		2.036.12.13
2.08
2.09
2.13
2.08
Table 3. Summary of UCS test results for the four mixtures analyzed after 7 days.
Table 3. Summary of UCS test results for the four mixtures analyzed after 7 days.
MixturesDry Density (g/cm3)UCS7 (MPa)UCS7 (MPa) MeanStandard
Deviation
100% Natural Aggregate2.368.9010.051.42
8.98
8.67
10.31
11.77
11.69
75% Natural Aggregate–25% RAP2.307.667.850.39
7.79
7.27
7.81
8.36
8.21
50% Natural Aggregate–50% RAP2.235.604.870.41
4.84
4.96
4.90
4.45
4.50
25% Natural Aggregate–75% RAP2.132.772.800.60
3.79
3.23
2.48
2.24
2.30
Table 4. Summary of maximum dry densities, optimum moisture and average UCS at seven days for the four mixtures analyzed.
Table 4. Summary of maximum dry densities, optimum moisture and average UCS at seven days for the four mixtures analyzed.
MixturesMaximum Dry
Density (g/cm3)		Optimum Moisture Content %UCS7 (MPa)
100% Natural Aggregate2.365.010.05
75% NA–25% RAP2.305.37.85
50% NA–50% RAP2.235.54.87
25% NA–75% RAP2.136.12.80
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Linares-Unamunzaga, A.; Gonzalo-Orden, H.; Aragón-Torre, Á.; Pérez-Acebo, H. Reclaimed Asphalt Pavement from BBTM Mixes as a Component in Soil–Cement: Preliminary Effects on Density, Moisture and Unconfined Compressive Strength. Appl. Sci. 2026, 16, 9289. https://doi.org/10.3390/app16189289

AMA Style

Linares-Unamunzaga A, Gonzalo-Orden H, Aragón-Torre Á, Pérez-Acebo H. Reclaimed Asphalt Pavement from BBTM Mixes as a Component in Soil–Cement: Preliminary Effects on Density, Moisture and Unconfined Compressive Strength. Applied Sciences. 2026; 16(18):9289. https://doi.org/10.3390/app16189289

Chicago/Turabian Style

Linares-Unamunzaga, Alaitz, Hernán Gonzalo-Orden, Ángel Aragón-Torre, and Heriberto Pérez-Acebo. 2026. "Reclaimed Asphalt Pavement from BBTM Mixes as a Component in Soil–Cement: Preliminary Effects on Density, Moisture and Unconfined Compressive Strength" Applied Sciences 16, no. 18: 9289. https://doi.org/10.3390/app16189289

APA Style

Linares-Unamunzaga, A., Gonzalo-Orden, H., Aragón-Torre, Á., & Pérez-Acebo, H. (2026). Reclaimed Asphalt Pavement from BBTM Mixes as a Component in Soil–Cement: Preliminary Effects on Density, Moisture and Unconfined Compressive Strength. Applied Sciences, 16(18), 9289. https://doi.org/10.3390/app16189289

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