Analysis of the Viability of Manufacturing MASAI Mixtures at Ambient Temperature

Abstract

The main goal of this study is to evaluate the feasibility of designing high-performance MASAI mixtures produced at ambient temperature. For this purpose, the impacts of certain variables, such as the type and amount of asphalt emulsion and the use or non-use of RAP, on its performance are evaluated. Subsequently, its stiffness modulus, tensile strength, permanent deformation, and resistance to thermal cracking were evaluated and compared against a conventional dense-graded asphalt concrete (AC 16) and an open-graded (BBTM11B) hot-mix asphalt used for wearing courses. The results showed that these materials could represent more sustainable and good solutions for the rehabilitation of some types of pavements.

1. Introduction

In a global context marked by growing concerns regarding climate change, the depletion of natural resources, and the need to reduce pollutant emissions, civil engineering faces the challenge of developing more sustainable infrastructure. The first and most widely used flexible pavement is Hot-Mix Asphalt (HMA). HMA offers superior structural performance. Nevertheless, it consumes large amounts of raw virgin materials and requires high production temperatures (150–190 °C) [1], leading to environmental concerns. In this regard, sustainable pavements have emerged as key alternatives to mitigate the environmental impacts of conventional transportation networks. These pavements aim to optimize the use of recycled materials, reduce energy consumption during manufacturing and construction processes, and enhance the durability of road structures. Through the application of innovative technologies, the Laboratory of Construction Engineering of the University of Granada (LabIC.UGR) has created the conception of sustainable and smart asphalt materials, named MASAI (acronym of the Spanish words for Sustainable, Automated, and Intelligent Asphalt Material) mixtures, which are intended to make future asphalt pavements more sustainable and efficient [2]. MASAI mixtures are defined by the combination of the following characteristics:

(1)

High-performance asphalt materials.

(2)

Asphalt materials manufactured at a maximum temperature of 140 °C.

(3)

Asphalt materials modified with a minimum of 0.5% in weight of local reclaimed polymers (e.g., crumb rubber, polyethylene, etc.) using dry or wet processes.

(4)

Asphalt materials manufactured with a minimum of 20% in weight of Reclaimed Asphalt Pavement (RAP), or a minimum of 5% of other waste from other local industries.

(5)

Asphalt materials that incorporate sensors and/or other devices that send/receive information to offer new functions for improvement, such as road safety and traffic assessment.

Depending on the number of characteristics presented (1 and 2 are mandatory), a variety of grades of MASAI can be found. To obtain MASAI Grade I, it is necessary to comply with characteristics 1 and 2, and either 3 or 4. To obtain MASAI Grade II, it is necessary to comply with all four characteristics. Furthermore, if a MASAI (I) or MASAI (II) also presents characteristic 5, then it is classified with the grade ‘+’. Thus, we can have MASAI (I+) for those that present either 1, 2, 3, and 5, or 1, 2, 4, and 5, and MASAI (II+) for those that present all five characteristics (1, 2, 3, 4, and 5) [2].

In recent years, MASAI has been successfully used in the construction and rehabilitation of hundreds of kilometers within the Andalusian Road network (a region in Spain with the most kilometers of roads and highways), with over 800,000 tons implemented to date. This has enabled the recovery of more than 90,000 tons of waste materials from the demolition of deteriorated roads, as well as over 2500 tons of recycled polymers (including plastics and end-of-life tires) and the reduction of more than 48,000 tons of CO₂eq.

Nevertheless, LabIC.UGR continues to advance the development of more sustainable MASAI mixtures. To this end, this research study evaluates the feasibility of designing high-performance MASAI mixtures produced at ambient temperature (Cold-Mix Asphalt, CMA). CMA is typically produced by blending water, aggregates, mineral fillers, and asphalt emulsion. CMA does not require heating during mixing or laying, making it more environmentally friendly (reduced emissions, reduced fuel consumption, and better working conditions for laborers) compared to HMA (Hot-Mix Asphalt) and WMA (Warm-Mix Asphalt) [3,4,5,6,7]. CMA is also a beneficial material in regions characterized by significant distances between the construction site and the plant [8]. Apart from all these advantages, one of the main drawbacks compared to HMA and WMA is the lower mechanical resistance to heavy and repeated loads. Furthermore, moisture damage is one of the main problems associated with CMA [4,7,9]. Since water is used in the preparation of CMA, and the aggregates are also not heated, moisture affects the interaction between binder and aggregates, which results in reduced adhesion, which further creates various forms of CMA pavement distress, such as rutting, fatigue cracking, striping, etc. [4,7]. The curing time is also long, as it requires time for the evaporation of solvents or water, which delays the opening to traffic as compared to HMA and WMA, where roads can be opened for traffic just after construction. Additionally, difficulty in compaction, high air void content in the compacted mixtures, and poor initial strength could not be as efficient as in HMA and WMA, which would leave voids and reduce the pavement’s service life [10,11].

These drawbacks limit its use for structural layers or base courses in primary roadways and have commonly restricted it to low-traffic roads, rural road construction, car parks, walkways, sidewalks, or temporary pavements [3,7,12]. Considering this situation, and in line with MASAI mixtures, this study focuses on the analysis of the viability of designing sustainable CMA with enhanced mechanical performance. To this end, the main design variables that influence the mechanical behavior of CMA will be analyzed for obtaining a MASAI (I) manufactured at ambient temperature. These variables will be the type and amount of asphalt emulsion and the use or non-use of RAP. In this respect, some researchers have found that the use of RAP in CMA could improve its mechanical properties, reduce costs, and reduce consumption of natural resources [3,5,13,14,15,16,17,18]. Furthermore, the emulsion contents and types would directly affect the final performance of the designed MASAI (I).

2. Materials and Methods

2.1. Materials

The aggregates selected for the mineral skeleton of the MASAI (I) mixture (gravel and sand) were limestone An active filler (Portland cement) was used to achieve the high mechanical performance required by MASAI mixtures. Additionally, a 20% constituent of 0/6 mm RAP was incorporated to fulfill the characteristics of MASAI mixtures. The main properties of RAP are summarized in

Table 1. Analysis of the gradation of reclaimed asphalt pavement.

Sieve Size (mm)	31.5	22.4	16	12.5	11.2	8	4	2	0.5	0.25	0.063
(% Passing)	100	100	86	77	74	62	45	31	13	10	8.3

and

Table 2. Results of tests of asphalt extracted from reclaimed asphalt pavement.

Binder content RAP [%]	4.0
Penetration (25 °C) [0.1 mm]	8
Softening point [°C]	85

.

Two types of bituminous emulsions were selected as binders: an anionic type, with its base binder being a common bitumen (classified as A67BFM in accordance with UNE 51603 [19]), and a cationic type, with its base binder being a polymer-modified bitumen (classified as C67BPF3 MBA in accordance with UNE-EN 13808) [20]. Both emulsions were medium setting with a recommended application temperature range of 30–60 °C. Emulsion contents varied, ranging from 5 to 7% of the total mixture weight. The main properties of both asphalt emulsions are summarized in

Table 3. Properties of the emulsions used.

Property	A67BFM	C67BPF3 MBA
Binder content (%, EN 1428)	67	67
Maximum residual binder penetration at 25 °C (dmm, EN 1426)	330	220
Minimum residual binder softening point (°C, EN 1427)	39	35

.

Figure 1 shows the physical aspects of the principal materials employed in this study.

2.2. Testing Plan

The present study was structured in two phases (Figure 2). In the first phase, a MASAI mixture manufactured at ambient temperature was designed, and the impacts of key variables on its performance were evaluated. Subsequently, in the second phase, its mechanical behavior was evaluated and compared against two conventional asphalt mixtures: a dense-graded hot-mix asphalt (AC 16 B50/70) and an open-graded hot-mix asphalt (BBTM11B PMB 45/80-65). This comparison allows evaluation of the performance of CMA MASAI against mixtures with different gradations and placement temperatures, considering mechanical properties, durability, and traffic-related behaviors. The gradation of different mixtures studied is shown in

Table 4. Analysis of the gradation of different mixtures studied.

Sieve Size (mm)	40	31.5	22.4	20	16	11.2	12.5	8	4	2	0.5	0.25	0.063
CMA (% Passing)	100	100	-	100	-	-	100	87	43	38	-	-	-
CMA MASAI (% Passing)	100	100	-	99	-	-	95	80	32	26	-	-	-
AC16B50/70S (% Passing)	-	100	100	-	99		-	83	54	35	12	9	4.0
BBTM11B PMB45/80-65 (% Passing)	100	100	100	100	100	97	-	59	26	19	10	-	5.1

.

During the design of the MASAI mixture in the first phase, it was established that, to remain competitive across various traffic levels (both low and high heavy vehicle intensities), the mixture must be applied in thin layers (maximum thickness of 3 cm). Additionally, this mixture should offer adequate macro-texture and permeability in order to guarantee road safety. Therefore, a gap-graded mineral aggregate skeleton with a maximum aggregate size of 12 mm was selected (in accordance with the Spanish Technical Association of Bituminous Emulsions [21]) to provide adequate surface texture and facilitate rainwater drainage. This skeleton consisted of 60% gravel (6/12 mm), 35% sand (0/3 mm), and 5% Portland cement as added filler, aiming to achieve an air void content between 12% and 18%. Additionally, to comply with MASAI specifications, an alternative mineral skeleton was studied, composed of 60% gravel (6/12 mm), 20% RAP (0/6 mm), 15% sand (0/3 mm), and 5% Portland cement.

Based on these parameters, the initial design stage investigated the influence of emulsion content on the properties of the MASAI mixture. For this purpose, both mineral skeletons (the reference and the MASAI with 20% RAP) were manufactured using different contents of the conventional A67BFM emulsion (5%, 6%, and 7% by total mass of the mixture). The minimum bitumen emulsion content was determined from the aggregate specific surface area, as recommended by the ATEB [21] specifications. From this reference value, two higher emulsion contents were selected to assess the effect of emulsion content and to identify the optimal content of the manufactured mixture. The emulsion contents considered were relatively high, trying to obtain a flexible and cohesive mixture despite the high air void content used in the mineral skeleton. During the mixing process of the aggregates, prior to the incorporation of the emulsion and the filler, 1% water (by aggregate weight) was added to moisten the aggregate surfaces and, thus, promote optimal interaction with the emulsion. Each of these job-mix formulas was evaluated based on different tests according to UNE-EN 12697-5 [22] and UNE-EN 12697-6 [23] (maximum and bulk densities), UNE-EN 12697-8 (air void content) [24], and UNE-EN 12697-17 [25] (particle loss) to determine optimal dosage, as specified in the ATEB specifications [21]. For this purpose, cylindrical specimens were prepared and compacted with 50 blows on each side using a Marshall hammer. Once compacted, the specimens were placed on a perforated surface inside an oven at 75 °C for 48 h. After this period, the specimens’ surfaces were examined for any bitumen drainage. If no drainage was observed, the temperature was increased to 90 °C, and the specimens were conditioned for an additional 5 days. After conditioning, the specimens were allowed to cool and then demolded for testing.

Once the optimum emulsion content for the MASAI mixture (containing 20% RAP) was selected, the influence of the emulsion type on its mechanical resistance was evaluated. To this end, an additional job mix formula was formulated using the same mineral skeleton, with the conventional emulsion replaced by a modified emulsion (C67BPF3 MAB). Based on the results of the aforementioned tests, the MASAI job mix formula with the optimal design was determined for comparison with the AC 16 B 50/70 S and BBTM PMB45/80-65 mixtures during the second phase of this study.

In the second stage of the research, the mechanical performance of the MASAI mixture—manufactured at ambient temperature with emulsion and 20% RAP—was compared with two conventional asphalt mixtures: a dense-graded hot-mix asphalt (AC 16 B50/70) and an open-graded hot-mix asphalt (BBTM11B PMB45/80-65). The mineral skeleton of this reference mixture, hot-mix asphalt (AC 16 B 50/70 S), consisted of natural limestone aggregates (including gravel, sand, and filler) without RAP. It was manufactured at 165 °C with a conventional B50/70 bitumen content of 4.5% by total mass of the mixture. The mineral skeleton of the open-graded hot-mix asphalt BBTM11B consists of coarse trachyte aggregate, fine limestone aggregate, cement filler, and PMB 45/80-65 polymer-modified bitumen (4.75% by total mass of the mixture) manufactured at a temperature of 165 °C. In addition to the previously mentioned tests for maximum and bulk densities (UNE-EN 12697-5 [22] and UNE-EN 12697-6 [23]), air void content (UNE-EN 12697-8 [24]), and particle loss (UNE-EN 12697-17 [25]), a comparative analysis of the mechanical response was conducted. This stage included indirect tensile strength (ITS) tests at 15 °C (UNE-EN 12697-23 [26]), stiffness modulus at 20 °C (UNE-EN 12697-26 Annex C [27]), and wheel-tracking tests (WTTs) for permanent deformation resistance at 60 °C under both dry and wet conditions (UNE-EN 12697-22 [28]). Furthermore, resistance to thermal cracking was evaluated via the TSRST procedure (UNE-EN 12697-46 [29]).

An indirect tensile stiffness modulus test (UNE-EN 12697-26 (Anexo C)) [27] was conducted to analyze the bearing capacity and stress distribution of the tested mixtures (Figure 3). This parameter is important because of its link to the material’s resistance to permanent deformations and fatigue. The procedure was carried out at 20 °C for cylindrical specimens, in which 3 cylindrical specimens were prepared with a diameter (D) of 101.6 mm and compacted with 50 blows to each side by a Marshall hammer for the CMA and BBTM11B mixtures. For the AC 16 mixture, 75 blows were applied on each side. During the test, 5 semi-sinusoidal test pulses were applied along the vertical diameter of each specimen, with a rise time of 124 ± 4 ms in a strain-controlled (5 μm) regimen. The test procedure was then repeated in the perpendicular diameter. The stiffness modulus of the mixtures was recorded from the average stiffness of the five pulses registered in the two diameters tested.

To evaluate the characteristics of the resistance of the mixture to failure of the mixtures and, hence, their cohesion, the indirect tensile test (UNE-EN 12697-23) [26] was carried out on the cylindrical specimens, in which 3 cylindrical specimens were prepared with a diameter (D) of 101.6 mm and compacted with 50 blows to each side by a Marshall hammer for the CMA and BBTM11B mixtures. For the AC 16 mixture, 75 blows were applied on each side at a temperature of 15 °C, applying a diametral load at a constant deformation speed of 50 ± 2 mm/min until the peak load (P) was reached (Figure 4).

A wheel-tracking test (UNE-EN-12697-22) [28] under dry and wet conditions is used to assess the extent to which asphalt mixtures can resist permanent deformation under conditions that simulate traffic effects (Figure 5). The manufacturing procedure of 4 parallelepiped test specimens with dimensions of 408 × 206 × 60 mm (for the AC 16 mixture) and 4 parallelepiped test specimens with dimensions of 408 × 206 × 40 mm (for the CMA and BBTM11B mixtures) were manufactured using a roller compactor. The compaction process was carried out with controlled compaction energy. Two specimens, prepared by mixing, were pre-heated for 4 h and tested at a temperature of 60 °C in air, and the other two specimens were submerged in water at 60 °C for preheating and testing. The test ended after 10,000 passes of the loaded wheel or until the deformation depth reached 20 mm. The deformation slope (WTS) was then determined based on the rut depth between 5000 and 10,000 cycles.

A thermal stress restrained specimen test (TSRST), conducted in accordance with UNE-EN 12697-46 [29] was used to determine the resistance to thermal cracking (Figure 6). In this test, parallelepiped specimens were manufactured using a roller compactor, from which test specimens were cut. Four specimens of 160 × 40 × 40 mm (for the CMA and BBTM11B mixtures) and 160 × 40 × 60 mm (for the AC 16 mixture) were fixed at their ends and conditioned in the climatic chamber at 20 °C. The temperature of the chamber was then reduced at a rate of 10 °C/h, thereby inducing their shrinkage and breakage due to the tensile stresses caused by temperature reduction. The results were obtained in terms of the temperature of fracture and the tensile stress generated in the specimen before its breakage. The lower the fracture temperature, the lower the slopes, the lower the transition temperatures, and the lower the material’s susceptibility to cracking from thermal shrinkage.

3. Analysis of Results

Figure 7 and Figure 8 present the results for the reference CMA and the CMA MASAI with 20% RAP, showing density, air void content, and resistance to particle loss as a function of emulsion content. In these figures, the limitation established by the ATEB for each test for this type of mixture is represented by a dashed line (Figure 1, recommended void percentage range, and Figure 2, particle loss greater than 25%). These values were obtained from tests conducted on three specimens for each emulsion content and mixture type. These figures demonstrate that, for a given compaction energy, an increase in emulsion content leads to higher density and lower air void content. Thus, it is shown that a higher binder proportion improves the workability of both mineral skeletons. Furthermore, although the MASAI mixture with 20% RAP exhibits a higher air void content for the same emulsion percentage, its resistance to particle loss is superior to that of the reference mineral skeleton. This indicates that the presence of the RAP binder contributes to enhancing the material’s internal cohesion. Based on these considerations, an optimum emulsion content of 7% was selected for the design of the CMA MASAI with 20% of RAP, as this dosage met both the target air void content and the required particle loss resistance. On the other hand, it is observed that, despite the lower density and higher air void content, the incorporation of RAP is able to improve particle loss at low and medium emulsion contents. However, at high emulsion contents (7%), the results are practically identical regardless of the presence of RAP (Figure 8). The bitumen contained in the RAP contributes to performance improvement, provided that the mixture exhibits a certain binder deficiency, as evidenced by the higher loss values obtained at low and medium emulsion contents. Conversely, when the emulsion content is sufficiently high (7%) for the mixture to achieve adequate performance on its own (Figure 8), the additional bitumen supplied by the RAP is no longer necessary, and no differences in cohesion are observed between virgin aggregate and RAP. Consequently, the results do not differ from those of the mixture without RAP.

In the second stage of the MASAI CMA mixture design, the feasibility of substituting the conventional emulsion with a polymer-modified emulsion was evaluated. The results (Figure 9 and Figure 10) demonstrate that the use of modified emulsion slightly reduces the mixture’s workability—yielding lower density and higher air void content for the same compaction energy (both mixtures were compacted by applying 50 blows per face using a Marshall hammer)—without improving resistance to particle loss. Consequently, it was decided to proceed with the study using a conventional emulsion.

Once the CMA MASAI with 20% RAP was designed, a comparative analysis of its mechanical performance was conducted against traditional AC 16 B 50/70 S and BBTM11B PMB 45/80-65 hot-mix asphalt. Figure 11 shows that the AC 16 B 50/70 S has a significantly higher density and a much lower air void content (characteristic of this type of mixture) than the designed CMA MASAI and BBTM11B PMB 45/80-65. Accordingly, its structural capacity (stiffness) and indirect tensile strength (ITS) are also substantially higher than those of the cold-produced mixture and the other HMA (BBTM11B PMB 45/80-65) (Figure 12). On the other hand, despite having similar air void contents (BBTM 11B and CMA MASAI), the BBTM 11B mixture exhibits a higher indirect tensile strength (ITS) and stiffness than the CMA MASAI mixture. These differences could be attributed to the higher manufacturing temperature and the use of a polymer-modified binder (PMB 45/80-65) in the BBTM 11B. The CMA mixture exhibited particle loss values comparable to both hot-mix mixtures (AC and BBTM 11B). Furthermore, its permanent deformation (under both dry and wet conditions, the latter indicating satisfactory moisture susceptibility) was lower than that of the AC mixture and similar to that of BBTM 11B, indicating that the CMA MASAI mixture provides comparable surface durability and rutting resistance. However, regarding thermal stress cracking, both mixtures, BBTM 11B and CMA MASAI, have comparable air void contents. The superior low-temperature performance of the BBTM 11B mixture, with a lower fracture temperature, is mainly associated with the use of a PMB 45/80-65 binder, which enhances elasticity and thermal stress relaxation at low temperatures. In contrast, the CMA mixture, produced with bituminous emulsion and containing RAP, fractured at higher temperatures and very low fracture stress levels, indicating a more brittle response. Compared with the AC mixture, which exhibited the lowest binder and air void contents, the poorest low-temperature performance was observed. The AC mixture, manufactured with a conventional B 50/70 binder and showing the highest stiffness, indicates a brittle response under restrained thermal contraction (Figure 13, Figure 14 and Figure 15).

The MASAI mixture exhibits a mechanical response comparable to those of the other mixtures.

4. Conclusions

This paper summarizes the results obtained in a research study whose main objective was to analyze the viability of manufacturing MASAI mixtures at ambient temperature. From the results obtained in this study, the following conclusions can be drawn:

-

Emulsion content is a key parameter in improving the wear resistance of MASAI mixtures produced at ambient temperature. As the emulsion content increases, the percentage of particle loss decreases, reaching an optimum at 7% emulsion.

-

The presence of RAP does not affect the mechanical performance of open-graded mixtures produced at ambient temperature.

-

It was demonstrated that the presence of an optimal emulsion content has a greater influence on the mechanical properties of these mixtures than the type of emulsion used.

-

The dense-graded AC mixture exhibited the highest stiffness and indirect tensile strength due to its low air void content, although this also resulted in a more brittle response, particularly under low-temperature conditions. Among mixtures with comparable air void contents, the BBTM 11B outperformed the CMA MASAI in stiffness and tensile strength, which can be attributed to its hot-mix production process and the use of a polymer-modified binder. In contrast, the CMA MASAI mixture displayed particle loss, permanent deformation, and moisture susceptibility comparable to those of conventional hot-mix references, demonstrating satisfactory surface durability and resistance to rutting. Based on these considerations, these materials could represent more sustainable and competitive solutions for the rehabilitation of pavements that do not require structural restoration.

The methodology proved useful for evaluating low-temperature mixtures containing 20% RAP, which exhibited mechanical performance comparable to that of hot-mix asphalt. Future research should address long-term behaviors through fatigue testing and assess reproducibility in both plant production and field application.