Sustainable Reclaimed Asphalt Emulsified Granular Mixture for Pavement Base Stabilization: Prediction of Mechanical Behavior Based on Repeated Load Triaxial Tests

Abstract

The stabilization of asphalt pavement bases with granular soil and aggregates emulsified with asphalt is a widely used technique in road construction and maintenance. It aims to improve the mechanical properties and durability of the lower pavement layers. Currently, there is no consensus on the most suitable method for designing emulsified granular aggregates with reclaimed asphalt pavement (RAP), as it is very complex. Therefore, the methodology is generally based on compliance with one or more volumetric or mechanical parameters established in the highway regulations for conventional asphalt mixtures, which does not guarantee the optimization and characterization of the recycled mixture in the base course. In this study, granular mixtures were developed, including five with emulsion and one emulsion-free as a control mix. Granular RAP mixes were designed in this study, including five with emulsion and one emulsion-free as a control mix. The five mixes ranged from 1% to 5% emulsion and were characterized by multi-stage triaxial tests with repeated load resilient modulus (RM) and permanent deformation (PD) to evaluate their mechanical behavior. The results showed that the mixes had RM values between 350 and 500 MPa, consistent with literature values. However, they showed similar levels of accumulated deformation to the control mix without RAP emulsion. The sample with 1 % RAP emulsion exhibited a satisfactory RM value and better performance in PD than the control mix (5 mm) and showed accumulated PD values of up to 4 mm. In contrast, the other samples exhibited deformations of up to 6 mm. In this study, the multi-stagge triaxial RM and PD tests were found to be an effective predictive method for characterizing the behavior of RAP materials in base courses, regardless of the types of admixtures contained. Multi-stage resilient modulus and PD tests can be considered as a predictive method for the behavior of milled material in base courses. They were able to provide initial data for interpreting the behavior of ETB mixtures.

1. Introduction

Emulsified treated base (ETB) is one of the cold stabilization techniques for the base course of roads in which the aggregates are treated with a bitumen emulsion. Due to the higher cohesion and shear strength, ETB shows better structural performance compared to conventional unbound granular materials in the base course, which is due to the bonding effect of the thin bitumen film surrounding the aggregate particles. Although ETB can ensure better performance compared to conventional base courses, the initial construction cost remains an issue. Andrews et al. [1] found that the cost of ETB with emulsion is almost double that of a conventional base course, and this cost increases by almost 40% for every 1% increase in emulsion.

The incorporation of recycled materials, such as reclaimed asphalt pavement (RAP), in emulsified treated bases (ETBs) has been investigated, aiming to reduce costs and minimize the need for virgin aggregates (VAs) [2,3]. However, the financial benefits of using RAP can only be achieved when it can replace a substantial amount of VA [4]. It is typically recommend to limit the RAP percentage in ETB mixtures to 50% to 60% in order to achieve the necessary aggregate gradation for both mechanical and workability purposes [5]. One of the main obstacles to the use of RAP, imposing limitations on its maximum content, is the lack of material homogeneity, caused by variability in the original pavement from which it came, which might combine RAP from various sources, pavement ages, states of damage, and milled layers [4].

Due to its composition and the particular characteristics of heterogeneous gradation, the type of residual binder, the specific weight, and the source rock, RAP has a different mechanical behavior than natural aggregates. Understanding how these properties affect the elastic and plastic behavior under load is important information for the design of road pavements. The literature indicates that RAP has a higher resilience modulus (RM) compared to VA, suggesting better performance. However, to properly understand and evaluate damage to flexible pavements, the study of permanent deformation (PD) in granular materials is one of the fundamental parameters [6,7,8,9,10]. Base pavement layers, which include RAP material, are very sensitive to the accumulation of PD under repeated loads. Stolle et al. [11] found that the addition of RAP tends to increase the accumulated permanent deformation. Thakur et al. [12] pointed out that the permanent deformation of aggregates containing RAP increases with increase in RAP content.

Previous studies have focused on characterizing RAP based only on its RM. Indeed, RM is considered the sole parameter for material characterization in the pavement structure design process, according to mechanistic empirical pavement design guidelines (MEPDGs) [13]. However, in Brazil, the mechanistic–empirical design method and the program proposed by the Transportation Research Institute (IPR) and the National Department of Infrastructure and Transportation (DNIT), the MeDiNa software, version 1.1.9, considers, in addition to RM, the PD characteristic as input data for the design and analysis of flexible pavements, using as the prediction model proposed by Guimarães et al. [8] for PD. Furthermore, the Brazilian standard DNIT 179 [14] describes the single-stage permanent deformation test method and indicates the criterion for accommodation verification. Other international standards such as the European BS EN 13286-7 [15], Australian AG: PT/T053 [16], and New Zealand NZTA/2014 [17] encompass multi-stage tests. For instance, multi-stage tests are used where different stress states are applied for certain numbers of cycles in the same specimen.

Clearly, a study aimed at understanding and predicting the behavior of RAP in granular layers is of great importance due to its heterogeneous characteristics. The ability to predict the performance of these mixtures with a minimum number of tests not only saves resources but also speeds up the process of developing and implementing new technologies. Therefore, the present investigation aims to use multi-stage repeated load triaxial testing (RLT) as a means of predicting the behavior of emulsified RAP in mixtures with granular materials under a variety of loading conditions, using a minimal number of required tests.

2. Materials and Methods

2.1. Materials

2.1.1. Reclaimed Asphalt Pavement (RAP)

Particulate milled RAP was obtained from the asphalt plant of the municipality of Rio de Janeiro, Brazil, produced as a result of pavement maintenance carried out in the city. During the beneficiation process, the original milled material was divided into three distinct samples: (i) RAP 1 (material retained on sieve #3/$8^{″}$ (9.5 mm)); (ii) RAP 2 (material retained on sieve #4 (4.8 mm)); and (iii) RAP 3 (material passing through sieve #4 (4.8 mm)). Illustrations of beneficiated samples are shown in Figure 1.

The particle size distribution of the recycled mixture, shown in Figure 2, without asphalt binder (black gradation curve) was obtained using the trial-and-error method, in which the goal is to fit the gradation curves in a way that allows the mixture to fall within the chosen and anticipated range specified in the Brazilian standard DNIT 141 [18] (see

Table 1. The particle size distribution by the trial-and-error method.

% Passing		RAP1		RAP 2		RAP 3		DNIT 141 [18]		Adopted Curve
# (pol.)	(mm)	Sample	Attempt	Sample	Attempt	Sample	Attempt	Range B
								min	max
$2^{″}$	50.8	100	50	100	45	100	5	100	100	100
$1^{″}$	25.0	81.2	40.6	100	45	100	5	75	90	90.6
3/$8^{″}$	9.5	2.5	1.25	100	45	100	5	40	75	51.25
n.° 4	4.8	1.7	0.85	82.1	36.94	99.95	4.99	30	60	42.79
n.° 10	2.0	1.6	0.8	69.4	31.23	91.3	4.56	20	45	36.59
n.° 40	0.42	1.4	0.7	33.8	15.21	25	1.25	15	30	17.16
n.° 200	0.07	0.75	0.37	13.6	6.12	1.87	0.09	5	15	6.58

). In other words, the mass percentage of each mixture is chosen to compose the adopted final mixture. The fitting was performed for Range B, using sieves $2^{″}$ (50.8 mm), $1^{″}$ (25.4 mm), 3/$8^{″}$ (9.5 mm), #4 (4.8 mm), #10 (2.0 mm), #40 (0.42 mm), and #200 (0.074 mm). The established composition was 50% RAP1, 45% RAP2, and 5% RAP3 (see Figure 2). Using the asphalt binder extraction method in the electric rotarex equipment, as described in the Brazilian standard DNER053 [19], the extracted asphalt binder content from the RAP samples before the addition of the emulsion was found to range from 4.5% to 5.3%.

2.1.2. Mineralogical Verification and Microscopic Structure

Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) tests were conducted for physical visualization of the structural arrangement of particles and qualitative assessment of the chemical components present.

The sintered samples were morphologically analyzed using the QUANTA FEG 250 microscope (FEI).The samples were not coated. SEM analysis was carried out under the following parameters: electron beam power of 20 kV, working distance ranging between 10.5 and 13 mm, spot size of 5, and image magnification at 400× and 1600×, utilizing the secondary electron detector. For EDS analysis and compositional mapping, a detector from Bruker was employed, coupled to the microscope column.

3. Emulsion

In principle, there are two types of asphalt emulsion used in cold mix design. One is cationic asphalt emulsion and the other is anionic asphalt emulsion. This study utilized a cationic medium-breakage (MB-1C) asphalt emulsion due to its higher resistance and modulus of elasticity compared to anionic asphalt emulsions. The properties of this emulsion are presented in

Table 2. Characteristics of the emulsion used (MB-1C).

Characteristics	Unit	Test Method (Brazilian Standard)	Results	Limits (Min–Max)
Asphalt residue	% (m/m)	NBR 14376  [20]	62.0	Min 62.0
Viscosity Saybolt Furol, 50 °C	SSF	NBR 14491 [21]	22.0	20–200
Sieved, retained on the 0.84 mm ${\mathrm{sieve}}^{″}$.	% (m/m)	NBR 14393 [22]	0	Max 0.1
Settlement, 5 days, difference in residue between top and bottom	% (m/m)	NBR 6570 [23]	1.2	Max 5
Particle load	% (m/m)	NBR 6567 [24]	Positive	Positive
De-emulsification	-	NBR 6569 [25]	17.9	Max 50.0
Water resistance	%	NBR 14249 [26]	85.0	Min 80.0
Distilled solvent	% (v/v)	NBR 6568 [27]	0	Max 12.0

.

4. Methods

In this research, six types of cold mixtures were designed: five mixtures with emulsion and one control mixture. The control mixture was prepared with granular RAP without the addition of emulsion and labeled as RAPC. The remaining 5 mixtures were designed with granular material, using emulsion additive amounts from 1% to 5%, with an increment of 1%. The mixture 100% RAP with 1% emulsion was named R1. Likewise, R2, R3, R4, and R5 indicate a recycled cold mixture with 2%, 3%, 4%, and 5% emulsion, respectively.

The compaction procedures were based on the methodological procedures recommended by the Brazilian standard DNIT 443 [28]. The mixtures were compacted using a tripartite mold, with dimensions of 100 mm in diameter and 200 mm in height, using modified Proctor energy with 21 blows. Following the rationale of Zhen et al. [29] and acknowledging the importance of balancing road closure time and curing period to ensure swift road reopening while preserving the integrity of the performance of cold recycled asphalt emulsion mix pavements, after compaction, the samples underwent a curing time of approximately 48 h, outdoors, at an ambient temperature of approximately ±25 °C. The 48 h curing time was considered by the researchers as a period that could be adapted to on-site conditions.

4.1. Experimental Program

4.1.1. Triaxial Tests with Repeated Loads

Figure 3 illustrates the main components of the equipment used for conducting triaxial tests on granular materials for pavement. The setup consists of a pneumatic press, a triaxial cell or chamber, an axial load transducer, and a vertical displacement measurement system using a linear variable differential transformer (LVDT), along with a tripartite cylindrical mold with a base and two steel clamps and a complementary ring (collar), properly shown in top view and in section.

Figure 4 presents the sequence of homogenization of samples, compaction, and testing.

4.1.2. Resilient Modulus (RM) Test

The RM test depends on the nature of the soil, texture, plasticity of the fine fraction, moisture content, density, and stress state [30]. In pavement mechanics, the RM is defined as the ratio between the cyclic load applied and the elastic or recoverable deformation of the material [31]. It is a parameter aimed at characterizing the elastic behavior of materials, such as soils and aggregates, under repeated loading, either in the laboratory or by the repeated actions of vehicle loads on the pavement. The RM is obtained from the results of repeated load triaxial tests, defined as the ratio between the deviator stress (${\sigma }_1$ − ${\sigma }_3$) and the axial resilient strain, ${\mathsf{\Delta }}_r$, which is understood as the ratio ${\mathsf{\Delta }}_h$ by $h_0$, where ${\mathsf{\Delta }}_h$ is the maximum vertical displacement and $h_0$ is the initial reference length of the cylindrical specimen. We aimed to reproduce in the laboratory the loading conditions of traffic loads on the pavement structure. Such a relationship for most pavement materials is nonlinear, unlike other elastic solids, with a strong dependence on the applied stresses [32].

In this research, the test was conducted according to the Brazilian standard test method DNIT RM 134 [33], using a Brazilian-made dynamic triaxial testing machine (Owntec-Brazil, Santa Cruz do Sul, Brazil) (MS-151). During the resilience modulus test, 18 pairs of confining stresses (${\sigma }_3$) and deviator stresses (${\sigma }_d$) were applied after the specimen conditioning phase. The loading cycle duration was 1 s, with 0.1 s of loading application and a frequency of 1 Hz (60 cycles per min). In the conditioning stage, the specimens were exposed to three sets of stresses and subjected to 500 loading cycles for each set. Subsequently, they were subjected to an additional 18 sets of stresses, with 100 loading cycles for each set, totaling 3300 cycles per test. The values of the applied stresses are presented in the

Table 3. Pairs of stresses for resilience modulus test.

Conditioning phase
Pair	${\sigma }_3$ (KPa)	${\sigma }_d$ (KPa)	${\sigma }_3$/${\sigma }_1$
1	70	70	2
2	70	210	4
3	105	315	4
Loading phase
Pair	${\sigma }_3$ (KPa)	${\sigma }_d$ (KPa)	${\sigma }_3$/${\sigma }_1$
1	20	20	2
2		40	3
3		60	4
4	35	35	2
5		70	3
6		105	4
7	50	50	3
8		100	2
9		150	3
10	70	70	2
11		140	3
12		210	4
13	105	105	2
14		210	3
15		315	4
16	140	140	2
17		280	3
18		420	4

.

4.1.3. PD of the Multi-Stage Type

Although there is a possibility of recording permenent deformation (PD) during the RM test, specific tests with more loading cycles are necessary to determine PD and the respective prediction model. Details such as the number of cycles and analysis of accommodation trend are specific to the PD test. In Brazil, the National Department of Infrastructure and Transportation regulates and describes the PD test of soils and granular materials using the repeated load triaxial equipment. The PD test consists of applying a large number of repeated load cycles to a stress state in each test specimen. Considering a frequency of 5 Hz for the application of 100,000 cycles, the test lasts approximately 6 h. To perform tests at nine different stress levels, at least four days of testing and nine different samples are required. This protocol demands a large amount of material and testing time, along with the use of specific equipment.

An alternative for this evaluation is the multi-stage PD test, in which a single sample is subjected to different stress states. Thus, in this study, the multi-stage PD test was used to assess the performance of the RAP mixture emulsion under a variety of stress conditions. For this particular, the European standard for triaxial testing was used as a reference [15]. The confinement stresses were estimated based on the literature. Barksdale [34] reported that, in the compacted base pavement layer, the confinement stress is approximately 70 kPa due to the locked-in stresses resulting from compaction. The literature also states that confinement stresses in the upper half of the base layer range between 35 and 50 kPa, while, in the lower half of the base layer, stresses range from 20 to 35 kPa, but never exceed 50 kPa [35].

Although confinement stresses above 50 kPa are not usual in the literature, the study by Ullah et al. [36] evaluated a confinement stress of 140 kPa to develop the shakedown envelope of the material. Therefore, in this research, it was deemed relevant to assess confinement stresses of 50, 70, and 100 kPa. Each stress pair (see

Table 4. Stress levels implemented for conducting multi-stage repeated triaxial loading.

Test N°	Stage	${\mathit{\sigma }}_{\mathit{d}}$	${\mathit{\sigma }}_3$	${\mathit{\sigma }}_{\mathit{d}}$/${\mathit{\sigma }}_3$
Test I	I	50	50	1.0
	II	75	50	1.5
	III	100	50	2.0
	IV	125	50	2.5
	V	150	50	3.0
Test II	Ia	70	70	1.0
	IIa	105	70	1.5
	IIIa	100	70	2.0
	IVa	125	70	2.5
	Va	150	70	3.0
Test III	Ib	100	100	1.0
	IIb	150	100	1.5
	IIIb	200	100	2.0
	IVb	250	100	2.5
	Vb	300	100	3.0

) was applied at a frequency of 5 Hz over 5000 cycles, totaling 25,000 cycles on a single specimen, resulting in approximately 1 h and 30 min of testing time per test. The multi-stage repeated triaxial loading (RTL) provides the foundation for understanding material behavior under a variety of loading and stress conditions with a minimal number of tests required. However, multi-stage RTL tests have their own drawback, commonly referred to as the stress history effect, on the magnitude of total accumulated deformation [37]. Nevertheless, in this research, the objective of applying the test was to predictively and comparatively characterize the influence of emulsion on the mechanical behavior of recycled material in the base layer.

5. Results and Discussions

5.1. Mineralogical Properties and Microscopic Structure

Figure 5 shows the micrographs of RAP2 before and after binder extraction. The sample without the presence of the binder appeared porous, where it was possible to observe a large number of pores in the fracture region. For the sample with the binder, it was observed that, primarily, a hydrocarbon compound composed of hydrogen and carbon molecules caused a reduction in porosity, as the fracture surface exhibited a greater number of deformation propagation marks, caused by the absence of pores.

The results of the chemical composition of RAP2, with binder extraction, are illustrated in Figure 6, according to the quantification obtained in the EDS test.

In the quantitative chemical analysis obtained from the EDS test of RAP2, after binder extraction, a significant presence of carbon (C) and oxygen (O) is observed, which are the main constituents of calcium carbonate (${\mathrm{CaCO}}_3$), a common mineral in limestone rocks. Additionally, calcium (Ca) is present in a significant proportion in the sample, suggesting that the primary natural material of this RAP2 is likely a limestone aggregate.

5.2. Proctor Compaction Test

Through the compaction test, the optimum moisture content and maximum dry density parameters necessary for the mechanical performance tests were obtained. The results of the Proctor compaction parameters for the RAPC, optimum moisture content (W%), and density are presented in

Table 5. Compactation result of the control mixture (RAPC).

Data RAPC
W Optimum (%)	Density (g/cm3)
4.54	1.98
Data R1 à R5
EAP (%)	Density (g/cm3)
1%	1.86
2%	1.85
3%	1.84
4%	1.85
5%	1.94

.

When comparing the results obtained in this research with other studies, it was noticed that the determined values were similar. In general, the densities vary between 1.8 and 2.3 $\mathrm{g}/{\mathrm{cm}}^3$, depending on the characteristics of the materials used [38,39,40,41].

5.3. Resilient Modulus (RM)

Based on the results of the triaxial repeated load tests for 18 stress pairs, the average values of the resulting resilient modulus were obtained through an arithmetic mean.

Table 6. RM results obtained in the laboratory.

Sample	Average RM (MPa)
RAPC	339.20
R1	441.22
R2	428.41
R3	412.23
R4	422.13
R5	496.34

presents the results for the average RM of each sample.

The average RM values determined in our tests are between 340 and 500 MPa. However, the results presented by Santos [41] show that samples with 100% RAP, regardless of the specific formulations (RAP-100% RAP-RS and M5-100% RAP-BR), have RM values between 358 and 387 MPa. This comparison suggests that emulsion-containing mixtures may have contributed to a significant increase in RM values compared to pure RAP. This could be due to the bonding and reinforcing properties of the emulsion leading to greater resistance to deformation under repeated loading.

The modulus values determined in the present study agree with the statements of Bernucci et al. [42] on the typical range of values for granular materials in the range of 100 to 400 MPa and with the range of 200 to 350 MPa proposed by Balbo [43]. In particular, our results show that recycled asphalt material with emulsion exhibits elastic properties consistent with those of granular materials such as dense-graded aggregates, as proposed in the literature [42]. They are also consistent with a number of other studies. For example, the results of Lima et al. [44] (273–397 MPa) for crushed stone and by Puppala et al. [45] for recycled asphalt concrete (180–340 MPa) and cement-stabilized recycled asphalt concrete (200–515 MPa) are in the same range as our results. The values by Arulrajah et al [46] for recycled concrete aggregate as 239–357 MPa and recycled ceramic block as 301–319 MPa are close to the values observed in our study. The wide range of values for construction and demolition waste (CDW) (200–500 MPa, as reported by Leite [47]) is also consistent with the variability observed in our results.

The comparison with the higher RM values for natural aggregates determined by Arshad [48] illustrates the expected difference between recycled (181–518 MPa) and natural (584–685 MPa) materials. In summary, these variations highlight the importance of evaluating the performance of recycled asphalt mixtures in each application context and considering PD tests alongside RM results, especially when data vary. By analyzing the results of PD tests together with RM results, a more comprehensive understanding of the mechanical behavior of the material can be gained.

5.4. Permanent Derformation

Figure 7 displays the results of multi-stage PD tests on emulsion-treated RAP material mixtures, compared to a control mixture RAPC.

The PD curves of the milled ETB mixtures show a higher degree of accumulated deformation, except in Figure 7b. In Figure 7b, the mixtures exhibited similar levels of PD in Stage I; within the first 5000 cycles, they show similar PD values for all three inclusion conditions. Figure 7c–f shows that the ETB mixtures have similar PD values to the emulsionless RAP mixture up to Stage III, but, with increasing deviatoric stresses, the ETB specimens accumulate larger deformations. This indicates that increasing the emulsion content decreases the resistance of the mixtures. Chakravarthi et al. [49] also observed a similar characteristic and reported that the residual binder present in the RAP and the emulsified asphalt content affect the deformation properties of cold mixes and that lower permanent deformations were observed in mixes with emulsified asphalt content below the optimum level.

Data analysis revealed a significant difference in the size of PD between the R1 mixtures and RAPC, as shown in the results in Figure 7a,b. Although the addition of 1% emulsion resulted in a reduction in PD compared to the mixtures without emulsion, it is important to emphasize that the focus in mechanistic road design should not only be on the total amount of PD accumulated, but rather on preventing the continuous accumulation of PD under cyclic loading [36]. In this respect, the continuous accumulation of permanent deformation observed towards the end of the test for the R1 specimen highlights the importance of strategies to mitigate this effect aimed at ensuring the durability and integrity of road structures.

Overall, the performance of RAP mixtures in unbound pavement layers (base course and sub-base) is an area that needs to be investigated, as the international literature provides contradictory results. For example, some studies report higher permanent deformations for RAP-VA mixtures compared to VA, while in other cases the opposite is observed [36]. This discrepancy is also confirmed by the experimental results of Plati et al. [50]. It is therefore obvious that this is an aspect that needs to be further investigated.

The use of RAP with an asphalt emulsion content of more than 1% resulted in a softer mix and lower rut resistance. These results are consistent with the findings of Jin et al. [51], who highlighted the feasibility of using such a material in low-traffic areas. In addition, Meena et al. [52] found that a lower amount of asphalt emulsion is required to increase the proportion of RAP in cold recycled asphalt mix. In summary, these studies confirm the conclusions from this investigation.

6. Conclusions

This study investigated the effects of adding reclaimed asphalt pavement (RAP) emulsion on the performance of granular mixes. Different proportions of the emulsion (1%, 2%, 3%, 4%, and 5% of the dry aggregate weight) were investigated together with RAP to improve the mechanical properties of these mixtures. The following conclusions can be drawn from this study:

• Our initial hypothesis was that the addition of emulsion to RAP could increase the resistance to deformation. However, the results showed that the resistance to PD decreased at higher emulsion contents, although the RM benefited;
• The tests showed that the addition of emulsion to RAP significantly affects the mechanical properties of paving mixes. The RM values indicated a satisfactory range of elastic deformation resistance under repeated loading, while the PD values showed a decrease in resistance with increasing emulsion content. Although the aim was not to determine the optimum binder content of the ETB with an emulsion content of 100%, it was observed that sample R1 with 1% emulsion had a satisfactory RM value and better performance in PD than RAPC (5 mm), showing cumulative PD values of up to 4 mm. In contrast, the other samples (R2, R3, R4, and R5) showed deformations of up to 6 mm. This indicates that higher additions of emulsified asphalt make the ETB with 100% RAP aggregate susceptible to PD, even at higher RM values;
• Performing multi-stage MR and PD tests proved to be an effective method for predicting the behavior of emulsion-treated pavement mixtures. The results provided valuable insight into the influence of emulsion on the resistance of mixtures to elastic and permanent deformation and contributed to a better understanding of the behavior of these materials;
• In summary, multi-stage resilient modulus and PD tests can be considered as a predictive method for the behavior of milled material in base courses. They were able to provide initial data for interpreting the behavior of ETB mixtures.