Comparative Study of Mechanistic-empirical Structural Design Methods Applied to Sustainable Asphalt Mixtures

2.1. Asphalt Mixtures Description

The asphalt mixtures considered in this study are:

2.1.7. WMA: Warm-Mix Asphalt

Semi-dense graded warm mix asphalt elaborated with super-stable cationic bituminous emulsion as a binder (being its base binder CA30-type bitumen). For the production of this mixture, the aggregates are heated at approximately 120 °C and the bituminous emulsion at 60°C [14].

2.2. Characterization

Table 1 describes the main physical-mechanical characteristics of the studied mixtures, including their volumetric properties.

Concerning the analysis of the mixtures' behaviour, Fig. (2) demonstratesthe master curves of dynamic modulus as a function of temperature for a frequency of 10 Hz. (UNE-EN-12697-26 Annex F).

To apply a mechanistic-empirical method like Back ViDe, it is necessary to know the failure criteria of the structural materials. These criteria are obtained from laboratory tests and are adjusted by a shift factor that considers their real in situ behaviour [7].

Fig. (3) shows the fatigue cracking failure criteria of the different asphalt mixtures evaluated, derived from Four-Point Bending Beam Tests (4PBT) (UNE-EN-12697- 24).

Fig. (4) displays the rutting evolution in the Wheel Tracking Test (WTT) (UNE-EN-12697-22) for the different mixtures studied as a function of the number of load applications.

2.3. Structural Analysis

2.3.2. Structural Design Methods

To proceed with the structural behaviour evaluation of the different structures, the Mechanistic-Empirical Design Method BackViDe is applied, conducting a structural analysis for initial and incremental conditions (deterioration).

2.3.2.1. BackViDe Initial Conditions

Firstly, an empirical-mechanistic method based on initial conditions is used, such as the “VIDA” component of the BackViDe software. The analysis under initial conditions allows us to determine the structure’s service life in terms of years or ESALs, using a fixed dynamic module, Ed, at 10 Hz and in correspondence with the mean temperature of the asphalt mixture in an average Argentinian location. The software considers a 30% cracked surface as a fatigue failure criterion and a 12 mm rut depth as a criterion for cumulative deformation. It also establishes that failure in the bound layers occurs due to cracking (repetition of Ɛt, which is a specific tensile strain at the bottom of the asphalt base), and in the unbound layers, failure occurs due to permanent deformation (accumulation of Ɛp, which is a specific compression strain on the subgrade).

2.3.2.2. BackViDe Incremental Conditions

The evaluation under Incremental Conditions is carried out through the “DETERIORO” module of the BackViDe software. The analysis under incremental conditions allows us to understand the evolution of the damage the pavement will sustain over time in terms of pavement distress and functional performance (cracking, rutting, and roughness). This methodology takes into account the variation of the asphalt layers' modulus based on temperature. Climate data from the typical location of the road and the estimated traffic evolution are used for this purpose.

3.1. Initial Conditions Model: VIDA

After defining each of the involved layers and processing the different structures using the VIDA module of BackViDe, the results presented in Fig. (6) are obtained, showing the service life and the adopted Ed. It is highlighted that all structures reach the end of their service life due to fatigue failure of the 10 cm-thick asphalt base..

The results indicate that the incorporation of recycled plastics, as well as rubber, has improved the characteristics of asphalt mixes compared to those of the control mix. Additionally, it is observed that all mixes with sustainable features have a longer service life than the HMA. The differential performance of the HIMA mix is also evident, surpassing 30 million ESALs, which represents a service life well above 20 years.

3.2. Incremental Conditions Model: DETERIORO

Firstly, surface rutting is analyzed after processing the structures through the DETERIORO module in BackViDe. Though the contribution of all layers is considered, the permanent deformation occurring at the subgrade level is the most significant one. In Fig. (7), the evolution of rutting and the adopted failure criterion are shown, and the intersection of both defines the service life of each structure.

Secondly, the progression of surface cracking (“bottom-up” type) is evaluated. Fig. (8) shows the adopted failure criterion and the cracked surface percentage, whose growth is caused by crack propagation. Notably, Fig. (8) does not include the HIMA mix because, due to its characteristics, cracking initiation occurs beyond the 30 years studied.

The results indicate that all studied structures fail due to rutting. As expected, mixes with shorter service lives are HMA and WMA, showing similar performance as they both have the same base binder. The other sustainable mixes evidently improve their performance, attaining service lives greater than that of the control mix. Mixes with recycled plastics have a life increase between 90% and 150%, approximately doubling their lifespan. A similar case occurs with PMA. The mix with ELTs shows a 175% increase in life and 3M, a 250%. Finally, the HIMA mix has a five-time-longer lifespan.

In Fig. (9), the results obtained from the initial (VIDA) and incremental condition (DETERIORO) analyses are compared. Additionally, just for comparative purposes, the number of ESALs to failure that is obtained by the AASHTO 93’ method, still used in Argentina, is indicated. As the thickness of each layer remains constant, the expected life of every structure differs. In subsequent studies, pavement structures designed for a given lifespan of 15 years will be processed, and the differences in the thicknesses of the base material required to achieve this will be compared.

3.3. Environmental Considerations

Having analyzed the behaviour of sustainable mixes through different models and concluded that they have a longer lifespan and better performance than conventional mixes, it is considered that all of them are suitable to be included in a flexible pavement structure. However, this is not sufficient, and therefore the next step is to evaluate the environmental benefits of each by using a Life Cycle Assessment (LCA) (ISO 14040 & ISO 14044) approach.

Pratico et al. (2023) presented a method for comparing pavement technologies from a broader perspective, including LCA-based criteria [ 16 ]. Based on these studies, one question that arises is whether the process of recycling, i.e., carrying out all the necessary waste processing for their incorporation into asphalt mixes (e.g., collection in generation areas, transportation, pelletization, and micronization treatments, etc.) is not more detrimental to the environment than other end-of-life treatments, such as incineration or landfill disposal. With this perspective, it can be concluded that the mere introduction of waste does not guarantee environmentally positive results [ 17 ]. Therefore, there is a need to conduct a Life Cycle Assessment (LCA) that considers the balance of these positive and negative aspects.

In the initial stage, a Life Cycle Assessment (LCA) is conducted considering the Global Warming Potential (GWP) in [kg CO2 eq] of the production of 1 ton of asphalt mix for each one of the studied mixtures. The system boundaries are indicated in Fig. (10). The LCA_Pave software [18] developed by FHWA is utilized, and its inventory is supplemented with bibliographic information. Tables 2 to 5 include the input data. The impact assessment method employed is TRACI 2.1, considering a horizon of 15 years.

Table 4.

Description	120 °C	160 °C	180 °C
SBDW	-		-
SBWW	-		-
PDDW	-		-
PDWW	-		-
3M	-		-
ELT	-		-
WMA		-	-
PMA	-	-
HIMA	-	-
HMA	-		-

Table 6 shows that WMA generates the lowest emissions per ton produced, followed by the conventional HMA. The other mixes can be considered polymer-modified asphalts, which incur higher impacts due to waste processing, the need to mix the binder with the residues when they are incorporated via a wet process, and the higher mix-manufacturing temperatures.

However, it is crucial not to overlook the impact that is avoided when choosing to recycle, i.e., the impact that would occur if waste were given a different end-of-life treatment than recycling [19-21]. This avoided impact is considered an “environmental credit,” and it should be subtracted from the values in Table 5. A comparative graphic is presented in Fig. (11), showing the decrease in GWP values due to this consideration. It is worth noting that the end-of-life treatment alternatives for each type of waste will depend on each region's current legislation and practices.

It is essential to note that this analysis overlooks the performance of the mixes when they are included as part of a structure, meaning the environmental benefits of having an asphalt mix with better mechanical properties and greater durability are not considered. Considering that all base layers have the same thickness (10 cm) but are made of different materials, they will not all have the same life span. Therefore, it is crucial to evaluate the usage stage, which includes road maintenance and rehabilitation processes for a 15-year horizon. It is important to clarify that the initial construction of all pavement alternatives implicates the same environmental impact and therefore, its consideration is not relevant to this comparative analysis.

To delve deeper into this issue, the focus is now turned to the sustainability of structures that incorporate the studied mixes. As an example, the Global Warming Potential (GWP) in [kg CO2 eq] caused by the rehabilitation of 1 km of road for the structure with HMA is evaluated and compared with that of a mix incorporating recycled plastics. The structure with the SBDW mix is chosen for this comparative study.

For both alternatives, rehabilitation involves milling and replacement. The milling depth is 1 cm, followed by a 2 cm sand-asphalt mix layer and then a 3 cm overlay of conventional asphalt mix. It is assumed that the first rehabilitation occurs when the rut depth reaches 12 mm, and the deterioration evolution after rehabilitation follows the same pattern as the initial one. After BackViDe proces-sing, it is concluded that the HMA structure requires two interventions over the 15-year period studied, while the SBDW structure requires only one (Figs. 12 and 13).

Fig. (14) illustrates the system boundaries. The hot mix asphalt is transported a distance of 20 km from the production plant to the construction site, and for the bituminous emulsion for coating, 300 km is considered.

After processing the data in LCA_Pave, it is determined that the Global Warming Potential (GWP) associated with the rehabilitation of 1 km of roadway (two lanes of 3.35 m) is 61,534.0 kg CO2 eq. This value corresponds to approximately 77 kg CO2 eq per ton of in situ mix, similar to results obtained by other authors who conducted similar studies [22, 23].

For the structure with CA30, two of these rehabilitations are necessary over the studied 15 years, while for the SBDW alternative, only one is required. Therefore, the HMA structure implies double the impact. The comparative advantage of SBDW lies in the improved mechanical properties that recycled plastic imparts to it, making it a more resistant and durable material. This positively impacts the reduction of the need for mainte-nance and rehabilitation over time.

From another perspective, if one wishes to design a structural package with a specific lifespan, the use of asphalt mixes with improved mechanical properties results in thinner base layers, leading to a reduction of impacts due to the lesser use of resources and energy [24].

3.4. Final Balance of Selection Criteria

It is desirable for construction professionals and public workers alike, to find the most beneficial flexible pavement structure solution. For a certain asphalt mixture to achieve this characteristic, it needs to perform adequately in different aspects: technical, environmental and economical. The weighing of each aspect depends on the priorities that the professional has on every particular task so the decision cannot be final in this study. Fig. (15) shows the performance of each mixture in the studied areas. Clearly, HIMA and ELT mixtures exhibit promising results. SBWW and PDWW have potential as well, regard-less of the chosen weighing.