1. Introduction
Increasing urban development trends have resulted in the construction of dense road infrastructure. Conventionally used Hot-Mix Asphalt Pavement (HMA) caused various environmental impacts from its material production to operations and disposal. This has prompted the government agencies and designers to search for appropriate ways to reduce the detrimental environmental impacts of road construction and maintenance, such as consumption of natural resources and production of emissions that are harmful to the health of the workers and residents. An increasing focus on the use of reclaimed asphalt pavement (RAP) in the construction and maintenance of roads around the world has been noticed in the recent past. RAP holds several environmental and economic benefits gained from recycling in comparison to other recycling technologies [
1].
RAP materials are produced from road maintenance activities by milling or crushing the existing HMA. Reusing RAP as a surface course [
2] or base and sub-base layers [
3] is the primary advantage, while it can also be mixed with other materials to improve the performance, such as using crumb rubber [
4] and Portland cement concrete [
5]. Cold in-place recycling (CIR) technology is an alternate to make use of RAP. Saudi Arabia started exploring the possibility of using this technology on a large scale after launching the 2030 Vision that aims to establish a sustainable economy, preserve natural resources, and reduce harmful emissions. Besides the economic and environmental benefits of recycled pavements, the need for specialized equipment and skilled labor is among some of the anticipated barriers to their implementation. Evaluation of the recycled pavements, encompassing all the three dimensions of sustainability, has yet to be evaluated in Saudi Arabia.
CIR technology follows a continuous process of cold milling of the pavement surface and remixing with asphalt emulsion or other modifiers to improve the qualities of the reprocessed material, followed by screeding and compaction of the reprocessed materials [
6]. Chemical additives are often used to improve the efficiency of the CIR process. The process starts with a milling machine pulverizing to deteriorate the top 50–100-mm layer of HMA. To achieve the necessary gradation, the milled material is crushed and screened on site. Subsequently, the milled grains are mixed with binding agents, such as emulsion, cement, lime, or fly ash. The mixture is reapplied to the roadway, which is subsequently graded to the final elevation [
6]. CIR holds several advantages over HMA in terms of reducing aggregate usage, material transportation, and energy consumption. Furthermore, using CIR is environmentally sustainable as it reduces carbon dioxide (CO
2) emissions by 9% over the lifecycle as opposed to conventional mixtures; the CO
2 emission reduction is 54% when just considering the recycling process [
7]. According to Schwartz (2016), CIR technology reduces CO
2 emissions by 80% as compared to conventional HMA applications and saves 60% of bitumen content [
8]. Presently, the RAP is being used for low-traffic roads in Saudi Arabia, where a simple surface treatment is all that is required. Examples of such work include restoring a damaged pavement, excessive cracking, extreme rutting, and an unstable base or subgrade [
9,
10,
11]. The quality of the old milling materials influences the strength of the cold recycled asphalt mixture [
12]. The sustainability of CIR for the construction of major highways in Saudi Arabia has not been investigated to date.
To investigate how the RAP temperature affects the strength of the CIR mixture components, Kim and Lee (2011) prepared samples of foamed CIR at various temperatures and performed indirect tensile strength [
13]. They determined that the temperature of RAP materials has a substantial effect on the wet indirect tensile strength of CIR foam mixtures, and the optimal foam quality varies with temperature after checking the samples. RAP sources and properties, as well as residual binders, influence CIR mechanical properties and efficiency. In comparison to traditional HMA blends, it is clear that the literature indicates that the CIR technology has quality engineering properties and field efficiency to be effectively used in constructing low-volume traffic roads.
Construction and maintenance of roads require an extensive amount of material and energy that significantly impact both the physical environment and natural resources. The Federal Highway Administration (FHWA) considers that a sustainable pavement should meet the basic human needs, use available resources effectively, and conserve adjacent environment [
14]. Sustainability of pavements can be achieved during the entire life cycle, including all the processes initiating from material production, pavement design, construction, operation, and maintenance/rehabilitation, to the end of pavement life [
15].
CIR has a lot of potential for repairing, strengthening, and recycling asphalt pavements. However, estimating the design thickness of the pavement layers from CIR compared to HMA for different traffic loads is not yet clear in the literature. The main objective of the present research was to develop a sustainability evaluation methodology for CIR and HMA pavements. The thickness of an equivalent CIR pavement that behaves similarly to the conventional HMA was estimated for different traffic loads. Scenario analysis of both the CIR and HMA paving structures for varying traffic classes and thicknesses for the base and sub-base layers was performed. The study also aimed to find the difference in thickness of the paving layers for each method and use this data to determine the difference in the cost and environmental and social impacts of CIR compared to HMA. Finally, sustainability of all the pavement scenarios was evaluated using fuzzy-based multicriteria analysis.
4. Discussion
The results presented in
Figure 3 show an overall supersedence of CIR over the HMA pavement for all traffic classes due to its low cost and minimal use of natural resources. Interestingly, in the case of ‘very light’ traffic class, HMA with a 0-cm sub-base, 15-cm base, and 16-cm wearing coarse (A1.1: 0SB-15B-16HMA) came out to be the second-best scenario based on the overall sustainability rank. The sustainability rank score of the top-ranked CIR scenario with the same thicknesses of sub-base and base and almost two times thicker wearing course (A1.2: 0SB-15B-29CIR) was almost 10% higher (1.0) than the HMA scenario (0.894). The results could be different if a higher relative weight is given to C3: need for advanced equipment and skilled labour. In the same traffic class, the third and fourth ranks were also obtained by the HMA. These results suggest the use of HMA for the farm-to-market roads with very-light traffic (<300,000 ESALs), where the availability of both the advanced equipment and skilled labors could be a primary constraint to using CIR pavements.
In the case of the ‘light’ traffic class, the CIR scenarios superseded the HMA scenarios with the top two sustainability ranks. In the comparative evaluation of the two pavement types for a given thickness of sub-base and base, CIR scenarios also obtained higher ranks. For instance, A2.5:10SB-20B-17HMA ranked at third place while the CIR scenario (A2.6:10SB-20B-24CIR) obtained second rank. Similarly, A10:20SB-25B-19CIR outperformed A2.9:20SB-25B-113HMA by getting the fifth rank in comparison to the seventh rank of its counterpart. CIR attained the first four ranks in the overall classification for the medium traffic class (7,000,000 ESALs), except for an equal thickness of sub-base and base (20 cm) where HMA got fifth rank and CIR the seventh.
Figure 3 illustrates an analogous behavior for ‘heavy’ and ‘very heavy’ traffic classes where all the CIR scenarios were visibly surpassing their comparable HMA scenarios.
The proposed evaluation of CIR and HMA clearly revealed that CIR technology is comparable to HMA for all traffic classes with some additional thickness of the surface layer. This approach will not only minimize the environmental impacts on non-renewable natural resources (aggregates) but also mitigate the visual nuisance due to open dumping of the asphalt mix generated from road maintenance. The proposed methodology will help the pavement engineers and managers for applying CIR as a sustainable construction and maintenance technology in Saudi Arabia and elsewhere.
5. Conclusions
The present research investigated the design thickness of the recycled-based pavement (CIR) equivalent to the conventional HMA pavement under the influence of different traffic loading classes. Five design scnearios with varying thicknesses of sub-base, base, and wearing course for each traffic class were investigated that generated a total of 50 scenarios, 25 each for CIR and HMA pavement types. All the scenarios were evalauted for their economic (construction cost), enviornmental (natural resource depletion), and social (need of advanced equipment and skill labour) sustainbility.
A thicker CIR surface layer behaves identically to that of HMA. Moreover, the ratio between the CIR and HMA layers reduces as the thickness of base and sub-base layers increases. The difference of 4 cm for very light (250,000 ESALs) and 12 cm for very heavy (31,000,000 ESALs) traffic classes was obtained for a 15-cm base layer in the absence of a sub-base. The difference reduced to 1 cm for very light and 10 cm for very heavy traffic classes for a 10-cm sub-base and 20-cm base layer. Interestingly, the same thickness (7 cm) was found for both the CIR and HMA for very light traffic with equal thickness (20 cm) for both the base and sub-base, while 9 cm was found for very heavy traffic.
The results of the sustainability evaluation menifest a clear dominance of CIR over the HMA for all classes except ‘very light’ traffic. This means that CIR is a more sustainable pavement overlay for feeders, collectors, main urban streets and highways, expressways, and heavily trafficked highways in industrial areas where ESALs range from 2,000,000 to more than 31,000,000. In the particular case of the ‘very light’ traffic class, HMA was found be a more sustainable option, keeping in view the limited availability of advanced equipment and skilled labour for construction of farm-to-market-roads. A recycled pavement mixture could present an economic design alternative to the conventional mixtures and should be considered when designing and rehabilitating roads. This sustainable alternative is not only economically attractive but also it conserves the environment with less use of natural materials. The study will help the design engineers and infrastructure asset managers for planning pavement construction and rehabilitation programs in Saudi Arabia and elsewhere. Future studies can investigate deatiled lifecycle costing and lifecycle assessment of CIR pavements.