1. Introduction
In recent times, there has been a growing awareness that the development of societies should be as sustainable as possible. The road sector is not oblivious to this demand and is making augmented efforts to develop products and processes to increase the contribution of road infrastructures to sustainability. In particular, in recent years, there has been a lot of research aimed at reducing the environmental impact of asphalt mixtures for flexible pavements, not only from the point of view of reducing the carbon footprint of their vital process, but also by turning them into active tools that recycle by-products or waste material. The ultimate goal is to design asphalt mixtures with similar or superior technical performance to conventional asphalt mixtures, but with a lower environmental impact.
One of the most used asphalt mixtures is asphalt concrete (AC) because it can handle high traffic loads. However, because there is an increase in the demand for higher and different load configurations each year, as well as constant temperature changes caused by global warming, this mixture is prone to failures of various kinds, such as plastic deformation and cracking. To solve these problems, different additives or modifiers have been studied to improve the durability of AC mixtures, such as polymer modified bitumen (PMB) [
1,
2,
3], fibres [
4,
5], plastics [
6,
7], etc. However, these additives also have disadvantages: the main drawbacks are the cost, which turns out to be high; the increase in mixing temperature, which leads to higher environmental pollution; and the problems of segregation between the asphalt-polymer phases during transport to the paving site or during static storage at high temperature [
1].
At the same time, the consumption of polymer products has increased worldwide in recent decades from 230 million tons in 2009 [
8] to 368 million tons in 2019 [
9], an increase of 60% in only 10 years. This trend has resulted in a large waste stream that must be properly managed to avoid environmental damage. For this reason, this article focuses on residual plastics as an additive in asphalt mixtures, especially recycled plastic from municipal solid waste treatment plants, which could be used to promote a circular and sustainable economy and thus reduce production costs. This kind of facility combines automatic and manual sorting processes for the separation of recoverable fractions from the municipal solid waste mixture. In the first stage, bulky waste is separated manually. Subsequently, different automatic processes are applied to separate and classify the waste according to composition, colour, material, etc. Once the residual fractions have been separated, they are subjected to a mechanical process consisting of crushing, washing, centrifuging and drying.
In order to integrate residual plastic samples in bituminous mixtures, it is necessary to subject them to a grinding process to obtain particle size fractions of 2/10 mm, which is achieved by means of a knife mill. Once shredded, these fractions are washed in order to remove debris and other residues that may have adhered to them, as well as other elements such as labels or stickers. For this washing, water and different concentrations of surfactants or surfactants with a high soda content are used at a temperature ranging from 25 °C to 40–70 °C. Any traces of surfactants and soda that may have adhered to the particles are removed by successive rinses with clear water. In addition, there are alternative cleaning routes which allow the reduction in organic surface contamination that do not involve the use of water, and are based on dry cleaning, by friction of the surface between the plastic particles. The last pre-treatment step consists of drying the cleaned particles to achieve complete removal of moisture. This treatment has been used to obtain the plastics that will be analysed in this article.
Once the plastics have been prepared for inclusion in the mixture, there are two ways of incorporating them into it: a wet or dry process. The first one consists of crushing the plastics and adding them to the hot bitumen; subsequently, the plastic-modified bitumen is added to the mixer as a binder. This is the most common method, but the principal disadvantages are the necessity for specialized plants for the digestion process at high temperatures, expensive costs and compatibility problems [
10,
11,
12]. In addition, the maximum amount of plastic that can be added to the mixture is smaller (8%) than by the dry method (more than 15%) [
13], reducing the impact on the recycling rate. The second one consists of adding the plastics directly into the mixer together with the aggregates and bitumen. This technique is less developed than the wet method [
14,
15,
16,
17,
18]; however, it is easier to apply since it is not necessary to make major modifications to the asphalt plant and requires less energy. This facilitates the spread of the process and facilitates the reuse of polymeric waste. Furthermore, it is more economically profitable [
19] and allows the incorporation of larger quantities of plastics (greater than 15%) [
13].
Until now, the impact has been mostly assessed with conventional bitumen only. In this case, the study includes new properties analysed, such as cracking energies, and the extent to which they reach the performance of PMB. All the information provided so far leads to the study carried out by this research, which consists of the design of new asphalt mixtures incorporating polymeric wastes using the dry method. This work evaluated the suitability of different residual plastics to replace virgin bitumen by analysing the mechanical behaviour of AC mixtures in which a binder fraction has been replaced by three different types of plastic fibre waste, whose cost is lower than other polymeric materials.
2. Materials and Methodology
2.1. Materials
The materials used are those normally employed in the manufacture of asphalt mixtures, except for plastic fibre waste of difficult valorisation. Ophite, a type of porphyry igneous rock commonly employed in the north of Spain, was used in the coarse fraction, while limestone was used in the fine fraction and filler. A conventional 50/70 penetration grade bituminous binder and a commercially available polymer-modified bitumen (PMB 45/80-65) were used to manufacture the mixtures. The properties of the aggregates and bitumen used are shown in
Table 1 and
Table 2, respectively.
Plastic fibre waste materials have to meet the technical requirements of homogeneity and cleanliness. The former is a key point because the properties and composition of the plastic waste should be approximately the same over time, in order to achieve a similar behaviour, independently of the moment the asphalt mixture is manufactured. This point is difficult to achieve because some plastic waste packages depend highly on the season (summer, Christmas, etc.). Cleanliness is another key issue, in order to avoid adding organic materials to the mixer. Apart from fulfilling these conditions, it is necessary that enough plastic waste is generated so that big quantities can be used in road manufacturing.
The selected plastic waste of this project fulfils these technical requirements. They were segregated fractions in the municipal solid waste treatment plant of Algimia (Castellón, Spain) managed by the company “Residuos Palancia Belcaire” (
Figure 1). The theoretical composition and recovery technique applied to each one is detailed below:
PLASTIC-1 (PLA-1) is a mixture of baskets. This fraction is separated manually at the bulky waste triage stage.
PLASTIC-2 (PLA-2) is a mixture of drums, pipes, toys, etc. This fraction is separated manually at the bulky waste triage stage.
PLASTIC-3 (PLA-3) is a PP/PE polymer blend obtained as a residual fraction from a solid waste sorting process by means of optical sorting.
Different technical tests were carried out to characterize the plastics used.
Figure 2 represents the granulometry (UNE-EN 933-2) of each residual plastic. It can be seen that 8 mm was the maximum size and the particle distribution was quite similar, especially between PLA-1 and PLA-3, which had an almost identical granulometry.
The exact composition of the residual plastics was calculated using infrared spectrophotometry (FT-IR) and differential scanning calorimetry (DSC) according to EN ISO 11357-1 and EN ISO 11357-3 standards, respectively. In addition, the density was obtained through the ISO 1183-1 standard, method B: liquid pycnometer. The components and density of each sample were as follows:
PLA-1: Blend of low-density polyethylene (LDPE), medium-density polyethylene (MDPE), ethylene-vinyl acetate copolymer (EVA), and polypropylene (PP) in a minor quantity, with the presence of SiO2 and CaCO3. This plastic had a density of 0.902 ± 0.030 g/cm3.
PLA-2: High density polyethylene (HDPE) with the presence of SiO2 and CaCO3. Density was 0.879 ± 0.021 g/cm3.
PLA-3: Polypropylene (PP), medium-density polyethylene (MDPE) and low-density polyethylene (LDPE) with the presence of SiO2 and CaCO3. In that case, the plastic’s density was 0.936 ± 0.009 g/cm3.
2.2. Sample Preparation
The manufacturing temperature was determined according to the information provided by the bitumen supplier; therefore, reference and experimental mixtures were produced at the same temperature (150 °C), because they used the same 50/70 penetration grade binder. Only the control mixture, which was produced with PMB 45/80-65 binder, was different; in this case, the manufacturing temperature was 165 °C according to the supplier.
For the preparation of the experimental mixtures, part of the virgin bitumen was replaced with residual plastics. This replacement was made in volume, trying to modify the structure of the reference mixture as little as possible. The incorporation of waste plastic was carried out using the dry process, pouring them directly into the mixing drum at room temperature. This method was selected because even though the plastics are joined with the hot material, they are not pre-heated, which minimizes the generation of gases [
15]. In addition, this technique is a simple alternative that has been shown to be effective when making experimental mixtures, and it can be easily replicated in practically any asphalt plant. Two possible methods were studied within the options presented by the dry process:
Method A: Incorporation of residual plastics in the coarse fraction. The plastic was poured over the coarse fraction before incorporating the rest of the aggregates and bitumen, so that the plastics soften forming “bonds” with this type of aggregate.
Method B: Incorporation of waste plastics after pouring bitumen. When bitumen was added to the coarse and fine aggregate fraction, it formed a film around them, so that the plastics were mostly embedded in the matrix which forms the mortar of the mix.
A scheme of the two methods can be seen in
Figure 3. The mixing time was increased for one minute at the time of incorporating the residual plastics to ensure a correct dispersion of the residual plastic in the mixture, which was checked visually. In addition, the particle size distribution was the same in all mixtures (
Figure 4), varying only the amounts of bitumen and residual plastics used.
2.3. Mixture Designs
Ten experimental asphalt concrete (AC) mixtures were developed for this work. The first 6 designs were produced by varying the type of waste plastic (PLA-1, PLA-2 and PLA-3) and the method of incorporation (method A and B). While the last 4 experimental mixtures were produced by raising the percentage of bitumen replaced and varying the type of compaction (normal or over-compacted). The nomenclature of the different designs was defined based on the type of plastic, the type of incorporation method and compaction. Thus, the designed mixture PLA1-A-N corresponds to the AC mixture with PLA-1 manufactured using method A and normal compaction. Similarly, the mixture PLA2-B-OC corresponds to the AC mixture with PLA-2 manufactured using method B and compacted with twice as many conventional blows.
Table 3 details the different designs carried out.
Experimental mixtures were compared with a reference and a control AC mixture for surface layer, whose difference was the type of bitumen used in their manufacture. Both mixtures had a void content of approximately 5%. This characteristic was achieved with a bitumen content of 4.3% by weight of mixture in both cases. A double comparison was conducted to analyse how much the experimental mixtures improve with respect to a conventional bitumen and, at the same time, to check if this improvement was comparable with that obtained using PMB.
2.4. Experimental Work Plan
The first milestone of this research was to obtain a residual plastic that provided the greatest functional improvements to the asphalt mixture, as well as the most efficient manufacturing method. To achieve this milestone, the results of the following tests were evaluated: air void content (EN 12697-8), the Marshall test (EN 12697-34), the water sensitivity test (EN 12697-12) and the wheel tracking test (EN 12697-22). All of them were performed on experimental mixtures containing 15% virgin bitumen replaced with waste plastics, always incorporated using the dry process.
The second milestone of the research was more ambitious and consisted of optimizing the virgin bitumen content that could be replaced, while maintaining the feasibility of manufacturing asphalt mixtures and the mechanical behaviour. To do so, the percentage of virgin bitumen replaced with plastic waste was increased to a maximum percentage of 25%. During this process, it was found that the impact of plastics was particularly significant on the density of the mixtures, increasing the percentage of voids, so it was decided to use two different compaction energies for the final design of the experimental mixtures. The first one was a normal compaction, which means that the same energy was applied as was applied to the reference mixtures. On the other hand, the energy used for the over-compacted specimens was twice that used in the reference mixtures.
To accomplish the second milestone, in addition to the mechanical test mentioned before and in order to check the cohesion of the experimental mixtures due to their high percentage of voids, the Cantabro test (EN 12697-17) was also performed on them.
Finally, to fully characterize the asphalt mixtures, stiffness (EN 12697-26) and fatigue resistance (EN 12697-24) were evaluated to check the dynamic performance of the over-compacted mixtures which showed the best balanced.
2.4.1. Air Voids and Marshall Tests
To measure bulk density and air voids in accordance with the European EN 12697-8 standard, Marshall specimens were used, compacted by 75 blows on each side in accordance with the European EN 12697-30 standard. It should be noted that the over-compacted specimens received 150 blows per face. Subsequently, the Marshall test was performed even though it is not currently included in the Spanish regulations, since it is one of the tests that has historically been most used to design bituminous concretes. Four replicates were performed for each mixture.
2.4.2. Water Sensitivity Test
The purpose of the water sensitivity test (EN 12697-12) is to determine the loss of cohesion caused by saturation and the action of water on a bituminous mixture. To do so, 8 cylindrical specimens were manufactured, compacted at 50 blows per face, except for the over-compacted ones that received 100 blows per side, which were divided into two batches of equal size. While one batch was left in dry conditions, the other was submerged in water for 3 days at 40 °C before it was broken. The indirect tensile strength (ITS) was determined in both dry and wet conditions (ITS
dry and ITS
wet) and the moisture susceptibility was obtained and expressed as a percentage according to Equation (1).
2.4.3. Cracking Energy Test
For the measurement of toughness, many researchers agree that the indirect tensile test is the most suitable test due to its simplicity [
20]. Stress–strain curves were recorded when the indirect tensile strength was determined (EN 12697-23). The fracture energy (FE) and post-cracking energy (PE) were calculated as the area under the curve before and after peak stress was reached, respectively, as shown in
Figure 5. The former (FE) was considered representative of the cracking resistance while the second (PE) showed the resistance against cracking propagation [
21,
22]. Cracking toughness was measured as the sum of both parameters. The performance of the mixtures was analysed in dry and wet conditions, so the impact of water damage was also assessed in relation to the propagation of fissures.
2.4.4. Wheel Tracking Test
To evaluate the rutting resistance of the experimental mixtures in accordance with the European standard EN 12697-22, the wheel tracking test was used. In this analysis, two prismatic specimens per mixture were made with the dimensions 410 mm × 260 mm × 50 mm. Conditioning and testing were carried out at a temperature of 60 °C. In this case, the result was determined by the slope, calculated using Equation (2).
where WTS was the inclination of the wheel track in mm for 10
3 loading cycles, and d
5000, d
10,000 were the bearing depth after 5000 and 10,000 load cycles in mm.
2.4.5. Cantabro Particle Loss Test
This test is traditionally applied for the evaluation of particle loss in porous asphalt mixtures. In this study, it was used on over-compacted mixtures as a method to check the cohesion of the mixtures, since a high percentage of voids could be considered as a possible risk. In accordance with the EN 12697-17 standard, the specimens were subjected to abrasion in the Los Angeles machine to measure the particle loss obtained after 300 turns. This loss is expressed as a percentage and is calculated by Equation (3).
where m
i and m
f were the initial and final mass of the specimens.
2.4.6. Stiffness and Fatigue Resistance Tests
These tests are key to evaluating the performance of the pavement as they condition the transmission of loads and its service life in the passage of axles. Moreover, they must be analysed together since the stiffness of a bituminous mixture is directly related to the deformation it undergoes and, therefore, to its fatigue damage.
Following the EN 12697-26 (Annex B) standard, a stiffness modulus analysis was carried out by means of the four-point bending test (
Figure 6). In the case of fatigue strength, the EN 12697-24 (Annex D) standard and the four-point bending test were used.
For these tests, eight specimens of each mixture with dimensions of 410 mm × 60 mm × 60 mm were required, which were obtained by cutting a specimen of 80 mm in height. Both tests were performed at 20 °C. The stiffness test was carried out in controlled deformation mode with a deformation amplitude of 50 µm/m at different frequencies, from 0.1 Hz to 30 Hz. The fatigue test was carried out by applying a frequency of 30 Hz in controlled deformation mode. The main test parameters obtained from this test were the fatigue law calculated through Equation (4) and the deformation at one million cycles.
where N is the number of loading cycles for a given level of strain ε (m/m); C
1 and C
2 are the fatigue constants.
2.4.7. Statistical Analysis
The results obtained from the experimental mixtures were statistically analysed using Minitab software to determine if the differences with the REF and CONTROL mixtures were significant. To do this, first, the normality of the data and the homogeneity of the variances were checked through the Kolmogorov–Smirnov and the Levene statistical test, respectively. Depending on the results obtained, Student’s t-test was used when a normal distribution of results and homogeneity of variances were observed, and the Mann–Whitney U-test was used otherwise. For all cases, a 95% confidence interval (p-value 0.05) and a significance level of 5% were considered.
4. Conclusions
In the present research, the effect of replacing virgin bitumen with different plastic fibre wastes was experimentally evaluated. After analysing different residues, optimizing the addition process and evaluating the mechanical performance, the following conclusions were reached:
The feasibility of replacing virgin bitumen with plastic waste was demonstrated from a mechanical point of view. The 25% replacement rate is technically feasible as long as the compaction energy is increased to control the increase in voids, although cracking performance should be analysed in more depth.
Method B was selected to incorporate the plastics into the manufacture of the mixture, which consists of adding the plastics after the bitumen using the dry process, embedding them in the mortar of the bituminous mixture.
Experimental mixtures showed a significant increase in voids due to differences in the viscosity of residual plastics with respect to virgin bitumen.
Regarding the residual plastics used, PLA-2 and PLA-3 showed better mechanical results and are considered the priority alternatives from a technical point of view.
The increase in resistance to plastic deformation is particularly noteworthy. As for the rest of the properties, the experimental mixtures obtained similar or moderately better results than the reference mixture. However, it should be noted that the cracking energy in wet conditions is lower than the energy of the reference mixture.
Mixtures, despite improving with respect to reference mix, are not as good as those manufactured with commercial polymer-modified bitumen, although in the case of resistance against plastic deformation they are quite similar.
After these conclusions, some properties that have not been assessed are the recyclability of the experimental mixtures, the possible modification of skid resistance and the potential generation of microplastics due to wear caused by vehicles on the roads. Therefore, future lines of research have been opened up, focused on analysing this technology.