Edited by: Castorina Silva Vieira, University of Porto, Portugal
Reviewed by: Xiaoming Huang, Southeast University, China; Gholam Hossein Hamedi, University of Guilan, Iran; Decheng Feng, Harbin Institute of Technology, China
This article was submitted to Sustainable Design and Construction, a section of the journal Frontiers in Built Environment
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
The use of additives in asphalt mixtures has been an important option in order to prolong the durability of the road infrastructure. Several works have reported that the utilization of synthetic fibers (e.g., plastic fibers) visibly enhance the overall mechanical performance of asphalt mixtures. The objective of this experimental study is to understand the physicochemical mechanisms leading to this superior response. With this in mind, two synthetic fibers were selected in order to analyze their effect within a conventional dense asphalt mixture used in Switzerland. The mechanical performance of the experimental mixtures along with the thermal properties of the fibers were evaluated. First, it was concluded that the fiber reinforced asphalt mixtures could perform similarly to the standard mixtures prepared with polymer modified binder. In addition, it was confirmed by imaging analysis that the reinforcing effect was associated to the physical presence of the fibers within the asphalt matrix.
The increase of heavy traffic and the effect of the climate change are the main questions that new designs for asphalt roads must address in the near future. The overall objective of road transportation must include a reduction of the maintenance frequency of road works usually linked to economic costs involving construction and utilization of new materials as well as higher commuting times for the users. In order to build better road infrastructures with longer service life, new solutions must be investigated and adapted to improve their current mechanical performance by reinforcing the asphalt matrix (
Recently, in order to increase their durability, different types of synthetic fibers have been used as additives to the asphalt mixtures for road construction (
The aim of the current research work is to experimentally evaluate the reinforcing effect of different synthetic fibers on the performance of a conventional dense asphalt mixture for base layers. This investigation is mainly focused on understanding the fundamental mechanisms governing the microstructural properties behind the overall behavior. The added functionality of the fiber reinforced materials is analyzed by using advanced characterization techniques. The mechanical performance of several asphalt mixtures is correlated to specific factors such as thermal properties of the fibers, their distribution within the asphalt matrix or the rheological response of the recovered binders after the fiber incorporation.
In this study, two commercially available fibers, designated as Type A and Type P, were selected to investigate the effect of their incorporation into conventional dense asphalt mixtures. Type A supplied by FORTA® Corporation (United States) consists of a combination of aramid (A1) and polyolefins (A2) fibers of 38 mm and 18 mm lengths, respectively. The ratio of this blend defined by the supplier was approx. 1:7 (aramid:polyolefins). The other type is polyacrylonitrile fibers (type P) of 4 mm length and nominal diameter ca. 10 mm supplied by Lambda Furtherance B.V. (Netherlands). Images of both types of fibers are shown in
Images of the different fibers evaluated. Type P
Four dense asphalt mixtures base course AC B 22 H according to the Swiss standard SN 640-436 (
Dense asphalt mixtures (AC B 22 H) prepared for the experimental study.
Name | Bitumen (4.2%) | Fiber |
ACB22H – Control | PmB 45–80/65 | – |
ACB22H – Reference | 50/70 | – |
ACB22H – Type P | 50/70 | P (0.15%) |
ACB22H – Type A | 50/70 | A (0.05%) |
Following guidelines provided by the fiber producers, the dry process was used to prepare the FRAMs as follows, fibers are added directly with the aggregate fraction and no previous modification of the bitumen is done. Although fibers are incorporated directly into the mixer in the asphalt plant, a special protocol for mixing at lab scale was recommended for fiber Type A in order to improve the fiber distribution within the mixtures. It was suggested that the aramid fibers (A1) were mixed first with the preheated mineral aggregates whereas the polyolefins fibers (A2) were added with the hot bitumen. First, the aramid fiber content was separated into two equal parts. Once the preheated (160°C) coarse aggregate fraction was added to the mixer, one part of the aramid component was introduced. Afterward, half of the content of fine aggregates plus the remaining content of aramid component were added to the mixer followed by the rest of fines. Finally, the entire polyolefins component fiber was incorporated to the pre-heated bitumen (4.2%wt.) at 160°C to immediately be poured into the mixer for 2.5 min mixing. In the case of the Type P fiber, the entire fiber content was added to the preheated mineral aggregates before pouring the hot bitumen. For the reference and control asphalt mixtures which were prepared without fibers, the conventional mixture procedures according to the European standards were followed. Finally, several cylindrical specimens and slabs were compacted.
In order to investigate the thermal transitions of these three types of fibers, a differential scanning calorimeter (DSC) from PerkinElmer (United States) was used. Samples of 5 ± 1 mg were prepared for the DSC analysis. The heating ramps were conducted at a rate of 20°C/min. Furthermore, the thermal stability of the fibers was analyzed by thermal gravimetric analysis (TGA) under a nitrogen atmosphere using TGA 209 from NETZSCH (Germany).
Water sensitivity tests were performed according to the European standard EN 12697-12 (
where
The fatigue resistance was investigated based on the adapted German standard AL-Sp-Asphalt 09, which has been included in the European Standard EN 12697-24 (
According to the standard, two parameters were selected to evaluate the fatigue resistance: the numbers of loading cycles (
Additionally, the Wöhler line (Eq. 4) can be used to express the material’s fatigue function. Hence, a minimum of three different strain amplitudes is required for each testing condition.
where, εel is the horizontal elastic initial strain and C1, C2 are fitting constants.
Furthermore, from the fatigue line the classical parameter ε6 defined as strain to reach one million cycles was also calculated.
Rutting resistance (permanent deformation) tests were carried out by using the large device wheel tracking test according to the European Standard EN 12697-22 (
Samples of the FRAMs were evaluated using the Environmental Scanning Electron Microscope (ESEM). For this purpose, specimens (28 mm × 47 mm × 10 mm) were cut from the center of cylindrical specimens. The samples were impregnated with resin and polished using sand paper and water as lubricant. More details about the sample preparation can be found elsewhere (
To understand the chemical effect, if any of the different types of fibers on the behavior of the experimental mixtures, samples of bitumen were extracted from the different asphalt mixtures with toluene. Then, the bitumens were recovered by rotatory evaporator according to the European Standard EN 12697-3 (
In an asphalt plant, the blending conditions of the asphalt binder components depend on the type of asphalt mixing adopted. Hot mixing of asphalt at temperatures from 130°C to 160°C, depending on the binder viscosity, is the common technology for preparing asphalt mixtures. The required mixing temperature should provide sufficiently low viscosity of asphalt binder to ensure full coating of all aggregates. Therefore, the fibers must be able to survive these temperatures and mechanically tough mixing conditions. In order to investigate such conditions, the fibers type A (aramid and polyolefins) and type P (polyacrylonitrile) were subjected to heating ramps using TGA. The measured weight loss of the fibers with the increase of temperature is shown in
Weight loss of fibers versus temperature during TGA heating ramps. Type P (polyacrylonitrile fiber) and type A (aramid and polyolefins fibers).
It can be observed that for both fibers forming type A (aramid and poyolefins), the weight of the sample is kept practically constant until temperatures above 400°C. This means that, at conventional temperatures for hot asphalt mixes (ca. 160°C), the fibers will not suffer any thermal degradation. Regarding the fiber type P (polyacrylonitrile), the weight loss starts at a temperature of 280°C. It can be also seen that the curve has different steps that indicate the degradation of components that decompose differently when exposed to temperature. Nevertheless, these temperatures are still far from the mixing temperatures normally used in standard asphalt mixing procedures, so, it can be confirmed that no thermal effect is expected for this kind of fiber either.
The fibers were also thermally analyzed for a large temperature range (from 20°C to 500°C) by differential scanning calorimetry in order to study their thermal transitions. The heat flow curves for fibers type A and P are given in
Heat flow curves versus temperature for the different fibers studied. Aramid fiber
During the mixing process at high temperatures, the fibers could melt and hypothetically react chemically with the bitumen and modify it. This could then change the binder properties affecting the performance of the asphalt mixtures. In
Concerning the polyacrylonitrile fibers (type P), the DSC measurement (
Indirect Tensile Strength (ITS) results for the mixtures in dry and wet conditions.
The water sensitivity can be interpreted using the indirect tensile strength ratio (ITSR). These results are shown in
Water sensitivity testing results by using ITSR (%).
Fatigue testing results for three different loading amplitudes at
In
Rutting (wheel tracking) testing results for different mixtures.
In parallel to the mechanical characterization, imaging analyses were carried out for different samples to visualize the fibers within the asphalt matrix in order to understand their role based on this aspect. The use of the EDX technique helped to identify the fibers thanks to the analysis of the chemical elements. First, single analyses of the fibers were done to characterize their chemical compositions. ESEM images of polyolefin, aramid and polyacrylonitrile fibers and their elemental analyses of characteristic points are shown in
Elemental analysis (atom%) of the fibers.
C | N | O | Na | S | Si | |
Aramid (Type A1) | 61.24 | 21.08 | 17.28 | 0.29 | 0.11 | – |
Polyolefin (Type A2) | 100.00 | – | – | – | – | – |
Polyacrylonitrile (Type P) | 52.30 | 38.73 | 8.78 | – | – | 0.19 |
ESEM micrographs of aramid fibers (type A1), polyolefin fiber (type A2), and polyacrylonitrile fibers (type P).
The EDX analysis of the polyolefin fibers (type A2) indicates that they contain mainly carbon elements. This means that, if these fibers were not melted during the mixing process, as discussed in the previous section, this fact will make it more difficult to identify them within the asphalt matrix which also contains hydrocarbons. Concerning the elemental analysis of the aramid fibers (type A1), it can be seen that more atoms appear (N, O or S) which reveals the presence of their characteristic amide groups. These particularities will later help to analyze the micro-images from the FRAM modified with fibers type A. Likewise, the EDX analysis of the fibers type P (polyacrylonitrile) indicates that they primarily consist of carbon (C), nitrogen (N) and oxygen (O). This is directly related to the presence of the acrylonitrile groups. In this case, the presence of nitrogen will be crucial to identify these fibers mixed with the asphalt and mineral aggregates.
Next, the ESEM images from the sample of the FRAMs using fibers type A (aramid + polyolefins) and type P (polyacrylonitrile) are shown in
ESEM images of samples from the FRAM with type A fibers
Similarly, the presence of parts of fibers type P can be clearly observed in
Finally, samples from the different mixtures were extracted and the bitumen recovered in order to study whether the fibers had any chemical effect on the binders. A potential bitumen modification was mainly expected as a consequence of the melted polyolefins fibers. The samples of the different bitumens recovered from the experimental mixtures were first tested to evaluate their softening point. These results, along with the properties of both bitumens in their original state, are shown in
Bitumen properties before and after mixing and extraction processes.
Softening point | |||
Original | Reference | 50/70 | 49.0°C |
Control | PmB | 66.4°C | |
After recovery | Reference | 50/70 | 55.0°C |
Control | PmB | 66.8°C | |
ACB22H – P | 50/70 + type P | 55.5°C | |
ACB22H – A | 50/70 + type A | 56.1°C |
First, it can be seen that there is an aging effect on the unmodified bitumen 50/70 which leads to an increase of the softening point from 49°C to 55°C. This effect was not observed in the polymer modified bitumen used for the control mixture. Regarding the influence of the fibers, at mixing temperature (160°C), a chemical modification of the bitumen could be expected by the polyolefin fibers (type A2) due to their melting temperature (124°C). Nevertheless, it can be confirmed that there was no chemical modification due to any of the fibers. Values obtained for the bitumens recovered from the FRAMs are quite similar to that obtained for the reference mixture. Therefore, the increase of the softening point must be attributed to the aging effect during the mixing process.
This behavior could be observed in more detail in
Master curves
In this work, the suitability of certain fibers for fiber reinforce asphalt mixtures (FRAMs) is experimentally investigated. Two commercially available fibers were used for this purpose, i.e., type A (a blend of aramid and polyolefins fibers) and type P (polyacrylonitrile fibers). Four different AC B 22 H mixtures with and without fibers were prepared for comparison purposes. Water sensitivity, rutting resistance and fatigue resistance tests were performed. Furthermore, the thermal characterization of the selected fibers, image analysis of samples of the FRAMs and the evaluation of the bitumen recovered were carried out. These studies aimed at understanding the chemo-mechanical mechanisms behind the performance obtained for the FRAMs.
Based on the testing results, the following conclusions can be drawn:
Similar water sensitivity properties were found between the FRAMs prepared with fibers and the reference one prepared with polymer modified bitumen (PmB). The positive effect of the addition of the fibers was observed when comparing it with the reference mixture prepared with the same unmodified bitumen (50/70) used as a base bitumen. In this sense, it was confirmed that FRAMs would fulfill the Swiss normative water sensitivity requirements of ITSR for this type of mixtures by reaching rates higher than 70%.
After analyzing the performance obtained for the rutting resistance test, it can be concluded that the FRAM modified with Type P fibers behaves similarly to the control mixture (PmB). Whereas, the FRAM with type A fibers showed deeper ruts at 60°C. Therefore, only FRAM with fiber type P satisfied the current Swiss requirements.
The incorporation of both types of fibers improved the fatigue response of the mixture with respect to the unmodified bitumen (reference) at lower strain levels. Nevertheless, this modification would not reach the general performance against fatigue distresses observed for the control mixture prepared with PmB.
The imaging analysis with ESEM plus EDX method has confirmed the presence of polyacrylonitrile (type P) fibers and aramid fibers (type A1) within the asphalt matrix after going through the mixing and compaction process. This confirms the improvement of the mechanical properties of the FRAMs is directly related to a physical influence of the fibers.
The thermal analysis of the fibers indicate that no degradation will happen during the mixing process due to the temperature. Besides, since their melting temperatures were found to be higher than the mixing temperatures, polyacrylonitrile (type P) fibers and aramid fibers (type A1) will remain in solid state. However, the melting temperature for the polyolefin fibers (type A2) was found to be lower, thus, they could melt and modify the bitumen. This effect could be amplified for the addition of these type of fibers directly to the hot bitumen.
Nevertheless, after extraction and recovery of the bitumens from the different asphalt mixtures, no significant modification due to the addition of the fibers was observed. The possible melting of the polyolefin fibers (type A2) does not affect the bitumen properties. The chemical composition of these fibers (mainly carbon) made it impossible to distinguish them within the asphalt matrix in the ESEM analysis. Therefore, it is not clear if these fibers survive the mixing process (small mixing times) and play a reinforcement role like the other fibers evaluated in this study.
Although the results obtained in this study are promising, additional experimental support is needed by extending the present research effort to the investigation of more types of asphalt bitumen and fiber types as well as construction of test sections to determine their
The datasets generated for this study are available on request to the corresponding author.
MB and LP designed the experiment plan, supervised the findings, and discussed the results. MB wrote the main manuscript. LP reviewed the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.