The interest in using bio-materials in pavement engineering has grown significantly over the last decades due to environmental concerns about the use of non-recoverable natural resources. In this paper, bio-materials are used together with Reclaimed Asphalt (RA) to restore some of the properties of the aged bitumen present in mixtures with high RA content. For this purpose, two bio-materials are studied and compared to conventional and polymer modified bitumens. Blends of these materials with RA bitumen were produced and studied to simulate a 50% RA mixture. The rejuvenating effect of the two bio-materials on RA has been assessed and compared with the effect of the conventional binders. Apparent Molecular Weight Distribution of the samples (obtained by the δ-method) and different rheological parameters were used for this purpose. Results revealed the power of bio-materials to rejuvenate RA bitumen, showing their capability to be used as fresh binders in high-RA content mixtures.
Bio-materials that are used as binders in asphalt mixtures are termed “biobinders” and were defined by Peralta et al. (
According to the literature, biobinders can be used in three different ways to decrease the demand for fossil fuel based bituminous binders: (
An alternative way to use biobinders in asphalt mixtures is combining them with the aged binder present in Reclaimed Asphalt (RA). Binders in RA are known to be brittle and stiff due to their exposure to climate changes and traffic loading during their service life. This fact could promote the prompt appearance of non-desired distresses in the pavement (such as fatigue and thermal cracking) and prevent authorities and constructors from using RA in high contents (
The aim of this paper is to study the potential of biobinders to totally replace conventional bitumen for the manufacture of asphalt mixtures with high RA content. For this purpose, recycled, bio and conventional binders have been rheologically tested and compared. Then, bio and conventional binders were blended with a reclaimed asphalt binder. The rejuvenating effect of the different materials over the recycled one was assessed using different techniques. Among others, the innovative δ-method developed in IFSTTAR (
A Reclaimed Asphalt (RA) source in France was selected for the study. RA binder was recovered following the EN 12697-4:2005 Fractionating Column by distillation (
In order to compare the behaviour of these two bio-materials with standard ones, a 70/100 penetration grade bitumen and SBS-polymer modified binder were selected as reference binders since they are soft materials usually used as binders in RA mixtures. Finally, a 50/70 penetration grade bitumen was included in the study in order to have a control binder which is often used in standard asphalt mixtures. Conventional properties of the six binders are shown in
Binders’ characterisation
BINDER | ROLE in the study | NAME | PENETRATION @ 25° (dmm) ( |
SOFTENING POINT (°C) ( |
---|---|---|---|---|
Reclaimed asphalt binder | Recycled binder | RA | 8.7 | 75.8 |
80% pine resin + 20% linseed oil | Bio-material | BB | 235 | 40 |
Biophalt® | Bio-material | BP | 147 | 73.5 |
70/100 penetration grade bitumen | Reference for BB | 70/100 | 86 | 46 |
SBS-polymer modified bitumen | Reference for BP | PMB | 85 | 67 |
50/70 penetration grade bitumen | Control | 50/70 | 68 | 47.6 |
The bio-materials, considered as a total virgin bitumen replacement, and reference binders were blended with RA binder in order to study their rejuvenating effect. The percentage of blend between RA and the different binders was calculated according to the Replaced Virgin Binder (RVB) concept (
Where,
For the production of the four blends of RA and bio or reference materials, RA was heated to 160°C (due to its hardness) and the other binders were heated to 140°C. Once the proportions of each material were weighted and put together in a tin, they were introduced into an oven at 150°C for one hour. After that, binders were blended using a vertical propeller coupled to a motor able to control the revolutions per minute (fixed at 200 rpm) in a temperature-controlled environment for 15 minutes.
Binders and blends were tested in the Dynamic Shear Rheometer (DSR). DSR used was a Kinexus Pro+. Frequency and temperature sweeps were carried out from 1 Hz to 25 Hz and from -10°C to 60°C. For this purpose, two parallel-plate geometries were used: 8 mm diameter parallel-plates from -10°C to 30°C, and 25 mm diameter parallel-plates from 20°C to 60°C with 1 mm gap. Tests were carried in strain-controlled mode at 0.06% strain in order to stay within the linear viscoelastic region of the binders’ behaviour. For each binder, correct geometries were selected for 20°C and 30°C depending on results in order to achieve a smooth Black diagram (
Using these data, master curves of the norm of the complex modulus and phase angle of the different binders were produced at 15°C for comparison following the procedure developed by Chailleux at al. (
A relationship between rheological properties of bitumen and its molecular weight distribution (MWD) is generally assumed in the literature (
The Huet-Such model is a combination of one spring (G∞), two parabolic creep elements (0<h<k<1) with one coefficient (δ) that regulates the balance between them and a dashpot (β) placed in series with the other elements. This model and its respective equation are represented in
Huet-Such model and equation.
A detailed description of the δ-method can be found in Themeli et al. (2015). The molecular weight distribution obtained by δ-method is termed “apparent” because the proportionality condition to the phase angle, proved for polymers, is assumed for bitumen in this study. Therefore, δ-method provides the Apparent Molecular Weight Distribution (AMWD).
Black diagram of original binders.
Once the rheology of the individual materials is known, the blends’ rheology is shown in
Black diagram of blends.
Storage modulus (G’) master curves at 15°C.
Loss modulus (G’’) master curves at 15°C.
Temperature dependency of shift factors.
In order to further evaluate the effect of the different binders over the RA, cross-over frequencies vs. rheological indexes (R-values) space (
Crossover frequencies vs. R-values.
Fatigue (G*sinδ) and rutting (G*/sinδ) parameters have been used to evaluate the rejuvenating effect of biobinders and reference binders on fatigue and rutting resistance on RA. For this purpose, G*sinδ = 5000 kPa and G*/sinδ = 1 kPa at 1 Hz were taken as thresholds for obtaining critical temperatures of the different materials for comparison purposes.
Rutting and fatigue performance temperatures.
In this section, the Apparent Molecular Weight Distribution (AMWD) for all blends is plotted and compared to that of the RA binder. δ-method application is restricted to materials that exhibit a continuous monotonic curve on the Black diagram; therefore, its application was only possible for RA, 50/70 bitumen and blends, and not for the original materials such as BP and PMB (see
Huet-Such rheological model parameters and WLF coefficients for RA binder and blends at T = 0°C
Bitumen / Parameter | G∞ (MPa) | δ | k | h | β | τ (s) | C1 | C2 |
---|---|---|---|---|---|---|---|---|
RA | 627.68 | 5.15 | 0.24 | 0.62 | 348.22 | 4.25E+00 | 25.87 | 154.27 |
RA+BB | 551.71 | 2.44 | 0.30 | 0.85 | 3.64 | 1.12E−01 | 18.22 | 126.82 |
RA+BP | 581.51 | 3.50 | 0.25 | 0.73 | 1.53E15 | 3.526E−02 | 20.71 | 126.93 |
RA+PMB | 579.14 | 6.14 | 0.30 | 0.71 | 422.04 | 1.51E−01 | 22.23 | 136.56 |
RA+70/100 | 623.42 | 5.17 | 0.26 | 0.65 | 53.32 | 8.93E−02 | 19.22 | 113.74 |
50/70 | 661.22 | 6.96 | 0.32 | 0.76 | 6.50 | 7.61E−02 | 27.28 | 195.75 |
Norm of the complex modulus (|G*|) master curves at 0°C and Huet-Such model fitting.
Phase angle (°) master curves at 0°C and Huet-Such model fitting.
In the case of the RA+BP blend, the modelling is on the limit of δ-method application for this case. The β value seems inconsistent, the increasing accumulative curve for the phase angle achieves a plateau at 70° instead of continue growing towards 90° as the other blends. This may be due to the high content of polymers, limiting at the same time the calculus of parameters and the application of the δ-method, so for the RA+BP AMWD curve the values are indicative. On the other hand, this constraint does not exclude the use of this tool to compare and verify the rejuvenating effect induced by the blending between RA and BP.
The AMWDs are plotted with the apparent molecular weight (AMW) in g/mol on the abscise axis and the corresponding probability density f(AMW) on the ordinate in
AMWD of RA and its blends.
Firstly,
Regarding the blends of RA with the different binders, the main effect that can be observed in
Nevertheless, rejuvenating effect is clear in all cases, proving the strength of this tool to enhance the identification of the rejuvenating effects of the different bitumen blends. In addition, the movement to the left of the cross-over points from the RA to those of the blends can be seen as a rejuvenation indicator.
The investigation presented in this paper shows that biobinders have the potential to replace conventional and modified binders in the production of high-RA content mixtures (50%) at the binder level of study. For this purpose, two biobinders and two more traditional petroleum-based binders (conventional and modified) have been analysed and blended with RA to study the rejuvenating effect that they produce over RA. This effect has been assessed by means of rheology and the application of the δ-method to obtain their molecular weight distribution.
Rheological characterisation of the biobinders revealed that they are thermo-rheologically simple materials with desired properties, such as high viscous response (BB) or capability to have low phase angles at high temperatures (BP), to produce rejuvenation in high-RA content mixtures. All the biobinders and conventional binders that were studied produced a rejuvenating effect over RA that can be easily appreciated in the cross-over frequencies versus rheological indexes space. In this regard, the blend of biobinders with RA generates less elastic and softer binders, having a comparable effect to traditional petroleum-based binders.
In terms of performance prediction, blends of RA and biobinders show equivalent rutting critical temperatures compared to 50/70 pen bitumen and RA blends with traditionally used binders, which would lead to similar rutting resistance in the asphalt mixture. Blends of RA and biobinders showed improved fatigue resistance in comparison to the rest of blends. The fact that RA fatigue performance can be enhanced with biobinders without compromising rutting behaviour is a desirable rejuvenating effect.
A more fundamental evaluation was undertaken to assess low-temperature properties of the blends. To this end, the materials response was fitted at 0°C with Huet-Such model. Parameters obtained for biobinders seem reasonable in comparison with those of conventional binders (but for the RA+BP blend which is at the limit of its application). From these parameters, δ-method was applied to determine the apparent molecular weight distribution of RA, 50/70 binders and RA blends. This AMWD allowed visualising the rejuvenating effect of binders over RA through the shift of the RA distribution to lighter molecular weights. In this regard, it can be highlighted that RA+BB and RA+BP blends show similar AMWD to the 50/70 pen bitumen, which is not the case of the RA+PMB and RA+70/100 blends having a rejuvenation effect less pronounced. The δ–method seems to be a powerful tool for the quantification of structural evolutions of bitumen blends, giving a clear visualization of the structural modifications induced. These results tend to confirm that a structural interpretation of recycling and the rejuvenating effect through these types of blends is possible with the δ-method.
In summary, biobinders have comparable characteristics and produce comparable or greater rejuvenating effect on RA than traditionally used binders. Therefore, they show great potential to be used in high-RA content mixtures as fresh (virgin) binders. However, these materials are relatively new and need further development in order to fully understand their performance and how they would affect asphalt mixtures in the field. In this regard, future research is now being focused on the study of mechanical performance and ageing of high-RA content mixtures manufactured with biobinders as the virgin binder.
The research presented in this paper is carried out as part of the Marie Curie Initial Training Network (ITN) action, FP7-PEOPLE-2013-ITN (