Characterization of neat and modified asphalt binders and mixtures in relation to permanent deformation

3.1 Materials and methods

Three different asphalt cements (AC) were studied in this research: one neat 50/70 penetration grade asphalt binder (AC1); one polymer-modified asphalt binder with 2.1% of reactive elastomeric terpolymer Elvaloy® RET 4170 and 0.18% of polyphosphoric acid 116 (AC2); and another polymer-modified binder with 1.9% of terpolymer Elvaloy® RET 5170 and 0.18% of polyphosphoric acid 116 (AC3). Elvaloy® RET 4170 is an EnBAGMA (ethylene n-butyl acrylate/glycidyl methacrylate) terpolymer and Elvaloy® RET 5170 is an EEGMA (ethylene ester/glycidyl methacrylate) terpolymer. The two modified asphalt binders were produced after the modification of the same AC 50/70 studied in the present research. Rheological tests were performed on each material using the dynamic shear rheometer (DSR) and asphalt mixtures constituted by these three binders were compacted and tested in terms of rutting resistance on the Laboratoire Central des Ponts et Chaussées (LCPC) wheel-track testing equipment (EN 12697-22, 2003). The asphalt mixtures were composed by a blend of basaltic aggregates collected from a quarry in the State of São Paulo (Brazil) with the addition of hydrated lime (1.5%) and 5.0% binder content. Figure 1 presents the aggregate size distribution of the three mixtures.

Figure 1

Aggregate size distribution

The characterization of the asphalt binders was done in order to provide the linear viscoelastic properties of the materials, which might be considered in design simulations for field performance prediction. The parameter Jnr was obtained by means of the MSCR test and it might be used for classification purposes and comparison between different materials, being also a powerful tool for rutting resistance prediction. In relation to the asphalt mixtures, the rutting resistance test normally validates an asphalt mixture to be used on an asphalt pavement roadway, by limiting the maximum percentage of deformation.

3.2 Master curves

The linear viscoelastic behavior of the asphalt materials indicates that their properties are dependent of loading frequency and temperature. Dynamic shear modulus (|G*|) master curves of the three binders studied were plotted after the frequency-temperature sweep test. The tests were performed in three specimens for each binder, and they were all tested in their unaged and short-term aged conditions. The results were obtained after a frequency sweep (from 1 to 100 rad/s) at different temperatures with 0.01% of strain amplitude. For the neatAC, the temperature range was 46 to 76^∘C with increments of 6^∘C. For the modified materials, the temperature of 82^∘C was also included, because these materials are capable of bearing higher temperatures than the neat binders. The master curves, using 52^∘C as reference, are presented in Figure 2. The authors observed that there was no significant variation among the different specimens (a total of two) of the same binder, indicating that the test did not present variability and that average values are a good representation of each binder tested.

Figure 2

Master curves of: (a) dynamic shear modulus, |G*| and (b) phase angle, δ

The master curves are used to analyze the behavior of the materials studied at different temperatures and loading frequencies, which are related to the variation of field conditions. At low frequencies, equivalent to high temperatures for viscoelastic materials, the neat binder presented lower values of |G*| if compared to the values of the modified binders. This could be an indication that the modified materials are more likely to have better rutting resistance, because asphalt mixtures with high stiffness tend to resist more to traffic loads.At high frequencies, equivalent to low temperatures, the three materials had similar behavior. A comparison between the two modified binders shows that both materials behave almost equally in relation to |G*| values at any frequency or any temperature conditions. Regarding the aging processes, the asphalt binders were also characterized in terms of their master curves after the use of RTFOT, and the results had the same trends as found for the unaged specimens.

3.3 Superpave rutting parameter (|G*|/sinδ)

The dynamic shear modulus (|G*|) and the phase angle (δ) of the binders were obtained through oscillatory tests run in the DSR according to ASTM D 7175 (2008). The tests were performed within a range of six different temperatures at the frequency of 10rad/s. Figure 3 presents the average results from three specimens of the Superpave parameter |G*|/sinδ for unaged and aged specimens (using RTFOT). The standard deviation values found between the three specimens for each temperature are plotted as error bars; however, these results are too low for most cases analyzed.

Figure 3

Superpave rutting parameter: (a) unaged and (b) RTFOT-aged specimens

The modified binders have similar behavior in relation to the Superpave parameter analyzed at each testing temperature (at the frequency of 10 rad/s). The neat binder presented lower values for both aging conditions. According to the Superpave specification, the minimum value for |G*|/sinδ should be 1.0 kPa for unaged specimens, i.e., the neat binder might have an adequate behavior for permanent deformation until approximately 68^∘C, while the modified binders might present a good resistance for this distress until approximately 80^∘C. This indicates that the addition of reactive polymer enhanced the rutting resistance of both modified materials. For the aged samples, the traditional Superpave specification indicates a minimum value of 2.2 kPa. The neat binder presents a good behavior up to approximately 68^∘C, with the modified binders up to 78^∘C. In terms of thermal susceptibility, both modified binder presented smaller slopes as the testing temperature increased, indicating better behavior.

3.4 Multiple stress creep and recovery (MSCR) test

The MSCR test is included as part of the current Superpave specification (AASHTO M 332, 2014) and evaluates three major parameters: the percent recovery (%R), the non-recoverable creep compliance (Jnr), and the stress sensitivity (Jnr_diff ) of the asphalt binders. The test consists in the application of creep and recovery periods for 1s and 9 s, respectively, under two different stress levels (100 and 3,200 Pa). The AC specimens were tested after short-term aging (RTFOT), which is described by ASTM D 2872 (2012). Figure 4 presents the evolution of the strain for both stress levels during the test. Only one cycle of 10s is represented in the graphs. The asphalt binders were tested at three temperatures (64, 70, and 76^∘C), and the results for Jnr and Jnr_diff are shown in Figure 5. Each result represents average values from three replicates, and the error bars correspond to one standard deviation from the mean value.

Figure 4

MSCR test results: (a) stress of 100 Pa and (b) stress of 3,200 Pa

Figure 5

MSCR results at three different temperatures: (a) Jnr at 3.2 kPa and (b) Jnr_diff

As expected, the neat asphalt binder has higher values of Jnr for all the temperatures tested. The comparison between the two modified binders indicates that AC3 has a better rutting resistance, with Jnr values 25% lower than the ones obtained for the AC2 in all three temperatures. The Jnr_diff parameter was lower than the specification limit of 75%, although this limit is under discussion on the technical community. Therefore, the materials studied could be used in asphalt mixtures regarding their stress sensitivity response.

The Jnr values for the different materials tested were chosen according to the maximum temperature obtained for the |G*|/senδ results previously presented. Considering the temperature range provided by the PG specification, the respective values for the neat and the modified binders were 64 and 76^∘C, respectively. Figure 6 presents the results (average values from three replicates at each condition) of Jnr and %R for both stress levels applied during the test. In general, the %R values have inversely proportional correlation with the Jnr values, i.e., as the %R increases, the Jnr decreases.

Figure 6

MSCR parameters at the PG temperatures: (a) Jnr and (b) %R

The modification of the neat asphalt binder by reactive polymers decreased the values of Jnr in at least 50% of the original value (note that the testing temperature was different for the modified binders). Both modified binders presented similar results, but AC3 resulted in even lower values of non-recoverable compliance, which might indicate that this binder tends to have lower permanent deformation than AC2. A Student’s t-test was done to evaluate if the Jnr values for the two modified asphalt binders were statistically similar, and the results obtained indicated that they are different (for a significance level of 5%).

AASHTO M 332 (2014) provides a classification for the asphalt binders that includes the maximum limit values for Jnr according to different traffic loads: standard (S), heavy (H), very heavy (V) and extreme (E), which is based on the number of AASHTO’s accumulated equivalent single axle loads (ESALs) passing on the asphalt pavement during its life. Table 1 presents these limit values.

According to the specification, the neat binder could only be used for standard traffic (PG 64S-XX). As for the modified binders, AC2 could be used for heavy traffic (PG 76H-XX) and AC3 could be used for very heavy traffic (PG 76V-XX).

The MSCR test has some advantages in comparison to the traditional Superpave parameter (|G*|/senδ). It provides the possibility of evaluating the dependency of polymer-modified binders in relation to stress levels, which is not possible with the traditional test because the latter is performed at the linear viscoelastic (LVE) region of the asphalt binder. Besides that, previous studies [47, 48, 49, 50, 51, 52] indicated that the Jnr value has provided better correlation to the permanent deformation of field and laboratory asphalt mixtures in relation to the correlations done with the parameter |G*|/senδ.

3.5 Laboratory traffic simulation

After the characterization of the asphalt binders, the present research performed a study on the asphalt mixtures. Two specimens of asphalt concrete slabs for each asphalt mixture was produced win the French roller compactor (EN 12697-33, 2003). These specimens were then subjected to the permanent deformation test performed by the French wheel tracking traffic simulator (EN 12697-22, 2003). The mixing and compaction temperatures were 153 and 140^∘C for the mixture constituted by the neat binder, respectively; and 165 and 155^∘C for the modified binders, respectively. For the preparation of the slabs, a pugmill mixer was used to blend the materials, simulating filed mixing conditions. After the mixing process, the asphalt mixes were kept in the oven for a period of 2h at the compaction temperature to simulate the time between the mixing and placement of the mixtures in the field.

The slab specimens were prepared by the LCPC French slab compactor, in which the asphalt mix was compacted by roller compactor (similar to real field compaction) after a sequence of passing of standard tires with pressure varying from 0.3 to 0.6MPa. The rutting resistance test was performed by the LCPC traffic simulator. For each mix, two 50 mm-thick, 180mm-width and 500mm-length slab specimens were tested. The test was done at the temperature of 60^∘C and consists in applying a load of 5kN to each sample at a frequency of 1.0 Hz. After 100, 300, 1,000, 3,000, 10,000 and 30,000 cycles, the rut depth was measured in each sample. In terms of recommended limits for rutting, in Brazil, a maximum value of 5% of rutting after 30,000 cycles is normally considered for use of the asphalt mixture in heavy traffic roads [53]. Figure 7 presents the average results obtained for all the three mixtures produced in terms of rut depth. For each mixture, two slab specimens were submitted to traffic simulation.

Figure 7

Rut depth results

The conventional mixture produced by the neat binder had the worst behavior in terms of rutting (5.3% after 30,000 cycles). In relation to the mixtures using the modified binders, the rut depth values were very similar (2.7% at the end of the test), indicating the both mixtures have the same permanent deformation resistance behavior. It is important to note that the mix design of all three mixes was the same (same gradation and same AC content). The air voids content of the asphalt mixtures were measured from one specimen of each asphalt mixture (Figure 8) to evaluate the influence of this parameter on the results of rut depth, considering that the same binder content was used for the three mixtures. The results show that the modified asphalt mixtures performed were not influenced by their air voids content, but these values were very similar for both those mixtures. The unmodified asphalt mixture presented higher values of rut depth despite having lower air voids content.

Figure 8

Air voids contents

Correlations between rut depth and |G*|/sinδ values (Figure 9a) and rut depth and Jnr values (Figure 9b) are provided. There is good linear correlation with the latter (R² value of approximately 0.95), although the authors are aware that more data should be provided in further research. One of the main reasons for the good indication of rutting resistance provided by the asphalt binders might be explained by the changes that normally occur after the polymer-modification. The asphalt binders tend to have higher stiffness at high temperatures and tend to be more elastic. These aspects can be noticed by the master curves of each material. It is important to note that the aggregates’ properties and size distribution were the same for the three mixtures evaluated in the present study, but other gradations and other aggregate sources could lead to different results and lower values of R². Despite resulting in a fair correlation, the Superpave parameter should be avoided because it does not address well the modified asphalt binders’ performance with respect to permanent deformation. The higher values of |G*|/sinδ for the AC3 would indicate better resistance to permanent deformation, but that was not observed in the wheel-track simulation test, which resulted in almost identical rut depth values for the two modified binders.

Figure 9

Comparison of rut depth with: (a) |G*|/sinδ and (b) Jnr