Technical suitability of wet or dry processing of a dense rubberized warm asphalt mixture

4.1.1 Compaction energy

In this study, most tests were performed on cylindrical specimens manufactured with a gyratory compactor (EN 12697-31). However, Spanish regulations include specifications for specimens manufactured with a Marshall compactor (EN 12697-30), which indicate that for an AC16 S mixture, the compaction energy must be 75 blows on each side. Therefore, we first determined the number of rotations that needed to be applied by the gyratory compactor to reach the density achieved with 75 blows on each side by using a Marshall compactor. For this purpose, three specimens were compacted per study point, with 120, 140 or 160 rotations. Figure 2 presents the bulk density (saturated surface-dry, SSD) of the specimens according to the number of rotations. The bulk density of 2.387 g/cm³ (corresponding to 75 blows on each side) was reached after approximately 130 rotations. Therefore, this number of rotations was selected as a reference for the wet-process mixes.

4.1.2 Compactability

Likewise, a compactability study was performed with a gyratory compactor, and the manufacturing temperatures were varied to determine whether the WMA additive allowed for the same levels of densification to be maintained despite the decrease in production temperatures. These results are presented in Figure 3.

As can be seen in Figure 3, the reference mixture, the rubberized mixture without the WMA additive, was manufactured at 165 °C. Mixtures with additive, manufactured at 165 °C and at 145 °C, both were located above the reference curve. Thus, the WMA additive facilitated the densification of the mixture with sufficient efficiency that the manufacturing temperature could be decreased more than 20 °C. However, the densification of the mixture manufactured at 125 °C, was lower than that of the reference mixture. Thus, in terms of compactability and volumetric properties of bituminous mixtures with the wet process, the additive allowed the manufacturing temperature to be decreased approximately 30 °C. However, because the mechanical properties might have been worsened, testing of the water sensitivity and the resistance to plastic deformation were performed.

4.1.3 Water sensitivity

The water sensitivity of the mixtures was evaluated through the indirect tensile test, before and after immersion. We first determined the compaction energy that needed to be applied to the specimens for this test. When the specimens were manufactured with 75 blows/side with a Marshall compactor, the water sensitivity specimens were compacted with 50 blows/side. Decreasing the compaction energy subjects specimens to more critical conditions: the density decreases, the air void content increases, and the asphalt mixture is ultimately more vulnerable to the action of water.

However, for compaction with a gyratory compactor, insufficient evidence was available to establish the magnitude of the energy decrease that needed to be applied. Because in surface layers manufactured with AC type mixtures, the compaction reaches 98% of the reference (75 blows/side), in this investigation, the number of rotations was decreased so that the density for the reference mixture decreased approximately 1-2%. In this case, the 130 established rotations were decreased to 100 rotations. Thus, the reference mixture (without additive and manufactured at 165°C), decreased in density from 2.387 g/cm³ to 2.344 g/cm³, or 1.5%. The immersion conditions were 72 hours at 40 °C. All specimens, both dry and submerged, were finally tested at 15 °C. The results obtained are presented in Table 4 and Figure 4.

Decreasing the temperature resulted in more effective water attack, although the increase in the air void content did not exceed 1%. Hence, the additive allowed for a decrease of temperature in terms of densification, but the adhesiveness between the binder and the aggregates appeared to lose quality, as indicated by the decrease in the ITSR parameter while the manufacturing temperature was decreased.

ITSR values ≥ 85% for the surface layer and ≥ 80% for the intermediate layer are usually required. Therefore, decreasing the manufacturing temperature below 145 °C for surface layers and above 140 °C for intermediate layers is not advisable (Figure 4).

4.1.4 Resistance to permanent deformation

The addition of the WMA additive at 165 °C did not appear to introduce marked differences in the resistance to plastic deformation. However, for the mixtures incorporating the additive, as the manufacturing temperature decreased, the bituminous mixture became more prone to plastic deformation. The mixture manufactured at 125 °C (Figure 5) already exceeded the maximum of WTS air = 0.07 mm/1,000 cycles allowed for AC16 S mixture subjected to the wheel tracking test, according to Spanish specifications (PG-3). Therefore, decreasing the manufacturing temperature below approximately 135 °C would not be reasonable; otherwise, the mixture could experience rutting. This result may relate to the decrease in density, as shown in Table 4, at a 125 °C working temperature, thus making the mixtures more prone to post-compaction and resulting in a deeper track during the testing.

4.2.1 Compaction energy

As explained before, we first established the number of rotations that needed to be applied in the gyratory compactor to reach a density similar to the Marshall compaction energy with 75 blows/side. Figure 6 shows the bulk density (SSD) of the specimens for the indicated rotations. The apparent density of 2.375 g/cm³ (corresponding to 75 blows/side) was reached with approximately 120 rotations. Therefore, this number of rotations was selected as a reference for the compaction of dry process mixtures.

This energy was lower than that of wet process mixes, for which 130 rotations had been established. This result might have been because the gyratory compactor, as a result of its kneading effect, performed better than the Marshall compactor in addressing the compaction difficulties caused by the rubber particles incorporated by the dry process.

4.2.2 Compactability

For the wet process, a compactability study was performed with the gyratory compactor. The reference mixture (without additive and manufactured at 170 °C) was studied, as well as mixtures with the WMA additive manufactured at temperatures of 170, 150 and 130 °C (Figure 7).

The results obtained did not indicate a clear trend, but the decrease in temperature from 170 to 150 and 130 °C resulted in lower densities than that of the 170 °C reference mixture. As shown in Figure 8, the reference mixture with no additive and produced at 170 °C, was reached by only the mixture with additive at 170 °C. Nevertheless, as soon as the production temperature decreased to 150 and 130 °C, the densification curves, respectively, decreased, thus indicating an inefficient compaction process.

These results might have been because the rubber particles needed to be digested in the bituminous mixture in the dry process (in the wet process, they were already digested in the binder). Because digestion requires high temperatures, above 160 °C (1010. Rodríguez-Alloza, A.M.; Gallego, J. (2017) Mechanical performance of asphalt rubber mixtures with warm mix asphalt additives. Mater. Struct., 147, 1-9. https://doi.org/10.1617/s11527-017-1020-z.
), the mixtures produced herein by the dry process at 150 and 130 °C had not sufficiently digested the rubber particles and therefore remained in an elastic condition, thereby hindering the compaction and densification of the bituminous mixtures.

This finding is in line with results from previous research indicating that, when rubber is added, if the compaction temperature is not sufficiently high, the use of rubberized mixes results in inadequate volumetric properties (higher air voids and uneven density distribution) and poor short-term and long-term performance, because of uncertain factors (1616. Akisetty, C. (2008) Evaluation of Warm Asphalt Additives on Performance Properties of CRM Binders and Mixtures. Civil Engineering. Clemson University, Clemson, USA.
).

4.2.3 Water sensitivity

The test conditions were the same as those described in Section 4.3 for the study of wet process mixtures. However, the compaction conditions of the specimens differed. In this case, the compaction reference was 120 rotations in the gyratory compactor. The density obtained for the reference mixture with that energy level was 2.375 g/cm³. As in the case of the wet process mixtures, we determined the number of rotations after which the density decreased to no more than 98% of the reference to be 90 rotations, with a bulk density of 2.340 g/cm³, 98.5% of the reference. Therefore, all water sensitivity specimens were compacted with 90 rotations in the gyratory compactor. The results obtained are presented in Table 5.

In terms of resistance to the action of water, the manufacturing temperatures could not be decreased (Figure 9), despite the incorporation of the WMA additive. Even for a temperature of 150 °C, the ITSR was far from the 80% usually required for an intermediate layer and the 85% required for a surface layer (1111. Yu, H.; Leng, Z.; Dong, Z.; Tan, Z.; Guo, F.; Yan, J. (2018) Workability and mechanical property characterization of asphalt rubber mixtures modified with various warm mix asphalt additives. Constr. Build. Mater. 175, 392-401. https://doi.org/10.1016/j.conbuildmat.2018.04.218.
).

This result was attributed to the lack of digestion of the rubber particles, because of the need for higher temperatures for the dry process (1010. Rodríguez-Alloza, A.M.; Gallego, J. (2017) Mechanical performance of asphalt rubber mixtures with warm mix asphalt additives. Mater. Struct., 147, 1-9. https://doi.org/10.1617/s11527-017-1020-z.
, 1212. Hu, J.; Ma, T.; Yin, T.; Zhou, Y. (2022) Foamed warm mix asphalt mixture containing crumb rubber: Foaming optimization and performance evaluation. J. Clean. Prod. 333, 130085. https://doi.org/10.1016/j.jclepro.2021.130085.
). In short, the selected WMA additive, which might have had other effects, did not promote the digestion of the rubber particles, which remained in an elastic state among the aggregates. Therefore, compaction was difficult, and the rubber-bitumen interface did not achieve sufficient affinity and, consequently, became vulnerable to water attack.

To overcome this situation, three alternatives can be suggested: a decrease in the rubber particle size, which in this case was <0.5 mm, to smaller particles, as reported by Da Silva et al. (1515. Mogawer, W.; Austerman, A.; Mohammad, L.; Kutay, M.E. (2013) Evaluation of high RAP-WMA asphalt rubber mixtures. Road Mater. Pavem. Design. 14 [sup2], 129-147. https://doi.org/10.1080/14680629.2013.812846.
) ; the selection of rubber pretreated to facilitate the digestion; or a decrease in rubber content, which was 1% of the mixture by weight. These solutions would decrease the digestion time at standard temperature and probably allow for the working temperatures to be decreased.

Although this aspect is previously undescribed, according to the protocol for compaction in a gyratory compactor (EN 12697-31), the bituminous mixture must remain in an oven at the compaction temperature for 2 hours, within the mould. In short, all studied mixtures, including those incorporating dry rubber, underwent 2 hours of digestion in an oven (1212. Hu, J.; Ma, T.; Yin, T.; Zhou, Y. (2022) Foamed warm mix asphalt mixture containing crumb rubber: Foaming optimization and performance evaluation. J. Clean. Prod. 333, 130085. https://doi.org/10.1016/j.jclepro.2021.130085.
). However, this treatment did not solve the compactability and water sensitivity problems. Hence, these problems do not appear to be addressable by increasing the digestion time if the manufacturing temperatures are not increased as well (contrary to the objectives of this investigation). The feasibility of using rubberized WMA mixtures with rubber with the dry process, at least with the additive selected for this study, does not appear to be ensured, on the basis of the results in this investigation. Additional research is needed.

4.2.4 Resistance to permanent deformation

Regarding the resistance to plastic deformation, the addition of the WMA additive at the reference temperature (170 °C) did not appear to introduce appreciable differences. In contrast, in the mixtures incorporating the additive, decreasing the manufacturing temperature resulted in no significant changes in the resistance to ruts (Fig. 10), because for all temperatures, the values obtained were below WTS air = 0. 07 mm/1,000 cycles, the maximum allowed for this type of bituminous mixture by the Spanish specifications (1111. Yu, H.; Leng, Z.; Dong, Z.; Tan, Z.; Guo, F.; Yan, J. (2018) Workability and mechanical property characterization of asphalt rubber mixtures modified with various warm mix asphalt additives. Constr. Build. Mater. 175, 392-401. https://doi.org/10.1016/j.conbuildmat.2018.04.218.
). The manufacturing temperatures could feasibly be decreased, although this hypothesis was already rejected because of compactability reasons and vulnerability to water attack.