The influence of regenerant with different dosages on the performance of aged SBS modified asphalt

2.1. Experiment design

SBS modified asphalt is used as the test material, and its main physical property indexes are tested and compared under five different aging times. Six different doses of regenerants are added to each RTFOT aging period to form recycled asphalt, and the performance indexes of the recycled asphalt and the original asphalt are analyzed to study the development pattern of asphalt properties.

To prepare specimens representing different aging degrees, the aging test time periods are set as 160, 200, 240, 280 and 320 min, and the aging oven temperature is set at 163 °C. The softening point, ductility and penetration of the specimens are tested after aging for different periods to analyze and compare the changes of their indexes. In order to prepare the regenerated asphalt, different doses of regenerants are added to the aged asphalt. According to the change law of asphalt performance before and after regeneration, the best dosage of regenerant for different aging time asphalt is achieved.

2.2. Experiment

3.1. Changes in technical indexes and data analysis after the addition of regenerant

In accordance with the requirements of “Highway Engineering Asphalt and Asphalt Mixture Test Procedure JTG E20-2011”, the needle penetration, softening point and ductility of the recycled asphalt specimens are tested, and the results are shown in Table 3-7.

From Table 3 to 7, it can be seen that the indicators of needle penetration and ductility of recycled asphalt recovered well, while the indicators of softening point did not recover significantly.

Line charts are drawn to directly reflect the relationship between the physical indexes of recycled asphalt and the variation of the regenerant dosage, shown in Figs. 1-3.

From Figs. 1-3, it can be seen that with the increase of regenerant dosage, all the performance indexes of aged asphalt are restored, with the softening point of regenerated asphalt reduced, and the ductility and needle penetration increased. Once the asphalt is aged, the proportion of aromatics decreases, and the proportion of asphaltenes increases. The mechanism of asphalt regeneration is to readjust the content of the components of aged asphalt, that is, by adding regenerating agents and other methods to obtain recycled asphalt that can meet the performance of the road [8].

3.2. Analysis of dynamic shear rheological test results

Rutting factor ($G^{*}/\mathrm{s}\mathrm{i}\mathrm{n}\delta$) can characterize the high-temperature performance of asphalt materials. The higher the rutting factor, the stronger the resistance to denaturation under high temperature conditions. The change trend of 70 ℃ rutting factor with aging time and the amount of regenerant is shown in Fig. 4. It can be seen from the figure that the rutting factor of recycled asphalt decreases with the increase of the amount of regenerant, indicating that the addition of regenerant will affect the high temperature performance of asphalt. This is because the addition of regenerant can improve the rheological properties of recycled asphalt, but when the amount is too high, it will have an adverse impact on the high-temperature shear resistance of asphalt, so the appropriate amount of regenerant should be selected.

Fig. 1The relationship between needle penetration and regenerant dosage

Fig. 2The relationship between ductility and regenerant dosage

Fig. 3The relationship between softening point and regenerant dosage

3.3. Analysis on the optimal dosage of regenerant

In Table 3, after 160 mins of RTFOT on SBS modified asphalt, the regenerated asphalt with 8 % regenerant addition receives the closest needle penetration and softening point to the original modified asphalt. So, 8 % regenerant is selected as the optimal dosage of regenerant for asphalt performance recovery after 160 mins of RTFOT. In Table 4, for SBS modified asphalt after 200 mins of ageing, the needle penetration and softening point of the recycled asphalt obtained by adding 8 % regenerant and 10 % regenerant are closer to the original modified asphalt, so the index of needle penetration is compared to select optimal dosage of regenerant. The interpolation equation is shown in Eq. (1). In Table 5, the regenerated asphalt with 10% regenerant addition receives the closest needle penetration and softening point to the original modified asphalt. So, 10 % regenerant is selected as the optimal dosage of regenerant for asphalt performance recovery after 240 mins of aging. In Table 6, for SBS modified asphalt after 280 mins of ageing, the needle penetration and softening point of the recycled asphalt obtained by adding 10 % regenerant and 12 % regenerant are closer to the original modified asphalt, so the index of needle penetration is compared to select optimal dosage of regenerant. The interpolation equation is shown in Eq. (2). In Table 7, the regenerated asphalt with 12 % regenerant addition receives the closest needle penetration and softening point to the original modified asphalt. So, 12 % regenerant is selected as the optimal dosage of regenerant for asphalt performance recovery after 320 mins of aging:

Fig. 4Relationship between rutting factor and regenerant dosage

Since the optimal dosages of 8.8 % and 11.3 % are calculated, in order to describe the recovery of the indicators of recycled asphalt more visually, the recovery of the indicators of the original asphalt and the recycled asphalt with 8 %, 10 % and 12 % regenerant are as shown in Fig. 5.

Fig. 5Technical indexes recovery of regenerated asphalt

a) Needle penetration

b) Ductility

c) Softening point

As can be seen from Fig. 5, the regenerant can recover needle penetration and softening point of aging asphalt effectively, but the recovery of its ductility is not obvious. The reason is that the regenerant ignores the low temperature plasticity of asphalt. Therefore, in order to achieve better regeneration effect, a certain amount of the same new asphalt is added along with regenerant. Regenerant works as supplement to the aromatics and helps to adjust the chemical components and content, but it cannot repair the degraded molecular chain. The new asphalt added can not only adjust its chemical components, but also join chemical grafting reaction with aging asphalt degradation products based on its original epoxy groups. Thus, the broken SBS molecular chains of aged asphalt are reconnected, and the ductility of aged asphalt can be well restored.

The relationship between asphalt aging time and optimal regenerant dosage is summarized in Table 8, and their fitted figure is shown in Fig. 5.

From Fig. 6, the fitted linear relationship between the optimal regenerant dosage and asphalt aging time is obvious, when the correlation index R2 is greater than 0.987, the correlation is better. The linear equation between the asphalt aging time and the regenerant dosage is in Eq. (3):

where $X$ is the ageing time, (min); $Y$ is the dosage of regenerant, (%).

In practical applications, the three major indicators of the original asphalt pavement recycling asphalt can be measured and compared with those of asphalt without regenerant in Table 3-7. Then, according to the aging degree of asphalt, the aging time of the tested asphalt can be determined by using numerical interpolation, and then be substituted into the above linear equation to find the optimal regenerant dosage.

Fig. 6Fitted figure of asphalt aging time and optimal regenerant dosage