Low temperature properties of UV aged asphalts containing layered double hydroxides modifier

Materials

Base asphalts with 60/80 penetration (pen) grade (S70), 80/100 pen grade (S90), and SBS-modified asphalt (SA) were studied in this research. The softening point, penetration, ductility, viscosity of the three applied asphalts are shown in Table I. LDH is a layered material that displays a special 2D-organized structure, and the specific formula of LDH used in this paper is: Mg_2/3Al_1/3(OH)₂(CO₃)_1/6•H₂O

Preparation of the LDH-modified asphalt

A high shear mixer was applied to prepare the LDH-modified asphalt, and the mass ratio of LDH was 3%. Firstly, asphalt was heated to 150^oC in a mixing container, then a certain amount of LDH was added into asphalt. LDHs and asphalt binder were sheared at a rotation speed of 3500 rev/min with the temperature ranging from 145^oC to 155^oC. Finally, 60 min later, the LDH-modified asphalt was successfully created. Pristine asphalts (SA70, SA90, SA) also experienced the same preparation process. In this paper, abbreviations like SL70, SL90 and SL are used, which represent the LDH-modified S70, LDH-modified S90 and LDH-modified SA, respectively.

Ageing procedure

Consistent with ASTM D 1754, thermal ageing in the mix process was carried out by thin film oven test (TFOT). Then the residuals of the samples from TFOT were placed in a UV chamber to simulate the UV ageing on pavement. The UV density in this ageing procedure was controlled within 500 ± 5 uw/cm², and the weight of asphalt in the ageing iron pan (Φ150 ± 0.5 mm) was 50 g. In order to ensure the same ageing conditions for different asphalt samples, in this procedure, a rotary table was applied to hold the samples and a constant temperatures of 60°C was selected. Here, ST70, ST90, ST, SLT70, SLT90, and SLT are the abbreviations of residues of S70, S90, SA, LDH-modified S70, LDH-modified S90, and LDH-modified SA from TFOT ageing, while SU70, SU90, SU, SLU70, SLU90, and SLU are the abbreviations of residues of S70, S90, SA, LDH-modified S70, LDH-modified S90, and LDH-modified SA from UV ageing.

Physical property tests

The penetration (15°C, 25°C and 30°C, 0.1 mm) and softening point (°C) of the binders were tested in accordance with the standards ASTM D564 and ASTM D3626, respectively. Equivalent brittle point (T_1.2) was calculated based on the penetration (Pen.) at 15°C, 25°C and 30°C.

Dynamic shear rheometer (DSR)

The fatigue factor of asphalts after both TFOT ageing and UV ageing were tested by the DSR (MCR101, Anton Paar, Austria). Strain-controlled mode was used in temperature sweep test with a constant frequency of 10 rad/s. During the test, the temperature was raised from 10°C to 30°C with an increment of 2°C per minute. Besides, plates with Φ8 mm and 2 mm gap were adopted.

Equivalent brittle point (T_1.2)

Brittle point is usually applied to evaluate the cracking temperature of asphalt. However, the repeatability of brittle point results is not always ideal, and in some parallel tests the error of the test results for the same asphalt sample is too large to be tolerated. According to BEP, equivalent brittle temperature (T_1.2) could be calculated, since T_1.2 also evaluates the cracking temperature of asphalt and its error is smaller than that of brittle point. In this research, T_1.2 is used to study the properties of aged and non-aged LDH-modified asphalt at low temperatures. Figure 1 shows the linear fitting curves between Log (Pen.) and temperature for three different samples (SLU70, SLU90, SLU). It can be seen that the R² values of all the three equations are above 0.995, and according to these equations, T_1.2 for each sample can be calculated.

It can be seen from Table II that T_1.2 of SLU70 and SLU90 are lower than that of SU70 and SU90, which means the anti-cracking properties of SLU70 and SLU90 are better than those of SU70 and SU90. From the statistics of ∆T_1.2, after UV ageing, ∆T_1.2 between SL70 and SL90 are much smaller than that between S70 and S90, indicating that LDHs could decrease the ageing rate when asphalt experience undergoes the UV ageing process. Interestingly, T_1.2 of SU is even lower than that of ST. As we know, ageing will lead to hardening of asphalt binders and thus increase T_1.2. The reason why it shows a reduced T_1.2 might be explained by the degradation of SBS modifier induced by the UV radiation. The degradation of SBS modifier could soften asphalt binders, thus resulting in a lower T_1.2. And for the same reason, ∆T_1.2 of SLU is much larger than that of SU, which also implies that LDHs could slower the degradation of SBS modifier in asphalt binders.

Fatigue factor (FF)

According to the Strategic Highway Research program (SHRP), the FF can be applied to estimate fatigue cracking property of asphalt at low temperatures. A higher FF means poorer fatigue cracking property of asphalt. In the SHRP specification, the FF of pressurized ageing vessel (PAV) aged asphalt should be less than 5 Mpa at certain temperatures. As a consequence, in this paper, the temperature when FF reaches 5 Mpa is treated as the FF temperature (FFT). A higher FFT indicates a more severe ageing degree obtained during the UV ageing process.

Figure 2 shows that FF of SLU90 is lower than that of SU90. The FFT of SLU90 is 19.3°C, and the FFT of SU90 is 20.5°C, the difference of FFT between SLU90 and SLT90 is 0.7°C, which is 1.4°C lower than the difference of FFT between SU90 and ST90. These results illustrate that LDHs modifier can remarkably reduce the ageing index of SL90 in the UV ageing process.

Table III shows more statistical results about the FFT of UV aged and non-UV aged asphalts. Similar to the results of SL90, the FFT of SLU 70 is lower than that of SU70, and the ∆FFT between SLU70 and SLT70 is less than half of the ∆FFT between SU70 and ST70. It can be seen from the reduced ∆FFT that LDHs can significantly prevent the UV ageing degree of asphalt. However, the trend of change in FFT for SL is opposite to that for SL70 and SL90. After being UV aged, the FFT of SU is lower compared to that of ST. However, FFT of SLU shows a slight increase compared to that of SLT. The tendency is similar to the results of EBP. It can be also explained by the fact that the UV ageing results in deterioration of SBS modifier in asphalt, which makes ∆FFT between SU and ST negative. The ∆FFT between SLU and SLT is positive, which means that LDHs slows down the degradation of SBS modifier.