The general performance test results of SBR emulsified asphalt are shown in Fig. 2. When the ductility is greater than 100 cm, it is recorded 100 cm. It can be seen from the figure that: With the increase of SBR mixing, the softening point and ductility of asphalt increase, and the degree of penetration decreases. The softening point of the original emulsified asphalt is low, the degree of penetration is high, and the asphalt is brittle in the 5 ℃ ductility experiment. When the dosage of SBR goes up to 3%, the softening point of the modified emulsified asphalt has increased by 11.4% compared to the original sample of emulsified asphalt. Ductility at 25 ℃ went up increased to 100 cm, the penetration decreased to 57.6 (0.1 mm). This indicates that the low-temperature cracking resistance of SBR-modified emulsified asphalt was enhanced and the high-temperature performance was improved. This is because the addition of SBR absorbs the light oil from asphalt and produces a swelling reaction, with the asphalt changing from the solgel type to the sol-gel type. A more stable mesh structure takes shape after the molecules inside the asphalt are restrained.
Temperature scanning tests were carried out on emulsified asphalt with varied SBR dosages. The results are shown in Fig. 3. When the temperature is determined, the addition of SBR induces reduction the phase angle (δ) of emulsified asphalt along with increment in both complex shear modulus (G*) and rutting factor (G*/sin δ). It shows that the addition of emulsified asphalt leads to a more stable spatial structure of SBR, and the reduction in light oil content lowers the temperature sensitivity of asphalt and improves the high-temperature deformation resistance and rutting resistance of asphalt as well. When SBR doping remains unchanged, the phase angle tends to increase and the complex shear modulus and rutting factor decrease with the increment in temperature. At this moment, the internal elastic component of asphalt is transformed into a viscous along with weakened deformation resistance of asphalt.
Fluorescence microscope with 400 times magnification was used to observe the distribution of SBR and asphalt. As shown in Fig. 4, fluorescence microscope image on the left hand side is darker than on the right hand side. But there is a similarity in proportion for fluorescent area which shows noticeable 3D characteristics. The emulsified asphalt image shows indistinct fluorescence emitted from the emulsifier, and the rest of the images show noticeable fluorescent material emitted from the SBR. When the dosage of SBR stands at 1%, SBR exists in the continuous phase of emulsified asphalt in the form of small particles that are well dispersed. When SBR is 3%, spatial network structure takes shape in modified emulsified asphalt along with high structural strength, improved high temperature performance and enhanced softening point as opposed to that for the conventional performance. When the dosage of SBR continues to increase to 4%, the latex begins to aggregate with a poor degree of dispersion. It shows from an overall point of view that it is best for SBR’s content to stand 3%.
Digital Image Processing of fluorescence microscope images of emulsified asphalt modified with SBR was performed using MATLAB. First of all, the fluorescence microscope images of asphalt with SBR of varied dosage were converted to grayscale image. Due to noticeable difference in gray levels between the SBR region and the background region in the image, unequal threshold segmentation of asphalt images with SBR of varied dosages was carried out to accurately extract the pixels in the target region when the original image was converted to grayscale image. The ratio of the SBR region and the background region is finally obtained, and the data obtained for each sub-block region is averaged to obtain the data results of the global image. The segmentation is shown in Fig. 5, where the black part is asphalt and the white part is SBR latex.
The binarized image of SBR emulsified asphalt was analyzed at each dosage to gain the ratio of the number of SBR pixel points to the number of pixel points of the modified emulsified asphalt in this image, namely the area percentage of SBR at each dosage. The results are shown in Fig. 6, and the formula is shown in Eq. (1) (2).
where i—pixel points of SBR in the sub-block;
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j—all pixel points in the sub-block;
k—the number of sub-blocks;
R—the area percentage of the sub-block;
\(R^{ - }\)—the area percentage of the total block, namely the average of the sub-block summation.
It is indicated in Fig. 6 that the area percentage of SBR is linearly distributed with the increase of butadiene doping. The area percentage is only 5.52% at 1% of SBR doping. As the dosage of SBR increases, the area percentage also increases linearly. The percentage of area reached 12.37% at 3% of SBR. The linear fitting of the data shows that the linear pattern of the dosage of SBR and its area percentage is good, and the correlation coefficient can go up to 98.9%, which indicates that the SBR and emulsified asphalt are highly compatible.
Infrared spectroscopic tests were performed on the evaporated residues of as-received emulsified asphalt and 3% SBR emulsified asphalt. These results are shown in Fig. 7. The same absorption peaks appeared in the two infrared spectral curves, including cm−1 and cm−1 for the absorption peaks of the stretching vibration of -CH2 alkanes, and cm−1 and cm−1 for the absorption peaks of the bending vibration of -CH3 alkanes. In addition to the above-mentioned absorption peaks, absorption peaks of butadiene and styrene components of the SBR composition appeared in the 3% SBR emulsified asphalt. These include 966 cm−1 for trans-butadiene, which is an out-of-plane bending vibration of trans-C-H olefin, and 699 cm−1 for styrene, which is an out-of-plane bending vibration of monosubstituted benzene ring C-H olefin. As a result, it can be seen that there is no new functional group in the emulsified asphalt of 3% SBR. Therefore, no change was found in the chemical composition of SBR and emulsified asphalt which was just subject to a physical reaction in Fig. 8.
According to the first law of thermodynamics, the work consumed by the molecules inside a substance to migrate to the surface is the potential energy of the molecules on the surface, called the surface free energy (\(\gamma\)). When asphalt adheres to the aggregate, the aggregate will adsorb the asphalt to reduce its own surface free energy under the influence of the force field. As a result, the surface energy is an intrinsic factor that affects the adhesion performance of asphalt. Surface energy by the polar component (also known as Lewis acid-base component, \(\gamma^{P}\)) and dispersion component (also known as van der Waals component, \(\gamma^{d}\)). The polar component in turn consists of Lewis acid (\(\gamma^{ + }\)) and Lewis base (\(\gamma^{ - }\)). The expression for the surface energy expression is show as follows:
The relation between the surface energy parameters for the liquid and bitumen are expressed as follows:
where \(\gamma_{L}\)—surface energy of the liquid;
\(\gamma_{{\text{a}}}^{d} ,\gamma_{L}^{d}\)—Dispersive component of bitumen and liquid;
\(\gamma_{{\text{a}}}^{ + } ,\gamma_{L}^{ + }\)—Lewis acid fraction of the bitumen and liquid;
\(\gamma_{a}^{ - } ,\gamma_{L}^{ - }\)—Lewis base fraction of the bitumen and liquid.
The modified emulsified asphalt with SBR of varied dosages was subjected to contact angle tests using three test liquids: water, glycerol and formamide. The surface free energy parameters of each liquid at 25 ℃ are shown in Table 3 below:
As shown in Fig. 9, the contact angle results of emulsified asphalt for each SBR dosage are slightly different. The contact angle results obtained for all three liquid reagents showed an increasing trend with the increase of SBR dosage, in which the contact angle of emulsified asphalt with water was the greatest, and the contact angle with formamide was the smallest. The coefficients of variation of the contact angles measured for the three liquids were around 2% with good reproducibility. The results of the linear analysis of the surface energy of the three liquids tested and its product with the cosine value of the contact angle are shown in Fig. 9. The figure shows that the correlation coefficients of the linear fits of the six SBR doped emulsified asphalt are all greater than 90%, indicating a good linear relationship. It indicates that the selected fluids are applicable to the surface energy testing of SBR modified emulsified asphalt.
The results of the contact angle test were used in Eqs. (3) (4), and the results are shown in Fig. 10: With the addition of SBR, the surface energy of the modified emulsified asphalt increases, and the surface free energy is more prone to decreasing when combined with the aggregate, and the asphalt-aggregate adhesion performance is better. This is due to the formation of a stable 3D network structure between the SBR and the asphalt, which enhances the adhesion of the asphalt.
The performance of asphalt is subject to mixable duration when asphalt is used during a project. Required mixable duration is necessary when modified emulsified asphalt is applied to micro-surfacing. In this paper, emulsified asphalt dosage is 10%, water dosage is 5% and cement dosage is 1% in the mixable test. The above-mentioned materials were poured into 100 g of the graded aggregate and mixed to observe the mixing duration. The temperature during testing was 23 ℃ and the measured solid content of the modified emulsified asphalt was 63%. The mixing duration of the asphalt mixtures are shown in Table 4. With increasing amount of SBR, the mixing duration becomes longer along with increasing fluidity. This may be the result of formation of a more stable spatial network structure of the modified emulsified asphalt, which becomes stronger when combined with the mixture.
The 1 h and 6 d wet wheel abrasion tests were conducted on modified emulsified asphalt mixtures with SBR of varied dosages using the above-mentioned mixing test formulations. The wet wheel abrasion values are shown in Fig. 11. From the 1 h and 6 d wet wheel abrasion results, it can be concluded that the wet wheel abrasion values of the mixes are higher when no SBR is added. As the dosage of SBR increases, the adhesion and consolidation of asphalt and aggregate become more noticeable, and the abrasion resistance and water damage resistance of the mixture increase. As shown in Fig. 12, the cohesion between aggregates is weak when SBR is not added, and the falling of stone in shape of large particles can be noticeably found. And when the SBR dosing is 3%, the emulsified asphalt and aggregate have strong adhesion ability and the mixture loss is less. This may be the result of the stable structure formed by SBR and asphalt, which inhibits the movement of molecules within the asphalt, thus reducing the effect of water immersion on the asphalt mixture. However, when the dosage of SBR was increased to 4%, the abrasion resistance and water damage resistance of the mixture decreased instead of increasing. Asphalt mixture’s resistance to water damage was the highest at 3% SBR dosage.
The load wheel rolling test was conducted on the SBR emulsified asphalt mixture, and the rutting deformation test results are shown in Fig. 13. The width deformation rate and rutting depth rate of SBR modified emulsified asphalt mixture both become higher first and lower later with the increment SBR doping, which shows that the rutting resistance of modified emulsified asphalt mixture is enhanced first and weakened later. The improvement in the performance of the mixture is the result of the swelling reaction between SBR and asphalt, which restricts the movement of molecules within the asphalt, and the asphalt becomes harder, which enhances the rutting resistance after combining with the mixture. However, when the dosage of SBR is overly high, they will be aggregated. Consequently, the structure of modified emulsified asphalt becomes weaker and the performance of SBR-emulsified asphalt mixtures will deteriorate.
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