Keywords

1 Introduction

As of 2022, China’s has realized over 170,000 km of highway mileage with 5 million km of road mileage, ranking first in the world. There has been a highway maintenance mileage of 5,350,300 km, accounting for 99.9% of highway mileage [1], China’s highway maintenance industry is undergoing a period of rapid development. Emulsified asphalt features such advantages as excellent fluidity at room temperature, no heating during construction, green and low-carbon [2, 3]. It is widely used in highway maintenance projects, and it has just become one of the core materials for highway maintenance [4,5,6]. As improvement has been made in latex preparation process in recent years, polymer-modified emulsified asphalt has become a research hotspot in the industry. Such modified latex as styrene butadiene styrene [7, 8], SBR rubber [9, 10] and waterborne epoxy resin [11,12,13] have been widely used in emulsified asphalt. Of them, SBR latex has significantly improved the low-temperature resistance to cracking and adhesion of emulsified asphalt while improving the high temperature performance of emulsified asphalt, having become the most widely used modifier. Domestic and foreign counterparts have carried out relevant researches such as Meng [14], Wang [15] and Che [16] found that SBR latex can improve the storage stability and high temperature performance of emulsified asphalt, and further enhance the rutting and cracking resistance of the mixture. SBR latex blending amount is recommended to be 3%–5%. Yang GM [17], Yang TW [18], Gong R [19] and Hu FG [20] studied the conventional properties of SBR emulsified asphalt, rheological properties, fluorescence microscopy and road performance of SBR asphalt mixtures, and found that the high temperature resistance to deformation, low-temperature resistance to cracking and relaxation characteristics of the modified asphalt as well as the shear capacity of the mixture have been improved with the increase of SBR doping, and the SBR doping should not be more than 4%. However, there are few reports both at home and abroad which describe the microscopic phase structure and dichotomization analysis, surface energy and adhesion concerning emulsified asphalt modified with SBR latex.

This paper starts with the effect of SBR latex dosing on the properties of emulsified asphalt, and further reveals the role of SBR latex on the performance enhancement of emulsified asphalt and its mixtures through conventional test, DSR, fluorescence microscopy, contact angle and infrared spectroscopy so as to further reveal the role of SBR latex on the performance enhancement of emulsified asphalt and its mixtures, with a purpose to provide a theoretical basis at certain level and a basis of data for the application of real engineering.

2 Materials and Testing

2.1 Raw Materials

  1. (1)

    Matrix asphalt

    70 # matrix asphalt is employed. Asphalt specific conventional performance indicators are shown in Table 1.

  2. (2)

    Modified SBR latex

    The modified SBR latex is a high solid content latex provided by a company in the United States with a solid content of 63%, of which the proportion of styrene is 24% and the proportion of butadiene is 76%.

  3. (3)

    Aggregate

    The aggregate consists of 0–5 mm limestone and 5–10 mm basalt, and all its properties are in compliance with the existing specifications.

Table 1. The basic Properties of Matrix Asphalt.
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2.2 Preparation of Modified Emulsified Asphalt

In this paper, the method where emulsification is followed by modification is employed. The emulsifier is of a slow-cracking and fast-setting cationic emulsifier with a dosage of 2%. SBR emulsified asphalt is prepared as follows: 0%, 1%, 2%, 3%, 4%, 5% SBR (and emulsified asphalt mass ratio) were added to the matrix emulsified asphalt respectively, and stirred well for later use; the mixing duration is 3 min with a mixing rate of 500 rpm. Then SBR modified emulsified asphalt was formed. The preparation process is shown in Fig. 1.

Fig. 1.
figure 1

SBR Modified Emulsified Asphalt Preparation Process.

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2.3 Micro-surfacing Grading Design

The micro-surfacing grade composition is shown in Table 2.

Table 2. Micro-surfacing Grading.
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The composition of the gradation at the micro-surfacing was finalized by the tests and the amount of water added was determined to be 5%, the amount of cement was 1% and the optimum oil/gravel ratio was 6.3%.

2.4 Test Methods

Conventional Performance Test.

According to JTG E20-2011 “Highway Asphalt and Mixture Test Procedure”, the evaporated residue of emulsified asphalt modified with SBR at varied dosages was tested for penetration at 25 ℃, softening point and ductility at 5 ℃ in order to characterize the effect of SBR on the conventional properties of emulsified asphalt.

DSR Test (Temperature Scanning Test).

The temperature scanning test was carried out on the emulsified asphalt modified with SBR using an AR 1500ex dynamic shear rheometer. The test conditions were described as follows: the angular velocity was 10 rad/s, the control strain was 12%, the scanning temperature was 30–66 ℃, and the temperature interval was 6 ℃.

Microscopic Phase Structure.

The microstructure of SBR emulsified asphalt was observed by LW300LFT-LED type fluorescence microscope with a magnification of 400 times. And its fluorescence microscope pictures were subjected to digital image processing to convert the effective information in the figure into digital information. The area percentage of SBR was employed thus to further analyze the effect of SBR on the phase structure of emulsified asphalt.

A Therom Scientific Nicolet iS5 Fourier infrared spectrometer was used to test the changes in functional groups and chemical composition of SBR emulsified asphalt. The tested wave numbers range from 400 cm−1 to 4000 cm−1, and the number of scans was 32 with a resolution of 4 cm−1.

Adhesion Test.

HARKE-CA contact angle tester was used to measure the contact angle of SBR emulsified asphalt by lay-drop method and calculate free energy on its surface, and thus to study the interfacial adhesion strength of SBR emulsified asphalt from the point of view of work and energy, and to gain the effect of SBR blending on the adhesion performance of emulsified asphalt.

Mixture Test.

Mixable tests, wet wheel abrasion tests and rutting tests were carried out on the mixtures to analyze the effect of SBR on the abrasion resistance, water damage resistance and rutting resistance of modified emulsified asphalt micro-surfacing mixtures.

3 Results

3.1 Conventional Performance

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.

Fig. 2.
figure 2

Conventional performance of Emulsified Asphalt with Different SBR.

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3.2 Rheological Performance

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.

Fig. 3.
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Rheological Properties of Emulsified Asphalt with Varied SBR.

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3.3 Microscopic Phase Structure

Fluorescence Microscope.

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%.

Fig. 4.
figure 4

Fluorescence Microscope Images of Emulsified Asphalt with SBR of varied dosages.

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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.

Fig. 5.
figure 5

Grayscale Plot and Binarized Image of Emulsified Asphalt with SBR of Varied Dosages.

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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).

$$ \begin{gathered} R = \frac{{\sum\limits_{i = 1}^{n} i }}{{\sum\limits_{j = 1}^{m} j }} \times 100\% \hfill \\ \hfill \\ \end{gathered} $$
(1)
$$ R^{ - } = \frac{{\sum\limits_{R = 1}^{k} R }}{K} $$
(2)

where i—pixel points of SBR in the sub-block;

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.

Fig. 6.
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The Area Percentage of SBR.

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Infrared Spectrum.

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 2919 cm−1 and 2849 cm−1 for the absorption peaks of the stretching vibration of -CH2 alkanes, and 1456 cm−1 and 1376 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.

Fig. 7.
figure 7

Infrared Spectrums of SBR Emulsified Asphalt.

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Fig. 8.
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Contact Angle Pictures.

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3.4 Adhesion Performance

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:

$$ \gamma = \gamma^{d} + \gamma^{p} = 2\sqrt {\gamma^{ + } \gamma^{ - } } $$
(3)

The relation between the surface energy parameters for the liquid and bitumen are expressed as follows:

$$ \gamma_{L} (1 + \cos \theta ) = 2\sqrt {\gamma_{a}^{d} \gamma_{L}^{d} } + 2\sqrt {\gamma_{a}^{ + } \gamma_{L}^{ - } } + 2\sqrt {\gamma_{a}^{ - } \gamma_{L}^{ + } } $$
(4)

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:

Table 3. Surface Free Energy of Liquids.
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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.

Fig. 9.
figure 9

Contact Angle Results of SBR Modified Emulsified Asphalt.

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Fig. 10.
figure 10

Surface Free Energy of SBR Modified Emulsified Asphalt.

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3.5 Mixture Test

Mixable Test.

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.

Table 4. Mixable duration of SBR emulsified asphalt Mixture.
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Abrasion Resistance and Resistance to Water Damage.

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.

Fig. 11.
figure 11

Wet Wheel Wear Values of SBR Modified Emulsified Asphalt Mixture.

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Fig. 12.
figure 12

Wet Wheel Abrasion Specimens of SBR Modified Emulsified Asphalt Mixture.3.5.3. Rutting resistance.

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Rutting Deformation Test.

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.

Fig. 13.
figure 13

Rutting Deformation Rate of Emulsified Asphalt of SBR Modified Emulsified Asphalt Mixture.

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Fig. 14.
figure 14

Dynamic Stability of SBR Modified Emulsified Asphalt Mixture.

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Rutting Test.

A modified asphalt mixture rutting test was employed as a rutting test method. The 5 cm-thick rutting plate specimen spares set aside a top at a height of 1 cm, and the SBR emulsified asphalt mixture of each dosage was put into the surface of other graded asphalt mixtures, simulating the asphalt structure at the time of micro-surfacing. The results of the dynamic stability of the rutting test at 60 ℃ are shown in Fig. 14. Figure 14 shows that the dynamic stability of the mixture increases as SBR dosage is increased, and the rutting resistance at the micro-surfacing is improved, and the dynamic stability of the mixture is higher and the rutting resistance is stronger when SBR dosage is increased to 3%. This is because the emulsified asphalt and aggregate with 3% of SBR has better combining ability, high structural strength and strong resistance to deformation. Similarly, the rutting resistance of SBR emulsified asphalt mixtures decreases when SBR is doped at 4% and higher.

4 Conclusion

In summary, the results of conventional test, rheological test, microscopic phase structure, adhesion test and Fourier infrared spectroscopy test performed on the SBR emulsified asphalt showed that the SBR can effectively improve the high and low temperature performance and adhesion properties of emulsified asphalt; the type of modification of SBR and emulsified asphalt is of physical modification; a stable spatial network structure can be formed via 3% SBR and emulsified asphalt, when the doping is overly high, SBR Aggregation occurs. Thus, it is not recommended to overly dope SBR; after the fluorescence microscope images of SBR emulsified asphalt were digitized, it is concluded that the compatibility between SBR and emulsified asphalt is good.

Tests on SBR emulsified asphalt mixtures showed that the water damage resistance, abrasion resistance and rutting resistance of SBR emulsified asphalt are improved by increment in SBR. The mix performance of emulsified asphalt reached its optimum at 3% of SBR dosing. In an overall consideration of the results of test on SBR emulsified asphalt and its mixture, dosage of SBR is recommended to be at 3%.