Abstract
Geosynthetic interlayer systems are effective techniques to control reflective cracking and have better waterproof performance in damaged asphalt pavements. The interlayer bonding performance is critical to its function. This research investigated the interfacial adhesive property and road performance of asphalt overlay with geotextile interlayer. First, shear damage energy characteristics of geotextile interlayer have been investigated by varying tack coat types (SBS-modified asphalt, virgin asphalt, and emulsified asphalt), application rates (0.3 kg/m2–1.2 kg/m2), spraying times (1–2), upper mixture paving temperature (140°C–180°C), service temperature (5°C–45°C), and soaking time (0 h–96 h). In addition, bending fatigue tests, rutting tests, and semicircular bending tests were carried out to evaluate the comprehensive road performance of the asphalt overlay. The results showed that the interface adhesive property of asphalt overlay with geotextile interlayer using SBS-modified asphalt is best when the application rate is 0.8 kg/m2. The higher paving temperature is beneficial for the geotextile interlayer adhesion performance when the SBS-modified asphalt is sprayed at one time, while the paving temperature has almost no effect when the SBS-modified asphalt is sprayed at two times. The geotextile interlayer adhesion performance is less affected by temperature and more affected by water immersion. Compared with the common asphalt overlay types, the asphalt overlay with geotextile interlayer using SBS-modified asphalt shows better overall performance.
1. Introduction
Rutting, cracks, potholes, delamination, and deformation are the main manifestations of the asphalt pavement overlay diseases, among which the reflection cracks are the most general form [1–4]. Cracks in the asphalt overlay will result in moisture intrusion into asphalt pavement structures and significantly reduce the service level [5]. Traditional cracks prevention methods mainly involve increasing overlay thickness [6], optimizing overlay material composition [7], and paving geosynthetic material layer [8, 9], among which the setting of geotextile interlayer is a more economical option [10, 11]. In addition, the permeability coefficient of the geotextile interlayer reduces approximately two orders of magnitude with impregnation conditions [12].
Although geotextile materials are gradually popularized and applied in asphalt pavements [13], the evaluation of the effects depends on the perspective of the studies [14]. The result may be caused by different bonding conditions under various tests, which has prompted scholars to start paying attention to realize that the bonding performance between the geotextile interlayer and the asphalt layer is the key factor affecting the effect of geotextile interlayer material. In addition, the poor bonding condition between the geotextile layer and the asphalt overlay will accelerate slippage cracking [15]. Therefore, it is necessary to conduct in-depth research on the bonding properties of geotextile interlayers.
The influence of the geotextiles on the bonding performance of asphalt interlayer was studied through the oblique shear test and pull-out test. It was found that the interlayer bonding performance decreased to a certain extent after adding geotechnical materials to the asphalt surface layer [16]. The new test equipment was developed to evaluate the bonding properties of geotextiles and asphalt layers, and the feasibility of the test equipment was verified [17]. The shear strength of the mixture-geotextile interface is influenced by the physical properties of the material itself. The weight and thickness of geotextile at the interface between two layers of pavement structure have been found to have a significant impact on bond and shear strengths [18]. Many external factors, such as the roughness of the interface and the service temperature, were also investigated to the influence trend of bonding performance of geosynthetic interlayer [19]. Compared with other geotextiles, the geotextile designed as an interlayer has some advantages, and it is determined whether these advantages can be exerted depending on the bonding performance. However, the current research on bonding performance is not systematic, and the correlation between bonding performance and the other road performance has not been well demonstrated [20, 21].
Given this background, the research presented in this study aims to perform a systematic evaluation of the bonding properties of geotextile interlayers in asphalt pavement and the correlation between adhesive properties and road performance. For this purpose, the effect of type, spraying quantity, spraying times of tack coatings, upper layer paving temperature, soaking time, and the ambient temperature on the shear characteristics of nonwoven polyester geotextile (NWPG) were investigated through the direct shear test. Furthermore, bending fatigue tests, rutting tests, and semicircular bending tests were carried out on the asphalt overlay specimens in the laboratory. The reference systems (interlayer without NWPG) were also included for comparison purposes.
2. Materials and Methods
2.1. Materials
Nonwoven polyester geotextile (NWPG) has been selected as an interlayer due to its appropriate flexibility and permeability (Figure 1). The physical and mechanical properties of NWPG are given in Table 1. SBS-modified asphalt, virgin asphalt (A-70#), and PC-3 emulsified asphalt were used as tack coats. The main technical indicators are given in Tables 2 and 3, which all satisfy the specification requirements (technical specifications for construction of highway asphalt pavements) [22]. The dense-graded AC-16 and AC-20 are adopted in this study, and their gradations are shown in Figure 2, respectively. Limestone is used as gravel and filler. The corresponding optimal asphalt content of AC-16 and AC-20 is 4.8% and 4.3%, respectively.
2.2. Specimen Preparation
In order to simulate the structure of asphalt pavement with a geotextile interlayer, the combination of “asphalt overlay + geotextile interlayer + base layer” was used as the experimental group. The combination of “asphalt overlay + bonding layer + base layer” was used as the reference group. The detailed process for sample preparation is as follows:(1)The AC-20 asphalt mixture plate with a size of 300 mm (width) × 300 mm (length) × 50 mm (height) was compacted, and then, the mixtures were demolded when they have reached a room temperature (usually 24 h).(2)Tack coats (SBS-modified asphalt, virgin asphalt, and PC-3 emulsified asphalt) were sprayed on the surface of the plate (Figure 3(a)). For the twice-spread process, half of the tack coats were applied first, and then the remaining half is applied after laying the geotextile material.(3)For the experimental group, the cut geotextile was tiled on the surface of the specimen and then rolled 5 times using a wheel grinding instrument (Figures 3(b) and 3(c)). The binder is enabled to fully penetrate geotextile, simulating the site construction rolling process.(4)For the reference group, emulsified asphalt was evenly spread on the surface of AC-20. Subsequent operations are performed without laying geotextiles after emulsifier demulsification.(5)The prepared specimen was placed into the rutting sample mold with a size of 300 mm × 300 mm × 100 mm. The AC-16 asphalt mixture was compacted to the target void ratio (4.5 ± 0.5%) to form a double-layer rut slab (Figure 3(d)). A double-layer rut slab with an interlayer of geotextile was used for rutting tests (Figure 3(e)).(6)A cylindrical sample with a diameter of 100 mm and a height of 100 mm was obtained by coring from the double-layer rut slab for the direct shear test (Figure 3(f)). The semicircular sample cut from the cylindrical core with a height of 50 mm (25 mm AC-16 + 25 mm AC-20) was used for semicircular bending test (Figure 3(g)). The bending fatigue test has been used to evaluate the NWPG against cracking reflection. The composite beam for the bending fatigue test was sawed in smaller specimens of 250 mm × 50 mm × 50 mm (length × width × height) from a double-layer rut slab. The initial crack of the pavement was simulated by making a notch (height 5 mm and width 3 mm) in the middle of the bottom of the composite beam (Figure 3(h)).
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2.3. Methods
2.3.1. Layer-Parallel Direct Shear Test
The direct-shear test was used to evaluate the bond performance of the NWPG and asphalt mixture. The shear strength of the material was measured by MTS universal material testing machine under displacement control at a rate of 1 mm/min, as shown in Figure 4. For each direct shear test, the area of the load-shear displacement curve envelope can be expressed as interface shear damage energy (J) [23], as described in the following equation:where J is the interface shear damage energy, N·mm; f(t) is the load at time t, N; s(t) is the shear displacement at time t, mm; T is the time of peak load, s.
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The damage energy coefficient C is used to characterize the difference in adhesion performance between the experimental and reference groups:where Jt is the shear damage energy of experimental group, N·mm; Jc is the shear damage energy of reference group.
An experimental program was designed to investigate the effect of spraying quantity, spraying times, mixture paving temperature, test temperature, and soaking time on the interlayer bonding performance of NWPG. In the experimental group, the preferred spraying quantity ranges of SBS-modified asphalt, 70# asphalt, and PC-3 emulsified asphalt were 0.3–1.2 kg/m2, 0.3–1.2 kg/m2, and 0.6–1.5 kg/m2, respectively. A reference group was spread with 0.4 kg/m2 PC-3-emulsified asphalt according to Technical Specifications for Construction of Highway Asphalt Pavements (JTG F40-2004) [22]. The paving temperature of the upper layer varies due to differences in the construction environment and field dispatch. Thus, three paving temperatures were selected, including140°C, 160°C, and 180°C. In addition, considering the influence of ambient temperature on the NWPG bonding performance, the direct-shear test was conducted at 5°C, 25°C, and 45°C in an air environment, respectively. Both single and twice spraying methods have been considered based on construction site experience. The effect of water immersion on the interlayer bonding properties of the NWPG was investigated at different immersion times. In the beginning, the specimens were saturated in a 45°C water bath for 0 h, 24 h, 48 h, and 96 h, respectively. Then, these specimens were taken out and maintained in an environment chamber (25°C, with no water) for 2 h before testing. Four parallel specimens have been evaluated using this test. The detailed experimental scheme is given in Table 4.
2.3.2. Layer-Parallel Direct Shear Test
The ability of the NWPG to resist reflective cracking in pavement overlay was investigated through bending fatigue experiments. This test method reproduces the conditions that lead to the appearance of fatigue cracking in pavements using a specimen (with optimal spraying quantity of tack coat) placed on two fixed supports, as shown in Figure 5. All the specimens were maintained at 25°C for 4 h before testing. The device can generate and propagate a controlled fatigue cracking process using 10 Hz dynamic sine loads. The minimum and maximum values of the dynamic load were set to 0.01 kN and 0.1 kN, which ensured that the loading head was in contact with the specimen to avoid impact load. When the macrocrack propagates throughout the entire specimen causing the fracture of the two layers, the tests were ended.
2.3.3. Long-Term Rutting Test
In order to analyze the influence of NWPG interlayer on the long-term high-temperature rut resistance of pavement structure, the rutting experiment was carried out according to the Test Specification of Asphalt and Asphalt Mixture in Highway Engineering (JTG E20-2011) [24]. Different from the standard test process, all the tests were performed for 3 h. Rutting depths were measured to compare between the experimental group (NWPG interlayer with SBS-modified asphalt at an optimal spraying quantity of 0.8 kg/m2) and reference group specimens. Through three independent parallel tests, dynamic stability (DS) could be expressed as follows:where d45 and d60 are the rutting depths at 45 min and 60 min in every hour, respectively.
2.3.4. Semicircular Bending Test
The semicircular bending (SCB) test was used to evaluate the low-temperature crack resistance of asphalt mixtures. The fracture energy (G) was calculated as the evaluation index [25, 26]. The setup for the SCB test consists of a loading ramp at the top center of the sample and two supporting rollers at two sides of the bottom, with a distance of 80 mm, as shown in Figure 6. A constant displacement rate of 1 mm/min at −10°C was applied to the sample until the cracking failure occurred. According to the optimum tack coatings scheme, three parallel tests were carried out by applying the SBS-modified asphalt experiment and the reference group as the test subject. The fracture energy calculation is shown in equation (4). The greater the G value, the more energy is required for the composite structure to form cracks:where G is the fracture energy, J/m2; W is the total area within the load-displacement curve , J; A is the fracture area, which can be approximated as the product of specimen thickness and radius, m2.
3. Results and Discussion
This section may be divided by subheadings. It should provide a concise and precise description of the experimental results, their interpretation, and the experimental conclusions that can be drawn.
3.1. Interlayer Adhesion Performance
3.1.1. Effect of Tack Coat Type and Its Spraying Quantity
The adhesion of NWPG interlayer to the asphalt overlay was tested at different tack coat spraying quantities. The test results are shown in Figure 7.
The shear damage energy of the NWPG with SBS-modified asphalt to the asphalt overlay was the highest, followed by the 70# asphalt, while the emulsified asphalt was the weakest. To explain the observed results, we may consider the differences between various types of tack coatings [27]. In addition, thermally bonding methods were used when the tack coatings were SBS-modified asphalt and 70# asphalt. The shear damage energy of all test samples showed a trend of firstly increasing and then decreasing with the spraying quantity of tack coatings. The shear damage energy of the 1# experimental group (emulsified asphalt + NWPG) was smaller than that of the reference group, indicating that the adhesive performance of the “geotextile-asphalt” structure was weaker than that of the “asphalt-asphalt” structure. The interface adhesive performance decreases with the introduction of geotextile. Meanwhile, the texture depth of the base surface was reduced, which led to difficulties in forming an interlocking structure between the upper and lower asphalt mixture layers, thus reducing the contribution to the shear resistance of the interlayer. The shear damage energy of the 2# experimental group (70# asphalt + NWPG) at an optimal tack coat spraying quantity (0.9 kg/m2) is similar to that of the reference group (2256 N∙mm). The shear damage energy of the 3# experimental group (SBS-modified asphalt + NWPG) exceeded that of the reference group in the spraying range of 0.6–1.0 kg/m2 for SBS-modified asphalt.
For emulsified asphalt, 70# asphalt, and SBS-modified asphalt, the tack coatings content corresponding to the peak of the damage energy curve was 1.2 kg/m2, 0.9 kg/m2, and 0.8 kg/m2, respectively. However, weak bonding due to improper tack coat type and/or unsuitable application rate contributes to slippage cracks. It is difficult to completely infiltrate the geotextile and achieve excellent adhesion performance between the upper and lower layers when the spraying quantity of tack coating was insufficient. The lubricating layer was formed by the excessive tack coatings, which negatively affected the interlayer adhesion performance.
3.1.2. Effect of Upper Paving Temperature and Spraying Times of Tack Coatings
In order to evaluate the influence of the upper paving temperature and the spraying times of the tack coatings on the interlayer bonding performance, the shear test results of the “0.8 kg/m2 SBS-modified asphalt + NWPG” structure are given in Table 5. Two paired sample t-tests were employed to analyze the differences between the experimental groups. This method could be useful to find the difference between the two experimental groups, excluding the factors of the operator, instrument, and samples. Student’s t-test results for distinct mixture paving temperature and tack coat spraying times are given in Tables 6 and 7, respectively (sig.<0.05 was considered statistically significant) [28].
As given in Table 6, the mean difference is negative, indicating that the higher paving temperature positively affects the geotextile interlayer adhesion performance. The mean difference of shear damage energy at 140°C under the once sprayed process is significant compared with the results at 160°C and 180°C (sig.<0.05). It proved that the geotextile interface shear damage energy increased with the paving temperature (Table 5). However, the mean difference of shear damage energy between the experimental groups at 160°C and 180°C was not significant (sig.>0.05), which illustrated that significant and sustained improvement in the adhesion properties of geotextile layers would not be observed after the paving temperature exceeds 160°C. The possible reasons are as follows: (1) it is beneficial for the tack coatings to fully immerse into the geotextile and form a unit with the asphalt overlayer under a higher temperature. The tack coatings lack the motivation to “transport” upward at lower temperatures. Therefore, it mostly congregates under the geotextile. (2) The upper surface of the geotextile tends to melt at high temperature, and this section will be combined with the asphalt overlayer as a unit [20].
However, the mean difference of shear damage energy between experimental groups at different temperatures was not significant when the tack coatings were sprayed twice. This phenomenon may be because the NWPG interlayer can achieve good adhesion with the mixture, which is less affected by temperature under the two-time spreading process. As given in Table 7, the sig. value of the experimental group under different coating processes was 0.037 (at 140°C), 0.608 (at 160°C), and 0.982 (at 180°C), respectively. The results show that the spraying times of tack coatings significantly affect the adhesion performance of the geotextile interlayer when the paving temperature of the asphalt overlayer is 140°C. The mean difference at this temperature was negative, indicating that the two-time spreading process was better than the single. The two-time spreading process of tack coatings can make up for the poor bonding between the geotextile interlayer and asphalt overlayer. Therefore, this spraying process was suitable for projects performed under lower paving temperatures.
3.1.3. Effect of Service Temperature and Soaking Time
To further study the influence of ambient temperature and soaking time on interface bonding performance, the shear damage energy of the structure of asphalt overlay with geotextile interlayer was evaluated by using shear tests under optimal tack coatings. The test results are shown in Figure 8.
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As shown in Figure 8(a), the shear damage energy of the composite structures rapidly decreases with increasing temperature, indicating the increase of temperature leads to the adhesion reduction of tack coatings [27]. The “emulsified asphalt + NWPG” interlayer structural damage energy coefficient was 49% at 5°C, 45% at 25°C, and 67% at 45°C. The shear damage energy coefficients of the “70# asphalt + NWPG” experimental group at different temperatures were 87% (at 5°C), 93% (at 25°C), and 123% (at 45°C), respectively. The adhesion property difference between the experimental and the reference group was not obvious when the coefficient of shear damage energy was closer to 1. The damage energy coefficient is increased because the geotextile has a certain adsorption effect on the excess asphalt at high temperatures, reducing the lubricating effect between pavement layers. Therefore, the decrease of adhesion performance of the interlayer with geotextile is slighter than that of without geotextile. The damage energy coefficients of the “SBS-modified asphalt + NWPG” experimental group were significantly higher than that of the reference group, which were 105% (at 5°C), 110% (at 25°C), and 150% (at 45°C). The reasons for this result include 2 points: (1) the adsorption of geotextile and (2) a higher softening point and the better thermal stability of SBS-modified asphalt were observed compared to emulsified asphalt.
As shown in Figure 8(b), the shear damage energy of each group decreased continuously with the increasing soaking time. Based on the results of different groups, it can be seen that the soaking time had significant effects on interface bonding performance. More and more moisture entered the bonding interface with the soaking time. The interlayer bonding property decreased with water immersion. The longer soaking time led to a greater peeling effect of the tack coat layers. After setting the geotextile layer, interlayer bonding performance is more sensitive to moisture absorption, which may also lead to the absorbed moisture ascending. Peeling between “asphalt-geotextile” is easier than “asphalt-asphalt” after absorbing moisture. The moisture diffusing into the geotextile layer was unfavorable to maintain good adhesion, leading to severe interface damage.
3.2. Antireflective Cracking Performance
A positive correlation between the adhesive properties of geotextile interlayers and their antireflection cracking performance are shown in Figure 9. The shear damage energy of the “SBS-modified asphalt + NWPG” group (2556 N∙mm) was best, followed by the reference group (2256 N∙mm) and “70# asphalt + NWPG” group (2108 N∙mm), “emulsified asphalt + NWPG” group (1010 N∙mm) was the last. The service life of geotextile laying groups with a strong bond layer (SBS-modified asphalt and 70# asphalt) was significantly higher than the reference group. The service life of the “emulsified asphalt + NWPG” group was less than the reference group due to the poor adhesion performance of emulsified asphalt bonding layer, although the geotextile interlayer has been set. This result illustrated that the antireflection cracking effect of the geotextile interlayer could not be achieved without a guarantee of its adhesive properties.
3.3. High-Temperature Antirutting Performance
Rutting disease is the main factor leading to poor comfort and safety of pavements and shortened service life. The rutting depth of the experimental group (“SBS-modified asphalt + NWPG”) was 2.137 mm (1 h), 2.856 mm (2 h), and 3.275 mm (3 h) respectively. Compared with the first hour, the rutting depth growth rate has been significantly reduced in the second and third hours. By comparing the test groups, the vertical deformation of the experimental group was less than that of the reference group. Therefore, laying a geotextile interlayer improved the rutting resistance of the pavement structure. The dynamic stability (DS) under different schemes was calculated according to equation (2), and the results are given in Table 8. The dynamic stability value shows a significant increasing trend with the loading times. The dynamic stability of the experimental groups was 131%, 132%, and 121% of the reference groups, respectively, which again proved that laying a geotextile interlayer would improve the rutting resistance of the pavement. The NWPG (using SBS-modified asphalt) interlayer changes the contact conditions between asphalt overlayers and the old pavement, which improves the interlayer bond ability. Consequently, the shear stress on the asphalt overlay would reduce significantly, slowing the shear damage of the whole pavement structure [29]. In addition, geotextile interlayer, as a monolithic material, limits the dislocation movement of aggregate particles caused by loading under high-temperature conditions [14], which also helps to improve the high-temperature deformation resistance of the structural layer.
3.4. Low-Temperature Crack Resistance
Figure 10 shows a typical load-displacement curve of the SCB test performed at −10°C. The variation trend of the curve in the SCB test of each scheme is the same. The vertical load increases with increasing displacement and then decreases sharply. It was found that the low-temperature crack characters were different between the reference group and the experimental group (“SBS-modified asphalt + NWPG”). Fracture failure of the reference group occurred suddenly at the peak load and was accompanied by the generation of a certain “sound”, which showed a brittle failure characteristic. However, the low-temperature fracture damage of the experimental group was divided into two stages. With the displacement increment, the vertical load firstly increased and then decreased. In the first stage, the variation trend of the curve was similar to that of the reference group. The vertical load reached its peak value when the displacement was 2 mm. Then, the load dropped rapidly to about 2 kN after the sample cracked. The geotextile interlayer in the experimental group has a high fracture elongation and higher impact toughness, absorbing a large amount of energy released by crack tips. In the second stage, the geotextile interlayer was the major load-bearing component, and cracks expanded with the continuous loading and eventually penetrated the entire specimen.
As given in Table 9, the peak load of the experimental group was slightly larger than the reference group, which was attributed to the partial tensile stress bearing by the geotextile. Based on the fracture energy (G), the test can simulate the crack propagation behavior and comprehensively reflect the low-temperature crack resistance of asphalt pavement. The larger the fracture energy was, the better the crack resistance of samples was at low temperatures. For the reference group, the difference between Gf and Gt was lower than 7%. In contrast, Gf and Gt in the experimental group showed a higher variance. Gf and Gt in the experimental group increased by 25% and 68%, respectively, compared with the reference group. It indicated that asphalt pavement structures with geotextile interlayers require higher fracture energy for low-temperature damage. Therefore, laying a geotextile interlayer significantly affects the low-temperature crack resistance of the pavement overlay engineering.
4. Conclusion
The results obtained during this experimental investigation allow several conclusive remarks to be made concerning the layer-parallel direct shear test, bending fatigue tests, long-term rutting tests, and SCB tests.(1)The bonding performance of the geotextile interlayer tends to increase first and then decrease with tack coat application rates. The corresponding optimal spraying rates are 1.2 kg/m2 (emulsified asphalt), 0.9 kg/m2 (70# asphalt), and 0.8 kg/m2(SBS-modified asphalt), respectively. The bonding performance of geotextile interlayers using SBS-modified asphalt is better than those using emulsified asphalt and 70# asphalt and can reach the level of interlayer bonding of conventional asphalt overlay pavement structure. Therefore, SBS-modified bitumen is recommended for the tack coat when the geotextile interlayer is applied in engineering.(2)When SBS-modified asphalt tack coat is sprayed once, the upper layer mixture paving temperature, which is lower than 160°C, has an adverse effect on the bonding performance of the geotextile interlayer. When the SBS-modified asphalt tack coat is sprayed twice, the upper layer mixture paving temperature of 140°C–180°C has no significant influence on the bonding performance of the geotextile interlayer. In view of this, once the spreading process is allowed for the SBS-modified asphalt tack coat when the asphalt overlay adopts a higher mixture paving temperature, the SBS-modified asphalt tack coat should be two-time sprayed when the asphalt overlay paving temperature is limited (≤160°C).(3)Increased service temperature or soaking time of the pavement will significantly reduce the bonding performance of the geotextile interlayer. Compared with conventional pavement structures, the bonding performance of asphalt overlay structure with geotextile interlayer is less affected by temperature but more affected by water peeling.(4)There is a good consistency between the antireflection crack property and the interface bond property of the asphalt overlay pavement structure with a geotextile interlayer. The service life of emulsified asphalt experimental group with poor interlayer adhesion is less than that of the reference group. The service life of the 70# asphalt experimental group and SBS-modified asphalt experimental group is 2–3 times higher than that of the reference group.(5)The antireflective crack performance, rutting resistance, and low temperature crack resistance of the asphalt overlay with geotextile interlayer using SBS-modified asphalt have significant advantages compared to common asphalt overlay types
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The authors appreciate the financial support from the Science and Technology Department of Henan Province (202102310263 and 212102310986) and the Open Fund of Key Laboratory for Special Area Highway Engineering of Ministry of Education (Chang’an University) (300102210505).