Beijing Key Laboratory of Traffic Engineering, Beijing University of Technology, Beijing 100124, China
Abstract
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The overall rigidity of the cement concrete pavement is high, but there are defects such as easy cracking and insufficient anti-slip performance. The epoxy resin ultra-thin wearing course overlay can effectively solve these issues. However, there is still a lack of knowledge about the long-term performance of epoxy resin ultra-thin wearing course overlay on cement concrete pavement. Therefore, this article analyzed the interlayer adhesion and durability of epoxy resin ultra-thin wearing course overlay through the Hamburg rutting test and a series of shear tests under damp heat, thermal oxygen aging, and ultraviolet (UV) aging conditions. Shear test results indicated that the shear performance of epoxy resin overlay grew with the increase in epoxy resin content and was severely affected by high temperature, and the optimal content was set as 3.4 kg/m 2 . The Hamburg rutting test results showed that the epoxy resin overlay exhibited satisfactory high-temperature performance and water resistance. For the damp heat effect, it was revealed that damp heat led to more significant shear strength loss compared with the overlay specimens without damp heat. The water immersion caused the shear strength decline due to the water damage to the overlay interface. As for the thermal oxygen aging effect, it was reflected that the short-term thermal oxygen aging had a minor impact on the shear performance of the epoxy resin overlay. However, with the increase in thermal oxygen aging duration, the shear strength of the epoxy resin overlay significantly decreased due to the aging of epoxy resin binders. Regarding the UV aging impact, it was also found that the shear performance of the epoxy resin overlay rapidly decreased as the UV aging duration grew whether at 20 °C or 60 °C. Moreover, UV aging led to a more significant impact on the shear performance of the epoxy resin overlay than thermal oxygen aging.
Keywords:
Introduction
The resin ultra-thin wearing course overlay is a critical coating technology that can effectively solve early pavement damage such as cracks, peeling, and potholes in cement concrete pavement; meanwhile, it can also improve driving comfort and anti-slip performance [1,2,3,4,5]. Sprinkel [6] conducted a long-term tracking investigation on a road surface paved with the resin wearing course and found that it had excellent anti-slip performance and long service life. Hong [7] proposed a new type of tunnel pavement, i.e., polyurethane ultra-thin wearing course, which exhibited satisfactory mechanical properties and wear resistance, and had nearly no harmful emissions due to its room-temperature construction. Compared with traditional asphalt mixtures, the resin ultra-thin wearing course was proved to have excellent mechanical properties, slip resistance, sound absorption, flame retardancy, and other functional properties, as well as good environmental performance.
The resin ultra-thin wearing course is mainly composed of aggregates and resin-based binders. Aggregates are used to enhance the friction coefficient of road surfaces and play a role in anti-slip. Wear-resistant and anti-slip materials such as ceramic particles and quartz sand can be selected. Deng et al. studied the performance and nano adhesion behavior of the overlay using experimental and molecular dynamics simulation methods [8,9] and found that using diamond sand with a particle size of 2–3 mm as the ultra-thin overlay aggregate exhibited more favorable wear and skid resistance than ceramic particles. The resin binders play a bonding role and are often selected from resin materials such as polyurethane or epoxy resin, which have good bonding performance, stability, and wear resistance. Compared with an asphalt binder, most of these resins exhibit thermosetting behavior as the temperature rises, and the molecular networks created during hardening are refined to form other networks. In contrast, asphalt binders tend to soften at high temperatures. As a result, these binders perform better, especially in high temperatures where the use of an asphalt binder can cause the binder to bleed out. Wu et al. confirmed that using an epoxy resin ultra-thin wearing course overlay can significantly improve the poor anti-slip performance of steel bridge decks, and the ultra-thin wearing course has good high-temperature resistance and mechanical properties [10]. After 7 months of use, the overall condition of the road surface was good, with a structural depth of over 1.4 mm, and the bond strength was still stable at 3.71 MPa. The experimental results from Stenko et al. [11] showed that the compressive strength of the epoxy resin ultra-thin wearing course material was about 40 MPa and the bending strength was about 13 MPa. Horn et al. [12] compared different types of resin concrete and established that the performance of epoxy resin concrete was superior to that of methyl acrylic acid resin concrete in all aspects. Attanayake et al. [13] found that regardless of the concrete age at the time of overlay application, the bond strength of an epoxy overlay under elevated temperatures was less than 1.7 MPa. The primary failure type was a bond failure at the concrete/overlay interface. Freeseman et al. [14] proposed that the accelerated freezing and thawing exposure greatly affects the bond strength of epoxy overlays.
The epoxy resin has certain chemical activity and can be ring-opened, cured, and cross-linked by compounds containing active hydrogen to form a network structure [15], Therefore, the epoxy resin ultra-thin wearing course overlay can play a role in anti-slip and extending the service life of the road surface [16,17]. However, the long-term performance of epoxy resin ultra-thin wearing course overlay is affected by rain, high temperatures, and strong ultraviolet radiation during service, and its durability varies with different environmental influences. Therefore, the long-term durability of the epoxy resin ultra-thin overlay layer is closely related to the impact of aging. Epoxy resin is prone to aging during use, manifested in surface yellowing, loss of gloss, cracking, and overall mechanical properties degradation [18]. Epoxy resin aging is mainly divided into three types: thermal oxygen aging, humid heat aging, and ultraviolet aging. Among them, when the epoxy resin is thermally excited in an oxygen-containing environment, the molecular chain will absorb the oxygen in the environment and generate hydroperoxide, which will lead to instability of hydroperoxide, rearrangement reaction of polymer main chain, and chain breaking or cross-linking, which will degrade the performance of polymer materials and lead to thermal–oxidative aging [19]. Yu conducted a 30-day thermal aging of an epoxy resin matrix at 130~160 °C to study the effect of thermal oxygen aging on epoxy resin [20]. The FTIR results indicated that during the thermal aging process, the epoxy resin sample undergoes oxidation and molecular rearrangement. The bending test results indicated that the thermal aging significantly reduced the fracture strain, while the bending strength was only slightly affected and the modulus increased. When epoxy resin materials underwent high humidity or rainwater environments during service, they will experience significant damp heat [21,22], resulting in a performance degradation and an inability to meet usage requirements. Wang et al. conducted an in-depth study on the effects of humidity and time on the structure and mechanical properties of phenolic epoxy resin during the wet heat aging process [23]. The results indicated that the moisture absorption rate increased linearly with the square root of aging time and follows Fick’s second law. There were two main types of reactions during wet heat aging: the first type was the post-curing process, which led to a higher crosslinking density and reduced internal stress; another was the plasticization and degradation of epoxy resin caused by the entry of moisture. The unsaturated bonds or polar groups such as benzene rings and ether bonds contained in epoxy resin, as well as impurities introduced during the polymerization stage and processing, were prone to absorbing ultraviolet radiation and causing photooxidation reactions in epoxy resin [24]. Affected by aging, the bonding and shear properties of epoxy resin decrease. Therefore, the firmness of the bonding between the epoxy resin ultra-thin overlay layer and cement concrete pavement is one of the key issues that needs to be solved when adding the epoxy resin ultra-thin overlay layer to cement concrete pavement.
Overall, it was noted that there is still a lack of knowledge about the long-term performance of epoxy resin ultra-thin wearing courses. Firstly, the optimum dosage and high temperature resistance of epoxy resin ultra-thin wear layer covering were evaluated by oblique shear test and Hamburg rutting test. Secondly, the effects of different aging conditions, including damp heat, thermal oxygen aging, and ultraviolet aging, on the shear resistance of epoxy resin ultra-thin wearing course overlay were investigated by means of oblique shear tests. This study was expected to provide an experimental theoretical basis for the engineering feasibility of the epoxy resin ultra-thin wearing course overlay on cement concrete pavement.
Materials and Experiment Methods
2.1. Materials
The performance of the epoxy resin was the key to ensuring the quality and longevity of the epoxy resin ultra-thin wearing course. The epoxy resin materials were divided into two components, A and B, and were blended in proportion to the application. Two kinds of commercial epoxy resin materials were shown in Figure 1, i.e., RS epoxy resin and RA epoxy resin, were selected. The main performance indicators after the blending reaction are shown in Table 1.
2.2. Preparation
The cement concrete panel was made of C40 strength grade concrete. In order to simulate the actual situation of cement concrete pavement with epoxy resin ultra-thin overlay, composite rutting specimens were formed by using special rutting plate test molds. Firstly, according to the requirements of the “Test Procedure for Cement and Cement Concrete in Highway Engineering” [25], 30 × 30 × 7 cm 3 cement concrete specimens were prepared by vibratory compact forming method using asphalt mixture rutting plate. The cement concrete panel was then cleaned and dried naturally, placed under standard curing conditions, and cured for 28 d to reach the design strength; the cured cement concrete panel was put into a 30 × 30 × 7 cm 3 mold, the epoxy resin components A and B were mixed and stirred nicely, and the designed amount of resin was evenly applied using a brush. After the resin was applied, the gravel was spread in time according to the design requirements after the first layer of resin was cured and stabilized, the unadhered aggregate was swept away and the second layer of resin material was applied, the gravel was spread in time after the resin was cured and stabilized, the unadhered aggregate was swept away, and the specimen was cured for 3 days at room temperature. The automatic rock cutting machine was used to cut the cured composite rutting slab into composite specimens of 8 × 8 × 7 cm 3 , which were subjected to the 45° oblique shear test and pull-out test, with three parallel specimens in each group. The forming process is shown in Figure 2.
2.3. Test Methods
2.3.1. Shear Test
The sample group was fixed to conduct the shear test using the 45° oblique shear fixture and oblique shear instrument [26,27]. Evaluation of shear resistance was carried out in an electro-hydraulic servo mechanical test system with a measurement accuracy of 1 N for the pressure measuring unit, 0.1 mm for the displacement sensor, and 0.1 °C for the ambient room temperature sensor. The shear stress and shear strain were calculated according to Equation (1). The tests started with the preparation of thin layers on cement concrete panels at room temperature with different resin dosages selected and tested for shear resistance, the main test procedure is shown in Figure 3 below. The load was then applied at a rate of 1.0 N/s until shear damage occurred to the specimen. The value of the applied load at the time of damage was recorded and the shear strength of the shear surface was calculated.
τ = P × sin α S
P —load (N); S —shear area of the specimen (mm 2 ); τ —shear strength (MPa); α —angle between shear surface and horizontal surface, which, in this test, was 45 degrees.
