3.1. Materials
ESSO 90# asphalt binder and Jinyang emulsifier from Tianjin Province, China, were selected to prepare the emulsified asphalt. The properties of asphalt and prepared emulsified asphalt are shown in
Table 1 and
Table 2, respectively. Two types of waterborne epoxy resins were selected, named X waterborne epoxy and Y waterborne epoxy, respectively. Two types of curing agents were also selected, named A curing agent and B curing agent, respectively. The properties of the waterborne epoxy used in this work are displayed in
Table 3. The limestone aggregate with a nominal maximum size of 9.5 mm and gradation shown in
Figure 1 was produced from a local quarry in Shanxi Province, China, has a density of 2.702 g/cm
3 and compressive strength of 135 MPa.
3.2. Optimum Curing Agent Content
There is difference between the actual optimal ratio and the optimal stoichiometric ratio of the reactants, and this difference can lead to a reduced performance of the reaction product. The optimal reactant stoichiometric ratios were 3.33% for curing agent A and 39% for curing agent B, respectively. Five different percentages of curing agent A content (by weight of waterborne epoxy system), namely 1%, 2%, 3.33%, 4%, and 5% and five for curing agent B including 30%, 35%, 39%, 45%, and 50% were considered.
Compressive strength is one of the parameters to evaluate the curing state of waterborne epoxy mortar (WEM) and the bonding performance of waterborne epoxy resin (WER). According to Chinese specification DL/T 5193-2004, the WEMs were cast into a cube specimen with the size of 40 mm [
16]. In this study, we prepared specimens with curing ages of 3 d, 7 d, 14 d, and 28 d, respectively at a curing temperature of 23 ± 2 °C. The specimens were loaded with a continuous and uniform load using a universal testing machine (UTM) with a loading speed of 45 N/(mm
2∙min). The maximum load when the sample is broken is recorded, and the compressive strength of the sample is obtained from the ratio of the maximum breaking load to the pressure-bearing area of the sample. Each test was repeated three times, and the compressive strength test results are shown in
Figure 2.
From
Figure 2a,b, it can be seen that the compressive strength of WEM increased with the increase in the curing agent content, and reached a peak at a certain content. Therefore, the actual optimal reactant ratios of the X waterborne epoxy system were 3.33% for curing agent A and 45% for curing agent B, respectively. Similarly, the actual optimal reactant ratios of the Y waterborne epoxy system were 3.33% for curing agent A and 45% for curing agent B, respectively, as can be seen in
Figure 2c,d. Compared with X WEM, Y WEM presented a greater advantage in terms of compressive strength. However, it was observed that Y WEM was more brittle than X WEM during testing. Therefore, the selection of waterborne epoxy for pavement patch materials requires comprehensive consideration of the compressive strength and flexibility of the resin.
The flexural strength test uses a revised bending test method based on the Chinese standard 0715-2011 [
13]. The flexural strength test of WEM was conducted on Universal Testing Machine (UTM) to evaluate the difference in flexibility between the two waterborne epoxy resins. In order to obtain the product with the highest degree of curing, the samples were prepared at the actual optimal ratio of the waterborne epoxy and curing agent at 25 °C for 28 d. The mortars were prepared into beams with the size of 40 mm× 40 mm× 160 mm. The test was controlled at a displacement rate of 5 mm/min, and the test temperature was set to 25 °C. The flexural strength test results are displayed in
Table 4 and
Figure 3.
It can be observed from
Table 5 and
Figure 3 that the Y waterborne epoxy is more sensitive to the type of curing agent than the X waterborne epoxy. The ultimate flexural strength of Y WEM with curing agent A was similar to that of the X WEM with curing agent A. However, the ultimate flexural strength of Y WEM containing curing agent B increases significantly.
The honeycomb-like voids on the mortar surface and comminuted failure of the specimens were observed due to the poor compatibility of the Y waterborne epoxy with curing agent A. The failure load of WEM with curing agent B was three times or more than that of WEM with curing agent A. In addition, the displacement of X WEM was larger than that of Y WEM, which means that the flexibility of X WEM is generally better, and the failure of Y WEM is brittle fracture.
3.3. Evaluation Method of Initial Strength and Forming Strength
The proportion of waterborne epoxy emulsified asphalt (WEEA) to the mixture was determined to be 1:4, combining the existing experimental data of the mixture performance. The compaction molding method of WEEAM was designed by introducing a two-stage compaction method, according to the Chinese standard JTG F40-2004 [
12]. The total number of compaction increased from 50 to 75, and the selected three types of two-stage compaction method are shown below: (i) compacted 25 times for the first stage, then curing at a constant temperature of 110 °C for 24 h, and compacted 25 times for the second stage; (ii) compacted 35 times for the first stage, then curing at a constant temperature of 110 °C for 24 h, and 40 times for the second stage; and (iii) compacted 50 times for the first stage, then curing at a constant temperature of 110 °C for 24 h, and compacted 25 times for the second stage. Specimens were prepared by using the methods described above and the indicators were measured, as shown in
Table 5.
The Marshall test results presented in
Table 5 indicate that the performance of WEEAM is closely related to the compaction molding method. The density of the samples increases and the air voids decrease with the increase in the compaction times. The stability of the mixtures was significantly improved with the increase in the number of compactions from 25 times to 35 times during the compaction of WEEAM. However, the aggregates were slightly broken and the emulsion splashed out of the molds when the number of compactions was increased from 35 to 50 in the first stage, which led to a lower density, higher air voids, and lower stability. Therefore, 35 times of compaction in the first stage, and 40 times of compaction in the second stage after curing were selected for further research. The compaction molding method of WEEAM is not only conductive to the formation of its strength, but also conforms to the compaction process of the cold-patching materials under load.
The preparation method of the modified Marshall was classified into room temperature curing and high temperature curing to conduct an initial strength test and forming strength test, respectively. The two-stage compaction method was proposed to conduct the test. Samples were compacted 35 times in the first stage and 40 times in the second stage after curing at 110 °C for 24 h, then curing at 25 °C for 48 h, 7 d, 14 d, and 28 d, respectively after demolding. The dehydration rate and the compressive strength of X WEEAM are given in
Figure 4.
It can be observed from
Figure 4 that the evaporation of water is slow at 25 °C, so the formation of strength is also relatively slow. Currently, the basic requirement of the initial strength is above 3 KN, so it is suitable to evaluate the initial strength of WEEAM by using the stability of the specimen after curing at 25 °C for 48 h.
The forming strength was evaluated by measuring the stability of the immersed specimen soaked in a 60 °C water bath for 30 min. Samples were compacted 35 times in the first stage and 40 times in the second stage after curing at 60 °C and 110 °C for 24 h, respectively, then curing at 25 °C for 48 h, 7 d, 14 d, and 28 d after demolding. The dehydration rate and compressive strength are shown in
Figure 5.
From
Figure 5, it can be clearly seen that the evaporation of water under the 60 °C curing was slower than that under 110 °C curing, and therefore the formation of the samples’ strength was also relatively slow. However, the specimen drained the water almost completely after curing at 110 °C for 48 h, and the final strength was almost formed. Hence, it is suitable for evaluating the forming strength of WEEAM by using the stability of the specimen after curing at 110 °C for 48 h.
3.4. Test Methods
The wheel tracking test is widely utilized to simulate the application of an actual wheel load on pavement structures at high temperature. The modified two-stage rolling method was proposed to prepare rutting specimens according to the existing research results and the material characteristics in this research. First, the amount of material was calculated according to the density of the Marshall specimen, and then the uniformly mixed mixture was poured into a mold with the size of 300 mm × 300 mm × 50 mm according to Chinese specification T0719-2011 [
13]. Then, the mixture in the mold was compacted back and forth seven times, and was then compacted seven times again after curing in a 110 °C thermostatic container for 24 h. Finally, the specimen was cured at room temperature for 24 h. A solid-rubber wheel travelling at a speed of 42 cycles/min and a wheel-pressure of 0.7 MPa were used to correlate with rutting. Two parameters, the DS and rutting depth at 60 min, were employed to feature high-temperature stability of the tested mixtures. Four specimens were tested for each group.
The low-temperature flexural test was employed to evaluate cracking resistance with a repeated load at low-temperature conditions according to Chinese standard 0715-2011 [
13]. Asphalt mixtures were first fabricated by the wheel tracking device of the same dimension as the specimens for the wheel tracking test. Then, the specimens were cooled for 24 h, after which they were cut into beams with dimensions of 250 mm (length) × 30 mm (width) × 35 mm (height). The test was conducted at −10 °C with a loading rate of 50 mm/min. The maximum flexural tensile strain and stress at the bottom of the tested beams before failure were employed to characterize the anti-cracking performance of the tested mixtures at low temperature. Four specimens were tested for each group.
Moisture susceptibility is one of the most important cold patch asphalt mixture performances. The immersed Marshall test and freeze–thaw splitting test were carried out to evaluate the moisture susceptibility of WEEAM. Specimens were prepared by the method of the forming strength test. The immersed Marshall test is similar to the standard Marshall test, except that it was soaked in a 60 °C water bath for 48 h. Residual Marshall stability was used to evaluate the moisture susceptibility of WEEAM. In the freeze–thaw split test, four of these specimens were stored in room temperature under dry conditions. The other four specimens were processed following the procedures of Chinese T0729-2000, in which four specimens were subjected to continuous freezing at −18 °C for 16 h and thawing at 60 °C for 24 h. Then, the loading speed rate was 50 mm/min. Finally, the splitting test was carried out for all eight specimens after being immersed in 25 °C water bath for 2 h, according to Chinese standard T0715-2011 [
13]. The parameter, freeze–thaw splitting tensile strength ration (TSR), was utilized to characterize the moisture susceptibility of the tested mixtures.
The Cantabro test was used to evaluate the raveling resistance of WEEAM [
13]. The specimens were prepared by the method of the forming strength test. Four groups of specimens were prepared for each different type of mixture, and each group had three duplications. The specimen was put inside a Los Angeles Abrasion machine drum without steel balls, and the drum was turned to 300 revolutions at room temperature for 10 min. One specimen was tested at a time. The percentage of mass loss during the test was measured to evaluate the raveling resistance of different types of mixtures.