3.1. Recovery Rate of Creep Deformation
Herein, in order to quantitatively characterize the creep and recovery deformation in a single cycle, the creep-recovery ratio (CRR) is defined as the deformation recovery rate in every single cycle:
where
Dcreep is the deformation in the 1 s creep process, and
Drecovery is the deformation recovery in the 9 s recovery process.
In the repeated creep recovery test (RCRT), after 50 cycles of loading, the development of the binder was considered to be stable, and the influence of delayed elasticity decreased. Therefore, the data of the 50th and 51st cycles of RCRT were usually used to analyze the creep-recovery deformation and fit the creep compliance [
20].
In order to characterize the strengthening effect of fiber on CA composite, first, the creep deformation and recovery rate of fiber-reinforced CA with loading time were captured by RCRTs.
Figure 4 shows the creep recovery curves of CA with polyester fiber in the 50th and 51st cycles. Compared to the plain CA, the creep recovery curves of CA with polyester fiber decrease significantly. In addition, when the polyester fiber content was 2% or 4%, the creep recovery curves showed little difference. However, for 6% polyester fiber-reinforced CA, the creep recovery curve experienced a significant decrease. Based on the creep deformation and CRR of CA with polyester fiber in the 50th and 51st cycles, shown in
Table 4, the following conclusions were obtained:
After 49 cycles of creep and recovery, the accumulated strain of CA with polyester fiber was only about half or less than that of CA. It is apparent that fiber has a good strengthening effect on CA. Consequently, the deformation resistance of fiber-reinforced CA greatly increased.
The CRR of CA with polyester fiber in the 50th and 51st cycles, both of which were above 70%, exhibited obvious improvement over CA, which indicates that most of the strain generated by 1 s loading in each cycle can be recovered by itself, thereby having a positive effect on the deformation resistance and elastic recovery ability of the composites. In addition, it should be noted that the CRR of 4% polyester fiber-reinforced CA was slightly lower than that of composites with 2% polyester fiber in the 50th and 51st cycles, but the CRR of the composite with 6% polyester fiber exhibited a significant increase and was already more than 80%. The increase proves that there is an obvious improvement in the internal microstructure of the composite system when the polyester fiber content changes from 4% to 6%.
Similarly, the creep recovery curves of brucite fiber-reinforced CA in the 50th and 51st cycles are shown in
Figure 5.
In comparison to
Figure 4, some new characteristics appear in
Figure 5. In the 50th and 51st cycles, the accumulated strains of CA with 4% and 6% brucite fiber were less than 1. Moreover, as shown in
Table 5, the CRRs of CA with 2%, 4%, and 6% brucite fiber were 79.80%, 84.54%, and 93.31%, respectively, which are higher than those of polyester fiber-reinforced CA under the same conditions; they are also much higher than that of CA (i.e., 43.73%). The results show that brucite fiber-reinforced CA had better deformation recovery ability.
Unlike polyester fiber, when brucite fiber content changes from 2% to 4%, the creep recovery curve decreases greatly, but the decline is relatively small when the brucite fiber content is 4% to 6%. The difference between the two fibers is affected by the properties and distribution characteristics of the fiber. Specifically, the tensile strength of brucite fiber is greater than that of polyester; meanwhile, its diameter and length are less than polyester fiber. Therefore, brucite fibers are more likely to form bridging and have a reinforcing effect even if only small additions are made.
3.2. Accumulated Strain
Due to the viscoelastic characteristic of fiber-reinforced CA, a residual deformation occurs in each cycle. Therefore, the accumulated strain is the sum of the residual deformation of 100 cycles. It reflects the deformation resistance ability and deformation recovery ability of fiber-reinforced CA.
The above analysis was based on the creep process and recovery process in a single cycle. The following analysis focused on the accumulated strain development of fiber-reinforced CA in 100 creep-recovery cycles.
Figure 6 shows the evolution of accumulated strains of polyester fiber-reinforced CA with time. In the beginning, the accumulated strain of the composite with 6% polyester fiber was even higher than that of the other two composites, and the accumulated strain of the composite with 4% polyester fiber was also greater than that of the composite with 2% polyester fiber content over a long period of time. The accumulated strain of the latter fiber content does not begin to exceed that of 4% fiber content until about the 50th cycle. In addition, similar to the accumulated strain curve of CA, the accumulated strain curves of the three polyester fiber-reinforced CAs increase linearly with time and the growth rates (i.e., the slope of the curve) decrease with the increase in fiber content. According to the final accumulated deformation value indicated in
Figure 6, when 2%, 4%, and 6% polyester fibers were added to CA, the accumulated strain of the composites after 100 creep-recovery cycles decreased by 44.1%, 47.8%, and 60.9%, respectively. When the polyester fiber changes from 4% to 6%, the accumulated strain of polyester fiber-reinforced CA decreases significantly.
Figure 7 shows the evolution of accumulated strains of brucite fiber-reinforced CA with time. Compared to CA, the addition of brucite fiber slows down the linear growth process of the accumulated strain of CA. In addition, after 100 creep recovery cycles, the total accumulated strain is reduced by 57.9%, 88.6%, and 94.3%, respectively. Moreover, unlike the rule governing polyester fiber-reinforced CA, the accumulated strain decreases greatly when the brucite fiber content changes from 2% to 4%, while the decrease is relatively small when the brucite fiber content changes from 4% to 6%. This situation is consistent with the analysis in
Figure 5 and
Table 5.
According to the above analysis, the accumulated strains of the two fiber-reinforced CAs have common characteristics, such as the total accumulated strain being significantly reduced and the accumulated strain decreasing with the increase in fiber content. However, the fiber-reinforced CAs with different fibers also contain the following differences. At the same fiber dosage, the accumulated strain of brucite fiber-reinforced CA is smaller, and the creep deformation recovery ability stronger. This result can be tied to the properties of the fibers. As shown in
Figure 1, polyester fibers are cylindrical bundles with a smooth surface, while brucite fibers are alkaline and consist of bundles or monofilaments with uneven thickness and rough surfaces. Therefore, although polyester fiber is more easily dispersed, brucite fiber has stronger adsorption to asphalt and cement hydration products. On the other hand, the tensile strength and other mechanical properties of brucite fiber are better than polyester fiber.