3.2. Effect of Carbon Fiber Content on Compressive Strength
With the increase of freeze-thaw cycles, the mechanical properties will decrease, in which the compressive strength [
8] is the most representative indicator. Before discussion of the relation of carbon fiber content and the frost resistance for the concrete, the effect of carbon fiber on compressive strength should be considered. Thus, compressive strength tests of the concrete were conducted at room temperature (
Figure 3). The effect of the carbon fiber amount on the compressive strength of the concrete is shown in
Table 4 and
Figure 4. The PC yielded a compressive strength of 36.9 MPa. The CFRC with a carbon fiber content of 0.25 wt.‰ (0.6 kg/m
3) exhibited the lowest compressive strength of 32.5 MPa. When the carbon fiber content was 0.50 wt.‰, the compressive strength of the CFRC was slightly lower than that of the PC. The compressive strength of the CFRC with a carbon fiber content of 1.00 wt.‰ was 37.9 MPa, which was 1.03 times that of the PC. The CFRC with a carbon fiber content of 1.50 wt.‰ had the highest compressive strength of 41.0 MPa. However, the CRFC with a fiber content of 2.00 wt.‰ did not show any improvement in the compressive strength compared with that of CF1.50. Hence, the results showed that when a small amount of carbon fiber was added, the compressive strength of the CFRC was lower than that of the PC, and when the carbon fiber content was more than 1.00 wt.‰, the compressive strength of the CFRC was higher than that of the PC. Thus, the optimal addition amount was 1.5 wt.‰ (3.6 kg/m
3).
The compressive strength trend, which shows a decrease at first and then an increase with the increasing of carbon fiber, has a close relationship with the development of cracks in the concrete. Cement agglutinate is mainly comprised of calcium silicate hydrate gel, micropores, and unhydrated cement particles. Micropores include gel pores and capillary pores. Powers [
18] proposed a formula for the compressive strength and gel-space ratio: F = AX
2.5–3, where F is the compressive strength of concrete, A is the compressive strength of concrete gel, which is in the range of 200–300 MPa, and X is the gel-space ratio, which represents the ratio of the hardened cement volume to the capillary pore volume. The capillary pore appearing in the form of a microcrack is one of the key factors affecting the physical properties of the concrete. Although there are several types of microcracks in the concrete, these can be classified into two types based on the age of the concrete [
6]. The first type is plastic shrinkage cracks, and the second type is cracks formed in the concrete hardening stage. Carbon fibers have a greater influence on the first type than the second type [
6]. Before hardening, i.e., the plastic deformation stage, the concrete has a small tensile strength. When it enters the constraint state in which internal moisture evaporation is accelerated, many cracks are produced in the interior of the concrete. The fiber reinforcement mechanism on the concrete is mainly the improvement of the crack resistance [
19,
20,
21]. A large number of fibers would be evenly distributed in the concrete as the fine reinforcement, which could effectively share the tensile stress of the concrete resulting from the deformation, and it could also effectively constrain the shrinkage of the concrete and reduce the number and scale of the cracks in the concrete, thus improving the performance of the concrete in all aspects [
22,
23,
24].
However, fibers induce interface defects into the composite material, and these defects have a harmful impact on the mechanical properties of concrete [
2]. The influence of fibers on the crack propagation in concrete is controlled by factors such as the fiber amount, fiber distance, drawing strength, interfacial bonds between the fiber and the matrix material, and the strength of the matrix [
25,
26]. Of these, the interfacial bonding between the fibers and the cement matrix is the most important influencing factor [
25,
26]. Carbon fibers, as a kind of ductile material, are combined to a concrete matrix by mainly bond forces. Given that stress is transmitted through the interfacial transition zone (ITZ) between the fibers and the matrix, the structure and properties of the ITZ are the key points that determine the performance of a CFRC. According to the research of Powers [
18], the increase in the solid volume caused by cement hydration is about 1.13, and the pore around the fiber is about 2.5 times that of the cement matrix. Thus, the ITZ near a fiber surface will have a high porosity, lower hardness, low strength, and more defects [
27,
28,
29,
30]. When the fiber content is increased, the induction of cracks by the ITZ will be more evident, which will lead to a reduction in the reinforcement effect of the fibers on the concrete, and even a negative effect can appear [
31,
32,
33]. As long as the fiber orientation is inconsistent with the direction of the principal stress, cracks will be induced, thus affecting the overall mechanical behavior of the concrete [
34,
35,
36].
As shown in this study, at first the fiber content was not sufficient to resist plastic shrinkage, and more weak planes were introduced into the concrete, so the compressive strength of CFRC was reduced. When the carbon fiber content increased, a large number of plastic deformation cracks that formed in the concrete were eliminated, and the positive effect was greater than the introduced weak area. Moreover, the effect of preventing and bridging makes the compressive strength of the concrete increase rapidly, showing an approximately linear strength growth. With the increase in the fiber content, the pores and weak plane reduced the strength significantly. As a result, the compressive strength stopped increasing and even declined.
3.3. Frost Resistance of CFRC
In the frost resistance tests, the effects of the fiber content on the mass loss ratio and the compressive strength are shown in
Table 5 and
Table 6 and
Figure 5 and
Figure 6, respectively. The PC showed a maximum mass loss of 3.79% after 300 cycles. Before 100 cycles, the PC displayed a moderate mass loss. Between 100 and 250 cycles, the PC displayed a slow mass loss. In contrast, after 250 cycles, the PC displayed a significant mass loss. The CF0.25 yielded a moderate mass loss at 1.64% after 300 cycles. As shown in
Figure 5 and
Table 5, after 300 cycles, the mass losses of CF0.50, CF1.00, CF1.50, and CF2.0 were 0.98%, 0.82%, 0.86%, and 0.93%, respectively. When the carbon fiber content was greater than 0.5 wt.‰, the mass loss of the CFRC decreased significantly, but the decreases were less than 1%. However, when the carbon fiber content was greater than 0.5 wt.‰, the mass loss of the CFRC did not change significantly with the increase in the fiber amount.
Figure 6 shows that the compressive strength of PC decreases with the number of freeze–thaw cycles. After 150 cycles, the decline of the PC compressive strength was 36.6%. According to GB/T 50082-2009 [
14], the frost-resistance mark of the concrete should be based on the maximum number of freeze–thaw cycles after which the compressive strength loss rate was not greater than 25% or the mass loss rate was not greater than 5%. Therefore, the frost-resistance mark of the PC was 100 freeze–thaw cycles, after which the compressive strength loss was 21.2%. After 100 and 150 cycles, the compressive strength values of the CF0.25 declined by 9.0% and 26.1%, respectively. The frost-resistance mark of the CF0.25 was also 100 freeze–thaw cycles, but the degree of decrease of the compressive strength was significantly less than that of the PC. For the CF0.50, the frost-resistance mark was improved to 150 cycles, and the strength loss was 11.8%. The strength losses of the PC, CF0.25, and CF0.50 showed steep declines after the maximum number of freeze–thaw cycles. The CF1.00, CF1.50, and CF2.00 showed high frost resistance performances with frost-resistance marks of 200, 250, and 250 cycles, respectively. After 200 and 250 cycles, the compressive strengths of the CF1.00 were 85.0% and 73.3%, respectively. After 250 cycles, the strength losses of the CF1.50 and CF2.00 were 20.2% and 22.8%, respectively.
Generally, the frost-resistance mark of concrete exhibited an increasing trend with the increase in the carbon fiber content before 1.5 wt.‰. When the carbon fiber content was greater than 1.5 wt.‰, the frost resistance of concrete did not improve further. When the water in pores suffered from frost, its volume swelled by about 9%, which would cause tensile stress and the formation of microcracks [
2]. Therefore, the compressive strengths of the concrete showed decreases with the increase in the number of freeze–thaw cycles [
2]. The improvement in the frost resistance of the carbon-fiber-reinforced concrete can be explained as the tensile resistance of the fibers that shared the stress of the cement matrix and prevented the formation of cracks. Therefore, the appropriate addition of carbon fibers can improve the frost resistance of concrete. Based on the cost, 1.50 wt.‰ is an optimal carbon fiber addition.