# Study on Frost Resistance of the Carbon-Fiber-Reinforced Concrete
## 3 Results and Discussion
### 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\({}^{3}\)). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Specimen**} & \multicolumn{5}{c}{**Compressive Strength (MPa)**} \\ \cline{2-6}  & **NO. 1** & **NO. 2** & **NO. 3** & **Mean** & **Range** \\ \hline PC & 37.4 & 35.8 & 37.6 & 36.9 & 1.8 \\ CF0.25 & 32.7 & 31.8 & 33.1 & 32.5 & 1.3 \\ CF0.50 & 35.9 & 34.5 & 36.1 & 35.5 & 1.5 \\ CF1.00 & 36.9 & 38.1 & 38.7 & 37.9 & 1.8 \\ CF1.50 & 40.3 & 40.6 & 42.0 & 41.0 & 1.7 \\ CF2.00 & 37.7 & 40.4 & 39.1 & 39.1 & 2.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Compressive strength of the concretes. Figure 3: Fracture specimen. 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. [PERSON] [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].