# Effect of Fireproof Coatings on the Post-Fire Behavior of CFRP Composite Sheets
## 3 Experimental Results and Discussion
### Large-Space Fire Condition  
Figure 6 depicts the uncoated and coated CFRP sheets after exposure to the large-space fire (before and after the removal of coatings). After 2 h heating, the surface of uncoated CFRP sheets became rougher, and cracks emerged. This observation indicates that the resin matrix had started to degrade at a temperature of 334.4 \({}^{\circ}\)C, leading to the gradual separation of the fibers. Furthermore, the fireproof coatings on the coated CFRP sheets displayed an oxidation phenomenon, resulting in a darkening of color. The morphology of CFRP sheets after fireproof coating removal showed minimal variance from that of CFRP sheets at ambient temperature without fire exposure. Figure 7 presents typical failure modes of CFRP bending test specimens without fire exposure and after exposure to large-space fire. After cooling down from elevated temperatures, the failure modes of coated CFRP specimens did not exhibit substantial differences from those without fire exposure. In the three-point bending tests, both the loads and deflections of the specimens were recorded. Figure 8 displays load-deflection curves of CFRP specimens with different thicknesses after exposure to the large-space fire. Each curve in the plot corresponds to a specific CFRP specimen, where the x-axis and y-axis represent the deflection and load, respectively. These load-deflection curves clearly demonstrated linear-elastic characteristics, devoid of  
Figure 5: Three-point bending test on the specimen. Figure 6: CFRP sheets exposed after a 2 h large-space fire: (**a**) without fireproof coatings; (**b**) with fireproof coatings; and (**c**) after the removal of coatings. apparent inelastic alterations. As loading progressed, the curve gradually rose linearly. Eventually, upon reaching a load threshold, the specimen suddenly failed and underwent brittle damage. The peak loads of uncoated CFRP sheets experienced a significant reduction, while those of coated CFRP sheets exhibited no obvious decreasing trend. The fracture deflection remained almost unchanged under different coating conditions. The flexural properties of CFRP specimens after exposure to the large-space fire are summarized in Table 3, which also includes the ambient-temperature group for comparison. The flexural stress is calculated using Equation (3). \[\sigma b=3 PL/2 wh^{2} \tag{3}\]Figure 8: Load–deflection curves of CFRP sheets with various thicknesses after large-space fire exposure: (**a**) 1.2 mm thickness; (**b**) 2 mm thickness; and (**c**) 3 mm thickness. In the above equation, \(\sigma b\) is the flexural stress. \(P\) is the loading force. \(L\) is the span. \(w\) and \(h\) are the width and thickness of specimens, respectively. In addition, the midpoint strain is calculated using Equation (4). \[\varepsilon=6D\text{maxh}/L^{2} \tag{4}\]  
In the above equation, \(\varepsilon\) is the midpoint strain. \(D\text{max}\) is the deflection at mid-span. \(h\) and \(L\) are consistent with Equation (3). The flexural modulus is calculated from the slope of the stress-strain curve. From the table, it can be seen that the flexural properties of CFRP specimens with different thicknesses at room temperature are different. Therefore, the reduction factor \(\eta\) is defined as the ratio of properties exhibited by CFRP sheets with different thicknesses under fire conditions to those with the corresponding thicknesses at ambient temperature to quantify the degree of property degradation. The post-fire flexural strength of 1.2 mm, 2 mm, and 3 mm thick CFRP sheets without fireproof coatings under the large-space fire condition was reduced to 0.232, 0.211, and 0.197 times, respectively, compared to that at ambient temperature. This underscores a substantial decline in the flexural strength of CFRP composites, resulting in a significant decrease in the load-carrying capacity of CFTR sheets after exposure to a large-space fire.