# Effect of Fireproof Coatings on the Post-Fire Behavior of CFRP Composite Sheets
## 3 Experimental Results and Discussion
### Large-Space Fire Condition
This can be attributed to the softening of the resin matrix at elevated temperatures, which leads to a decrease in the ability to transfer forces between the fibers and resin. Consequently, the wide application of CFRP composites in practical engineering necessitates fire protection. Remarkably, the application of fireproof coatings significantly enhanced the post-fire flexural strength of the coated CFRP sheets, but the strength was still reduced to a certain extent compared to that at ambient temperature. The enhancement of fireproof coatings is primarily influenced by three factors: coating thickness, coating type, and CFRP sheet thickness. With coating thickness increasing, the reduction factor gradually escalated, implying that thicker coatings provided better fire protection by retarding the temperature rise of the CFRP sheets under fire conditions and preventing the fire from propagating to the interior of the CFRP sheets. The strength reduction factors stabilized around 0.95 when the coating thickness exceeded 25 mm. Further increases in coating thickness were unlikely to cause a significant alteration in the strength reduction factor. This may be related to the properties of coatings, such as thermal insulation and flame retardancy, which might reach saturation beyond a specific thickness. In addition, when subjected to fireproof coatings of 15 mm thickness, CFRP specimens with Type C1 coating exhibited superior performance compared to those with Type C2 coating. However, this performance difference became less prominent with thicker fireproof coatings (25 mm and 35 mm). Moreover, for fireproof coatings of 15 mm thickness, the strength reduction factors for CFRP sheets of 1.2 mm thickness were higher than those for CFRP sheets of 2 mm and 3 mm thickness. Nonetheless, this phenomenon seemed to be insignificant when thicker fireproof coatings (25 mm and 35 mm) were applied. It suggests a trend or pattern where all three factors hold significance for thinner coating thicknesses. As fireproof coatings become thicker than 25 mm, coating thickness emerges as the dominant factor among the influencing factors, leading to a reduction in performance disparities between CFRP sheets with varying coating types and sheet thicknesses. The trend in post-fire flexural modulus followed a similar pattern to that in strength. For the uncoated CFRP sheets with thicknesses of 1.2 mm, 2 mm, and 3 mm, the reduction factors in flexural modulus were 0.242, 0.644, and 0.389, respectively. As the coating thickness increased, there was a gradual increase in flexural modulus. At a 15 mm coating thickness, CFRP specimens treated with Type C1 coating demonstrated superior flexural modulus compared to those coated with Type C2. Notably, the effect of CFRP sheet thickness on flexural modulus seemed less pronounced. The fracture deflection of uncoated CFRP sheets exhibited a slight reduction, while the deflection reduction factor of the coated CFRP laminates increased, exceeding 0.90. In accordance with Appendix B of British Standard 5950: Part 8 (2003) [40], structural steel may be reinstated for use after a fire incident, given that the mechanical performance of the steel structure has not been severely compromised and the deformation or damage to the components remains within acceptable engineering standards. Expanding upon this principle, this study proposed that CFRP composites, after fire exposure, could be considered for reuse if they exhibited at least 90% recovery of their flexural properties. This recovery percentage was utilized as an indicator to evaluate the fire-resistance duration of CFRP composites. After exposure to the large-space fire, the test-obtained reduction factors for the coated CFRP sheets with coatings of 15 mm thickness were generally less than 0.90, whereas those with coatings of 25 mm and 35 mm thickness exceeded 0.90. In addition, some reduction factors exceeded 1.00, possibly related to the discrete properties of CFRP composites and post-curing effects in epoxy resin.