As a composite material combining fire resistance and structural function, the fire resistance of galvanized fireproof iron sheet diminishes after repeated bending. This requires comprehensive analysis considering material properties, processing technology, and actual application scenarios. Its core performance depends on the integrity of the galvanized layer, the stability of the substrate's metal structure, and the synergistic effect of the fire-retardant coating. The bending process can indirectly affect the fire resistance by damaging the galvanized layer, inducing metal fatigue, or altering the coating distribution.
The galvanized layer is the first line of defense for fireproof iron sheets. Zinc's melting point is much higher than the temperature of common fires, and the dense zinc oxide layer formed by oxidation effectively isolates oxygen from the substrate. However, if cracks or peeling occur in the galvanized layer during bending, the exposed iron substrate will come into direct contact with the flame, accelerating the oxidation reaction. Especially when the bending radius is too small or repeated bending occurs, the zinc layer may fracture due to stress concentration, forming localized corrosion channels. For example, in curtain wall projects, if the galvanized fireproof iron sheet is damaged due to excessive bending during installation, its fire resistance time may be shortened from the designed 120 minutes to less than 90 minutes.
The stability of the substrate's metallic structure is equally crucial. Galvanized fireproof iron sheets are typically made of low-carbon steel or alloy steel, whose grain structure can deform during bending. Mild bending can improve strength through cold working, but excessive bending can lead to grain boundary slippage and the formation of microcracks. These cracks can become stress concentration points at high temperatures, accelerating material creep and even causing localized collapse. For example, in tunnel fire protection projects, if repeated bending of galvanized profiled steel sheets causes grain distortion, its fire resistance limit may drop from 1.5 hours to 1 hour, failing to meet regulatory requirements.
The synergistic effect of the fire-retardant coating is also significant. Some galvanized fireproof iron sheets are additionally coated with intumescent fire-retardant paint, whose components expand at high temperatures to form a charred layer, further insulating against heat. Bending can cause uneven coating thickness or peeling, weakening the insulation effect. For example, in fire escape route design, if the thickness of the fire-retardant coating on galvanized floor decking decreases from the design requirement of 15 mm to 10 mm due to bending, the temperature rise on the unexposed side may exceed the standard limit, resulting in substandard fire resistance.
Differences in actual application scenarios can amplify or mitigate the impact of bending. In curtain wall projects, galvanized fire-resistant sheet metal is typically wrapped around building columns by bending, with a fixed bending angle and a limited number of bends, resulting in minimal impact on fire resistance. However, in temporary facilities or irregularly shaped structures, where multiple bends are required to adjust the shape, the bending radius and number of bends must be strictly controlled. For instance, a data center project used galvanized profiled steel sheets as floor slabs. By optimizing the bending process (such as using specialized machinery for heating and pressurizing), the material maintained a 1.5-hour fire resistance limit while meeting complex shape requirements, validating the importance of process control.
Material thickness and galvanizing amount are also key factors. Thicker sheets (e.g., 1.5 mm or more) maintain structural integrity more easily when bent, while higher galvanization (e.g., 275 g/m²) provides more durable corrosion protection. For example, in fire-resistant partitions in chemical plants, thicker, high-galvanization fireproof iron sheets show significantly less fire resistance degradation even after repeated bending compared to thinner, low-galvanization products.
Maintenance and testing are equally important. Bent galvanized fireproof iron sheets require visual inspection, coating thickness gauge testing, or salt spray testing to assess the integrity of the galvanized layer, and a fire resistance test to verify overall performance. For instance, in a hospital project, after installing galvanized fireproof floor decking, some areas showed coating peeling due to bending. By promptly reapplying fire-retardant coating and increasing the sheet thickness, the project ultimately passed a 1.75-hour fire resistance test.
After repeated bending, the fire resistance of galvanized fireproof iron sheet may decrease due to damage to the galvanized layer, changes in the substrate structure, or uneven coating distribution. However, by optimizing material specifications, controlling the bending process, and strengthening quality inspection and maintenance, its fire resistance advantages can be preserved to the maximum extent. For critical projects, it is recommended to prioritize thicker specifications and products with high galvanization, and to use specialized machinery for bending to ensure that the fire resistance meets design requirements.
