Improving the crack resistance of indoor non-expansive steel structure fire retardant coatings requires a comprehensive approach encompassing material selection, substrate treatment, optimized construction processes, environmental control, additive improvement, and maintenance management. These interconnected aspects collectively determine the coating's crack resistance and durability.
Material selection is fundamental to crack resistance. Non-expansive indoor non-expansive steel structure fire retardant coatings must be suitable for heavy steel structures (such as columns and beams), prioritizing products with high compressive strength and strong adhesion. For example, thick-coat coatings with a compressive strength of at least 20 MPa and an adhesion strength of at least 0.3 MPa can effectively reduce cracking caused by shrinkage stress. Simultaneously, avoid using coatings with low expansion ratios to prevent cracking due to mismatch between the expansion layer and the substrate deformation. Furthermore, selecting inorganic binders with excellent weather resistance (such as modified sodium silicate) and lightweight aggregates (such as expanded perlite) can reduce coating density, improve flexibility, and reduce the risk of cracking.
Substrate treatment is a crucial prerequisite for crack resistance. The steel structure surface must be thoroughly cleaned of oil, rust, and dust, ensuring a rust grade of St3 (manual grinding) or Sa2.5 (sandblasting) to prevent hollowing and cracking due to insufficient adhesion. For welded joints, stress relief treatment (such as vibration aging or slow cooling after localized heating) is required to reduce the damage to the coating caused by welding stress. For concrete substrates, the compressive strength must be controlled to be no less than C20, and the moisture content no more than 8%. If necessary, an interface agent should be applied to enhance adhesion and prevent shrinkage cracking caused by moisture evaporation.
Refined control of the construction process directly affects the crack resistance. When applying in layers, the thickness of each layer must be strictly controlled within a reasonable range. For non-expanding coatings, the thickness of each layer should not exceed 1.0 mm to avoid excessive thickness leading to concentrated shrinkage stress. Each layer should be cured until surface dry (approximately 2-4 hours) before applying the next layer to ensure tight adhesion between layers. In terms of construction methods, airless spraying (pressure 0.5-0.6MPa) can reduce the introduction of air that can cause the coating to become loose. When applying by hand, use a trowel in a "cross" motion to ensure the coating is dense and free of air bubbles. For thick coatings such as steel columns, wire mesh can be wrapped around the coating when it reaches half its thickness to enhance crack resistance.
Environmental control is crucial for crack resistance. The ambient temperature during construction should be maintained between 5-35℃ to prevent low temperatures from causing moisture in the coating to freeze and expand, or high temperatures from accelerating moisture evaporation and causing shrinkage cracking. Relative humidity should be controlled below 85% to prevent the coating from drying too slowly and becoming loose. During the curing period, the coating should be covered with plastic film and regularly sprayed with water (2-3 times a day) to keep the coating moist but not waterlogged. The curing time should be no less than 14 days to ensure the coating is fully cured.
Additive modifications can significantly improve the crack resistance of the coating. Incorporating crack-resistant fibers (such as polypropylene fibers, at a dosage of 0.5-1.0 kg/m³) can enhance coating toughness. The fiber length should ideally be 3-6 mm to avoid agglomeration or exposure. Adding water-reducing agents (such as calcium lignosulfonate, at a dosage of 0.2-0.5%) can reduce water consumption and decrease drying shrinkage cracks. Some products further enhance crack resistance by introducing nanomaterials (such as nano-silica) to optimize the coating's microstructure.
Maintenance management is crucial for long-term crack resistance. After construction, a "one coating, one file" system should be established, recording data such as construction time, thickness, and ambient temperature and humidity to facilitate traceability of quality responsibility. Regularly check coating thickness (deviation ≤ ±15%) and adhesion (≥ 0.5 MPa) to ensure performance meets standards. For localized cracking, the damaged coating must be removed down to the substrate, the edges sanded into a bevel, a new anti-rust primer applied, and an indoor non-expansive steel structure fire retardant coating applied in layers to ensure repair quality.
