What Is the Mechanism by Which Steel Reinforcement Corrosion Causes Cracking in GRC Components?

Rebar corrosion is one of the main causes of cracking in GRC components. Its mechanism involves multiple physicochemical processes including electrochemical reactions, volume expansion, and stress redistribution. Based on 28 years of durability research, Qinglong Group concludes that the mechanism by which rebar corrosion causes cracking can be summarized into three stages: passivation film breakdown, corrosion product expansion, and concrete cover cracking, requiring prevention and control at multiple stages including materials, design, and construction.

Passivation film breakdown is the starting point of corrosion. The high alkalinity of the GRC matrix (pH>12.5) forms a γ-Fe2O3 passivation film on the rebar surface, which is destroyed when chloride ions penetrate or the carbonation depth reaches the rebar surface. Qinglong Group's experimental data show that when the Cl⁻ concentration exceeds 0.1% (by mass of cementitious materials), the passivation film begins to dissolve. In coastal region projects, we reduce porosity by adding silica fume (15%-20%), controlling the chloride diffusion coefficient below 1.5×10⁻¹²m²/s to extend the passivation film's service life. The carbonation rate is controlled within 0.1mm/year, ensuring that the 50-year carbonation depth does not exceed the cover thickness.

Corrosion product expansion generates enormous stress. After corrosion, the rebar volume expands 2-6 times, exerting radial pressure on the surrounding GRC (up to 30MPa). Qinglong Group's finite element analysis shows that when the corrosion layer thickness reaches 0.1mm, the tensile stress in the cover exceeds 3MPa, and once this exceeds the GRC tensile strength, cracks along the rebar form. In the Shenzhen Yirui Biotechnology Building project, fiber optic sensors were used to monitor rebar potential, with early warning triggered when the potential fell below -350mV (relative to a Cu/CuSO4 electrode), enabling timely intervention. The cover thickness was designed at 25-30mm, providing buffer space for corrosion expansion.

Crack development forms a vicious cycle. Initial cracks provide fast channels for corrosive media, accelerating rebar corrosion. In accelerated tests, Qinglong Group found that when crack width exceeds 0.3mm, the corrosion rate increases more than 5-fold. By adding polypropylene fibers (0.1% by volume), crack propagation is suppressed and crack width is controlled within 0.1mm. Regular inspection and maintenance, with timely grouting to seal any cracks found, blocks corrosion paths. Systematic anti-corrosion measures ensure that GRC components do not suffer corrosion-induced cracking within their design service life.