What Is the Mechanism by Which Steel Corrosion Causes GRC Component Cracking?

The corrosion of steel reinforcement (or steel embedded parts) causing GRC components to crack is a typical chain failure process of "electrochemical corrosion – volume expansion – stress cracking". Understanding this mechanism is essential for preventing such durability defects. Through the inspection and repair of GRC components in multiple old buildings, Qinglong Group has conducted an in-depth analysis of the entire process of corrosion-induced failure. Corrosion not only weakens the steel reinforcement itself; the hoop tensile stress exerted on the GRC cover by the volume expansion of corrosion products is the direct cause of cracking along the reinforcement and spalling of the cover. In coastal environments with high salt mist, this process accelerates significantly.

Initiation of Electrochemical Corrosion: Formation of Anodes and Cathodes

Corrosion is an electrochemical process that requires an anode, a cathode, an electrolyte, and a conductive path. When the GRC cover loses its alkaline protection due to carbonation or chloride ingress, or when existing cracks allow moisture and oxygen to penetrate, the passive film on the steel reinforcement surface is destroyed, forming a local anode (where Fe loses electrons to become Fe²⁺) while other areas become the cathode (where oxygen and water gain electrons to form OH⁻). The electrolyte is the concrete pore solution, and the steel reinforcement itself and the concrete provide the conductive path. Once this corrosion cell is formed, corrosion proceeds continuously.

Volume Expansion: The Physical Driving Force of Failure

Rust (mainly mixtures such as Fe₂O₃·nH₂O) has a density far lower than that of iron, and its volume can expand to 2-4 times that of the original steel. This expansion generates enormous hoop tensile stress in the GRC matrix around the reinforcement. Although the tensile strength of GRC is higher than that of ordinary concrete, it is still far lower than the expansion stress produced by corrosion. When this stress exceeds the tensile strength of GRC, the cover is "burst" open along the direction of the reinforcement, forming typical "cracks along the reinforcement".

Crack Development: The Start of a Vicious Cycle

The appearance of initial cracks provides a direct and rapid ingress channel for moisture, oxygen, and corrosive media (such as chloride ions and carbon dioxide), greatly accelerating the corrosion rate of the internal reinforcement. More corrosion products generate greater expansion stress, causing the cracks to widen, extend, and interconnect further, eventually leading to large-scale spalling of the cover, severe weakening of the reinforcement cross-section, and a significant decline in the component's load-bearing capacity. This is a vicious cycle with positive feedback.

Fundamental Prevention Strategy: Multiple Barriers and Active Protection

Based on this mechanism, Qinglong's prevention strategy is systematic: 1) Improve the quality and thickness of the cover: use low water-cement ratio, high-density GRC and ensure a sufficiently thick cover (typically ≥20mm outdoors) to delay the ingress of harmful media. 2) Break the corrosion cell: use epoxy-coated reinforcement, galvanized reinforcement, or stainless steel materials to prevent corrosion at the source. 3) Strict chloride ion control: strictly prohibit the use of sea sand and control the chloride content of raw materials and admixtures. 4) Refined structural detailing: ensure smooth drainage, avoid water accumulation, and reduce moisture retention. By cutting off any link in the corrosion chain, such failure can be effectively prevented.