Visual Analysis of the Protective Effect of Asphalt Coating on MOCC against Steel Corrosion and Rust Crack Formation

The constant current energization accelerated test was carried out to study the effect of asphalt coating on delaying reinforcement corrosion and rust-induced cracks in coated reinforced magnesium oxychloride cement concrete (CRMOCC). CRMOCC with different degrees of damage was scanned by adopting X-ray computed tomography (X-CT) technology, and the spatial information of rust and rust-induced cracks were analyzed visually and quantitatively. In addition, Scanning Electron Microscope (SEM) was used to analyze the non-uniformity of rust-induced crack formation. The results showed that the corrosion amount of coated reinforcement after energization was only 81.6% of that of uncoated reinforcement. Besides, no crack no cracking was observed on CRMOCC after 480h’s energization, while the rust-induced crack of reinforced magnesium oxychloride cement concrete (RMOCC) was 0.11 mm. The coating reduced the potential of rust-induced crack formation such that only three areas in CRMOCC were cracked, while RMOCC had cracks in eight areas. This study also found that interface transition zone (ITZ) played a dominant role in the formation of rust-induced cracks. The surface crack width in CRMOCC had an exponential relation with the crack volume and it had a linear relation with the volume of corrosion products. Further, there was an exponential relation between the volume loss of the reinforcement and the formation of rust-induced cracks. The correlation between surface crack width, corrosion products volume, rust-induced cracks volume and steel volume can be described by quadric surface. Results by SEM analysis indicated that in the ITZ of CRMOCC, Mg and O elements were the two main elements in the hydration products that were loose and needle-like. Furthermore, Si and O elements were the dominant materials in the non-interface transition zone (NITZ), resulting in relatively dense hydration products.