Effect of test methods on corrosion phenomena of steel in highly resistive concrete systems and data interpretations
Fly ash and limestone calcined clay cement (LC3) are being used/introduced in concrete to enhance chloride resistance. In this study, sixty specimens (with steel in three concrete systems, namely OPC, fly ash, and LC3 with surface resistivity of ≈10, ≈25, and ≈200 kΩ.cm, respectively) were subjected to impressed corrosion and the results were compared with 15 lollipop steel-mortar specimens subjected to natural corrosion under wet-dry/chloride conditions. It was found that the traditional way of impressed corrosion tests can induce microstructural changes in highly resistive concrete cover and at steel-concrete interface; hence, are not suitable for evaluating corrosion resistance (say, corrosion rate and corrosion-induced cracking) in highly resistive concrete systems. Further, the Raman spectra from the corroded steel surfaces indicated that the impressed corrosion and natural corrosion tests led to different forms of corrosion (i.e., uniform and pitting, respectively) and different compositions of corrosion products (i.e., ‑Fe2O3 and FeOOH phases). This led to different expansive stresses making the lab-to-field correlations inappropriate in case of highly resistive concrete systems. This paper recommends natural corrosion tests exposed to wet-dry conditions and not the impressed corrosion tests for assessing corrosion phenomena of steel in highly resistive concrete systems.