Probabilistic corrosion rates of cold-twisted deformed and thermo-mechanically treated steel in chloride-contaminated mortar

The overall service life of concrete structures can be divided into corrosion initiation and corrosion propagation phases. Although the corrosion propagation period (tp) is usually found to be smaller than the initiation period (ti) it is important to estimate tp for planning and budgeting for the repair activities. The tp depends on the corrosion rate (icorr) of the steel reinforcement. India has many old concrete structures built using the Cold Twisted Deformed (CTD) steel bars. Now-a-days, CTD steel (being highly vulnerable to corrosion) is rarely used and the Thermo-Mechanically Treated (TMT) or Quenched and Self-Tempered (QST) steel bars are extensively used. Quantitative information on icorr of the CTD and TMT/QST bars are required for estimating tp. However, very limited quantitative information is available on icorr to estimate tp. Therefore, the current practice is to assume that the icorr of both CTD and TMT/QST steels are equal to that of plain mild steel, which might result in unrealistic estimations. This paper provides icorr data obtained from 20-month long experimental program. The icorr data were obtained using linear polarization resistance (LPR) tests on CTD and TMT/QST steel bars embedded in mortar. Twenty-five specimens were cast, cured, and subjected to a cyclic wet-dry exposure using 3.5 % sodium chloride solution. It is observed that the icorr of CTD and TMT/QST steel bars can be represented as ∼3PLN(σ, μ, γ); with ∼3PLN(0.3, 3.9, -24) μA/cm2 and ∼3PLN(0.2, 3.6, -20) μA/cm2, respectively. This corresponds to an average icorr of 26.6 and 16.7 μA/cm2, respectively, for CTD and TMT/QST steels. It is also found that, in general, icorr of TMT/QST steel exhibits less scatter than CTD steel. This paper also provides the probabilistic estimations on tp using the measured icorr data and the tp model developed by Wang and Zhao (1993). Based on the estimations, it can be concluded that the median time-to-crack for a system with CTD steel can be approximately 1.8 times less than that of a system with TMT/QST steel - indicating that early notification is required for engineers to prepare an optimized repair strategy for deteriorating structures.