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It is widely accepted that chloride ions are responsible for causing local passive layer breakdown and subsequent corrosion of reinforcing steel bars (rebars) in concrete. There is general agreement in the existence of a certain Cl- concentration threshold value (ClTH) necessary to initiate the corrosion process. The ClTH may vary depending on concrete mix properties such as cement type, C3A content, blended materials, water to cement ratio, temperature, relative humidity, and the exposure conditions and source of chloride penetration, among others. Further than this one more question arises that needs a practical answer: how to determine this value be for a given structure in a particular environment. From our previous investigations [1,2], we have, correlated the evolution of Ecorr and the corrosion current density icorr, the values of r and the Cl-concentration at the rebar surface as a function of time by plotting the Cl- concentration as a funtion of concrete resistivity (r) determined for a given time. The ClTH value considered here was the Cl- ions concentration at the rebar surface corresponding to an active state of corrosion (Ecorr < -0.35 V vs. CSE and icorr > 0.2 mAcm-2). Two separated regions appear: the active corrosion area and the passive corrosion area. The limit between these would represent the correlation between ClTH values and concrete resistivity which can be expressed by the following expression ClTH = 0,009 r + 0,33 Thus, given a certain structure, the risk for Cl- induced corrosion can be estimated by measuring the resistivity of concrete, which in turn, implies using a simple non-destructive technique requiring neither expensive nor sophisticated instruments.

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