Reliable predication of corrosion initiation of rebars inside concrete is a key factor influencing life cycle evaluation of concrete structures under chloride-laden environments. Reliability of chloride threshold level (CTL) assessment mostly depends on reliable detection of corrosion initiation of rebars. It was aimed to assess the corrosion development on rebars within concrete subjected to an accelerated NaCl environment through continuous half-cell potential (HCP) monitoring using lead reference electrode in concrete and computer operated data logger. An AC impedance spectroscope is also employed to obtain HCP and other electrochemical parameters. Experimental study was carried out on two types of OPC and one fly ash concrete. It was found that the lead reference electrode is capable to detect corrosion onset of rebars in concrete accurately through half-cell potential monitoring. CTL was estimated for both the OPC concrete. The one year immersion test depicted that resistance against chloride penetration in fly ash concrete is very low compared to other OPC concrete. TG/DTA analysis confirmed CTL relates with CH content in hardened OPC concretes.