Post-failure analyses of pretensioned concrete (PTC) structures have revealed the possibility of stress corrosion cracking (SCC) of prestressed steels in chloride environment with no visible signs/stains of corrosion on the concrete surface until it is too late. An understanding of passive-to-active (P-to-A) transition of prestressing (PS) steel, which marks the onset of SCC, is necessary for both service life-based design (SLD) of new PTC structures and corrosion assessment of existing ones. This thesis work focusses on (i) proposing a mechanism for initiation of SCC in PS steel-cementitious systems using electrochemical and microanalytical characterization techniques (EIS, SEM/EDAX, XRD, etc.) (ii) identifying the key factors influencing P-to-A transition (iii) developing a test method (named as psd-ACT test) to determine chloride threshold (Clth) of prestressed steel for estimating service life and (iv) proposing a methodology to assess the corrosion condition of existing PTC structures. The results showing that the electrochemical response from quenched and self-tempered (QST/TMT) steels are different from that of PS steels will be presented. The effect of cementitious environment (say, type of binder or corrosion inhibiting admixtures) on Clth of PS steel was also quantified. The estimates of Clth of prestressed steel and its impact on service life, and a methodology for routine corrosion condition assessment of PTC structures will be presented.