Premature deterioration of reinforced/prestressed concrete structures due to corrosion is of considerable current concern. One detrimental effect of corrosion of reinforcing/prestressing steel is the reduction of bond between steel and concrete due to the development of corrosion products at the interface. This thesis examines the influence of localized corrosion of reinforcing bars/ untensioned prestressing strands on their bond strength in concrete. In addition, an analytical study is conducted in order to investigate the complex behaviour due to cracking of cover concrete, non-uniform corrosion, presence of softened paste layer at the interface between steel and concrete, and stress relaxation due to creep in relation to the bond strength. The conditions of severe localized corrosion were simulated electrochemically. The main variables were cover-to-bar diameter ratio, reinforcement type, and corrosion rate. The influence of reinforcing/prestressing steel corrosion and cracking of the concrete cover on the behaviour of bond were studied at different stages of corrosion: non-corrosion, precracking, cracking, and postcracking levels. It was found that the bond strength increased with corrosion up to a certain amount. However, with the progressive increase in corrosion, the bond strength decreased very rapidly until the cracking of cover concrete, and then decreased at a very slow rate in the postcracking stage. The untensioned prestressing strands showed almost similar general behaviour but were found relatively more deteriorated due to corrosion in the postcracking stage under the similar corrosive conditions. The structural properties of steel such as yield strength and ultimate tensile strength were influenced significantly due to the enormous local reduction in cross-sectional area of the reinforcing/prestressing steel by corrosion. The test data showed that the percentage corrosion required to cause cracking of cover concrete varied linearly with cover-to-bar diameter ratio. Bond-slip studies at different stages of corrosion indicated that bond stiffness increases and then decreases with the increase of corrosion of reinforcing steel in concrete. Corroded prestressing strands exhibited a nonlinear bond-slip relationship. Corrosion rate was found to be a significant variable. Pullout bond specimens using deformed bars were exposed to the current densities of 4.0,2.0, 1.0,0.5,0.25,0.15,0.09, and 0.04 mA/cm'. Bond behaviour was studied at the cracking stage and after 20% corrosion. The results indicated a significant and non-linear effect of corrosion on bond strength. Both corrosion to cause cracking and bond strength as a ratio of the non-corroded bond strength increased with an increase of current density up to about 0.15-0.25 mA/CM2 , and then decreased with a further increase in current density. This explains the different results obtained by previous researchers at different corrosion rates. Finite element analysis of the effect of concrete cover at the stages of internal and surface cracking confirmed the behaviour found in the laboratory study. The presence of an interfacial softened paste layer showed no significant effect on the expansive pressure. The study of the relaxation of stresses due to creep showed that the high degradation in bond strength at high rate of corrosion was significantly contributed by the relaxation of stresses due to creep at the cracking stage. However, in the postcracking stage, creep showed little effect on the stresses in the concrete surrounding the reinforcing bar. It is concluded that it is extremely difficult to extrapolate laboratory data to field conditions. However, a simple assessment rule is proposed.