The corrosion of reinforcing steel has emerged as the most prevalent factor causing deterioration of reinforced concrete structures in aggressive exposures, such as the Middle East and Arabian Gulf States. Recently, the effect of using embedded steel bars passing through different types of concrete has been found to exacerbate the corrosion activity. This may occur with a steel bar passing from a foundation manufactured with sulphate resistance Portland cement (SRPC) into a beam or column manufactured with ordinary Portland cement (OPC). This thesis aims at elucidating the behaviour of reinforcing steel (RC) embedded in concrete mixes of different types by incorporating both OPC and SRPC and blends with ground granulated blast furnace slag and pulverised fuel ash. Two different types of RC specimens were used: Single bars in a single mix and electrically connected bars in same or different mix types sharing a common electrolyte in the absence and presence of a specific interface. The RC specimens were exposed to wet-dry cycles of chloride and sulfate solutions. The results of steel bars that were electrically connected, and were embedded in concrete cubes manufactured with different cement types and without a direct interface exhibited reduced times to onset of corrosion when subjected to chloride ion ingress. In the worst case, the connection of the a bar in OPC (0.45) concrete and one in SRPC (0.45) concrete caused the bar in the SRPC (0.45) mix to start corroding after only 3 cycles (6 weeks) compared to 15 cycles (30 weeks) for a single unconnected bar in the same SRPC (0.45) concrete. Moreover, measurements of the half-cell potential, polarisation resistance and corrosion current, all confirm that a single bar passing the interface between two different concretes manufactured with different cement types can suffer the formation of anode and cathode regions that depend on the local environment. For example, a steel bar passing from an OPC (0.45) concrete to a SRPC (0.45) concrete will preferentially form an active anode within the SRPC concrete. This has implications for concrete structures manufactured using SRPC foundations and OPC superstructure, as often used in the Arabian Gulf States