This paper deals with replacement of cement in concrete by ground granulated blast furnace slag (GGBFS) with different replacement levels. The strength and chloride penetration of concrete mixes determined by replacing 40%, 50%, 60% and 70% of cement with GGBFS. Total five slag based concrete mixes of different mix proportions were used. For each mix standard sizes of cubes, cylinders and prisms were cast and tested for mechanical properties and rapid chloride penetration at different ages of curing. The obtained results were compared with control concrete and the mix contains 50% replacement with GGBS perform better strength and chloride penetration compared with all other concrete mixes.