The present article deals with the condition assessment and durability characterization of the oldest Algerian concrete structures exposed to the Mediterranean sea for around 100 years, taking as a study case the port of Ghazaouet, located in the extreme northwest of Algeria (province of Tlemcen). This study includes inspections of the condition of the harbour structures and in-situ tests conducted in atmospheric zone to characterise the mechanical characteristics of concrete (sclerometer rebound), and durability indicators determination as porosity (Mercury Intrusion Porosimetry) and chloride diffusion coefficient calculated with total and free chloride profiles. Diagnosis inspections on the port's concrete structures indicate localized damage with acceptable mechanical resistance. However, an increase in porosity with microstructure change was noted and extensive chloride penetration have been recorded. Finally, simulation analysis of the valuable data of diffusion coefficient (Da) and surface concentration (Cs), showed the great effect of the chlorides concentration at the surface on long-term exposure of concrete behaviour in seawater.