A one component sealant specially designed for sealing reinforced concrete structures. Reinforced concrete structures that are continuously exposed to water can be sealed reliably and permanently with FlexproofX® 1 because the special composition of FlexproofX® 1, which is based on a patented, prepolymer mixture, allows practically no water absorption to take place even with continuous water exposure. The reactive process of FlexproofX® 1 in particular with hydroxyl groups (as they can also occur in the composition of concrete), makes FlexproofX® 1 the ideal, permanent sealant for reinforced concrete structures.