The NSR-10 classifies concrete exposed to moisture and to chlorides from seawater, salt or sea spray as exposure class C2 (table C.4.2.1). For that class it requires a maximum water-cementitious materials ratio of 0.40, a minimum strength of 35 MPa and a water-soluble chloride content of no more than 0.15% by weight of cement in reinforced concrete (table C.4.3.1).
Why so much
Chlorides travel through porous concrete, reach the steel and corrode it. Rust takes up more volume than steel, breaks the concrete cover and leaves the bar exposed. A mix with less water is less porous and slows that process down. That is why, for class C1 (moisture without chlorides), the code asks for 0.50 and 17 MPa, and for C2 it goes up to 0.40 and 35 MPa.
Concrete cover
The NSR-10 sets minimum covers for cast-in-place concrete: 75 mm against the ground, 50 mm for No. 6 bars or larger exposed to the weather, and 40 mm for protected beams and columns (C.7.7.1). In corrosive environments it asks for suitably larger covers or other protection (C.7.7.6). A 2 cm cover on a facade facing the sea is an invitation to corrosion.
What to check in your project
That the drawings state the exposure class of each member; that the specified mix meets the water-cement ratio, not just the strength; that spacers of the right size are used on site; and that curing is complete. We go into more detail in chlorides and reinforcement.
Source: NSR-10, Title C, chapters C.4 and C.7.
Frequently asked questions
What strength should concrete near the sea have?
For exposure class C2, the NSR-10 requires at least 35 MPa and a maximum water-cementitious materials ratio of 0.40 (table C.4.3.1).
What cover should the steel have?
At least 75 mm against the ground, 50 mm for No. 6 bars or larger exposed to the weather and 40 mm for protected beams and columns; in corrosive environments the code asks for more (C.7.7).
