Seawater and unreinforced concrete
Concrete needs to be of a sufficient quality to resist seawater attack.
Information on the exposure condition for unreinforced (mass) concrete exposed to seawater is given in BS 8500-1: 2023 clause A.4.6 Table A.13.
Table A.13 gives the minimum quality of concrete as a maximum w/c ratio is 0.55 and the minimum cement content 300kg/m3, assuming a 20mm maximum aggregate size.