A well designed concrete structure is waterproof. The greater the strength of the concrete the greater is its water-resistence (4000 psi or above is recommended). High strength concrete has a low water:cement ratio. The use of an air entraining agent also increases the strength of the concrete and reduces its porosity. Ensuring that the concrete cures slowly and does not dry out at the surface (or where it makes contact with shuttering) reduces surface cracking.
Structural grade concrete does not require any additional surface treatment to prevent water penetration. But a large number of waterproof treatments is available. Why? (1) to stop the movement of water vapour through concrete (2) to make good defects (3) to provide additional security in the event of a defect arising (4) to waterproof structures which were not originally designed to be waterproof.
The range of concrete and masonry sealant treatments recommended for concrete include: epoxy resin, latex paint and neoprene. Other treatments, used but not recommended, include: linseed oil, paint and chlorinated rubber.
Additional protection can also be provided with a sand-cement render (plaster), hot-applied mastic ashpalt or ceramic tiles.
Poorly detailed non-structural concrete could be water-proofed by tanking.