Most concrete does not require waterproofing either because it is used where it is not exposed to water under pressure or because it is of such high quality that water cannot penetrate it. Not only does concrete in dry exposures like interior walls and floors above grade need no waterproofing, but exterior walls above grade usually get along well without waterproofing since in most climates their exposure to water or wind-driven rain is only intermittent. On the other hand, concrete roofs as well as walls and floors below grade are frequently subjected to sufficient water under pressure to require waterproofing. The far more severe exposures to water under pressure in tunnels, water-holding structures and bridges require serious attention to design and construction of waterproof barrier systems.
When concrete of low permeability is needed, the contractor should make certain that the ready mix producer knows the purposes for which the concrete is to be used. The producer may able to give valuable advice regarding the mix. Even tanks and reservoirs are sometimes built without use of waterproof membranes. For such structures the mix should have a water-cement ratio no higher than 0.53. In larger structures such as dams it may be possible to use water-cement ratios as high as 0.67 because of their thicker sections, but in concrete of thin sections the water-cement ratio should be limited to 0.44.