Is concrete waterproof?

The most popular classes of concrete, such as C 16/20 or C20/25, are not waterproof in the same way as, e.g., the majority of plastics. Of course, water does not leak through a concrete structure freely, but if the composite is exposed to long-term humidity or the presence of high pressure water, then, after some time, water stains can be observed appearing on the concrete surface.
This happens because of the microscopic structure of the composite. Its entire volume contains micropores, which are no big barrier against penetrating water. This means that increasing the waterproofness level of concrete is synonymous with limiting the number and size of micropores. This can be done in a few ways. The most popular ones are vibrating the laid mixture with a suitably low water-cement ratio (0.55 at maximum, the lower the indicator, the more waterproof the concrete) thoroughly and using special admixtures.
The proper aggregate, which grains have to fill up all the free space inside the concrete tightly, should be selected as well. If possible, the concrete surface should be impregnated with specialised agents protecting against the destructive effects of water and humidity. Furthermore, in case of some components by definition exposed to the presence of humidity, e.g. foundations or foundation walls, a proper waterproofing layer should be additionally provided – film, asphalt or latex emulsion, etc.
It is important to know that in case of concretes used for special applications, e.g. for constructing dams or bridges, the waterproofness criterion is employed as well. In such a case, the waterproofness level assumes various values (W-2, W-4, W-6, W-8, W-10 and W-12). This value is related to the ability to combat the flow of water through the composite. The number in the above designation specifies the maximum water pressure (in MPa) where no signs of leakage occur, multiplied by ten. The test is conducted on a 150 mm thick sample and concerns specialised composites primarily.


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