Concrete waterproof coating, breathable?

[dcceecy]

I am working on a concrete diving platform. We specified a type of waterproof coating to covered all the concrete surfaces. So the pool water wil not reach steel rebar in concrete. The contractor said that if we seal all the side of concrete with non-breathable membrane, it will cause problems. I also feel that it is not a good idea to encapsulate the whole deck.

So I am check other waterproof coating products. some products says breathable (not a vapor barrier). if it is not a barrier, how it can be waterproof?

What's the difference between waterproof and vapor barrier?

Thanks.

 

[a2mfk]

Are you sealing the underneath of the platform?

From a curing and shrinkage standpoint I would certainly want to wait as long as possible.

Water vapor is gaseous water, versus water in liquid form. I'm rusty in my chemistry in this area but I believe water molecules in liquid state are attracted to each other due to chemical bonding, which is affected mainly by pressure and temperature. When the temp and pressure conditions are right, individual water molecules "escape" the surface into the air in gaseous form, as individual H2O molecules. Surface tension also plays into this in keeping the water molecules together and preventing evaporation, as is evidenced by a drop of water on a hard surface.

So individual water vapor molecules are very small compared to water droplets ("clumps" of water molecules if you will), like the difference between sand and small pieces of gravel. Going with the soil metaphors- A vapor barrier must therefore be a fine sieve, where a water barrier may be a coarse sieve (measured in "perms" or permeability). So a water vapor barrier, or more correctly, a retarder (since nothing is a vapor BARRIER, not even steel), may allow some vapor to pass through while stopping water droplets.

This principal is used/misused in wall and roof assembly details all of the time, and you often want to have a water barrier but you want to allow vapor transmission...

http://www.airbarrier.org/about/difference_e.php

 

[dcceecy]

Thanks for your reply. I should have mentioned that the platform have been there for 20 years.

the purpose of the coating is to prevent pool water with cholride ions reaching the steel rebar. Small amount of water molecules may be trapped by the conrete cover before they reach the steel. thanks.

 

[a2mfk]

If the bottom of the slab remains uncoated it can still breathe, but I don't know what happens to the Cl ions in this process. I'd seek the advice of my cousin who is a Chem E in the building science industry, but then again you don't know my cousin. But maybe seek the guidance of a similar person who works for one of the sealant manufacturers, as I am sure this situation is deal with all of the time.

I would think a chemical water barrier would prevent the pool water from infiltrating the top of the slab but still allow vapor to escape.