Yes, concrete is a very porous material. It starts off as a dry mixture made of cement, sand and stone. Once clean water is added, the cement chemically reacts and forms a paste that binds the sand and aggregate together. As wet concrete dries, hardens and cures, water evaporates out. This leaves behind a network of very small tunnels where water used to be. On average, 10%-15% of a concrete slab has these tiny pores. You can’t see them with your eye but they’re why concrete absorbs water like a sponge. So is all concrete porous? And is there anything you can do about it? Yes, all concrete is porous. No matter how you alter the mixture, water must evaporate out during curing which leaves behind pores. And yes, while you can’t stop the pores from forming, you can fill them later with a good quality sealant.
Concrete is a porous material, though the extent of those pores depends on the type of concrete. It’s impossible to create concrete that is not porous due to the materials it’s made of and evaporation, but the size of the pores can be controlled a bit. All concrete is porous, but some batches are more porous than others.
Concrete pores are very small, but still much larger than water or gas. Water vapor and soil gases, including radon gas, can flow right through a concrete slab. It’s very easy for thin slabs of concrete to draw water from the soil and into your basement. Water and gasses are attracted to the difference in pressure between a basement and the soil. Gases like radon flow even easier than water because of how small they are. This is where a good plastic barrier can help.
Is Concrete Watertight?
No, concrete is not water tight. However, when it’s poured extremely thick, such as with a large footing, it can appear to be.
It’s very unlikely that pores will work they’re way entirely through a thick slab. It’s easy for water to pass right through a 4″ – 6″ slab because pores can run from end to end. But when a slab is 2 foot thick it’s very unlikely. This can make a concrete footing or wall appear water tight. But it’s not. The water is actually just absorbing on the surface because the tunnels and pores get broken as the concrete gets thicker. If a pore got lucky and ran through the 2 foot of concrete without being broken then so will the water.
Did you know that concrete can actually float? When concrete is poured into the correct shape it can actually float really well. This is due to surface area and displacement of water and not because the concrete is water tight. In the past, boats, walkways and barges have actually been made out of concrete. So why don’t they absorb lots of water and sink?
Because it’s poured thick enough that water doesn’t easily pass through it.On average concrete is only 10%-15% porous so that leaves lots of solid pass to float.A sealant can be used to keep water out.
Dams are made out of concrete and they hold back tons of water. Are they water tight? No,damns leak in lots of places and the concrete absorbs water just like all concrete does. But it can be poured over 10 foot thick in some areas which makes it very unlikely that water would penetrate.
So no, concrete is not water tight.
