Is concrete porous?

Vapor retarder permeance

≤ 0.1 permsASTM E1745

Vapor retarder thickness

10 mil (0.25 mm) minNRMCA CIP 29

Sub-slab relative humidity

close to 100%NRMCA CIP 28

Called a vapor barrier

below 0.01 permsafter conditioning

Short version

• Concrete absorbs water and transmits vapor even when it looks solid.
• Permeability is driven mainly by the water-to-cementitious-materials ratio.
• Porosity is what makes freeze-thaw scaling, efflorescence, and flooring failures possible.
• Vapor retarders under interior slabs need permeance of 0.1 perms or less per ASTM E1745.
• Pervious concrete is the deliberate opposite: engineered voids that let water pass straight through.

Why is concrete porous?

Concrete is porous because more water goes into the mix than the cement can chemically combine with. The excess occupies space in the paste, and as hydration proceeds and that water leaves or is consumed, it leaves behind a connected system of capillary pores.

The volume and connectivity of those pores is what permeability means in practice. NRMCA states the relationship plainly: permeability is affected by the water-to-cementitious-materials ratio and by the type and proportions of cementitious materials in the mix.

Lowering the w/cm reduces porosity, which is why durability requirements in ACI 318 pair a maximum w/cm with a minimum specified strength. Since the w/cm cannot be verified at delivery, strength tests serve as the acceptance basis.

Supplementary cementitious materials reduce permeability further. Fly ash, slag cement, and silica fume refine the pore structure compared with mixtures containing only portland cement, and they also improve resistance to chemical attack.

Porosity is therefore a design variable, not a fixed property. Two slabs of the same thickness can differ enormously in how much water they take in.

How does porosity cause freeze-thaw damage?

Porosity is the precondition for freeze-thaw damage, because concrete has to be saturated before freezing can hurt it. When water-filled concrete approaches freezing, the water expands as it forms ice, and that expansion generates stress inside the material.

Repeat the cycle enough times and the surface starts to flake — scaling. Light scaling stays in the mortar; severe scaling exposes the coarse aggregate.

Air entrainment is the engineered defense. Air-entrained concrete contains millions of tiny bubbles that give expanding water and ice somewhere to go, preventing the stress from building. Those voids are deliberate porosity used to protect against the harmful kind.

Deicing chemicals make everything worse. NRMCA notes that deicers increase the saturation of the concrete at the surface and increase the number of freeze-thaw cycles, which is why exterior flatwork in cold regions needs entrained air, low permeability, and restraint about salt.

Why does moisture come up through a slab?

Moisture rises through a slab because concrete transmits water vapor and the ground underneath is effectively always wet. NRMCA is explicit: regardless of where a building is located, the relative humidity of the subgrade below a slab on ground measures close to 100%.

There are several sources. Liquid water rises by capillary action from the water table when fine-grained soil sits below the slab. Water vapor rises and saturates the soil regardless of water-table depth. Granular fill between the vapor retarder and the slab can take on additional water and feed it upward.

Residual mix water is its own source. It can take weeks or months, even in favorable interior conditions, for a slab to dry to the level flooring manufacturers accept, and slab thickness, mix design, curing, and ambient conditions all change the rate.

The consequences land on whatever is installed on top. Vapor moving through a slab causes adhesive failures, delamination, distortion, and discoloration of flooring, plus the potential for fungal growth and odors.

How do vapor retarders control it?

Vapor retarders control slab moisture by putting a low-permeance sheet between the ground and the concrete. They are specified under ASTM E1745, which requires the material’s permeance to be no greater than 0.1 US perms when tested under ASTM E96 or ASTM F1249.

Thickness matters for two reasons. A minimum of 10 mil (0.25 mm) is recommended both for reduced vapor transmission and for durability during and after installation — a sheet that tears while the crew works on it stops functioning. Membranes with after-conditioning permeance below 0.01 perms are called vapor barriers rather than retarders.

Installation determines performance. Tears, punctures, and improperly sealed penetrations turn the sheet into a conduit, and a granular blotter layer above the retarder can carry moisture laterally to those openings.

Vapor retarders are for interior slabs, where flooring, coatings, or moisture-sensitive equipment will be present. Exterior slabs on grade generally do not use them.

Does sealing make concrete non-porous?

Sealing reduces absorption but does not make concrete impermeable. Penetrating sealers line the pores near the surface so liquid water beads instead of soaking in; film-forming sealers add a thin surface layer. Neither one changes the pore structure in the body of the slab.

That distinction matters for moisture moving the other direction. A film-forming sealer applied over a slab that is still releasing construction moisture traps that vapor underneath, which is what causes blistering, whitening, and adhesion failure.

Efflorescence is porosity made visible. Water travels through the pore network, dissolves salts, and evaporates at the surface, leaving the salts behind as a white deposit. NRMCA lists efflorescence as a source of light-colored patches on concrete surfaces.

Sealers are a supplement to good concrete, not a substitute. Low w/cm, adequate entrained air where freezing applies, proper finishing, and full curing do more for permeability than any surface treatment.

What is pervious concrete?

Pervious concrete is concrete engineered to be porous on purpose. It uses a high-porosity structure built from an interconnected void system, achieved with an aggregate blend and just enough cementitious paste to coat the particles while keeping those voids connected.

Water from rainfall passes through the pavement into an aggregate base layer that stores it and lets it percolate into the ground. The layer thicknesses are designed around the design storm, the soil subgrade, and the traffic load.

The application is stormwater management. EPA recognizes pervious concrete pavement systems as a Best Management Practice for first-flush pollution control and stormwater management, and NRMCA notes that they reduce runoff, can shrink or eliminate detention ponds, and recharge groundwater.

Typical uses are parking areas, light-traffic pavements, and pedestrian walkways. Pervious concrete demonstrates the underlying point of this page: porosity in concrete is a property engineers control, dialing it down for durability or up for drainage.

Common mistakes

• Treating a hard, dry-looking slab as waterproof — the pore network passes vapor continuously, and flooring adhesives are what find out.
• Installing flooring without a moisture test — construction moisture can take weeks or months to leave, whatever the calendar says.
• Puncturing the vapor retarder during the pour — an unsealed tear defeats a sheet that met ASTM E1745 on paper.
• Relying on a sealer to fix a high-w/cm slab — surface treatments do not change the permeability of the concrete underneath.
• Using deicing salts on porous exterior concrete — they raise surface saturation and multiply freeze-thaw cycles, which is how scaling starts.