Is Concrete Waterproof? (The Surprising Answer)
No. Concrete is porous like a hard sponge. Water wicks through it. You must seal it to make it hydrophobic.
Is Concrete Waterproof? The Surprising Truth
Most people assume concrete is waterproof, but it's not. Concrete is porous like a hard sponge—water wicks through it. Understanding this fundamental property helps you protect your concrete investment and make informed decisions about sealing and protection.
The Surprising Answer: No, Concrete Is Not Waterproof
Concrete is not waterproof. Despite its hard, solid appearance, concrete contains countless microscopic pores that allow water to pass through.
Why Concrete Is Porous
Concrete's porosity comes from its composition:
The Structure
- Cement paste: The binding material contains microscopic pores
- Aggregate: Gaps between aggregate particles create pathways
- Hydration products: Chemical reactions create porous structure
- Air voids: Entrapped air during mixing creates additional pores
Like a Hard Sponge
Think of concrete like a hard sponge:
- Looks solid: Appears impermeable from the outside
- Actually porous: Contains countless interconnected pores
- Wicks water: Water travels through these pores via capillary action
- Absorbs moisture: Can hold significant amounts of water
How Water Moves Through Concrete
Water moves through concrete via capillary action—the same principle that allows a paper towel to absorb water.
Capillary Action
- Tiny pores act like capillaries: Microscopic channels draw water through
- Water wicks through: Moves from wet areas to dry areas
- Travels significant distances: Water can travel several feet through concrete
- Occurs continuously: Happens whenever there's a moisture gradient
What This Means
If water is on one side of concrete, it will:
- Travel through to the other side
- Soak into the concrete
- Carry dissolved substances with it
- Create moisture problems
Why This Matters
Understanding that concrete is porous explains many common problems:
Moisture Problems
- Basement leaks: Water travels through foundation walls
- Floor moisture: Slab-on-grade wicks moisture from ground
- Staining: Water carries substances that create stains
- Freeze-thaw damage: Water in pores freezes and expands
Why Sealing Matters
Since concrete is porous, protection is essential:
- Sealing is mandatory: Not optional for most applications
- Prevents moisture entry: Stops water from wicking through
- Protects from damage: Prevents freeze-thaw and other moisture-related problems
How to Make Concrete Hydrophobic (Water-Resistant)
To make concrete resist water, you must seal it.
Penetrating Sealers
Penetrating sealers make concrete hydrophobic:
- How they work: Penetrate into concrete pores and create a water-repelling barrier
- Result: Water beads up and rolls off instead of soaking in
- Types: Silane, siloxane, and other penetrating sealers
- Benefit: Invisible protection that doesn't change appearance
Film-Forming Sealers
Film-forming sealers create a barrier on the surface:
- How they work: Create a film that blocks water from entering
- Result: Surface protection that prevents water entry
- Types: Acrylic, epoxy, urethane sealers
- Benefit: May enhance appearance (glossy or matte finish)
Applications Where Waterproofing is Critical
Some applications require true waterproofing (not just sealing):
Below-Grade Applications
- Foundation walls: Must be waterproofed, not just sealed
- Basement floors: Need moisture protection
- Retaining walls: Back side needs waterproofing
Above-Grade Applications
- Driveways: Sealing prevents water damage and staining
- Patios: Sealing protects from moisture and freeze-thaw
- Walkways: Sealing prevents moisture problems
Testing Porosity
You can test how porous your concrete is:
Simple Water Test
- Pour water on concrete surface
- Watch how quickly it soaks in
- Fast absorption = very porous
- Slow absorption = less porous (but still porous)
Professional Testing
- Moisture meters: Measure moisture content
- Absorption tests: Measure how much water concrete absorbs
- Vapor emission tests: Measure moisture vapor coming through
The Bottom Line
No, concrete is not waterproof. It's porous like a hard sponge—water wicks through it via capillary action. This is why sealing is essential. Penetrating sealers make concrete hydrophobic (water-repelling), while film-forming sealers create a barrier. Understanding that concrete is porous explains moisture problems and why protection through sealing is mandatory for most applications.
Don't assume concrete is waterproof—always seal it to protect your investment from moisture damage.
Need help sealing your concrete? Contact Local Concrete Contractor. We can assess your concrete and recommend the right sealer to protect it from water damage.
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