Coating Concrete

Concrete is coated to protect it from chemicals, abrasion, water and freeze–thaw damage, to make floors cleanable and safe, and to improve appearance. Unlike steel, concrete does not corrode, but it degrades in other ways — and it is a far more variable substrate. Two slabs poured a week apart can behave very differently under the same coating.

This article explains how concrete’s properties affect coatings, how to prepare and test it, and which coating families suit different services.

Concrete as a substrate

Several characteristics set concrete apart from metal substrates:

• Porosity. Concrete is full of capillary pores that absorb primer, hold moisture and release air when warmed.
• Alkalinity. Fresh concrete is highly alkaline (pH typically around 12–13), which attacks alkali-sensitive binders such as some alkyds.
• Moisture. Slabs on grade can receive moisture vapor from below indefinitely, and new concrete contains a great deal of mix water.
• Weak surface layer. Laitance — a thin, weak, cement-rich skin — forms on troweled surfaces and must be removed.
• Cracking and movement. Shrinkage, thermal movement and structural loading crack concrete; rigid coatings crack with it.
• Tensile strength. Because concrete is relatively weak in tension, pull-off tests on well-bonded coatings often fail within the concrete itself.

Moisture, curing and timing

A common rule of thumb is that new concrete should cure for around 28 days before coating, but age alone is not reliable. Moisture testing tells you whether the slab is ready:

• ASTM F2170 — in-situ relative humidity probes placed in drilled holes.
• ASTM F1869 — calcium chloride test of moisture vapor emission rate.
• ASTM D4263 — plastic sheet test as a simple qualitative check.

Acceptable limits depend on the coating and are stated on the product data sheet. Where moisture is high, options include waiting, using moisture-tolerant primers or vapor-mitigation systems, or selecting breathable coatings. Detail is covered in concrete moisture testing.

Surface preparation

Good preparation removes anything that weakens the bond and opens the surface so the primer can penetrate. Typical steps:

1. Assess the slab. Note cracks, joints, contamination, previous coatings, curing compounds and sealers.
2. Remove contaminants. Degrease oil-soaked areas; remove curing compounds, old coatings and sealers.
3. Profile mechanically. Diamond grind, shot blast or scarify to remove laitance and achieve the specified CSP from ICRI 310.2R — thin coatings need lower profiles, thick mortars higher.
4. Repair defects. Fill bugholes, spalls and cracks with compatible repair materials; treat joints according to whether they move.
5. Clean thoroughly. Vacuum all dust before priming.
6. Verify. Check tensile strength and adhesion with pull-off testing to ASTM D7234 where specified.

See concrete surface preparation and CSP for methods in detail. Acid etching is generally considered less reliable than mechanical methods for performance coatings.

Coating options for concrete

For floor-specific systems, see concrete floor coatings. Breathable water repellents are covered in silane and siloxane water repellents.

Common problems

• Blistering and delamination from moisture vapor pressure or osmotic effects.
• Pinholes and bubbles from air escaping out of the pores as the concrete warms — see pinholes and outgassing. Applying while the slab temperature is stable or falling helps.
• Reflective cracking where rigid coatings bridge moving cracks or joints.
• Efflorescence — salt deposits carried to the surface by moisture, which can push coatings off.
• Peeling from laitance, curing compounds or contamination left in place.

Vertical and structural concrete

Walls, columns, tanks and bridge elements face different stresses from floors: weathering, carbonation, chloride ingress and water pressure. Breathable acrylic or elastomeric coatings, anti-carbonation coatings, and penetrating sealers are common for exterior structures, while epoxy, polyurea and cementitious linings protect tanks, sewers and containment.

Frequently asked questions

How long should new concrete cure before coating?

A 28-day rule of thumb is common, but moisture testing is more reliable. Some primers are designed for earlier application on damp concrete.

Is acid etching enough preparation?

For performance coatings, mechanical methods such as grinding or shot blasting are generally preferred because they remove laitance more consistently and produce a measurable profile.

Why did my concrete coating blister?

Usually moisture vapor or osmotic pressure from below, or contaminants trapped under the film. Moisture testing and correct primer selection are the main preventives.

Can coatings bridge cracks in concrete?

Flexible coatings like polyurea can bridge small, static cracks. Moving cracks and joints need sealants or joint treatment rather than relying on the coating alone.