Concrete Is Not Just Gray Steel
Much of what works on steel carries over to concrete. Much of it does not, and the part that does not is where most concrete coating failures start.
Four ways concrete differs from steel
It is porous. Concrete breathes and holds moisture.
It is alkaline. Surface pH of 12 to 13 attacks some coatings from underneath. An alkyd applied to concrete turns soft and gummy because the alkali converts the oil binder into soap.
It is weak in tension. Concrete cracks, and a rigid coating over a crack cracks with it.
It is rarely dry. Water vapor moves through a slab constantly. A coating that blocks that movement can be pushed off.
How the approach changes
On steel, the sequence is blast and coat. On concrete, it includes steps steel never needs:
Test moisture per ASTM F2170 (in-slab relative humidity) or ASTM F1869 (calcium chloride).
Remove laitance, the weak, dusty layer on the surface of a poured slab.
Watch for outgassing, air in the pores rising into fresh coating.
Consider slab age carefully. The familiar 28 days is a strength milestone, not a dryness milestone. A fully cured slab can still be too wet to coat.
Surface preparation
Concrete is not blasted to white metal. It is shot blasted, ground or otherwise abraded to a concrete surface profile (CSP) defined by ICRI 310.2R, matched to the thickness of the system going on. Preparation follows SSPC-SP 13/NACE No. 6.
What to check in your spec
Moisture testing method and acceptance limit from the coating manufacturer.
Laitance removal and a target CSP range per ICRI 310.2R.
Crack and joint treatment.
Application timing to limit outgassing.
Further reading: Tnemec Technical Bulletin 03-43, Application of Coatings over Concrete. Standards: ICRI 310.2R, SSPC-SP 13/NACE No. 6, ASTM F2170, ASTM F1869.
Talk to a consultant. Atlantic Coating Consultants helps owners, engineers and contractors in PA, NJ, DE, MD, DC, WV and OH with specification writing, on-site condition assessment, training, product recommendations and field support. Call 484-644-5303 or email atlantic@tnemec.com.
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