Before coating industrial concrete, repair in this order: remove the cause (water, corroded rebar, movement), remove deteriorated concrete down to sound concrete, rebuild spalls and potholes with a suitable mortar, treat cracks according to whether they are dormant or active, rebuild joint edges and fill joints, level, then let the repairs cure and dry as the data sheet requires before applying the coating.
A floor or protective coating is thin compared with the concrete it covers. It follows every defect, transmits every crack and debonds where the substrate is weak. Repairing first is therefore not optional: it is the condition for the coating to last. This guide is written for maintenance managers and project managers of plants, warehouses, mine sites and energy facilities in Quebec and Ontario. It complements our guide to industrial floor coating types.
Why must concrete be repaired before a coating is applied?
Because the coating does not correct the substrate: it reproduces its defects. An unrepaired spall becomes a hollow that holds water, an active crack runs through the film, and weak concrete pulls the coating off with it.
Three reasons justify repairing first:
- Adhesion. The coating bonds to the concrete surface. If that surface is cracked, delaminated or contaminated, it is the surface that fails.
- Continuity. A film meant to protect against chemicals or water must be continuous. Potholes and broken joint edges create uneven thicknesses and holidays.
- Service life. A repair made after the coating is installed means cutting it out, repairing and tying back in, with a poorer result and one more shutdown.
How do you assess the concrete before repairs?
Assess the concrete with a field survey: hammer or chain-drag sounding to find hollow areas, mapping of cracks and joints, checks for contamination, moisture testing and, where needed, pull-off tests. The survey sets the quantity and type of repairs.
ACI 546R provides a general framework for concrete repair. On site, record:
- Delaminated or hollow areas, marked out with chalk before removal.
- Corroded or exposed rebar, a sign of corrosion that will continue under the repair if it is not addressed.
- Cracks, with their width, length and behaviour (dormant or active).
- Contamination by oils, grease or chemicals, which may call for degreasing or deeper removal.
- Moisture, measured with the methods described in our article on concrete moisture before coating.
- Surface strength, checked by pull-off testing to ASTM D7234 when the concrete looks weak.
How do you repair spalls, potholes and joint edges?
Saw-cut the perimeter of the area for clean edges, remove deteriorated concrete down to sound material, prepare and clean the bottom, then apply the primer and mortar specified on the data sheet. Thin feathered edges should be avoided with most cementitious mortars.
Key steps:
- Mark out and saw-cut. A saw cut around the area gives a minimum mortar thickness all around and avoids crumbling edges.
- Remove. Light chipping or mechanical tools, without cracking the adjacent sound concrete. Around corroded rebar, ICRI 310.1R describes how to expose the bar all the way around and clean the steel.
- Prepare the bottom. Rough, clean and dust-free surface; pre-wetted for a cementitious mortar if the data sheet requires it, dry for an epoxy mortar.
- Apply. Bonding primer if required, mortar compacted in lifts within the allowed thickness, finished flush with the surrounding concrete.
- Joint edges. Broken arrises are rebuilt with a mortar able to take wheel loads, then the joint is re-established.
Epoxy mortar or cementitious mortar: which one?
Epoxy mortar suits thin to medium repairs that must return to service quickly and receive a resin-based coating; polymer-modified cementitious mortar suits thicker repairs and large areas. ICRI 320.2R helps choose based on the properties required.
| Repair type | When to choose it | Strengths | Limits |
|---|---|---|---|
| Epoxy mortar | Spalls, potholes, joint edges under a resin coating | Adhesion, fast cure, chemical resistance | Sensitive to cold, thermal movement different from concrete in thick sections |
| Polymer-modified cementitious mortar | Thick repairs, large areas, vertical surfaces | Behaves like concrete, moderate material cost | Curing and drying before coating, shrinkage to manage |
| Rapid-setting repair mortar | Short shutdowns, traffic areas | Fast return to service | Short working time, compatibility to check |
| Cementitious underlayment or self-leveller | Flatness, slopes, large worn areas | Even surface before coating | Does not fix deep defects, moisture to control |
| Epoxy grout | Equipment foundations and bases | Load bearing, vibration resistance | Specific use, see heavy machinery epoxy grouting |
The basic rule: the repair mortar must be compatible with the coating that will go over it. The coating data sheet lists accepted substrates and the time before overcoating.
How do you treat cracks and joints before coating?
Treat a dormant crack as a repair, by filling or injecting it, and an active crack as a joint, by leaving it free to move. Expansion joints are always carried through the coating; control joints are filled or carried through depending on the system.
- Dormant, dry cracks. Routing and filling with resin, or epoxy injection if the crack must regain continuity; see our comparison of epoxy vs polyurethane crack injection.
- Active or wet cracks. Polyurethane injection to stop water, then treatment as a flexible joint in the coating.
- Control joints. Cleaning, edge repair and filling with a semi-rigid filler where hard wheels cross them, as ACI 302.1R recommends; the coating can then run over them if the manufacturer allows.
- Expansion and isolation joints. Never covered: they are carried through the coating with a suitable flexible sealant.
Our concrete crack injection service covers these treatments before coating.
How do you level and reprofile before coating?
Level with a compatible underlayment or self-leveller when flatness or slopes to drains are not adequate, after spot repairs and before the coating. The coating itself is not designed to make up large deviations.
Points to check: slopes to drains and trenches, to avoid ponding; thresholds and transitions with uncoated areas; equipment bases, which are often rebuilt with grout as in our article on pump foundation repair; and the maximum thickness per lift allowed by the product. The target flatness tolerance comes from the project specification.
How long should repairs cure before coating?
It depends on the product and conditions: an epoxy mortar can often be overcoated once cured, whereas a cementitious mortar or underlayment must first cure, then dry to the moisture level the coating accepts. The data sheet sets the time; a moisture test confirms it.
Common mistakes:
- Coating a cementitious repair too early. Residual moisture causes blisters or debonding.
- Skipping the cure. A cementitious mortar that dries too fast cracks and dusts at the surface.
- Working cold. Below the product's minimum temperature, epoxy cures poorly and cementitious mortar sets slowly; heat and enclose the area, as for cold weather coating application.
- Forgetting final preparation. Repairs are ground or shot blasted with the rest of the slab before coating.
How do you plan repairs and get a quote?
Plan by including repairs in the same job as the coating: survey, repairs, curing, final preparation, coating, inspection. Repair quantities are often confirmed after preparation, when hidden defects appear; the quote should therefore set a clear method for measuring them.
Repairs are part of our protective coatings and industrial epoxy flooring offer, and of concrete protection more broadly. Our crews and qualified subcontractors work from LaSalle, including in Montreal, in Abitibi-Témiscamingue and in Ontario. Send photos of the defects, the areas, the use of the zone and the shutdown window through the quote request: pricing on consultation, reply within 24 business hours.
Frequently asked questions
Can a coating go directly over cracked concrete?
No, not without treatment. A dormant crack is filled or injected before coating; an active crack is treated as a flexible joint, otherwise it reappears through the film.
Should the edge of a spall always be saw-cut before repair?
It is the recommended practice for most mortars, because it avoids thin edges that crumble. Some resin-based mortars accept thinner sections; the data sheet says so.
Do epoxy mortar repairs show under the coating?
They can remain slightly visible depending on the finish, especially under a thin glossy coating. A full skim coat or a thicker coating evens out the appearance.
What if the rebar under the slab is corroded?
The steel must be exposed and cleaned, the cause of corrosion addressed and the section rebuilt with a suitable mortar before any coating. Covering corroding rebar moves the problem without solving it.
Hicham M, P Eng, PMP
Engineer and project manager (PMP) at Induscoat. Over 16 years of experience in industrial coatings, composite repairs and wear protection on mining, energy and petrochemical sites in Canada and internationally.