Подготовка поверхности бетона перед нанесением покрытия: загрязнения, дефекты и методы

As with steel, the success of any coating system on concrete depends on the surface beneath it. Concrete, though, brings its own family of contaminants and defects — most of them invisible to an untrained eye — and its preparation standards are less rigidly defined than steel's. This guide covers what must be removed, what must be repaired, which methods to use, how to protect workers from silica dust, and how to prove the surface is ready.

What counts as a contaminant on concrete?

Any material on a concrete surface that affects the adhesion or application of a coating is a contaminant and must be removed. The usual suspects:

• Laitance — a thin, weak layer of cement fines and partially hydrated cement that forms at the surface of fresh concrete. It is not tightly bonded; coat over it and the coating delaminates with the laitance.
• Efflorescence — a powdery white salt deposit left as water migrates to and evaporates from the surface. Remove it and address the moisture source, or it returns.
• Form-release agents — oils, waxes, greases or silicones applied to formwork to release the cured concrete. They transfer to the surface and destroy adhesion.
• Curing compounds — films applied to fresh concrete to retard water loss during cure. Unless certified compatible with the coating, they must be removed.
• Hardeners and sealers — applied to floors to reduce permeability and improve abrasion resistance. They leave a dense, low-energy surface that resists coating.
• Oils, chemicals and in-service contamination — on older surfaces these penetrate deeply and are among the hardest to remove.

Which surface defects must be repaired?

Cleaning is only half the job; the geometry of the surface matters too.

• Bug holes, honeycomb and air pockets — voids formed during placement and vibration of the pour, often hidden under a thin laitance skin. Break the skin and the void is exposed; coat over it and air and solvent escape through the film as pinholes and blisters.
• Tie-bar holes — depressions left when formwork ties are removed.
• Fins and protrusions — sharp, uneven ridges that prevent an even film; grind them smooth before coating.
• Parge coat — a hand-applied cement-sand mortar that, once dry, behaves like laitance and pulls away under a coating.

In most cases all such defects must be repaired before coating — always to the coating manufacturer's repair recommendations.

Why concrete behaves nothing like steel

Steel is dense, impermeable and predictable; concrete is porous, alkaline and full of moisture. Those three properties drive most concrete-coating failures. Porosity means a coating can be pulled into the surface or trapped over voids; alkalinity (fresh concrete is typically above pH 12) attacks alkali-sensitive coatings such as alkyds; and the slab's own moisture, migrating outward for months or years, lifts coatings from beneath. A method that works perfectly on steel with mill scale can be wrong for concrete.

Preparation methods

Concrete methods are less standardised than steel's; SSPC-SP 13/NACE No. 6 sets out the expected cleanliness, strength, profile and dryness of prepared concrete, and ASTM D4258 (cleaning) and D4259 (abrading) describe the procedures. The common methods, roughly in order of escalation:

• Chemical cleaning — should precede every other method. Uses alkaline detergent solutions or proprietary emulsifying cleaners to lift oil and grease, followed by thorough rinsing. Do not wipe with solvents: they dissolve the contaminant and spread it over a larger area and deeper into the pores.
• Scarification — power-driven cutters or serrated discs abrade the surface; useful for thick coatings or heavy contamination, but can remove too much concrete and leave a very rough surface.
• Abrasive blast cleaning — widely used and very effective; procedures mirror steel, with nozzle distance, media and dwell adjusted for the softer substrate.
• High-pressure water jetting — very effective at removing laitance and weak concrete; slightly slower than abrasive blasting, and abrasive injection increases cutting. It adds water to the slab, so allow drying time before coating.
• Centrifugal wheel (shot) blasting — portable equipment throws and recovers steel shot, removing laitance and opening the profile with integrated dust collection.
• Acid etching (ASTM D4260) — a last resort when no mechanical method is possible. It uses a dilute hydrochloric (muriatic) acid solution, produces only a light profile, and requires careful handling, thorough rinsing and a pH check afterwards. It does not remove oil, curing compounds or sealers.

Silica dust: protecting the crew

Grinding, scarifying, shot blasting and abrasive blasting concrete all release respirable crystalline silica, which is why method selection is also a health decision. In the United States, OSHA's construction silica standard (29 CFR 1926.1153) sets a permissible exposure limit of 50 µg/m³ as an 8-hour time-weighted average and specifies engineering controls — water delivery, or shrouded tools with vacuum dust collection — for common concrete tasks, with respiratory protection where those controls are not enough. Choose equipment with integrated dust extraction, keep surfaces damp where the method allows, and see MontiPower's overview of surface preparation safety for the broader principles.

Moisture: the silent killer

More concrete coatings fail from moisture than from any single contaminant. New slabs need time — often weeks — to dry, and slab-on-grade concrete may never fully dry if there is no vapour retarder beneath it. Beyond the simple plastic-sheet test (ASTM D4263), specifiers increasingly require in-situ relative humidity probes (ASTM F2170) or calcium chloride moisture-vapour-emission tests (ASTM F1869) to quantify how much water is still moving through the slab. If the number is too high, no surface preparation will save the coating; specify a moisture-tolerant primer or wait.

Profile has a standard too: ICRI CSP

Just as steel has an anchor-profile specification, concrete has the ICRI Concrete Surface Profile (CSP) scale (ICRI Guideline No. 310.2), running from CSP 1 (very smooth, typical of acid etching) to CSP 10 (very rough, typical of jackhammering). Profiles are assessed by visual and tactile comparison with ICRI replica chips, or measured with replica putty (ASTM D7682). Thin coatings and sealers want a low CSP; high-build coatings and polymer overlays want a deeper one. Specifying "clean concrete" without a CSP target is the concrete equivalent of specifying "clean steel" without a cleanliness grade — an invitation to dispute.

Match the method to the coating

The right preparation method is the one that produces the CSP the coating needs without harming the slab. Acid etching produces only the lightest profiles and is a last resort; abrasive and shot blasting suit most high-build systems; scarification and grinding handle thick removals and levelling. Always remove contamination by chemical cleaning first — abrading an oily surface only drives the oil deeper.

Repair before you coat

Bug holes, tie-bar holes and honeycomb are filled with compatible cementitious or epoxy mortars; fins and protrusions are ground flush; parge coat and laitance are removed entirely. The repair material must be compatible with both the slab and the coating, and cured before coating — another reason to confirm the manufacturer's repair recommendations in advance. Get the substrate sound, dry, clean and correctly profiled, and the coating can do its job; compromise any one of them and the system fails from the bottom up.

How to verify the surface is ready

After preparation, prove the surface is acceptable before coating:

• Dust test — wipe with a dark, dry cloth or apply and remove a strip of tape; visible white powder means the surface is too dusty to coat.
• Water-break test — sprinkle water; if it spreads and soaks in rather than beading, the surface is free of oil, grease and sealers.
• pH (ASTM D4262) — after chemical cleaning or acid etching, measure the pH of the rinsed surface to confirm that residual chemicals have been removed; the specification or coating manufacturer sets the acceptable range.
• Dryness (ASTM D4263) — tape a clear polyethylene sheet to the surface for at least 16 hours; condensation under the sheet or darkened concrete means the slab is too wet to coat. Use F2170 or F1869 where a quantitative limit applies.
• Profile — compare with ICRI CSP chips or measure with replica putty against the specified CSP.

These checks map directly onto the hold points described in coating inspection.

Coating systems for concrete

Once the surface is sound, dry, clean and profiled, the system is chosen for the service. Penetrating sealers and thin-film coatings suit light-duty floors and want a low CSP; high-build epoxies and novolacs handle chemical exposure and secondary containment; moisture-tolerant and vapour-permeable primers address damp slabs; and self-levelling and broadcast systems rebuild worn floors. The substrate preparation and the coating choice are a matched pair — the wrong profile undermines the right coating. For the chemistry behind these systems, see generic types of protective coatings.

Why concrete coatings fail

The recurring failures all trace to the surface: coating over laitance or a parge coat; trapping slab moisture beneath an impermeable film; bridging bug holes that later outgas; coating an alkali-sensitive material over fresh concrete; or skipping the chemical clean and abrading oil deeper. None is a coating defect — each is a preparation or specification defect, which is why the discipline matters as much on concrete as on steel.

A practical preparation workflow

Sequence matters as much as method. A reliable order of operations:

1. Assess — identify substrate age, contaminants, defects and moisture state.
2. Chemical clean to remove oil and grease before any abrasion.
3. Abrade to remove laitance and coatings and reach the CSP the coating requires.
4. Repair bug holes, tie-bar holes, fins and honeycomb with compatible materials, and let them cure.
5. Verify with dust, water-break, pH, moisture and profile checks.
6. Coat within the window the surface and weather allow.

Skipping or reordering steps — abrading before cleaning, coating before verifying moisture — is where most concrete-coating projects quietly fail.

The standards to cite

Concrete preparation references its own standards just as steel does: SSPC-SP 13/NACE No. 6 for prepared-surface requirements, ICRI 310.2 (CSP 1–10) for surface profile, ASTM D4258/D4259 for cleaning and abrading, ASTM D4260 for acid etching, ASTM D4262 for pH, and ASTM D4263, F1869 and F2170 for moisture. Naming them turns a vague concrete specification into an enforceable one — exactly as on steel.

Key takeaways

• Concrete is porous, alkaline and full of moisture — it fails differently from steel.
• Remove laitance, efflorescence, form-release agents, curing compounds, hardeners and oils; repair voids and fins.
• Chemical-clean first; never wipe with solvents that spread the contaminant.
• Specify a profile against the ICRI CSP scale and verify moisture (D4263, F1869 or F2170) before coating.
• Control respirable silica dust whenever concrete is ground, scarified or blasted.

The principle is universal

Surface preparation is the most important part of any engineered corrosion-protection programme, and any compromise made here compromises the entire system. Whether the substrate is concrete or steel, the rule holds — the coating can only be as good as the surface under it. For the steel side, see the substrate-by-substrate guide to heavy-duty coatings and the controlled preparation system that makes a variable surface predictable.

Frequently asked questions

How long must concrete cure before coating?

Conventionally about 28 days, but cure time is not the same as dryness. Always verify moisture with the plastic-sheet test (ASTM D4263), in-situ relative humidity probes (ASTM F2170) or a calcium chloride emission test (ASTM F1869), and compare the result with the coating manufacturer's limit rather than relying on the calendar.

What is laitance and why does it matter?

Laitance is a weak, poorly bonded surface layer of cement fines that forms on fresh concrete. It looks like sound concrete but has little strength, so a coating applied over it fails with the laitance. It is removed by abrasive blasting, shot blasting, grinding or, as a last resort, acid etching.

What concrete surface profile (CSP) does a coating need?

The coating manufacturer specifies it. The ICRI scale runs from CSP 1, nearly smooth as after acid etching, to CSP 10, very rough as after jackhammering. Penetrating sealers and thin films need a low CSP, while high-build coatings and polymer overlays need a deeper profile, typically produced by shot blasting or scarifying.

Do curing compounds have to be removed before coating?

Usually yes. Curing compounds form a film that reduces adhesion, so they must be removed unless the coating manufacturer confirms in writing that the specific compound is compatible. Removal is normally mechanical, by abrasive or shot blasting or grinding, followed by the dust and water-break checks.