Preparing Concrete for Protective Linings

Concrete Is Not a Uniform Surface

Concrete is porous and variable. Its condition is affected by its original mix, placement, finishing, curing, exposure, repairs, contamination, and moisture history.

Two areas that look similar may absorb primer differently or have very different surface strength. One area may be sound while another contains weak laitance, oil contamination, hidden delamination, or moisture moving through the slab.

Review the Lining Requirements First

Surface preparation must match the lining system. A thin coating and a reinforced lining may require different surface profiles, repairs, primers, and moisture conditions.

Before selecting preparation equipment, confirm:

• The required concrete age and cure
• The specified Concrete Surface Profile
• The required surface tensile strength
• The acceptable moisture condition
• The required treatment of cracks and joints
• The approved repair and resurfacing materials
• The primer and recoat requirements
• The environmental limits for application
• The inspection methods and acceptance criteria

Final preparation requirements should come from the project specification and the lining manufacturer's current written instructions.

Has the Concrete Cured Long Enough?

New concrete contains substantial moisture and continues to change while curing. Many lining manufacturers establish a minimum concrete age, but age alone does not prove that the concrete is ready.

Mix design, slab thickness, temperature, humidity, curing method, finishing, vapor barriers, and site conditions all affect moisture and surface condition. Concrete that has reached a specified age may still exceed the lining system's moisture limit.

Curing compounds and surface sealers can also interfere with adhesion. They must be identified and removed when required.

Evaluate Concrete Soundness

The lining must bond to sound concrete. Weak material at the surface or below it can separate while the lining itself remains firmly attached to the failed layer.

Look and listen for:

• Laitance and weak surface paste
• Scaling, dusting, and soft concrete
• Spalling and exposed aggregate
• Delamination or hollow-sounding areas
• Honeycombing and poorly consolidated concrete
• Freeze-thaw deterioration
• Chemical attack and softened surfaces
• Corrosion around reinforcing steel
• Loose or incompatible previous repairs

Sounding, probing, coring, pull-off testing, or engineering evaluation may be required. The contractor should document questionable areas and obtain an approved repair direction before proceeding.

Remove Laitance and Weak Surface Material

Laitance is a weak layer of cement paste and fine material that can form at the concrete surface. It may appear hard and smooth but often lacks the strength needed to support a high-performance lining.

Surface preparation must remove laitance, weak concrete, loose aggregate, curing compounds, sealers, and other material that interferes with adhesion. The objective is to expose sound concrete with an open texture.

Grinding only the highest spots may leave weak material in low areas. The selected method must treat the entire lining surface, including edges, corners, drains, penetrations, and vertical transitions.

Concrete Surface Profile

Concrete Surface Profile, commonly called CSP, describes the texture created by preparation. ICRI benchmark profiles range from relatively smooth surfaces to very rough surfaces used for heavy repairs and overlays.

The required CSP depends on the thickness and type of lining. Thin coatings generally require a finer profile than heavy overlays or mortar systems. Contractors should compare the prepared surface with the specified CSP benchmark and confirm that the complete area is consistent.

The preparation method does not automatically guarantee a particular CSP. Equipment condition, operator technique, concrete strength, number of passes, and existing surface condition affect the result.

Mechanical Preparation Versus Acid Etching

Mechanical methods are commonly preferred for industrial lining work because they remove weak concrete and create a physical profile. Acid etching may be permitted for certain products and conditions, but it has important limitations.

Acid must contact clean, unsealed concrete. Oil, grease, coatings, and some curing compounds prevent uniform etching. The reaction must be controlled, residues must be completely removed, and the concrete must reach the required moisture condition before lining.

Acid etching should not be substituted for specified mechanical preparation without written approval.

Oil and Chemical Contamination

Concrete can absorb oil, grease, chemicals, wastewater, cleaners, and process materials. The contamination may extend below the visible surface and migrate back after cleaning.

Signs of contamination include:

• Dark staining or oily appearance
• Persistent odors
• Water beading on the surface
• Softened, discolored, or chemically attacked concrete
• Residue returning after cleaning
• Unusual surface pH
• Adhesion failure in previous coatings

Cleaning may require detergents, degreasers, steam, hot-water washing, mechanical removal, or removal and replacement of contaminated concrete. Cleaning products and residues must be compatible with the subsequent system.

Efflorescence and Soluble Contamination

Efflorescence appears as a white or light-colored deposit formed when moisture carries soluble material to the surface. Removing the visible deposit does not necessarily correct the moisture source.

Investigate leaks, groundwater, process moisture, failed joints, and vapor movement. The surface must be cleaned, but the underlying cause must also be addressed when it can affect lining adhesion.

Cracks Require Classification

Before repairing a crack, determine whether it appears stable, active, structural, leaking, or associated with a joint. Different conditions require different repair methods.

Stable cracks may be routed, filled, injected, or reinforced according to the approved lining detail. Active cracks may require a flexible treatment or engineered joint system. Structural cracks require evaluation by the owner or qualified engineer.

Rigidly filling an active crack can move the failure to the edge of the repair. Spraying an elastomeric lining over it without an approved detail is also not a guarantee of long-term performance.

Joints Must Remain Joints Unless Redesigned

Expansion joints, isolation joints, and other designed movement joints should not be filled and coated as though they were random surface cracks unless the project design specifically calls for that treatment.

Joint preparation may require removal of old sealant, rebuilding joint edges, installation of backer material, application of a compatible sealant, and use of a reinforced or flexible lining detail.

Wall-to-floor transitions, drains, penetrations, embedded steel, and equipment bases also require approved termination details.

Bugholes, Voids, and Honeycombing

Bugholes and open pores can trap air. As the concrete or trapped air warms, expanding air can travel into a wet lining and create pinholes, craters, or bubbles.

Open voids should be cleaned and filled with a material approved for the lining system. Honeycombed or poorly consolidated concrete may require removal to sound material and rebuilding.

A skim coat or resurfacer may be required to provide a continuous surface. The repair material must be properly cured and prepared before lining.

Repairing Deteriorated Concrete

Remove unsound material until sound concrete is reached. Exposed reinforcing steel may require cleaning, evaluation, and treatment before the concrete is rebuilt.

Repair materials should be selected for:

• Compatibility with the existing concrete
• Compatibility with the primer and lining
• Required thickness and placement orientation
• Service temperature and chemical exposure
• Required strength and movement characteristics
• Available curing time
• Moisture and application conditions

Smooth, dense repair materials may require profiling before the lining is applied. Repair edges and transitions should not leave sharp steps or feather-edged weak material.

Moisture and Vapor

Moisture may enter concrete from curing water, groundwater, cleaning, leaks, process exposure, hydrostatic pressure, or vapor transmission through the slab.

Moisture can interfere with primer penetration, react with certain materials, reduce adhesion, create pinholes, or cause blistering after the lining is placed in service.

Use the moisture-test method and acceptance criteria required by the specification and lining manufacturer. Visual appearance and surface touch are not acceptable substitutes for testing.

Because concrete moisture requires careful evaluation, Article 13 examines when moisture conditions should stop a lining application.

Managing Concrete Outgassing

Air in concrete pores expands as the substrate warms. If primer or lining is applied while this air is escaping, bubbles and pinholes may form.

Application during stable or falling concrete temperatures can sometimes reduce outgassing, but this must remain within the manufacturer's temperature, humidity, and dew-point limits.

Other controls may include sealing bugholes, applying an approved pore-filling primer, using a grout coat, and following the specified application technique.

Removing Dust and Debris

Preparation creates dust, loose aggregate, slurry, and debris. These materials must be removed from the surface and from cracks, joints, bugholes, drains, corners, and ledges.

Industrial vacuuming is commonly used after dry preparation. Water-prepared surfaces must be rinsed as required and allowed to reach the specified moisture condition.

Do not use contaminated compressed air. Equipment used for cleaning should not deposit oil or water on a surface that has already been accepted.

Protect the Prepared Surface

After preparation, restrict traffic and protect the area from dust, water, spills, condensation, exhaust, and other trades. Workers entering the area should use clean footwear and appropriate protective coverings.

If contamination or damage occurs, the affected area must be cleaned, re-prepared, and reinspected before primer application.

Final Inspection Before Priming

The final hold-point inspection should confirm:

• Concrete is sound and free of weak surface material
• The specified CSP has been achieved
• Oil, chemicals, sealers, coatings, and curing compounds are removed
• Cracks and joints have received the approved treatment
• Bugholes, voids, and honeycombing have been repaired
• Repair materials are cured and properly prepared
• Dust, debris, slurry, and cleaning residues are removed
• Moisture conditions meet the system requirements
• Environmental conditions are acceptable
• The area is protected from recontamination

Record inspection results and obtain the required release before applying primer or lining.

Concrete-Preparation Safety

Grinding, shot blasting, scarifying, chipping, and cutting concrete can create respirable crystalline silica, noise, flying debris, vibration, and electrical hazards. Waterjetting introduces high-pressure injection and slip hazards.

• Evaluate silica and other contaminants before work begins.
• Use dust collection, ventilation, water delivery, or other required controls.
• Use properly guarded and maintained equipment.
• Provide required respiratory, hearing, eye, face, hand, and foot protection.
• Use ground-fault protection where electrical equipment may contact water.
• Control slurry, runoff, debris, and slip hazards.
• Follow confined-space procedures where applicable.
• Keep unauthorized workers outside the preparation area.

Common Preparation Failures

• Preparing only the visible high spots
• Leaving laitance or weak concrete beneath the lining
• Producing a profile that is too smooth or too aggressive
• Applying over oil, curing compounds, or previous sealers
• Failing to remove contaminated concrete
• Coating over active cracks without an approved detail
• Filling designed movement joints as though they were cracks
• Leaving bugholes and voids open
• Using incompatible or uncured repair materials
• Ignoring concrete moisture and vapor movement
• Allowing dust or other trades to recontaminate the surface

Contractor Field Checklist

Knowledge Check

Key Takeaway