Secondary containment coating: chemical resistant coatings for concrete dikes, pits and sumps

A secondary containment coating is a continuous, chemical resistant lining applied to a concrete dike, pit or sump so that a leak from the primary tank stays contained until it is recovered. Bare concrete cannot do that job: it is porous, and acids, sulfates and chlorides attack the cement paste or the reinforcing steel.

The concrete provides structure and volume; the coating provides liquid-tightness and chemical resistance. Assuming that a well-poured basin is a containment by itself is a common reason containment areas fail their first real spill or their first serious inspection.

Why bare concrete is not a chemical containment

To a liquid, concrete is a network of capillary pores, bleed channels, shrinkage cracks and cold joints. A spilled product migrates into it, and from there into the soil under the slab. Four mechanisms explain why an uncoated basin degrades.

• Porosity and cracking. Even sound concrete absorbs liquid, and every crack, joint and tie hole is a direct path through the section. Concrete that looks dry after a spill has often absorbed the product.
• Acid attack on the cement paste. Hardened cement paste is strongly alkaline. Acids dissolve the calcium hydroxide and calcium silicate hydrates that bind the aggregate, layer by layer: the surface turns sandy, the aggregate stands proud, the section loses thickness.
• Sulfate attack. Sulfates react with the aluminate phases of the cement to form expansive products that crack the concrete from the inside.
• Chlorides and reinforcement. Chlorides reach the reinforcing steel and break down the passive film that alkaline concrete maintains on it. The corrosion products occupy more volume than the steel, and the cover cracks and spalls. A wall can lose structural capacity this way long before it leaks.

A sealer reduces absorption but does not change the chemistry. For acids, solvents and most process chemicals, the concrete has to be isolated by a resin-based barrier.

Secondary containment requirements: what the coating has to achieve

The obligation to provide secondary containment does not come from the coating industry. Depending on the product, the site and the province, it comes from the National Fire Code of Canada as adopted by the province, from provincial regulations on petroleum products, hazardous materials or the environment, and from the site's insurer. Retained volume, rainwater drainage and impermeability to the stored product are set by that framework; read the applicable text for each installation rather than copying a previous project.

For the coating, those requirements become four criteria:

1. Liquid-tightness over the entire wetted surface, including joints, penetrations and wall-to-floor transitions.
2. Chemical resistance to the stored products until a leak is recovered. Resistance charts often distinguish immersion from splash; specify on immersion when leak detection is slow.
3. Durability on site: freeze-thaw, UV on exposed walls, maintenance traffic, standing rainwater.
4. Inspectability: visual checks and, where required, continuity testing after repairs.

Preparing concrete for a chemical resistant coating

The parallel with NACE No. 2 / SSPC-SP 10 preparation of steel is direct: a defined standard, a measurable profile, a hold point before the first coat. For concrete, the joint standard is SSPC-SP 13 / NACE No. 6, supported by two ASTM practices and one ICRI guideline.

Concrete surface profile (CSP). ICRI 310.2R defines ten profiles, from CSP 1 (nearly flat, as after acid etching) to CSP 10 (heavy scarification). Thin-film coatings need a light profile; thick mortars and reinforced systems need a heavier one. The target CSP is not a general rule: it comes from the data sheet of the selected system, and the ICRI replica chips are used on site to confirm it.

Laitance and contamination. The weak, cement-rich surface skin is removed down to sound concrete. Curing compounds, form-release agents, old sealers and oil prevent adhesion; oil that has penetrated has to be removed with the contaminated layer, not washed, as for pump foundations repaired with epoxy grout.

Repairs before coating. Spalls, honeycombing and exposed reinforcement are repaired first with a compatible mortar, which is itself cured and prepared before coating.

Moisture testing before coating concrete

Moisture is the concrete equivalent of soluble salts on steel: invisible, and behind many blistering and disbondment failures. Three tests are common:

• ASTM D4263 (plastic sheet method): a taped plastic sheet; condensation or darkening indicates moisture. Qualitative, a screening test.
• ASTM F1869 (calcium chloride test): measures the moisture vapour emission rate from the slab surface over a defined period.
• ASTM F2170 (in situ relative humidity probes): measures relative humidity inside the slab at a defined depth.

The acceptance limit is not universal: it is the one in the system's data sheet, for the method it names. Some systems tolerate damp concrete, others do not, and the wrong one on a wet slab blisters or disbonds.

Young concrete and cure

New concrete releases water and shrinks for weeks after placement. The common reference is 28 days of cure, but the real criterion is the moisture test and the data sheet, since some systems are formulated for earlier application. Concrete also releases air when its temperature rises; priming while the concrete temperature is stable or falling limits outgassing pinholes. Winter work adds constraints on temperature and cure, covered in our guide on epoxy coating application in cold weather.

Choosing a concrete chemical resistant coating system

There is no single best containment coating. Selection starts from the stored products, their concentration and temperature, then checks each family against the manufacturer's resistance data.

Two rules follow. The lining is chosen for the most aggressive product that can reach it, not the most common one. And service life depends as much on thickness, continuity and cure as on resin family: the failure mechanisms described in our article on epoxy coating service life apply to concrete as they do to steel.

Detailing joints, cracks, coves, penetrations and sumps

Containment linings rarely fail mid-floor. They fail at the details.

• Joints. Static joints are filled and coated over. Moving joints (isolation, expansion) receive a chemical resistant sealant and a flexible or reinforced detail; a rigid coating across a moving joint cracks along it.
• Cracks. Dormant cracks are routed and filled before coating; active cracks are treated as joints or bridged with a reinforced strip. Coating a slab that is still settling only moves the crack into the lining.
• Coves at wall-to-floor transitions. A sharp inside corner concentrates stress and is hard to coat to full thickness. A rounded epoxy mortar cove at the base of each wall gives a continuous, thick transition.
• Penetrations. Pipes, tank anchors and drain valves are sealed with collars or sealant, and the lining is turned up around them. Terminations are keyed into a saw cut or chase so the edge cannot lift.
• Sumps and drain points. The sump sees the product first, longest and most concentrated. It usually receives the heaviest system of the basin, often reinforced.

Quality control on a containment coating

The quality plan has the same hold points as a steel lining, with methods adapted to concrete.

• Before coating: CSP compared with ICRI chips, moisture test to the method named in the data sheet, cleanliness, ambient and substrate temperature, dew point.
• Thickness: magnetic gauges do not work on concrete. Thickness is controlled with wet-film gauges and material consumption per area during application, and after cure with ultrasonic gauges (ASTM D6132) or destructive checks at agreed locations.
• Continuity: ASTM D4787 covers continuity verification of liquid or sheet linings applied to concrete. Concrete is not a reliable conductor, so the method, voltage and any conductive layer have to be agreed before application; every holiday found is repaired and retested.
• Adhesion: ASTM D7234 pull-off testing with a portable tester measures bond to the concrete. The failure mode is recorded along with the value: a failure within the concrete indicates that the bond was stronger than the substrate.
• Cure: the containment is not returned to service until the cure required by the data sheet at the actual temperature is reached.

Inspection and maintenance of a coated containment

A containment is only tested when it is needed, so inspection is scheduled. It looks for cracks, blisters, disbondment at edges and terminations, softening or discoloration from chemical attack, damaged sealants, and mechanical damage.

Standing rainwater reduces the available volume and keeps the lining permanently wet. After any real spill, the lining is inspected once cleaned. Local repairs work when the surrounding lining is sound; widespread blistering points to a preparation or moisture problem a patch will not correct. Relining versus repeated repairs is a life-cycle question, covered in our analysis of the total cost of ownership of a coating.

FAQ

Is a concrete sealer enough for secondary containment? Not for chemical storage. A penetrating sealer reduces water absorption but does not isolate the concrete from acids or solvents, nor bridge cracks.

Can a containment coating be applied on new concrete? Yes, once it has cured and passes the moisture test named in the data sheet, with curing compounds and laitance removed. Some systems allow earlier application on damp concrete.

Which is better for a containment dike, epoxy or polyurea? It depends on the products. Epoxy novolac covers a broad range of acids and solvents; polyurea cures fast and bridges small cracks, but its chemical resistance varies by grade. Compare the resistance charts for the actual chemicals.

How is a coating on concrete tested for pinholes? By continuity testing under ASTM D4787. Concrete is a variable conductor, so the method is agreed before application; some systems include a conductive layer for that purpose.

Does the old coating have to be removed before relining a containment? If it is disbonded, blistered or chemically attacked, yes. A sound coating may be abraded and overcoated if compatible; adhesion tests decide.

In practice

Induscoat prepares and lines concrete secondary containments, sumps and trenches through its protective coatings service for plants in Quebec and Ontario, with surface profile, moisture, thickness, continuity and adhesion recorded at each hold point. See also our concrete protection application. To start, send the stored products, the basin dimensions and a few photos, and request a quote. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.