Coating plant concrete trenches and pits: selection and details

A plant concrete pit or trench is coated according to its dominant attack: novolac epoxy or vinyl ester for strong chemicals, urethane cement for hot water and thermal shock, filled resin mortar for abrasion by solids. Coating life depends as much on details as on the product: coves at corners, joints carried through, terminations in anchor grooves around drains and edge angles, concrete prepared down to sound material and a continuity test before return to service.

Pump pits, sumps, process trenches and washdown gutters receive whatever the floor drains: spills, hot washdown water, slurries, sometimes continuously. Bare concrete is attacked faster there than anywhere else, and the rebar follows. This guide is written for maintenance managers and project managers of chemical, metallurgical, food, pulp and paper and mining plants in Quebec and Ontario. It complements our article on secondary containment coating, which covers large containment areas.

Why do pits and trenches deteriorate faster than the floor?

Because they concentrate every attack in one place: products accumulate instead of draining, water stands, washdown heat builds up, solids scour and cleaning is frequent. The concrete is in almost permanent contact with the service environment.

Typical mechanisms:

• Chemical attack. Acids dissolve the cement paste and expose the aggregate; some salts and strong alkaline solutions also degrade concrete.
• Thermal shock. Hot water followed by cold water makes the coating and the concrete move differently, causing cracks and debonding.
• Abrasion. Slurries, sand and chips wear bottoms and corners.
• Rebar corrosion. Once the concrete cover is attacked, the steel rusts, expands and spalls the concrete.
• Gases. In wastewater structures, H2S produces an acid that attacks the walls, as our article on H2S concrete corrosion explains.

Which coating for which attack?

Choose first by the most severe attack, then check temperature, abrasion and the shutdown window. The table gives the families generally considered; the manufacturer's chemical resistance chart, for the actual chemicals, concentrations and temperatures, decides.

Our comparison of vinyl ester vs novolac epoxy helps choose for severe chemical service, and our article on urethane cement vs epoxy helps in hot-water areas.

How do you detail joints, corners and construction joints?

Round corners with mortar coves, carry movement joints through the coating with a compatible sealant, and treat construction joints and cracks before coating. Failures almost always start at corners and joints.

• Coves. A sharp corner between floor and wall is hard to cover at the intended thickness; a resin mortar cove gives a continuous transition.
• Movement joints. Never bridged with a rigid film: they are carried through with a sealant resistant to the same service as the coating.
• Construction joints and cracks. Treated before coating; cracks that let water through are injected first, see our comparison of epoxy vs polyurethane crack injection.
• Top edges. At the top of the wall, the coating ends in an anchor groove or runs up onto the adjacent floor so liquid cannot get behind it.

How do you tie the coating into drains, gratings and edge angles?

Tie the coating into metal parts by preparing and coating them continuously when the system allows, or by ending the coating in an anchor groove sealed with sealant. A termination that is simply feathered against the metal eventually lifts and lets liquid under the film.

• Edge angles and grating frames. The steel is prepared by blasting or power tools, then primed with a compatible primer before the coating, or the coating stops in a groove along the angle.
• Drains and floor sinks. The joint uses a clamping ring when the drain has one, otherwise a groove and sealant.
• Pipe penetrations. Sealed sleeves or collars, with the coating turned up the pipe as the data sheet allows.
• Gratings. Removed during the work and protected separately, for example by galvanizing or painting.

How do you prepare wet or contaminated pit concrete?

Remove attacked concrete down to sound material, degrease and rinse contaminants, rebuild sections, then mechanically prepare the surface to the required profile and let it dry to the moisture level the coating accepts. In a pit, this is often the longest step.

Concrete soaked with oil or chemicals can prevent adhesion even after careful cleaning; it is then removed to a sufficient depth. Hydro blasting works well for removing deteriorated concrete without microcracking sound concrete, but it requires drying before coating. Repairs follow our guide to concrete repair before coating, and final preparation our guide to concrete preparation and CSP profiles. On damp concrete, some systems tolerate moisture: the data sheet says so.

How do you work during a short shutdown?

Work during a short shutdown by preparing everything possible before it starts, splitting the structure into sections, choosing a system whose cure fits the window and controlling temperature. Return to service follows the full cure time required for chemical exposure, not just the tack-free time.

Usual levers:

• Before the shutdown. Survey, adhesion tests, system selection, equipment and products on site.
• Bypass. Effluent diverted or pumped to keep the pit dry during the work.
• Fast systems. Some resin-based systems cure quickly, including at low temperature; their chemical resistance must still fit.
• Heating and ventilation. To hold the product's minimum temperature and remove vapours.
• Confined space. A pit is often a confined space: permits, attendant, gas detection and rescue plan apply.

Our article on planning coating work during a plant shutdown details how to prepare the window.

How do you inspect the coating before return to service?

Check thickness, appearance, continuity by holiday test to ASTM D4787 for coatings on concrete, and adhesion by pull-off test to ASTM D7234 where needed. Results are recorded before return to service.

A single holiday in a chemical pit is enough for liquid to get under the film and attack the concrete from below. The continuity test is therefore especially useful in these structures. Corners, coves, drain connections and edge terminations deserve the closest look, because that is where the film is hardest to build and where failures usually begin. Our coating inspection service can support these checks and the close-out file.

How do you prepare the project and get a quote?

Prepare the project by documenting, for each pit or trench, the chemicals and temperatures in contact, dimensions, concrete condition, metal parts and the shutdown window. This data sets the system, the details and the phasing.

Pit and trench coating is part of our protective coatings and concrete protection work. Our crews and qualified subcontractors work from LaSalle, including in Bécancour, Montreal and Ontario. Send photos, dimensions, the list of chemicals and temperatures and your shutdown constraints through the quote request: pricing on consultation, reply within 24 business hours.

Frequently asked questions

Can a pit that stays damp be coated?

Yes, with a system that tolerates moisture per its data sheet, after water inflows are stopped and the concrete is prepared. If water comes through the wall, stop it first by injection or drainage.

Does a floor coating work in a process trench?

Not necessarily. The trench receives more concentrated, hotter chemicals for longer than the floor. Check the system's chemical resistance for prolonged contact, not just for splashes.

Should trench angles and gratings be coated?

Edge angles are prepared and tied into the coating, or the coating stops in a groove along them. Gratings are removed and protected separately.

How long before a pit goes back into service?

The time depends on the system, temperature and exposure. For chemical immersion, wait for the full cure stated on the data sheet, which is longer than the time for foot traffic.