Manhole Rehabilitation: A Technical Guide to Coatings and Methods

Manholes are among the most corrosion-susceptible elements of the municipal sewer collection system. Their position at the interface between the submerged sewer pipe and the above-ground atmosphere creates the conditions that produce the highest hydrogen sulfide concentrations and most aggressive sulfuric acid generation in the collection system. Simultaneously, manholes are among the most frequently inspected and accessed sewer structures, making their condition directly visible to operations staff and the public, and their deterioration one of the most common triggers for rehabilitation investment decisions.

Why Manholes Are Vulnerable

The mechanisms that make manholes particularly vulnerable to biological corrosion are related to their geometric and hydraulic characteristics. At manhole inlet and outlet connections, changes in pipe diameter, direction, and gradient create turbulence that accelerates gas transfer from the wastewater stream to the sewer atmosphere. The manhole invert — the trough connecting inlet and outlet pipes — is typically shallower than the connecting pipes, increasing wastewater surface area and gas transfer rate per unit of wastewater volume. The confined geometry of the manhole barrel concentrates H₂S in the air space above the waterline, producing higher crown-zone H₂S concentrations than adjacent straight-run gravity sewer.

The result of this elevated H₂S exposure is that manhole crowns and upper walls deteriorate 3-5 times faster than comparable sections of the adjacent gravity sewer. Manholes that are nominally less than 50 years old are commonly found in conditions requiring rehabilitation, while the connecting sewer sections remain structurally sound. Systematic manhole rehabilitation programs that address corrosion before structural failure occurs are therefore essential components of collection system asset management.

Assessment and Condition Rating

Accurate condition assessment is the prerequisite to cost-effective manhole rehabilitation. The National Association of Sewer Service Companies (NASSCO) Manhole Assessment and Certification Program (MACP) provides a standardized inspection protocol that characterizes defects by type, location, and severity using a consistent coding system that enables objective condition rating and prioritization across a manhole inventory of any size.

Inspection methods include visual inspection by certified inspectors using portable lighting, close-circuit television (CCTV) cameras deployed on pole-mounted or rope-lowered mounts, and laser profiling systems that measure interior surface geometry and identify corrosion-related dimensional loss. Concrete core sampling and laboratory testing quantify the remaining concrete strength and depth of deterioration for structures where visual inspection suggests advanced structural compromise.

Polyurea as the Standard of Care

For manhole rehabilitation in sewer environments, polyurea spray-applied coating represents the current standard of care based on service life, crack-bridging performance, application speed, and return-to-service time. A standard manhole rehabilitation specification using polyurea involves high-pressure water jetting to prepare all interior surfaces, repair of active infiltration points and structural defects with compatible cementitious materials, prime application where specified by the polyurea manufacturer for the existing surface conditions, and spray application of a minimum 125-mil polyurea coating to all interior surfaces including walls, cone, chimney, and invert.

Application of this thickness at typical productivity rates requires 2-4 hours for a standard 48-inch diameter, 10-foot depth manhole using a two-person crew. The structure can typically be returned to service within 2 hours of coating completion, allowing the majority of rehabilitation work to be accomplished within a single shift without extended traffic or system disruption.

Grouting and Infiltration Control

Active water infiltration through manhole walls and pipe-to-manhole connections must be eliminated before coating application. Chemical grouting, using hydrophilic polyurethane or acrylamide gel grouts injected through drilled ports in the manhole wall, provides effective sealing of active leaks. The grout reacts with the inflowing water to form an expanding, water-reactive foam or gel that fills the crack or void and stops water movement. After grout injection and confirmation of infiltration elimination, cementitious repair materials are used to restore the surface profile before polyurea application.