What a manhole is up against
Manhole rehabilitation repairs service in Denver, Colorado exists because a manhole lives in the harshest spot on the whole sewer system. Sewage drops into it, splashes, and turns over, releasing hydrogen sulfide gas. Bacteria on the damp walls above the flow line convert that gas to sulfuric acid, which eats concrete, mortar and brick from the inside out. Add traffic loads pounding on the frame and cover, freeze and thaw where that applies, groundwater pushing in through every joint, and the occasional utility contractor knocking into the chimney, and a structure that looked permanent when it was set can be crumbling within a few decades.
Types of manhole damage
- Surface corrosion. The wall has gone soft and sandy, aggregate is exposed, and a screwdriver sinks in. Wall thickness is being lost year by year.
- Infiltration. Clean groundwater streams in through the chimney, the joints between precast sections, the pipe penetrations or the bench. It overloads the system in wet weather and carries soil with it.
- Missing mortar and loose brick. In older brick manholes the mortar goes first, then bricks fall into the channel.
- Bench and invert damage. The channel that guides flow across the bottom erodes, causing turbulence, splashing and more gas.
- Frame and chimney failure. The adjustment rings under the frame crack from traffic, letting in surface water and letting the cover rock.
How a manhole is rehabilitated
The structure is taken out of service briefly, or flow is plugged and bypassed around it. The interior is pressure washed to remove loose material and the corrosion layer, and active leaks are stopped with fast-setting hydraulic cement or injected chemical grout. Where wall thickness has been lost, a cementitious mortar, often modified with calcium aluminate for acid resistance, is sprayed or hand-applied to rebuild the profile to its original dimension. The bench and invert are reformed so flow moves cleanly across the bottom. Then, in an aggressive environment, an epoxy or similar protective lining is applied over the rebuilt surface at a specified thickness, typically in two coats, to create a seamless barrier that acid and water cannot penetrate. The chimney and frame are sealed with a flexible internal seal or rebuilt with new rings and an adjusted frame.
The whole sequence for one manhole is usually a single day, sometimes two if the rebuild is heavy or the epoxy needs a longer cure before flow returns. Because it is done from inside, the road above is not opened and the structure is not replaced. This is the companion work to manhole and main rehabilitation in Denver: mains and manholes are two halves of the same system, and rehabilitating one while leaving the other to corrode wastes the money spent.
Surface prep decides whether an epoxy lining lasts. Epoxy applied over a wet, dusty or still-corroded wall will peel in a season. The specification should call for the substrate to be cleaned to sound material, leaks stopped, moisture checked, and the cementitious layer cured before the coating goes on. Ask for the thickness readings and the holiday test results as deliverables.
Cementitious, epoxy or both
A cementitious rebuild alone restores structure and is often sufficient in a manhole with moderate deterioration and low hydrogen sulfide exposure. An epoxy or polyurethane lining alone works on a structurally sound manhole that needs corrosion protection. The two together are used where the wall has been lost and the atmosphere remains corrosive, which describes most manholes at pump station discharges, force main outlets and downstream of long, flat runs where sewage goes septic. A rehabilitation contractor should measure hydrogen sulfide or at least identify these locations before choosing the system, because the difference in service life between a bare mortar rebuild and a coated one in a corrosive manhole is large.
Where manholes fit in a lining program in Denver
Most sewer relining projects on private mains and campus systems inspect and, where needed, rehabilitate the manholes at the same time. The bypass and traffic control are already set, the crew is already there, and a liner terminated into a leaking manhole will show groundwater running in around its ends on the first post-project inspection. Property managers and associations with older shared systems should have every manhole on the survey list, not just the pipes between them. Reading about large-diameter and structure lining work shows how the two are scheduled together, and verified CIPP installers serving Denver identifies installers who have documented their manhole and structure rehabilitation capability alongside their pipe lining.
Service life and maintenance
A properly rebuilt and coated manhole is generally expected to last decades in service, with the coating manufacturers stating long design lives when the specification is followed. A visual check every few years from the surface, looking for coating blisters, new infiltration or a rocking frame, catches problems early. The structures most worth checking are the ones just downstream of any point where sewage is likely to turn septic, because that is where the corrosion clock runs fastest.
Common questions in Denver
Does manhole rehabilitation in Denver require replacing the manhole?
Rarely. Corroded, leaking or crumbling manholes are almost always rebuilt in place with cementitious mortar and, where needed, an epoxy lining. Replacement is reserved for structures that have collapsed or shifted.
What is the difference between cementitious rebuild and epoxy lining?
The cementitious layer restores lost wall thickness and structure; the epoxy layer is a seamless protective coating that keeps acid and water off the rebuilt wall. Many manholes need both.
How long is a manhole out of service?
Typically one working day. Flow is plugged or bypassed while the interior is cleaned, rebuilt and coated, and returned once the materials have cured.
Why do manholes corrode faster than the pipes?
Turbulence in the manhole releases hydrogen sulfide from the sewage, and bacteria on the damp walls convert it to sulfuric acid, which attacks concrete and mortar above the flow line.
Should manholes be done at the same time as sewer lining?
Yes, whenever possible. The bypass and access are already in place, and a lined main terminated in a leaking manhole will still admit groundwater at the ends.