Short answer
Concrete pipe corrodes from the crown down. Hydrogen sulphide from sewage collects in the air space at the top of the pipe, bacteria convert it to sulphuric acid, and the acid dissolves the cement. The bottom stays sound while the top thins, spalls and cracks. Concrete that is still round with its crown intact can be lined; concrete with the crown gone or aggregate exposed needs replacement.
The chemistry, briefly
Sewage gives off hydrogen sulphide. In a partly full pipe the gas collects in the air space above the flow, where a family of bacteria living on the damp crown convert it to sulphuric acid. The acid attacks the cement paste, loosens the sand and gravel, and the crown thins. Meanwhile the invert — the bottom of the pipe — is continually washed and stays sound. That is why a concrete line can look fine to a cable and yet be paper-thin on top.
What it looks like on camera
- Early: a rough or "sandy" texture on the upper half of the bore while the bottom is smooth.
- Middle: exposed aggregate at the crown — you can see individual stones; longitudinal cracks along the top.
- Late: missing pieces of crown, soil visible above the pipe, root curtains hanging in through the gap, and a flattened profile as the top settles.
Joints are the other weak point. Concrete sections were joined with mortar, which cracks, so concrete lines carry the same joint and offset problems as clay on top of the corrosion.
Ask the inspector to pause the camera at the worst-looking stretch and tilt the head up to the crown. The bottom of a concrete pipe can look fine for its whole length while the top is missing; a report written from a camera that only looked forward can miss it.
Where it is common
Concrete was the common choice for a lot of interwar and postwar work. We see it in the 1920s–40s houses of Green Lake, Laurelhurst and Magnolia, in postwar Lake City, and in the older parts of Shoreline, Edmonds and Kirkland. Long, flat runs with low flow corrode fastest, because the flow never fills the pipe and the crown is always exposed to gas.
Line or replace?
| Camera finding | Recommended path |
|---|---|
| Rough crown, no cracks, round bore, roots at joints | Line — the liner seals the joints and stops further gas contact with the concrete |
| Cracks along the crown, wall intact, round | Line — the liner supplies structure the crown has lost |
| Exposed aggregate over long stretches | Line only if still round; get a candid view on remaining wall thickness |
| Missing crown, soil or roots entering from above, flattening | Replace — usually by bursting; the host cannot hold a liner |
| One collapsed section in an otherwise lineable run | Spot repair that section, then line the rest |
Why lining works well on mid-stage concrete
A cured-in-place liner is a structural pipe in its own right once cured; it does not rely on the concrete for strength, only for shape during installation. It also puts a chemically inert surface between the sewer gas and what is left of the concrete, which stops the corrosion. So a concrete line caught at the "rough crown, some cracks" stage can be given a new working life without a trench — the UV-cured liners we use are designed for exactly this kind of host.
Why waiting is expensive with concrete specifically
Clay fails at joints and tends to stay lineable for a long time. Concrete keeps dissolving. The window between "lineable" and "collapsing" is narrower, and once the crown goes, the only options are trenchless replacement or excavation. If a camera has shown crown corrosion, it is worth acting while lining is still on the table — the difference in disruption between the two paths is large, especially where the line runs under a driveway or a mature tree.
Questions people also ask
Why does concrete fail at the top and not the bottom?
Because the acid forms in the air above the flow. The bottom of the pipe is wet and washed; the crown is where gas collects and where the corrosion happens.
How old is concrete side sewer in Seattle?
Most was installed between the 1920s and the 1960s, so it is 60 to 100 years old. Some later houses used concrete for the street end of the run and another material near the house.
Can a concrete line with cracks be lined?
If the pipe is still round and the cracks are not accompanied by missing wall, yes — the liner restores structure. Once the crown is gone or aggregate is falling in, the host cannot hold the liner's shape and replacement is the answer.
Is exposed aggregate on camera serious?
Yes. Exposed stones mean the cement between them has dissolved. The wall is thinner than it looks and the section is approaching collapse.
More of the questions we hear every week are on the FAQ page .
If this is your problem
Sewer lining
The service page — what we do, how we decide, and what you get in writing.
OpenOr just ask us
(425) 478-3133Open 24/7/365. Estimates and second opinions are free — see our current offers .