They can be frustrating,
Easier to just do it yourself,
Not rocket science or an interstate highway,
I have 2 steel culvert pipes, 36” diameter and 60’ long.
Installed in the 50’s
Bottoms rusted out in the 90’s
Slope is good, ordered 3 yards of concrete for each one.
Mixed it soupy and covered the bottom with concrete.
I did make a drag to help with the flow.
Used the concrete chute to analyze the proper consistency.
Still working after 30 years.