Strip it, dig out the crumbly bits, fill with cement?
Maybe drill in some wire mesh, and make a form, if there are really big crumbly bits.
Hadn't thought about cement. But that's a good shout.
@NotThamesWater started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
Strip it, dig out the crumbly bits, fill with cement?
Maybe drill in some wire mesh, and make a form, if there are really big crumbly bits.