"EricP" wrote
| It may be concrete cancer. If they have iron reinforcing inside
| them, it's rusting and causing the cracks. If so, replacement
| is the only solution as no matter what you fill with, the rust
| is unlikely to stop.

In the meantime a temporary way of holding things together might be to drill
holes either side of the crack and well clear of it, chisel out a
recess/trough linking the two holes, and drop a U shaped steel staple into
epoxy into the holes, to hold the two halves together. Then make good over
to prevent water/frost ingress.

Owain