Instead of jumping to the conclusion that an expensive safety device is faulty and replacing it, look at the wiring itself. Somewhere, likely near where the concrete was broken up, there are loose or damaged wires. The fact that the old breaker and the new one are tripping is a very strong indicator that they're doing their job of saving your life and preventing your house from burning down.
You indicate that you opened & inspected all boxes on the circuit. Either you missed one, or a wire was cut between boxes, or your inspection wasn't thorough.
I'd suggest a re-inspection:
- Power off (easy one)
- Open a box (preferably closest to the breaker)
- Pull the devices out of the box but do NOT disconnect anything.
- Label cables to identify where they go.
- Take a picture of all the wiring with the labels showing
- Remove all the cables, put wire nuts on them all
- See if the breaker will turn on
- if the breaker turns on, the problem was in the wiring in the box or further down the line.
- If the breaker doesn't turn on, it's between the breaker & this box.
- Wire up just one device in this box (ignoring everything down stream).
- Test the breaker for functionality again.
Lather. Rinse. Repeat. those last 3 steps - disconnect everything, test the breaker, add one item at a time. So long as the breaker/GFCI will turn on and stay on after each step, you're doing it right. Make sure you turn the power back off before going back to the box to do more work!
If/when you get to a device that will trip the breaker every time it's turned on, then you've got a bad device (it's possible, they do fail over time). Replace that one device, then continue your process.
Be sure to label every wire & device and take pictures before you disassemble to ensure you get it put back together the same way. Assuming no wiring changes were made, you'll have a correctly wired system when you're done, too.