Heh - I just got back from looking at the same kind of garage - with leaks as you describe.
Generally speaking, the leaks can come from a number of sources.
1. Joint failure in construction joints in the topping slab.
2. Joint failure in expansion joints.
3. Degredation of the concrete along the joints (chloride attack on the concrete matrix will create a more "powdery" concrete that is more porous - i.e. leaking without cracks. This is evidenced by weathered, granular-like surfaces near the joints.
4. Micro cracking in the concrete away from joints.
5. Leaking along perimeter walls, edges, where joint sealants have failed.
6. Leaking around drains or trench drains where the concrete-to-drain seal is bad.
All of the above must be combined with a failure of the waterproof membrane below the topping slab. The problem with membranes such as this is that you NEVER know exactly where the membrane leak is. It isn't necessarily right at the joints that have failed.
The only methods to respond to this (in ascending order of cost and effectiveness:
Option 1 (lowest cost and effectiveness)
1. Grind out all joints and replace new sealant.
2. Re-build damaged joint edges with patching mortar where necessary.
3. Replace and Re-install new expansion joints.
4. Cover with a concrete water resistant sealer.
5. Don't put chlorides on the concrete in winter.
6. Maintain the joints.
Option 2 (better)
1. Do all of Option 1 above
2. Instead of water resistant sealer, provide a traffic bearing waterproof membrane (see Neogard, Carlisle, or Euclid products.)
3. Don't put chlorides on the concrete in winter.
4. Maintain the joints.
Option 3 (best)
1. Remove the topping.
2. Remove the membrane and replace with a new membrane.
3. Replace the topping
4. Provide good joint details
5. Provide water resitant (breathable) sealant.
6. Don't put chlorides on the concrete in winter.
7. Maintain the joints.
I'm sure there are other Options out there - these just come to mind right now.
