Several techniques are available to repair and strengthen reinforced concrete.  The problem will be choosing the one that fixes the problem, causes the least disruption to service, and costs the least.

If the beam is fully accessible and temporary shoring can be erected to unload the beam, all or part of the beam can be removed and then new concrete placed along with appropriate dowels and additional reinforcing steel.

Other techniques that may be useful, but also have limitations include: epoxy injection of cracks, attaching fiber reinforced composite (FRP) material on the exterior faces of the beam, adding new support members to change the loading conditions of the beam, fully casing the existing beam with new reinforced concrete, "stapling" across cracks with structural steel bolted through drilled holes in the beam, etc.

The possibilities really are quite extensive, but the most important thing to realize is that if a repair is made, but the problem that caused the need for that repair in the first place is not identified and fixed, then the repaired beam will eventually end up in the same state again (needing another repair).

~dison