When about 25 years old our garage floor began to disintegrate, but only from the front edge. Thin, parallel cracks would form, then enlarge, and pieces would then break off. This continued for about 10 years, by which time the edge of the slab had retreated about a foot from the door. Last year, when installing a new driveway, the contractor sawed the floor slab back about 3 feet to what seemed to be entirely sound concrete. But now the same process of cracking and disintegration is starting again, even though the sawed edge is protected by the adjacent new and perfectly sound concrete. It seems apparent that the old concrete floor was probably made with a weak mix but that the subgrade has been firm enough to keep it from cracking anywhere except at the exposed edge. Is there any chemical treatment that would harden and strengthen the concrete?
It is not possible to pin down the cause of your trouble without examining the floor. It is possible that it may be an example of D-cracking (see "Preventing D-cracking in Pavements and Flatwork," CONCRETE CONSTRUCTION, July 1975, page 287), an uncommon trouble associated only with some aggregates. The usual chemical surface hardeners such as silicates and fluosilicates would not be of any help. You might try an epoxy resin, preferably one of very low viscosity that would penetrate well into the concrete and around exposed pieces of aggregates when squeegeed liberally over the surface. A reliable epoxy manufacturer should be able to give advice. Anything you could do to keep moisture out of the joint between the old and new concrete will help stop further deterioration.