The most frequent construction-related causes of partial-depth concrete patch failure include the following:
Failure to square the hole:
The shape of repair is important. Repairs should always be square or rectangular. Long, narrow repairs have a much greater tendency to crack than even sided (square) repairs.
Failure to remove all deteriorated material:
A material placed on a weak or deteriorating surface will bond to the surface. However, there is a good chance that the surface will delaminate from itself. Always remove material until the surface is sound.
Good surface preparation is critical for durable repairs. When a repair material is placed on a dusty or dirty surface it will bond to the dust. This is not good! Make sure the surface is clean of all contaminants including oils, sealers, debris, etc. and has a rough profile.
Lack of bond:
Mechanical bond of a fresh material to hardened concrete is dependent on the surface profile. Be sure the hardened concrete is SSD. A rough surface is best. A slurry made from the repair material will also help increase surface tension and bond.
Failure to re-establish the joint:
This will cause compression failure. If you start a repair with a joint(s) the repair needs To be finished with the joint(s).
Variability of the repair material:
Old material (especially in bags) loses its performance and strength characteristics over time.
Deep repairs need to be rodded or vibrated to ensure all air pockets are removed.
To determine the cause of failure, look on the back of the repair material that has been removed or has fallen out. A dusty surface (dust/dirt that was not properly cleaned from the repair zone) or some of the substrate (parent material) indicates the cause of failure.