Bonding fresh, plastic concrete to old, hardened concrete increases the strength of the composite material. Bond is not, however, guaranteed. It must be ensured throughout proper surface preparation, material choice and use, and curing.
All damaged, loosened, or unbonded portions of existing concrete should be removed by chipping hammers or other mechanical methods.
The edges of the defective area should be chipped or cut straight and at right angles to the surface, or slightly undercut to provide a key at the edge of the patch.
The most practical and economical bonding agents are sand-cement and water-cement grouts. Epoxy resin grouts specially formulated for each application also are on the market. ACI 503 details the use and specification of epoxies for bonding fresh to hardened concrete. Regardless of the bonding medium, a minimum bond strength is required. Based on laboratory and field tests, the bond strength of 200 psi is generally used as a guide in designing bonding media.