[mazz44]

Let the bonding agent become tacky and I would recommend drilling some 1/2×1” deep holes in existing for better bond. Use a masonry bit.

[Karliner]

Let the bonding agent become tacky and I would recommend drilling some 1/2×1” deep holes in existing for better bond. Use a masonry bit

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Good idea, drill holes, cut rebar, make a type of lattice with rebar in to the drill holes. Fill holes with concrete and finish.