How do you bond new concrete to an existing hardened surface?

• A. Roughen, clean, and apply a bonding agent or epoxy
• B. Moisten surface
• C. Apply more water
• D. Use solvent

Answer: (A)

To bond new concrete to a hardened surface, you need both a good mechanical key and a chemical adhesive action. Start by roughening the old surface to create a key for the new concrete, then clean thoroughly to remove laitance, dust, oil, and curing compounds. After that, apply a bonding agent or epoxy specified for concrete. This combination—mechanical roughness plus a bonding agent—creates the strong adhesion needed for a durable bond between old and new concrete. Simply moistening the surface or adding water won’t provide the necessary interlock or chemical adhesion, and solvents won’t establish the bond either; they may even leave residues that hinder cement hydration.