How to Effectively Bond New Concrete to Old: A Step-by-Step Guide

Understanding the Challenges of Concrete Bonding

Bonding new concrete to old can be tricky, as fresh concrete doesn’t naturally adhere well to existing dry surfaces. However, with the right techniques and materials, you can create a strong, lasting bond between old and new concrete.

Preparing the Existing Concrete Surface

1. Clean the surface thoroughly, removing all dirt, debris, and loose particles using a pressure washer or garden hose with a strong nozzle.

2. For stubborn stains or deeply embedded contaminants, use a wire brush or consider mechanical abrasion methods.

3. Address any structural issues or deep cracks before proceeding with the bonding process.

Choosing the Right Bonding Method

There are two main approaches to bonding new concrete to old:

1. Using a liquid bonding agent: Apply the agent to the existing concrete and allow it to become tacky before adding the new concrete.

2. Creating a “scratch coat”: Mix a small batch of concrete to a soupy consistency and brush it onto the old surface just before applying the new concrete.

Applying the Bonding Layer

1. If using a liquid bonding agent, paint it onto the existing concrete with a brush and let it dry until tacky.

2. For the scratch coat method, mix a small amount of concrete to a thin, paintable consistency.

3. Apply the cement paint to the dampened old concrete surface using a paintbrush, ensuring full coverage.

Mixing and Applying the New Concrete

1. Prepare your concrete mix according to the manufacturer’s instructions, aiming for a peanut butter-like consistency.

2. Add water to your concrete mix just before application to ensure optimal workability.

3. Pour the new concrete onto the prepared surface immediately after applying the bonding layer or while the scratch coat is still wet.

Finishing and Curing

1. Smooth and shape the new concrete to match the surrounding surface texture.

2. Allow the repaired area to cure properly, keeping it damp for at least 24 hours to ensure proper bonding and strength development.

3. Protect the newly repaired area from foot traffic or heavy loads for several days to allow for complete curing.