To effectively bond wet concrete to dry concrete, you need to use a specialized bonding adhesive.
Pouring new concrete directly over old concrete without preparation typically results in a weak bond. This is because, as highlighted in the reference, dried concrete "has no natural bonding agents and won't bond or stick well on another concrete material." Therefore, "You'll need a bonding adhesive to get wet concrete to bond to existing concrete."
Why Bonding Adhesives Are Necessary
When concrete cures and dries, it becomes a rigid, non-porous material that doesn't readily adhere to new, wet concrete. The surface of the old concrete often lacks the necessary chemical or physical properties to create a strong, lasting connection with fresh mix. Without a bonding agent, the new layer is likely to crack, delaminate, or separate from the existing slab over time, especially under stress or traffic.
Bonding adhesives act as a critical intermediary layer, creating a strong link between the old and new concrete surfaces.
What is a Concrete Bonding Adhesive?
A concrete bonding adhesive (sometimes called a bonding agent or primer) is a liquid or powder material applied to the surface of the existing dry concrete before the wet concrete is placed. These products work in several ways:
- Improving Adhesion: They contain polymers or other chemicals that create a sticky surface, allowing the new concrete to grip the old.
- Sealing Pores: Some seal the surface pores of the old concrete, preventing the rapid loss of water from the new concrete, which can weaken the bond.
- Enhancing Durability: They help create a more durable transition zone between the two layers.
Using a Bonding Adhesive: Key Steps
While specific product instructions should always be followed, the general process involves preparing the surface and applying the bonding agent before placing the new concrete.
Preparation is Crucial
Before applying any bonding agent or new concrete, the existing dry concrete surface must be properly prepared. This typically involves:
- Cleaning: Remove all dirt, dust, grease, paint, sealers, and any loose or deteriorated concrete. Pressure washing, wire brushing, or even mechanical grinding may be necessary.
- Making it Sound: Ensure the old concrete is structurally sound. Loose or cracked areas must be repaired or removed.
- Wetting (Optional but Recommended for Some Adhesives): For some bonding agents (like cementitious slurries), the surface may need to be saturated with water beforehand, ensuring it is damp but has no standing water (SSD - Saturated Surface Dry).
Applying the Bonding Agent
- Apply the bonding agent according to the manufacturer's instructions. This is usually done with a brush, roller, or sprayer.
- Ensure even coverage over the entire area where the new concrete will be placed.
- Allow the bonding agent to reach the proper tackiness or drying state specified by the product before placing the new concrete.
Placing the New Concrete
- Place the wet concrete over the prepared surface with the applied bonding agent while the agent is still active (tacky or wet, depending on the product).
- Ensure good compaction of the new concrete to eliminate air pockets and maximize contact with the bonded surface.
Summary Checklist
Here's a quick reference table for the bonding process:
| Step | Description |
|---|---|
| Clean Surface | Remove all debris, grease, loose material from old concrete. |
| Prep Surface | Ensure old concrete is sound; potentially wet surface (SSD). |
| Apply Bonding Agent | Apply evenly to the old concrete surface per product instructions. |
| Wait (if needed) | Allow bonding agent to reach specified tackiness/state. |
| Place New Concrete | Pour and compact new concrete over the prepared, bonded surface. |
By utilizing a concrete bonding adhesive, you create a strong, reliable connection between existing dry concrete and newly poured wet concrete, ensuring the durability and integrity of the repaired or expanded structure.
