Understanding Three-Sided Bonding in Concrete Joints
Understanding why three-sided bonding occurs—and how to prevent it—is one of the keys to achieving long-lasting concrete joint repairs.
Why Three-Sided Bonding Causes Joint Sealant Failure
One of the most common mistakes made when installing joint sealants or crack fillers is allowing the material to bond to the bottom of the joint. This is known as a three-sided bond, and it can dramatically reduce the amount of movement the joint material can accommodate before failure.
What Is a Three-Sided Bond?
Ideally, a joint sealant or crack filler should bond only to the two vertical sides of the joint.
As concrete expands and contracts because of changes in temperature, moisture, loading, and other environmental conditions, the joint material stretches and compresses between those two bonded surfaces.
If the material also bonds to the bottom of the joint, it becomes restrained on three sides instead of two. Rather than stretching freely, the material becomes locked in place, creating much higher internal stresses as the joint moves.
The result is often premature failure through tearing, adhesive failure, or cohesive failure.
How Do You Prevent a Three-Sided Bond?
The solution is simple.
Before installing the joint sealant or crack filler, place a properly sized closed-cell backer rod or another suitable bond breaker into the joint.
The bond breaker prevents the material from adhering to the bottom of the joint while still allowing it to bond securely to the two sidewalls.
This allows the joint sealant or crack filler to flex as the joint opens and closes, maximizing its ability to accommodate movement.
Additional Benefits of a Bond Breaker
A bond breaker also serves another important purpose.
It helps keep the material where it belongs.
Without a backer rod or bond breaker, the material may continue flowing down into the crack below the joint. In some cases it may even flow into voids beneath the slab or into the underlying soil.
Using a bond breaker reduces unnecessary material consumption while helping maintain the proper sealant geometry within the joint.
Selecting the Proper Backer Rod
The backer rod should generally be slightly larger than the width of the joint so it fits snugly without stretching.
Install it at the proper depth to achieve the recommended joint dimensions for the sealant or crack filler being used.
Closed-cell backer rod is suitable for many applications. In situations where outgassing may be a concern, other types of bond breakers may be appropriate.
Summary
A properly installed bond breaker is one of the simplest and least expensive steps you can take to improve long-term joint performance.
By preventing three-sided bonding, the joint material can move as it was designed to move, reducing internal stresses, improving durability, and extending the service life of the repair.
