Concrete is one of the most popular construction materials in the world because of its high strength and longevity. Today, you’ll find many modern homes made of concrete. They’re easy to maintain, look beautiful and offer other benefits.
But one drawback that must be considered is delamination. Delamination, if left unchecked, can eventually cause a structure or slab to fail.
It’s important to understand what delamination is, what causes it and how to fix it.
What is Delamination?
Delamination is when thin layers of concrete detach from the concrete surface. Once the concrete starts peeling, it becomes prone to damage from the environment and can weaken the structure.
If left unchecked, delamination can cause concrete to fail.
One of the clearest signs of delamination is hearing a hollow sound when hitting the concrete with a heavy chain drag or hammer.
Even reinforced concrete can suffer from delamination when metal reinforcements begin to corrode. Eventually, stresses can lead to cracks that form and spread to nearby cracks from corroded rebar.
Typically, it’s the first 1/8-1/4″ of the concrete that’s affected by delamination. The size of the affected area can range from just a few square inches to a few square feet.
What Causes Concrete Delamination?
Delamination can occur if troweling operations are performed before the concrete’s initial setting time.
When fresh concrete is poured, the solids (aggregate and cement) start to settle. The settling process displaces trapped air and excess water in the mix. This process is known as bleeding.
Naturally, the lighter materials move up toward the surface.
If troweling operations begin before the concrete finishes bleeding, the displaced water and air will become trapped under the surface mortar.
Over time and as the concrete hardens, these trapped elements create subsurface voids that weaken the concrete. Eventually, this can cause layers of concrete to detach.
Delamination can also occur because of other reasons, such as:
- A high evaporation rate
- Use of sticky mortar with a high concentration of fines
- Rushed finishing work
If delamination is isolated to one area (typically 1”-3”), it may not affect the performance of the concrete. However, widespread delamination can quickly become a serious problem and needs to be addressed as soon as possible.
How to Fix Delaminated Concrete?
When delamination occurs, it’s possible to resolve the problem with patching. To patch the area, the surface layer must be removed, and the underlying concrete must be thoroughly cleaned.
- Concrete should be removed to a depth where only sound concrete remains.
- Shotblasting, hydrodemolition or grinding can help remove the defective concrete.
- Once the defective concrete has been removed, a new surface can be applied.
Resurfacing products are available that are formulated to bond with a sound substrate and create a new working surface. It’s best to perform a patch test first to make sure the resurfacing product will work as desired.
How Do You Prevent Delamination?
The most effective and simplest way to prevent delamination is to make sure that the bleeding process is complete before performing the finishing processes.
Once the bleeding and consolidation have been completed, there’s nothing left to be trapped and cause voids.
Knowing when the bleeding process is complete can be tricky, even for experienced finishers.
As a general rule of thumb, you want to:
- Avoid closing or sealing the concrete too early
- Use a broom or light steel-troweled finish for air-entrained concrete
- Keep subgrades covered until you’re ready to place the concrete if temperatures are below 40F
Determining when concrete is done bleeding is both an art and a science, but it’s worth mastering to prevent delamination.
Can You Put a Thin Layer of Concrete Over Concrete?
When delamination occurs, it may be tempting to simply pour another thin layer of concrete to resolve the problem. But does this work?
It is possible to pour concrete over concrete. However, if you do not address the cause of the problem – the voids beneath the surface – it will not resolve the issue.
Putting concrete over concrete is fine as long as it is done properly. Layers can be as thin as just 1.5” over the old concrete, although some experts recommend a thickness of at least two inches.
If you want to pour new concrete over your old, delaminated concrete, make sure that you’re first addressing the delamination by eliminating the defective concrete first. Otherwise, these issues will just carry over into the new concrete.
To make your new concrete pour work, you’ll need to use a bonding agent. Bonding agents will ensure that the new concrete will stick to the sound concrete below.
Without bonding agents, you’ll wind up with two separate layers of concrete rather than one solid one.
There are a few options when joining old and new concrete layers:
A fully bonded overlay, or bonded concrete overlay (BCO), is placed on top of a textured concrete surface and bonds the two layers of concrete together.
If your new concrete layer is less than 3” thick, a fully bonded overlay is the way to go.
Another option here is to use reinforcing bars or steel mesh.
A partially bonded overlay is one that is applied directly onto the concrete surface without a barrier or bond. Generally, partially bonded overlays are not an ideal option for delamination in concrete.
If you want to create two separate layers of concrete, an unbonded overlay may be the right option. With this type of overlay, a layer of material is placed over the existing concrete before a new concrete overlay is placed.
The material creates two separate layers. The separation barrier can help prevent cracks from spreading to the overlay.
Materials for the separation barrier can include crushed stone, sand, plastic sheets, tar paper or even Styrofoam.
Delamination is a concern for slabs and for concrete home owners in general. If the concrete is given enough time to finish the bleeding process, delamination shouldn’t be an issue. However, it’s important to understand how to recognize the signs of this issue and how to remedy the problem before it spreads.