Concrete Blisters: How to Fix Them

What Are Concrete Blisters?

You've just finished a beautiful new concrete slab, and a few days later, you notice small, hollow bumps—about the size of a dime or a quarter—appearing on the surface. These are called blisters. If you step on one or hit it with a hammer, it will "pop," revealing a hollow void underneath. While they aren't usually a structural threat, they are an eyesore and can eventually lead to surface flaking.

What Causes Blistering?

Blisters are caused by air or water being trapped under the surface during the finishing process. This typically happens for three reasons:

• Premature Finishing: If the contractor starts troweling the surface too early, they "seal" the top layer while the concrete underneath is still releasing air and "bleed water." That air and water have nowhere to go, so they push up the top layer, creating a blister.
• Over-troweling: Using a power trowel or hand trowel too aggressively can "burn" the surface, closing the pores and trapping gases inside.
• High Air Content: If the concrete mix has too much "entrained air" and is then finished with a heavy trowel, blisters are nearly certain.

How to Fix Concrete Blisters

If you only have a few blisters, you can fix them individually. If the entire slab is covered, you'll need a different approach.

Method 1: The "Pop and Fill" (Small Scale)

1. Pop the blister: Use a screwdriver or small hammer to break the thin "skin" of the blister.
2. Clean the void: Use a wire brush or compressed air to remove all dust and loose fragments.
3. Apply a patching compound: Use a high-strength, polymer-modified concrete patch. Squeegee it into the hole and level it with the surrounding concrete.
4. Sand smooth: Once dry, lightly sand the patch to match the surrounding texture.

Method 2: Grinding and Grouting (Large Scale)

If your slab looks like bubble wrap, you can't fix them one by one.

1. Diamond Grinding: Use a concrete grinder to remove the top layer of the slab. This will pop every blister and expose the voids.
2. Slurry Grout: Spread a specialized concrete slurry (a mix of cement, fine sand, and polymer) over the entire slab. This slurry fills every void and "grout" the holes.
3. Final Hone: Once the grout has set, perform a final light grind to remove the excess slurry and leave a smooth, uniform surface.

How to Prevent Blisters (The Local Concrete Contractor Way)

We prevent blisters by practicing **patience**. We never start finishing concrete until the bleed water has stopped and the slab has "gas out." We also use "flat" trowel angles during the initial passes to keep the pores open as long as possible. If we know we're using a high-air mix, we skip the heavy power trowel finish in favor of a broom or slightly more open texture.

The Verdict

Blisters are a sign of a rushed job, but they don't mean your driveway is ruined. Whether you need a few spots patched or a full grind-and-grout, Local Concrete Contractor can restore your surface to its intended beauty. Contact us today for a free assessment.