How to Glue Concrete to Concrete | A Bond That Actually Holds

Getting new concrete to stick to old concrete is a job where the prep work decides everything. If you bond to a dirty or smooth surface, the joint fails no matter which adhesive you buy. The working method for gluing concrete to concrete comes down to surface preparation, the right product, and following the cure time exactly.

This guide covers the full process, the common mistakes that wreck bonds, and what to look for in a product before you start. If you are comparing adhesives right now, our roundup of the best concrete glue options breaks down the top choices tested on real projects.

What Kind of Glue Bonds Concrete to Concrete?

Two product types handle this job: structural epoxies and concrete bonding adhesives. Epoxies like Sika’s Sikadur line are two-part systems that create a strong, rigid bond. Bonding adhesives like QUIKRETE’s Concrete Bonding Adhesive (No. 9902) are designed to permanently bond new concrete, plaster, and stucco to existing concrete, plaster, and stucco.

Sika’s product lineup shows there is no universal glue. Their Sikadur-31+, Sikadur-31 SBA Cold Weather, Sikadur-31 Hi-Mod Gel, Sikadur-32 Normal, and Sikadur-30 each have different application limits, cure behavior, and prep requirements. Match the product to your specific job rather than grabbing whatever is on the shelf.

Epoxy structural adhesives are meant for specific structural or repair use cases, not as a general-purpose construction glue. Read the technical data sheet for your project before buying.

How Do You Prepare Concrete for Bonding?

Sika’s official guidance for Sikadur-31+ says to mechanically prepare surfaces to an open-textured profile, then apply the mixed adhesive with a spatula, trowel, notched trowel, or gloved hand.

Here is the preparation sequence that works:

• Clean thoroughly. Remove dust, laitance, grease, curing compounds, waxes, loose material, ice, and other contaminants. Sika states substrates must be sound, clean, and dry or matt damp with no standing water.
• Roughen the surface. Use abrasive blast cleaning, scabbling, grinding, needle gunning, or similar methods to create an open-textured profile. A smooth surface will not hold.
• Verify the concrete is cured. Sika specifies concrete and mortar should be at least 28 days old before bonding.
• Check for moisture. The surface can be damp but never have standing water, ice, or frost present.

For damp prepared concrete, Sika says to brush the adhesive well into the substrate. Working the epoxy into the surface matters more than people realize.

What Is the Right Way to Apply the Adhesive?

Mix both components exactly as directed until you achieve a uniform color and consistency. Sika’s cold-weather epoxy calls for mixing a minimum of 3 minutes at low speed, scraping the sides of the pail as you go. Do not mix only part of a prebatched unit when the instructions prohibit it.

Application thickness depends on the product. Sika’s Sikadur-31 SBA Cold Weather guidance for bonding hardened concrete to hardened concrete says to spread the mixed epoxy 1/8 in. (3 mm) to one face or 1/16 in. (1.5 mm) to both faces, depending on project requirements. The glue line for Sikadur-31 Hi-Mod Gel should not exceed 1/8 in. (3 mm).

For optimum adhesion, apply adhesive to both surfaces when the product specifies it. Press the parts together firmly, then support heavy vertical or overhead components until fully hardened. Watch the open time and pot life, and place fresh concrete or parts while the adhesive is still tacky where applicable.

Common Mistakes That Ruin a Concrete Bond

The failures people see with concrete adhesives almost always trace back to the same few errors. Bonding to a dirty, dusty, or greasy surface instead of a clean, open-textured substrate tops the list. Sika’s official documentation lists ice, dirt, oil, grease, coatings, laitance, efflorescence, and loose friable material as contaminants that must be removed.

Applying to wet surfaces with standing water, ice, or frost present is another frequent cause of failure. Skipping the mechanical roughening step or trying to bond to a smooth surface also guarantees a weak joint. Exceeding the specified glue-line thickness, leaving mixed epoxy past its pot life, and not supporting vertical pieces during cure round out the common errors.