Bonding Old to New Concrete at Vertical Faces: A Step-by-Step Timeline

Define what the repair or new-to-old joint must do: carry the intended loads, resist moisture intrusion where required, and remain durable under the site’s expected movement and environmental conditions.

Common failure modes to prevent include delamination at the interface, shear slip under load, and differential movement that opens gaps or induces cracking; check product data sheets and design documents for compatible solutions and limits.

Intended performance criteria

The main goal is to create a strong, durable bond between old and new concrete at vertical faces. Here’s what we’re aiming for:

Bond Strength: At least 60% of the strength of the weaker material. This ensures proper load transfer.

Shear Capacity: Enough to handle expected loads without slipping or cracking at the interface.

Durability: Resistance to weathering, corrosion, and other environmental factors over the structure’s lifespan.

Typical failure mechanisms

Without proper bonding, you’ll face issues like delamination or shear slip. Here’s how:

Lack of Keys: Without proper keys (dowels, grooves), the new concrete can’t grip the old, leading to slippage under load.

Poor Surface Preparation: Inadequate cleaning, roughening, or wetting can result in weak adhesion and eventual failure at the interface.

Incorrect Adhesive Choice: Using the wrong adhesive (like cement slurry for high-strength applications) can lead to insufficient bond strength and early failure under load.

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Keys are geometric features—cast recesses, milled pockets, or formed lips—intended to provide positive interlock and back up adhesive or grout bond with mechanical shear transfer.

Choose a key type and layout that provides redundancy and aligns with the structural intent; verify dimensions and edge details against engineering drawings or manufacturer guidance before forming or milling.

Cast-in vs milled keys

Keys can be formed during casting (cast-in) or created after the concrete has cured (milled).

Cast-in keys are cheaper and faster. They’re formed using wooden or metal strips placed in the formwork before pouring.

However, they require precise formwork setup and can be damaged if not properly protected during curing. Also, any issues with the formwork will transfer to the key.

Milled keys, on the other hand, are cut into cured concrete using diamond blades or other tools. They’re more flexible as they can be created after any issues with the existing surface have been addressed.

But milling is slower and more expensive due to the additional labor and equipment required.

Key geometry and placement considerations

When specifying keys, consider these factors:

Depth: Deeper keys provide better interlock but require more concrete. Aim for at least 1.5 inches (38mm) deep.

Width: Wider keys distribute stress better. A good rule of thumb is to make them at least 2 inches (50mm) wide.

Spacing: Closer spacing increases shear transfer but also requires more concrete. Aim for no more than 6 inches (150mm) between key centers.

Pattern: Keys can be arranged in various patterns like staggered, grid, or herringbone. Consult with your structural engineer and manufacturer instructions to determine the best pattern for your project.

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Dowels and shear connectors transfer tensile and shear forces across a vertical plane, reducing reliance on the bond line alone and helping control relative movement between old and new concrete.

Select connector type (straight bars, hooked ends, swaged bars, or mechanical couplers) and spacing to match the design load path, and coordinate locations with existing reinforcement and embedment requirements as specified by the engineer or product instructions.

Dowel types and anchorage methods

Dowels are the backbone of load transfer across vertical faces. Let’s look at common dowel types and their anchorage options.

Through-cast dowels: These are simple, embedded in both old and new concrete. They’re easy to install but lack additional strength.

Epoxy-anchored dowels: Epoxy provides extra grip, boosting load capacity. Use them where higher loads are expected or when space is tight for grout-filled sleeves.

Grouted sleeve dowels: Sleeves allow for easy installation and adjustment. Grouting provides additional strength but requires more time and material.

Layout, embedment, and edge distance guidance

Proper layout and embedment are crucial for dowel performance. Here’s how to get it right.

First, consult your structural drawings: They’ll tell you the exact dowel spacing, size, and type needed.

Next, consider embedment depth: Dowels should be embedded at least 5 times their diameter in both old and new concrete for optimal performance. More embedment is better, up to a point – too much can lead to stress concentrations.

Lastly, maintain proper edge distance: Keep dowels away from edges to prevent splitting. A safe rule of thumb is 10 times the dowel diameter from any edge or face.

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Structural epoxies bond through chemical adhesion and can provide high immediate strength, while cementitious slurries or bonding agents rely on cement hydration and mechanical interlock with the substrate.

Compare cure behavior, moisture sensitivity, service limitations, and compatibility with the substrate and new concrete; consult the adhesive or bonding agent technical literature and the project’s performance requirements to decide which system fits the job.

Advantages and limitations of epoxy

Epoxy adhesives are the go-to choice when you need a strong, reliable bond between old and new concrete at vertical faces. They offer several advantages:

High Adhesive Strength: Epoxies create a powerful chemical bond with both old and new concrete surfaces.

Gap-Filling Ability: Epoxy can fill small gaps and irregularities, making it ideal for bonding uneven or damaged surfaces.

However, epoxies have their limitations. They are sensitive to moisture and temperature changes during curing. This means you need to prepare the surface properly and ensure optimal curing conditions. If not, the bond could weaken over time.

Advantages and limitations of cement slurry or mortar bonding

Cementitious slurries or mortars are another option for bonding old to new concrete. They work differently than epoxies but offer their own set of advantages:

Mechanical Bonding: Cement slurries rely on mechanical interlocking with the concrete surface, similar to how keys and dowels work.

Chemical Bonding: They also form a chemical bond with the concrete, but this is less strong than with epoxies.

Cement slurries are more compatible with concrete’s natural movement due to moisture changes. This makes them a good choice when there might be some movement in the structure over time. However, they don’t fill gaps as well as epoxies and may not be as strong under high loads.

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Good bond starts with the substrate: remove laitance, loose material, oils, and contaminants and establish an adequate surface profile to allow the bonding system to engage the concrete mechanically and/or chemically.

Use appropriate methods—mechanical scabbling, shotblasting, grinding, or pressure washing—and confirm cleanliness and profile per the product recommendations or testing protocols before applying adhesive or new concrete.

Methods: grinding, sandblasting, hydrodemolition, scabbling

Preparing the surface for bonding is crucial. Here are some common methods:

Grinding: Uses diamond blades to create a rough profile. It’s clean but can be slow and may damage edges.

Sandblasting: Blasts away contaminants with high-pressure sand. It’s fast but creates dust and can cause substrate damage if not done right.

Hydrodemolition: Uses high-pressure water jets to remove concrete. It’s clean, fast, and won’t damage reinforcement, but it’s expensive.

Scabbling: Uses a machine with rotating blades to roughen the surface. It’s quick and creates a good profile, but it can be messy.

Cleaning and moisture condition before bonding

Before applying any adhesive or slurry, clean the surface thoroughly:

Vacuum or blow off loose material. Rinse to remove any remaining dust or contaminants.

Moisture matters: Check the substrate’s moisture content. Most adhesives and slurries need a damp surface for best results. Too much moisture can cause bonding issues, too little can lead to poor adhesion.

Use a moisture meter if you’re unsure. Always follow the manufacturer’s guidelines for your chosen product.

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Follow a logical sequence: verify design and materials, prepare and inspect the surface, install keys and connectors, apply the chosen adhesive or bonding slurry, place the new concrete, and protect during cure.

Emphasize coordination between trades, timing of adhesive open time or slurry recoat windows, and quality-control checkpoints such as connector placement, bondboard checks, and post-installation inspection; when in doubt, refer to manufacturer instructions or the engineer’s sequence.