Introduction
The process of bonding old to new concrete at a vertical face involves careful planning and execution. Understanding the timeline is crucial for DIY enthusiasts to ensure they allocate enough time and resources for each step.
In this guide, we’ll walk you through the entire process, breaking it down into manageable steps with approximate durations to help you plan your project effectively.
For the full guide, see Bonding Old to New Concrete at Vertical Faces: Keys, Dowels, and Epoxy vs Cement Slurry.
The Bonding Timeline: Step-by-Step
Bonding old to new concrete at a vertical face involves several stages that require careful attention and time management. Here’s a step-by-step timeline to guide you through the process:
- Surface Preparation (1-3 days): Clean, roughen, and prime surfaces as required.
- Mechanical Interlock Installation (2-4 hours per linear foot): Install keys or dowels based on your project’s needs.
- Bonding Agent Application (1-3 hours): Apply epoxy or cement slurry according to manufacturer instructions.
- Curing and Inspection (7-28 days): Allow the bonding agent to cure, then inspect the bond before proceeding with further work.
Total project duration can range from 10 to 35 days, depending on factors such as surface condition, chosen methods, and weather conditions. Keep in mind that these are approximate durations, and your specific project may take more or less time.
Key takeaways
The timeline for bonding old to new concrete at vertical faces begins with thorough surface preparation, which can take up to three days depending on the condition of the existing surface.
- Align project goals with failure modes: cracking, delamination, and poor shear transfer.
- Mechanical keys require clean depth contact and full engagement for transfer.
- Dowels and shear connectors must be placed per plan to resist vertical and lateral loads.
- Epoxy vs cementitious bonding: follow manufacturer instructions; test adhesion on sample area.
- Surface prep and cleanliness: remove contaminants, roughen surfaces, and prime where required.
- Safety: wear eye/hand protection; manage fumes and dust; follow site rules.
Table of Contents
- Introduction
- Key takeaways
- Project Goals and Failure Modes to Avoid
- Mechanical Keys: Types and Design Intent
- Dowels and Shear Connectors: Selection and Placement
- Epoxy Adhesives Vs Cementitious Slurry/Bonding Agents
- Surface Preparation and Cleanliness
- Step-by-Step Installation Procedure for Vertical Joints
- Material Specs, Standards, and Testing to Verify Bond
- Tools, Materials Checklist, and Safety Considerations
- Conclusion
- FAQ
Project Goals and Failure Modes to Avoid
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.
Mechanical Keys: Types and Design Intent
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.
Dowels and Shear Connectors: Selection and Placement
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.
Epoxy Adhesives Vs Cementitious Slurry/Bonding Agents
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.
Surface Preparation and Cleanliness
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.
Step-by-Step Installation Procedure for Vertical Joints
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.
Step-by-Step Installation Process
This section guides you through the practical steps of installing vertical concrete joints, ensuring a strong bond between old and new.
- Preparation: Safety first! Wear appropriate PPE. Clear work area, secure tools, and check weather conditions.
- Surface Preparation: Refer to earlier sections for methods like grinding or sandblasting. Ensure surfaces are clean, dry, and free of contaminants.
- Key and Dowel Placement: Follow layout plans from earlier. Install keys and dowels using proper anchorage methods, ensuring correct embedment and edge distances.
- Mix Epoxy/Cement Slurry: Follow manufacturer’s instructions for mixing epoxy or cement slurry. Ensure intimate contact with the surface by applying within 15-30 minutes of preparation.
- Place New Concrete: Pour new concrete into forms, ensuring it makes proper contact with old surfaces. Use consolidation methods like vibrating or rodding to eliminate voids and ensure a strong bond.
- Curing and Protection: Keep newly poured joints moist and protected from freezing for at least 7 days. Monitor progress and make adjustments as needed.
Temporary supports, formwork, and shoring
Temporary supports, formwork, and shoring are crucial for vertical joints to maintain alignment and restraint during placement.
When needed: When existing structures lack stability or when new concrete volume is significant. Also, consider using when pouring against gravity (e.g., overhead pours).
Plan for temporary supports by: 1) Assessing the load-bearing capacity of existing structures; 2) Designing and installing formwork that fits snugly against old surfaces; 3) Using shores, walers, and braces to provide additional support.
Ensure proper alignment by using leveling tools and checking vertical and horizontal lines. Regularly inspect supports during placement to maintain integrity.
Mixing, application, and consolidation best practices
Correct mixing, application technique, and void elimination are vital for a strong bond between old and new concrete.
For epoxy: Mix components thoroughly according to manufacturer’s instructions. Apply using a trowel or sprayer within the recommended time frame (typically 15-30 minutes) to ensure intimate contact with the surface.
For cementitious slurries: Mix cement, sand, and water until smooth. Apply using a trowel or brush, ensuring full coverage and proper contact with old surfaces within 30-60 minutes of preparation.
Consolidate new concrete to eliminate voids: Use internal vibrators for large pours or hand-held vibrators/rods for smaller ones. Ensure proper consolidation by checking for air pockets and maintaining consistent vibration time.
Material Specs, Standards, and Testing to Verify Bond
Identify the required material properties and standards to check: adhesive type and classification, bond strength or pull-off values, cure time, and compatibility with concrete and reinforcement detailing.
Confirm performance with specified lab or field tests (for example, pull-off testing or witness panels) and follow the test method and acceptance criteria referenced in the project specifications or product technical data sheet.
What to Request from Manufacturers and Spec Writers
When specifying materials for bonding old to new concrete at vertical faces, don’t just pick the cheapest option. Here’s what you should ask for:
Technical Data Sheets: These detail material properties like bond strength, cure time, and temperature/moisture limits. Study them closely.
Adhesion Test Results: Ask for results from adhesion tests on similar substrates to ensure the product works well with your existing concrete.
Remember, understanding these details helps you make informed decisions and avoid costly mistakes.
Field and Laboratory Tests (Pull-Off, Shear, Visual)
To verify bond quality, use a mix of field and lab tests. Here’s when to use each:
Pull-Off Test: This destructive test measures bond strength by pulling a loaded disc off the surface. Use it after curing is complete.
Shear Test: Another destructive test, shear testing measures resistance to sliding forces. It’s useful when expecting high shear loads.
Visual Inspection: Before and during application, visually inspect surfaces for cleanliness, moisture, and any signs of deterioration. After curing, check for cracks or delamination.
Tools, Materials Checklist, and Safety Considerations
Prepare a real-world checklist: surface-prep equipment, mixing and dispensing tools for adhesives or slurries, reinforcement and connector hardware, formwork and curing materials, consumables, and appropriate PPE.
Include permits, confined-space or fall protection planning, ventilation for solvents or epoxy fumes, and review the safety data sheets and manufacturer instructions for handling, storage, and disposal before starting work.
Typical tools and consumables
Before you start, make sure you have all the necessary tools and consumables for a smooth bonding operation. Here’s a checklist to help you.
- Surface prep tools: Grinders, sandblasters, hydrodemolition equipment (if needed), scabbling hammers.
- Mixing equipment: Mixers, buckets, mixing paddles, scales for accurate material measurement.
- Injection/placement tools: Injection pumps, placement guns, trowels, brushes.
- Curing aids: Curing compounds, plastic sheets, damp cloths.
- Safety gear: Safety glasses, gloves, ear protection (for surface prep tools).
- Miscellaneous: Tape measures, levels, chisels, hammers, wedges for temporary support.
- Consumables: Check compatibility with chosen materials – epoxy resins, cement slurry, primers, accelerators, retarders.
Quick rule: Always verify tool and consumable compatibility with your chosen bonding materials to avoid failures.
Safety, handling, and environmental precautions
Safety is paramount when working with concrete and chemicals. Here’s a checklist to keep you safe and environmentally responsible.
- PPE: Wear appropriate gloves, safety glasses, and protective clothing for both cementitious materials and epoxies.
- Ventilation: Ensure adequate ventilation when working with chemicals. Use fans or respirators if needed.
- Spill control: Have absorbents ready to contain and clean up spills promptly.
- Disposal: Follow local regulations for disposal of concrete waste, epoxy waste, and contaminated materials.
- Temperature limits: Check material data sheets for temperature ranges. Avoid extreme temperatures that could compromise bond strength.
- Moisture work limits: Ensure surfaces are dry before application (for cementitious materials) or within specified moisture range (for epoxies).
- Storage and handling: Follow manufacturer’s guidelines for storage, handling, and disposal of chemicals.
Quick rule: Always follow material data sheets and safety guidelines to protect yourself and the environment.
Conclusion
Bonding old to new concrete at vertical faces is about doing the right preparation, choosing the right connection method, and proving the bond before you trust it with load or weight. Safety, durability, and a clean look depend on following the steps, testing what you did, and avoiding shortcuts.
First, confirm your scope with a practical check: identify the failure mode you’re addressing, select keys or dowels per the design intent, decide between epoxy or cement slurry, scrub and etch the surface, mix and apply only within the approved specifications, and install the joint in the planned sequence with proper cure and verification. Then inspect before moving on: verify material specs, confirm tool readiness, follow the step-by-step procedure for vertical joints, run the cure and bond tests, and document the results. Keep safety front and center, and never skip the cleanliness or testing steps.
Common mistakes to avoid are skipping surface preparation, using the wrong adhesive or slurry, ignoring temperature and cure requirements, and underestimating the need for thorough testing. Don’t rush the cure, don’t overwork the bond, and don’t run joints if the surface is contaminated or wet. A few plain rules that help: test on a small area first, stay within the material specs and standards, and use PPE and proper ventilation when mixing epoxy. If anything looks off, back up, re-clean, and re-test rather than guessing.
If you hit unclear conditions, hidden reinforcement, or big structural questions, call in a professional instead of risking a costly failure. When in doubt, pause and verify each step against the project goals and safety considerations, then proceed with confidence and a clear plan for inspection and verification. You’ve got this—steady progress and careful checks will yield a strong, durable joint that stands up to use.
FAQ
How do I know if the vertical joint is worth bonding at all, or if I should replace the section?
Look for major cracking, spalling, or loose sections. If the old face is unstable, bonding may not hold well and replacement is safer. When in doubt, consult the project notes and local guidelines for durability concerns.
What should I do to ensure clean, sound surfaces before bonding on vertical faces?
Scrape away loose material and dust. Use a stiff brush or grinder to expose fresh concrete. Follow up with a thorough wipe to remove debris before applying adhesives or slurry.
How do epoxy and cementitious slurry differ, and when is one better for vertical joints?
Epoxy gives tougher, more gap-bridging bonds but needs clean, dry surfaces and proper mixing. Cementitious slurry can be easier and more forgiving but may not handle heavy loads as well. Always check the product instructions and compatibility with your existing concrete.
What are common failure signs after bonding, and how can I reduce risk during installation?
Cracking, debonding, or moisture bleed are signs of trouble. Ensure proper surface prep, follow product timing, and avoid applying in freezing conditions or on actively damp surfaces. If unsure, stop and verify with the product label or manufacturer guidance.