Tips for Bonding Old and New Concrete for a Strong, Monolithic Connection | Conmix

One of the most common challenges in renovation, rehabilitation, and structural extension projects is this: how do you ensure that new concrete truly bonds with existing concrete?

It sounds simple, but in practice, it is one of the most frequently mishandled tasks on construction sites. Simply having new concrete “stick” to the surface of old concrete is not enough—both must bond structurally and perform as a single, monolithic element.

If this bond fails, cracks will inevitably develop along the joint line. And that means costly repairs, wasted labor, and project delays.

Why Old and New Concrete Are Difficult to Bond

Before discussing the solution, it is important to understand the challenge.

Existing concrete has already completed its hydration and hardening process. Its surface is typically smooth and may be contaminated by dust, oil, laitance, or other substances that prevent proper adhesion. It no longer has the natural bonding capacity required to integrate with fresh concrete.

Meanwhile, newly placed concrete shrinks as it cures. This shrinkage can create micro-gaps at the interface. In addition, differences in the modulus of elasticity between old and new concrete cause them to respond differently under load, increasing the likelihood of cracking at the joint.

According to the American Concrete Institute, failures at old-to-new concrete interfaces (construction joints) are among the leading causes of premature deterioration in repaired concrete structures.

Step-by-Step Guide to Bonding Old and New Concrete

Step 1 – Prepare the Existing Concrete Surface

This is the most critical step. The existing concrete surface must be clean, rough, and free of any contaminants that could interfere with bonding.

Required Preparation:

• Roughen the surface using a steel wire brush, chipping, or sandblasting until coarse aggregate is exposed
• Remove all dust, oil, paint, laitance, and debris using a high-pressure water jet
• For oil-contaminated surfaces, clean with acetone first
• If reinforcing steel is exposed and corroded, remove rust and apply a corrosion inhibitor
• Pre-wet the surface with clean water approximately 2 hours before placement
• The surface should be in SSD condition (Saturated Surface Dry)—moist but without standing water

Recommended Product: CONMIX HKRMA-1 — Anti-Corrosive Agent

If the existing reinforcement has begun to corrode, HKRMA-1 is an ideal solution. This gray powder:

• Increases concrete density
• Inhibits alkali-aggregate reaction
• Improves sulfate erosion resistance
• Enhances compressive strength by up to 25% at 120 days
• Can exceed 60% strength improvement after one year

Step 2 – Apply a Bonding Agent

Once the old concrete surface is clean and in SSD condition, the next step is to apply a bonding agent. This material acts as an adhesive bridge between the old and new concrete.

It penetrates the existing concrete surface, fills micro-pores, and creates a strong transition layer, enabling both chemical and mechanical bonding.

Recommended Product: CONMIX HK-JMJ — Concrete Interface Agent

A single-component dry mortar formulated with microcrystalline technology, specifically designed for old-to-new concrete bonding.

Application:

• Mix with water at a ratio of 1:4–7
• Apply using a roller or mechanical spray
• Place new concrete while the bonding layer is still active

Performance Specifications:

• Shear bond strength:
• 7 days: ≥1.0 MPa
• 14 days: ≥1.5 MPa
• Tensile bond strength:
• ≥0.4–0.6 MPa under various exposure conditions
• Impermeability:
• No leakage after 24 hours under a 50 cm water column

Suitable for concrete, brick walls, aerated blocks, floors, basements, and swimming pools.

Step 3 – Choose the Right Repair Mortar or Concrete

Not all concrete or mortar is suitable for bonding applications. The ideal repair material should provide:

• High early strength
• Low shrinkage or shrinkage compensation
• Excellent compatibility with existing concrete

Recommended Product: CONMIX HK-JHW — Polymer Waterproof Mortar

Key advantages:

• High bond strength to existing concrete
• Minimal drying shrinkage
• Excellent corrosion resistance
• Shrinkage-compensating performance

Application:

• Mix with water at a ratio of 1:5–7
• Apply using a trowel or stiff-bristle brush
• Use within 0.5–1 hour
• Coverage: 7–8 kg/m² at 8–10 mm thickness

Step 4 – Use Grouting Mortar for Narrow Joints

For narrow gaps, voids, or prestressed ducts where conventional concrete cannot penetrate, non-shrink grouting mortar is the best option.

Recommended Product: CONMIX HK-GJL — Grouting Mortar

Features:

• Non-shrink
• Self-leveling
• High flowability
• Suitable even for underwater applications

Compressive Strength:

• 1 day: ≥25 MPa
• 3 days: ≥40 MPa
• 7 days: ≥50 MPa
• 28 days: ≥60 MPa

Expansion Rate at 27°C: 0.30–1.40%

Ideal for:

• Anchor bolts
• Bridge bearings
• Machinery foundations
• Precast concrete
• Underwater structures

Step 5 – Prevent Cracking at the Joint

Even with proper bonding, joint areas remain vulnerable to shrinkage cracking. The best solution is to incorporate an anti-crack admixture into the new concrete.

Recommended Product: CONMIX HK-KF — Expansive Fiber Anti-Crack Waterproofing Agent

Contains uniformly distributed polypropylene fibers (6–9 mm) that act as micro-reinforcement.

Benefits:

• Reduces early cracking by 60–80%
• Improves waterproofing performance up to P20
• Increases impact and abrasion resistance by over 50%

Recommended Dosage: 8–12% of cement weight

Specialized Solutions for Different Joint Conditions

For Road Repairs and Horizontal Surfaces

Horizontal surfaces are subjected to heavy traffic loads, vibration, and temperature fluctuations.

Recommended Product: CONMIX HK-XB — Road Quick Repair Mortar

Strength Performance:

• 4 hours: ≥20 MPa
• 28 days: ≥30 MPa

Simply add 13–15% water, mix thoroughly, and the repaired area can be reopened to traffic within 1 hour.

For Wet or Underground Environments

Basements, tunnels, and damp areas require waterproof bonding systems.

Recommended Product: CONMIX HK-DL — Sealing King

A rigid waterproof sealing material that can be applied even under flowing water conditions.

Coverage:

• Non-permeable surfaces: 2.0 kg/m²
• Highly permeable surfaces: 3.0 kg/m²

Ideal for:

• Active leak stopping
• Emergency sealing
• Pre-treatment before structural repair

Common Mistakes to Avoid

•  Pouring directly onto an unclean surface
•  Skipping the bonding agent
•  Applying to a surface that is too dry or too wet
•  Using ordinary concrete for repairs
•  Neglecting curing (minimum 7 days is essential)

International Standards and References

• ACI 318-19 — Building Code Requirements for Structural Concrete
• ACI 546R-14 — Guide to Concrete Repair
• ASTM C881 — Epoxy-Resin Bonding Systems for Concrete
• SNI 03-2847-2019 — Structural Concrete Requirements for Buildings
• EN 1504-4 — Structural Bonding Systems for Concrete Repair

Summary of CONMIX Products for Concrete Bonding

Conclusion

Bonding old and new concrete is not a task that can be done casually. It requires proper surface preparation, the right bonding agent, high-quality repair materials, and correct placement techniques. When all these elements work together, they create a strong, durable, and truly monolithic bond.

PT Far East Conmix Indonesia offers a complete range of solutions for every stage of this process—from bonding agents and repair mortars to grouting materials and anti-crack admixtures.

All CONMIX products are manufactured under ISO 9001:2015 and ISO 14001:2015 standards and have been successfully used in major Indonesian infrastructure projects, including the Jakarta–Bandung High-Speed Railway, projects by Waskita Karya, and developments by Wijaya Karya.

International quality, local technical support, and competitive pricing—that is the CONMIX advantage.