When Does a Concrete Crack Need Epoxy Injection?
Not every crack in concrete requires structural repair. Hairline shrinkage cracks in non-structural elements can often be left alone or sealed with a surface coating. But when cracks compromise structural integrity, allow water ingress, or indicate ongoing movement, epoxy injection is the standard repair method.
Cracks that typically require epoxy injection include structural cracks wider than 0.2mm in load-bearing members, cracks in water-retaining structures regardless of width, cracks showing active movement that needs to be arrested, and cracks in prestressed concrete where corrosion protection of tendons is critical.
Crack Assessment: The First Step
Before selecting a repair material, the crack must be characterized:
Width measurement: Use a crack comparator card or digital microscope. Width determines which resin viscosity to use.- Less than 0.3mm → Low-viscosity resin (gravity feed or low-pressure injection)
- 0.3mm to 1.0mm → Low or medium-viscosity resin (pressure injection)
- Greater than 1.0mm → Medium-viscosity resin or epoxy paste
Choosing the Right Injection Resin
We manufacture two primary injection resin viscosities, each designed for specific crack widths:
Low-Viscosity Crack Injection Resin (XQ-LF-L)
- Viscosity: 150-300 mPa·s at 25°C
- Penetrates cracks as narrow as 0.1mm
- Tensile strength: ≥ 40 MPa (stronger than concrete itself)
- Bond strength to concrete: ≥ 3.5 MPa
- Best for: Fine structural cracks, slab cracks, basement wall cracks
Medium-Viscosity Crack Injection Resin (XQ-LF-M)
- Viscosity: 1,000-2,000 mPa·s at 25°C
- Designed for wider cracks (0.3mm to 5mm)
- Higher gap-filling capability
- Tensile strength: ≥ 35 MPa
- Best for: Wider structural cracks, construction joints, areas with moderate movement
When to Use Polyurethane Instead
Our polyurethane crack sealant (XQ-LF-PU) is designed for active cracks that continue to move. Unlike rigid epoxy, polyurethane remains flexible after curing and can accommodate crack movement of up to 25% of the original width. Use polyurethane when the crack cause has not been resolved and ongoing movement is expected.Step-by-Step Injection Procedure
1. Surface preparation Clean the crack surface by removing loose concrete, dust, and contamination. Compressed air and wire brushing are typically sufficient. 2. Install injection ports Bond injection ports (T-ports or surface-mounted ports) along the crack at intervals equal to the member thickness. For a 200mm thick wall, space ports at 200mm intervals. 3. Seal the crack surface Apply surface seal paste (XQ-LF-SS) along the entire visible crack length between injection ports. This prevents resin from leaking out during injection. Allow the seal to cure (typically 4-6 hours) before injecting. 4. Inject resin Starting at the lowest port (for vertical cracks) or one end (for horizontal cracks), inject resin at low pressure (0.2-0.4 MPa). When resin flows from the adjacent port, cap the injected port and move to the next one. Continue until all ports show resin flow. 5. Monitor and top-up Some cracks will absorb more resin as it penetrates deeper. Return to each port after 10-15 minutes and top up if the level has dropped. 6. Remove ports and finish After full cure (24-48 hours), remove injection ports and grind the surface seal smooth.Complete Crack Repair System
A successful crack injection requires more than just the resin. Our complete system includes:
- Injection resin (XQ-LF-L or XQ-LF-M): The structural adhesive that restores load transfer
- Surface seal paste (XQ-LF-SS): Prevents resin leakage during injection
- Injection ports: Provide connection points for injection equipment
- Crack comparator: For accurate width measurement