Hairline cracks in concrete (typically 0.05–0.2 mm wide) are common but can lead to long-term durability issues if left untreated.   This article covers how to assess, repair, and prevent these microcracks effectively.

Understanding Hairline Cracks

Typical Size & Characteristics

• Width: 0.05–0.2 mm (barely visible, similar to a human hair).

• Depth: Usually surface-level (<10 mm), but some may extend deeper.

• Common Causes:

• Plastic shrinkage (during curing)

• Thermal contraction

• Minor settlement

Do Hairline Cracks Need Repair?

✔ Repair if:

• Located in load-bearing areas (beams, columns, slabs).

• Exposed to water, chemicals, or freeze-thaw cycles.

• Showing rust stains (indicating rebar corrosion).

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✖ May not need repair if:

• In non-structural areas (e.g., interior floors).

• Stable (no widening over time).

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Best Repair Methods

A. Epoxy Injection (HM-120L)

Best for: Structural cracks (0.05–0.5 mm).
Steps:

1. Clean the crack (use compressed air or a wire brush).

2. Seal the surface with HM-9 epoxy putty (to prevent leakage).

3. Install injection ports (every 20–30 cm).

4. Inject low-viscosity epoxy (pressure: 0.1–0.3 MPa).

5. Cure for 24–48 hours.

✅ Pros: High strength, long-lasting.
❌ Cons: Requires skill; not for moving cracks.

B. Polymer-Modified Slurry (Surface Repair)

Best for: Non-structural cracks (0.1–0.3 mm).
Steps:

1. Dampen the crack (prevents premature drying).

2. Apply slurry (acrylic-modified cement).

3. Smooth with a trowel.

✅ Pros: Easy DIY solution.
❌ Cons: Less durable than epoxy.

C. Penetrating Sealers (Preventive Protection)

Best for: Multiple microcracks.
Materials: Silane/siloxane sealers.
Application: Spray or roll onto the surface.
Effect: Waterproofs without changing appearance.

✅ Pros: Prevents future damage.
❌ Cons: Doesn’t structurally repair cracks.

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