Repair of a Delaminated Shotcrete Area
During a routine inspection of a 6-month-old wet-mix shotcrete wall, you discover a 1 m x 2 m area that sounds hollow when tapped. The area is located near a construction joint. The specified shotcrete thickness is 150 mm, and the mix had a w/c of 0.45 with 8% silica fume. The ambient temperature during placement was 30°C. The contractor proposes to simply apply a new layer of shotcrete over the hollow area after cleaning the surface. The owner is concerned about structural integrity and wants a proper repair.
1. What are the likely causes of the delamination, and why is the contractor's proposed repair inadequate?
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Delamination near a construction joint is often caused by improper surface preparation of the joint, excessive rebound trapped at the interface, or poor bonding due to dust or laitance. The high temperature (30°C) may have caused rapid drying, leading to a weak bond. The contractor's proposal to simply apply a new layer over the hollow area is inadequate because it does not address the underlying delamination; the new layer would also be unbonded and could fail. Delamination indicates a lack of bond between layers or between shotcrete and substrate. Proper repair requires removal of the delaminated material to sound concrete, preparation of the substrate, and replacement with new shotcrete.
2. Describe the proper repair procedure for this delaminated area, including surface preparation, material selection, and application method.
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The repair procedure should be:
- Delineate the delaminated area by sounding; mark boundaries extending at least 25 mm beyond the hollow area.
- Remove the delaminated shotcrete using chipping hammers or saw cutting to a square edge. The removal should extend to sound, bonded concrete. The edges should be squared off (not feathered) to provide a key.
- Clean the exposed surface by abrasive blasting or high-pressure water to remove dust, laitance, and loose material. The surface should be roughened to CSP 5–7.
- Pre-wet the surface to SSD condition.
- Apply a bonding agent if required by specification; ensure it is compatible with the repair material.
- Use a repair mix similar to the original shotcrete, possibly with a slightly higher cement content or a shrinkage-compensating additive. For small areas, a trowel-applied repair mortar may be used, but for 150 mm thickness, shotcrete is appropriate.
- Apply the repair shotcrete in layers (50–75 mm) using proper nozzle technique, ensuring good compaction and bond. Allow each layer to set before the next.
- Cure the repair properly (moist curing for 7 days).
- Verify the repair by sounding and possibly pull-off testing.
- Document the repair.