Surface Preparation and Bonding on Existing Concrete
You are preparing to apply a 75 mm wet-mix shotcrete overlay on an existing concrete wall that is 20 years old. The surface has areas of carbonation, some oil stains, and a few cracks. The specified bond strength is 1.0 MPa. The ambient temperature is 10°C and the surface is dry. The crew plans to use a bonding agent and then apply the shotcrete in one pass. The nozzleman has a 50 mm hose and is using a mix with w/c 0.45 and 10% silica fume. The air pressure at the nozzle is 7 bar. The contractor wants to start spraying immediately after applying the bonding agent.
1. What surface preparation steps are necessary before applying the bonding agent and shotcrete?
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Proper surface preparation is critical for bond. The following steps are required:
- Remove all loose material, oil stains, and carbonation. Use high-pressure water blasting (minimum 200 bar) or abrasive blasting to achieve a roughened, clean surface. Oil stains may require degreasing or mechanical removal.
- Repair cracks: Inject epoxy or route and seal cracks to prevent reflective cracking.
- Check substrate moisture: The surface should be saturated surface-dry (SSD) to prevent absorption of water from the shotcrete. Since the surface is dry, pre-wet it and allow excess water to drain.
- Verify surface profile: A profile of CSP 5–7 is recommended for bond.
- Apply bonding agent according to manufacturer's instructions; typically, it should be applied to a damp surface and allowed to become tacky before shotcreting. Do not let it dry completely.
- Ensure the bonding agent is compatible with the shotcrete mix; some agents require a specific application method.
- The nozzleman should not spray until the bonding agent is properly prepared. Starting immediately after application may be too soon; check tackiness.
2. Is it acceptable to apply 75 mm in one pass? What considerations should be made for the low temperature and bonding?
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75 mm in one pass on a vertical wall is generally acceptable if the mix is properly designed and the surface is prepared. However, for overhead or vertical, 75 mm is at the upper limit; typically 50–75 mm is allowed. Given the low temperature (10°C), the shotcrete will set slower, increasing the risk of sagging. The nozzleman should monitor the build-up and may need to reduce layer thickness to 50 mm. For bonding, the low temperature may affect the bonding agent's curing; ensure it is applied within the manufacturer's temperature range. Also, the substrate should be kept above 5°C. The shotcrete mix may need acceleration to compensate for slow set. The nozzleman should check bond by tapping with a hammer after set. If bond is critical, consider applying a scratch coat first.