Rebound and Overspray on Overhead Ceiling
You are part of a crew applying wet-mix shotcrete to a tunnel ceiling (overhead). The mix contains 10% silica fume, 5% polymer fibers, and a water/cement ratio of 0.40. The specified thickness is 100 mm. The nozzleman is using a 38 mm hose and a nozzle with a 45° elbow. The air pressure at the nozzle is 6.5 bar. The crew is spraying in a back-and-forth pattern across the ceiling. After the first pass, the nozzleman observes significant rebound (about 30%) falling onto the floor and some overspray on the side walls. The temperature is 25°C and humidity is 50%. The contractor is concerned about material waste and wants to reduce rebound.
1. What factors are contributing to the high rebound, and what changes can the nozzleman make to reduce it?
Read the worked answer
High rebound in overhead work is common but 30% is excessive. Contributing factors:
- Nozzle angle: The nozzleman may not be holding the nozzle perpendicular to the surface. For overhead, the nozzle should be aimed slightly ahead of the impact point to allow the material to build up.
- Distance: Too great a distance (>1.5 m) increases rebound. Ideal distance is 0.5–1.0 m.
- Air pressure: 6.5 bar may be too low for overhead; increase to 7–7.5 bar (check manufacturer recommendations).
- Mix design: High silica fume and fibers can increase cohesion but may also increase rebound if not properly proportioned. The w/c ratio of 0.40 is low, making the mix stiff; slightly increasing water (if allowed) or using a water reducer could improve workability.
- Spray pattern: A steady, consistent pattern with overlapping passes is needed. The back-and-forth pattern may be too fast; slow down and ensure each pass covers the previous one by 50%.
- Nozzle type: A 45° elbow may cause turbulence; consider a straight nozzle or a 30° elbow. Immediate actions: Adjust nozzle angle to perpendicular, reduce distance to 0.8 m, increase air pressure slightly, and slow the pass speed. Also, ensure the surface is clean and properly prepared; rebound can be higher on dry or dusty surfaces.
2. How should the nozzleman manage the rebound material and overspray to minimize waste and ensure quality?
Read the worked answer
Rebound material must not be reused in the shotcrete. It should be collected and disposed of properly. To minimize waste:
- Use a catch net or board under the work area to collect rebound for disposal.
- Adjust the spray pattern to direct rebound away from finished surfaces.
- Overspray on side walls should be removed immediately before it sets; use a trowel or scraper.
- The nozzleman should focus on building thickness gradually in layers (50 mm max per pass for overhead) to reduce rebound.
- Consider using a set accelerator to allow faster build-up and reduce rebound, but only if approved.
- Monitor rebound rate; if it remains high, stop and adjust mix or equipment. Quality: Ensure that rebound is not entrapped in the shotcrete layer, which can cause voids. The nozzleman should visually inspect each layer and remove any loose rebound before applying the next layer.