Nozzleman Adjusts for Rebound and Thickness on Vertical Wall
You are a nozzleman placing wet-mix shotcrete on a 4.5 m high vertical wall. The mix design has a water/cement ratio of 0.45, 10 mm maximum aggregate, and 8% silica fume. The specified thickness is 150 mm. The crew is using a 50 mm diameter hose with a 90° elbow at the nozzle. The air pressure at the compressor is 7 bar, but the gauge at the nozzle reads 5.5 bar. The first 2 m of the wall shows noticeable sagging and a rebound layer of about 25 mm at the base. The contractor wants to finish the wall in one pass to save time. The ambient temperature is 32°C and the wind is 15 km/h.
1. What are the likely causes of the sagging and excessive rebound, and what immediate adjustments should the nozzleman make?
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The sagging and rebound are likely due to insufficient air pressure at the nozzle (5.5 bar vs. 7 bar at compressor) and possibly improper nozzle angle or distance. The pressure drop is 1.5 bar, which is high for a 50 mm hose with a 90° elbow; check for kinks, leaks, or excessive hose length. The nozzleman should first verify air pressure at the nozzle with a calibrated gauge, then increase pressure to the recommended 6.5–7 bar (or per manufacturer). Also, ensure the nozzle is held perpendicular to the wall and at a distance of about 0.5–1.0 m. The mix may be too wet; check slump (should be 50–75 mm for wet-mix). The high temperature and wind can cause rapid evaporation, leading to stiffening; consider using a set retarder or adjusting mix water. For the sagging area, remove loose material and rebound before continuing. The 25 mm rebound layer at the base indicates improper spraying technique; the nozzleman should start spraying at the bottom and work upward in a consistent pattern, using a slight circular motion.
2. Should the contractor place the full 150 mm thickness in one pass? Explain why or why not, and recommend a placement sequence.
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No, placing 150 mm in one pass on a vertical wall is not recommended. Wet-mix shotcrete should be applied in layers of 50–75 mm for vertical surfaces to avoid sagging and ensure proper compaction. The first layer should be 50–75 mm, then allow it to set enough to support the next layer (typically 15–30 minutes, depending on mix and temperature). The second layer can be applied to reach the specified thickness. For the first 2 m that already show sagging, the affected material should be removed back to sound concrete before reapplying. The nozzleman should use a thickness gauge or probe to check each layer. Given the high temperature, the setting time may be shorter, so monitor carefully. Also, consider using a set accelerator if the mix is too slow, but avoid over-accelerating as it can reduce strength.