Nozzle Technique and Encapsulation Around Rebar
A nozzleman is placing a 2-inch dry-mix lining inside a manhole. The reinforcement is a single mat of #4 bars at 8-inch centers with 1.5 inches of cover specified. The crew is shooting from one side, holding the nozzle about 5 feet from the surface at a 45° angle, and moving in a circular pattern. After the first lift, the inspector finds voids behind the bars and shadowing on the far side of the steel. The nozzleman wants to increase air pressure to 'blow the material in tighter' and continue.
1. Evaluate the nozzle technique and the proposed air pressure change.
Read the worked answer
The technique is the problem. A 5-ft standoff at 45° is too far and too oblique for encapsulation around #4 bars at 8-inch centers with 1.5-inch cover - the material velocity drops before it reaches the steel, and the angle creates shadowing on the far side of the bars. Circular motion without regard to angle of incidence produces inconsistent compaction.
Increasing air pressure does not correct a standoff and angle problem; it can increase rebound, drive material past the steel, and create more voids and shadowing. The correct approach is to reduce standoff to roughly 2-3 feet, hold the nozzle closer to perpendicular to the surface (with a slight lead angle for vertical work), and work material around and behind the bars deliberately rather than spraying across them.
2. What placement sequence and verification would you require for the remaining lifts?
Read the worked answer
Sequence:
- Build the first lift to just past the back of the bars, working the nozzle close and directing material behind the steel to eliminate shadowing.
- Allow initial set before the next lift; do not place wet-on-wet beyond the manufacturer's limit.
- Build subsequent lifts to final thickness, maintaining cover over the bars.
- Keep the nozzle perpendicular with a controlled lead angle; move in consistent passes rather than random circles.
Verification: hammer-tap or probe the placed lining for voids and shadowing before the next lift, and after cure perform a visual and sounding inspection at the bars. Any void or shadowed area must be cut out and replaced, not covered over.