Surface Preparation, Application Technique, and Finish
Core concepts
Bond starts before the first shovel of material leaves the nozzle. Surface preparation means a clean, sound, rough substrate: remove loose material, dust, oil, and laitance; chip or scarify to expose sound concrete; and pre-wet absorptive surfaces so they do not pull water from the fresh shotcrete. On rock or concrete, a rough, open texture improves mechanical keying.
Reinforcement: ensure rebar and mesh are clean and properly supported. The nozzleman must be able to shoot behind the bar - if the bar is too close to the surface or the spacing is too tight, voids form behind steel. Encasement requires shooting from an angle that fills behind the bar, then building out.
Application technique: shoot in lifts, keep the nozzle square, and work from low to high on vertical surfaces so material does not sag over fresh work. On overhead work, keep lifts thin and let them stiffen. Cold joints occur when a lift is allowed to set before the next is applied; the surface should be cleaned and, where required, roughened or bonding-agent treated.
Finishing: wet-mix can be floated, troweled, or broom-finished while still plastic. Overworking draws water to the surface and weakens it. Curing is mandatory - moist curing or a curing compound - because shotcrete has high surface-area-to-volume and dries quickly.
Traps
- Skipping pre-wetting and blaming the mix for rapid stiffening.
- Shooting over a lift that has already set without preparing the cold joint.
- Troweling too early or too hard, producing a weak, dusty surface.
Worked example
A wall shows a void line behind horizontal rebar. Likely cause: the nozzle was held too far from the bar or shot at a shallow angle so material bridged over the bar instead of filling behind it. Fix: shoot from a steeper angle to the bar, fill behind it first, then build out.