Shotcrete Materials, Mixture Proportions, and Equipment
Core concepts
Wet-mix shotcrete is batched like conventional concrete - cementitious material, aggregate, water, and admixtures - then pumped and sprayed. Aggregate gradation matters more than in cast concrete: a well-graded combined aggregate with a sensible coarse fraction (commonly up to about 10–12 mm / 3/8–1/2 in for typical nozzle work) pumps and consolidates well. Too much coarse aggregate or a gap-graded blend increases rebound and blockage.
Cementitious content is typically higher than cast concrete to give cohesion and shootability. Supplementary cementitious materials (fly ash, slag, silica fume) improve pumpability, reduce rebound, and boost durability; silica fume especially improves bond and permeability but needs proper dispersion and curing.
Admixtures: air-entraining agents for freeze–thaw exposure, water reducers, retarders for long hauls, and accelerators. In wet-mix, accelerators are added at the nozzle to set the material quickly on the surface - never batched into the truck in a way that would set it in the drum. Fiber reinforcement (steel or synthetic) is common; fibers raise toughness but can increase rebound and wear on equipment.
Equipment: a consistent, steady pump output and a properly sized hose and nozzle are essential. A pre-orifice or air ring at the nozzle introduces air to shape the stream; too little air gives a slug-like stream, too much gives a fluffy, low-velocity spray.
Traps
- Assuming accelerator is a batch-plant admixture in wet-mix - it is nozzle-added.
- Confusing slump of the delivered mix with shootability; wet-mix is often placed at a moderate slump and made shootable by admixtures and gradation.
Rule of thumb
If the stream looks like a rope or slug, add air; if it looks like fog, cut air back. The target is a firm, cohesive cone.