ACI Shotcrete Nozzleman Wet-Mix cheat sheet

Nozzleman controls the final water content at the discharge point; adding water at the nozzle is the primary adjustment for wet-mix consistency, but total w/cm must stay within the approved mix design.

Keep the nozzle perpendicular (90 degrees, +/- about 10 degrees) to the receiving surface; angled impact causes rebound, uneven compaction, and shadowing behind reinforcement.

Hold the nozzle 0.6 to 1.5 m (about 2 to 5 ft) from the surface; closer increases rebound and gouging, farther reduces impact velocity and compaction.

Maintain a steady circular or elliptical motion so each pass overlaps the previous one; a single thick pass causes sagging and trapped voids.

Build thickness in controlled lifts: typically 50 to 100 mm (2 to 4 in) per pass on vertical and overhead work; overhead lifts are kept thinner than vertical or floor lifts.

For overhead work, apply the first pass thin and let it tack before building thickness; material that falls away indicates too-wet mix, too-thick lift, or excessive nozzle distance.

Rebound is highest on overhead work and behind reinforcement; never reuse rebound in the work and never trowel rebound back into the fresh section.

Start spraying at the bottom of a vertical wall and work upward, keeping the lower edge of the placed material from collecting rebound; encase reinforcement by spraying from behind/around it.

Typical wet-mix cementitious content is roughly 350 to 450 kg/m3 (about 590 to 760 lb/yd3); higher paste content improves pumpability and nozzle flow but raises shrinkage.

Total water includes batch water plus any water added at the nozzle; w/cm is evaluated on the total, and typical field w/cm is about 0.40 to 0.50.

Maximum aggregate size for wet-mix shotcrete is generally limited to about 10 to 12 mm (3/8 to 1/2 in) to allow pumping and reduce rebound and blockage.

Supplementary cementitious materials (fly ash, slag, silica fume) improve pumpability, cohesion, and sulfate resistance; silica fume also reduces rebound and permeability.

Air-entraining admixtures are used where freeze-thaw exposure occurs; entrained air typically 5 to 8 percent depending on aggregate size and exposure.

Accelerators (non-chloride for permanent work with embedded steel) are added at the nozzle to shorten set and allow thicker overhead lifts; dosage must be controlled to avoid flash set or strength loss.

Pump output, hose diameter, and nozzle air supply must be matched; typical delivery line is 38 to 50 mm (1.5 to 2 in) and air pressure at the nozzle is commonly 0.5 to 0.7 MPa (70 to 100 psi) or per equipment manual.

Pre-dampen the pump and lines and keep a steady, continuous pumping rate; surging causes slug flow, inconsistent nozzle discharge, and blockages.

Remove loose material, dust, oil, and deteriorated concrete; concrete substrates are typically roughened (e.g., chipped or scarified) and cleaned before shooting.

Pre-wet absorptive substrates to a saturated-surface-dry condition so the new shotcrete does not lose water to the base; standing water must be removed.

Reinforcement must be clean and free of rust scale, oil, or form release; shotcrete must fully encase bars, so provide adequate clearance and shoot from the correct angle to fill behind them.

For overhead and vertical work, apply a thin first pass (scratch coat) to establish bond before building thickness; allow initial set between lifts where required.

Finish timing is critical: floating or troweling must occur while the mix is still plastic; working too late tears or delaminates the surface.

Use a rod or cutting tool to trim and shape fresh shotcrete to line and grade; do not overwork the surface, which draws paste and weakens the skin.

Construction joints are formed by cutting back to sound, square edges; wet the joint surface before resuming; avoid feathered edges.

Curing is required after finishing: keep shotcrete moist or use a curing compound; curing is especially important for thin sections and overhead work.

Pre-construction (mock-up) panels are shot to demonstrate the nozzleman's technique and to provide test specimens; panels are typically at least 500 mm (about 18 in) square and shot in the same orientation as production work.

Core samples are taken from panels or in-place shotcrete; compressive strength is evaluated on cores, and results are compared to the specified strength.

Bond strength is evaluated by pull-off or core testing; poor bond indicates inadequate surface prep, dry substrate, or incorrect nozzle angle.

Rebound and thickness are checked during placement; thickness is verified by probes, cores, or depth gauges, and results must meet the specified minimum.

Nozzleman qualification is demonstrated through the mock-up panel; the qualified nozzleman and the shooting orientation are tied to the approved procedure.

Safety: nozzleman and crew wear eye protection, hearing protection, gloves, and respiratory protection as required; silica exposure controls (wet methods, ventilation, respirators) are essential.

The nozzleman controls the air and material flow and must be able to shut off material feed and air quickly in an emergency; keep the nozzle pointed away from personnel.

Repair of defects (voids, delamination, low-strength areas) requires removal to sound material, surface prep, and re-shooting or patching with an approved repair material.

Nozzleman controls the final water content at the discharge point; adding water at the nozzle is the primary adjustment for wet-mix consistency, but total w/cm must stay within the approved mix design.

Keep the nozzle perpendicular (90 degrees, +/- about 10 degrees) to the receiving surface; angled impact causes rebound, uneven compaction, and shadowing behind reinforcement.

Hold the nozzle 0.6 to 1.5 m (about 2 to 5 ft) from the surface; closer increases rebound and gouging, farther reduces impact velocity and compaction.

Maintain a steady circular or elliptical motion so each pass overlaps the previous one; a single thick pass causes sagging and trapped voids.

Build thickness in controlled lifts: typically 50 to 100 mm (2 to 4 in) per pass on vertical and overhead work; overhead lifts are kept thinner than vertical or floor lifts.

For overhead work, apply the first pass thin and let it tack before building thickness; material that falls away indicates too-wet mix, too-thick lift, or excessive nozzle distance.

Rebound is highest on overhead work and behind reinforcement; never reuse rebound in the work and never trowel rebound back into the fresh section.

Start spraying at the bottom of a vertical wall and work upward, keeping the lower edge of the placed material from collecting rebound; encase reinforcement by spraying from behind/around it.

Typical wet-mix cementitious content is roughly 350 to 450 kg/m3 (about 590 to 760 lb/yd3); higher paste content improves pumpability and nozzle flow but raises shrinkage.

Total water includes batch water plus any water added at the nozzle; w/cm is evaluated on the total, and typical field w/cm is about 0.40 to 0.50.

Maximum aggregate size for wet-mix shotcrete is generally limited to about 10 to 12 mm (3/8 to 1/2 in) to allow pumping and reduce rebound and blockage.

Supplementary cementitious materials (fly ash, slag, silica fume) improve pumpability, cohesion, and sulfate resistance; silica fume also reduces rebound and permeability.

Air-entraining admixtures are used where freeze-thaw exposure occurs; entrained air typically 5 to 8 percent depending on aggregate size and exposure.

Accelerators (non-chloride for permanent work with embedded steel) are added at the nozzle to shorten set and allow thicker overhead lifts; dosage must be controlled to avoid flash set or strength loss.

Pump output, hose diameter, and nozzle air supply must be matched; typical delivery line is 38 to 50 mm (1.5 to 2 in) and air pressure at the nozzle is commonly 0.5 to 0.7 MPa (70 to 100 psi) or per equipment manual.

Pre-dampen the pump and lines and keep a steady, continuous pumping rate; surging causes slug flow, inconsistent nozzle discharge, and blockages.

Remove loose material, dust, oil, and deteriorated concrete; concrete substrates are typically roughened (e.g., chipped or scarified) and cleaned before shooting.

Pre-wet absorptive substrates to a saturated-surface-dry condition so the new shotcrete does not lose water to the base; standing water must be removed.

Reinforcement must be clean and free of rust scale, oil, or form release; shotcrete must fully encase bars, so provide adequate clearance and shoot from the correct angle to fill behind them.

For overhead and vertical work, apply a thin first pass (scratch coat) to establish bond before building thickness; allow initial set between lifts where required.

Finish timing is critical: floating or troweling must occur while the mix is still plastic; working too late tears or delaminates the surface.

Use a rod or cutting tool to trim and shape fresh shotcrete to line and grade; do not overwork the surface, which draws paste and weakens the skin.

Construction joints are formed by cutting back to sound, square edges; wet the joint surface before resuming; avoid feathered edges.

Curing is required after finishing: keep shotcrete moist or use a curing compound; curing is especially important for thin sections and overhead work.

Pre-construction (mock-up) panels are shot to demonstrate the nozzleman's technique and to provide test specimens; panels are typically at least 500 mm (about 18 in) square and shot in the same orientation as production work.

Core samples are taken from panels or in-place shotcrete; compressive strength is evaluated on cores, and results are compared to the specified strength.

Bond strength is evaluated by pull-off or core testing; poor bond indicates inadequate surface prep, dry substrate, or incorrect nozzle angle.

Rebound and thickness are checked during placement; thickness is verified by probes, cores, or depth gauges, and results must meet the specified minimum.

Nozzleman qualification is demonstrated through the mock-up panel; the qualified nozzleman and the shooting orientation are tied to the approved procedure.

Safety: nozzleman and crew wear eye protection, hearing protection, gloves, and respiratory protection as required; silica exposure controls (wet methods, ventilation, respirators) are essential.

The nozzleman controls the air and material flow and must be able to shut off material feed and air quickly in an emergency; keep the nozzle pointed away from personnel.

Repair of defects (voids, delamination, low-strength areas) requires removal to sound material, surface prep, and re-shooting or patching with an approved repair material.