ACI Dry-Mix Nozzleman cheat sheet

Dry-mix shotcrete: cementitious binder, aggregate, and additives are pre-blended dry; water is added at the nozzle.

Water-cementitious materials ratio (w/cm) controls strength and durability; typical nozzle w/cm is 0.35–0.45.

Aggregate gradation per ASTM C33; for dry-mix, coarse aggregate maximum size typically 3/8 in. (9.5 mm) or 1/2 in. (12.7 mm).

Cement content typically 600–800 lb/yd³ (356–475 kg/m³) for dry-mix; higher for overhead or wet areas.

Silica fume addition (5–12% by mass of cement) improves cohesion, reduces rebound, and increases strength.

Accelerators (chloride-free) reduce set time; dosage per manufacturer, typically 1–4% by mass of cement.

Fibers (steel, synthetic) improve toughness and crack resistance; dosage per project specification (e.g., 30–100 lb/yd³).

Pre-bagged dry-mix must be protected from moisture; use within shelf life and discard caked material.

Rebound increases with larger aggregate, higher air pressure, and greater nozzle-to-surface distance.

Dry-mix gun: rotor/stator or pneumatic feed; maintain consistent feed rate to avoid pulsation.

Air pressure at the gun typically 60–100 psi (4–7 bar); adjust for hose length, elevation, and material.

Hose diameter typically 1.25–2 in. (32–50 mm); keep hose as straight as possible; avoid kinks and sharp bends.

Nozzle: water ring introduces water; check for even water distribution and worn water ring.

Water pressure should exceed air pressure at the nozzle by about 10–20 psi to ensure proper mixing.

Air compressor must supply sufficient cfm (typically 100–300 cfm) and be equipped with water trap.

Pre-shift inspection: check hose for wear, couplings, nozzle, water ring, and air line; test safety interlocks.

Nozzleman must have clear communication with gun operator (radio or hand signals).

Hold nozzle perpendicular to the surface; maintain distance of 3–6 ft (0.9–1.8 m) for dry-mix.

Use consistent circular or elliptical motion; avoid dwelling in one spot to prevent build-up and shadows.

Start water at the nozzle before material; stop material before water to prevent clogging.

Adjust water at the nozzle to achieve a slightly dark, moist appearance with no dry pockets or sagging.

Shoot from bottom to top in vertical work; for overhead, apply in thin layers to avoid fall-out.

Maintain a consistent layer thickness: typically 1–3 in. (25–75 mm) for vertical, less for overhead.

Keep the nozzle at a 90° angle; if angle must be changed, keep it within 15° of perpendicular.

Use a test panel or board to verify mixture and technique before starting production.

Surface must be clean, rough, and sound; remove loose material, dust, oil, and laitance.

Pre-wet the substrate to saturated-surface-dry (SSD) to prevent absorption of mixing water.

For overhead or vertical, use a bonding agent or scratch coat as specified; follow manufacturer instructions.

Placement temperature: ambient and substrate between 40°F (4°C) and 90°F (32°C); protect from freezing.

Apply in layers; allow initial set before next layer; remove rebound and loose material between layers.

Finishing: strike off excess with a straightedge, then float or trowel as specified; avoid overworking.

Curing: begin curing as soon as finishing is complete; use curing compound, wet curing, or plastic sheeting.

Cold joints: roughen and clean surface, pre-wet, and use bonding agent before placing new layer.

Pre-construction test panel: simulate field conditions, including orientation, thickness, and reinforcement.

Test panel evaluation: visual inspection, thickness, compressive strength (cores), and bond strength.

Compressive strength: sample from test panel or in-place cores; typical 28-day strength 4000–6000 psi (28–41 MPa).

Bond strength: pull-off test per ASTM C1583; minimum typically 100 psi (0.7 MPa) or as specified.

Thickness: verify with depth gauge or cores; tolerance typically ±1/4 in. (6 mm) of specified thickness.

Inspection: check for voids, delamination, dry patches, and rebound pockets; tap with hammer for delamination.

Record keeping: document mixture proportions, water addition, test results, and environmental conditions.

Quality control testing frequency: per project specification, often one set per 50 yd³ (38 m³) or per shift.

PPE: wear NIOSH-approved respirator, safety glasses, face shield, gloves, and hearing protection.

Silica exposure: dry-mix contains crystalline silica; follow OSHA 29 CFR 1910.1053 (respirable crystalline silica).

Use engineering controls: water mist, ventilation, and vacuum dust collection; avoid dry sweeping.

Nozzleman and helper must not stand directly in line with the hose; high-pressure air/water can cause injury.

Compressed air safety: never exceed hose rated pressure; secure couplings with safety pins.

Waste disposal: contain rebound and wash water; prevent entry into storm drains per local regulations.

Material Safety Data Sheets (SDS) must be available on site for all products.

Confined space entry: follow OSHA 29 CFR 1910.146 if applicable.

Dry, dusty material at nozzle: increase water pressure or check water ring for blockage.

Sagging or sloughing: reduce water content, reduce layer thickness, or increase accelerator dose.

Pulsating flow: check for kinked hose, worn rotor/stator, or inconsistent feed; maintain steady air pressure.

Clogged hose: stop material, reduce air, and clear hose safely; never look into hose end.

Rebound pockets or voids: remove loose material, pre-wet, and fill with fresh material.

Delamination: remove unsound material, roughen, clean, and apply bonding agent before repair.

Repair procedure: square-cut edges, clean, pre-wet, apply bonding agent, and place repair material.

Post-repair inspection: verify bond, thickness, and finish; cure repaired area as specified.