ACI Shotcrete Inspector cheat sheet

Shotcrete is mortar or concrete pneumatically projected at high velocity onto a surface; both dry-mix and wet-mix processes use the same basic cementitious constituents.

Dry-mix places cement and aggregate dry at the nozzle, adding water there; wet-mix pre-mixes all ingredients including water before pumping.

Typical dry-mix water/cement ratio at the nozzle falls near 0.35-0.45; wet-mix is batched at conventional ratios near 0.40-0.50 with water-reducing admixtures.

Silica fume at roughly 5-12% of cementitious mass increases cohesion, reduces rebound, and raises compressive strength and impermeability.

Aggregate grading for dry-mix shotcrete generally peaks at 10-12 mm (3/8 in); wet-mix may use up to 16 mm (5/8 in) depending on nozzle and hose size.

Accelerators shorten set time: alkali-free liquid accelerators are preferred over powdered aluminates because they reduce strength loss and dust.

Fiber reinforcement (steel or synthetic) is dosed by volume, commonly 30-60 kg/m3 steel, and is not a substitute for structural reinforcement unless designed as such.

Pre-bagged proprietary mixes must be used with the manufacturer's stated water content; field-added water beyond the bag recommendation changes w/c and strength.

Dry-mix uses a pressure vessel or rotary gun feeding a hose; wet-mix uses a concrete pump (piston or rotor-stator) delivering material to the nozzle.

Nozzle distance from the surface is typically 0.9-1.5 m (3-5 ft); greater distance reduces impact velocity and increases rebound.

Nozzle should be held perpendicular to the surface; the spray angle should not deviate more than about 15 degrees from normal.

Compressed air supply at the nozzle typically requires roughly 7-10 m3/min at 0.5-0.7 MPa for dry-mix; insufficient air causes surging and poor consolidation.

Nozzleman technique: steady circular or elliptical motion builds uniform thickness; dwelling in one spot creates lumps and shadowing.

Hose size for dry-mix is commonly 32-50 mm; wet-mix lines are typically 50-65 mm with reducers at the nozzle.

Rebound is the material that bounces off; it is higher for dry-mix and for overhead work, and rebound must never be reused in the work.

Equipment must be cleaned and the line blown clear between mixes; a plugged line is cleared only after depressurizing and with the nozzle pointed away from personnel.

Surface must be clean, sound, and free of dust, oil, loose material, and standing water before spraying.

Concrete substrates receive a roughened profile (typically CSP 5-7 by acid etching, abrasive blasting, or scarifying) to promote mechanical bond.

Substrate should be pre-wetted to saturated-surface-dry (SSD) so it does not draw mixing water from the shotcrete; free water is not permitted.

Reinforcement must be clean and free of release agents; minimum cover to reinforcement is typically 15-20 mm for shotcrete in non-corrosive exposure.

Spray the first pass into corners and behind reinforcement first, building up in layers rather than one thick pass.

Maximum single-pass thickness is roughly 50-75 mm for walls and less (about 25-50 mm) for overhead work; subsequent layers are applied while the previous layer is still workable or after proper preparation.

For overhead work, apply material in thin lifts and allow initial set before adding more to prevent sagging and fall-out.

Joints and construction interfaces should be square-cut or tapered and cleaned before resuming; cold joints require roughening and wetting.

Compressive strength is evaluated on cores or on test panels sprayed under field conditions; test panels must be cured and cored per project specification.

Core diameter is typically 75 mm (3 in) minimum; cores are trimmed, capped, and tested in compression to determine field strength.

Acceptance often requires field strength to meet a specified fraction (commonly about 70-85%) of the design or laboratory strength to account for field variability.

Thickness is verified by probe, core, or depth gauge; the average of measurements must meet the specified thickness with no single point below a stated minimum.

Rebound and overspray are estimated by placing a rebound board or measuring material collected at the base of the work.

Bond strength is assessed by pull-off or core inspection; a sound, continuous bond line with no visible delamination is required.

Inspection includes verifying nozzleman qualification, mix design approval, water content at the nozzle, and curing method.

Curing must begin promptly after final set; wet curing, curing compound, or moist cover is required to prevent plastic and drying shrinkage cracking.

Defective shotcrete (voids, delamination, honeycomb, low strength) is removed to sound material with square edges before repair.

Repair material must be compatible with the parent shotcrete in strength, modulus, and thermal expansion to avoid debonding.

Shallow repairs use a bonding agent or roughened, SSD-prepared surface; deep repairs are built up in layers matching the original placement limits.

Safety: nozzleman and crew wear eye protection, respiratory protection for dry-mix dust and silica, and hearing protection near compressors.

Silica dust controls include wetting, ventilation, and NIOSH-approved respirators; crystalline silica is a known health hazard.

Compressed air lines and nozzles are a struck-by hazard; never look into a nozzle or stand in line with the hose during pressurization.

Standards and references: ACI 506 (Guide to Shotcrete) and ACI 506.2 (Specification for Shotcrete) govern materials, application, and inspection; ASTM C1140 covers making and testing shotcrete panels.

Inspector qualification and nozzleman certification (e.g., ACI or equivalent) are commonly required by specification before production work begins.