ACI Shotcrete Nozzleman Wet-Mix flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Nozzle angle

The nozzle should be held perpendicular (about 90 degrees, plus or minus 10 degrees) to the receiving surface to maximize compaction and bond while minimizing rebound and shadowing behind reinforcement.

Nozzle distance

A typical nozzle-to-surface distance is 0.6 to 1.5 m (about 2 to 5 ft); too close causes gouging and rebound, too far reduces impact velocity and compaction.

Rebound

Material that bounces off the surface during shooting; it is highest on overhead work and behind reinforcement and must never be reused or troweled back into the work.

Lift thickness

Shotcrete is built up in controlled passes, typically 50 to 100 mm (2 to 4 in) per pass on vertical and overhead work, with thinner first passes overhead.

Nozzle motion

A steady circular or elliptical motion with overlapping passes distributes material evenly and avoids sagging, trapped voids, and localized overbuild.

Shooting sequence

On vertical walls, start at the bottom and work upward so the lower edge does not collect rebound; encase reinforcement by shooting from behind and around it.

Water at the nozzle

In wet-mix shotcrete the nozzleman may adjust water at the discharge point for consistency, but total water-to-cementitious-material ratio must remain within the approved mix design.

Cementitious content

Typical wet-mix shotcrete contains roughly 350 to 450 kg/m3 (about 590 to 760 lb/yd3) of cementitious material; higher paste content aids pumpability but increases shrinkage.

Water-cementitious ratio

Total water (batch water plus any nozzle water) divided by cementitious material; typical field values are about 0.40 to 0.50 and must not exceed the approved design.

Maximum aggregate size

Wet-mix shotcrete aggregate is generally limited to about 10 to 12 mm (3/8 to 1/2 in) to permit pumping and reduce rebound and line blockage.

Supplementary cementitious materials

Fly ash, slag, and silica fume improve pumpability, cohesion, and durability; silica fume also reduces rebound and permeability of shotcrete.

Air entrainment

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

Accelerators

Admixtures added at the nozzle to shorten set time and allow thicker overhead lifts; non-chloride types are used where embedded steel is present, and dosage must be controlled.

Delivery line size

Wet-mix shotcrete is commonly pumped through 38 to 50 mm (1.5 to 2 in) lines; line size, pump output, and nozzle air supply must be matched to avoid blockages.

Nozzle air pressure

Air pressure at the nozzle is commonly about 0.5 to 0.7 MPa (70 to 100 psi), or as specified by the equipment manufacturer, to propel material onto the surface.

Pumping continuity

A steady, continuous pumping rate prevents slug flow and surging, which cause inconsistent nozzle discharge and blockages; lines are pre-dampened before shooting.

Surface preparation

Remove loose material, dust, oil, and deteriorated concrete; roughen and clean the substrate so the new shotcrete can develop bond.

Pre-wetting substrate

Absorptive substrates are brought to a saturated-surface-dry condition before shooting so the new shotcrete does not lose water to the base; standing water is removed.

Reinforcement encasement

Bars must be clean and free of scale, oil, or form release; shotcrete must fully encase reinforcement, so shoot from the correct angle to fill behind bars.

Scratch coat

A thin first pass applied on vertical and overhead work to establish bond before building thickness; subsequent lifts follow after initial set as required.

Finishing timing

Floating or troweling must be done while the mix is still plastic; working too late tears or delaminates the surface and weakens the skin.

Trimming to line and grade

Fresh shotcrete is trimmed and shaped with a rod or cutting tool to line and grade; overworking the surface draws paste and weakens it.

Construction joints

Joints are formed by cutting back to sound, square edges; the joint surface is wetted before resuming, and feathered edges are avoided.

Curing

After finishing, shotcrete must be kept moist or coated with a curing compound; curing is especially important for thin sections and overhead work.

Mock-up panel

A pre-construction test panel, typically at least 500 mm (about 18 in) square, shot in the same orientation as production work to demonstrate nozzleman technique and provide specimens.

Core testing

Cores taken from panels or in-place shotcrete are tested for compressive strength and compared with the specified strength to verify acceptance.

Bond strength testing

Pull-off or core tests evaluate bond; poor results point to inadequate surface preparation, a dry substrate, or incorrect nozzle angle.

Thickness verification

Placed thickness is checked with probes, cores, or depth gauges during and after placement and must meet the specified minimum.

Nozzleman qualification

The nozzleman demonstrates competence through the mock-up panel; qualification is tied to the shooting orientation and the approved procedure.

Silica exposure control

Respirable crystalline silica controls are required: wet methods, ventilation, and respiratory protection, along with eye, hearing, and hand protection for the crew.

Emergency shutdown

The nozzleman must be able to shut off material feed and air quickly; the nozzle is always pointed away from personnel.

Defect repair

Voids, delamination, and low-strength areas are removed back to sound material, the surface is prepared, and the area is re-shot or patched with approved repair material.