ACI Dry-Mix Nozzleman key terms

A placement method in which cementitious material and aggregate are blended dry, conveyed pneumatically through a hose to a nozzle, and combined with water only at the nozzle outlet during spraying.

Adding a small, controlled amount of water to the dry blend before or during pneumatic conveying so the material flows more uniformly and dust is reduced, without fully hydrating the cement.

The mass of water divided by the mass of cementitious material in a mixture; in dry-mix work it is controlled at the nozzle by the operator rather than measured in a mixer.

The distribution of particle sizes in the sand or aggregate portion of a dry blend, which governs pumpability, cohesion at the nozzle, and the amount of rebound.

The proportion of fine particles, generally passing a No. 100 sieve, in the dry blend; adequate fines help the material cohere on the surface and reduce rebound.

A material such as fly ash, slag, silica fume, or metakaolin used to replace part of the portland cement to improve workability, durability, or resistance to chemical attack.

The maximum stress a hardened specimen can carry in compression before failing, typically reported from cores or molded specimens and used to judge whether placed material meets specification.

The mechanical device that receives dry material from a hopper or pressure vessel and feeds it into the delivery hose using compressed air, often through a rotating or vibratory feed mechanism.

The flexible pressure line that carries the dry blend and conveying air from the gun to the nozzle; its diameter, length, and condition strongly affect material flow and output.

The assembly at the discharge end of the hose where dry material meets water and is directed onto the work surface; its size, angle, and condition control spray pattern and quality.

A perforated ring or manifold at the nozzle that injects water into the dry material stream so hydration begins just before the material leaves the nozzle.

The compressed-air pressure supplied to the gun and hose, which controls conveying velocity and output; too little causes surging or blockage, too much causes excessive rebound and dust.

The pressure of water supplied to the nozzle water ring; it must exceed the conveying pressure at the nozzle so water penetrates the material stream evenly.

The machine that supplies compressed air to the dry-mix gun and hose; its capacity and the condition of its filters and moisture traps affect consistent material flow.

The trained operator who holds and directs the nozzle, controls the water addition and spray distance, and is primarily responsible for the quality of the placed material.

The perpendicular distance from the nozzle tip to the receiving surface, typically held at about 0.6 to 1.5 meters so material lands with proper compaction and minimal rebound.

The angle between the nozzle axis and the work surface; the nozzle is normally kept perpendicular to the surface so material is compacted on impact and rebound is minimized.

Material that bounces off the receiving surface during spraying because it is too dry, poorly graded, or sprayed from an incorrect distance or angle; rebound must be removed, not reused.

Placing material completely around and behind reinforcement so the steel is fully embedded with the specified cover, achieved by spraying from the correct angle and distance.

Cleaning and roughening the substrate, removing loose material, dust, oil, and laitance, and pre-wetting as needed so the sprayed material bonds properly.

Moistening an absorptive substrate, such as concrete or masonry, before spraying so it does not draw water out of the freshly placed material and cause dry, weak bond zones.

A material applied to a prepared substrate to improve adhesion of the sprayed layer, used only when specified and compatible with the dry-mix material.

A seam formed where fresh material is sprayed against material that has already stiffened, producing a weak plane unless the older surface is properly prepared and bonded.

Working the freshly sprayed surface with tools such as a float, trowel, or broom to close the surface, achieve the specified texture, and avoid overworking material that has begun to stiffen.

Maintaining adequate moisture and temperature in freshly placed material for a defined period so cement continues to hydrate and the material reaches its intended strength and durability.

A trial area of material sprayed under production conditions to confirm mixture, equipment settings, and nozzle technique before work proceeds, and often used for subsequent testing.

A cylindrical specimen drilled from hardened in-place material and tested to verify thickness, density, and compressive strength of the placed work.

The ease with which water or other fluids pass through hardened material; low permeability is achieved through proper water-cement ratio, compaction, and curing.

The thickness of material between the outer surface and the nearest reinforcement, which protects steel from corrosion and fire and must meet the specified minimum.

Separation of a sprayed layer from the substrate or from a previous layer, often caused by poor surface preparation, dry material, or spraying onto a contaminated surface.

A restriction in the hose or nozzle that stops or surges material flow, commonly caused by oversized aggregate, wet material, low air pressure, or a kinked hose.

A sudden release of compressed air and material from the hose or nozzle, often when a blockage clears; it can cause serious injury and requires safe hose handling procedures.

The violent movement of a pressurized hose when a coupling fails or a blockage releases; hoses must be restrained and couplings inspected to prevent this hazard.

Gear worn to reduce injury risk, including eye protection, hearing protection, gloves, respiratory protection against silica dust, and protective clothing for the nozzleman.

A respirable mineral component of sand and aggregate that can cause silicosis and other lung disease when dust is inhaled; controls include wet methods, ventilation, and respirators.

Measures such as pre-dampening the dry mix, enclosing transfer points, and using ventilation or water sprays to keep airborne dust, including silica, below exposure limits.

Removing defective material and replacing it with properly sprayed material after the area has been prepared, so the finished work meets thickness, strength, and appearance requirements.