ACI Shotcrete Nozzleman Wet-Mix key terms

Material that bounces off the receiving surface during spraying instead of adhering. It is richer in coarse aggregate and lower in cement paste than the as-batched mix, so it must never be reused in the work.

The angle between the nozzle axis and the receiving surface. Holding the nozzle perpendicular (90 degrees) minimizes rebound and produces the densest, best-compacted in-place material.

The standoff between the nozzle tip and the surface being sprayed. Too close causes gouging and uneven build; too far increases rebound and reduces impact velocity and compaction.

Complete coating of aggregate particles and reinforcement by cement paste and fines during impact. Good encapsulation is the main goal of proper nozzle technique and depends on velocity, angle, and mix consistency.

Densification of the shotcrete as particles strike the surface at high velocity. It is achieved by the impact energy of the stream rather than by mechanical vibration.

A manifold around the nozzle that injects compressed air into the material stream. It controls the velocity and pattern of the spray and helps break the mix into a uniform, well-compacted stream.

A manifold at the nozzle of a dry-mix system that introduces mixing water into the dry material stream. In wet-mix work the water is already in the mix, so the ring is not used to add mixing water.

Shotcrete in which cement, aggregate, water, and admixtures are pre-mixed before entering the delivery line. Only air (and sometimes accelerator at the nozzle) is added during spraying.

Shotcrete in which dry cement and aggregate are conveyed pneumatically and water is added at the nozzle. The nozzleman controls the water content by feel and observation of the material.

An admixture added at the nozzle to shorten setting time and increase early strength, especially on overhead or vertical work. Dosage must be controlled to avoid excessive strength loss or durability problems.

A metering pump that injects liquid accelerator into the material stream at the nozzle. Its rate must be calibrated and monitored so the accelerator is uniformly and correctly proportioned.

A measure of the consistency of a wet concrete or shotcrete mixture by the standard slump cone test. Wet-mix shotcrete typically uses a relatively low slump to reduce rebound and sagging.

The workability or flow behavior of the wet mix as it moves through the pump and hose and lands on the surface. It affects pumpability, rebound, and the ability to build thickness.

The ability of a wet mix to be conveyed through the pump and delivery line without segregation, blockage, or excessive pressure. It depends on gradation, paste content, and consistency.

The hose or pipe that carries the wet mix from the pump to the nozzle. Its diameter, length, and routing affect pressure, flow, and the quality of the stream at the nozzle.

The terminal fitting of the delivery line where the material stream is formed and directed. It includes the air ring and, where used, the accelerator injection point.

The skilled operator who directs the spray and controls build-up, angle, distance, and pattern. The quality of the finished shotcrete depends heavily on the nozzleman's technique and experience.

A historical term for dry-mix shotcrete, originally applied to pneumatically placed mortar. It is not a synonym for wet-mix shotcrete and is best avoided in modern specifications.

Cleaning and roughening the receiving surface before spraying so the shotcrete bonds properly. It includes removing loose material, dust, oil, and laitance, and often chipping or sandblasting.

A weak, cement-rich layer that forms on the surface of fresh or hardened concrete. It must be removed before shotcrete is applied because it prevents a strong bond.

The adhesion between shotcrete and the substrate, or between successive layers. It depends on surface preparation, moisture condition, and proper spraying technique.

A plane between two layers of shotcrete that were not properly bonded, often because the first layer had already set. It is a weak plane that can allow delamination or water penetration.

A single layer of shotcrete applied in one continuous operation before the next layer is placed. Lift thickness is limited by the mix, orientation of the surface, and whether accelerator is used.

The process of accumulating shotcrete to the required thickness by repeated passes of the nozzle. Overhead and vertical work require thinner passes to avoid sagging and fall-out.

Downward slumping or drooping of freshly placed shotcrete on vertical or overhead surfaces, usually caused by too wet a mix, excessive thickness, or insufficient accelerator.

Shotcrete material that detaches and drops from the surface after being placed, often on overhead work. It signals poor adhesion, excessive thickness, or an unsuitable mix.

Spraying shotcrete around and behind reinforcing steel so the bars are fully surrounded by material. The nozzleman must angle the stream to fill behind the bar without leaving voids.

A void or poorly filled zone behind reinforcement caused when the bar blocks the stream from reaching the surface. It is avoided by correct nozzle angle and spraying from several directions.

A sample of shotcrete sprayed under site conditions, used for testing strength, density, or bond. It should be produced by the same nozzleman and with the same mix and technique as the work.

A cylindrical sample drilled from the in-place shotcrete or from a test panel for strength or density testing. Cores show the actual quality of the placed material.

The load-carrying capacity of shotcrete in compression, usually measured on cores or cubes. It is the primary acceptance property and is affected by mix design, compaction, and curing.

Maintaining adequate moisture and temperature so the shotcrete continues to gain strength and durability. It is especially important for thin layers that dry quickly.

A joint formed before spraying where fresh shotcrete meets an existing surface, such as a construction joint or a previously placed layer. It must be cleaned and prepared for bond.

Restoring shotcrete that is defective, damaged, or out of tolerance. It typically involves removing unsound material to a sound edge, preparing the surface, and re-spraying or patching.

Safety gear worn during shotcrete work, including eye protection, hearing protection, gloves, and respiratory protection. It protects against rebound, dust, and noise hazards.

Airborne crystalline silica particles released when handling cement and aggregate or when spraying. It is a serious health hazard and requires dust controls and respiratory protection.

The practice of collecting and reusing rebound material in the work. It is prohibited because rebound has lost fines and paste and will produce weak, non-conforming shotcrete.