scope:

Work specified (shotcreting process)-The work should be classified as either structural or nonstructural. Shotcrete having a specified compressive strength of 4000 psi (28 MPa) or greater is considered structural shotcrete. Shotcreting can be applied by one of the two processes: wet-mix or dry-mix. Shotcrete is further described according to the size of aggregate used (coarse or fine).

Dry-mix process-The dry-mix process consists of five steps and are as follows:

1. All dry ingredients, except water, are thoroughly mixed. Dry ingredients are predampened to contain approximately 6% moisture.

2. The cementitious aggregate mixture is fed into a special mechanical feeder or gun called the delivery equipment.

3. The mixture is usually introduced into the delivery hose by a metering device such as a feed wheel, rotor, or feed bowl. Some equipment uses air pressure alone (orifice feed) to deliver the material into the hoses.

4. The material is carried by compressed air through the delivery hose to a nozzle body. The nozzle body is fitted inside with a water ring through which water is introduced under pressure and thoroughly mixes with the other ingredients.

5. The material is jetted from the nozzle at high velocity onto the surface to be shotcreted.

We-mix process-The wet-mix process consists of five steps and are as follows:

1. All ingredients, including mixing water, are thoroughly mixed.

2. The shotcrete mixture is introduced into the chamber of the delivery equipment.

3. The mixture is metered into the delivery hose and moved by positive displacement.

4. Compressed air is injected at the nozzle to increase velocity and improve the shooting pattern.

5. The concrete is jetted from the nozzle at high velocity onto the surface to be shotcreted.

Comparison of processes-Either process can produce shotcrete suitable for typical construction requirements. Differences in capital and maintenance cost of equipment, operational features, suitability of available aggregate, and placement characteristics, however, may make one or the other more attractive for a particular application.

Coarse aggregate shotcrete-Although in its early years shotcreting was performed with fine-aggregate-based materials (mortar), most of today's applications (both wet-mix and dry-mix) include larger coarse aggregate (refer to Table 1.1.1 Grading No. 2).

There are six reasons for adding coarse aggregate to shotcrete:

1. Less cementitious materials required: The reduced surface area of coarse aggregate versus fine aggregate permits reducing cementitious content.

2. Reduced shrinkage: Coarse aggregate reduces drying shrinkage by reducing water and cementitious content.

3. Pumpability: The addition of coarse aggregate may improve pumpability for wet-mix.

4. In-place-density: The addition of coarse aggregate into plastic shotcrete improves the in-place density.

5. Improved economy: Reduction of cementitious content improves economy.

6. Sustainability: Reduction of cementitious content contributes to sustainability.

For both the dry-mix and wet-mix processes, however, coarse-aggregate shotcrete with more than 30% coarse aggregate as a percentage of total aggregate has greater rebound, is more difficult to finish, and cannot be used for thin layers. Coarse aggregate shotcrete requires the use of larger-diameter hoses and may create craters in the plastic shotcrete.

Types of shotcrete

Conventional shotcrete-Conventional shotcrete (shotcrete without special admixtures) is the most commonly used application for shotcrete and includes the following:

(a) New structures-Roofs, thin shells, walls, prestressed tanks, buildings, reservoirs, canals, swimming pools, boats, sewers, foundation shoring, ductwork, shafts, and artificial rock

(b) Linings and coatings-Over brick, masonry, earth, and rock; underground support, tunnels, slope protection, erosion control, fireproofing of steel, steel pipeline, stacks, hoppers, bunkers, steel, wood, and concrete; pipe protection; and structural steel encasement

(c) Repair-For deteriorated concrete in bridges, culverts, sewers, dams, reservoir linings, grain elevators, tunnels, shafts, waterfront structures, buildings, tanks, piers, seawalls, brick, masonry, and steel structures

(d) Strengthening and reinforcing-To strengthen and reinforce concrete beams, columns, slabs, concrete and masonry walls, steel stacks, tanks, and pipes; used for seismic rehabilitation of shear walls, boundary elements, beams, columns, overhead joists, and slabs, and for strengthening of existing masonry and concrete walls; and used in structural interiors and exteriors because of its speed and flexibility of application

(e) Ground support-Extensively used as temporary and permanent ground support. It has become the primary method of ground support in mining and tunneling (ACI 506.5R). Shotcrete is also used extensively as lagging instead of wood for soldier pile and lagging shoring systems, and as the lagging in soil nailing (Society of Mining Engineers 2011). Soil nailing using shotcrete is a method of shoring that is used for both temporary and permanent ground support of soil retention systems

Refractory shotcrete-Shotcrete applications using high-temperature binders and refractory aggregates in refractory construction began in the mid-1920s, where it was used primarily for repair and maintenance of furnace linings. The refractory industry favors shotcrete because of the speed of installation and general effectiveness of the process. Shotcrete has become a major method of installation for all types of linings from several inches to several feet thick and is used in new construction and for repair and maintenance in steel and nonferrous metal; chemical, mineral, and ceramic processing plants; steam power plants; and incinerators.

Refractory shotcrete provides a viable alternate to traditional methods of refractory construction. Hot gunning procedures for high-temperature installations and bench shooting for thick layers have opened new fields for refractory shotcrete use.

Special shotcrete-Special shotcrete includes proprietary mixtures for corrosion- and chemical-resistant protection. Portland cement with admixtures or other types of cements are used to produce special corrosion- and chemical-resistant properties. Special cements include magnesium phosphate cement and calcium aluminate cement. Special shotcrete applications are used for caustic and acid storage basins, chimneys and stacks, process vessels, chemical spillage areas, sumps, trenches, pollution control systems, and concrete repair in other highly aggressive environments. The application of polymer (latex) shotcrete is not recommended due to numerous failures and is not covered in this document.

Fiber-reinforced shotcrete-The addition of steel or synthetic fibers in conventional and refractory shotcrete has been gaining favor during the past four decades. Fibers at normal addition rates (typically between 0.3 and 1.0 % volume fraction) can provide improved flexural and shear capacity, fracture toughness, and impact resistance. For refractory shotcrete, stainless steel fibers increase resistance to thermal shock, temperature cycling damage, and crack development. The addition of polyolefin fibers and other low-melting-point fibers improve the fire resistance of shotcrete due to the fibers creating steam relief vents when exposed to fire (Tatnall 2002).

Some specific applications where fiber-reinforced shotcrete (FRS) can be cost effective are slope protection; ground support in tunnels and mines; concrete repair; swimming pools; thin shell configurations; and refractory applications such as boilers, furnaces, coke ovens, and petrochemical linings (Sections 2.1.7.4, 2.3, and 2.4.8).