MATERIALS

Shotcrete, a pneumatically applied concrete, plays a vital role in modern construction and repair. Its effectiveness largely depends on the careful selection and combination of various constituent materials, each contributing unique properties to the final product.

Constituent Materials of Shotcrete

1. Cement: The primary binding material in shotcrete, typically Portland or blended cement. It's used alone or combined with supplementary cementitious materials, adhering to Australian standards​​.
2. Supplementary Cementitious Materials: These materials enhance the properties of the cement. They must comply with specific Australian standards to ensure quality and consistency.
3. Aggregates: A crucial component, comprising sand and coarser aggregates, which contribute to the strength and texture of shotcrete​​.
4. Mixing Water: The water/binder ratio is critical as it affects the strength, shrinkage, and durability of the shotcrete​​.
5. Chemical Admixtures: Used to modify the properties of the shotcrete for specific requirements, such as improving workability or reducing water content​​.
6. Fibre Reinforcement: Often added to enhance the tensile strength and reduce the potential for cracking.
7. Steel Mesh or Bar Reinforcement: Used in certain applications to provide additional structural strength.
8. Other Additives: May include colored pigments, additives for permeability, shrinkage-control, or internal curing​​.

Mix Design

• General
• Wet-Mix Shotcrete
• Dry-Mix Shotcrete
• Swimming Pool Mix Design
• Special Mixes

Properties of Shotcrete Materials

• Bleeding: Refers to the movement of water from inside to the surface, caused by the separation of water from solid ingredients in the mix​​.
• Build-up: The increase in thickness with successive passes of shotcrete, crucial for achieving the desired structural dimensions and strength​​.
• Cohesion: The ability of the mix to remain homogeneously bound together during transport and application, crucial for the structural integrity of shotcrete​​.

Mix Design Considerations

• Application-specific Requirements: The mix must be designed to be easily sprayable for the intended application, with minimal rebound and maximum adherence to the substrate​​.
• Comparisons with Conventional Concrete: The major differences between shotcrete and conventional concrete include aggregate gradation, cementitious content, conveyance method, placement, and chemical admixtures selection​​.

The selection and combination of materials in shotcrete are critical to its performance in various applications. Understanding these materials and their interactions is key to achieving the desired properties in the final shotcrete product, be it for construction, repair, or reinforcement purposes.

Shotcrete Constituents

Shotcrete consists of cementitious materials, fine and coarse aggregates, water, additives (pigments) and admixtures and often fibres. The water/binder (w/b) ratio is the mass of water divided by the total mass of binder comprising all cementitious materials in the shotcrete mix. The water/binder ratio is important as it has a major effect on the strength, shrinkage, and durability of shotcrete.

Cement
Cement - A hydraulic binding material comprising Portland or blended cement complying with Australian Standard AS 3972 alone or in combination with one or more supplementary cementitious materials complying with the applicable part(s) of AS 3582.

Grade 1 Fly Ash
Fly ash is a finely-divided inorganic pozzolan material which can be added to concrete and mortar to improve or achieve certain properties in the fresh and/or hardened states. Grade 1 fly ash for use in shotcrete should comply with the requirements of AS 3582.1.

Silica Fume
Silica fume is a form of amorphous silica (AS 3582.3) and is a finely-divided, densified highly-reactive inorganic pozzolanic material which can be added to shotcrete to improve or achieve certain properties in the fresh and/or hardened states.

Slag – Ground Granulated Blast Furnace Slag (GGBFS)
Ground granulated iron blast-furnace slag (AS 3582.2) is a fine granular latent hydraulic binding material which can be added to concrete and mortar to improve or achieve certain properties in the fresh and/or hardened states.

Aggregates

All aggregates should comply with Australian Standard AS 2758.1. Each individual aggregate in the mix should have a consistent grading in accordance with the allowable variation of AS 2758.1 from the original aggregate proposed for use.

Mixing Water

Water quality can have a significant effect on shotcrete performance. Mixing water should be drawn from a source of acceptable quality complying with Australian Standard AS 1379 and comprise potable water if possible.

Chemical Admixtures

Admixture – any material added to concrete before or during mixing, other than cementitious materials, water, aggregates and fibre reinforcement.

General
Chemical admixtures and their use should comply with Australian Standard AS 1478.1 where applicable.
Shotcrete set accelerators and other admixtures, which are added to the shotcrete at the nozzle or at the delivery hose, should be dispensed by calibrated mechanical means at dose rates not exceeding the maximum recommended by the manufacturer.

Water Reducers
Water reducers are used to improve workability and/or reduce the water/binder ratio.
Refer to manufacturer’s recommendations and AS 1478.1 for specific details. Water reducers may be formulated to have neutral setting, set retarding or set accelerating characteristics.
According to AS 1478 water reducing admixtures can be classified into three major group:

• Water Reducers (WR)
• Mid Range Water Reducers (MWR)
• High Range Water Reducers (HRWR) /Superplasticisers

Hydration Control Admixtures
Concrete that is required to be transported for considerable distances from the batching plant to the site, delays in construction schedules as well as plant and equipment breakdowns ensure that much of the concrete actually sprayed is beyond its workable life. HCA admixtures can extend the workability of the shotcrete mix.

Fast Set Accelerators
Fast set accelerators are primarily used to aid the placement of shotcrete by promoting the early setting of the mix which may allow subsequent layers of shotcrete to be applied sooner and at greater thickness. They may also accelerate the early strength development.

Fibre Reinforcement
Fibres - short slender reinforcing elements typically of high tensile capacity. Commercially-available fibres are normally composed of either steel, polymers, or Alkali Resistant (AR) glass. Fibres are widely incorporated in shotcrete to increase toughness. Fibres generally can be categorized as structural (steel and macro-synthetic fibres) and non-structural (micro fibres). Structural fibre post-crack performance should be specified in terms of toughness. Micro-fibres are generally only used to control plastic shrinkage cracking but are also useful for reducing rebound in addition to spalling of shotcrete when subjected to fire loading.

Steel Mesh or Bar Reinforcement
As in conventionally-reinforced concrete, steel mesh or bars is used in situations where shotcrete is required to withstand tensile stresses.

Other Additives
These include coloured pigments, additives for permeability and shrinkage-control, or internal curing, together with others listed in AS 1478.1.

 
Mix Design

General

Many of the principles of normal concrete technology can be applied to the mix design of shotcrete, particularly that produced by the wet-mix process. The major differences between conventional concrete and shotcrete are in aggregate gradation, cementitious content, method of conveyance and placement, and selection of chemical admixtures. The mix design process in particular needs to consider, but is not limited to, issues including:
Pumpability – The mix must be sufficiently workable to pump the shotcrete from the pump hopper to spray nozzle.

• Sprayability – the mix must be capable of being conveyed and placed for the particular application with minimum rebound. Applications may have horizontal, vertical, or overhead surfaces.
• Strength – it must satisfy early-strength and long-term strength requirements, depending on the application. The effect of set accelerators on long-term strength needs to be considered.
• Compaction – the mix must be able to be compacted to form a dense, homogeneous material.
• The design, testing and trialling of a shotcrete mix should be based on the anticipated conditions which will prevail on site so that, under these conditions and with the nominated application method and nozzle operators, shotcrete of the quality specified will be produced.

Wet-Mix Shotcrete
Wet-mix Shotcrete - Shotcrete in which all of the ingredients, including the mixing water, are mixed together before being pumped into the delivery line. Compressed air is introduced to the material flow at the nozzle in order to project the material toward the substrate.

The choice of mix proportions for shotcreting of major infrastructure work is usually based on a specified compressive strength, slump limits, density, flexural strength/toughness, drying shrinkage, permeability, durability (including exposure classifications where nominated), and site application. Pumped mixes normally contain a higher percentage of sand/fines than normal, for lubrication and to eliminate segregation.

Dry-Mix Shotcrete
Dry-mix Shotcrete – Shotcrete in which all the ingredients are conveyed in a dry state by compressed air to the nozzle, where water is added, and the resultant shotcrete is projected onto the substrate via compressed air at high velocity.

Swimming Pool Mix Design
Australian Standard AS 2783 sets out requirements for the structural design and construction of swimming pools constructed wholly or partly of either in situ or pneumatically-applied reinforced concrete.

28-day compressive strength capability of the proposed shotcrete mix to be verified prior to supply by compressive strength testing. It is recommended that the minimum acceptable 28-day compressive strength test result should be 32 MPa for standard cylinder specimens cast from the shotcrete as supplied and 25 MPa for specimens taken from test panels.

Special Mixes
Shotcrete is occasionally required to exhibit special properties (eg. low density , insulating qualities, resistance to heat, resistance to acids, requirements of a special aggregate finish).