Shotcrete or Sprayed Concrete or Gunite -Properties, Materials, Uses

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Shotcrete or sprayed concrete is also called as gunite or shotcrete is used for rapid concrete construction. Shotcrete is used for repair of damaged buildings, underground construction such as storage reservoir, tunnel linings as well as for new constructions. Properties, materials and applications of sprayed concrete is discussed. Stabilization and support for the structures can be attained with best quality and performance with the help of an innovative construction technique, called the "sprayed concrete". The method of sprayed concrete makes use of man, machine, and the concrete. This is a high - performance material, where the person who is assigned as the nozzle man for this purpose must gain great skill and dedication in the application. A complete rely on the machine and the sprayed concrete is ensured, as this is a " three component success", i.e. man, machine, and concrete. The quality and the interaction between these three materials governs the ultimate strength and the performance of the sprayed concrete. It is a flexible, economic and a rapid construction method that requires mechanization at a higher rate and works who are trained or specialized in the same.

History of Sprayed Concrete

Concrete accounts to be more versatile construction material, that is strong and benefitted more massive structures. The fluidity of concrete, before it starts setting, helps to attain any required shape. The only drawback is that we must bring formworks or shuttering works with respect to the shape the structure must take. In 1895, the curator Dr. Carlton Akeley was looking for a device that will help in making models of prehistoric animals. Even if the skeleton shape could be formed, it was difficult to make the mortar to take that shape with the convention method. For this, a device was developed by Dr. Akeley to facilitate the spraying of mortar mixture. A single chamber pressure vessel was developed, that contained a mixture of cement, sand, and water. When the vessel is pressurized with compressed air, the mix will be sprayed out through a nozzle. A water spray was inbuilt inside, as when the cement and sand are pressurized they get hydrated at the water spray area and come out as a mortar mix. This equipment is called as the "cement gun". This system works based on the "dry mix process", as the dry miss is hydrated separately. This was later altered by the "true gun method". The true gun method initially mixes all the three elements, i.e. the cement, sand, and water before being pumped. This system hence makes use of the "wet process".

Sprayed Concrete Vs. Shotcrete

The sprayed concrete is also called as the Gunite. In some areas, the sprayed concrete is also called as Shotcrete. But the Shotcrete is designated as a mix that has aggregate with a maximum size of 10mm.

Properties of Sprayed Concrete

When compared to the poured concrete, the sprayed concrete gain certain properties that make it unique and superior. The properties that are obtained uniquely is because of the different methods that are employed for mixing, transporting and placing rather than the component materials that are used. The properties of sprayed concrete based on following are explained below:

• Water-Cement Ratio
• Strength
• Density and Permeability
• Freeze and thaw resistance
• Adhesion and Bond Strength
• Thickness

Water Cement Ratio of Sprayed Concrete

The sprayed concrete possesses a water cement ratio that is lesser than a conventional poured concrete. This condition is seen more in the dry process, where sagging is avoided by a lower water-cement ratio. In the case of a wet process, this low value of water cement ratio is obtained with the help of a plasticizer.

The Strength of Sprayed Concrete

It is observed that the sprayed concrete can gain higher compressive strength at an early stage, by employing a lower water-cement ratio. The compaction of the sprayed concrete mass is carried out by spraying at a higher velocity. Hence compaction tends to increase the strength. When using batched materials, rapid strength gain is achieved. It is accounted that 30 % higher strength than the poured concrete is obtained for the sprayed concrete.

Density and Permeability

As mentioned above, compaction of the mix is employed by the high-velocity application. This compaction makes the mix dense and free from voids. This will decrease the permeability of the sprayed concrete structure.

Freeze and Thaw Resistance of Sprayed Concrete

The problem of freezing and thawing exist with an increase of permeability. As in the case of sprayed concrete, we have high impermeability property that makes them resistant towards freezing and thawing problems. This will help in reduction of crack formation and propagation. The resistance in terms of abrasion is also increased due to its watertight property. Using fiber reinforced concrete will contribute more to the sprayed concrete resistance properties.

Adhesion and Bond Strength

The sprayed concrete facilitates good bonding and surface tension with the substrate. This property of sprayed concrete will reduce the use of bonding agents or coatings, except in certain conditions.

Sprayed Concrete Thickness

The fluidity of sprayed concrete make them be sprayed in higher speed and volumes. After excavation, the tunnel linings or the retaining wall lining can be immediately sprayed with the lining mass. In the USA, walls that have a thickness of 1 m have been constructed with reinforcement provided at a higher density. The application of sprayed concrete in multiple thicknesses will help in reduction of thermal stresses in the construction.

Constituent Materials for Sprayed Concrete

The sprayed concrete can be manufactured either by the dry process or the wet process. The materials for the process is either ready mixed supplied or pre-proportioned in the factory. The materials based on-site batching of sprayed concrete is explained below:

Cements for Sprayed Concrete

For most of the sprayed concrete applications, Portland cement is used. They should comply with the codes BS12 and EN 197. In general class 42.5 or Class 52.5R are the main two types of cement that are recommended for the this concrete. Based on the national standards, regulations and the area where it is used, other cement can be employed. There are other options that are alternative for the OPC. The calcium aluminate cement can be used in areas of higher temperature, natural cement for lower heat generation and faster setting needs and the sulfate resisting cement to resist the sulfate attack. To have a general rule in selection, higher reactivity in terms of setting time and strength gain at early stages can be obtained by having a cement of higher C3A content and higher specific surface.

Additions to Cement

The following choice of additions are applicable: Pulverized Fuel Ash: Pulverized fuel ash is inorganic pozzolanic material that is finely divided in structure. It can be added to the concrete either during the wet or the dry batching process. This addition helps in improving the plastic properties and certain properties of concrete in the hardened state. The fly ash used here should be based on the EN 450 and BS 4328. Ground Granulated Blast Furnace Slag (GGBS): A latent hydraulic binding material. This improves the properties of concrete in fresh and the hardened state. BS66999: 1992 provides the recommendation for the use of GGBS in this  concrete manufacture. Condensed Silica Fume: This is a highly active inorganic material that can be added during the wet or dry batching of concrete. The structure is finely divided, improving the pumpability, adhesion property, and cohesiveness of the mix. A Certain improvement in hardened properties is also observed. Pigments: The code EN 206 give the recommendation for sprayed concrete requirements. Table-1: Maximum Addition Level of Supplementary Cementitious Material

Supplementary Materials	Maximum Addition Amount in %
Silica Fume	15% of the Portland Cement
Fly ash	30% of OPC 15 % of Fly Ash Cement 20% of Portland Blast Furnace Slag
GGBS	30% of the Portland Cement

Aggregates for Sprayed Concrete

The code BS 882 gives the recommendation for the aggregates that is used in sprayed concrete. The aggregates must be checked for their susceptibility towards the alkali-aggregate reaction. The figure-1 given below shows the aggregate gradation curve. This is employed for both the wet and dry process manufacture of sprayed concrete. The aggregate should lie in respective envelopes as shown in the figure-1.

Figure.1: The Aggregate Grading Curves for Aggregates employed for Wet and Dry Process of Sprayed Concrete Manufacture.

Choosing the most appropriate aggregate for the manufacture of this concrete lies in the hands of the contractor in charge. A maximum moisture content of 6% is only accepted for the aggregates that are employed for the dry mix process. It is also advised to have aggregates that have a constant moisture content. If necessary lightweight aggregates can be used with special reference and specification of the product.

Mixing Water Quality

This Concrete make use of potable water. Other sources of water that will comply with BS 3148 is suitable. The temperature of the water during mixing will influence the final temperature of the sprayed concrete mix. Hence a check on the same must be made

Admixtures for Sprayed Concrete

The following admixtures can be employed in increasing the properties of this concrete:

Accelerators

The overhead stability and thickness gain can be achieved by a concrete that has early setting property. This is facilitated by accelerators, which increases the stiffening rate as well as the setting time. The accelerator dosage should be maintained properly to have better cohesion and long-term performance. Now different range and type of the accelerator are available in the market. Each product must be properly studied and must take assistance from the respective manufacturer, before using it in the mix.

Superplasticizers and plasticizers

This admixture helps in increasing the workability without the addition of water. This also reduces the water content without compromising the quality. Superplasticizers provide better water reduction compared to plasticizers.

Hydration Control Admixtures

To facilitate the open time for transportation these admixtures are used. This maintains the workability so that nothing is lost if any delay in hauling is taking place. The final quality of the sprayed concrete mix is not affected.

Retarders

To reduce the setting time of the concrete, the retarders can be employed. Before the project commences, pre-construction test with the actual materials and the mix design can be carried out. This will help in knowing the correct dosage requirement for the manufacture.

Use of Fibers in Sprayed Concrete

The below are the gained properties of sprayed concrete with the addition of fibers:

• The plastic shrinkage cracks are reduced.
• The thermal cracking is controlled
• The fire resistance of the structure is improved
• The ductility and the toughness properties of the concrete is increased
• The tensile and the flexural strength of the concrete structure is increased
• More abrasion and impact resistance is gained

Steel Reinforcement for Sprayed Concrete

The flexural strength is increased with the use of steel reinforcement. The reinforcement also helps in the control of crack and its propagation. For layers of sprayed concrete structure that are less than 50mm, steel reinforcement in the form of fabric is best suggested. In most of the cases, steel meshes of 50 to 150mm along with a wire diameter greater than 10mm are widely employed.

Curing Agents

The water evaporation that is seen in sprayed concrete mix will affect the hydration of the structure. Liquid curing agents will help in reducing the evaporation and hence promotes hydration. There are two main types of curing agents that are available:

1. Externally spray applied curing agents
2. Internally applied curing agents

Based on the technical instructions that are provided by the respective manufacturers, the agents must be used carefully.

Applications and Uses of Sprayed Concrete

Following are the different applications and uses of sprayed concrete in construction works:

• Underground Construction: Storage reservoir construction, tunnel linings
• New Construction: Silo structures, Bank Vaults, Caissons, Retaining Wall, Shell roofs and Domes
• Free Formed Structures: Theme parks, swimming pools, climbing walls, landscaping etc.
• Strengthening and repair: Housing damages, reinforcement corrosion damage of concrete, fire damaged structure etc.
• Protective Coatings: For rock and soil stabilization, refractory lining, pipeline encasements, structural steel fire protection works

Read More: Sprayed Concrete Mix -Process, Methods, Equipments And Advantages Special Concrete Types of Concrete