Shotcrete is a construction material that is pneumatically applied at high velocity onto a surface. It is a method of placing concrete or mortar using a spray gun. The process involves a mix of cement, aggregates, and water, which is typically pre-blended, being propelled through a hose and sprayed onto a substrate.
Shotcrete is commonly used for structural repairs, tunnel construction, swimming pool construction, and stabilization of slopes.
What Is Shotcrete?
Shotcrete is a method of applying concrete proposed at high velocity primarily on a vertical or overhead surface. It is a mortar or high-performance concrete conveyed through a hosepipe and pneumatically shot at high force at the back surface. The force of this spraying action leads to compaction of the concrete which then forms layers of concrete to the necessary thickness.
The impact created by the application consolidates the concrete. Even though the hardened properties of shotcrete are related to those of conservative cast-in-site concrete, the nature of the settlement process results in an excellent bond with most substrates, and rapid capabilities, particularly on composite forms or shapes.
It has proved to be the greatest method for the construction of curved surfaces. Tunnel linings and domes are now much easier to construct with the advent of shotcrete technology.
The shotcrete process needs less formwork and can be more cost-effective than traditionally placed concrete. It is applied by a wet- or dry-mix process. The wet-mix shotcrete method mixes all components with water before the introduction into the delivery hose.
The dry-mix process adds water to the fusion at the nozzle. It is used in repairs and new construction also it is suitable for curved and thin elements.
Types of Shotcrete
There are two basic types of shortcrete
Dry mix shotcrete (DMS)
Wet Mix shotcrete (WMS)
It is based on the period at which the water is added to the concrete mix or mortar. In the Dry mix shotcrete (DMS), water is added at the nozzle, while water is added in the mortar mix or concrete in the wet mix shotcrete (WMS).
1. Dry Mix Process for Shotcrete (DMS)
Dry-mix shotcrete the water is added at the nozzle.”
The Principle of Dry Mix Shotcrete (DMS)
It involves the mixing of cement and wet aggregates at the required ratio before supplying it to the device. The systematically mixed ingredients are placed on the device hopper. For the period of the operation, the mix, under the act of compressed air is taken from the hopper to the nozzle through the delivery hosepipe of the apparatus.
The quantity of water included in the dry-mix process is controlled by the nozzleman. The amount of water is controlled using a valve. In limited access areas, the dry mix process is applied.
Advantages of the Dry Mix Process
- Improved Bonding ability and bonding strength.
- Water content is controlled at the end of the nozzle.
- Less waste is generated during the dry process.
- Low investment.
Disadvantages of the Dry Mix Process
- Required skilled nozzleman to control the water content mixed through nozzle tuning.
- The spraying procedure is tough.
- High dust release.
- The Water-cement ratio of the mix is based on the nozzleman.
- Difficulty in addition to additives.
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2. Wet Mix Process for Shotcrete (WMS)
The wet mix process involves the mixing of all components to form concrete or mortar through the required water content. The mix is prepared before placing it in the equipment. The delivery equipment used can be a pneumatic -feed.
The Principle of Wet Mix Shotcrete (WMS)
The procedure involves pushing the wet mix to the nozzle through the delivery hosepipe using compressed air. The mix shot at the high force on the surface. The mixed ingredient is delivered to the pump using a transit truck.
The rate and amount can be controlled manually. As the wet mix requires transportation facilities. It works in large-area construction.
Advantages of Wet Mix Process
- The water content (WC) is managed more accurately.
- The cost of the process is lower.
- Less labor is required.
- The spraying process is easy.
- The consumption of air is less.
- Additives can be added as per the design requirement.
Disadvantages of the Wet Mix Process
- Wastage of materials is high
- Transportation of mix will require high control.
Shotcrete Wall
A shotcrete wall is constructed by using a special mix of high-strength concrete, which is sprayed at high velocity on the wall using compressed air, to form a retaining structure, drain, or wall.
shotcrete walls provide a permanent and cost-effective solution for retaining wall construction. Shortcreting on wall strengthens and protect the natural earth embankment as well as covers sheet-piled walls.
Shotcrete Wall Benefits
Shotcrete Wall Benefits are as follows,
- Provides a permanent solution for retaining wall
- Shotcreting strengthen and protect the earth’s banks
- Shotcrete walls are cost-effective compared to normal retaining walls
- Higher construction speed compared to cast in situ concrete walls
- ACI specifications are available for shotcrete wall construction
- Utilize shotcrete technology as an amendment order during construction.
- It offers High durability, High strength & low permeability
- As concreting is done very fast with spraying, labour cost is low
- Provides good bonding strength to the earth’s surface
- It reduces the use of
- forming material up to 100%
- Eliminate the need for extra footing for the retaining wall
- Reduce the time required for form setting and concrete pouring
Shotcreting Equipment
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Most shotcrete is batched and mixed in the site using transferrable mixing equipment or delivered in trucks from a local ready-mixed plant. The mixing equipment is of the consignment or the continuous type.
In the continuous type, individual ingredients are served to a mixer screw using variable speed augers, belt-feed systems, or a pattern of both. A hopper is occasionally used in high manufacturing units to collect and feed the mixture as necessary.
Water-metering systems are also available to re-dampen the mixture. Batching and mixing equipment must be efficient in maintaining an acceptable and continuous flow of uniform material.
Batching by mass is ideal and will normally be mandatory. Water may be batched by mass or volume.
For minor work, approval may be given to batching by a volumetric container, as long as periodic weight checks are made since many shotcrete jobs have a low production rate and are in isolated locations, mixing is often done by a small drum mixer at the job site.
Applications of Shotcrete
The range of shotcrete for a specific application should be based on information, understanding, and a careful study of necessary and feasible material performance. The achievement of the shotcrete for that application is contingent upon proper scheduling and administration, plus the skill and continuous attention provided by the shotcrete applicator.
1. Repairs
It can be used to repair the damaged surface of concrete, timber, or steel structures provided there is access to the surface needing repair. The following examples specify a few ways in which shotcrete can be used in repairs:
2. Bridges
Its restoration can be used for bridge deck rehabilitation, but it has normally been uneconomic for general full-thickness repairs. It is useful for beam repairs of flexible depths, caps, columns, abutments, wing walls, and under decks from the angle of technique and cost.
3. Buildings
In building maintenance, It is generally used for the restoration of fire and earthquake damage and wear, strengthening walls, and enclosing structural steel for fireproofing.
The restoration of structural members such as beams, columns, and joints is common for structures damaged by an earthquake.
4. Marine Structure
Destruction of marine structures can result from a relapse of the concrete and the reinforcement. Destructive circumstances are erosion of the steel, freezing and defrosting action, bearing loading, structural distress, physical abrasion from the action of waves, sand, gravel, and chemical occurrence due to sulfates.
5. Spillway Surfaces
High-velocity flow may be effective by cavitation erosion or abrasion erosion. Its repairs are beneficial because of the comparatively short outage required to complete the repairs.
6. Underground Excavation
It is used in underground excavations in rock; but on occurrence, it has been effectively used in the expansion of tunnels through rehabilitated cohesion-less, and loose soils.
7. Slope and Surface Protection
It is frequently used for temporary shelter on unprotected rock surfaces that will deteriorate when exposed to the atmosphere.
It is used to permanently cover slopes that may grind down in time. Slope protection should be appropriately drained to avoid damage from extreme uplift pressure.
8. New Structures
Where large areas and thin sections are involved, shotcreting can be used effectively to save time.
9. Pools and Tanks
10. Shotcrete Domes
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