Shotcrete, gunite, or sprayed concrete is concrete or mortar that is conveyed through a hose and is pneumatically projected at high velocity on the surface. Shotcrete is a mixture of cement, aggregates, and water that is usually reinforced by conventional steel rods, steel mesh, or fiber.
Shotcreting is performed with the special unit, which consists of a mortar gun or concrete sprayer and a compressor.
Here we learn about shotcrete, methods of shotcrete, advantages & diadvantages of shotcrete.
Introduction to shotcrete:
Shotcrete has proved to be the best method for manufacturing curved surfaces such as dome, tunnel, etc.
Technology improvement gives advanced control the tasks and completed with greater economy in terms of both time and investment.
Why shotcrete is preferred:
- Shotcrete is more economical than conventional concrete because it requires less formwork.
- It only requires a small space for construction and its location.
- In the case of the shotcrete, cement content is high, so it is durable.
- It is resistant to fire, disasters, mold, worms and also has low permeability.
- It also has a good thermal resistance mass.
Properties of shotcrete:
1. Resistance to freezing and thawing:
Wet-mix shotcrete frost resistance is ensured by entering a proper air-void system.
Typically, a mixture of 8 to 12% air content has a proper air-zero system in place.
Although many dry-mix process have performed when subjected to mild freezing and thawing, dry-mix shotcrete’s is more subject to cold and thawing problems than wet-mix shotcrete’s.
This is due to the difficulty in entering the air and creating an adequate air-void system in the dry-mix shotcrete.
The ratio between compressive and flexural strength appears to be similar to that of conventional concrete.
Relationships between water-cement ratio and strength also appear to follow the general pattern with higher strength and lower water-to-cement ratio.
The initial strength can be very high, reaching 7N/mm2 in 5 hours and 21N/mm2 in 24 hours.
Fibers can be added to the shotcrete as a result of a product exhibiting significant load-carrying capacity after the first crack occurance.
The relation of post-crack load capacity to load capacity in the first crack is defined as stiffness.
The type, size, shape, and quantity of fibers determine the extent of this performance.
The density and permeability can be excellent provided good practices are followed in the field.
Manufacturing Process for Shotcrete:
Wet Mixing Process:
In wet-mix process: cement, aggregates, mixtures, and water are mixed together before being ductile and aerated.
Typically, a concrete truck supplies a wet-mix and is pumped through a shotcrete rig for placement.
The wet-mixes are used for large quantities of placements with easy accessibility to construction automobiles.
Advantages of wet process:
- Little or no formwork is required.
- Cost-effective method for placing concrete.
- Ideal for irregular surface applications allows easy material handling in hard to reach areas.
Dry Mixing Process:
In the dry-mix process, cement, aggregate, and admixture are mixed together, conveyed pneumatically through a tube and then through a water ring in the nozzle.
Water is applied uniformly throughout the mixture.
Typically, dry-mixes are used for small to medium volume placements or in confined vehicle access conditions.
Shotcrete is used in situations where the formwork is cost-prohibitive or impractical.
Advantages of dry process:
- Easy to start-up, shutdown and cleaning.
- Control of materials is on site.
- The nozzle man can move the gun to 1000 feet horizontally or 500 feet vertically.
Advantages of Shotcrete:
- In this process, little or no structure is required.
- It cost-effective process for placing concrete.
- Ideal for irregular surfaces.
- It allows for easy material handling in hard-to-reach areas.
- Also, easy for start-up, shutdown and cleaning.
- Increases the load-carrying capacity due to redistribution of stress.
- It provides excellent corrosion resistance.
Disadvantages of Shotcrete:
- The rainwater washes the cement away from the sandy surface or it may saturate and trigger slouching or breaking.
- A strong wind will separate the material between the nozzles, the purpose of the deposit is to reduce energy.
- This requires expert and skilled labours & detailed geotechnical information.
- Improperly utilized shotcrete’s can produce a lot worse situation than an untreated situation.
Application of Shotcrete:
- It was used in underground openings in tunnels, mines, drainage audits, and exploitative audits.
- In rock slope, combined with rock and soil fixing techniques provides stabilization and support for excavation foundations.
- It is used for protecting channel linings, bridge safety, and stabilizing debris flows.
- Also, used for the rehabilitation of reinforced concrete constructions such as bridges, chemical processing, and handling crops.
- Used for restoration of deteriorating marine constructions such as bulkheads, piers, seas, and so on.
Also read: Papercrete, Concrete Joint filler & Concrete Joints
Shotcrete is a possible alternative to superstructure repair (unstructured) and substructure repair (unstructured).
It is efficient for manual lay-up technique for vertical and overhead tasks using rapid set mortar.