What is Shotcrete?

Shotcrete, also known as air-blown or pneumatically applied concrete, is fired through a hose at high speed onto surfaces. This method is adopted in challenging locations where conventional concrete pouring is impractical. Shotcrete serves to strengthen existing structures and provide protection against fire, corrosion, and various types of damage.

What are the Types of Shotcretes or the process of Shotcrete?

1. Mix Process
2. Wet Mix Process

1. Dry Mix Process

In the dry mix process, a mixture of cement and moist sand is delivered to the nozzle through a delivery hose. Most of the water mixture is under pressure at the nozzle. This process includes the following steps:

Step 1: Mixing cement and moist sand: The cement and moist sand are mixed thoroughly.

Step 2: Pouring the Mix: The mixed cement-sand combination is fed into a special machine, often called a gun or mechanical feeder (in this standard it is called a delivery device).

Step 3: Hose Delivery: A feed wheel or distributor forces the mixture into a delivery hose.

Step 4: Compressed Air Suspension: The mixed material is transported by compressed air through the delivery hose to the nozzle.

Step 5: Nozzle Process: The nozzle, equipped with a perforated manifold inside, receives the mixture. The water is forced under pressure and mixed thoroughly with the other ingredients.

Step 6: Jetting the Surface: The prepared mortar is then thrown from the nozzle at high velocity onto the surface where concreting is required.

1. Controlling the water-cement ratio is the responsibility of the nozzleman and may vary within a limited range. Too little water creates excessive dust and hinders mixture compaction. Conversely, too much water increases workability, reduces strength, and induces shrinkage.
2. The use of suitable steel or other fibers, microsilica, polymers, and powder admixtures can yield better results in terms of strength and durability.(IS Code)
3. Under no circumstance should rebound be reused. Rebound loss depends on factors such as surface nature, application direction, mix design, and the nozzleman’s skill. Rebound loss is generally higher in overhead shooting compared to vertical surfaces
4. The optimal distance between the nozzlemen and the surface to be sprayed should be kept around 1.0 meter.

2. Wet-Mix Process

The wet-mix process involves pre-mixing concrete or mortar components with water before delivering them through a hose and spraying onto the surface. This common process allows for better control of mixing.

Step 1: Thorough Mixing: Combine all ingredients, including mixing water, ensuring a thorough mixture.

Step 2: Introduction into Delivery Equipment: Introduce the mixed mortar or concrete into the chamber of the delivery equipment.

Step 3: Forcing into Delivery Hose: Force the mixture into the delivery hose, utilizing compressed air or alternative means.

Step 4: Conveyance to Nozzle: Convey the mixed material through the delivery hose, utilizing compressed air or alternative methods, to a designated nozzle.

Step 5: Additional Air Injection: Inject additional air at the nozzle to enhance velocity and refine the shooting pattern.

Step 6: High-Velocity Jetting onto Surface: Jet the mortar or concrete from the nozzle at high velocity onto the targeted surface for shotcrete.

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Be careful during dry-Mix Shotcrete: Below are the Important Points

1. The wet process of shotcrete, requiring a larger volume to mix at a time with high output, is generally less suitable than the dry process for repairing concrete structures.
2. Wet mix shotcrete is typically used for large volumes, thin sections, and confined spaces.
3. If specified, fibres of steel, polypropylene, or other materials, as may be specified, could be used with admixtures to modify the structural properties of the concrete/mortar.
4. Ready-to-use pre-bagged dry mix micro-concrete with 3 mm aggregate and admixed with polymers, micro silica, and superplasticizer is available in the market. This material consistently yields better results than the conventional cement-aggregate mix by reducing shrinkage, achieving high strength, and improving repair quality.
5. The shotcrete mix design must undergo laboratory tests and field trials, approved by the Engineer-in-charge.

What are the Properties of Shotcrete?

• Properly applied shotcrete is structurally sound and durable, forming excellent bonds with various materials like concrete, masonry, and steel.
• Favourable properties depend on meticulous planning, supervision, and the continuous attention of operating staff.
• The water-cement ratio for shotcrete typically ranges from 0.35 to 0.50 by mass, lower than most conventional concrete mixes.
• Physical properties of sound shotcrete are comparable to conventional mortar or concrete of the same composition.
• Reported values for 2-day strength range from 20 to 50 N/mm²; strengths higher than 25 N/mm² are recommended only for carefully executed shotcrete jobs.
• Drying shrinkage of shotcrete, influenced by mix proportions, generally falls within 0.06 to 0.10 percent.
• This is higher than low-slump conventional concrete, potentially leading to more shrinkage, cracking, and requiring closer joint spacing.
• Shotcrete exhibits good durability in laboratory tests and under field exposure.
• Advantages over conventional concrete include cost-effectiveness due to lesser forming requirements.
• Particularly beneficial in various new construction and repair projects.

Slope Stabilization:

• Shotcrete is utilized to strengthen and stabilize slopes and embankments, preventing erosion and landslides.

Tunnel Construction:

• Shotcrete is applied to tunnel walls for controlling and supporting the stability of rock and soil during excavation.
• A notable example is the Silkarya Tunnel, where shotcrete was crucial in preventing collapse after an initial incident on day 1. For a detailed case study, click here.

Swimming Pool Construction:

• Shotcrete is commonly employed in swimming pool construction due to its adaptability to various shapes and sizes.

Repair and Rehabilitation:

• Shotcrete is a go-to solution for repairing and strengthening existing structures like bridges and buildings.
• It provides increased strength and durability, enhancing the lifespan of structures.

Underground Construction:

• Due to its efficiency in creating stable and durable linings, shotcrete is widely used in the construction of underground structures, including mines and subways.

What are the Benefits of Shotcrete?

Versatility: Suitable for a variety of applications, including irregular surfaces and confined spaces.

Rapid Application: Speeds up manufacturing processes compared to traditional methods.

Enhanced Durability: Provides a strong and durable finish.

Adaptable thickness: Can be applied to various thicknesses to meet structural requirements.

Get the Free IS Code, IS 9012,1978