What is Shotcrete?
Shotcrete is a method when we spray concrete or mortar with a high velocity (130–170 m/sec) on a vertical surface. In the shotcrete method, a nozzleman sprays concrete or mortar on a vertical surface to compact it with concrete.
A single shotcrete layer is 10–15 mm thick and has high mechanical strength (40–70 meganewtons per sq m), density, water tightness, and cold resistance.
The hardened property of shotcrete is the same as conventional concrete but the process through which we apply it helps it in making an excellent bond with the substrates. This method is applied specifically in new construction & repairs suitable for curved and thin elements. Shotcrete is also known as sprayed concrete.
For example, the construction of tunnel linings and domes has become much easier after the introduction of the shotcrete method. The care of concrete, in this case, should be taken as any other concrete. Shotcrete also required adequate curing for better efficiency and prevention of cracks.
It must be cured with water for 14 days from the time the concrete was sufficiently set. Adequately cured concrete can attain strengths around 450-550 kg/cm2 in 28 days.
Also read: Complete Guide on Grades of Concrete
Applications of Shotcrete
- The shotcrete method is used in the construction of the thin overhead vertical and horizontal surfaces.
- It’s especially used in curved and folded surfaces such as tunnel lining, domes, canals, or reservoirs.
- Other structures for which it’s used are water-retaining structures, swimming pools, and pre-stressed tanks.
- It’s applied for the stabilization of rock slopes and temporary protection of freshly excavated rock surfaces.
- It’s utilized for protection against long-term corrosion of piling, coal bunker, oil tanks, steel-frame structures, and other structures. Moreover, it’s also used for encasing structural steel for fireproofing.
What Material is Used in Shotcrete?
The following materials can be used in shotcrete
- Cement
- Sand
- Coarse Aggregates
- Water
- Chemical additives
- Compressed Air
Also Read: Planning and Site Preparation for Concrete
Materials, Proportioning of Shotcrete
The material composition of shotcrete is an important part, the following proportion can be used while making shotcrete:
- Chemical admixtures are added for the acceleration of setting and hardening.
- Sodium Carbonate = ½ Kg per 50 Kg bag of cement
- Sodium Aluminate = ½ Kg per 50 Kg bag of cement
- Calcium Carbonate = 1 Kg per 50 Kg bag of cement
Alternatively, the following additives can also be used:
- Superplasticizer @ 1 % by weight of cement
- Â Accelerator (say sodium Silicate) @ 5 % by weight of cement
| Materials | Quantity |
| Cement |  450 kg/m³ |
| Sand | 1100 kg/m³ (0.70 m³) |
| Coarse aggregate (5 mm – 10 mm size) |  500 Kg/M³ (0.30 M³) |
| Proportion by weight | 1:2.44:1.11 |
Shotcrete Mix Design
The following grades of concrete and ratios of concrete mix design used in shotcrete:
| Grades of Concrete | Ratios of Concrete mix design |
| (Cement: Sand: Aggregate) | |
| M5 | 1:5:10 |
| M7.5 | 1:4:8 |
| M10 | 1:3:6 |
| M15 | 1:2:4 |
| M20 | 1:1.5:3 |
| M25 | 1:1:2 |
| M30 | 1:0.75:1.5 |
| M35 | 1:0.5:1 |
| M40 | 1:0.25:0.5 |
Properties of Shotcrete
- In this method, cement content is high and a small maximum size aggregate is used.
- It increases durability in most cases.
- The conventional concrete is compacted by vibration. While the shotcrete is compacted by high-velocity jet impinging on the surface.
- This process increases the cement content and also brings about the different air-void systems that will affect the durability of the concrete.
- There is a greater effect on the properties of in-place concrete by the application procedures than the mix proportions.
- Shotcrete specimen is sewed from test panels of about one meter square and 75mm thick made by gunning out the plywood form, which is known as shotcrete.
Also Read:Â 11 Basic Steps In Construction Of Multi Storey Buildings
Advantages of Shotcrete
- Shotcrete has a rapid capability to get hardened.
- This method usually works better on complex forms.
- It requires less formwork than the conventional method of concreting.
- It’s economical than the conventional method.
- Doesn’t require much space and resources for batching and mixing.
- Good adhesion, compact shotcrete.
- less rebound
- less dust.
Disadvantages of Shotcrete
- To execute the shotcrete, you need a skilled nozzleman who can control the amount of water mixed through nozzle tuning.
- The spraying procedure is very tough. It requires immense attention and skills.
- You have to face the problem of high dust release.
- The water-cement ratio is based on the nozzleman.
- You might have to face the difficulty in addition to additives.
Also Read: Different Types of Concrete Mixer or Concrete Mixing Machines
Method of Shotcrete
There are basically two methods of applying the shotcrete:
- Wet-Mix method
- Dry-Mix method
What is the Wet-Mix Method?
The water is mixed with the ingredients before it enters into the delivery hose.
In the Wet-Mix method, the wet concrete is transferred from the Ready-mix truck to the concrete pump, which is then sent to the nozzle under the effect of compressed air.
The rate and amount of mix are controlled manually since this method requires much transportation, it’s applied at the larger construction sites.
Types of Equipment Used in Wet-Mix Shotcrete
The following are the main types of dry-mix shotcrete equipment:
- Wet Concrete
- Shotcrete Pump
- hoses
- nozzle
- Air Compressor
Advantages of Wet Mix Method
• The water content is controlled more accurately.
• Not much labor is required in this method
• Due to lesser requirements of labor and maintenance, the cost goes lower in this method.
• Spraying is easy in the Wet-Mix method.
• The consumption of air is lesser as compared to other methods.
• Additives can be added as per the requirement.
Disadvantages of Wet Mix Method
• A lot of wastage is produced in this method.
• The transportation of mix will require high control.
What is the Dry Mix Method?
The water is added to the ingredients at the nozzle. In this method, the mixed ingredients are first placed at the hopper. Next, the ingredients are sent from hopper to nozzle through the delivery hosepipe of the apparatus.
The water in the mix is controlled by the nozzleman using a valve and the Dry-Mix method is applied where we have limited access areas.
Types of Equipment Used in Dry-Mix Shotcrete
The following are the main types of dry-mix shotcrete equipment:
- Pressure vessels;
- Dry-mix shotcrete guns;
- Batch plants; and
- Predampeners.
- Gunite Machine
- hoses
- nozzle
- Air compressor
Advantages of Dry Mix Method
- It offers better bonding ability and strength.
- Water content can be controlled at the end of the nozzle.
- The Dry Mix process generates lesser wastage.
- The investment required for this process is lesser.
Disadvantages of Dry Mix Method
- It requires a skilled nozzleman to control the water content. Otherwise, it would be difficult to have the efficient mix.
- The spraying procedure is tough in this method.
- High dust is released in the process.
- The water-cement ratio is based on the nozzleman.
- It would be totally dependent on the manual process to make the mix efficient.
- You may face difficulty in the addition of additives.
IS Code for Shotcrete
IS I9011 – 1978 code can we used for shotcrete, this Indian Standard was adopted by the Indian Standards Institution on 30 November 1978, after the draft finalized by the Cement
and Concrete Sectional Committee had been approved by the Civil Engineering Division Council.
Download IS Code for Shotcrete – IS:9012 – 1978
What is Rebound in Shotcrete?
During the shotcrete process, rebound aggregate and cement paste ricochet off the surface due to collisions with the hard surface, reinforcement, or aggregate particles themselves.
The amount of rebound varies with the position of the working angle of the nozzle, air pressure, cement content, water content, size and grading of aggregate, amount of reinforcement, and thickness of the layer. Depending on the position of the work, the variation in the amount of rebound maybe
as below:
| Surface | Â Percentage of Rebound |
| Floor or slabs | 5-15% |
| Sloping and vertical walls | 15-30% |
| Overhead work | 25-30% |
Precautions to be Taken in Shotcrete
- After completion, the finished shotcrete layer must be kept wet for at least a week.
- During the operations, there will be rebounds that will not be used for shotcrete.
- The time difference between the first and second layers should be as short as possible for example less than 8 hours.
- After the shotcrete is finished and the mix is set, the surface should be thoroughly sounded with a hammer to look for drumming areas caused by rebound pockets, lack of bond, and other issues that should be cut out and replaced with new or fresh shotcrete.
- Where the dry process is adopted the jetting of water should be synchronized with that of the concrete mix for homogeneous shotcrete.
- Where wet mix is adopted the specified water-cement ratio is to be strictly enforced.
Also Read: What are the different varieties of cement
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