Shotcrete is that concrete which is sprayed to the surface under external pneumatic pressure and high velocity. Earlier it had been reinforced with steel reinforcement which nowadays, gets replaced by steel mesh or steel fibre. Shotcrete is essentially utilized in tunnels (as a short lived support), For slope protection with wire mesh and in some confined spaces where normal concreting can’t be done
- Coarse Aggregate (below 10mm)
- Fly ash/microsilica
1. Dry-Mix Shotcrete
2. Wet-Mix Shotcrete
1. Dry-Mix Shotcrete:
It involves placing the dry ingredients into a hopper then conveying them pneumatically through a hose to the nozzle. The nozzleman controls the addition of water at the nozzle. The water and therefore the dry mixture aren’t completely mixed, but is completed because the mixture hits the receiving surface. this needs a talented nozzleman, especially within the case of thick or heavily reinforced sections.
Advantages of the dry mix process are that the water content are often adjusted spontaneously by the nozzleman, allowing simpler placement in overhead and vertical applications without using accelerators.
The dry mix process is helpful in repair operations when it is necessary to stop at regular intervals, as the dry material is easily discharged from the hose.
Dust will be more as we are using the dry ingredients. It is not applicable where the shotcrete quantity is reqiured more frequently.
2. Wet-Mix Shotcrete:
In the wet-mix shotcrete, ready-mix concrete is used, conveyed through hose pipe to the nozzle where compressed air and accelerator is added at the nozzle point.
i) Less rebound .
ii) Lesser dust compared to dry mix shotcrete.
iii) Larger volume can be placed in less time.
i) Highly skilled man is required in spraying.
2) W/c pressure and accelerator quantity has to be precised.
Therefore, we are going to discuss about Wet-mix Shotcrete which is widely using around the world. As per Is Code 9012:1978 para 3.2.1 and 3.2.2, Table 1, the grading of Coarse and fine aggregate should be as follows:
Fine aggregate i.e. Sand for shotcrete is preferred of Zone II or Zone III for proper density and compaction purpose.
Before learning about spraying, we need to go through the various components of shotcrete machine.
Components of Shotcrete Machine:
A concrete hopper is the component where concrete is dropped from the chute of the transit mixer. At the bottom of the hopper two cylindrical pistons are attached which force concrete to go to the hose pipe with some external pressure.
This component is made up of steel or flexible rubber through which concrete is passed up to the nozzle. Multiple pieces of pipe are connected with clamps. Before clamping, rubber washer is used to avoid air entering into the concrete resulting choking issue.
The final concrete outlet is done via nozzle which is having diameter of 60-70mm made up of rubber. The air pipe and accelerator unit are connected at this junction having holes at its periphery.
Compressor is the main component which receives air from outside air unit and provides compressed air of 6-7 Bar air pressure. For Shotcrete, we require 5-6 bar pressure.
Accelerator Tank Unit:
It is 1000 Litre of tank which is filled with accelerator and is connected with dispenser unit. This dispenser unit is regulated by display unit to regulate the dosage of accelerator.
This unit has calculation part in which we have to enter the quantity of cement and percentage fixed for shotcrete.
Joystick is the last but not the least unit used to control the moment of the boom, rotation of nozzle and is operated by skilled person.
Application of Shotcrete:
1. After detail understanding of different components, we need to know about the application of shotcrete machine.
2. All loose material and debris have to be removed, and the surface has to be cleaned from dust, using water, air or a combination of both.
3. The air pressure which guarantees the adhesion or bonding of the shotcrete to the surface is determined by the distance between the nozzle and the surface. A small distance results in a high quantity of rebound. With a distance too great, the impact force is too weak to provide proper bonding and compaction of the shotcrete. Ideal distance between nozzle and surface is 1-1.5m.
4. The angle between the nozzle and the surface should be 90° which results the correct compaction of the shotcrete and the reduction of rebound. The spraying has to be carried out in circular pattern over the whole surface to achieve a homogenous result.
5. Before applying the first layers, it is necessary to spray in holes and cracks of the terrain to make surface even.
6. Shotcrete slump should be high i.e.130-170mm for smooth flow and spraying.
7. Spraying should be done in layers and one layers should have thickness around 70-80mm and the time gap should be 7-9 mins so that shotcrete achieve the initial strength.
8. The percentage of accelerator at the sides of periphery should be 5% and 7-8% at top.
9. Regular curing for at least 7 days needs to be maintain for better compressive strength.
10. Regular training of the operator is very much essential for better productivity.
Rebound & Collapse:
Two terms are generally very important and common in underground projects i.e. rebound and collapse:
Rebound is the weightage of shotcrete that bounce back after hitting the hard surface of the rock while doing shotcrete. The amount of rebound depends upon the quantity of cement, dosage of accelerator, spraying technique, w/c ratio, aggregate type etc.
While collapse is the fall of a mass shotcrete together after 4-5 minutes of spraying. It generally occurs in overhead portion of the tunnel since self-weight of shotcrete is too heavy. The cause of collapse depends upon low dosage of accelerator, high seepage area, continue spraying at single point, efficiency of the operator etc.
As per IS 9012:1978, percentage of rebound is allowable as:
|Surface||percentage of rebound|
|Floor or slabs||5-15|
|Sloping and vertical walls||15-30|
Calibration of the Machine:
Calibration of the machine is a very important aspect in respective to the shotcrete. Its frequency is every 15 days to check the variation of dosage of accelerator against the input given in display unit. The calibration report has to be displayed near to the display unit of the machine so that operator could fix the value accordingly.
Problems During spraying of shotcrete:
Problems occur while doing shotcrete due to two major reasons:
1. Due to Concrete :
Harsh concrete (Zone I sand)
low slump (<130mm)
High bleeding (higher water content)
Mix design issue
Temperature of shotcrete (<15/>32) °C
Slurry powder or cement slurry is not used.
2. Due to Machine :
Low air pressure
Accelerator dosage is not as per design
Shotcrete spraying video : https://www.youtube.com/watch?v=rBEouCO74iE
Mr. Ankit Ranaut
Mr. Ankit is a Civil Engineer who worked as a Assistant manager in larsen and toubro (L&T) and has rich experience ( 6+) in tunnel projects. He is an Author, Editor and Partner at constructionblogs.in