what is shotcrete

What is Shotcrete

Shotcrete is mortar or very fine concrete deposited by jetting it with high velocity on to a prepared surface. The system has different proprietary in different countries such as Guncrete, Nucrete, Pneukrete, Torkrete, Blastcrete, etc. through the principle is essentially the same. Shotcrete offers advantages over conventional concrete in a variety of new construction concrete because of fewer formwork requirements, requiring only a small portable plant for manufacture and placement. It is capable of excellent bonding with a number of materials and this may be an important consideration.

 As with other concretes, shotcrete also requires adequate curing for the prevention of cracks and to assure a strong and weatherproof surface, shotcrete should be curved with water for a minimum period of 14 days from the time the concrete has attained sufficient set. Adequately cured spray concrete can attain strengths around 450-550 kg/cm2 at 28 days.

Shotcrete

Table of contents

• What is shotcrete
• Process of shotcrete
• Dry mix process
• Wet mix process
• Application of shotcrete
• Proproites of shotcrete
• Advantages of shotcrete

Processes of shotcrete

• Dry mix process
• Wet mix process

Dry Mix Process

In this process, the mixture of cement and damp sand is conveyed through a delivery hose pipe to a special mechanical feeder or gun called delivery equipment. The mixture is metered into the delivery hose by a feed wheel or distributor. This material is carried by compressed air perforated manifold in which water is introduced under pressure and mixed with other ingredients. The mortar is jetted from the nozzle at high velocity on to the surface to be shotcrete. In this process, any alteration in the quantity of water can be easily accomplished by the nozzleman. If the water content is more, then concrete tends to slump when jetted onto the vertical surface, The amount of water should be so adjusted that wastage of material be rebounding is minimum. The water-cement ratio should be between 0.33 and 0.50.
Several forms of equipment are available for shotcreting by this technique. The equipment ensuring a continuous supply of the mortar can convey the material to a distance of 300 to 500 m horizontally and 45 to 100 m vertically.

Wet Mix Process

 In this process, all the ingredients such as cement, sand, small-sized coarse aggregate and water, are mixed before entering the chamber of delivery equipment. The ready-mixed concrete is metered into 7 atmospheres to a nozzle, additional air is injected at the nozzle to increase the velocity and improve the gunning pattern. Equipment capable of placing concrete at the rate of  3 to 9 m3/hour is available.

 The phenomenon of falling back of a part of mortar or concrete jetted on to the surface to be treated, due to the high velocity of the jet, is called rebound and depends upon the water-cement ratio, and the nature and position of the surface treated. The approximate range of rebound is 5 to 15% for horizontal slabs, 15 to 30% for sloping and vertical surfaces and 20 to 50% for the treatment of overhead surface and corners.

The dry mix process is preferred in case the lightweight concrete is used. the lower water-cement ratio is used, result in higher strengths, less creep and drying shrinkage, and higher durability. Whereas in the wet process the higher durability can easily be achieved by using air - entraining agents. The water-cement ratio can be very accurately controlled in the wet process. The wet process does not cause most due problems. The larger capacity available in wet mix process results in higher rates of placing of concrete.

Properties Of Shotcrete

• A small maximum size aggregate is used and cement content is high.
• It should enhance durability in most cases.
• Whereas conventional concrete is consolidated by vibration, shotcrete is consolidated by the impact of a high-velocity jet impinging on the surface. This process not only increases the cement content due to rebound but also brings about different air-void systems affecting the durability of shotcrete.
• The application procedures have a greater effect on the in-place properties of shotcrete than the mix proportions.
• Shotcrete specimens are usually sawed from test panels of about one-meter square and 75 mm thick made by gunning out plywood from.

Applications Of Shotcrete

1. Shotcrete is widely used in different construction such as thin overhead vertical or horizontal surface, particularly the curved or folded sections, canal, reservoir, and tunnel lining.
2. It is also used in swimming pools, water retaining structure and prestressed tank.
3. Shotcrete has been successfully used in the stabilization of rock slopes and temporary protection of freshly excavated rock surface.
4. It is a utility for protection against long-term corrosion of pilling, coal bunker, oil tanks, steel building frames, and other structures, as well as in encasing structural steel for fireproofing.
5. These are developed for high-temperature applications, such as the refractory lining of kilns, chimneys, furnaces, etc.

Advantages of Shotcrete 

• The Concrete can be applied by a nozzle from a safe distance.
• More economical than conventional concrete requires less formwork.
• Excellent bonding in nature which makes the concrete layers very strong.