Shot Crete: Uses, Operations and Advantages | Materials | Concrete Technology

In this article we will discuss about:- 1. Meaning of Shot Crete 2. Preparation of Base for Shot Crete 3. Uses 4. Advantages 5. Disadvantages.

Meaning of Shot Crete:

The procedure of applying cement mortar or concrete through a hose and pneumatically projected at high velocity on the back up surface is known as shot creting or guniting and the mortar or concrete is known as shot crete or gunite. The technique of depositing very thin layers of mortars in each pass of the nozzle than that available with the shot crete is known as guniting. This is the difference between guiniting and shot create. The force of the jet impacting on the surface compacts the material such that it can support itself without sagging or sloughing even on a vertical face or overhead.

In general terms it may be called sprayed concrete. The shot crete has no different properties than that of conventionally placed concrete of similar proportions; it is the method of placing that imparts some significant advantages to it in some of its applications. However considerable skill and experience is required in the application of shot crete. Thus the quality of shot crete depends to a large extent on the performance of the operator.

Preparation of Base for Shot Crete:

All surfaces to be guniting should be cleaned thoroughly to obtain a good bond. Any loose or weak material should be removed by chipping, followed by sand blasting. Sand blasting can be done with the cement gun equipment using a sand blast nozzle and a low feed rate for the sand. In order to reduce the shrinkage stresses, the base which has dried, should be moistened thoroughly for a few hours before carrying out the guniting operation. Any absorptive surface to be gunited should be moistened to retard any excessive withdrawal of water from the shot crete, but excessive water should be avoided.

Grading of Sand:

The grading of sand to be used in guniting is very important. If the sand is very fine, it will produce a weak layer subject to excessive shrinkage, if too coarse, the amount of rebound will be excessive and a rough textured surface will be obtained. The maximum size of sand particle may be 6 mm and the fineness modulus may vary from 2.5 to 3.3.

The following grading is found most suitable:

The sand should have moisture content between 3 to 6% for satisfactory operating of the equipment, but in no case it should exceed 8%. 4% moisture content is found to give best results. If the sand is too dry it will not flow uniformly, but it will bursts out and dry patches will be the result. If too wet, it will clogg the equipment and dry sand will give more rebound.

Rebound:

The material which bounces back from the working face is known as rebound. It is largely the coarser particles of sand which rebound.

The amount of rebound depends upon the following factors:

1. Rebound is greater when guniting is done on vertical or on over hanging surfaces than level or sloping surfaces.

2. Greater the nozzle velocity, more the rebound.

3. Lower the water/cement ratio, greater the rebound.

The amount of rebound varies from 20% to 40% of the material handled. The approximate value of rebound is 5 to 15% for horizontal surfaces, 15 to 35% for sloping and vertical surfaces and 25 to 40% for overhead surfaces and corners.

Mix Proportion:

Mortar mixes having one part of cement and 4 to 4.5 part of moist sand by weight are found quite satisfactory. In general richer mixes shrink more and high shrinkage may destroy the bond with the base. For this reason richer mixes should be avoided. The usual water/cement ratio varies from 0.35 to 0.5 or 14 litres of water per 50 kg of cement is sufficient. The 7 day strength of 1:3 mixes can be of the order of 70 MPa.

Mixing and Placing:

Before putting the dry materials into the cement gun, the sand and cement should be mixed thoroughly for 1½ minutes till the homogeneous mixture of one colour is obtained. Any mixed material not used within 45 minutes of mixing should be rejected.

For placing the mortar, the hose length should be kept as small as possible as long hose requires greater pressures for air and water. Though hose length upto 110 m have been used but preferably they should not be longer than 30 to 45 m. For the shorter lengths the air pressure should be about 3.5 kg/cm² at the nozzle and the water pressure at the nozzle should be 4.2 kg/cm². Both air and water pressure should not be more than 5.25 kg/cm² for a long hose. Though the volume of air required depends upon the size of the nozzle and air pressure, but, for an average work using 3 cm diameter nozzle about 7.25 cubic metre air per minute is required.

Operation:

After filling sand cement mix in vessel C, air at a pressure of 2.2 kg/cm² to 3.5 kg/cm² is sent from compressor A to vessel C through moisture extractor B. At B the moisture in air is absorbed and dry air reaches in the vessel C. This air pushes the cement- sand mixture through pipe D and ejects it through nozzle E as shown in Fig.24.9. Through another pipe, air is sent from compressor A at a pressure of 3.0 to 4.6 kg/cm² to water tank F. From this tank air is delivered through pipe G and ejected at E. Pressures are controlled at E.

While guniting, the nozzle should be held normal to the surface and about 1 to 1.5 m from it. In order to obtain a uniform layer, it should be kept moving. In case for thicker layer, it should be provided in layers not thicker than about 2.5 cm using stiffer mix as thicker layer tends to slough from vertical and overhead surfaces.

Successive layers should be applied before the first layers have dried. All rebound material should be removed carefully, as there is a tendency for the rebound material to collect in the forms or to coat the surface of completed work. Wooden templates should be used at comers, edges and on surfaces where it is necessary to obtain true lines and proper thickness.

If a smooth surface is desired, steel troweling can be done, but it should be done at least after 1 hour of placing the mix. The process of guniting should be suspended when wind is blowing as it will not be possible to maintain the consistency of the mix.

Curing:

The completed work should be protected from direct sun rays at least for 3 days and should be moist cured for at least 14 days.

Uses of Shot Crete:

Shot crete usually is used in the following situations:

1. For the construction of thin, lightly reinforced sections such as curtain walls.

2. For the construction of shell or folded plate roofs.

3. For tunnel lining and for protective covering of soft rocks.

4. For pre-stressed tanks.

5. For repairing of deteriorated concrete and for stabilizing of rock slopes.

6. For encasing steel for fire proofing.

7. For repair or laying new canal and reservoir lining.

8. It can also be applied to a surface covered by running water in such cases an accelerator producing flash set such as washing soda should be used. Though it adversely affects its strength, but makes repair work possible.

9. For repairing of old buildings.

Advantages of Shot Crete:

Shot crete has been found to have following advantages and disadvantages:

1. The shot crete layer can be made very strong by applying pneumatic pressure.

2. 3 cms to 5 cms thickness of short crete lining is sufficient.

3. Shot crete lining does not need expansion and construction joints.

4. Short crete can be applied even on uneven base successfully.

5. Shot crete can be used for repair of disintegrated and leaking lining etc.

6. Shot crete can be applied at least from a distance of 1 m, hence it is very useful to apply lining while tunneling.

Disadvantages of Shot Crete:

1. The success of shot crete depends on the performance of the operator.

2. The cost of construction of shot crete is more than ordinary concrete of same proportion and same thickness.

3. It is less durable than ordinary lining of the same thickness.

4. Shot crete lining in canals is damaged very soon due to settlement, shrinkage and hydrostatic pressure etc.

5. To obtain perfect bond with the base is impossible. Equipment

One of the type of guniting equipment known as cement gum is shown in Fig. 24.8. It consist of two compression chambers one above the other. The upper chamber serves as feeding hopper for the lower chamber which is under constant pressure, while the upper chamber is alternately under pressure and free of pressure. By suitable valves the pressure in the upper chamber can be increased to that, in the lower chamber and the connecting gate opened, and the dry mortar then flows into the lower chamber.

The connecting gate is then closed, the upper chamber opened up and refilled. In this way cycle is repeated. The delivery of material from the lower chamber to the hose is regulated by a feed wheel driven by an air motor. A separate hose carries water to the water ring in the nozzle where radial sprays wet the cement sand mixture. In the Fig. 24.8 H is the upper chamber and L lower chamber K and J are bell values, where M and N are gear wheel and small opening respectively.