Question

What is shotcrete? Describe the difference between the wet and dry processes.

Please I want the answer written here not on paper

Answer 1

SHOTCRETE: Shotcrete can be defined as mortar which is conveyed through a hose and pneumatically projected with large speed on the desired surface and it offers the advantage of placing and compaction of concrete at the same time. Shotcreting is generally reinforced by steel mesh and fibres. There is not much difference between guniting and shotcreting. Gunite was first used in the early 1900 and this process is mostly used for pneumatical application of mortar of less thickness, whereas shotcrete is a recent development on the similar principle of guniting for achieving greater thickness with small coarse aggregates.

There are two different processes in use for making shotcrete, namely the “Wet-mix” process and the “dry-mix”process. The dry mix process is more successful and generally used.

The Dry-mix Process

The dry mix process consists of a number of stages and calls for some specialized plant.
The stages involved in the dry mix process is given below:
(a) Cement and sand are thoroughly mixed.
(b) The cement/sand mixture is fed into a special air-pressurized mechanical feeder termed as ‘gun’.
(c) The mixture is metered into the delivery hose by a feed wheel or distributor within the gun.
(d) This material is carried by compressed air through the delivery hose to a special nozzle. The nozzle is fitted inside with a perforated manifold through which water is sprayed under pressure and intimately mixed with the sand/cement jet.
(e) The wet mortar is jetted from the nozzle at high velocity onto the surface to be shotcreted.

The dry-mix methods makes use of high velocity or low velocity system. The high velocity Shotcrete is produced by using a small nozzle and a high air pressure to produce a high nozzle velocity of about 90 to 120 m/s. This results in exceptional good compaction. The lower velocity shotcrete is produced using large diameter hose for large output. The compaction will not be very high.

The Wet-mix Process
In the wet-mix process the concrete is mixed with water as for ordinary concrete before conveying through the delivery pipe line to the nozzle, at which point it is jetted by compressed air, onto the work in the same way, as that of dry mix process.
The wet-mix process has been generally discarded in favour of the dry-mix process, owing to the greater success of the latter. However the amount of waste rebounded is less as compared to the dry mix process. Also there is better control of water cement ratio in wet mix but in dry mix the fact that this ratio can be kept very low overcomes the objection of the lack of accurate control.