WET AND DRY SHOTCRETE
There are two ways of shotcrete—“dry” and “wet” method. “Wet” method, in turn, is divided into “wet” one with a solid stream and “wet” one with a rarefied stream. Each of these methods has its advantages and disadvantages.
The dry shotcrete method was first used in the early 20th century. Under this method, a dry mix of cement, fillers (sand, gravel), and additives (if necessary) are previously prepared. The mixture is fed into the dry shotcrete machine. With the help of compressed air, the mix is fed through a hose to the nozzle, it is wetted in it with water supplied by another hose, and is ejected at a high speed (130-170 m/s) onto the gunned surface.
The process of building the gunite (shotcrete) level is as follows:
· in the first period (in milliseconds), practically the whole mix bounces off the surface; only the laitance remains on the surface, which forms an adhesive layer;
· the next moment, the smallest filler remains on the surface, and everything else bounces off;
· in the future, an ever larger filler remains on the surface, until the outbound is stabilized.
The resulting gunite (shotcrete) level has the following positive properties due to the application method:
· high adhesion due to the fact that the boundary layer is formed by laitance with the smallest filler;
· high density and cohesion due to the high speed of flight of the particles of the mix – the particles are literally driven into the shotcrete layer applied;
· optimal water-to-cement ratio – an experienced nozzleman submits a quantity of water not exceeding the required for the hydration reaction;
· the possibility of applying a layer of up to 10-15 cm in one pass – a high consistency of the composition applied, all that is badly fixed – bounces off.
However, the outbound takes up to 25% of the mix.
“Wet” shotcrete with a solid stream
“Wet” shotcrete has arisen half a century later. This was due, above all, the development of construction chemistry. Under this method, the ready-mixed concrete is fed into the “wet” shotcrete machine (concrete pump) and hydraulically (as a solid stream) is fed through a hose to the nozzle. Compressed air is supplied to the nozzle via another hose, by means of which a solid stream of the mix is broken and delivered to the gunned surface. The speed with which shotcreting is performed is much lower. As a result, the outbound amount is reduced to 10%.
The high quality of the shotcrete is achieved by the introduction of additives:
· adhesion to the surface – by forcing the adhesive layer;
· applying a thick layer – introducing setting accelerators which mortars are supplied to the nozzle via a separate hose;
· optimal water-to-cement ratio – using plasticizers.
“Wet” shotcrete is further consistent with the industrial construction method. A concrete mix can be prepared at a specialized enterprise (concrete plant), delivered by a mixer truck and when applied, the qualification of a nozzleman plays a smaller role.
However, the need to introduce additives significantly increases the cost of the mix applied. In addition, transportation through the hose of wet concrete precludes the possibility of even short breaks in the shotcrete without a labor-intensive hose flushing operation (the concrete mix seizes in the hose).
“Wet” shotcrete in a rarefied stream
Under this method, the ready concrete mix is placed in a pneumatic concrete pump and, with the aid of compressed air, is delivered to the nozzle, from where it is ejected at high speed to the surface. This method occupies an intermediate position between the above methods. There is the possibility of preparing a ready (wet) mix in a concrete plant and there is no need to introduce plasticizers.
The shortcomings of this method include a high outbound up to 25%, the need to apply an adhesive layer, the use of setting accelerators, and the mandatory washing of the material hose during breaks.