Admixtures for shotcrete and sprayed concrete
Additives for shotcrete, a technique currently indispensable in the procedures of support and structural coating of tunnels and slopes.
To clarify some concepts:
- Gunitar is understood as the commissioning of a concrete or mortar sprayed with pressurized air through a hose, at high speed on a support.
- The shotcrete is a concrete whose maximum size of aggregates is greater than 8 mm, and that applied by machine, is projected at high speed on a support through hose and nozzle.
- The projected mortar is a mortar whose maximum aggregate size will not exceed 8 mm, and which, when applied by machine, is projected at high speed on a surface through a hose and nozzle.
Currently three different processes are used, which are:
- The wet mixing process involves the use of more services, but its use is widespread for large applications.
- The semi-wet mixing system, which consists in the dosing of water, approximately 5 m before the nozzle, is a process that fundamentally prevents the dry mixture from dispersing (especially the cement) when making the projection.
- The Dry Mixing System, whereby all the concrete components are pre-mixed, except water, which is added to the nozzle before the mixture is projected, the dry mixture being transported through hoses pneumatically until nozzle.
The most commonly used additives and additives in dry-cast concrete are accelerators (dust or liquid), silica smoke (dust or slurry), setting stabilizers, steel fibers and fly ash.
The most commonly used additives and additions in wet shot concrete are accelerators (liquid or powder), superplasticizers, silica fume (powder or slurry), setting stabilizers, rebound reducers, steel fibers or polypropylene and fly ash.