SHOTCRETE – AN OLDIE BUT A GOODIE
It’s in the news every day: As world population increases, the need for affordable housing, mass transportation, updated infrastructure and water preservation is more critical than ever. Pressure on the construction industry isn’t likely to let up, while worries about labor shortages and the need to adopt more sustainable building methods abound.
A 100-year old method of applying concrete - known as shotcrete, Gunite or sprayed concrete - is increasing in usage because it is cost-effective, flexible, and ideally suited for the construction projects that are in the most demand. The global market for sprayed concrete is expected to grow at a compound annual rate of more than eight percent by 2021. Underground construction alone - transportation tunnels, sewers, irrigation and drainage channels, excavations - accounts for 37 percent of the sprayed concrete market.
Gunite was trademarked in 1909 by American taxidermist Carl Akeley, who repaired a crumbling museum wall in Chicago using a hose with compressed air to blow out dry material while injecting water at the nozzle as it was released. His “cement gun” (hence “Gunite”) was patented in 1911. By 1951, shotcrete was the official generic name of the sprayed concrete process—whether it utilizes a wet or dry process. It is normally reinforced with steel rods, wire mesh or fibers. The main difference between concrete and shotcrete is the placement process.
In the dry mix method, dry ingredients are placed into a hopper and transferred pneumatically through a hose to the nozzle. The dry mixture and the water are mixed as they land on the surface. This method requires a skilled person operating the nozzle. For the wet process, wet concrete (usually ready-mix) is shot through the nozzle. The wet-mix process allows for a larger amount to be applied in less time than with the dry method.
Shotcrete is very versatile. It can be sprayed onto any type of surface, making it ideal for covering vertical or overhead areas. Swimming pools, water features and skate parks can be created in record time with shotcrete. In developing countries, shotcrete can be used to build sea and river walls, aqueducts, reservoirs and dams, and other water retention facilities. Mines can be made safer with underground pathways lined with shotcrete. Cliffs weakened by storms or erosion can be stabilized with shotcrete. Shotcrete offers so many benefits:
- Cost-efficient –up to 50% labor savings
- No need for cranes and other large equipment – less damage to site
- Sustainable – more accurate use of materials, protects remote areas; lower water-to-cement ratio
- Perfect for complex shapes – no cast-in-place fabrication
- Can apply to new or existing structures
- Very durable – less porous, so it creates a stronger bond
- Chemical and weather resistant