Silica-Fume/ Polymer modified shotcrete
Silica fume is a fine nanocrystalline pozzolanic material that used to enhance concrete characteristics. Silica fume improves concrete compressive strength, bond strength, impact resistance, and it reduces concrete permeability. Using silica fume will increase the concrete structure's durability and strength. In shotcrete, silica fume used to improve the bonding strength of shotcrete with the existing surface. Additionally, silica fume will improve shotcrete cohesiveness. Thicker layers of shotcrete can be applied in a single pass owing this to increasing of cohesion. Moreover, silica fume will increase the resistance of fresh shotcrete to wash out where fresh shotcrete is subjected to water flow. Finally, silica fume will significantly reduce the rebound of shotcrete.
figure 1The adding of silica-fume will improve hardened and un-hardened shotcrete concrete properties. Silica fume concrete has been used in tunnel construction. Silica fume combined with fibers added to shotcrete often to control shrinkage cracks. The reduced permeability of shotcrete concrete made it suitable for capping landfill and other waste areas which seal the surface to prevent surface water infiltration.
The polymer is introduced to shotcrete in two ways. In one method, the polymer replacing the binder totally. In other words, polymer shotcrete formed by this method, and the hydraulic cement totally replaced by the polymer. In the second method, polymer emulsion added to the shotcrete mixture. The water is replaced partially by polymer emulsion, or it is totally replaced. The polymer emulsion disperses through the shotcrete mixture creating a polymer matrix. This is termed polymer-portland-cement shotcrete.
A styrene-butadiene polymer is frequently used in shotcrete concrete. While Acrylic polymer latexes and epoxy resins, polymers are less frequently used for shotcrete concrete. Adding polymer to shotcrete will enhance the tensile and flexural strength, bond strength, and it will reduce concrete permeability.
In the past accelerating admixture used extensively in shotcrete concrete. Accelerating admixtures will significantly reduce the setting time of concrete. In spite of that the using of accelerating admixture may decline as a result of using silica fume. Accelerating admixture produced in powdered and liquid form. The adding of liquid admixture for dry mix shotcrete will be through the nozzle. Accelerated shotcrete is beneficial in tunnel construction. It will allow for a rapid section build-up, the rapid gaining of strength and sealing the water leakage. Also, it can be used for marine structures in the splash zone. The early development of strength will resist the washing of shotcrete concrete by incoming tides.