Shotcrete

Shotcrete

Shotcrete is mortar or concrete conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique. Shotcrete undergoes placement and compaction at the same time due to the force with which it is projected from the nozzle. It can be impacted onto any type or shape of surface, including vertical or overhead areas with or without reinforcement. Shotcrete is usually an all inclusive term that can be used for both wet mix and dry mix versions. In the pool construction trade however, the term shotcrete refers to wet mix and 'gunite' refers to dry mix, these two terms are not interchangeable.

Types of shotcrete
Fibre reinforced shotcrete
Unreinforced shotcrete is a brittle material that cracks and displaces when subjected to tensile stresses and strains. Addition of fibres to the shotcrete mixture adds ductility to the material, energy absorption capacity and impact resistance. Composite material is capable of sustaining post crack loadings and often displays increased ultimate strength - tensile strength. Fibers used in shotcrete are available in three general forms, steel fibers, glass fibers, and synthetic fibers. Typical application for fiber structures that may be subjected to high deformations or where crack control is needed.

Silica fume shotcrete
Silica fume shotcrete is a very fine non crystalline pozzolanic material. Silica fume is used in shotcrete to increase strength, decrease permeability and enhance cohesion and adhesion. Specific advantages of silica fume in shotcrete are the improved bond strength of shotcrete to substrate surfaces, the improved cohesion of the shotcrete and the resulting ability to apply thicker layers of shotcrete in a single pass to vertical and overhead surfaces. More resistant to washout, and rebound is significantly reduced. Silica fume shotcrete have improved resistance to aggressive chemicals. Silica fume shotcrete is widely used in tunnel construction often combined with fibers to control shrinkage cracking. Because of inherent improvements in permeability, silica fume shotcrete has been used to cap landfils and other waste areas to be sealed from surface water infiltration. Performance in high strength application is more easily accomplished with silica fume.

Polymer modified shotcrete
Polymers are incorporated into shotcrete in two ways. The entire binders is composed of a polymer material. No hydraulic cement but only polymer-polymer shotcrete. Otherwise polymer emulsion is added to the cement mixture, with partial replacement of the mixing water, or as total replacement. This is polymer cement shotcrete. The advantage of polymer modified shotcrete are polymers improve flexural and tensile strength, improve bond, and reduce absorption because of lower permeability.

Accelerated shotcrete
Accelerating admixtures are used extensively in shotcrete. Highly effective accelerators have been developed for rapid setting of shotcrete. Often considered super accelerators, these are commonly used with dry mix shotcrete. With the increasing use of silica fume, the use of accelerators may decline same what. In the past, these accelerators, were exclusively powdered materials added to dry mix shotcrete materials. Now  powdered and liquid admixtures are used in dry mix and wet mix shotcrete. Accelerators with a wet mix process is added at the nozzle rather than batchedwith the other materials. Applications include tunnel support and linings, seawalls, portions of dams, roof construction, slope protection, and water retention structures such as canals, thick concrete sections applied vertically or overhead, rapid repairs and leaks sealed with flashset shotcrete.

Shotcrete process
Shotcrete can be applied by two distinct application techniques, the dry mix process(guniting) and wet mix process(shotcrete).

Dry mix shotcrete

Cementitious material and aggregate are thoroughly mixed and either bagged in a dry condition, or mixed and delivered directly to the gun. Mixture is normally fed to a pneumatically operated gun which delivers a continuous flow of material through the delivery hose to the nozzle. Interior of the nozzle is fitted with a water ring which uniformly injects water into the mixture as it is being discharged from the nozzle and propelled against the receiving surface.

Advantages of dry mix shotcrete process

1. Increased bonding ability and bonding strength
2. The water content is controlled at the nozzle
3. Less waste is created during dry shotcrete
4. Low investment

Disadvantages of fry mix shotcrete process

1. Experienced nozzleman is required to control the water content varied through nozzle adjustment.
2. Possibilities of making shotcrete with variable water content ratio and water content is limited
3. Higher cost
4. Spraying cost is tough
5. High dust emission
6. Water content ratio of the mix is dependent of the nozzle operator
7. Difficulty in addition of admixtures

Wet mix  shotcrete

The cementitious material, aggregate, water and admixtures are thoroughly mixed as would be done for conventional concrete.The mixed material is fed to the delivery equipment, such as a concrete pump, which propels the mixture through the delivery hose by positive displacement or by compressed air. Additional air is added at the nozzle to increase the nozzle discharge velocity.

Advantages of wet mix shotcrete process

1. The water content is controlled more precisely
2. Admixtures can be added as per design requirement
3. Less rebound
4. Cost of the process, energy and labour is required
5. The spraying operation is easy as the mix is prepared before

Disadvantages of wet mix shotcrete process

1. Bond strength and tensile strength is less
2. Wastage of materials
3. Transportation of wet mix will require high control