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Shotcrete : Competitive Alternative for Repair and Restoration of Concrete Structures

While some view shotcrete simply as another means of placing concrete and mortar, it is much more than that. Modern shotcrete technology, as used in infrastructure rehabilitation, is a sophisticated process that can provide high-quality, durable solutions for repair/retrofit challenges that likely couldn't be met as effectively, either technically and/or economically, with conventionally cast concrete procedures. Shotcrete is now a mature technology for a wide range of new construction and repair/retrofit applications.
Shotcrete, in the broadest sense, refers to mortar or concrete pneumatically projected at high speed on previously prepared surfaces. It is considered one of the most effective methods, in terms of cost, quality and productivity to restore the structural integrity of defective reinforced concrete, increase the concrete cover over reinforcement bars or for both of these applications. 

Shotcrete is a very versatile construction material that can be readily placed and successfully used for a variety of concrete repair applications. Shotcrete has been used to repair canal and spillway linings and walls, the faces of dams, tunnel linings, highway bridges and tunnels, deteriorating natural rock walls and earthen slopes, and to thicken and strengthen existing concrete structures. Provided the proper materials, equipment, and procedures are employed, such shotcrete repairs can be accomplished quickly and economically.

Properly formulated shotcrete material guarantees the integrity and durability of the repaired sections. The first step to developing an effective repair product is to understand key aspects of a project that include the ultimate strength requirement, strength development rate, maximum application thicknesses at vertical and overhead surfaces and the general site conditions (i.e. exposure to chemicals, weathering conditions, freeze/thaw cycles and more).

There are two basic types of shotcrete—dry mix and wet mix. In dry mix shotcrete, the dry cement, sand, and coarse aggregate, if used, are premixed with only sufficient water to reduce dusting. This mixture is then forced through the delivery line to the nozzle by compressed air. At the nozzle, sufficient water is added to the moving stream to meet the requirements of cement hydration. For wet mix shotcrete, the cement, sand, and coarse aggregate are first conventionally mixed with water, and the resulting concrete is then pumped to the nozzle where compressed air propels the wet mixture onto the desired surface.

The two types of shotcrete produce mixes with different water contents and different application characteristics as a result of the distinctly different mixing processes. The dry mix shotcrete process tends to have higher bond strength with the substrate. In addition, the dry shotcrete process results in longer pumping distance but it usually suffers high dust generation and rebound losses varying from about 15 percent to up to 50 percent. The dry shotcrete mix design will use a higher cement amount to compensate for the loss of aggregate due to rebound. Wet mix shotcrete must contain enough water to permit pumping through the delivery line. Wet mix shotcrete, as a result, may experience significantly more cracking problems due to the excess water and drying shrinkage. Advances in the development of the high range water reducing admixtures, pumping aids, and concrete pumping equipment have greatly reduced these problems, and wet mix shotcrete is now being used more frequently in repair construction.

The properties of both wet- and dry-process shotcrete can be further enhanced through the addition of many different admixtures or additives such as:

Admixtures : With wet mix shotcrete water reducing admixtures are often incorporated in the mixes to lower water content and air entraining agents may be used, especially when the placed shotcrete will be exposed to freeze/thaw cycles. Water reducing and air entraining admixtures are not used with the dry shotcrete. For overhead and vertical applications, set accelerators are used in both the dry and wet shotcrete. Strength development accelerators might be used when a high early strength is required.

Silica Fumes : Silica fume is a very fine non-crystalline pozzolanic material composed mostly of silica. Silica fume is used in concrete and 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. The material is more resistant to "washout," where fresh shotcrete is subject to the action of flowing water, and rebound is significantly reduced. Shotcrete containing silica fume may have improved resistance to aggressive chemicals.

Fiber-Reinforced : The addition of fibers to the shotcrete mixture adds ductility to the material as well as energy absorption capacity and impact resistance. The composite material is capable of sustaining post crack loadings and often displays increased ultimate strength, particularly tensile strength. Fibers used in shotcrete are available in three general forms: steel fibers, glass fibers, and other synthetic fibers made of polypropylene, polyethylene, rayon and more. Typical applications for fiber-reinforced shotcrete are for tunnel linings, surface coatings on rock and soil, slopes, structures, embankments, or other structures that may be subject to high deformations or where crack control is needed.

Polymer-Modified : In some cases, where higher bond, tensile and flexural strengths are required along with having a more waterproof shotcrete mix, polymer emulation can be used to partially or totally replace the mixing water. This is one of the most common forms of polymer-modified shotcrete that can be easily done onsite.

The shotcrete process is suitable in a number of concrete restoration situations such as: 

Where formwork is not practical 
Where formwork can be reduced or eliminated 
Where normal casting into formwork can't be employed 
Where a thin and/or variable thickness layer is required 
Where access to the work area is difficult 

The numerous advantages of the shotcrete process are especially evident when viewed from a sustainability point of view.

Formwork savings of 50 to 100% over conventional cast-in-place construction;
Formwork does not have to be designed for internal pressures;
Complex shapes require very little, if any, formwork;
Crane and other equipment savings or elimination;
Labour savings of at least 50% in repair applications;
Speed of repair reduces or eliminates downtime;
Better bonding to the substrate, which enhances durability;
Adaptability to repair surfaces that are not cost-effective with other processes; and
Ability to access restricted space and difficult-to-reach areas, including overhead and underground.