While fibers are randomly oriented in reinforced concrete, their orientation can affect the strength of the concrete. Fibers that are aligned parallel to the load provide more tensile strength and durability than those perpendicular or randomly distributed.
Durability Of Cracked Fibre Reinforced Shotcrete Poolsl
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Shotcrete can be used in lieu of conventional concrete for reasons of convenience or, less frequently, of cost. Shotcrete is advantageous in the situations when formwork is cost prohibitive, impractical or altogether impossible, due to limited access to the work area. Very thin layers of shotcrete can be achieved (up to 3-4 cm when reinforcement is included), considerably thinner than normal casting techniques that require at least 10-12 cm of thickness when reinforcement is present. This makes shotcrete ideal for reinforced concrete jackets.
Fibres are used to increase the ultimate strength, particularly the tensile strength of shotcrete, as well as its ductility and energy absorption capacity. Furthermore, fibres allow for better crack control and may decrease the width of shrinkage cracks in the material. Fibres are discontinuous and unlike conventional reinforcement, they are distributed randomly throughout the concrete matrix. In shotcrete they are available in the following general forms [US Army Corps of Engineers, 1993]:
Steel fibres are found in different shapes, round, flat, or irregular, while additional anchorage is provided by deformations along the fibre length or at the ends, e.g. fibres with hooked or flat ends, with crimps, corrugations, or undulated fibres (Figure 3). Steel fibres provide a modest increase in the tensile and flexural strength, but limited increase in the compressive strength of shotcrete, however they contribute to improved load carrying capacity after the cracking of the member.
b. Synthetic and polypropylene-fibres. The technology of using synthetic fibres in shotcrete is relatively new, compared to steel fibres, but their use is growing rapidly, and nowadays they are preferred to steel fibres (Figure 4). A wide range of types is available: aramid, polypropylene, polyethylene, polyester, or rayon, but polypropylene is mostly used in retrofit applications. The most common specified lengths for polypropylene are between 2 and 4 cm, however longer lengths can also be accommodated. The typical amount to be added to the shotcrete mix is between 1.0 and 3.0 kg/m, and it does not require any change in the mixing ratios of the other concrete materials.
The most important use of synthetic fibres is to control plastic shrinkage, but synthetic fibres also increase the toughness and the tensile, and to a limited extent the compressive, strength of the shotcrete.
c. Glass fibres. The fibres consist of alkali-resistant glass (designated AR glass), which is protected from the alkalinity of cement. Alkali resistance is achieved by adding zirconia to the glass, the higher the zirconia content the better the resistance. The application of glass-fibre shotcrete cannot be done with the conventional equipment for shotcrete, but requires a special gun and delivery system, hence glass reinforced shotcrete is not widely used in strengthening and retrofit operations.
Long-term durability is also significantly improved as the steel fibres provide a concrete matrix with discontinuous reinforcement, able to withstand corrosive elements carried by subsurface water infiltration. As a result, steel fibre reinforced shotcrete can be used under conditions that would otherwise require the use of traditional and expensive support methods, including rock bolting. 076b4e4f54