30-05-2013, 04:05 PM
FIBER REINFORCED CONCRETE & IT’S RECENT DEVELOPMENT
FIBER REINFORCED.pptx (Size: 673.15 KB / Downloads: 67)
FRC INTRODUCTION
PCC has low tensile strength, limited ductility, little resistance to cracking. In pcc ,cracks develops due to self weight and drying shrinkage . The width of the crack will increase to few microns in few days ,but length and height will increase in larger extent. It will reduce the total strength of the structures.
To rectify these defects research studies have revealed that addition of small closely spaced uniformly dispersed Fibers to concrete acts as a crack arrestor and this concrete with Fibers is known as “FIBRE REINFORCED CONCRETE.”
In RCC, fibers are mainly used to have a good bond between steel and concrete. It will attract it’s surrounding concrete towards itself.
FRC is mainly used in place where water tightness is required, earthquake resistant structures, roads, bridges, water decks etc.
PROPERTIES AND TYPES OF FIBRES USED
STEEL FIBRES:
Only few Fibres are suitable for concrete they are steel fibres, polypropylene, asbestos, nylon, glass and carbon. They reduce micro cracks.
Fibres can be circular or flat. They can be expressed in “ASPECT RATIO”. Aspect ratio ranges from 30 to 150. up to 75 increase in aspect ratio increase in ultimate strength beyond 75 strength reduced.
Steel fibre is most commonly used, it’s diameter varies from 0.25 to 0.75. In steel fibres corrosion will takes place only at surface but the core remains unaffected.
SYNTHETIC FIBRES:
Polypropylene and nylon fibres are increase the impact strength. Polypropylene fibres are hydrophilic
Asbestos , a mineral fibre , is a most successful of all the fibres. Tensile strength varies between 560 to 980N/mm2.
Glass fibre is a recent introduction. Tensile strength varies between 1020 to 4080 N/mm2.
STEEL FIBRE REINFORCED CONCRETE
Steel fiber-reinforced concrete is basically cheaper .
Easier to use in the form of rebar reinforced concrete.
Having greater structural strength and reduces cracking
SFRC is mainly used in place where there is increased in static and dynamic tensile strength and better fatigue strength.
The uniform dispersion of fibres provides isotropic properties not common to conventional methods
SYNTHETIC FIBRE REINFORCED CONCRETE
The dosage of super plasticizer in concrete has to be adjusted to get the desired slump.
General dosage per cum is 0.9 Kg .
The water demand is nil for polypropylene fibre reinforced concrete.
FRC mixes usually contain a high mortar volume as compared to conventional concrete mixes
SNFRC reduce the plastic shrinkage crack area due to their flexibility and ability to conform to form.
The vapor pressure In SNFRC is released in newly formed voids and explosive sapling is significantly reduced
HANDLING OF FRC
A fibre with double end hooks as anchorage elements gives the right optimization of performance and workability.
The maximum drum rotating speed should be 12-15 revolutions per minute.
Each truck mixer should be rotated at full speed for 8 -12 minutes after adding the fibres
The concrete with steel fibres can be pumped, with some adjustments to the mix design may be necessary depending on the fibre dosage requirements.
Recommended slump SFRC pumping is 150mm to avoid balling.
The concrete can be compacted and finished normally. Toweling will help to embed the fibres in to the concrete surface. Hence, Toweling after laying is advisable.
The fibers should not float to the surface nor sink to the bottom in the fresh concrete.
Synthetic fibres are commercially available in water disposable bags.