FIBRE REINFORCED CONCRETE:
Plain concrete possesses a very low tensile strength, limited to ductility and little resistance to cracking.
Internal micro-cracks are inherently present in the concrete and its poor tensile strength is due to the propagation of such micro-cracks, eventually leading to the brittle failure of the concrete.
It has been recognized that the addition of small, closely spaced and uniformly dispersed fibres to concrete would act as crack arrester and would substantially improve its static and dynamic properties.
Fibre Reinforced Concrete is therefore defined as the concrete made with cement, containing fine or fine and coarse aggregate and discontinuous discrete fibres.
The fibres can be made from
Natural Material . :- Such as Asbestos, Sisal, & Cellulose
Manufactured Products :- Such as Glass, Steel, Carbon, & Polymer (e.g. Polypropylene, Nylon etc.)
Fibre reinforcement improves the impact and fatigue strength, and reduces the shrinkage.
Fibre is a small piece of reinforcing material possessing certain characteristic properties.
The quantity of fibre used is small, typically 1 to 5 percent by volume.
To render them effective as reinforcement:
– The tensile strength
– Elongation at failure, and – Modulus of elasticity
of the fibres need to be substantially higher than the corresponding properties of the matrix.
Some other significant characteristics of the fibres are:
– Aspect Ratio (ratio of length to mean diameter) – Shape and surface texture
– Length and – Length and
The fibre can withstand a maximum stress f, which depends on the aspect ratio (L/D), Viz.:
(1)
Where, = Interfacial bond strength d = Mean diameter of fibre
d ) ( L
f
LC is the critical length of the fibre such that, If L < LC
the fibre will pull out of the matrix due to failure of bond, and If L>LC then,
the fibre itself will fail in tension.
The length of the fibre should be greater than the maximum size of the aggregate particles.
According to Eq.(1), the higher the interfacial bond strength the higher the maximum stress in the fibre.
The interfacial bond strength is improved by fibres having:
A deformed or roughened surface,
Enlarged or hooked ends, and
By being crimped.
The type of fibres used may be of Steel, Polypropylene or Nylon, Asbestos,
Glass and Carbon.
Steel Fibres:-
It is one of the most commonly used fibres.
Generally round fibres with a diameter ranging from 0.25 to 0.75 mm are used.
Use of steel fibres makes significant improvements in flexural, impact and fatigue strength of concrete.
It has been extensively used in various types of structures Such as :
Overlaying of Roads
Airfield Pavements and
Bridge Decks
Thin Shells and Plates have also been constructed using steel fibres.
Polypropylene And Nylon Fibres:-
They are found to increase the impact strength.
They possesses very high Tensile Strength.
Their low Modulus of Elasticity and higher Elongation do not contribute to the flexural strength.
Asbestos Fibres:-
It is a mineral fibre and has proved to be most successful of all fibres, and can be mixed with Portland cement.
Tensile strength varies from 5600 to 9800 Kg/cm2 Glass Fibres:-
Glass fibres are the recent introduction in making fibre concrete.
It has a very high tensile strength varying from 10200 to 40800 Kg/cm2
Glass fibre was fund to be affected by alkaline condition of cement, therefore alkali-resistant glass fibre by the trade name of “CEM-FIL” has been developed and used.
The alkali resistant glass fibre reinforced concrete shows considerable improvement in durability when compared to the conventional glass fibre concrete.
Carbon Fibres:-
Carbon fibres, perhaps possesses very high tensile strength (21120 to 28150 Kg/cm2) and Young’s modulus.
It has been reported that cement composite made with carbon fibre as reinforcement will have very high modulus of elasticity and flexural strength and good durability.
Properties & Applications
It has been increasingly used on account of increased
Static and dynamic tensile strength,
Energy absorbing characteristics and
Better fatigue strength.
The uniform dispersion of fibres throughout concrete provides isotopic properties not common to conventional reinforced concrete.
It has been tried on:
Overlays of airfield
Road pavements
Industrial floorings
Bridge decks
Canal linings
The FRC can also be used for the fabrication of pre-cast products like pipes, boats, beams, stair case steps, wall panels, roof panels, manhole covers etc.
The FRC sometimes called fibrous concrete, is manufactured under the trade name “Wirand Concrete”, and after extensive research has been extensively used in USA.
With the development of ‘CEM-FIL’ the alkali resistant glass fibre by the U.K. Building Research Establishment and Pilkington Glass, UK, a wide ranging applications of fibrous concrete is being made in various areas of building construction.
Glass reinforced cement consist of 4 to 4.5 per cent by volume of glass fibre mixed into cement or cement sand mortar.
This glass reinforced cement mortar is used for fabricating concrete products having a section of 3 to 12 mm in thickness.
The GRC has been used for cladding of buildings, temporary or permanent form-work, pressure pipes, door and door frames decorative grills, sun breakers, bus shelters, and park benches etc.
