is 4.5 MPa + 1.65 S …….. (2.1)

is based on the target flexural strength. The target mean flexural strength of quality concrete is given by Equation 2.1:

fct

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=

4.5 MPa

+

1.65 S

                                                                        ……..  (2.1)

Where,

S

=

Standard deviation of flexural strength derived by conducting test on minimum 30 beams.

2.3  Steel -Polypropylene Fibre Reinforced Concrete

The Steel Polypropylene Fibre Reinforced Concrete(SPFRC) is composite of conventional Concrete and uniformly dispersed steel & polypropylene fibre. In this steel fibre provides the structural improvement where as polypropylene fibre enhances the resistance to plastic shrinkage cracking.

Chang et al., 1995, studied the behaviour of fibres-reinforced concrete. They suggested that with the use of steel fibres; increases in flexural strength and ductility of concrete due to the ability of the fibres to restrain cracks. All concrete contains flaws which could increase in size under loads which were less than 50% of the ultimate load. The fatigue failure mechanism for concrete or mortar develops in three stages:

(i)     The first stage, flaw initiation, is an inherent quality of concrete such as the presence of air voids and weak or debonded regions between aggregate and paste.

(ii)   The second stage, the slow growth of flaws to a critical size, is a complex mechanism in a heterogeneous material such as concrete. The growth of the inherent flaws under static loading of concrete is called micro cracking. From the previous studies it was stated that there would be some flaws of a shape, size and orientation in the stress field that may grow slowly in a stable manner to a critical size and then increase rapidly.

(iii) In this third stage, when a sufficient number of unstable cracks join to form a continuous crack, failure of the member follows quickly.

The first and third stages of the failure mechanism cannot be prevented, but there is the possibility of retarding the growth of the flaws in the second stage by using closely spaced and randomly dispersed steel & polypropylene fibres as reinforcement in the concrete or mortar.

2.3.1   Behaviour of Steel & Poly-Propylene Fibres in Concrete

The experimental work is oriented towards concrete reinforced with steel and polypropylene fibres, so it is most important to understand how these fibres behave in the concrete mix. The