For further information refer to concrete society

For further information refer to Concrete Society technical reports
TR63
[35]
and TR65
[36]
.
Fibre quantity assessment
The following procedure should be used in conjunction with fibre stock
control and the recording of the number of bags/boxes added to each
load to ensure that the correct quantity of fibres has been used.

21
Concrete Industrial Ground Floors 4 th Ed.
Table 6.1: Strength properties for concrete.
Symbol Property
Strength class Units Explanation
C25/30 C28/35 C30/37 C32/40 C35/45 C40/50
f
ck
Characteristic compressive
strength (cylinder)
25 28 30 32 35 40 N/mm 2 Cylinder strength
f
cu
Characteristic compressive
strength (cube)
30 35 37 40 45 50 N/mm
2
Cube strength
f
cm
Mean compressive strength
(cylinder)
33 36 38 40 43 48 N/mm
2
f
ck
+ 8
f
ctm
Mean axial tensile strength 2.6 2.8 2.9 3.0 3.2 3.5 N/mm
2
0.3 f
ck
(0.67) note 1
E
cm
Secant modulus of elasticity 31 32 33 33 34 35 kN/mm
2
22 ( f
cm
/10) 0.3
Note 1 : For concrete strength class above C50/60 the expression for determining f
ctm
is:
f
ctm
= 2.12 ln[1 + ( f
cm
/10)]
The test procedure for steel fibre content and homogeneity is in
accordance with EN 14721 [37] Test method for metallic fibre concrete.
Measuring the fibre content in fresh and hardened concrete and for macro-
synthetic fibre content EN 14488-7 [38] Testing sprayed concrete. Fibre
content of fibre reinforced concrete . In both Standards, three samples are
taken from the first, middle and last third of the load and individually
tested. It is recommended that the sample container capacity is 10 litres.
Test compliance criteria are given in Table 6.2.
In the light of the variability of fibre distribution, it may be necessary to
specify a target fibre dosage that is higher than the required design value.
Table 6.2: Identity criteria for fibre content of fresh concrete [39] .
Application Criterion
Every sample ≥ 0.80 of specified minimum value
Average of three samples from a load ≥ 0.85 of specified minimum value
6.2.3 Micro-synthetic fibres
Micro-synthetic fibres do not provide any post-crack ductility. They do not
control cracking of the hardened concrete and therefore cannot be used in
lieu of other reinforcement. They are not considered in the design process.
6.3 Moment capacity
6.3.1 Plain concrete
The moment capacity of plain concrete per unit width of slab is given by:
M
un
= f
ctd,fl
( h
2
/ 6) Equation (2)
where f
ctd,fl
= design concrete flexural tensile strength.
Note that neither fabric nor fibres increase the cracking moment, so where
the required design limit is the onset of cracking, the moment capacity
should be derived on the basis of plain (unreinforced) concrete. This applies
in particular to the negative (hogging) moment in ground-supported slabs.
6.3.2 Fabric-reinforced concrete
The moment capacity M
pfab
per unit width of slab is calculated from:
M
pfab
= 0.95 A
s
f
yk
d / γ
m
Equation (3)
where A
s
= area of steel
f
yk
= characteristic strength of steel
d = effective depth.
The coefficient 0.95 is adopted as the reinforcement content is small
enough to make a specific calculation of the lever arm unnecessary.