Fire Safety and Concrete Structures - Febelcem

The rules of Eurocode 2 fire also specify the cases in which
special measures must be taken against the spalling:
For concrete classes C55/67 to C80/95, the rules given for normal
concrete above for the spalling apply, provided the maximum
level of silica fumes is less than 6 % of the weight of the cement.
For higher levels of silica fumes, the rules given for concrete
classes C, with C80/95 < C ≤ C90/105 apply.
For concrete classes 80/95 < C ≤ 90/105, it is advisable to apply at
least one of the following 4 methods:
Method A: place a mesh of reinforcements with a nominal cover
of 15 mm. It is best for this mesh to include wires with a
diameter greater than or equal to 2 mm, with a pitch less than or
equal to 50 x 50 mm. It is best for the nominal cover of the main
reinforcement to be greater than or equal to 40 mm.
de-rating factor for resistance
1
0,9
curve 1
0,8
curve 2
0,7
curve 3
0,6
0,5
0,4
0,3
0,2
0,1
0
0 200 400 600 800 1000 1200
temperature (°C)
NOTE: We advise against this method because it is not certain
that this mesh will be able to be kept in place during concreting.
The mesh may be close to the surface in the 20 mm superficial
zone of the concrete with the associated risks of carbonation. The
prescribed nominal cover is less than those prescribed in EN
1992‐1‐1 for all exposure classes.
Method B: use a type of concrete for which it has been shown
(by local experience or by tests) that there is no risk of the
concrete spalling when exposed to fire.
Method C: use protective casings for which it has been shown
that there is no risk of the concrete spalling when exposed to
fire.
Method D: use a mix of concrete which contains more than
2 kg/m 3 of monofilament polypropylene fibres.
This last method is the one we recommend in the absence of a
demonstration by methods B and C.
7.2.4. Self-compacting concretes
These concretes do not require vibrating to be installed. For
more details on the technology of these concretes, we refer the
reader to the cement bulletin nr 36 [59] which deals specifically
with this subject.
The small amount of experimental results show that the
reduction in compressive strength and behaviour on blow‐out
do not present any significant differences in relation to vibrated
concretes of a similar composition. Tests carried out in France
allowed them to be used.
7.2.5. Steel
Figure: de‐rating factor ks(θ) applicable for the characteristic strength
(fyk) of tension or compression reinforcement (class N = recommended
class) [107]
Legend:
Curve 1: tension reinforcements (hot‐laminated steel) for strain ≥ 2 %;
Curve 2: tensioned reinforcements (cold‐formed steel) for strain ≥ 2 %;
Curve 3: compression reinforcement or tension reinforcement for strain
< 2 %.
The difference between the curves is linked to the fact that the
experimental results show that the plateau of plasticity of steels
disappears when hot, and therefore that the factor ks(θ) depends
on the elongation at rupture.
Curve 1 is the same as in NBN ENV 1993‐1‐2 [112] for the
calculation of profiles in steel frame constructions.
Why are the deformations of the compression reinforcement
limited (curve 3)?
In more concrete terms, in columns or in support zones of
continuous beams, reinforcements may be situated in hightemperature
zones where the concrete can cope with greater
deformations, beyond the elastic deformation limit of 0,2 % for
steel. In limiting the strains, the use of curve 3 warns against the
danger of premature buckling of reinforcements between
stirrups, thereby guaranteeing the compatibility of the
deformations of concrete and steel.
Resistance of pretensioning steel
The drop in strength is much more rapid in pretensioning steels.
This explains the increase of concrete cover in the tabulated
value methods compared to ordinary steels: 10 mm for bars and
15 mm for strands.
Resistance of steel for reinforced concrete
The de‐rating factor for the characteristic strength of the steel in
reinforced concrete as a function of temperature θ is illustrated
in the following figure. This factor varies according to the type
of steel (hot‐ or cold‐laminated) and according to the steel strain:
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