Civil Engineering Reference
In-Depth Information
With regard to the time-related behaviour of compression members wrapped with CF
sheets, only the experimental studies of
AlChamietal.
[140] were available at the
time of carrying out the work on which the provisions of the DAfStb guideline [1, 2]
are based. The provisions for describing the time- and load-dependent deformation
behaviour of confined compression members in the DAfStb guideline [1, 2] were
drafted on the basis of the relationships known from annex B of DIN EN 1992-1-1
[20, 21], taking into account the results of the experiments reported in [140]. A value
of 2.7 was determined for the stress intensity factor
α
σ
on the basis of the results given
in [140].
Generally, the creep coef
cient
φ
(
t
,
t
0
) according to duration of loading (
t
,
t
0
) at the time
considered
t
is calculated as follows:
φ
t
;
t
0
φ
0
?
β
c
t
;
t
0
(7.28)
Here, the notional creep coefficient
φ
0
is given by
?
β
t
φ
0
φ
RH
?
β
f
cm
(7.29)
The coefficient
φ
RH
describes the contribution of drying creep to the notional creep
coefficient. For customary concrete members to DIN EN 1992-1-1 [20, 21], this
coef
cient is determined depending on the relative humidity of the ambient air, the
geometry of the member and the compressive strength of the concrete. In their own
series of tests,
Naguib
and
Mirmiran
[141] observed that a full wrapping with glass-
fibre sheet saturated with epoxy resin functions like a vapour-tight sealing layer and
therefore shrinkage deformations can be ignored. Drying creep, which makes a
signi
cant contribution to the creep deformation of unsealed concrete elements, is
almost eliminated by a wrapping over the full surface area. The coef
cient
φ
RH
was
therefore set to 1 for the full wrapping with CF sheets - applied using cold-curing,
low-viscosity epoxy resins - dealt with in the DAfStb guideline [1, 2].
The coef
cient
β
(
f
cm
) describes how the compressive strength of the concrete in
uences
the notional creep coef
cient depending on the mean cylinder compressive strength
of the concrete
f
cm
after 28 days. The formulation in EN 1992-1-1 [20] has been included
in the DAfStb guideline [1, 2] without modi
cation.
The system coefficient [
k
7
] for
β
(
t
0
) proposed in the DAfStb guideline [1, 2] in order to
consider how the age of the concrete at the onset of loading influences the notional creep
coefficient was specified as 0.39 - an average figure that results from the conditions of
the experimental studies of
Al Chami et al.
[140]. The onset of loading in these tests was
about four to six weeks after casting the specimens. Using this suggested value, the
factors for the much higher concrete age and the preloading, which have a generally
positive effect on strengthening measures, are ignored
an approach that lies on the safe
side. However, suitable tests can be carried out on preloaded test specimens to establish
a different, more realistic value. In such tests it may also be necessary to reproduce
experimentally the spectrum of properties of the concrete aggregates relevant to creep
for the range relevant in practice.
-
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