Civil Engineering Reference
In-Depth Information
The following expression fromModel Code 90 [142] was incorporated to describe the
chronological development of creep after the onset of loading
β
c
(
t
,
t
0
). In contrast to
the corresponding relationship in DIN EN 1992-1-1 [20, 21], this formula also
contains the time-based variable
t
1
. A reference time period
t
1
=
1.7 d for describing
the creep behaviour of concrete compression members wrapped with CF sheets was
determined on the basis of the tests by
Al Chami et al.
[140].
0
:
3
t
t
0
=
t
1
β
c
t
;
t
0
(7.30)
β
H
t
t
0
=
t
1
The coef
cient
β
H
takes into account the relative humidity of the air
RH
and the effective
member thickness relevant to drying creep
h
0
. Owing to the full wrapping, this
coefficient is specified for the range of applicability of the DAfStb guideline [1, 2]
without taking into account these two values and only depending on the compressive
strength of the concrete.
(
for
f
cm
35 N
=
mm
2
250
β
H
(7.31)
mm
2
250
?
α
3
for
f
cm
>
35 N
=
0
:
5
35
f
cm
α
3
(7.32)
In the DAfStb guideline [1, 2] the creep deformation of reinforced concrete compression
members with a circular cross-section and a full wrapping of CF sheet over the period of
loading
Δ
t
=
t
t
0
was related to the elastic deformation variable. The latter is deter-
mined from the creep-effective compressive stress
σ
cp
and the modulus of elasticity,
which in contrast to DIN EN 1992-1-1 [20, 21] is included as a secant modulus
corresponding to the calibration of the model:
σ
cp
E
cm
ε
cc
Δ
β
t
?
β
c
Δ
?
β
f
cm
?
β
0
;
k
?
(7.33)
0
:
3
Δ
7
β
H
Δ
t
=
t
=
1
:
β
c
Δ
t
(7.34)
1
:
7
8
f
cm
N
16
:
β
f
cm
q
(7.35)
=
mm
2
e
2
:
7
?
k
σ
0
:
45
β
0k
(7.36)
Figure 7.8 compares the approach of the DAfStb guideline [1, 2] with selected results
from the work of
Al Chami et al.
[140]. This approach supplies an estimate that also lies
on the safe side for the results of creep tests on reinforced concrete cylinders wrapped
with aramid
bres by
Wang
and
Zhang
[143] and also the studies of
Berthet et al.
[144],
which have in the meantime been published.
Search WWH ::
Custom Search