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International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
also be taken into account (total heat release, concrete thermal conductivity,… [8]). Besides,
the initial concrete temperature has a significant effect on the maximal temperature but to
avoid this third parameter, we used a relationship between the external temperature T
ext
and
the
initial concrete one T
ini
, proposed by Torrenti and Buffo-Laccarrière [8].
Figure 2: Temperature difference between the core and the surface in a massive wall
(thickness 1.2m) with respect to the wind velocity and of the external temperature.
The results of this study are resumed on figure 1 and 2. One can see on the figure 2 a slight
effect of wind velocity on maximal temperature. This can be explained by the fact that, as the
thermal concrete conductivity is rather low, the thermal exchange conditions at the core are
close to adiabatic ones. On the contrary, the evolution of the maximal temperature due to the
external temperature is quasi-linear. This can be explained by the fact that the initial
temperature is assumed to be a linear function of the external temperature. On contrary to the
previous results, the wind velocity has a high effect on "temperature gradient" (Figure 2),
whereas the external temperature has a slight one. This indicates that with regards to these
two phenomena, there are no optimal casting conditions (autogenous and drying shrinkage are
not considered).
2.1.2 Dissymmetry of creep: effect on stresses
There is no consensus on the tensile creep of concrete at early-age and at long term. In most
cases, only compressive creep tests are performed, since they are easier to perform. Besides,
stresses (compression/tension) may change during service life of concrete structures. In order
to show the impact of this dissymmetrical behavior, material parameters were identified on
experimental results for creep of Briffaut et al. [9] (see the results in Figure 3 at left). Then,
numerical simulations were performed on a 1.2 m thick concrete structure [3]. The results
shown in Figure 3 (right) highlight a difference which is greater than the difference between
the results obtained with and without the taking into account or not of creep (after 10 days).
However, the difference occurs only after 4 days.
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International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
Figure 3: Comparison between simulated (thick lines) and experimental evolutions of strains
(left). Stress and temperature evolutions versus time in a 1.2 m thick massive wall [3] (right).
2.1.3 Simulation of RG8 specimen: influence of creep and creep/cracking coupling
Numerical simulations are performed on a large beam specimen realized for ConCrack
international Benchmark (for Control of Cracking in Reinforced Concrete Structures, [4]).
After casting, the structure is thermally isolated and protected from drying during 48 hours.
Then, the isolation and the formwork are removed and the structure is kept during two months
outside. During all the test, longitudinal strains of the structure are globally restrained by two
metallic struts.
Numerical simulations on an active thermal ring test [9] and on RG8 beam [4] show that the
coupling between creep and cracking should be taken into account in order to retrieve
experimental simulations (i.e. the occurrence of 1 crack at least). Indeed, as displayed in
Figure 4 and Table 1, no cracking is predicted with an approach where creep is not coupled
with damage (3 cracks appear during the experiment at different times). Taking into account
creep reduces drastically the crack opening. However, it should be emphasized that only 2
(considering the symmetry) cracks occur, as 3 cracks (the first one in the center of the
specimen) have been reporting during the experiments.
Figure 4: Damage field on the RG8 beam for different creep approach: 1 = without creep, 2 =
Creep without coupling with cracking, 3&4 = creep coupled with cracking for 2 different
meshes (3 is not displayed but it is very similar to 4).
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International RILEM Conference on Materials, Systems and Structures in Civil Engineering
Conference segment on Service Life of Cement-Based Materials and Structures
22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
Table 1: Predicted crack opening for different models
Model
Without creep
(1)
Creep without
coupling with
cracking (2)
Creep coupled
with cracking,
mesh 1 (3)
Creep coupled
with cracking,
mesh 2 (4)
Crack opening
[μm]
359 0 158 152
2.2. Effect of drying at long term
The effect of drying shrinkage is illustrated on a RC-tie specimen (w/c = 0.48, section of
10x10 cm², 1 m long reinforced with a steel bar of 12 mm diameter, see [10]). Ninety days of
drying are taken into account (only a quarter is meshed). Figure 5 shows the damage and axial
stress fields after 90 days. Along the interface, it can be observed a slight concrete
degradation (D around 0.3) due to the strain incompatibility between concrete and the couple
rebar-interface. However, considering a uniform drying shrinkage strain, the rebar restraint
induces significant tensile quasi-uniform axial stresses in the specimen. The axial stresses are
equal to about 0.75 MPa (=1/4 of the tensile strength). The mechanical response of the tension
test will be therefore influenced by the ‘‘initial’’ state of stress in the specimen.
Figure 5: RC tie: experimental set-up; stress and damage fields in concrete, after 90 days in
drying conditions (quarter of structure) – the steel rebar and the interface are not represented.
The tension test is then numerically carried out. The mechanical response is plotted in Figure
6. The response is greatly impacted by the drying shrinkage strain in the specimens. The first
cracking force decreases by about 30%, in accordance with the initial internal stresses before
loading. The first crack establishes the course of the further crack initiations. It is also
observed a slight loss of initial stiffness if drying. That is explained by concrete degradation at
the steel–concrete interface before loading (Fig. 5).