Proceedings of the International rilem conference Materials, Systems and Structures in Civil Engineering 2016
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- Bu səhifədəki naviqasiya:
- Constituant Weigth in Kg for 1m 3
- 3.1 Creep test results
- Compressive basic
- 3.2 Mechanical strength after creep loading
130 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
which were not subjected to creep test. Comparing to the specimens that were not subjected to the creep test, the strength decreasing for creep specimens that were under creep loading could reach 13% while the Young Modulus decreasing is about 25%. The Young modulus decrease is therefore due to both creep strains and previous loading of the structure to a stress level of 25, 35, 50 or 65% and could not be only attributed to only creep.
Figure 3: Constitutive law - compressive test before creep and after creep in compression had taken place. 3. Experimental campaign
For performing the compressive basic creep test, concrete cylindrical specimens (diam. 7cm, length 14cm) were used. For two concrete maturity (1 or 3 months; curing under water), two loading levels (stress/strength = 0.5 and 0.8) were applied for two months. The creep device is based on the principle of the roman balance which allows maintaining a constant loading (see figure 4). After the creep test, the residual mechanical properties of concrete were compared to control specimens from the same batch but which had not been submitted to sustain loading. For each strength value, three concrete specimen where tested.
The concrete mix design is close to one that was extensively caracterized in our laboratory under uniaxial and triaxial behaviour and is given in table 2. The low maximal aggregate size (8mm) allows to obtain a representative concrete specimen.
Without creep 25% 35% 50% 65% 131 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 4: Creep test device and instantaneous compressive test
Table 2: Concrete mix Constituant Weigth in Kg for 1m 3 Water
174
Cement (CEM I 52.5) 348
Sand (0 - 1.8mm) 826
Gravel (0.2 – 8mm) 991
Longitudinal mean strains during the creep test are plotted in figure 5 (measured by two strain gages for two concrete specimens). As expected, higher the stress/strength ratio is higher is the strains. The effect of the concrete maturity between 1 and 3 month remains slight.
Figure 5: Creep strains evolution for two loading rate (stress/strength = 80% and 50%) and two loading age (1month and 3 month)
Thanks to the strain measurements in two directions (parallel and perpendicular to the loading), an apparent creep Poisson ratio could be calculated. The results for the loading age Compressive basic
132 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
equals to 3 months and for two loading level tests, is displayed in the figure 6. One can remark that after a slight increase, theses lasts seem to tend around a value which is close to 0.2.
Figure 6: Apprarent creep Poisson ratio evolution for two loading rate (stress/strength = 80% and 50%)
From figure 8 which presents the compressive strength evolution due to the creep loading, different trends could be remarked. In the one hand, for the specimens which were loaded at one month after casting, a significant increase in compressive strength is observed for two loading levels, 50% and 80%. On the other hand, for the higher concrete maturity (3 months) a slight decrease is obtained for the both loading levels. It is therefore obvious that at least two phenomena are in competition. The first one is the microcracking at the cement paste/aggregate interface which could explain the strength decrease. The compressive strength increase could be explained by the solidificat increase of the hydration due to microcrack. Indeed, microcracks become new path for water to react with anhydrous cement and create inner hydration product. Another possibility is that creep reduces stress concentrations (especially when dealing with real aggregate shapes) and thus lead to an increase of the maximum strength. Yüklə 8,6 Mb. Dostları ilə paylaş:
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