Coupling Between Diffusivity and Cracks in Cement-Based Systems
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different levels of compressive stress, then tested by the diffusion test. The technical experiments and the results are presented in the first part. Then, a modelling of the relation between diffusivity and cracks is discussed. EXPERIMENT Compression tests The behaviour of mortar or concrete in compression follows 3 phases : in the first, the relation between compressive stress and strain is almost linear; in the second, it becomes non-linear with positive hardening ; then, after the peak load capacity, it softens. The non-linearity reflects the appearance of microcracks which are localised in a shear band at the maximum compressive capacity and then become organised into macrocracks [1]. A compressive test with a constant rate of circumference expansion allows the sample to go past the maximum compressive capacity with a stable softening curve [2], [3]. Consequently, it creates considerable cracking across the whole specimen. Actually, in the softening phase, lateral strains (perpendicular to the loading) become very large and continue to increase (cf. figure 1). For the study of the influence of cracks on diffusivity, the material was mortar [4]. The compressive tests were controlled by the lateral strain measured with a chain and a displacement 325 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
sensor placed around the middle of the cylindrical sample (diameter 113 mm and height 220 mm) [4]. The three levels of loading were (cf. figure 1): - level 0 : no loading - PEAK level: sample loaded up to a circumference strain almost equivalent to the
maximum compressive capacity
(E2 = 63 =
1,15.10-3).
POSTPEAK level: sample loaded up to a circumference strain such that the peak load capacity was passed with appearance of a localised macrocrack running across the specimen from side to side, visible to the naked eye on both sides. The POSTPEAK level was obtained after 4 cycles, so the control of the test was good (C2 = E3 = 3,76.10-3). Reproducibility in global behaviour (strain-stress law) was good, although cracking features were not exactly similar: softening behaviour was structural but the cracks spread with a diffuse damage zone at PEAK level whereas a macrocrack ran across every sample at POSTPEAK level. Figure 2 shows an example of the crack patterns (microcracks and macrocrack) observed by image analysis in PEAK and POSTPEAK samples. -
Lateral strain
-80 ±70
62 =3
.
Axial strain ..........
e1
............. ..........
-I-60 Sg
•r
a.
40 a.. -30 0n -20 -10
5
4
3 62 max
for post-pea 61
2
il
0
-1
-2
-3
-4
WO 2 max for peak load A
Strain (minim) Figure1. The three levels of loading in compression : level 0 corresponds to a virgin sample, PEAK level to the maximum compressive capacity and POSTPEAK level to the softening phase. For the POSTPEAK level, 4 cycles of loading and unloading were controlled by circumferential expansion (the first one up to the maximum compressive capacity -if the unloading was complete, the level was PEAK-, and the three others up to the sof
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