PSI - Issue 43

Vlastimil Bílek et al. / Procedia Structural Integrity 43 (2023) 107–112 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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3. Results and Discussion The experimentally obtained results from strength and fracture mechanics tests for foil- and water-cured specimens are shown in Table 3.

Table 3. Properties of concretes with various water to cement ratios and under two boundary conditions: foil and water. water to cement ratio 0.50 0.50 0.40 0.40 0.30

0.30

way of curing

water

foil

water

foil

water

foil

volume density  bending strength f b

(kgm -3 ) (MPa) (MPa) (MPa)

2218 ± 24 7.7 ± 0.1 52.8 ± 3.9 5.2 ± 0.2 34.7 ± 2.6 1.15 ± 0.02

2175 ± 15 7.1 ± 0.2 51.2 ± 1.0 5.0 ± 0.8 32.3 ± 2.3 1.05 ± 0.09

2365 ± 17 7.3 ± 0.7 69.1 ± 1.5 6.2 ± 0.5 38.4 ± 2.5 1.41 ± 0.19

2344 ± 11 8.2 ± 0.5 72.0 ± 3.6 6.4 ± 0.7 38.3 ± 1.8 1.29 ± 0.11

2365 ± 18 9.1 ± 0.3 72.5 ± 2.0 7.0 ± 0.4 49.3 ± 2.4 1.38 ± 0.09

2356 ± 13 9.3 ± 0.3 78.8 ± 3.5 7.0 ± 0.7 39.9 ± 1.4 1.48 ± 0.09

compressive strength f c modulus of rupture f r modulus of elasticity E ef fracture toughness K IC ef

(GPa)

(MPam 1/2 )

fracture energy G F

(Jm

-2 )

106 ± 7

108 ± 18

130 ± 13

153 ± 15

134 ± 9

151 ± 15

Compressive strength of concrete increases as w/c decreases. Higher values of the compressive strengths of foil enveloped specimens for w/c 0.4 seem interesting, especially for w/c = 0.3. For w/c = 0.5, all the mechanical properties are nearly in accordance with theory, Aitcin (2014), Kurdowski (2014), because the concrete contains enough water for hydration and also for filling the capillary pores. Bending strength for this concrete is higher for water-cured prisms, as water can penetrate inside the concrete and support the cement hydration and a lower shrinkage of concrete. In the case of w/c = 0.40, there is a similar situation for all mechanical properties thanks to the standard deviation of average values. There is an indication that the bending strength is slightly higher for foil-cured concrete. The concrete with w/c = 0.30 shows nearly the same values of f b , f r and K IC ef for both regimes of curing, but lower f c and E ef for foil-cured concrete. With respect to the higher compressive strength of foil-cured specimens, this indicates microcrack development. The microcracks were searched by CT. The results of CT are presented in Table 4 and Fig. 1. Fig. 1 shows a brighter shade as w/c decreases. The gray levels of individual voxels in tomographic sections are scaled to 256 levels, where black (value 0) represents the materials in the CT volume with the lowest density, lighter shades of gray represent materials with a higher density (densest materials = white = 255). A brighter shade in lower w/c means a higher density of the hydrated cement paste, which agrees with the assumption. Significant inhomogenities in all specimens are represented by pores and microcracks. It seems from the tomographic sections that in the foil-cured specimens there are slightly more separation cracks at the contact of the individual grains of concrete and its matrix. But the difference is low.

Table 4. Porosity of concretes for thresholds P1, P2 and P3. water to cement ratio 0.50

0.50

0.40

0.40

0.30

0.30

way of curing

water

foil

water

foil

water

foil

P1 (%) P2 (%) P3 (%) (P2-P1)/P1 (P3-P1)/P1

5.47 6.94 8.87

7.10 9.05

5.07 5.99 7.46

5.47 6.42 8.02

4.94 5.33 6.17

5.07 5.61 6.66

thresholds

11.00 0.275 0.549

0.269 0.622

0.181 0.471

0.174 0.466

0.120 0.296

0.136 0.348

ratios

To verify the assumption about the occurrence and amount of separation cracks, the porosity analysis was performed at three levels of the threshold values P1, P2, and P3 of the voxel determining the pore space, where P1 < P2 < P3. Since both test specimens were scanned at the identical tomographic scanning process settings, the voxel values in the tomographic volumes of both specimens are at a comparable level. The first threshold value was