PSI - Issue 42

Stanislav Seitl et al. / Procedia Structural Integrity 42 (2022) 1512–1519 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

1516

5

al. (1996), Medeiros et al. (2015). The number of cycles corresponding to the fatigue limit of cement composites was set to 2×10 6 cycles. In Seitl et al (2022), we showed that for description of concrete fatigue properties is most appropriate model postulated by Kohout- Věchet (2001). Kohout - Věchet model is the modification of the Basquin's model (Basquin (1910)) that is able to describe the whole range of the fatigue lifetime, including the low-cycle and high-cycle fatigue regions. Parameters B , C are calculated from σ 1 (value of the stress for the number of cycles N = l) and σ͚ (permanent fatigue limit).  ( ) = ( ( + + ) ) . (3) Fatigue tests lasted a long time, which is problematic from the point of view of the ageing of the material of the specimen. Therefore, the data obtained from the fatigue tests were normalized to 28 days static strength when defining the approximation curves for the compressive strength measurement over time. 4. Results and discussion 4.1. Facture mechanical properties of concrete with various water to cement ratio and D max

The fracture-mechanical properties of the studied concrete with various water to cement ratio and with the different maximum size of aggregate D max = 8 mm or 16 mm are mentioned in Tab. 2. For the reference concrete w / c = 0.5 in 28 days, the average compressive strength is 45 MPa. When the w / c is 0.4 and 0.3, the compressive strength is 69.9 and 78.6 MPa, which is 55 % and 74 % larger than of w / c = 0.5. The above results are in accordance with Abrams‘ law (Nagaraj & Banu (1996)), see in Fig. 3, where compressive strength is shown for various age. Based on the results above, it is clearly shown that the w / c ratio can significantly affect the fracture mechanical properties of studied concrete. The bending strength of concrete with various w / c ratio and different D max are mentioned in Tab. 2. It can be concluded that f b is significantly affected by w / c ratio whereas the maximum size of aggregate

Fig. 3. Relative compression stress versus water to cement ratio in different age, comparison with Abram’s law .

has no significant influence. After 28 days, the f b values practically stay constant for given w / c ratio. Concrete is usually assumed to be about 10 % as strong in tension as it is in compression. That tensile strength is the basis for its ability to resist bending, or its flexural strength. In the studied materials, the ratio tension/compression is in average of 12 % from 10 up to 16 (for w / c = 0.5 in 365 days). In ACI 207R is noted that concrete’s tensile strength is often taken as 6.7 times the square root of its compressive strength. For the studied materials the analytical tensile strength values are from 3.73 ( f c = 45 MPa) up to 5.43 ( f c = 95.3 MPa), this means that the studied concrete minimalize cracks initiating at the surface the less value for f c is 6.5 MPa.