PSI - Issue 43

Petr Miarka et al. / Procedia Structural Integrity 43 (2023) 124–129 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

125

2

changes are mainly associated with the progressive growth of internal microcracks, resulting in a significant increase in irrecoverable strain. At the macrolevel, this will manifest itself as a change in the material mechanical properties. Fatigue design of concrete structures is partly implemented in the standards for structural design, Eurocode (Eurocode 2004) or ACI (ACI 1997). These design approaches assume fatigue failure under compressive loading. On the other hand, the recommendations of CEB-FIB Model Code (Model Code 2010) consider fatigue failure under both compressive and tensile loads. Nonetheless, all these technical recommendations assume a reduction of material strength to prevent fatigue failure by a certain value. Moreover, MC 2010 provides S - N curves for fatigue design with the fatigue limit of 1 × 10 28 cycles, which is a rather unrealistic expectation for concrete structures.

Nomenclature a 0

initial notch length (mm) maximum applied force (kN) maximum applied stress (MPa) minimum applied stress (MPa)

P max  max

 min

load cycle (cycle)

N

load cycle to fracture (cycle)

N f

stress ratio (-)

R

SLS ULS 3PB HPC PE

serviceability limit state ultimate limit state

Polyethylene

three-point bending

high-performance concrete specimen width (mm)

W

span- distance between supports of specimen (mm)

S B

specimen thickness (mm)

Furthermore, fatigue design of a structure is considered for the ultimate limit state (ULS), while the serviceability limit state (SLS) is missing, i.e., the assessment of the crack width w c , crack length l and deflection δ . This missing SLS fatigue consideration in standards may lead to an unoptimized structure, which may eventually result in a visible damage of load-bearing structural components. This contribution analyses the internal damage in the high-performance concrete (HPC) mixture induced by cyclic load. The tested sample was exposed to 2  10 6 load cycles and afterwards, it was scanned by  CT tomography scan to localize inner damage i.e., cracks. Found experimental results are discussed. 2. Material and Methods 2.1. Fatigue test set up The fatigue experiment was conducted on a three-point bending (3PB) test setup using a Zwick/Roell Amsler HC25 servo-hydraulic testing rig with a maximum load capacity of 25 kN. The load frequency was set to 10 Hz in order to reach high cycle fatigue region in reasonable time. The applied asymmetry of the cycle R =  min/  max was set to 0.1, as it is the most common for load presence in the civil engineering structures. Similar test setup was used in studies by Seitl et. al. (2018), Seitl et. al. (2019), Šimonová et. al (2021) and by Miarka et. al. (2022). The tested sample had the following dimensions: span S of 240 mm, width W of 80 mm (ratio S/W = 3) and thickness B of 40 mm. The initial notch a 0 with a length of 8 mm was made by using a diamond saw. This notch allowed the crack initiation, which was crucial for the damage identification. The test setup together with the sample’s dimensions can be seen in Fig. 1.