PSI - Issue 47

Ezio Cadoni et al. / Procedia Structural Integrity 47 (2023) 331–336 Author name / Structural Integrity Procedia 00 (2023) 000–000

333

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Fig. 2. Experimental load versus time of a specimen 60 mm long and photos of failure state.

3. Results

The purpose of this preliminary experimental campaign was to highlight the impact of specimen size on the com pressive response of preloaded UHP(FRC) and UHPC samples. The experimental plan is summarised in Table 1, with at least 3 tests conducted for each material and condition. It was observed that under the same initial conditions, the strength of the specimen and the fracture time increased. There was a slight decrease in the stress rate (due to the specimen length), indicating the influence of specimen length on the material response as a result of structural e ff ects. A UHP(FR)C sample’s stress versus time curve and pictures during its fracturing phase are shown in Fig. 2. The fibres act as discontinuities in the matrix, causing micro-cracks to propagate. Later, they improve post-peak behaviour by serving as bridge elements between the two crack edges, connecting them. For the purpose of understanding the influence of strain conditions before the dynamic event, two preload levels of 1 / 3and2 / 3 of the failure load were chosen (Cadoni et al. (2015)). Dynamic tests were carried out at the same velocity, imposing 500 kN in the pre-tensioned bar, giving a stress rate of 1600 GPa / s. Fig. 3 shows the stress versus time curves of the UHPC (a) and UHP(FR)C (b) samples for the three preloading conditions. All tests show the same gradient of stress until the first 60 µ s. Afterwards, the stress versus time curve changes due to the presence of activated cracks during pre-loading process. Activated cracks and stress rate play a crucial role in enucleating the cracking process that results in specimen failure. The material had a more uniform response in tests with higher static pre-loading values, but its strength was lower due to damage caused by static pre-loading in the case of UHPC while UHP(FR)C materials do not present significant di ff erences. Fig. 4 shows the experimental stress versus time curves for UHPC and UHP(FR)C samples having di ff erent length such as: a) 30 mm; b) 45 mm; c) 60 mm. The peak load seems not be influenced by the presence of fibre, at the contrary the post-peak is governed by the fibre and this influence decrease with increasing the specimen length.

Table 1. Results of dynamic compressive tests obtained with same pre-load and pre-strain (Cadoni et al. (2018)). Material length ( mm ) stress-rate ( GPa / s ) strength ( MPa ) fracture time ( µ s )

failure time ( µ s )

UHPC UHPC UHPC

30 45 60 30 45 60

1567 (135) 1489 (41) 1431 (155) 1738 (91) 1503 (238) 1557 (117)

518 (35) 549 (50) 518 (15) 551 (41) 557 (38) 562 (10)

283 (29) 345 (32) 395 (14) 299 (70) 344 (44) 354 (44)

402 (27) 402 (25) 463 (32)

UHP(FR)C UHP(FR)C UHP(FR)C

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