PSI - Issue 28

Ezio Cadoni et al. / Procedia Structural Integrity 28 (2020) 933–942 Author name / Structural Integrity Procedia 00 (2020) 000–000

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oglu et al. (2013); Cadoni et al. (2001, 2019)]. Their structural performances directly depend by the dynamic response of these materials at high strain-rate and in particular tensile and shear strength. They are fundamental material proper ties of UHPFRC because they play a vital role in the applications field of defence and structural engineering. Although in the quasi-static regimes their determination seems less problematic, in dynamics the measurement becomes quite complex. Moreover, the shear behaviour of UHPFRC materials under the dynamic loading it is not well understood. Among the suggested methods for the quantification of this parameter the direct-shear test (punching test) is undoubt edly those that warrants minimisation of the bending stresses on the specimen, permits an easy sample preparation and can be applied on high strength materials. The direct-shear behaviour of UHPFRC has not been su ffi ciently in vestigated in particular at high strain rates. French et al. (2017) investigated direct shear behaviour under impact loads showing how the fibre reinforcement highly incremented the direct shear capacity of the matrix in terms of peak shear stress and associated slips. By comparing the quasi-static results to the impact results they found an increase in peak shear stress with increasing strain rates. Lukic and Forquin (2016) observed an increment of the direct shear strength with increasing strain-rate. They attributed this enhancement as result of radial confinement stress. Ngo and Kim (2018) experimentally observed how UHPFRCs are rate sensitive and significantly depended on the fibre volume contents. Hardening response was observed, even at high strain rates, for UHPFRCs with fibre-reinforcement of smooth steel fibres ranging from 0.5 to 1.5 %. With the aim to provide new data sets on the direct-shear behaviour of UHPFRCs an experimental research was set-up in order to compare the dynamic response of UHPFRC with di ff erent percentage of fibre reinforcement. This research is part of an ongoing multi-year research aimed at addressing various aspects related to the protection of sensitive infrastructures against against improvised explosive devices. The Ultra-High Performance Fibre Reinforced Concretes (UHPFRCs) analysed in this work are commercial prod ucts with very high strength and durability. They have extremely durable and highly resistant matrix (equivalent water / cement ratio = 0.17). Four di ff erent percentages of steel fibre reinforcement were studied (1%, 2%, 3% and 4% in volume). High carbon straight steel fibres, 13 mm long with a 0.20 mm diameter (aspect ratio l f / d f equal to 65) were used. The quasi-static mechanical characterisation was performed in compression with cubic specimens hav ing 100 mm side and in bending / compression on 40x40x160 mm 3 specimens according to EN196. The results of the compression tests after 28 days are reported in Table 1 while the others are shown in Table 2. They shows very high compression and flexural strength. The elastic modulus was 51 GPa. 2. Materials

Table 1. Quasi-static results of UHPFRCs in compression on cubes 100mm side. Material Density

Compression strength

(kg / m 3 )

(MPa)

UHPFRC 1% UHPFRC 2% UHPFRC 3% UHPFRC 4%

2323 ± 6 2393 ± 9 2449 ± 4 2512 ± 6

132.2 ± 1.4 138.2 ± 3.6 140.8 ± 4.4 151.5 ± 4.1

3. Experimental procedure

3.1. Specimen

In order to study the direct shear behaviour of the UHPFRC it is necessary to define the most appropriate specimen shape. Among the possible configuration regularly used only few are suitable for a dynamic test. In fact in shear test configuration depicted in Fig. 1a can be present strong eccentricity while others seem to be not adequate to transmit the wave because of high stress concentration. The attention was addressed to the configuration Fig. 1b) and c) and on these geometries a numerical assessment was performed by Ls-Dyna FE code. The results are shown in Fig. 2.