Investigation of Spalling Damage in Ultra-High Performance Concrete Through X-ray Computed Tomography

Ultra-high performance concretes (UHPC) are increasingly used to build protective structures such as headquarters, nuclear power plants or critical civil engineering structures. However, under impact or contact detonation, concrete is exposed to high-rate tensile loadings that can lead to intense damage modes. Such complex damage modes need to be correctly characterised. When a UHPC sample is subjected to a dynamic tensile loading by means of the spalling technique the post-mortem pattern shows a large number of fractures that cannot be seen with a classical observation of the external face (inner crack network). In the framework of the Brittle’s CODEX chair project, the fracturing process in spalled samples of UHPC is investigated with X-ray computed tomography. The tensile loading is applied thanks to a spalling technique that is based on the reflection of a compressive wave on a free boundary. The concrete samples are entirely scanned using X-ray tomography prior spalling test to identify the initial microstructure, and post spalling test to analyse the damage pattern. Image analysis tools are used in both steps. The main fracturing properties are related to the microstructure of the tested concrete.

Numerical modelling of dynamic spalling test on rock with an emphasis on the influence of pre-existing cracks

This article deals with numerical modeling of rock fracture under dynamic tensileloading and the related prediction of dynamic tensile strength. A special emphasis is laid on theinfluence of pre-existing natural microcrack populations as well as structural (articial) cracks.For this end, a previously developed 3D continuum viscodamage-embedded discontinuity modelis employed in the explicit dynamic nite element simulations of the spalling test. This modelis capable of modelling the eect of natural microcracks populations always present in rocks aswell as to capture the strain rate hardening eect of quasi-brittle materials. In the numericalsimulations of spalling test on Bohus granite, it is shown that the model can predict the pull-pack velocity of the free end of the intact rock sample and the eect of structural cracks witha good accuracy. According to the simulations, the effect of microcrack populations, modeledhere as pre-embedded discontinuity populations, is weaker than the corresponding eect underquasi-static loading

Investigation of Dynamic Fracture Behavior of Graphite

The strain-rate sensitivity of brittle materials, such as glass, ceramics or concrete-like materials, is usually easier to be performed in compression. However, also the tensile behavior, which affects phenomena such as spalling, scabbing and fragmentation, has to be investigated to achieve an exhaustive characterization. In last decades, a lot of researchers suggested spalling test as one of the best ways to characterize dynamically brittle materials. This type of test is based on propagation and reflection of elastic waves: the fracture for spalling occurs when, in the material, the tensile stress state, obtained by the reflection on a free surface of a compressive pulse, exceeds the strength limit. These conditions are usually reached using a SHPB setup: a striker bar is launched against the input bar, which is in contact with a long bar specimen free at the opposite surface. In this work, the spalling test has been performed to investigate the dynamic tensile behavior of graphite. The apparatus is actuated by a pneumatic gas-gun (1.5 m long). Striker and input bars are made of high-strength steel 10 mm of diameter. Different striker lengths are used (100 and 80 mm) to obtain different pulse lengths and amplitudes. The input bar is 3.4 m long and is instrumented in the middle. The specimens are 200 mm long and 10 mm of diameter, instrumented at 80 from the free surface with strain-gages.

Behavior of Densified Normal Strength Concrete under Elevated Temperature

This paper presents an experimental investigation on fire resistance of densified normal strength concrete (DNSC), at water/binder (W/B) ratios of 0.45, 0.36, and 0.32, of which compressive strength of 28-days ranged from 42.5 MPa to 56.3 MPa. The results of the spalling test reveal that DNSC encountered explosive under high temperature. Polymer fiber can be used to improve fire resistance of DNSC. DNSC subjected to high temperature lost its mechanical properties in a similar manner to that of high-strength concrete.