Characterisation of crack distribution of strain-hardening cement composites (SHCC) under imposed strain
The formation of multiple cracks under tensile load is believed to be a durability enhancing mechanism in strain-hardening cement composites (SHCC). The mechanism associated with multiple crack formation is control of individual cracks to widths in the micro-range, whereby new cracks form at close spacing, rather than old cracks widening. This is achieved by effective fibre bridging of the matrix cracks, enabling resistance to higher tensile loads without significant crack widening. To link the cracking characteristics of SHCC to durability, it is required to study crack formation and widening under various loading regimes, and relate them to physical and chemical processes of degradation. In this endeavour it is useful to find elegant ways to observe and characterise crack patterns, considering the high cracking densities, or small spacing of cracks at roughly 1-10 mm, combined with the complexities of crack tortuousity, branching and coalescence. In this paper crack observation with an ARAMIS non-contact, optical, 3D digital deformation observation device is reported. The system combines microscopy and digital image photography with speckle pattern recognition software, whereby local deformations can be computed with high resolution. For the results reported here, an observation area of 70 mm × 30 mm on a tensile specimen was used, whereby up to 60-70 cracks could be observed simultaneously. Crack patterns determined in this manner are reported for loaded and unloaded states. The methodology is used here to study the influence of cyclic loading and loading rate on crack width in SHCC. © 2010 Taylor & Francis Group, London.