Title:
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Crack growth in hardened cement paste
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This thesis describes the examination of the microstructure of
hardened cement paste [HCP], in particular, cracking in HCP, and the
measurement of subcritical crack growth in HCP.
A scanning electron microscope has been used to examine, fracture
surfaces, complementary fracture surfaces, polished surfaces, and
polished surfaces containing introduced cracks. This examination has
provided information about fracture paths in HCP, which shows that the
main hydration products [C-S-H gel and calcium hydroxide] form the
majority of crack path elements. The interpenetration of these
products during hydration, and the expected variations in strength of
these hydration products suggest that both must be recognised as
significant contributors to the mechanical properties of HCP. The
examination of cracks in polished surfaces appears to be a very useful
way of determining the microstructural aspects of cracking, and its
possible future applications are discussed.
Two methods of determining the subcritical crack growth parameter
[n] were performed, the first, load-relaxation in double torsion
plates yielded a value of 64, while the second, the variation in the
critical stress intensity of single edge notched beams with loading
rate yielded a value of 27. The much higher value determined by load
relaxation is thought to be due to both, the effect of crack resistant
inhcmogeneities in the microstructure which have a more marked effect
on load relaxation measurements, and a susceptibility to creep in the
double torsion plates which reduced the amount of crack growth. The
susceptibility to creep was identified by a shift of the crack
velocity-stress intensity [V-K] curves to higher stress intensities
with repeated loading. The value of subcritical crack growth parameter
of 27, determined by the variation of critical stress intensity with
loading rate, is therefore regarded as the more valid.
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