Published online by Cambridge University Press: 25 February 2011
The subcritical crack growth parameter (n) has been determined both from the variation in the crack velocity (V) with stress intensity (K) in double torsion specimens, and from the variation in critical stress intensity (Klc) with loading rate (x) in single edge-notch bend specimens. As in previous cases for hardened cement paste (HCP), and other inhomogeneous materials, the value of n determined using double torsion specimens (˜70) was higher than that determined using SENB (˜30).
Backscattered electron image examination of cracks in polished sections of HCP has shown that all of the components, both hydration products and anhydrous phases, are involved in determining crack paths although, as expected; calcium silicate hydrate and calcium hydroxide form the majority of the crack path; unnhydrated cement cores are resistant to crack growth and cause the crack to deviate around them. There is some evidence that crack growth occurs by the formation and coalescence of microcracks ahead of the crack tip, with only a limited formation of an actual process zone.