Next: Representative Volume for Up: Results Previous: Computation of Diffusivities

Comparison to Published Diffusivity Values

There are few experimental data sets in the literature to which the results of the current computer simulation study can be compared. Luping and Nilsson [28] do provide a set of measured chloride diffusion coefficients for concretes produced with w/c ratios of 0.32 and 0.7 at several different ages. The mixture proportions are also given so that the volume fraction of aggregates can be computed. While the age of the specimens at the times of test are given, no information is provided concerning the accompanying degrees of hydration. Thus, Eqn. 4 was used to determine the degrees of hydration that best fit the experimental diffusivity data. The results are summarized in Table 3. The estimated degrees of hydration for the two different w/c ratios are certainly plausible. Recent measurements on two cements mixed at a w/c ratio of 0.3 resulted in measured degrees of hydration of about 0.37, 0.53, 0.60, and 0.66 at 1, 3, 7, and 28 days, respectively [29]. While the 1-day value is in good agreement with that predicted by the Eqn. 4, the predicted hydration at the intermediate ages is somewhat less than these measured values. Of course, some variation will be observed due to the different particle size distributions and phase distributions of the cements [29]. Regarding the long term hydration values, Waller et al. [30] have recently proposed an equation for the long-term degree of hydration, _c as a function of w/c ratio of the form:

This equation predicts long-term degrees of hydration of 0.635 and 0.89 for w/c ratios of 0.32 and 0.7, respectively, once again in reasonable agreement with the values estimated for the greatest ages in Table 3, based on fitting Eqn. 4 to the measured diffusion data. Because w/c ratio is the only variable in Eqn. 5, it can be directly applied to estimating diffusivities for the two concretes. Thus, a diffusivity of 2.3 x 10^-10 m²/s is predicted for a w/c ratio of 0.7, approximately one order of magnitude higher than the experimentally observed 7-90 day values. The value predicted by Eqn. 5 for a w/c ratio of 0.32, 1.2 x 10^-12 m²/s is in reasonable agreement with the measured long-term values in Table 3.