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Concrete testing - microscopy

The petrographic analysis (DS 423.41-45) revealed that all concretes were well consolidated without encapsulated air. In terms of cracks no coarse or fine cracks were observed and only very limited paste cracking were found. As expected there was a tendency towards more paste cracks in slag based concrete, and in concretes containing microsilica.

The estimated parameters of “internal stability” and “paste homogeneity” of the 15 concretes are plotted below. There seems to be no apparent connection between the reported microstructure parameters and e.g. the chloride migration coefficients (NT Build 492) and chloride diffusion coefficients (NT Build 443). In terms of “internal stability” and “paste homogeneity” mix K is judged as having the worst microstructure of all concretes, while it has the lowest chloride migration and diffusion coefficients. In fact mix K does not fulfill the Danish requirement to paste homogeneity at maximum 1 for concrete in environmental classes E and A.
Mix O with the superabsorbing polymers exhibits good “paste homogeneity” and the best of all “internal stability”. This is in accordance with the previously reported internal curing effect of the superabsorbing polymer. On the other hand this is not translated into improved durability parameters for mix O, e.g. the chloride migration coefficient is similar to that mix E, which is the reference of mix O.

Internal stability