Frost resistance of concrete with high contents of fly ash - A study on how hollow fly ash particles distort the air void analysis

Cenospheres are hollow fly ash particles. When performing air void analysis on a contrast enhanced plane section, air inclusions in cenospheres are counted as air voids. In the present study, air void analyses for air entrained concrete mixtures with fly ash (up to 50% of binder mass) were corrected based on chord counting for non-air entrained paste samples with various contents of fly ash. The correction only lead to a small reduction of the total air content, but it increased the spacing factor up to 25%. The concrete mixtures were also exposed to salt frost scaling testing. The amounts of scaling were unacceptable for several mixtures with high dosages of fly ash. Inferior strength or inadequate air void structure could not explain this. Additional testing pointed to that chemical surface degradation aggravated the physical frost attack for concrete mixtures with high contents of fly ash.