2.4 Evaluation of air void content

The air entrainment was reduced in the SCC mix with the addition of finely divided mineral admixtures, such as fly ash and silica fume. Samples with the dimensions of 80 80 12 mm were acquired from both types of concretes at w/cm ratios of 0.30, 0.45, and 0.60 to study their air-entrained properties. Air voids larger than 200 microns only were considered for the analysis. The measurements showed that SCC samples exhibited half the amount of air voids compared to NC. Visual analyses of the "air voids" sizes and shapes revealed that SCC had smaller and more round-shaped voids than NC which presented slightly larger and irregular shaped voids (by 15–20%) (Figure 4).

#### Figure 4. Distribution of air voids (white spots) in: (a) NC and (b) SCC.

Moreover, while the air voids in the SCC appeared more clustered in certain areas of the cut specimen, the voids in NC were relatively well dispersed in the

specimens. These data indicate that the lower air void content also contributed to an increase in strength of the SCC as cracks initiate more easily in the cement paste with higher air voids. Some of these air voids represent "flaws" in the cement paste at the aggregate-paste boundary possibly forming microcracks or "bond cracks" in the ITZ which ultimately lead to the failure of the concrete due to the propagation of microcracks under localized tensile stresses.
