Air contents of concrete are necessary for concrete durability in freeze-thaw exposure. According to the Finnish concrete code, the target value for air content varies between 4% and 5.5% for XF - exposure classes. Lately in Finland, some cases showed an elevation of air contents up to 15% in fresh air-entrained concrete at construction site and in drilled concrete samples.
The objectives of this study were to investigate the stability of air entrainment by measuring the air content elevation 30 minutes and 60 minutes after concrete mixing and investigating the concrete sensitivity to segregation. Composition of concretes used in this study include 7 different combination of PCE based superplasticizer and air-entraining agent admixtures, cement content of 425 kg/m3, two consistency classes S3 with water to cement ration of 0.33 and F5 with water to cement ration of 0.38. One cement type was used for all concrete mixes. The concretes were mixed for 2 minutes and 5 minutes mixing times.
The results show that the elevation of the air content of fresh concrete depends on the consistency of the concrete and on the used combination of superplasticizer and air-entraining agents. The higher consistency classes concretes have more risk of air elevation with some combinations of PCE-based superplasticizers and air-entraining agents. The results also indicate that short mixing time would not be enough to achieve total effectivity of some air-entraining agents, especially for higher consistency classes concrete.
This paper presents the characteristics of air void systems in hardened concrete with the method of digital image analysis (DIA) coupled with Schwartz-Saltykov (SS) conversion. The results indicate that the DIA method coupled with SS conversion estimates the air content with more accuracy than it would without SS conversion; the correlation between air content obtained from the DIA method, and that from the thin section (TS) method is as good as the correlation observed between the pressure saturation (PS) method and the TS method. It was also found that the DIA method shows a better correlation with the TS method when the spacing factor without SS conversion is considered, while both methods show poor correlations when the corresponding specific surface is considered. In addition, it indicates that the peak of three-dimensional size distribution (3-DSD) of air voids after SS conversion falls in smaller voids, and 3-DSD of air voids shifts to a narrow size range, in comparison with the 2-DSD without SS conversion; the shape of the 3-DSD air voids remains constant irrespective of the class widths. Increasing the number of classes can minimise the standard deviation in the estimation, however, it also results in a leap in voids volume density, which will influence the estimation of air content.
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