Empirical relations between compressive strength and porosity

1 Empirical relations between compressive strength and porosity 

The physical properties of concrete are only partly determined by those of the cement paste that it contains. We shall consider these properties only for hep and to the extent that chemical or microstructural studies have contributed to an understanding of them. Other aspects are treated elsewhere (e.g. Ref. M73). 

Many empirical relations between compressive strength and one or more of these variables have been proposed. Thus, Feret s law (1892) states that the strength is proportional to [c/(c -I- w -h a)], where c, w and a are the volumes of cement, water and air, respectively, and various authors have reported equations based on regression analyses relating strength to cement composition and other variables (e.g. AI7,A18). The generality of such equations appears to be limited, probably because of the difficulty of taking into account all the relevant parameters. 

The porosity of the hardened paste, appropriately defined, is strongly correlated with strength. As was seen in Section 8.3.1, it depends primarily on the degree of hydration and the w/c ratio. In this discussion, the following symbols will be used  

Powers (P34) found that cement pastes of various degrees of hydration and w/c ratio conformed to the relation 

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