Relationship between Porosity & Strength of Concrete
Porosity can be defined as the total volume of the overall volume of the pores larger than the gel pores, expressed as a percentage of the overall volume of the cement paste. The effects of pores smaller than 20mm in diameter were found to be negligible.
Porosity Factors Affecting Concrete Strength
The strength of concrete is fundamentally a function of the volume of voids in it. The strength of concrete is influenced by
 The volume of all the voids in the concrete
 Entrapped Air
 Capillary Pores
 Gel Pores
 Entrained Air
Relationship between Compressive Strength and Porosity
The relation between the compressive strength and the porosity is as follows.

Fig.1. Comparison of predicted and observed compressive strength ( Image Courtesy: Influence of porosity on compressive and tensile strength of cement mortar Xudong Chen, Shengxing Wu, Jikai Zhou College of Civil and Transportation Engineering, Hohai University, Nanjing, China
The relation between the Flexural strength and the porosity is as follows.


Fig.2. Comparison of predicted and observed Flexural Strength ( Image Courtesy: Influence of porosity on compressive and tensile strength of cement mortar Xudong Chen, Shengxing Wu, Jikai Zhou College of Civil and Transportation Engineering, Hohai University, Nanjing, China 
The relation between the Splitting Tensile strength and the porosity is as follows.

Fig.3. Comparison of predicted and observed Splitting Tensile strength ( Image Courtesy: Influence of porosity on compressive and tensile strength of cement mortar Xudong Chen, Shengxing Wu, Jikai Zhou College of Civil and Transportation Engineering, Hohai University, Nanjing, China 
Higher the porosity, lower is the compressive strength
Porosity of Aggregates on Concrete Strength
The porosity of the aggregate influences the
 Bond between the cement paste and the aggregate
 Workability of the Fresh Concrete
 Strength of the Concrete