Isotropic triaxial tests of equal strength concrete with different mix compositions

•EOS of concrete specimens was determined from high pressure hydrostatic tests.•The effect of different mixtures at same compressive strength was examined.•Mix parameters were: water content, coarse aggregate amount and max aggregate size.•It was proven that EOS depends on mix properties and not on compressive strength.•The differences between hydrostatic and oedometric based EOS were determined. High intensity impact loading on a concrete structure induces three dimensional high intensity stresses, the analysis of which requires appropriate constitutive relationships describing the isotropic and deviatoric stress components. The isotropic component known as the hydrostatic relationship or the equation of state (EOS) is in the focus of this paper. The literature relates the EOS of concrete to its uniaxial compressive strength. This paper aims at refuting this relationship and validating the conjecture that the EOS depends on the concrete mixture. For that purpose, different concrete mixes of equal strength have been tested. The mixture parameters are the water content (with the same w/c ratio), the coarse aggregate content and the maximum aggregate size. Using a unique test setup, hydrostatic tests were carried out to extremely high pressures at the 1GPa level. The results clearly show the pronounced differences between EOS curves representing different mixtures having the same concrete strength, thus validating the above conjecture. As the EOS is sometimes derived from oedometric tests, the present investigation exploited oedometric tests results on exactly the same concrete mixtures and strength to compare the hydrostatic and oedometric resulted EOS curves. The comparisons show that for most cases the stress at a certain volumetric strain in the oedometric curves are somewhat lower than these in the hydrostatic EOS curves. However, for mixes with a large maximum aggregate size with a high aggregate content, the oedometric EOS merges with the hydrostatic EOS curves in the mid-to high pressure levels. Finally, statistical analysis using Analysis of Variance has been employed to identify the effect of different mixture parameters and their combination on the differences in the EOS. These new results enhance the insight on concrete behavior under triaxial high pressures and contribute to the development of a new generation of EOS curves.