Issue 29

A. Caporale et alii, Frattura ed Integrità Strutturale, 29 (2014) 19-27; DOI: 10.3221/IGF-ESIS.29.03 21 It is noted that the iterative procedure begins with the first iteration (  1 i ), where the void volume fraction   , 1 ,0 v i v f f is required: ,0 v f may represent a measure of the defects in the paste before the loading process. If this information is already contained in the constant elasticity   pp C of the pure paste then ,0 v f can be assumed equal to zero. The concrete two- phase composite has mortar as matrix and gravel as inclusions; at the i th iteration, the average stress   m σ in the mortar of the concrete two-phase composite subject to  σ is given by          , 1 m m v i f σ B σ (3) where      , 1 m v i f B is the average stress concentration tensor of the mortar in the concrete and depends on      , 1 c v i f D and      , 1 m v i f D , as well as on the constant compliance   g D of the gravel. Then, the average stress   m σ evaluated in (3) becomes the far-field stress applied on the mortar two-phase composite, which has paste as matrix and sand as inclusions; at the i th iteration, the average stress   p σ in the paste of the mortar two-phase composite subject to   m σ is           , 1 p p m v i f σ B σ (4) where      , 1 p v i f B is the average stress concentration tensor of the paste in the mortar and depends on      , 1 m v i f D and      , 1 p v i f D , as well as on the constant compliance   s D of the sand. Finally, the average stress   p σ evaluated in (4) becomes the far-field stress applied on the paste two-phase composite, which has pure paste as matrix and voids as inclusions. The average stress   pp σ in the pure paste of the paste two-phase composite subject to   p σ is evaluated by using the stress average theorem:          , 1 1 pp p v i f σ σ (5) Once   pp σ has been evaluated at the i th iteration by means of (5), it is possible to determine the corresponding average strain   pp ε in the pure paste:        pp pp pp ε D σ (6) where   pp D is the constant compliance of the pure paste. The value of the void volume fraction at the current i th iteration is given by            , ,0 , , pp pp v i v v m m v eq eq f f f f (7) where                                   11 22 33 2 , , 3 3 pp pp pp pp pp pp pp pp pp pp m ij ij m ij eq ij ij e e e (8) and    pp ij are the components of the second-order strain tensor   pp ε in (6), equal to the symmetric part of the displacement gradient. Relation (7) is the evolution law of the voids in the paste and depends on the following five parameters: ,0 v f , , v m f , , v eq f ,  ,  . If the error   , , 1 v i v i f f is less than or equal to a given tolerance then further iterations are not necessary and the average stress and strain evaluated in the current i th iteration are correct; otherwise, the void volume fraction , v i f in the current i th iteration provided by (7) becomes the value of v f to consider at the