MULTIFRACTAL NATURE OF CONCRETE FRACTURE SURFACES AND SIZE EFFECTS ONNOMINAL FRACTURE ENERGY

Experimental evidence of the fractality of fracture surfaces has been widely recognized in the case of concrete, ceramics and other disorder ed materials. An investigation post mortem on concrete fracture surfa ces of specimens broken in direct tension has been carried out, yieldi ng non-integer (fractal) dimensions of profiles, which are then relate d to the 'renormalized fracture energy' of the material. No unique val ue for the fractal dimension can be defined the assumption of multifra ctality for the damaged material microstructure produces a dimensional increment of the dissipation space with respect to the number 2, and represents the basis for the so-called multifractal scaling law. A tra nsition from extreme Brownian disorder (slope 1/2) to extreme order (z ero slope) may be evidenced in the bilogarithmic diagram: the nominal fracture energy L(F) increases with specimen size by following a nonli near trend. Two extreme scaling regimes can be identified, namely the fractal (disordered) regime, col responding to rite smallest sizes, an d the homogeneous (ordered) regime, corresponding to the largest sizes , for which an asymptotic constant value of L(F) is reached.