There is certain fracture process zone in front of macro crack in concrete structure.Considering its impact

critical equivalent crack length of instable expansion is no longer initial crack length a0

but the equivalent critical crack length ac corresponding to the maximum load

that is ac=a0+Δae

in which Δae is equivalent crack length of fracture process zone.Damage mechanics considers that the whole process of concrete from beginning of force action to final fracture is a process of formation

accumulation and development of the damage.In calculating equivalent crack length of fracture process zone

the influence of concrete′s initial damage and σ-ε curve′s upward segment on calculated results should be conidered.By use of grey theory

this paper calculates initial damage D0 and damage threshold Df of specimens in wedge-splitting experiment.Based on virtual crack model principle and damage mechanics theory

the concret′s wedge-splitting specimen with initial crack a0 was studied and the equivalent crack length Δae in fracture process zone and corresponding range of damage degree were deduced.