For example according to the "Guidelines fo Nonlinear Finite Element Analysis fo Concrete Strucutres" you can estimate the compression fracture energy to 250*G_f0*(f_cm/10MPa)^(0,7), with G_f0=0,025Nmm/mm² for a maximum aggregate size of 8mm; G_f0=0,030Nmm/mm² for a maximum aggregate size of 16mm and G_f0=0,058Nmm/mm² for a maximum aggregaten size of 32mm.
What is the relation of Mode 1 fracture energy of concrete with concrete compressive strength (fc') and concrete tensile strength (ft)? In literature for mode 2 fracture energy, we just multiply mode 1 fracture energy by 10 to get mode two fracture energy.
(1) B. Alfarah, F. López-Almansa, S. Oller, New methodology for calculating damage variables evolution in Plastic Damage Model for RC structures, Engineering Structures. 132 (2017) 70–86.
(2) A. Hillerborg, The theoretical basis of a method to determine the fracture energy Gf of concrete, Matériaux et Constructions, Vol. 18, No. 106, p. 291-296. It can be useful. Best regards,