采用跨高比为2.5的非标准三点弯曲梁试件，利用在试验中测得的最大荷载P_max及对应的裂缝口张开位移CMOD_c，根据渐近线性叠加假设，求得了砼裂缝亚临界扩展量∆a_c。在此基础上，采用虚拟裂缝模型计算了不同初始缝长、不同试件髙度的砼非标准三点弯曲梁试件的起裂断裂韧度Kⁱⁿⁱ_IC、失稳断裂韧度K^un_IC及临界裂 缝尖端张开位移CTOD_c。计算结果表明，砼K^un_IC：与试件高度及初始缝高比无关；而Kⁱⁿⁱ_IC在初始缝髙比a₀/h = 0.3〜0.5内，也是一稳定的常数且与试件高度无关。这说明砼双K断裂参数可以作为砼的材料常数。

Using the non-standard three-point bending beam specimens of concrete, which have the span/ height ratio of 2.5, and the maximum load P_max and the correspoding crack mouth opening displacememt CMOD_c measured in the experiment, the precritical crack propagation length ∆a_c are evaluated by means of linear asymptotic superposition assumption. On this basis, the initiation fracture toughness Kⁱⁿⁱ_IC, unstable fracture toughness K^un_IC and the critical crack tip opening displacement CTOD_c for the non-standard three point bending beams with different initial crack lengths and different spcimen heights are determined utilizing fictitious crack model. The calculated results show that K^un_IC is independent of the initial crack length and the specimen height. Meanwhile, Kⁱⁿⁱ_IC is size-independent too, when a₀/h = 0.3〜0.5. This indicates that the double-K fracture parameter can be considered as a constant for concrete materials.