Concrete Double-k Fracture Parameters And Their Size Effect

With the development of structural analyses, experimental methods and design means, the research on fracture properties of concrete has been extensively taken seriously. As one part of the project supported by the National Outstanding Youth Natural Science Foundation, the lowing research work had been accomplished in this paper: I. By using photoelastic coatings, the wholly integrated and audio-visual process of pre-cast cracks?initiation, stable propagation and final failure on all wedge splitting specimens, whose largest size is l400X l400X200mm3 and which have the same initial notch-to-depth ratio 0.4, were directly recorded and systematically analyzed. At the same time, the practically measured critical crack propagation length a . maximum load iax and several other concerned values were utilized to determine the double-K fracture parameter of concrete and the critical crack tip opening displacement CTODG.. It was found that, in the experimental limits, the crack initiation toughness K7 is size-independent; when the specimens?height h 600mm, the unstable fracture toughness K7 is size-independent on the whole; while the critical crack tip opening displacement CTODC still has size effect generally. 2. According to the theoretically analytical methods, the critical crack propagation length Aa, the double-K fracture parameter of concrete and the critical craCk tip opening displacement CTODC were determined by using the maximum load Ppa. critical crack mouth opening displacement CMODC, concerned parameters of specimens?size and fracture energy GF of specimens, which were measured through the computer data collecting system from all specimens mentioned above. It was found that when the specimens?height h~ 800mm. the fracture energy GF, the double-K fracture parameter of concrete (K~7! and K) that was considered G and the critical crack tip opening displacement C扵ODC are size-independent. 3. The relevant comparison between the practical value on photoelastic coatings and the theoretical value was taken to further analyze some parameters Aa/h, K?K and CTOD), verify the similarity of the practical and the theoretical values, and ascertain the conclusion rationality. It was found that the two values are fairly similar, and the result is relatively fine. But the differences still exist: take K, the practical value is size-independent for all specimens. while the theoretical value is the same only when the specimens?height h~ 800mm; take K7? the two values should meet different size requirements to be size-independent, which are h600mm for the practical and h800mm for the theoretical; take CTOD, the practical value has size effect on the whole, while the theoretical value is size-independent when the specimens?height h~800mm. The main reasons that cause the numerical differences above are as follows: 0 the uneven property of concrete; he measuring error in the practical value; ? the difference on utilizing stress equations, experiential formulas and simulated models.