Energy methods for fracture-toughness determination in concrete

The concepts for evaluating the energetic fracture parameter,Gic, experimentally for concrete have been disputed because of the inconsistency and diversity of the results obtained. In order to re-examine the validity of this fracture parameter, experimental investigations were conducted to compare various methods for determiningGic with considerations of effects of microcracking and slow crack growth prior to the onset of instability leading to failure. These studies were made using small beams in three-point bending and utilizing compliance calibration techniques to estimate crack lengths.The following approaches were used to determine the critical energy-release rate: (1) stress-intensity-factor method which uses linear-elastic fracture mechanics to relateGic toKic; (2) theJ-integral concept in which energy change per unit crack extension was measured experimentally; and (3) Petersson's Method (modified) in which the total energy absorbed to instability is divided by the uncracked cross-sectional area to obtain the energy per unit area during the fracture process.All of these approaches resulted in excellent agreement and also good consistency. Invariancy witha/W was obtained when extended crack lengths were considered.