Method and theory for nondestructive determination of fracture energy in concrete structures

Current methodologies for the determination of fracture energy in cementitious composites are based on laboratory tests. In the research presented in this paper, the maturity method is introduced as a technique for estimation of fracture energy as it exists in concrete structures. A new hypothesis is introduced stating that the rate offracture energy development in concrete is related to the combined effects of time and temperature during the hydration period. An experimental program is developed to examine the hypothesis and to acquire data pertaining to the effects of temperature and age on the rate Of fracture energy development in concrete. Experiments were performed for Mode I or opening mode of fracture in concrete. Fracture energy and strength gain data for concrete beams and cylinders cured at 14, 23, and 35 C were obtained at several different testing ages. The testing age for beam and cylindrical specimens ranged from a few hours to 45 days. Experimental results revealed that the gain in fracture energy under isothermal curing temperatures can be described by a three-parameter hyperbolic equation. This is in direct conformity with the hyperbolic relationship previously developed for strength gain in concrete. The practical implicatian derived from this study is a method for in-place determination of fracture energy in concrete structures based on the maturity principle. The work presented herein establishes the fact that the activation energies derived from fracture energy tests are the same as those obtained from the mechanical strength tests. This result is significant, since it has been firmly established that the activation energies evaluated from strength tests are equivalent to the ones obtained from heat of hydration studies. Therefore, results from the present study provide a one-to-one correspondence between the fracture energy of concrete and the activation energy involved in the hydration of cement paste.