The constant fracture angle model for cementitious materials
Department of Civil and Environmental Engineering
Doctor of Philosophy
Meegoda, Jay N.
Fracture mechanics--Mathematical models
Fracture mechanics of concrete has been investigated for the past two decades using linear elastic and nonlinear fracture mechanics concepts. The models proposed so far remain questionable largely due to specimen dependency of the proposed fracture parameters.
In this study, a new approach for modeling the fracture characteristics of concrete and fiber reinforced concrete is proposed. The model depends on the load CMOD relationship rather than the traditional load-deflection principle. Although energy consumed during fracture is definitely a direct function of the load displacement response, it was observed that traditional displacement measurement included an extraneous and erratic portion due to test setup and support crushing. The magnitude of this erroneous deformation was found to be of the same order as the actual displacement, leading to inaccurate determinations of fracture parameters. To overcome this problem, the load-CMOD relationship is a more reliable parameter for determining the fracture characteristics because it is unaffected by the specimen setup and any support crushing.
An important step towards the use of load-CMOD concept as a key fracture parameter depends on relating the CMOD to the traditional load line deflection. This investigation found that there is a unique linear relationship between the CMOD and the load-line deflection, provided that deflection is measured accurately. The exact numeric value of relationship between the CMOD and the deflection, that is, the slope of the line, is discovered to be a material property. This linear relationship between the deflection and CMOD can be understood physically as a constant fracture angle of the material. The proposed concept is therefore named the Constant Fracture Angle Model.
The model was evaluated for size dependency using several sizes of notched beams with different notch lengths. Different types of cementitious materials were also investigated to confirm the validity of the proposed model. The proposed model finds a problem with the existing RILEM recommendations for measuring the fracture toughness of concrete. A proposal to correct the problem is made.
This theoretical model can easily relate the fracture energy to the observed load-CMOD response. The model shows that fracture energy is a constant fracture parameter and independent of specimen and notch size. The model also provides a constant linear relationship of the deflection and CMOD, works with a range of specimen sizes to produce consistent fracture parameters, and the size of an equivalent micro cracked zone. In addition, it also generates a new concept for measuring the toughness index of fiber reinforced composites. Different types of fiber reinforced materials were studied and the same unique relationships were observed.
Finally, a new standard testing setup for measuring the fracture parameters of concrete is proposed if the traditional load-line deflection method is to be used. However, the present study strongly suggests that the CMOD response should be used as the new standard for any future fracture toughness testing and evaluation.
njit-etd1996-023 (154 pages ~ 4,409 KB pdf)
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Created June 9, 2004