Foundation, Concrete and Earthquake Engineering

Fracture Toughness


Fracture toughness is one of the most important properties of any materials for virtually all design applications which describes the ability of material containing a crack to resist fracture. It is a quantitative way of expressing a material’s resistance to brittle fracture when crack is present. If a material has large value of fracture toughness it will probably undergo ductile fracture.
Reason of Concrete Surfaces Cracking:

Improper design and construction practices cause majority of concrete cracks; such as :
1. Improper sub grade preparation.
2. The use of high slump concrete or addition of water on the job.
3. Improper finishing.
4. Inadequate or no curing.
5. Omission of isolation and control joints and improper jointing practices.
Material Susceptible to Low Fracture Toughness:

The high strength materials have low crack resistance (Fracture Toughness). i.e the residual strength under the presence of cracks is low. When only small cracks exists, structures designed in high strength materials may fail at stresses below the highest service stress, they were designed for. The structure is made fail safe by selecting materials with low growth rate and high residual strength and by adopting a design with inherent crack stopping capabilities.
Failure Mechanism:

Pre-existing crack will grow with time due to the application of repeated load or due to a combination of loads and environmental attack. The longer the crack, the higher the stress concentration induced by it. This implies that the rate of crack propagation will increase with time. The presence of crack reduces the strength of the structure. After a certain time the residual strength has become so low that the structure can not withstand accidental high loads that may occur in service.If such accidental high loads do not occur, the crack will continue to grow until the residual strength has become so low that fracture occurs under normal service loading.
Significance of Fracture Toughness:

Structural design must incorporate considerations of both the strength and durability of a component that contains cracks. The residual strength of a part in the presence of defects is a function of the material fracture toughness. So, to ensure the safety of a structure, the designer must estimate the load carrying capacity of a structure after the propagation of cracks.

The concrete industry and inspection agencies are much more familiar with traditional cylinder compression tests for control and acceptance of concrete.

Fracture toughness can be used for design purpose, but corresponding compressive strength must be considered. Anytime trial batches are made both fracture toughness and compressive tests should be made, so that a correlation can be developed for field control.

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