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What do we know about cracks?
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The subject of cracks, how they begin, how they grow, and if they stop,
are all part of the field called fracture mechanics. Here
we'll discuss some fracture mechanics which can be applied to large
concrete structures, like dams.
First some general ideas in fracture mechanics.
Forces
There has to be some sort of force acting on a material to make it
fracture or crack. There are two major forces we will look at:
- temperature variation (the material gets hot and cold, expands and
shrinks) -- this can be a problem in concrete. As you learned earlier in the
section on concrete, when concrete is mixed it
gets very hot and expands. When it tries to cool off, the concrete wants to
contract. If it can't contract (because it is connected to another wall or
rock) then cracks form.
- live load -- the general pulling or pushing
on a material. In a dam,
loads could be caused by settling or sliding of the foundation or abutments,
ice, silt, waves,
earthquakes, or even asteroids!
Modes |
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These forces are simplified into three different "modes", basically three
ways the forces can act and what kind of fracture they cause. The
simplest mode is Mode I, the opening mode. In Mode I, the forces are
perpendicular to the crack (the crack is horizontal and the forces are
vertical), pulling the crack open.
What would happen if both of the forces were pushing down on the crack?
Nothing. This would close the crack.
See Mode I for yourself!
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Mode I cracking happens in concrete dams. If there is an existing crack on
the upstream side, water can enter and open the crack:
How do the cracks know where to
start?
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