14 Flashcards
What direction do stress and compressive cracks form
1 stress is 90° to the direction of force
2. Compressive is parallel to the direction of force and is due to the mission effect
Why is tensile streugh a fraction of compressive strength
This is due to never micro-cracks that can only be seen by an electrics ricacescare
Describe stress in micro-cracks
They have highly concentrated stresses which cause the cracks to spread
Why is concrete strong in compression
Compression forces can pass through crack white in tensions it cannot
What is the poisson effect
When a compressive force is applied in one direction is cause a tensile force at 90° in a concrete test the compressive strength is determine when tension cracking begins to form
The 5 fundamental concepts of stresses in concrete
1 only a fundamental force exist in structural members tension and compression
2. Shear stresses are just components of ten and comp forces acting at an angle
3. Shear and comp strength in concrete is higher than ten strength
4. Two forces will always exist and they are arthagacial to each other
5. Only two modes of failure exist
What is shrinkage
The volumetric change of concrete from fluid state to solid state
What is shrinkage cracking
When a material is restrained it expresses resistance to shrinkage and this strain leads to cracking since its is in an immature fragile state
What parameters affect shrinkage cracking ‘(7)
- High water cement ratio
- Wind
- Ambient temperature
- High humidity
- Concrete generated heat
- Type of aggregate
- Rich mix in cement ratio
- Differential shrinkage
How to prevent shrinkage
- Last in 30° to 25°
- Spray with water to coal and prevent evaporations
- Sheltered from wind
- Cement and aggregates should be kept cool and shaded
- Should be cast in cooler points of the day
- Reduce water cement ratio
- How large amounts the pre cool the water and use additives
- Reduce restrainments to control cracking such as saw cuts
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