Volumetric changes in concrete Flashcards
What happens to concrete when wetted or dried?
Expands on wetting
Contracts on drying
If load is sustained what happens to concrete?
Creep
What causes swellage, shrinkage and creep?
Whater movement, especially at the C-S-H gel
What is capillary water?
‘free water’ not under the influence of the attractive forces of the solid surfaces
What is adsorbed water?
Water physically adsorbed onto the surfaces of hydration products (6 layers or 1.5 nm)
What relative humidity is needed to dry adsorbed water?
<50%
What is interlayer water?
Water that forms a single layer, held by strong hydrogen bonding
What relative humidity is needed to dry interlayer water?
<11%
What is capillary action?
The movement of a liquid up a narrow tube against gravity?
What allows capillary action to happen
Attraction of water molecules to the tube wall (adhesion) is strogner than the attraction between the water molecules (cohesion) pulling the molecules up
What causes swelling in concrete?
Occurs when water is absorped by the C-S-H
What is the magnitude of the strain due to swelling after 100 days?
1300uE
What is the magnitude of the swelling after 1000 days?
2000 uE
What is Autogeneous shrinkage?
Shrinkage with zero moisture content (0 moisture movement)
For normal strength concrete what is the strain due to shrinkage after 1 month and 5 years?
1 month: 40uE
5 years: 100u E
For high strength concrete what is the strain due to shrinkage at 0.2 w/c?
<700uE
What is the effect of drying?
shrinkage
in extreme cases this causes C-S-H to ‘collapse’, can cause cracking
Is shrinkage reversible?
partially upon rehydration
Define creep
The gradual increase in strain under a sustained constant stress after taking swelling, shrinkage, and thermal movement into account
How much larger is creep than the elastic strain?
2-5 x
What causes creep to occur?
rearrangement of C-S-H sheets, mircrocracking at the ITZ, when driying underload the magnitude of the creep is much higher
Is creep reversible?
Partially, there will always be a platic deformation
What are some negatives of creep?
Excessie deflection
Servicability problems
Loss of prestress in prestressed concrete
What are some positives of creep?
Relieve stress concentration
Reduces risk of microcracking
What is thermal movement?
A function of the material coefficient of thermal expansion and the magnitude of the temperature change.
Why is thermal effect such a big issue in large structures?
The heat of hydration can make large volume changes.
What are the two main concerns about heat in massive structures?
- Temperature rise in concrete due to heat of hydration
2. Subsequent shrinkage on cooling and potential cracking if appropriate measures are not taken
How do we mitigate thermal cracking?
changing the mix ingredients and composition.
Following good curing procedures
Adequatly designing reinforcements and adding cooling pipes
How are expansion joints made?
By sawing cuts into the surface
Why are expansion joints important?
they introduce a more ductile material to accomodate expansion or shrinkage