Volume changes Flashcards
What is the main hydration product in Portland Cement and how much of cement paste consists of it
Calcium Silicate Hydrate (CSH) makes up around 50-65% of the cement paste
Give 3 properties of CSH
complex structure with variable composition, high surface area, contains a lot of small pores
What are the three types of water in CSH
Capillary pore water, adsorbed water, Interlayer Water
What is the size of the interlayer gaps between CSH layers generally
1.5nm
Where is free water, water that doesn’t have attractive forces exerted on it, located in the CSH layers and what does this mean when drying occurs
Capillary pores, the water isn’t bounded to the surfaces so is easily removed during drying
What is the water that is stuck to the CSH layers called
Adsorbed water
What is the water between CSH layers
interlayer water
What is capillary action
Movement of water against gravity up a narrow tube due to the adhesion of the water molecules and the side walls being greater than the attraction between water molecules
Why does CSH swell when absorbing water
the water increases pore pressure which forces the CSH gel apart
what are the strain values for swelling of cement paste for 100 and 1000 days in water respectively and what about concrete
1300 and 2000 microstrain respectively - concrete is 100~150 microstrain after 6-12months
What is autogenous shrinkage, why does it occur and what is the typical strain
shrinkage of concrete that occurs despite the concrete being in a sealed environment. Occurs because the capillary pore water is being used up to fuel the hydration reactions. around 100 microstrain in 5 years
Where is water driven out of in these percentages of relative humidity in drying shrinkage
100-80
80-50
50-10
<10
large capillary pores
small capillary pores
adsorbed water
interlayers
Why does the concrete shrink a lot when the adsorbed and interlayer water is driven off and what is strain values for cement and concrete
The CSH layer collapse. cement 4000 microstrain, concrete 200-1200 microstrain
Is drying shrinkage reversible
It is partially reversible
Why is drying shrinkage not fully reversible
When the CSH collapses, they form bonds between the sheets so the water can never re-enter these interlayer pores that have now been sealed
What happens when shrinking is constrained either by other parts of concrete shrinking less or other objects
cracking
What is creep
Long term deformation under sustained load which results in gradual stress under constant stress
What is relaxation
A type of creep which results in a reduction in stress at the same strain
What is the reason why creep occurs in concrete. (micro structure wise)
Movement of adsorbed water and interlayer water to empty capillary pores which causes collapse and rearrangement of CSH layers.
Why does removal of load on concrete result in less reduction in strain than it gains in strain when initially loaded.
The concrete has hardened over time as more hydration reactions occur so the Youngs modulus is greater.
is creep reversible
it is only partially reversible
What affects creep and shrinkage
Time, Relative humidity, Temperature, Load (mainly for creep), Aggregate properties especially its young’s modulus (concrete shrinkage/creep). Aggregate resist shrinkage
What are the ranges of thermal expansion coefficients for aggregates and cement paste (microstrain per celsius)
Aggregate 4-12 cement 10-20
In what kind of structures is thermal expansion/shrinking a problem
mass structures. Expansion due heat release from reaction and subsequent cooling can cause cracking
What happens when concrete is heated up really quickly and why
The water will boil and since the vapor doesn’t have enough time to escape it causes an explosion
How can thermal cracking be controlled
cooling pipes, retarders, curing, use less/less reactive/lower specific surface cement, use well graded/ lower thermal coefficient aggregates.
