Volume changes

Question 1

What is the main hydration product in Portland Cement and how much of cement paste consists of it

Answer 1

Calcium Silicate Hydrate (CSH) makes up around 50-65% of the cement paste

Question 2

Give 3 properties of CSH

Answer 2

complex structure with variable composition, high surface area, contains a lot of small pores

Question 3

What are the three types of water in CSH

Answer 3

Capillary pore water, adsorbed water, Interlayer Water

Question 4

What is the size of the interlayer gaps between CSH layers generally

Answer 4

1.5nm

Question 5

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

Answer 5

Capillary pores, the water isn’t bounded to the surfaces so is easily removed during drying

Question 6

What is the water that is stuck to the CSH layers called

Answer 6

Adsorbed water

Question 7

What is the water between CSH layers

Answer 7

interlayer water

Question 8

What is capillary action

Answer 8

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

Question 9

Why does CSH swell when absorbing water

Answer 9

the water increases pore pressure which forces the CSH gel apart

Question 10

what are the strain values for swelling of cement paste for 100 and 1000 days in water respectively and what about concrete

Answer 10

1300 and 2000 microstrain respectively - concrete is 100~150 microstrain after 6-12months

Question 11

What is autogenous shrinkage, why does it occur and what is the typical strain

Answer 11

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

Question 12

Where is water driven out of in these percentages of relative humidity in drying shrinkage
100-80
80-50
50-10
<10

Answer 12

large capillary pores
small capillary pores
adsorbed water
interlayers

Question 13

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

Answer 13

The CSH layer collapse. cement 4000 microstrain, concrete 200-1200 microstrain

Question 14

Is drying shrinkage reversible

Answer 14

It is partially reversible

Question 15

Why is drying shrinkage not fully reversible

Answer 15

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

Question 16

What happens when shrinking is constrained either by other parts of concrete shrinking less or other objects

Answer 16

cracking

Question 17

What is creep

Answer 17

Long term deformation under sustained load which results in gradual stress under constant stress

Question 18

What is relaxation

Answer 18

A type of creep which results in a reduction in stress at the same strain

Question 19

What is the reason why creep occurs in concrete. (micro structure wise)

Answer 19

Movement of adsorbed water and interlayer water to empty capillary pores which causes collapse and rearrangement of CSH layers.

Question 20

Why does removal of load on concrete result in less reduction in strain than it gains in strain when initially loaded.

Answer 20

The concrete has hardened over time as more hydration reactions occur so the Youngs modulus is greater.

Question 21

is creep reversible

Answer 21

it is only partially reversible

Question 22

What affects creep and shrinkage

Answer 22

Time, Relative humidity, Temperature, Load (mainly for creep), Aggregate properties especially its young’s modulus (concrete shrinkage/creep). Aggregate resist shrinkage

Question 23

What are the ranges of thermal expansion coefficients for aggregates and cement paste (microstrain per celsius)

Answer 23

Aggregate 4-12 cement 10-20

Question 24

In what kind of structures is thermal expansion/shrinking a problem

Answer 24

mass structures. Expansion due heat release from reaction and subsequent cooling can cause cracking

Question 25

What happens when concrete is heated up really quickly and why

Answer 25

The water will boil and since the vapor doesn’t have enough time to escape it causes an explosion

Question 26

How can thermal cracking be controlled

Answer 26

cooling pipes, retarders, curing, use less/less reactive/lower specific surface cement, use well graded/ lower thermal coefficient aggregates.