Volume Changes Of Concrete

Question 1

1με is ? x ε

Answer 1

1x10^-6

Question 2

If concrete experiences [] or more it will start to crack

Answer 2

300με

Question 3

Swelling,shrinking,drying, shrinkage and creep are all caused by

Answer 3

Movement of water within the microstructure of cement paste

Question 4

What are the hydration products of cement?

Answer 4

Calcium silicate hydrates C-S-H

Question 5

CSH makes up what proportion of hydrated cement paste?

Answer 5

50~60%

Question 6

CSH has what kind of structure?

Answer 6

A variable composition, not well defined

Ranges from fibres to an amorphous gel

High surface area with gel pores

Question 7

Water within gel pores are called “free water” , meaning ?

Answer 7

This water is not under the influences of attractive forces exerted by the solid surfaces

Question 8

Water can stay in three places in the CSH gel pores, where are these?

Answer 8

Inside gel pore ‘free water’

Inter layer water

Absorbed water (on surfaces of hydration products)

Question 9

Up to [] layers of absorbed water can be held

Answer 9

6

Question 10

CSH sheets are held together by inter layer water molecules with

Answer 10

Strong hydrogen bonding

Question 11

What is capillary action?

Answer 11

Movement of liquid up a narrow tube agonist gravity

Question 12

In capillary action, attraction of water molecules to the tube wall is called what?

Answer 12

Adhesion

Question 13

In capillary action, attraction between water molecules is called

Answer 13

Cohesion

Question 14

In capillary action, which is stronger?
Adhesion or cohesion?

Answer 14

Adhesion forces pull water molecules up

Question 15

Capillary action is [] to tube size

Answer 15

Inversely proportional, the narrower the easier

Question 16

Swelling occurs when [] is absorbed by []

Answer 16

Water is absorbed by CSH

Question 17

What is autogenous shrinkage?

Answer 17

The loss of capillary pore water by cement hydration

Self starving self hydration

Question 18

Why does autogenous shrinkage occur?

Answer 18

When saturated concrete is exposed and is losing water to the environment

Question 19

What causes drying shrinkage?

Answer 19

Capillary pore tension generates negative pore pressure (suction), causing pores to collapse

Question 20

Following the collapsing of pores, how does drying shrinkage damage concrete?

Answer 20

Initial loss of capillary pore water causes little shrinkage

Further drying removes absorbed and inter layer water from CSH, making shrinkage go from little to big

Question 21

What is the root cause of capillary tension in drying shrinkage?

Answer 21

Removal of inter layer and absorbed water, making pores “zip” shut

Question 22

What is creep?

Answer 22

You

But seriously

Deformation caused by a constant load. Strain grows as stress is constant

Question 23

What is relaxation?

Answer 23

Impossible . . .

If a structure is kept at a constant strain, creep can also manifest itself as a decrease of stress over time

Question 24

Where does creep occur?

Answer 24

Hydrated cement paste

Question 25

What happens to cement paste in creep?

Answer 25

It rearranges itself, with the sliding of CSH sheets

Question 26

If concrete dries under load, what two things can happen to it?

Answer 26

Shrinkage and creep

Question 27

What are the negative effects of creep?

Answer 27

Excessive deflection

Loss of pre stress in prestressed concrete

Question 28

What can be some positive effects of creep?

Answer 28

Reduces stress induced by shrinkage and thermal movement

Reduces risk of micro racking

Question 29

What factors influence drying and shrinkage creep?

Answer 29

Humidity

Temperature

Applied load

Question 30

Cement hydration is []

(Think of the challenge of building the hoover dam)

Answer 30

Exothermic

Question 31

During hydration, what is the internal temperature at which expansion occurs?

Answer 31

60~70°C

Question 32

The greater the cement content, the greater the [] rise

Answer 32

Temperature rise

Question 33

Thermal movement =

Answer 33

ΔT x α

Where ΔT is change in temp

and α is coefficient of thermal expansion

Question 34

What does α describe?

Answer 34

Change in length per degree of temperature

Question 35

What’s are the values of α for aggregate and cement paste?

Answer 35

aggregate 4-12με per °C

Paste 10-30με per °C

Question 36

What is greater?
Thermal shrinkage or drying shrinkage

Answer 36

Thermal shrinkage

Question 37

How can we mitigate thermal shrinkage? (5)

Answer 37

Control casting size

Cooling pipes (hoover dam!)

Use less cement (lol no)

Retarder admixture

Aggregates with low heat coefficients

Question 38

What is setting concrete at night good for?

Answer 38

Combatting thermal shrinkage

Question 39

To mitigate thermal shrinkage, what do we want to keep temperature change below?

Answer 39

10°C

Question 40

What is a construction joint in concrete?

Answer 40

A deliberate cut into the concrete’s surface, attracting cracks to it

Question 41

What is the advantage of construction joints in concrete?

Answer 41

Reliving stress, mitigating further cracking

Question 42

Why is tensile strength less than compressive strength in concrete in the case of uniaxial tension ?

Answer 42

Cracks propagate easily under tension

Cracks form orthogonal to load

Stress conc. increases rapidly at crack zones

Question 43

Why is tensile strength less than compressive strength in concrete in the case of uniaxial compression ?

Answer 43

Cracks form in the direction of the load

Fracturing is relatively stable, many cracks form before failure

Question 44

Why is the stress-strain behaviour of concrete non-linear?

Answer 44

concrete is made of many materials with different behaviours