Volumetric changes in concrete

Question 1

What happens to concrete when wetted or dried?

Answer 1

Expands on wetting

Contracts on drying

Question 2

If load is sustained what happens to concrete?

Answer 2

Creep

Question 3

What causes swellage, shrinkage and creep?

Answer 3

Whater movement, especially at the C-S-H gel

Question 4

What is capillary water?

Answer 4

‘free water’ not under the influence of the attractive forces of the solid surfaces

Question 5

What is adsorbed water?

Answer 5

Water physically adsorbed onto the surfaces of hydration products (6 layers or 1.5 nm)

Question 6

What relative humidity is needed to dry adsorbed water?

Answer 6

<50%

Question 7

What is interlayer water?

Answer 7

Water that forms a single layer, held by strong hydrogen bonding

Question 8

What relative humidity is needed to dry interlayer water?

Answer 8

<11%

Question 9

What is capillary action?

Answer 9

The movement of a liquid up a narrow tube against gravity?

Question 10

What allows capillary action to happen

Answer 10

Attraction of water molecules to the tube wall (adhesion) is strogner than the attraction between the water molecules (cohesion) pulling the molecules up

Question 11

What causes swelling in concrete?

Answer 11

Occurs when water is absorped by the C-S-H

Question 12

What is the magnitude of the strain due to swelling after 100 days?

Answer 12

1300uE

Question 13

What is the magnitude of the swelling after 1000 days?

Answer 13

2000 uE

Question 14

What is Autogeneous shrinkage?

Answer 14

Shrinkage with zero moisture content (0 moisture movement)

Question 15

For normal strength concrete what is the strain due to shrinkage after 1 month and 5 years?

Answer 15

1 month: 40uE

5 years: 100u E

Question 16

For high strength concrete what is the strain due to shrinkage at 0.2 w/c?

Answer 16

<700uE

Question 17

What is the effect of drying?

Answer 17

shrinkage

in extreme cases this causes C-S-H to ‘collapse’, can cause cracking

Question 18

Is shrinkage reversible?

Answer 18

partially upon rehydration

Question 19

Define creep

Answer 19

The gradual increase in strain under a sustained constant stress after taking swelling, shrinkage, and thermal movement into account

Question 20

How much larger is creep than the elastic strain?

Answer 20

2-5 x

Question 21

What causes creep to occur?

Answer 21

rearrangement of C-S-H sheets, mircrocracking at the ITZ, when driying underload the magnitude of the creep is much higher

Question 22

Is creep reversible?

Answer 22

Partially, there will always be a platic deformation

Question 23

What are some negatives of creep?

Answer 23

Excessie deflection
Servicability problems
Loss of prestress in prestressed concrete

Question 24

What are some positives of creep?

Answer 24

Relieve stress concentration

Reduces risk of microcracking

Question 25

What is thermal movement?

Answer 25

A function of the material coefficient of thermal expansion and the magnitude of the temperature change.

Question 26

Why is thermal effect such a big issue in large structures?

Answer 26

The heat of hydration can make large volume changes.

Question 27

What are the two main concerns about heat in massive structures?

Answer 27

1. Temperature rise in concrete due to heat of hydration

2. Subsequent shrinkage on cooling and potential cracking if appropriate measures are not taken

Question 28

How do we mitigate thermal cracking?

Answer 28

changing the mix ingredients and composition.
Following good curing procedures
Adequatly designing reinforcements and adding cooling pipes

Question 29

How are expansion joints made?

Answer 29

By sawing cuts into the surface

Question 30

Why are expansion joints important?

Answer 30

they introduce a more ductile material to accomodate expansion or shrinkage