Volumetric changes in concrete

Question 1

How can early age change volume of concrete?

Answer 1

• Plastic settlement and plastic shrinkage
• Drying and bleeding -> cracks

Question 2

What type of material is concrete physically?

Answer 2

Brittle

Question 3

How can elastic strain change volume?

Answer 3

• Concrete deforms
  Only deforms a little so we use micro strains

Question 4

What is the tensile strain capacity for concrete cracking?

Answer 4

300 micro-strains

Question 5

What is caused by the movement of water within the microstructure of cement paste?

Answer 5

• Swelling
• Autogenous shrinkage
• Drying shrinkage
• Creep

Question 6

What does the microstructure of cement consist of?

Answer 6

• Solids
• Voids
• Water

Question 7

What are the different components of water?

Answer 7

• Free-capillary water (evaporable)
• Physically absorbed gel water
• chemically bound (non-evaporable)

Question 8

What is calcium silicate hydrate?

Answer 8

Main hydration product in Portland cement binder & source of concrete strength

Question 9

How much of the paste does calcium silicate hydrate make?

Answer 9

50-65%

Question 10

What are the features of C-S-H?

Answer 10

• High surface area - contain a lot of water with pore structures
• Morphology varies from fibres to amorphous gel

Question 11

What is capillary pore water?

Answer 11

• ‘Free’ water, not under the influence of attractive forces exerted by the solid surfaces
• Stored in large voids

Question 12

What is absorbed water?

Answer 12

• Physically adsorbed onto the surfaces of hydration products. Up to 6 layers of H2O (1.5 nm) can be physically held. Removed on drying < 50% r.h.

Question 13

What is interlayer water?

Answer 13

Between the C-S-H sheets held by strong hydrogen bonding. Removed on strong drying < 11% r.h.

Question 14

What is capillary action equal to?

Answer 14

= adhesion + cohesion + surface tension

Question 15

What is capillary action?

Answer 15

The movement of water within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension. (against gravity)

Question 16

What gives a greater adhesion?

Answer 16

Smaller tube size

Question 17

What does adhesion do?

Answer 17

Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upward.

Question 18

When will capillary action occur?

Answer 18

Occurs when the adhesion to the walls is stronger than the cohesive forces between the liquid molecules.

Question 19

When does swelling occur?

Answer 19

• Absorption of water by C-S-H
• Water reacts with these silicates and leads to the formation of alkali calcium silicate hydrate.

Question 20

What are the typical values for cement paste in terms of strain? (swelling)

Answer 20

2000 mircorstrains after 1000 days

Question 21

What are the typical values for concrete in terms of strain? (swelling)

Answer 21

100-150 mirco-strains after 6-12 months due to presence of aggregates → aggregates do not swell

Question 22

What is autogenous shrinkage?

Answer 22

Autogenous shrinkage is the uniform reduction of internal moisture due to cement hydration, which is typical of high-strength concrete.

Question 23

When does autogenous shrinkage?

Answer 23

Occurs in sealed conditions, i.e., no moisture movements in or out of concrete

Question 24

What is the typical value for strain in concrete? (autogenous)

Answer 24

100 micro-strain after 5 years - normal strength concrete

Question 25

What is drying shrinkage?

Answer 25

- Concrete exposed to drying externally to the environment - Water lost to environment

Question 26

When is water lost from smaller pores in drying shrinkage?

Answer 26

Water lost in larger capillary pores → then smaller pores when relative humidity gets lower

Question 27

When is cracking caused in drying shrinkage?

Answer 27

Cracking caused when shrinkage restrained

Question 28

What is the typical value for strain of concrete? (drying shrinkage)

Answer 28

Concrete - 200-1200 micro-strain (partially reversible)

Question 29

What is the typical value for strain of cement? (drying shrinkage)

Answer 29

4000 με

Question 30

What is creep?

Answer 30

- Deformation of structure under sustained load - Gradual increase in strain under constant stress

Question 31

When does creep occur?

Answer 31

In hydrated cement

Question 32

What are the mechanisms of creep?

Answer 32

- Involves internal movement of absorbed water and interlayer water to empty capillary pores - Sliding and re-arrangement of the C-S-H - Microcracking at the ITZ also contributes

Question 33

How does drying influence creep?

Answer 33

- If concrete dries while under load, shrinkage and creep occur simultaneously * Drying increases the magnitude of creep

Question 34

What are the negatives of creep?

Answer 34

- Excessive deflection - Serviceability problems (especially in high-rise buildings and long-span bridges) - Loss of prestress in prestressed concrete

Question 35

What are the +ve of creep?

Answer 35

- Reduces stress concentrations induced by shrinkage, thermal movement etc. - Reduces risk of micro-cracking - Adding rebar can help restrain deformation

Question 36

What does cement hydration generate?

Answer 36

Heat

Question 37

What gives a greater temp rise in concrete?

Answer 37

Greater cement content

Question 38

What temp can concrete get to?

Answer 38

60-70 degrees celsius

Question 39

What is the coefficient if thermal expansion?

Answer 39

(α) – change in length per degree of temperature

Question 40

How do you calculate thermal movement?

Answer 40

Thermal movement = ∆T × α

Question 41

What are the main concerns for mass concrete?

Answer 41

a) Temperature rise → expansion b) Subsequent cooling → thermal shrinkage c) Cracking if appropriate measures are not taken

Question 42

What is an example of a massive structure?

Answer 42

Dams

Question 43

How do you mitigate thermal cracks in mass concrete?

Answer 43

- Using low heat cement and less reactive - Avoid cement with high specific surface area - Use increased aggregate content to pack more into the system - Good curing

Question 44

What does having no joints lead to?

Answer 44

Uncontrolled cracking

Question 45

What can you use to construct adequate joints?

Answer 45

Saw

Question 46

What is the ratio of tensile-compressive strength?

Answer 46

~ 0.1

Question 47

Why is tensile strength << compressive strength?

Answer 47

- Cracks form orthogonal to direction of load - tensions - Fracturing is relatively stable and many cracks form before failure - compressions

Question 48

Where does micro-cracking exist?

Answer 48

Exists at the weak aggregate-paste interface (bond cracks)

Question 49

What determines number and width of cracks?

Answer 49

Number and width of cracks depend on curing history, bleeding, aggregate characteristics etc…