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CIVE40007 – Materials > Volumetric changes in concrete > Flashcards

Volumetric changes in concrete Flashcards

1
Q

How can early age change volume of concrete?

A
  • Plastic settlement and plastic shrinkage
  • Drying and bleeding -> cracks
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2
Q

What type of material is concrete physically?

A

Brittle

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3
Q

How can elastic strain change volume?

A
  • Concrete deforms
    Only deforms a little so we use micro strains
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4
Q

What is the tensile strain capacity for concrete cracking?

A

300 micro-strains

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5
Q

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

A
  • Swelling
  • Autogenous shrinkage
  • Drying shrinkage
  • Creep
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6
Q

What does the microstructure of cement consist of?

A
  • Solids
  • Voids
  • Water
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7
Q

What are the different components of water?

A
  • Free-capillary water (evaporable)
  • Physically absorbed gel water
  • chemically bound (non-evaporable)
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8
Q

What is calcium silicate hydrate?

A

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

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9
Q

How much of the paste does calcium silicate hydrate make?

A

50-65%

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10
Q

What are the features of C-S-H?

A
  • High surface area - contain a lot of water with pore structures
  • Morphology varies from fibres to amorphous gel
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11
Q

What is capillary pore water?

A
  • ‘Free’ water, not under the influence of attractive forces exerted by the solid surfaces
  • Stored in large voids
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12
Q

What is absorbed water?

A
  • 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.
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13
Q

What is interlayer water?

A

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

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14
Q

What is capillary action equal to?

A

= adhesion + cohesion + surface tension

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15
Q

What is capillary action?

A

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

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16
Q

What gives a greater adhesion?

A

Smaller tube size

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17
Q

What does adhesion do?

A

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.

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18
Q

When will capillary action occur?

A

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

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19
Q

When does swelling occur?

A
  • Absorption of water by C-S-H
  • Water reacts with these silicates and leads to the formation of alkali calcium silicate hydrate.
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20
Q

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

A

2000 mircorstrains after 1000 days

21
Q

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

A

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

22
Q

What is autogenous shrinkage?

A

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

23
Q

When does autogenous shrinkage?

A

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

24
Q

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

A

100 micro-strain after 5 years - normal strength concrete

25
Q

What is drying shrinkage?

A
  • Concrete exposed to drying externally to the environment
  • Water lost to environment
26
Q

When is water lost from smaller pores in drying shrinkage?

A

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

27
Q

When is cracking caused in drying shrinkage?

A

Cracking caused when shrinkage restrained

28
Q

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

A

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

29
Q

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

A

4000 με

30
Q

What is creep?

A
  • Deformation of structure under sustained load
  • Gradual increase in strain under
    constant stress
31
Q

When does creep occur?

A

In hydrated cement

32
Q

What are the mechanisms of creep?

A
  • 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
33
Q

How does drying influence creep?

A
  • If concrete dries while under
    load, shrinkage and creep
    occur simultaneously
  • Drying increases the magnitude
    of creep
34
Q

What are the negatives of creep?

A
  • Excessive deflection
  • Serviceability problems (especially in high-rise buildings and long-span bridges)
  • Loss of prestress in prestressed concrete
35
Q

What are the +ve of creep?

A
  • Reduces stress concentrations induced by shrinkage, thermal movement etc.
  • Reduces risk of micro-cracking
  • Adding rebar can help restrain deformation
36
Q

What does cement hydration generate?

A

Heat

37
Q

What gives a greater temp rise in concrete?

A

Greater cement content

38
Q

What temp can concrete get to?

A

60-70 degrees celsius

39
Q

What is the coefficient if thermal expansion?

A

(α) – change in length per degree
of temperature

40
Q

How do you calculate thermal movement?

A

Thermal movement = ∆T × α

41
Q

What are the main concerns for mass concrete?

A

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

42
Q

What is an example of a massive structure?

A

Dams

43
Q

How do you mitigate thermal cracks in mass concrete?

A
  • 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
44
Q

What does having no joints lead to?

A

Uncontrolled cracking

45
Q

What can you use to construct adequate joints?

A

Saw

46
Q

What is the ratio of tensile-compressive strength?

A

~ 0.1

47
Q

Why is tensile strength &laquo_space;compressive strength?

A
  • Cracks form orthogonal to direction of load - tensions
  • Fracturing is relatively stable and many cracks form before failure - compressions
48
Q

Where does micro-cracking exist?

A

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

49
Q

What determines number and width of cracks?

A

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

CIVE40007 – Materials (16 decks)

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