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CIVE40007 – Materials > Early age & mechanical properties of concrete > Flashcards

Early age & mechanical properties of concrete Flashcards

1
Q

What are the processes in concrete?

A
  • Batching
  • Mixing
  • Transporting
  • Placing
  • Compacting
  • Finishing
  • Curing
  • Formwork removal
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2
Q

What must fresh concrete have the ability to do?

A
  • Easily mixed
  • Easily transported
  • Easily placed
  • Easily compacted
  • Easily finished
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3
Q

What must fresh concrete have the ability to resist?

A
  • Bleeding
  • Segregation
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4
Q

What does batching mean?

A

The batching of concrete refers to the process of measuring and combining the ingredients required for the production of concrete in predetermined proportions.

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5
Q

What is involved in batching and mixing?

A
  • measuring ingredients
  • aggregate moisture checked
  • drum mixer
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6
Q

In what time is the fresh concrete placed?

A

1.5 hrs

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7
Q

How can you transport cement?

A

truck
conveyor belts
pumps
skips
wheelbarrows

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8
Q

How should you place the concrete and why?

A

Place concrete as near as possible to final its position, mix
remains cohesive, with no segregation.

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9
Q

What should you do with each layer before placing the next?

A

Make sure uniform and compact

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10
Q

How much of the volume of freshly placed concrete is trapped air?

A

5-20%

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11
Q

What methods for compaction are there?

A

Manual ramming
tamping
vibrators (poker, formwork, beam)

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12
Q

What does the air voids in concrete affect?

A
  • Strength
  • Density
  • Permeability
  • Durability
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13
Q

How does trapped air have an affect on bonding?

A

Affects bonds with rebar and corrosion initiation

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14
Q

What is the purpose of finishing?

A

To produce flat, level and dense surface

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15
Q

What is screeding?

A

To strike off excess with sawing action across the surface with a straight edge

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16
Q

What is darby/bull float?

A

When you smooth down high spots, embedded large aggregates fill small hollows left after screeding

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17
Q

What is floating/trowelling?

A

With flat wood or metal blades to compact the surface, brings paste to the surface & removes remaining imperfections.

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18
Q

What is brooming?

A

Uses a rake/broom if skid resistance is required

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19
Q

What are some test methods?

A
  • Slump test
  • Flow table test
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20
Q

What is a slump test?

A

A concrete slump test measures the consistency of a concrete batch to see how easily the concrete will flow

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21
Q

What does a greater slump mean?

A

More workable

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22
Q

What flow table test?

A

It is used primarily for assessing concrete that is too fluid (workable) to be measured using the slump test, because the concrete will not retain its shape when the cone is removed.

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23
Q

When flow table test used?

A

For high or very high workability

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24
Q

What is flow table test useful for?

A

Good for indicating mix cohesiveness and segregation

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25
Q

How do you work out average spread?

A

(A+B)/2 (mm)

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26
Q

What are many concrete innovation dependent on?

A

Rheology

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27
Q

What is roller compacted concrete?

A
  • Has zero slump
  • Compacted using a vibrating roller
  • Dry enough to prevent sinking but wet enough for distribution
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28
Q

What are the different types of placing of concrete?

A
  • Self compacting concrete
  • Pumping concrete
  • Shotcrete
  • Underwater concreting
  • Additive manufacturing
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29
Q

What is the aim of curing?

A

To keep concrete as nearly saturated as possible

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30
Q

What is needed for curing?

A

Should start as early as possible and be a continuous

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31
Q

What is curing?

A

Curing is the process of maintaining satisfactory temperature and moisture conditions in concrete long enough for hydration to develop the desired concrete properties.

32
Q

What are the methods for curing?

A
  • Impermeable sheets
  • Steam curing/autoclaving
  • Curing compounds
33
Q

What does curing promote?

A

Hydration and microstructure development

34
Q

What happens when capillary pores become discontinuous?

A

If capillary pores become discontinuous, concrete will be impermeable and durable

35
Q

What happens when you drop curing temp?

A

Strength drops

36
Q

What is needed for satisfactory strength development?

A

Not all cement to hydrate

37
Q

Which type of cement has the highest compressive strength?

A

Continuously moist

38
Q

What does poor compaction lead to?

A
  • Reduced strength
  • Reduced bond between rebar and concrete
  • Increases transport of aggressive agents
  • Creates visual blemishes
39
Q

How much does strength for poorly compacted concrete decrease by?

A

5-6% for every 1% vol.air

40
Q

What is segregation?

A

The separation of the constituent materials in freshly mixed concrete.

41
Q

What is segregation caused by?

A

Differences in density

42
Q

When can segregation be detected?

A

When handled on site

43
Q

How can segregation be controlled?

A
  • Improve aggregate grading
  • Increase fines content
  • Care in transporting, placing & compacting fresh concrete
  • Air entrainment – small air bubbles help keep particles in
    suspension, reduce sedimentation
44
Q

What is bleeding?

A

The process where free water in the mix is pushed upward to the surface due to the settlement of heavier solid particles such as cement and water.

45
Q

When does bleeding occur?

A

When rate of evaporation < rate of bleeding

46
Q

What are the negative effects of bleeding?

A
  • Laitance on surface, dusting - paste easily abraded
  • Reduce bond strength between rebar and concrete
47
Q

How can bleeding be controlled?

A
  • Increase cement fineness & content
  • Reduce water content
  • Increase fine aggregate content
  • Use air entrainment
48
Q

When does bleeding stop?

A

When concrete stiffens and sets

49
Q

Where are plastic settlement cracks common?

A

In deep sections

50
Q

What are plastic settlement cracks caused by?

A

Caused by excessive settlement and bleeding, restrained by large
obstructions

51
Q

What do plastic settlement cracks look like?

A

Crescent shaped void beneath rebar, initially filled with bleed water

52
Q

Where are plastic shrinkage crack common?

A

In thin sections

53
Q

What is plastic shrinkage cracks caused by?

A

Caused by rapid loss of water by evaporation or absorption

54
Q

What happens when fresh concrete dries and contracts?

A

Plastic shrinkage

55
Q

When do plastic shrinkage cracks form?

A

Shrinkage restrained by inner concrete, induces tensile stress, cracks when exceeds tensile strength

56
Q

How can plastic shrinkage be controlled?

A
  • Reduce surface evaporation rate to less than 1 kg/m2/h
  • Start curing early
  • Cool aggregates and mixing water
57
Q

Why is the compressive strength test the most common test for hardened concrete?

A
  • Used in structural design
  • Compliance and quality control
  • Formwork removal
  • Transfer of pre-stress
  • Easy to measure
  • Correlates to many properties
  • Measured by standard uniaxial compression test
  • Cubes or cylinders
58
Q

What are the compressive strength test results influenced by?

A
  • Ingredients & mix proportions
  • Early age conditions
  • Test parameters (sample type, size moisture condition, loading rate)
59
Q

Why is cube strength > cylinder strength?

A
  • Friction between plates of testing machine and specimen produces a restraining effect
  • Cylinders (larger H/D ratio) are less affected by end restraint
60
Q

What are tensile strength test useful for?

A
  • Resistance to cracking
  • Shear design
  • Assessment of fatigue performance
  • Design of highway & airfield pavements, unreinforced structures, dams
61
Q

What is a disadvantage to tensile strength testing?

A

More difficult to measure

62
Q

What is the direct tensile test?

A
  • Axially applied tensile load
  • Measures the stress and strain at failure of a specimen
63
Q

What is the splitting tension test?

A
  • Indirect test
  • Load in compression
64
Q

How does the splitting tension test work?

A

In this test, a standard cylindrical specimen is laid horizontally, and the force is applied on the cylinder radially on the surface which causes the formation of a vertical crack in the specimen along its diameter.

65
Q

What does tensile strength increase with?

A

Drying

66
Q

What is the flexural test?

A

It is a measure of an unreinforced concrete beam or slab to resist failure in bending.

67
Q

How is the max tensile stress calculated?

A

By finding the modulus of the rupture

68
Q

How much does the flexural test overestimate the axial tensile strength by?

A

50-100%

69
Q

For the flexural test what does tensile strength decrease with?

A

Drying

70
Q

What is the relationship between compressive and tensile strength?

A

As compressive strength increases, tensile strength increases but at decreasing rate

71
Q

What is the ratio of tensile to compressive strength?

A

0.1

72
Q

What is the modulus of elasticity?

A
  • Measures stiffness
  • Slope of stress-strain curve
73
Q

What is the modulus of elasticity used for?

A

Used to calculate elastic deflection & stresses induced by volume changes

74
Q

When does ductile failure occur?

A

Ductile failure occurs when a material is loaded beyond its yield strength and begins to plastically deform for a period before ultimately failing.

75
Q

Why is a ductile material better?

A

Ductile is better as it ismore predictable and controllable than brittle fracture, following a well-defined stress-strain curve and exhibiting deformation before failure

76
Q

What is strength of a concrete a function of?

A
  • Strength of aggregate
  • Strength of paste
  • Strength of aggregate-paste interface(ITZ)

CIVE40007 – Materials (16 decks)

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