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Materials > Early age and mechanical properties > Flashcards

Early age and mechanical properties Flashcards

1
Q

What is meant by the term ‘early age’ of concrete?

A

The time between batching and formwork removal when curing has completed.

It includes; mixing, transporting, placing, compacting, finishing, curing

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2
Q

Approximately how long does the ‘early age’ last?

A

~12 hrs for high early age strength concrete
~24 hrs for normal strength concrete
~48 hrs for concretes with high PFA (pulverised fly ash) or GGBS (ground granulated blast furnace slag)

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3
Q

Why is the early age so important?

A

The operations performed withing this time period can have a huge impact on the long term properties - these can have an economic impact

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4
Q

What is the definition of workability?

A

The ability to be easily mixed, transported, placed, compacted and finsihed using available equipment without seggregation and bleeding

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5
Q

What is batching?

A

The operation of measuring ingredients an putting them into the mixer

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6
Q

How are the materials involved in batching ususally measured?

A

weight (volume is used very occasionally)

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7
Q

What is mixing?

A

Rotating and stirring using a drum, pan or central batching plant

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8
Q

What is the objective of mixing?

A

To coat all the aggregates with a uniform homogeneous cement paste

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9
Q

What is important to check when batching?

A

Aggregate moisture content, on and off site quality

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10
Q

How long after mixing is completed should you place the concrete?

A

withing 1.5 hrs

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11
Q

How long does mixing usually take?

A

between 1 and 3 minuts dependent on the mixer, and longer if specialist equipment is available

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12
Q

Where should cocnrete be placed?

A

As near as possible to its final mixing position (to prevent segregation)

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13
Q

When placing a deep section how should you place the concrete?

A

In layers, each layer should be placed while the lower level is still plastic.

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14
Q

How is segregation controlled?

A

Using a down pipe, flexible chute or tremie

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15
Q

How do we compact concrete?

A

Using mechanical concrete

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16
Q

Why do we compact concrete?

A

To remove entrapped air. It decreases the internal particle friction, making the concrete more fluid. This helps with the packing of coarse aggregate
N.B. the difference between entrapped and entrained air

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17
Q

Why do we finish concrete?

A

To produce a flat dense surface. This helps to increase resistance to impact and abrasion. It als improves aesthetics and can better other properties such as skid resistance.

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18
Q

What are the key factors of rheology of fresh concrete?

A
  • Consistency: resistance to flow, ability to flow into formwork and around rebars
  • stability; behaviour when flowng, ability to remain homogeneous without bleeding or segregation
  • compactibility
  • finsihability
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19
Q

How to we test workability (rheology)?

A

Slump test (mainly), flow table test etc.

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20
Q

What is the main equipment for a slump test?

A

Tamping rod and slump cone

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21
Q

What are the different types of slump?

A

True slump, slump remains fairly in shape

Shear slump, slump deforms and test needs to be redone

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22
Q

How do you measure the consistency of the concrete in a slump test?

A

The heigh difference between the slump cone and the highest point of the slumped concrete in mm

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23
Q

What are the weaknesses of the slump test?

A

It is more sensitive for mixes with medium and high workability and is heavily operator dependent.
It also does not measure compactability

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24
Q

Why do we use the flow table test?

A

For flowing concrete that contains superplasticizer, also works for concrete with very high workability

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25
Q

What material is involved in the flow table test?

A

Slump cone, hinged steel plate board

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26
Q

How do you measure the workability?

A

flow = (A+B)/2 where A is the diammeter of the slump in one direction and B in the other

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27
Q

What flow (mm) shows high workability?

A

400-500 mm

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28
Q

What flow (mm) shows very high workability?

A

500-65mm

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29
Q

What are a couple advantages of the flow table test?

A

It indicates mix cohesivness and segregation

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30
Q

What are some innovations from rheology?

A

Roller compacted (0 slump) concrete, pumped concrete, self-compacting concrete, sprayed/shotcrete, underwater concrete and 3D printed concrete.

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31
Q

Why is concrete cured?

A

To keep concrete as nearly saturated as possible

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32
Q

What is the benefit of curing concrete?

A
  • Promote continuous hydration
  • Reduce porosity
  • Achieve good strength development and durability
  • Minimise plastic shrinkage
  • Protect from adverse weather
  • Prevent high temperature gradient
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33
Q

What type of water should be used in curing?

A

Any water that is suitable for mixing is suitable for curing (drinking water)

34
Q

When should curing start and how long should it last?

A

As early as possible and should be continuous and last as long as is economically viable (slide 33 lecture 8)

35
Q

What are the main methods for curing?

A
  • Impermeable Sheets
  • Curing compounds
  • Steam Curing/ Autoclaving (factory conditions)
36
Q

In order for the concrete to be impermeable and durable, what needs to happen to the capillay pores?

A

They need to become discontinuous

37
Q

What are some common problems during the early ages of concrete?

A
  • Poor compaction
  • Honeycombing
  • too dry a mix leads to cracking and not enough concrete around rebar
  • too wet a mix leads to slumps
  • Segregation
  • Bleeding
  • Plastic settlement cracks
  • Plastic shrinkage cracks
38
Q

What does poor compaction lead to?

A
  • Strength loss
  • Weakened bond between concrete and rebar
  • Increased transport of aggresive agents
  • visual blemishes
39
Q

What is the strength reduction for every 1% vol. of air?

A

5-6%

40
Q

What is segregation of concrete?

A

When the distribution of agregate particles is no longer uniform

41
Q

What causes segregation?

A
  • Difference in density and size of particles (excessive amount of large particles)
  • poor handling of fresh concrete
42
Q

How do we detect segregation?

A

Flow table test

43
Q

How do you control segregation?

A
  • Improve the grading of aggregates
  • Increase the fines content
  • Care in transporting, placing and compacting fresh concrete
  • Air entrainment - helps keep particles in suspension
44
Q

What are fines?

A

Aggregate smaller than 4 mm - another term for fine aggregate

45
Q

What is bleeding?

A
  • when mix water rises to the surface of freshly compacted concrete
46
Q

What causes bleeding?

A

The inability of solids to hold mix water when heavy particles settle .
Some water gets trapped under large aggregate and rebars

47
Q

What are the effects of bleeding?

A
  • a w/c ratio gradient, resulting in a weak top surface
  • a dusting of cement on the top that allows the paste to be easily abraded
  • reduces the bond strenght between rebar and concrete
  • reduces plastic shrinkage cracking
48
Q

What can be done to mitigate bleeding?

A
  • Increase cement fineness and content, reduce w/c
  • Air entrainment
  • Use accelerator or a rapid hardening cement
  • reduce evaporation rate (early curing)
49
Q

What increases bleeding?

A
  • Retarders
  • Deep sections
  • low ambient temperature
50
Q

What causes plastic settlement cracks?

A

Excessive settlement and bleeding, restrained by large obstructions

51
Q

How do you control Plastic settlement?

A

Reduce bleeding and segregation

  • air entraining (admixture)
  • adopt revibration
52
Q

Where is plastic settlement common?

A
  • Concrete with rebar, large aggregate and changes in depth

- In deep sections

53
Q

What causes plastic shrinkage cracks?

A

Rapid loss of water (evaporation or absorption)

54
Q

Where does platic shrinkage occur?

A

At the surface (top is under tension) - concrete dries and contracts

55
Q

What helps reduce plastic shrinkage?

A

Bleeding (one of the times where it is useful)

56
Q

Where is plastic shrinkage common?

A

Thin sections where bleeding is less common

57
Q

How do you control Plastic shrinkage?

A

Control rate of surface evaporation to less than 1 kg m^-2 per hour but ideally below 0.25
This is done by curing and cooling aggregated and mixing water

58
Q

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

A
  • It is used in structural design
  • For compliance and quality control
  • For formwork removal
  • Transfer of pre-stress
  • easy to measuer
  • correlates to many properties
59
Q

How do we measure compressive strength?

A

Standard uniaxial compression test on cubes or cylinders

60
Q

How long after casting do we measure compression?

A

28 days

61
Q

What size of cube correlates to aggregates <20mm ?

A

100mm

62
Q

What size of cube corelates to aggregates >20mm

A

150mm cube

63
Q

What size of cylinders are used in compression testing?

A

150x300mm

64
Q

What influences the test results

A
  • Ingredients and mix proportions
  • Early age conditions
  • Test Paramters (size, humidity, loading rate)
65
Q

What is the ration of cube strength to cylinder strenght and why is it difference?

A

cylinder strength ~0.8 * cube strength

This is because of friction between the platens of the testing machine and the cube - restraining effect

66
Q

Why is tensile strenght important?

A
  • It is resistant to cracking
  • Useful in shear desing
  • Assesment of fatigue performance
67
Q

How do we measure tensile strength?

A
  • Direct tension (axial)
  • splitting tension
  • flexure
68
Q

Why arent Direct tensile tests usually done on concrete

A

It is difficult to avoid secondary forces

69
Q

How does splitting tension test work?

A
  • Loaded in compression
  • Transverse tensile stress causes splitting along the vertial diammeter
  • overestimates axial tensile stress by ~10-15%
  • usually on cylinders
70
Q

What are the equations needed for the splitting tension test?

A

fsp = 2F/piLD

axial tensile strenghth = 0.9*fsp

71
Q

How does a flexural test work?

A

Beam is loaded at 2 points until failure
Single crack between two points
Not valid if crack is not in middle third of span

72
Q

What is the equation for the flexural test?

A

Modulus of rupture = FL/bd^2

73
Q

What are the equations for the relationship between compressive and tensile strength (empirical)?

A

For C50/60
fctm = 2.12*ln(1+(fck+8)/10)

Where fctm is the 28-day mean tensile strength and fck is the 29-day characteristic cylinder strength

74
Q

How is modulus of elasticity measured?

A

Slope of stress strain curve

75
Q

How is the secant modulus calculated

A

Slope of line drawn from origin to a point corresponidng to 0.4*ultimate stress

76
Q

How is the Chord modulus calculateed

A

slope of the line corresponding to 50uE from the point 0.4*ultimate stress

77
Q

What is the strenght of concrete a function of?

A

Porosity

78
Q

What is the relationship between strenght and porosity?

A

As porosity goes down, strenght goes up

79
Q

How much does dryin increase measured strength by?

A

10-25% - C-S-H boundaries

80
Q

Why is tensile strenghth < compressive strength?

A
  • Flaws propogate under tensile strength
  • Cracks orthogonal to the direction of load
  • Stress concentrations increase rapidly
  • Cracks in compression form in direction of load
  • cracking does not change the area under load

Materials (13 decks)

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