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Materials > Early Age Mechanical Properties of Concrete > Flashcards

Early Age Mechanical Properties of Concrete Flashcards

1
Q

When does the early age of concrete start and finish?

A

batching through to formwork removal

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2
Q

what are the 8 stages of early age concrete life?

A

batching
mixing
transporting
placing
compacting
finishing
curing
formwork removal

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3
Q

fresh concrete must have the ability to be . . . (5)

A

easily mixed
easily transported
easily placed
easily compacted
easily finished

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4
Q

fresh concrete must have the ability to resist . . . (3)

A

bleeding
segregation
sedimentation

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5
Q

what is meant by workability?

A

How east concrete is to mix, transport, place, compact and finish.

It describes the effort required to work manipulate fresh concrete with minimum loss of homogeneity

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6
Q

What goes into batching and mixing? (6)

A

measuring ingredients by mass

checking aggregate moisture content

coat aggregate surface with cement paste

produce uniform composition

wet mixing time 1~3 minutes

placed within 1.5 hours

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7
Q

what are the various methods of transporting and placing?

A

direct from truck
conveyor belts
pumps
skips
wheelbarrows

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8
Q

what is the aim of the transportation and placing of fresh concrete? (3)

A

place concrete as near as possible to its final position, mix remains and have no segregation

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9
Q

You must place fresh concrete in [] layers, compact each layer before placing the next

each subsequent layer placed whilst the underlying layer is still []

A

uniform layers

still plastic

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10
Q

What are some things you can do during placement to mitigate segregation?

A

Attach funnels to the end of chutes

placing concrete in deep sections using a flexible chute or long down pipe

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11
Q

Why do we compact concrete?

A

Affects strength, density, permeability and therefore durability

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12
Q

What are some methods of compaction?

A

Manual ramming/tamping

vibrators (poker, formwork, beam)

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13
Q

What is the aim of finishing concrete

A

producing a flat, level and dense surface

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14
Q

What might the process of finishing involve?

A

screeding - striking off excess with a straight edge

bull float/darby - smooth down high spots, embed large aggregate, fill small hollows

floating/trowelling- compact surface, bring paste to surface, remove remaining imperfections

brooming - done if some surface friction is favourable

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15
Q

What do the slump and flow table tests check for?

A

consistency of mix and any batch variations

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16
Q

The greater the slump in a test . . .

A

the greater the workability

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17
Q

What is the flow table test better at showing as opposed to the slump test?

A

inconsistencies in aggregate mixture

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18
Q

What does the flow table test ential?

A

Executing the slump test on a hinged table and lifting and slamming this hinge repeatedly, observing the movement of aggregate in the sample as you do so

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19
Q

the flow table test is more suited towards [] workability concrete

A

high

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20
Q

in the flow table test, what are the ranges for high and very high workability?

A

400-500mm high
500-600mm very high

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21
Q

What is zero slump concrete used for?

A

direct road paving, hydroelectric dams

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22
Q

What are some factors affecting workability?

A

ingredients: w/c , type of cement
admixtures
environment: temp, humidity, moisture
time
aggregates: graded or single sized?

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23
Q

What is curing concrete?

A

By providing suitable humidity and temperature at the surface of concrete, concrete stays as saturated as possible

24
Q

What are some curing methods?

A

impermeable sheets
steam curing for pre-poured blocks
Application of curing compounds

25
Q

What happens on a microscopic level during the curing of concrete?

A

hydration of cement
microstructure develops as capillary pores become discontinuous

26
Q

How does curing duration affect the strength of concrete?

A

increases rapidly initially, stabilises within a month after setting

27
Q

What is the general effect of temperature on strength?

A

generally increases as temperature at which concrete cures increases

28
Q

What does poor compaction of early age concrete result in?

A

reduced strength
reduced bond between concrete and rebar
increases transport of aggressive agents
visual blemishes

29
Q

strength decrease by how much []% for every [] vol. of air?

A

decreases 5-6% for every 1% volume of air introduced

30
Q

How would you describe the segregation of concrete?

A

distribution of aggregates is no longer uniform

31
Q

What is the segregation of concrete caused by?

A

differences in density and particles size in the aggregate

32
Q

How might you mitigate segregation?

A

improve aggregate grading
increase fine aggregate content
care in transportation, placing and compaction
use air entrainment to keep particles in suspension

33
Q

What does the bleeding of concrete involve?

A

rising of mix water to the top surface

34
Q

What is the bleeding of concrete caused by?

A

inability of solids to hold mix water when heavy particles settle

35
Q

all concretes bleed, but it is only seen when rate of [] < rate of []

A

rate of evaporation < rate of bleeding

36
Q

What are the effects of concrete bleeding?

A

change in w/c ratio, weak top surface

Laitance on the surface of concrete (dusty layer of fine cement particles which is easily abraded)

reduced bond strength within concrete

37
Q

What might increase bleeding in concrete?

A

retarders, deep sections, low ambient temperature

38
Q

How can concrete bleeding be reduced?

A

Increase cement fineness

Reduce water content

air entrainment

accelerator admixture

reduce rate of evaporation by curing early

39
Q

What are plastic settlement cracks caused by?

A

excessive settlement and bleeding and are restrained by large obstructions such as reinforcement and large aggregate

40
Q

What happens in plastic settlement cracking?

A

a crescent shaped void forms beneath rebar

41
Q

How might plastic settlement cracking be reduced

A

reduce bleeding & segregation

do not re-vibrate

42
Q

What are plastic shrinkage crack caused by?

A

rapid drying of fresh concrete, as concrete dries and contracts with loss of water

43
Q

where is plastic shrinkage cracking most common ?

A

Thin sections where little bleeding occurs

44
Q

How would you control plastic shrinkage cracking?

A

reduce rate of surface evaporation

cure as soon as possible

use windbreaks and sunshades, to mitigate rapid water loss

45
Q

Why is the standard uniaxial compressive strength test the most common for concrete?

A

used in structural design

easy to measure

correlates to many properties

46
Q

A cylinder of concrete is []% less strong than a cube of the same volume

A

20%

47
Q

Tensile strength in concrete is useful for . . .

A

resistance to cracking

shear design

design of vehicle pavements, unreinforced structures, dams etc.

48
Q

What methods are there for measuring tensile strength?

A

direct tension test

splitting tension

flexure

49
Q

What happens in the direct tensile test?

A

tensile load is applied axially to a sample

50
Q

How is the splitting tensile test carried out?

A

a concrete cylinder is placed between two packing strips which introduce a transverse tensile stress

the cylinder is actually under compression, but the compression puts the packing strips into tension

51
Q

By how much is axial strength over estimated in the splitting tensile test?

A

10~15%

52
Q

How is the Flexural test carried out?

A

Dual two point loading on beam until failure

single crack forms in central span

max tensile stress is reached at the bottom fibre

53
Q

The flexural test overestimates axial tensile strength by how much?

A

50~100%

54
Q

What is the relation between compressive and tensile strength in concrete?

A

As compressive strength increases, tensile strength increases but at a diminishing rate

55
Q

What is the modulus of elasticity of concrete a measure of?

A

stiffness

calculate elastic deflection & stresses induced by volume changes

56
Q

The strength of concrete is a function of

A

aggregate strength

paste strength

ITZ strength

Materials (13 decks)

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