Early Age Mechanical Properties of Concrete

Question 1

When does the early age of concrete start and finish?

Answer 1

batching through to formwork removal

Question 2

what are the 8 stages of early age concrete life?

Answer 2

batching
mixing
transporting
placing
compacting
finishing
curing
formwork removal

Question 3

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

Answer 3

easily mixed
easily transported
easily placed
easily compacted
easily finished

Question 4

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

Answer 4

bleeding
segregation
sedimentation

Question 5

what is meant by workability?

Answer 5

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

Question 6

What goes into batching and mixing? (6)

Answer 6

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

Question 7

what are the various methods of transporting and placing?

Answer 7

direct from truck
conveyor belts
pumps
skips
wheelbarrows

Question 8

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

Answer 8

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

Question 9

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

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

Answer 9

uniform layers

still plastic

Question 10

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

Answer 10

Attach funnels to the end of chutes

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

Question 11

Why do we compact concrete?

Answer 11

Affects strength, density, permeability and therefore durability

Question 12

What are some methods of compaction?

Answer 12

Manual ramming/tamping

vibrators (poker, formwork, beam)

Question 13

What is the aim of finishing concrete

Answer 13

producing a flat, level and dense surface

Question 14

What might the process of finishing involve?

Answer 14

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

Question 15

What do the slump and flow table tests check for?

Answer 15

consistency of mix and any batch variations

Question 16

The greater the slump in a test . . .

Answer 16

the greater the workability

Question 17

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

Answer 17

inconsistencies in aggregate mixture

Question 18

What does the flow table test ential?

Answer 18

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

Question 19

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

Answer 19

high

Question 20

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

Answer 20

400-500mm high
500-600mm very high

Question 21

What is zero slump concrete used for?

Answer 21

direct road paving, hydroelectric dams

Question 22

What are some factors affecting workability?

Answer 22

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

Question 23

What is curing concrete?

Answer 23

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

Question 24

What are some curing methods?

Answer 24

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

Question 25

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

Answer 25

hydration of cement microstructure develops as capillary pores become discontinuous

Question 26

How does curing duration affect the strength of concrete?

Answer 26

increases rapidly initially, stabilises within a month after setting

Question 27

What is the general effect of temperature on strength?

Answer 27

generally increases as temperature at which concrete cures increases

Question 28

What does poor compaction of early age concrete result in?

Answer 28

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

Question 29

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

Answer 29

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

Question 30

How would you describe the segregation of concrete?

Answer 30

distribution of aggregates is no longer uniform

Question 31

What is the segregation of concrete caused by?

Answer 31

differences in density and particles size in the aggregate

Question 32

How might you mitigate segregation?

Answer 32

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

Question 33

What does the bleeding of concrete involve?

Answer 33

rising of mix water to the top surface

Question 34

What is the bleeding of concrete caused by?

Answer 34

inability of solids to hold mix water when heavy particles settle

Question 35

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

Answer 35

rate of evaporation < rate of bleeding

Question 36

What are the effects of concrete bleeding?

Answer 36

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

Question 37

What might increase bleeding in concrete?

Answer 37

retarders, deep sections, low ambient temperature

Question 38

How can concrete bleeding be reduced?

Answer 38

Increase cement fineness Reduce water content air entrainment accelerator admixture reduce rate of evaporation by curing early

Question 39

What are plastic settlement cracks caused by?

Answer 39

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

Question 40

What happens in plastic settlement cracking?

Answer 40

a crescent shaped void forms beneath rebar

Question 41

How might plastic settlement cracking be reduced

Answer 41

reduce bleeding & segregation do not re-vibrate

Question 42

What are plastic shrinkage crack caused by?

Answer 42

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

Question 43

where is plastic shrinkage cracking most common ?

Answer 43

Thin sections where little bleeding occurs

Question 44

How would you control plastic shrinkage cracking?

Answer 44

reduce rate of surface evaporation cure as soon as possible use windbreaks and sunshades, to mitigate rapid water loss

Question 45

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

Answer 45

used in structural design easy to measure correlates to many properties

Question 46

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

Answer 46

20%

Question 47

Tensile strength in concrete is useful for . . .

Answer 47

resistance to cracking shear design design of vehicle pavements, unreinforced structures, dams etc.

Question 48

What methods are there for measuring tensile strength?

Answer 48

direct tension test splitting tension flexure

Question 49

What happens in the direct tensile test?

Answer 49

tensile load is applied axially to a sample

Question 50

How is the splitting tensile test carried out?

Answer 50

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

Question 51

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

Answer 51

10~15%

Question 52

How is the Flexural test carried out?

Answer 52

Dual two point loading on beam until failure single crack forms in central span max tensile stress is reached at the bottom fibre

Question 53

The flexural test overestimates axial tensile strength by how much?

Answer 53

50~100%

Question 54

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

Answer 54

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

Question 55

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

Answer 55

stiffness calculate elastic deflection & stresses induced by volume changes

Question 56

The strength of concrete is a function of

Answer 56

aggregate strength paste strength ITZ strength