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STRCTENG 201 > Concrete > Flashcards

Concrete Flashcards

1
Q

what is the 2nd most used material

A

concrete

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2
Q

what is concrete?

A

binder + filler

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3
Q

what four part mixture is concrete normally referred to?

A

coarse aggregates (eg gravel) + fine aggregates (eg sand) +(portland) cement + water

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4
Q

what is concrete “cement” compared to

A

cake “flour”

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5
Q

what are aggregates

A

the main volume and strength

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6
Q

what is a binder

A

normally portland cement

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7
Q

what is the binders purpose

A

to glue aggregates together

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8
Q

what is cement paste composed of

A

cement + water

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9
Q

what is mortar paste composed of

A

cement + water + sand

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10
Q

what is concrete composed of

A

cement + water + sand + coarse aggregates

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11
Q

what is grout composed of

A

cement + water + sand + smaller aggregates and “flowable”

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12
Q

what are the advantages of concrete?

A

high compressive strength, ability to cast, cheap (~10% the cost of steel by volume), durable, fire resistant, high mass gives energy efficiency, flexible construction (onsite and precast), aesthetic properties

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13
Q

what are the disadvantages of concrete?

A

low tensile strength, brittle/non-ductile, non volumetrically stable, low strength to weight ratio, on site fabrication can be complex, high embodies carbon

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14
Q

what is the compressive strength of concrete

A

20-70 MPa

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15
Q

what is the tensile strength of concrete

A

2-4 MPa

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16
Q

what is the modulus of elasticity of concrete

A

20-35 GPa

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17
Q

what is the Poisson’s ratio

A

~0.2

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18
Q

what is the coef of thermal expansion

A

10 x 10^6 /C

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19
Q

what is the density of concrete

A

~2400 kg/m^3

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20
Q

what are the uses of concrete

A

beams, columns, walls, floors, pipes, driveways, powerpoles, crash barriers, counter weights, kitchen benches…

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21
Q

who were the best known early users of concrete

A

romans

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22
Q

what concrete did romans use to use

A

non hydraulic cement

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23
Q

what is concrete that is made out of non-hydraulic cement

A

concrete that will not harden in contact with water, made using materials such as non-hydraulic lime, and hardens by lime drying

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24
Q

what is hydraulic cement

A

cement that hardens under water, can harden due to chemical reactions

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25
Q

what did romans use for hydraulic cement

A

volcanic ash (Pozzolana)

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26
Q

What is portland cement

A

calcium silicates + extras

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27
Q

what is 3CaO.SiO2 (the chemical formula)

A

tricalcium silicate

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28
Q

what is the weight percent of tricalcium silicate in the concrete mix

A

45-75 %

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29
Q

how is cement manufactured

A

lime and silica mix heated (1400 - 1600 C) to form clinker (rocks) - then ground to form cement powder

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30
Q

what is lime

A

limestone (calcium carbonate) CaCO3

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31
Q

what is the two types of silica used

A

clay or silt (Si + impurities: Alumino-silicate)

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32
Q

what are the two processes of cement manufacturing

A

wet and dry

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33
Q

what are the processes of wet cement manufacturing

A

lime + silica mixed as a slurry

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34
Q

what is the cement manufacturing process used nowadays

A

dry process

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35
Q

what are the processes of dry cement manufacturing

A

lime + silica mixed as powders.

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36
Q

what is the pros and cons of dry cement manufacturing

A

dustier process, fine particals, more health and safety regulations need to be in place. But it is more efficient.

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37
Q

what is the pros and cons of wet cement manufacturing

A

mechanically easy, energy-intensive process. water must be evaporated before clinker forms

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38
Q

what are the wet cement process steps

A
  1. raw material (limestone and marl) from quarry
  2. raw material travels up conveyors and through raw mills
  3. raw material is converted to slurry, blended in basins and pumped into the kilns
  4. slurry is dried and burnt in kilns to produce clinker; waste dust is converted to fertiliser
  5. the clinker travels up the conveyors to the silos where the gypsum is added
  6. the product is then processed in the cement mills and finally stored in the cement silos
39
Q

what are the dry cement process steps

A
  1. raw materials transported (silicon (SiO2) typically via clay and calcium (CaCO3) typically via limestone
  2. mix well
  3. extreme heat to drive off CO2 and form ‘clinker’ - a solid mix of Si and Ca Oxides
  4. Grind into powder
40
Q

why is wet cement process energy intensive

A

kiln requires large amounts of energy, cement production accounts for ~5% of global CO2 emissions

41
Q

why is dry process more environmentally friendly

A

more energy efficient, use of carbon neutral fuel (wood chips, recycled wood), reduction in clinker used for cement (e.g. use of supplementary cementitious materials)

42
Q

what is hydration

A

series of chemical reactions that cause cement to harden

43
Q

important aspects of hydration

A

rate of reaction, heat generated and strength of hydration product

44
Q

what is the target of hydration

A

to form calcium silicate hydrates (C-S-H) and to provide strength of cement paste

45
Q

what are the two general cement types

A

GP: General purpose, <10% mineral addition
GB: General blended, >10% mineral addition

46
Q

what are the four specialised cement types

A

HE: high early strength
LH: low heat
SR: sulphate resistant
SL: low shrinkage

47
Q

what are the two manufacturers of concrete in nz

A

Golden bay and Holcim

48
Q

what reinforcement is in concrete

A

steel reinforcing bars

49
Q

why does concrete need reinforcement

A

because concrete alone has low tensile strength and is a brittle material

50
Q

what are the two reinforcement grades

A

Grade 300E, fy = 300MPa (mild steel)
Grade 500E, fy = 500MPa (alloyed or tempered)

51
Q

what are the three grades of grade 500 reinforcement

A

Grade 500L : Low ductility
Grade 500N: normal ductility
Grade 500E: earthquake (high ductility)

52
Q

which 500 grade reinforcement should be used in NZ

A

500E

53
Q

why does concrete crack when exposed to effects of corrosion

A

as steel rusts, its volume increases six-fold and results in spalling of concrete

54
Q

what is the most effective way of preventing corrosion

A

using GFRP (glass fire reinforced polymer) - a non metallic reinforcement. but it is more expensive and not as ductile

55
Q

what is prestressing and what does it do to concrete

A

prestressing prevents corrosion - involves stretching reinforcement at the bottom, tightens and strengthens concrete. However, overtime prestressing can lose some of its force, needs a maintenance program to check force over time.

56
Q

what corrosion preventions are in place for concrete

A

prestressing
using low porosity concrete (SCMs)
galvanized rebar
stainless rebar
cathodic protection
non-metallic reinforcement

57
Q

when are non metallic reinforcements useful

A

in very corrosive environments and in situations where steel cannot be used

58
Q

what is the most common alternative of steel (non metallic reinfocement)

A

GFRP, Glass Fibre Reinforced Polymer

59
Q

what is the tensile strength and youngs modulus of GFRP

A

tensile strength: 500-800 MPa
Young’s modulus: 40-50 GPa

60
Q

why is concrete not really sustainable

A

because cement production releases greenhouse gases (burning fuel and calcining)

61
Q

How to make concrete more sustainable

A
  • using waste woodchip to burn fuel (less landfill)
  • using less cement, and adding more minerals
  • using an alternative binder
62
Q

what is a new kind of concrete being made

A

GPC - geopolymer concrete
less CO2 from cement manufacture

63
Q

what are the environmental issues of portland cement

A

it has an energy intensive production process
1 tonne of cement produced releases ~1 tonne of CO2

64
Q

what is inorganic polymer cement

A

silicon based material + aluminuim based material + alkaline liquid

65
Q

what temperature is inorganic polymer cement mixed at

A

ambient (surrounding air) temperatures

66
Q

what are the environmental benefits of inorganic polymer cement

A
  • 85% reduced carbon dioxide
  • raw ingredients can be a waste such as fly ash and glass
67
Q

what would waste paint be used as in concrete

A

as an admixture - for greater flowability

68
Q

what would glass be used as in concrete

A

as aggregates (however there are issues with bonds) - green star points

69
Q

what would recycled concrete be used as in concrete

A

as an aggregate

70
Q

what would pumice be used as in concrete

A

as a lightweight aggregate

71
Q

what are the benefits of concrete an insulator

A
  • good thermal mass (doesnt heat up in hot weather and doesn’t cool down too much)
  • good insulation
  • less energy needed for heating/cooling
72
Q

what are the benefits of concrete in fire

A

concrete does not melt or burn

73
Q

what are the tree main aims for a mix design concrete

A
  • economic
  • ensuring fresh concrete useable
  • meeting requirements for hardened concret
74
Q

what are the meeting requirements for hardened concrete

A

strength, durability, colour, weight… etc (its about achieving the correct balance)

75
Q

minimising cement =

A

minimising cost, reducing actual strength

76
Q

cost of concrete includes

A
  • plant manufacturing (size, scale, level of automation)
  • equipment, colour
  • transport
  • materials
77
Q

what is workability of concrete

A

how easy the concrete is to use

78
Q

which type of concrete is easier to use

A

“runnier concrete”

79
Q

more water =

A

more workable

80
Q

lower water:cement ratio =

A

more strength

81
Q

what is strength of concrete linked to

A

volume of voids in concrete

82
Q

what is the main specification for concrete

A

strength. Compressive strength fc (measured in MPa)

83
Q

what is one of the key parameters to describe a concrete mix

A

water:cement ratio

84
Q

how is water:cement ratio calculated

A

water/weight of cement

85
Q

what two very large effects does water:cement ratio have on concrete

A

strength and durability

86
Q

is strength inversely or directly proportional to weight/cement

A

inversely proportional

87
Q

what s the ratio of w/c required for complete hydration

A

0.42

88
Q

whats the importance of w/c for aggregates

A

the water added/absorbed by aggregates

89
Q

smaller surface area for aggregates =

A

more absorption by weight

90
Q

how to calculate water absorbed by fine aggregates

A

% absorbed x % of fine aggregates in mix x weight/volume

91
Q

how to calculate water absorbed by coarse aggregates

A

% absorbed x % of coarse aggregates in mix x weight/volume

92
Q

how to calculate total free water in mix

A

total water content - water absorbed by fine aggregates - water absorbed by coarse aggregates

93
Q

how to calculate actual w/c

A

total free water in mix / cement weight

STRCTENG 201 (3 decks)

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