Concrete Flashcards
1
Q
what is the 2nd most used material
A
concrete
2
Q
what is concrete?
A
binder + filler
3
Q
what four part mixture is concrete normally referred to?
A
coarse aggregates (eg gravel) + fine aggregates (eg sand) +(portland) cement + water
4
Q
what is concrete “cement” compared to
A
cake “flour”
5
Q
what are aggregates
A
the main volume and strength
6
Q
what is a binder
A
normally portland cement
7
Q
what is the binders purpose
A
to glue aggregates together
8
Q
what is cement paste composed of
A
cement + water
9
Q
what is mortar paste composed of
A
cement + water + sand
10
Q
what is concrete composed of
A
cement + water + sand + coarse aggregates
11
Q
what is grout composed of
A
cement + water + sand + smaller aggregates and “flowable”
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
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
14
Q
what is the compressive strength of concrete
A
20-70 MPa
15
Q
what is the tensile strength of concrete
A
2-4 MPa
16
Q
what is the modulus of elasticity of concrete
A
20-35 GPa
17
Q
what is the Poisson’s ratio
A
~0.2
18
Q
what is the coef of thermal expansion
A
10 x 10^6 /C
19
Q
what is the density of concrete
A
~2400 kg/m^3
20
Q
what are the uses of concrete
A
beams, columns, walls, floors, pipes, driveways, powerpoles, crash barriers, counter weights, kitchen benches…
21
Q
who were the best known early users of concrete
A
romans
22
Q
what concrete did romans use to use
A
non hydraulic cement
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
24
Q
what is hydraulic cement
A
cement that hardens under water, can harden due to chemical reactions
25
what did romans use for hydraulic cement
volcanic ash (Pozzolana)
26
What is portland cement
calcium silicates + extras
27
what is 3CaO.SiO2 (the chemical formula)
tricalcium silicate
28
what is the weight percent of tricalcium silicate in the concrete mix
45-75 %
29
how is cement manufactured
lime and silica mix heated (1400 - 1600 C) to form clinker (rocks) - then ground to form cement powder
30
what is lime
limestone (calcium carbonate) CaCO3
31
what is the two types of silica used
clay or silt (Si + impurities: Alumino-silicate)
32
what are the two processes of cement manufacturing
wet and dry
33
what are the processes of wet cement manufacturing
lime + silica mixed as a slurry
34
what is the cement manufacturing process used nowadays
dry process
35
what are the processes of dry cement manufacturing
lime + silica mixed as powders.
36
what is the pros and cons of dry cement manufacturing
dustier process, fine particals, more health and safety regulations need to be in place. But it is more efficient.
37
what is the pros and cons of wet cement manufacturing
mechanically easy, energy-intensive process. water must be evaporated before clinker forms
38
what are the wet cement process steps
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
what are the dry cement process steps
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
why is wet cement process energy intensive
kiln requires large amounts of energy, cement production accounts for ~5% of global CO2 emissions
41
why is dry process more environmentally friendly
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
what is hydration
series of chemical reactions that cause cement to harden
43
important aspects of hydration
rate of reaction, heat generated and strength of hydration product
44
what is the target of hydration
to form calcium silicate hydrates (C-S-H) and to provide strength of cement paste
45
what are the two general cement types
GP: General purpose, <10% mineral addition
GB: General blended, >10% mineral addition
46
what are the four specialised cement types
HE: high early strength
LH: low heat
SR: sulphate resistant
SL: low shrinkage
47
what are the two manufacturers of concrete in nz
Golden bay and Holcim
48
what reinforcement is in concrete
steel reinforcing bars
49
why does concrete need reinforcement
because concrete alone has low tensile strength and is a brittle material
50
what are the two reinforcement grades
Grade 300E, fy = 300MPa (mild steel)
Grade 500E, fy = 500MPa (alloyed or tempered)
51
what are the three grades of grade 500 reinforcement
Grade 500L : Low ductility
Grade 500N: normal ductility
Grade 500E: earthquake (high ductility)
52
which 500 grade reinforcement should be used in NZ
500E
53
why does concrete crack when exposed to effects of corrosion
as steel rusts, its volume increases six-fold and results in spalling of concrete
54
what is the most effective way of preventing corrosion
using GFRP (glass fire reinforced polymer) - a non metallic reinforcement. but it is more expensive and not as ductile
55
what is prestressing and what does it do to concrete
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
what corrosion preventions are in place for concrete
prestressing
using low porosity concrete (SCMs)
galvanized rebar
stainless rebar
cathodic protection
non-metallic reinforcement
57
when are non metallic reinforcements useful
in very corrosive environments and in situations where steel cannot be used
58
what is the most common alternative of steel (non metallic reinfocement)
GFRP, Glass Fibre Reinforced Polymer
59
what is the tensile strength and youngs modulus of GFRP
tensile strength: 500-800 MPa
Young's modulus: 40-50 GPa
60
why is concrete not really sustainable
because cement production releases greenhouse gases (burning fuel and calcining)
61
How to make concrete more sustainable
- using waste woodchip to burn fuel (less landfill)
- using less cement, and adding more minerals
- using an alternative binder
62
what is a new kind of concrete being made
GPC - geopolymer concrete
less CO2 from cement manufacture
63
what are the environmental issues of portland cement
it has an energy intensive production process
1 tonne of cement produced releases ~1 tonne of CO2
64
what is inorganic polymer cement
silicon based material + aluminuim based material + alkaline liquid
65
what temperature is inorganic polymer cement mixed at
ambient (surrounding air) temperatures
66
what are the environmental benefits of inorganic polymer cement
- 85% reduced carbon dioxide
- raw ingredients can be a waste such as fly ash and glass
67
what would waste paint be used as in concrete
as an admixture - for greater flowability
68
what would glass be used as in concrete
as aggregates (however there are issues with bonds) - green star points
69
what would recycled concrete be used as in concrete
as an aggregate
70
what would pumice be used as in concrete
as a lightweight aggregate
71
what are the benefits of concrete an insulator
- 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
what are the benefits of concrete in fire
concrete does not melt or burn
73
what are the tree main aims for a mix design concrete
- economic
- ensuring fresh concrete useable
- meeting requirements for hardened concret
74
what are the meeting requirements for hardened concrete
strength, durability, colour, weight... etc (its about achieving the correct balance)
75
minimising cement =
minimising cost, reducing actual strength
76
cost of concrete includes
- plant manufacturing (size, scale, level of automation)
- equipment, colour
- transport
- materials
77
what is workability of concrete
how easy the concrete is to use
78
which type of concrete is easier to use
"runnier concrete"
79
more water =
more workable
80
lower water:cement ratio =
more strength
81
what is strength of concrete linked to
volume of voids in concrete
82
what is the main specification for concrete
strength. Compressive strength fc (measured in MPa)
83
what is one of the key parameters to describe a concrete mix
water:cement ratio
84
how is water:cement ratio calculated
water/weight of cement
85
what two very large effects does water:cement ratio have on concrete
strength and durability
86
is strength inversely or directly proportional to weight/cement
inversely proportional
87
what s the ratio of w/c required for complete hydration
0.42
88
whats the importance of w/c for aggregates
the water added/absorbed by aggregates
89
smaller surface area for aggregates =
more absorption by weight
90
how to calculate water absorbed by fine aggregates
% absorbed x % of fine aggregates in mix x weight/volume
91
how to calculate water absorbed by coarse aggregates
% absorbed x % of coarse aggregates in mix x weight/volume
92
how to calculate total free water in mix
total water content - water absorbed by fine aggregates - water absorbed by coarse aggregates
93
how to calculate actual w/c
total free water in mix / cement weight
