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Concrete - General Flashcards

1
Q

What do these letters designate?:
C
S
H
A
S(bar)

A

C = CaO
S =SiO2
H = H2O
A = Al2O3
S(bar) = SO3

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2
Q

What do these chemical compounds designate?:
Tricalcium silicate
Dicalcium silicate
Tricalcium aluminate
Tetracalcium aluminoferrite
Gypsum

A

C3S
C2S
C3A
C4AF
CSH2

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3
Q

What is concrete and what are its 3 phases?

A
  1. Aggregates mixed with hydrated cement paste
  2. HCP (hydrated cement paste)
  3. ITZ (interfacial transition zone)
  4. Aggregates
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4
Q

What are these terms referring to:
Paste
Mortar
Concrete

A
  1. Cement + water
  2. Paste + fine aggregates (sand)
  3. Gravel + aggregates (gravel)
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5
Q

How do these chemical compounds affect concrete?
Tricalcium silicate
Dicalcium silicate
Tricalcium aluminate
Tetracalcium aluminoferrite
Gypsum

A

C3S = early strength
C2S = long-term strength (ulitmate strength)
C3A = flux, reduces temperature
C4AF = flux, reduces temperature
CSH2 = prevents concrete from setting too fast

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6
Q

How does C3S produce more early strength compared to C2S?

A
  1. C3S creates more CH which contributes to early strength
  2. C2S creates more CSH which contributes to ultimate strength
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7
Q

What is the reaction of calcium silicates?

A

CS + H2O = CSH + CH + Heat

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8
Q

What is the reaction of C3A?

A
  1. C3A + CSH2 (gypsum) + H2O = Ettringite + Heat
  2. Ettringite + C3A + H2O = Monosulfaluminate + Heat
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9
Q

What is a flux?

A
  1. Agent reducing temperature required to make concrete
  2. C3A, C4AF
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10
Q

Why can’t C3A react alone?

A
  1. C3A reacts too quickly
  2. Gypsum is combined with C3A to create ettringite and slow down the reaction
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11
Q

What is ettringite and what is its role?

A
  1. Ettringite is a byproduct from the reaction of C3A and CSH2
  2. Slows down the reaction
  3. Reacts with more C3A to create monosulfaluminate
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12
Q

What is the role of C4AF?

A
  1. Flux
  2. Iron (Fe) in C4AF makes concrete grey
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13
Q

What is the hydration process of cement paste?

A
  1. Dormant period
  2. Setting
  3. Hardening
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14
Q

How does concrete gain its strength (unrelated to early strength of ultimate strength)?

A
  1. Through hydration of the cement paste
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15
Q

What is the dormant period?

A
  1. Cement crystals are not set
  2. Cement crystals in suspension with water
  3. Adding water will affect w/c ratio
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16
Q

What is the initial set period?

A
  1. Cement crystals come into contact and form one big piece (matrix)
  2. Adding water will not affect w/c ratio
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17
Q

What is the curing or hardening period?

A
  1. Concrete continues to hydrate is there is water left
  2. Adding water will not affect w/c ratio
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18
Q

When can you add water to cement without affecting strength?

A
  1. After initial set when cement crystals form a matrix
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19
Q

When can you not add water without affecting strength and why?

A
  1. During the dormant period
  2. Cement crystals are not set yet
  3. Water will separate cement crystals even further
  4. More space between cement crystals reduces strength
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20
Q

What is porosity?

A

Overall proportion of void space in a material

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21
Q

What is permeability?

A

Property which allows fluids to pass through a material

22
Q

Porosity vs permeability

A
  1. High porosity does not equal high permeability
  2. You can have high porosity, but if the pores are not connected to the surface of the material, it is impermeable
  3. Conversely, you can have high permeability with a low porosity material
23
Q

What is the interfacial transition zone?

A

Zone where tiny cement crystals come into contact with big aggregate particles (wall)

24
Q

Why is strength lower at the ITZ?

A
  1. Wall effect : Crystals cannot compact efficiently against a wall compared in open space
  2. ITZ becomes a high porosity zone and high local w/c ratio
  3. Reduces strength at ITZ
25
Q

What are the 5 types of concrete?

A
  1. GU (10)
  2. MS or MH (20)
  3. HE (30)
  4. LH (40)
  5. HS (50)
26
Q

What is concrete type GU (10)?

A
  1. General use concrete
  2. Nothing special
27
Q

What is concrete type MS or MH (20)?

A
  1. Moderate sulfate resistant or moderate heat of hydration
  2. Moderately low C3S and C3A content
  3. Produces less heat
  4. Used when exposed to sulfates (low moisture and arid regions like California or Nevada)
28
Q

What is concrete type HE (30)?

A
  1. High early strength concrete
  2. Used for highways and buildings (1st floor has to be strong enough to support 2nd floor)
  3. Produces a lot of heat
29
Q

What is concrete type LH (40)?

A
  1. Low heat of hydration concrete
  2. Produces very low heat
  3. Low C3A and C3S content
  4. Used for high surface area to low volume structures (dam)
30
Q

What is concrete type HS (50)?

A
  1. Sulfate resistant concrete
  2. Lowest C3A and C2S content
31
Q

What are mineral admixtures or SCMs?

A
  1. Supplementary cement materials
  2. Replace cement
  3. Improve durability
  4. Improve workability
  5. Lower heat of hydration
  6. Cheaper
  7. Lowers heat of hydration
  8. Increases strength
32
Q

What are the 3 categories of SCMs?

A
  1. Pozzolanic
  2. Cementitious
  3. Non-reactive (fillers to reduce price)
33
Q

What is the one cementitious SCM?

A

Blast furnace slag

34
Q

Where does blast furnace slag come from and what does it do?

A
  1. Waste from steel furnaces
  2. Makes concrete stronger by producing more CSH
  3. Will react with water alone
35
Q

What are the types of pozzolan SCMs and what do they do?

A
  1. Silica fume
  2. Fly ash
  3. Calcined clay
  4. Pozzolan SCMs convert CH into CSH
  5. Increases strength (more CSH) and reduces permeability (less CH)
36
Q

What is silica fume and what does it do?

A
  1. Comes from silicon dust
  2. Spherical particles makes workability easier
37
Q

What is fly ash and what does it do?

A
  1. Ash from burning coal
  2. Spherical particles makes workability easier
  3. Fly ash particles are much smaller than silica fume particles
  4. Fly ash fills in voids at ITZ
  5. Increases strength
38
Q

What is calcined clay?

A
  1. Modified clay particles
39
Q

What is the difference between pozzolanic and cementitious SCM?

A
  1. Cementitious only needs water to react
  2. Pozzolanic needs water and CH to react
  3. CH comes from C2S and C3S in cement reaction
40
Q

What are chemical admixtures?

A

Materials added to concrete to change its physical properties

41
Q

What are the types of chemical admixtures?

A
  1. Air entraining
  2. Set accelerating
  3. Set reducing
  4. Water reducing
42
Q

What is air entraining admixture?

A

Intentionally add stable air bubbles into concrete mix

43
Q

How does air entraining work?

A
  1. Air bubbles have a protective layer of hydrophobic particles
  2. Prevents air bubbles from collapsing
44
Q

Why do we use air entraining admixture?

A
  1. Improves freeze-thaw resistance
  2. Improves sulfate resistance
  3. Improves workability
    BUT
  4. Reduces strength
45
Q

What is the difference between entrained air and entrapped air?

A
  1. Entrained air is intentionally added into concrete mix to resist freeze-thaw
  2. Entrapped air is accidentally added during mixing
46
Q

What is water reducing admixture?

A
  1. Cement particles are stuck together and create a structure with empty space in the middle (donut)
  2. Water fills in the empty space
  3. Prevents easy mixing
  4. Admixture is used to eliminate cement particle attraction and breaks structure
47
Q

What are the effects of water reducing admixture?

A
  1. Increases workability
  2. Reduce w/c ratio (increase strength)
48
Q

What does set accelerating admixture do?

A

1.Reduces time for concrete to set
2. Used in low temperature conditions

49
Q

What does set retarding admixture do?

A
  1. Increases time for concrete to set
  2. Used in high temperature conditions or long distance transportation
50
Q

Why are pozzolanic supplementary cementing materials
desirable as a Portland cement replacement when
designing a concrete mix for durability?

Construction (16 decks)

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