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CIVE 202 construction materials > 09 - concrete general > Flashcards

09 - concrete general Flashcards

1
Q

cement vs concrete

A

cement is one of the ingredients in concrete
- cement is completely converted into something else there is no more cement in concrete

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2
Q

concrete hardens as it dries T or F

A

false!
- cement reacts with water to form concrete and that reaction gives it strength
- so concrete gets weaker when it dries

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3
Q

concrete wont harden under water T or F

A

cement reacts with water - it is hydraulic so it hardens under water

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4
Q

concrete last forever T or F

A

false!
can only last if its in the right environment

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5
Q

when do you get a compression strength in concrete

A

never
- compression strength only when it fails in compression
- concrete is brittle so never fails in compression

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6
Q

how rare is structural failure

A

rare
- most are do to failure mechanisms with deterioration (resistance goes down)

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7
Q

what is concrete

A
  • particulate composite consisting of dispersed particles (aggregates) surrounded and bound together by a continuous matrix (hydrated cement paste)
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8
Q

what are the three distinct phases of concrete

A
  • hydrated cement paste (HCP)
  • aggregates
  • interfacial transition zone (ITZ)
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9
Q

ingredients of concrete

A
  • portland cement
  • water
  • aggregates
  • admixtures
  • SCM’s
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10
Q

paste formula
mortar formula
concrete formula

A
  • paste = cement + water
  • mortar = paste + fine aggregate
  • concrete. = mortar + coarse aggregate
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11
Q

tricalciums silicate formula and Fraction

A

C3S , 55

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12
Q

dicalcium silicate formulav and Fraction

A

C2S , 20

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13
Q

tricalcium aluminate formula and Fraction

A

C3A, 10

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14
Q

tetracalcium aluminoferrite and Fraction

A

C4AF, 8

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15
Q

gypsum formula and Fraction

A

CSH2, 5

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16
Q

what is the primary effect of tricalcium silicate on coement

A

early strength

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17
Q

what is the primary effect of dicalcium silicate on cement

A

utlimate strength

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18
Q

what is the primary effect of tricalcium aluminate on concrete

A

flux fast set and sulfate reactive

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19
Q

what is the primary effect of gypsum on concrete

A

prevents fast set

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20
Q

what is the primary effect of tetracalcium aluminoferrite in concrete

A

flux

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21
Q

chemical reaction of calium silicates

A

calcium silicates + water –> calcium silicate hydrate + calcium hydroxide

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22
Q

tricalcium aluminate chemical formula reaction

A

triaclcium aluminate + gypsum + water –> ettringite

ettringite + tricalcium aluminate + water -> monosulfoaluminate

23
Q

why dont we want to add water at the beginning of the hydration of cement paste

A
  • before microstructural skeleton, water added increases volume which makes more space = weaker therefore we want to minimize water
24
Q

why do we want to add water after the microstructural skeleton during the hydration of cement paste

A

after the microst. skeleton, the water cant increase the volume so itll just sit on the surface

25
Q

what is the microstructural skeleton

A

cement grains are bonded together and cant move

26
Q

pros and cons of CSH (calicum silicate hydrate)

A

pros: provides major cohesive force but intrinsically weak

cons: very insoluble (water in micropores doesnt freeze and has low mobility so low permeability)

27
Q

calcium hydroxide strength and durability

A

strength: contributes by reducing porosity. Cleavage may limit strength of high strength pastes

durability: blocks capillary pores and hence lowers permeability
- slowly leached by water, which causes efflorescence and increases permeability

28
Q

C4ASH12 impact on durability

A

causes sulfate attack by reforming ettringite and causing expansion

29
Q

unhydrated cement impact on durability

A

renewed hydration may cause autogeneous healing of internal microcracks

30
Q

capillary pores impact on strength and durability

A

strength: total porosity is the major factor influencing strength

durability: porosity influences permeability and diffusivity. Large pores increase water flow through concrete

31
Q

what type of pores increase water flow throuhg concrete

A

large pores

32
Q

what is highly modified in the vicinity of inclusions (aggregate fiber and steel)

A

the microstructure of hydrated cement paste

33
Q

primary features of hcp

A
  • increases porosity
  • less unreacted cement
34
Q

what is wall effect

A

inability of cement particles to pack efficiently in close proximity to the embedment
- raises local W/C ratio at the interface
- furtherrise in W/C due to localized bleeding

35
Q

what is the wall effect also know as

A

ITZ (Interfacial Transition Zone)

36
Q

why do we add mineral admixtures to the concrete

A
  • Replace cement
  • improve workability
  • enhance durability `
37
Q

what are the three main categories of admixtures

A
  • pozzolanic
  • cementitious
  • non- reactive
38
Q

what are the two types of supplenmentary cementing materials (SCM)

A
  • cementitious
  • pozzolanic
39
Q

what is the most common type of cecemtitious of SCM

A
  • blast furnace slag (waste from steel blast furnaces quenched in water to form glass)
40
Q

most common type of SCM pozzolanic materials

A
  • fly ash: ash from burning coal
  • silica fume: dust from the manufacture of silicon
  • calcined clay: modified natural clays
  • chemically react with calcium hydroxide and water to produce calcium silicate hydrate
  • doesnt react with water alone
41
Q

what type of particles are better in cement

A

small particles: improve durability and improve strength (wall effect: fine particle can go fill in the holes that the cement grains cant)

42
Q

benefits of SCM’s

A
  • economics
  • environmentally friendly
  • lower heat of hydration
  • improved durability
  • improved workability
  • increased strength
43
Q

types of chemical admixtures

A
  • water reducing
  • air entraining
  • set retarding
  • set accelerating
44
Q

how do water reducing admixtures work

A

surfactants which adsorb at the solid-water interface and prevent the cement grains from flocculating in water

45
Q

how are water reducing admixtrues used

A
  • plasticizer: increase in workability at the same W/C ratio
  • water reducer: decrease in water cement ratio at same workability
  • used for tightly packed rebar and pours requiring pumps/ chutes/ tremies
46
Q

how do air entraining admixtures work

A

surfactants which act at the air-water interface, causing water to foam during mixing

47
Q

what are the advantages and disadvantages of air entraining admixtures

A

advantages
- improves freeze/thaw resistance
- improves sulfate resistance
- improves workability (higher slump)
- reduces segregation

disadvantes:
- reduced strength (but without it the cement wont survive in environment)

48
Q

important characterisitcs of air entraining admixtures

A
  • 4-7%
  • must be dispersed but close
  • must be non-interconnected
49
Q

what do set retarding admixtures do

A

delay the setting of the concrete by slowing down the early hydration reactions and thus the rate of early strength dev.

50
Q

characteristics of set retarding admixtures

A
  • do not affect ultimate strength
  • do not significantly affect time to ultimate strength
  • may not affect slump loss
51
Q

what are set retarcing admixtures commonly used for

A
  • hot weather conditions
  • long delays between mixing and casting
52
Q

what do set accelerating admixxtures do

A

accelerate the setting of concrete by speeding up the early hydration reactions and thus the rate of early strength dev

53
Q

characteristics of set accelerating admixtures

A
  • do not affect ultimate strength
  • do not significantly affect time to ultimate strength
  • may affect slum loss
54
Q
A
  • cold wheather conditions
  • early form removal
  • repairs

CIVE 202 construction materials (10 decks)

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