Concrete-General Flashcards
What is concrete?
Concrete is a particulate composite consisting of dispersed
particles (aggregates) surrounded and bound together by a
continuous matrix (hydrated cement paste)
How is concrete important
Concrete the most heavily used man-made material in the world.
cite the three phase of concrete
-hydrated cement paste (hcp)
-aggregates
-interfacial transition zone (itz)
Ingredients of concrete (4)
- Portland Cement
- Water
- Aggregates
- Admixtures and SCMs
what is paste mortar and concrete
paste=cement+water
mortar=paste+fine aggregate
concrete=mortar+coarse aggregate
important discoveries historically
greeks use lime
greeks use volcanic ash
romans use pottery and bricks as aggregate and ash- concrete hardens under water
dirty limestones (contain high proportions of clay)
Manufacture of Portland Cement
(raw materials (5categories + 5 steps)
Categories: calcium, silicates, aluminium, iron, sulphates)
Raw materials: Limestone, Sand, Clay and Iron Ore
Processing: quarrying, crushing and mixing
Firing: burned and calcined at 450°C (produces Clinker)
Gypsum is added
Grinding(1-100micrometer)
=Portland Cement
why use chains for heat exchanger
use heat exchanger chains to shorten the kiln length
Effect of tricalcium silicate
early strength (C3S)
Effect of dicalcium silicate
Ultimate strength (C2S)
Effect of tricalcium aluminate
Flux, Fast Set, Sulfate Reactive (C3A)
Effect of tetracalcium aluminoferrite
flux (C4AF)
Effect of Gypsum
Prevents Fast Set (CSH2)
Hydration of cement is
Exothermic
Calcium Silicates+Water
Calcium Silicate Hydrate+Calcium Hydroxide
(CSH+CH)
Tricalcium Alumiante + Gypsum + Water
Ettringite
Ettringite + Tricalcium Aluminate + Water
Monosulfoaluminate
Tetracalcium Aluminoferrite + Calcium Hydroxide + Water
Tetracalcium Aluminate Hydrate + Ferric-Aluminum Hydroxide
(Flux)
C-S-H influence
provide major cohesive force but weaker because of microporosity. dry concrete is stronger than wet (van der waals bonds)
very insoluble water in micropores does not freeze and has low mobility (low permeability)
water loss from micropores cause shrinkage on drying and creep on loading at room temp
calcium hydroxide influence
Contributes by reducing porosity. Cleavage may limit strength of high strength pastes
blocks capillary pores and hence lowers permeability. slowly leached by water which causes efflorescence and increases permeability.
is dimensionally stable will restrain c-s-h deformations
C4ASH12 influence
causes sulfate attack by reforming ettringite and causing expansion
unhydrated cement effects
restrains c-s-h deformations
renewed hydration may cause autogeneous healing of internal microcracks
Capillary pores influence on concrete
total porosity is the major factor influencing strength.
fine pores contribute to shrinkage and creep
porosity influences permeability and diffusivity. large pores increase water flow through concrete.
c3s influence
controls normal setting
temp rise during hydration
responsible for early strength
creep and shrinkage
leaching of Ca(OH)2
c3a effects
cause premature stiffening
temp rise during hydration
minor effects on shrink and creep
c2s effects
long term strength
major contribution to shrink and creep
c3a effect
leads to sulfate attack
interfacial transition zone
The microstructure of hcp is highly modified in the vicinity of
inclusions (aggregate, fiber, steel).
Primary features:
- Increased porosity
- Less unreacted cement
wall effect
inability of cement particles to pack efficiently
in close proximity to the embedment.
Raises local W/C ratio at the interface. Further rise in W/C due to localized bleeding.
Standard Portland Cement types
general purpose GU
moderate heat of hydration MH
moderate sulfate resistance MS
high early strength HE
low heat of hydration LH
sulfate resistance HS
GU use
roads, bridges, floors, structures
(everything)
MS or HS use
concrete exposed to sulfates in soil or water
LH and MH types of cement used where
where there is lots of water (dam, dikes, buoys)
what are mineral admixtures
Finely ground solid materials added to the concrete, either as a separate ingredient or blended with the cement.
3 reasons to use admixtures
- Replace cement
- Improve workability
- Enhance durability
3 categories of admixtures
- Pozzolanic
- Cementitious
- Non-reactive
Supplementary cementing materials (SCM)
Mineral admixtures added to the concrete, either as a separate ingredient or blended with the cement, in which part of the cement is replaced by the mineral admixture.
2 types of SCM
- pozzolanic
- cementitious
Blast Furnace Slag
waste from steel blast furnaces, quenched
in water to form glass
Chemically react with water to produce aluminum-substituted calcium silicate hydrate
Will react with water alone
most common types of pozzolanic SCMs
fly ash
silica fume
calcined clay
how do pozzolanic SCMs work
Chemically react with calcium hydroxide and water to produce calcium silicate hydrate.
Will not react with water alone
calcium silicates + water
calcium silicate hydrate + calcium hydroxide
benefits of SCMs(6)
economics
environmentally friendly
lower heat of hydration
improved durability
improved workability
increased strength
SCM replacement rate
Ranges from 5% to 70% of total cementitious material
chemical admixtures
Materials added to the concrete in small amounts, usually dissolved in the mixing water, which bring about a desired effect on the physical properties of the fresh and/or hardened concrete.
Common Types of chemical admixtures (4)
Water Reducing
Air Entraining
Set Retarding
Set Accelerating
water reducing admixture
Surfactants which adsorb at the solid-water interface and prevent
the cement grains from flocculating in water
two types of water reducing admixtures and explain
plasticizer- increase in workability at same w/c ratio
water reducer- decrease in w/c ratio at same workability
air entraining admixtures
act at the air water interface causing water to foam during mixing.
bubbles remain stable and are locked into cement paste during hardening
Advantages (4)
Disdvantage (1)
(air entraining)
Improves freeze/thaw resistance
Improves sulfate resistance
Improves workability (higher slump)
Reduces Segregation
Reduced Strength (10-20%)
set retarding admixtures
Delay the setting of the concrete by slowing down the early hydration reactions and thus the rate of early strength development
properties of set retarding admixtures (5)
does not affect ultimate strength
does not affect time to ultimate strength
may not affect slump loss
used for hot weather conditions
long delays between mixing and casting
set accelerating admixtures
Accelerate the setting of concrete by speeding up the early hydration reactions and thus the rate of early strength development.
