Concrete - General Flashcards
What do these letters designate?:
C
S
H
A
S(bar)
C = CaO
S =SiO2
H = H2O
A = Al2O3
S(bar) = SO3
What do these chemical compounds designate?:
Tricalcium silicate
Dicalcium silicate
Tricalcium aluminate
Tetracalcium aluminoferrite
Gypsum
C3S
C2S
C3A
C4AF
CSH2
What is concrete and what are its 3 phases?
- Aggregates mixed with hydrated cement paste
- HCP (hydrated cement paste)
- ITZ (interfacial transition zone)
- Aggregates
What are these terms referring to:
Paste
Mortar
Concrete
- Cement + water
- Paste + fine aggregates (sand)
- Gravel + aggregates (gravel)
How do these chemical compounds affect concrete?
Tricalcium silicate
Dicalcium silicate
Tricalcium aluminate
Tetracalcium aluminoferrite
Gypsum
C3S = early strength
C2S = long-term strength (ulitmate strength)
C3A = flux, reduces temperature
C4AF = flux, reduces temperature
CSH2 = prevents concrete from setting too fast
How does C3S produce more early strength compared to C2S?
- C3S creates more CH which contributes to early strength
- C2S creates more CSH which contributes to ultimate strength
What is the reaction of calcium silicates?
CS + H2O = CSH + CH + Heat
What is the reaction of C3A?
- C3A + CSH2 (gypsum) + H2O = Ettringite + Heat
- Ettringite + C3A + H2O = Monosulfaluminate + Heat
What is a flux?
- Agent reducing temperature required to make concrete
- C3A, C4AF
Why can’t C3A react alone?
- C3A reacts too quickly
- Gypsum is combined with C3A to create ettringite and slow down the reaction
What is ettringite and what is its role?
- Ettringite is a byproduct from the reaction of C3A and CSH2
- Slows down the reaction
- Reacts with more C3A to create monosulfaluminate
What is the role of C4AF?
- Flux
- Iron (Fe) in C4AF makes concrete grey
What is the hydration process of cement paste?
- Dormant period
- Setting
- Hardening
How does concrete gain its strength (unrelated to early strength of ultimate strength)?
- Through hydration of the cement paste
What is the dormant period?
- Cement crystals are not set
- Cement crystals in suspension with water
- Adding water will affect w/c ratio
What is the initial set period?
- Cement crystals come into contact and form one big piece (matrix)
- Adding water will not affect w/c ratio
What is the curing or hardening period?
- Concrete continues to hydrate is there is water left
- Adding water will not affect w/c ratio
When can you add water to cement without affecting strength?
- After initial set when cement crystals form a matrix
When can you not add water without affecting strength and why?
- During the dormant period
- Cement crystals are not set yet
- Water will separate cement crystals even further
- More space between cement crystals reduces strength
What is porosity?
Overall proportion of void space in a material
What is permeability?
Property which allows fluids to pass through a material
Porosity vs permeability
- High porosity does not equal high permeability
- You can have high porosity, but if the pores are not connected to the surface of the material, it is impermeable
- Conversely, you can have high permeability with a low porosity material
What is the interfacial transition zone?
Zone where tiny cement crystals come into contact with big aggregate particles (wall)
Why is strength lower at the ITZ?
- Wall effect : Crystals cannot compact efficiently against a wall compared in open space
- ITZ becomes a high porosity zone and high local w/c ratio
- Reduces strength at ITZ
What are the 5 types of concrete?
- GU (10)
- MS or MH (20)
- HE (30)
- LH (40)
- HS (50)
What is concrete type GU (10)?
- General use concrete
- Nothing special
What is concrete type MS or MH (20)?
- Moderate sulfate resistant or moderate heat of hydration
- Moderately low C3S and C3A content
- Produces less heat
- Used when exposed to sulfates (low moisture and arid regions like California or Nevada)
What is concrete type HE (30)?
- High early strength concrete
- Used for highways and buildings (1st floor has to be strong enough to support 2nd floor)
- Produces a lot of heat
What is concrete type LH (40)?
- Low heat of hydration concrete
- Produces very low heat
- Low C3A and C3S content
- Used for high surface area to low volume structures (dam)
What is concrete type HS (50)?
- Sulfate resistant concrete
- Lowest C3A and C2S content
What are mineral admixtures or SCMs?
- Supplementary cement materials
- Replace cement
- Improve durability
- Improve workability
- Lower heat of hydration
- Cheaper
- Lowers heat of hydration
- Increases strength
What are the 3 categories of SCMs?
- Pozzolanic
- Cementitious
- Non-reactive (fillers to reduce price)
What is the one cementitious SCM?
Blast furnace slag
Where does blast furnace slag come from and what does it do?
- Waste from steel furnaces
- Makes concrete stronger by producing more CSH
- Will react with water alone
What are the types of pozzolan SCMs and what do they do?
- Silica fume
- Fly ash
- Calcined clay
- Pozzolan SCMs convert CH into CSH
- Increases strength (more CSH) and reduces permeability (less CH)
What is silica fume and what does it do?
- Comes from silicon dust
- Spherical particles makes workability easier
What is fly ash and what does it do?
- Ash from burning coal
- Spherical particles makes workability easier
- Fly ash particles are much smaller than silica fume particles
- Fly ash fills in voids at ITZ
- Increases strength
What is calcined clay?
- Modified clay particles
What is the difference between pozzolanic and cementitious SCM?
- Cementitious only needs water to react
- Pozzolanic needs water and CH to react
- CH comes from C2S and C3S in cement reaction
What are chemical admixtures?
Materials added to concrete to change its physical properties
What are the types of chemical admixtures?
- Air entraining
- Set accelerating
- Set reducing
- Water reducing
What is air entraining admixture?
Intentionally add stable air bubbles into concrete mix
How does air entraining work?
- Air bubbles have a protective layer of hydrophobic particles
- Prevents air bubbles from collapsing
Why do we use air entraining admixture?
- Improves freeze-thaw resistance
- Improves sulfate resistance
- Improves workability
BUT - Reduces strength
What is the difference between entrained air and entrapped air?
- Entrained air is intentionally added into concrete mix to resist freeze-thaw
- Entrapped air is accidentally added during mixing
What is water reducing admixture?
- Cement particles are stuck together and create a structure with empty space in the middle (donut)
- Water fills in the empty space
- Prevents easy mixing
- Admixture is used to eliminate cement particle attraction and breaks structure
What are the effects of water reducing admixture?
- Increases workability
- Reduce w/c ratio (increase strength)
What does set accelerating admixture do?
1.Reduces time for concrete to set
2. Used in low temperature conditions
What does set retarding admixture do?
- Increases time for concrete to set
- Used in high temperature conditions or long distance transportation
Why are pozzolanic supplementary cementing materials
desirable as a Portland cement replacement when
designing a concrete mix for durability?