09 - concrete general Flashcards
cement vs concrete
cement is one of the ingredients in concrete
- cement is completely converted into something else there is no more cement in concrete
concrete hardens as it dries T or F
false!
- cement reacts with water to form concrete and that reaction gives it strength
- so concrete gets weaker when it dries
concrete wont harden under water T or F
cement reacts with water - it is hydraulic so it hardens under water
concrete last forever T or F
false!
can only last if its in the right environment
when do you get a compression strength in concrete
never
- compression strength only when it fails in compression
- concrete is brittle so never fails in compression
how rare is structural failure
rare
- most are do to failure mechanisms with deterioration (resistance goes down)
what is concrete
- particulate composite consisting of dispersed particles (aggregates) surrounded and bound together by a continuous matrix (hydrated cement paste)
what are the three distinct phases of concrete
- hydrated cement paste (HCP)
- aggregates
- interfacial transition zone (ITZ)
ingredients of concrete
- portland cement
- water
- aggregates
- admixtures
- SCM’s
paste formula
mortar formula
concrete formula
- paste = cement + water
- mortar = paste + fine aggregate
- concrete. = mortar + coarse aggregate
tricalciums silicate formula and Fraction
C3S , 55
dicalcium silicate formulav and Fraction
C2S , 20
tricalcium aluminate formula and Fraction
C3A, 10
tetracalcium aluminoferrite and Fraction
C4AF, 8
gypsum formula and Fraction
CSH2, 5
what is the primary effect of tricalcium silicate on coement
early strength
what is the primary effect of dicalcium silicate on cement
utlimate strength
what is the primary effect of tricalcium aluminate on concrete
flux fast set and sulfate reactive
what is the primary effect of gypsum on concrete
prevents fast set
what is the primary effect of tetracalcium aluminoferrite in concrete
flux
chemical reaction of calium silicates
calcium silicates + water –> calcium silicate hydrate + calcium hydroxide
tricalcium aluminate chemical formula reaction
triaclcium aluminate + gypsum + water –> ettringite
ettringite + tricalcium aluminate + water -> monosulfoaluminate
why dont we want to add water at the beginning of the hydration of cement paste
- before microstructural skeleton, water added increases volume which makes more space = weaker therefore we want to minimize water
why do we want to add water after the microstructural skeleton during the hydration of cement paste
after the microst. skeleton, the water cant increase the volume so itll just sit on the surface
what is the microstructural skeleton
cement grains are bonded together and cant move
pros and cons of CSH (calicum silicate hydrate)
pros: provides major cohesive force but intrinsically weak
cons: very insoluble (water in micropores doesnt freeze and has low mobility so low permeability)
calcium hydroxide strength and durability
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
C4ASH12 impact on durability
causes sulfate attack by reforming ettringite and causing expansion
unhydrated cement impact on durability
renewed hydration may cause autogeneous healing of internal microcracks
capillary pores impact on strength and durability
strength: total porosity is the major factor influencing strength
durability: porosity influences permeability and diffusivity. Large pores increase water flow through concrete
what type of pores increase water flow throuhg concrete
large pores
what is highly modified in the vicinity of inclusions (aggregate fiber and steel)
the microstructure of hydrated cement paste
primary features of hcp
- increases porosity
- less unreacted cement
what is wall effect
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
what is the wall effect also know as
ITZ (Interfacial Transition Zone)
why do we add mineral admixtures to the concrete
- Replace cement
- improve workability
- enhance durability `
what are the three main categories of admixtures
- pozzolanic
- cementitious
- non- reactive
what are the two types of supplenmentary cementing materials (SCM)
- cementitious
- pozzolanic
what is the most common type of cecemtitious of SCM
- blast furnace slag (waste from steel blast furnaces quenched in water to form glass)
most common type of SCM pozzolanic materials
- 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
what type of particles are better in cement
small particles: improve durability and improve strength (wall effect: fine particle can go fill in the holes that the cement grains cant)
benefits of SCM’s
- economics
- environmentally friendly
- lower heat of hydration
- improved durability
- improved workability
- increased strength
types of chemical admixtures
- water reducing
- air entraining
- set retarding
- set accelerating
how do water reducing admixtures work
surfactants which adsorb at the solid-water interface and prevent the cement grains from flocculating in water
how are water reducing admixtrues used
- 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
how do air entraining admixtures work
surfactants which act at the air-water interface, causing water to foam during mixing
what are the advantages and disadvantages of air entraining admixtures
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)
important characterisitcs of air entraining admixtures
- 4-7%
- must be dispersed but close
- must be non-interconnected
what do set retarding admixtures do
delay the setting of the concrete by slowing down the early hydration reactions and thus the rate of early strength dev.
characteristics of set retarding admixtures
- do not affect ultimate strength
- do not significantly affect time to ultimate strength
- may not affect slump loss
what are set retarcing admixtures commonly used for
- hot weather conditions
- long delays between mixing and casting
what do set accelerating admixxtures do
accelerate the setting of concrete by speeding up the early hydration reactions and thus the rate of early strength dev
characteristics of set accelerating admixtures
- do not affect ultimate strength
- do not significantly affect time to ultimate strength
- may affect slum loss
- cold wheather conditions
- early form removal
- repairs
