TU 06 : aggregates Flashcards
what is the cheapest ingredient in concrete and asphalt
aggregates
what are the two types of aggregates
- natural sources
- artificial sources
what are the types of natural sourced aggregates
- naturally occuring (sand)
- artificially made (crushed, sorted, washed)
what are the different types of artificially sourced aggregates
- industrial waste
- reclaimed (recycled concrete)
- specially produced (lightweight)
what are the different aggregate properties
- shape
- texture
- density
- size and gradation
- moisture content
what does shape of aggregate affect
workability and bond characterisitcs
what does texture affect
mechanical properties of concrete
what does density affect
affects weight to volume relationships in mix proportioning
what does size and gradation affect
affects paste performance
what does moisture content affect
affects the water to cement ratio
what are the properties of a round shape aggregate
- low surface area to volume ratio
- fewer edges, discontinuities, corners
what is the influence of a round shape on concrete
- less inter-aggregate contact in mixing - more workability
- less surface bonding are - decreased mechanical properties ( ie strength)
- less cementitious material required to fully coat the surface
angular shape properties
- high surface area to volume ratio
- many edges, discontinuities, faces
influence of angular shape on concrete
- more inter-aggreagate contact - less workability
- more surface bonding area - increased mechanical properties (ie strength)
- more cementitious material required to fully coat the surface
- irregular shapes may induce stress concentrations
influence of smooth surfaces on concrete
- provides more workability (less friction)
- less surface-cement paste interlock with smooth surfaces which lowers strength
what are the rough surface influences on concrete
-less workable in fresh concrete bc more friction
- more surface-cement paste interlock with rough surfaces (higher strength)
choice of aggregates will determine if concrete is lightweight regular or heavy. What are the different density based on?
- apparent specific gravity (ASG) - density based on slid material - no pores
- bulk specific gravity (BSG) - density based on solid + pore phases (including water)
the max size of an aggregate is determine by what
its specific application
the larger the maximum aggregate …
the smaller amount of paste required
what do gradation curves record
the amount of material retained on the individual sieves
what do you plot to create gradation curves
plot cumulative percent passing (y) vs sieve size (x)
what are gradation curves
- putting aggregate sample through progressively smaller sieves
what are the different gradation
- linear
- uniform
- gap graded
what does gradation influence
- has an influence on the presence of air voids and hence the cement paste requirements
how does water get in and affect moisutre content
- aggregates are not perfectly dense
- they are capable of absorbing water into tiny pores in the surface
different types of water ratio
- ovendry - no potential absorption
- air dry < potential absorption
- saturated surface dry = to potential absorption
- wet - greater than absorption
which types of water contents are variable
air dry and wet
oven dry and saturated surface dry only exist at one point and are constant
what is absorption
amount of moisture an aggregate can absorb expressed as a percentage of the dry unit weigth
which is the ideal water content
saturated surface dry aggregates (bc no excess or no lack of water)
what does using a wet aggregate do
the excess water is immediately available
what does using air dry aggregates mean
the amount of water absorbed is a function of time
what is the ideal moisture state of aggregates to use in mixing?
what are the advantages of using a wet vs air dry state? what is changed in mixture design when using air-dry or wet aggregates?
- ideally use saturated surface dry becasue no excess or lack of water but this state is difficult to obtain
- with wet aggregates, the excess water is immediately available (provides a respresentative slump value)
- with air-dry aggregates the amount of water absorbed is a function of time (does not provide a representative slump value)
- in design goal is to adjust the mix water such that the moisutre absorbed by or shed by the aggregates is accountedd for and the amount of water specified in the concrete mix design is unaffected
what happens to the compressive strength of concrete if it is allowed to dry before testing? why do compressive strength testing standards specify concrerte to be completely saturated during testing?
- compressive strength concrete increases when allowed to dry
- compressive testing standards specify saturated state becasue it is a worst- case scenario (bc its weak)
- saturated is the easiest moisture content level to reach when testing you simply soak the specimen in water
why do we use aggregates?
- inexpensive filler
- dimensional stability (shrinkage/expansion control)
- durable (Wear resistance, chemical attack)
what is the maximum aggregate size
smallest sieve opening through which the entre aggregate sample will pass
what is nominal maximum aggregate size
ASTM allows 5-10% retention on the largest sieve size
how do you store aggregates
- avoid segregation
- do not store in tall cone-shaped piles
- wary of windy conditions
- do not let aggregates run down slope
what is the fineness modulus
describes the grading curve of a fine aggregate
- usefull to check uniformity of frading between aggregate samples
why isnt Fineness modulus not used for coarse aggregates
- less relevant
- high values
- low sensitivity
- fine sand increases workability
what is effective absorption
amount of water needed to bring the aggregate from the AD state to the SSD state
how to adjust moisture content
goal = adjust the mix water so that
- the moisture absorbed by or shedded by the aggregates is accounted for
- amount of water specified in the concrete mix design is unaffected
what is the fuction of time used for air dry water content of aggregate
30 min absorption
what is soundness
aggregate is unsound if volume changes that accompany environmental changes lead to deterioration of the concrete
what are the two forms of freeze-thaw deterioration due to aggregate unsoundness
- pop-outs
- D-cracking
