TU 06 : aggregates

Question 1

what is the cheapest ingredient in concrete and asphalt

Answer 1

aggregates

Question 2

what are the two types of aggregates

Answer 2

• natural sources
• artificial sources

Question 3

what are the types of natural sourced aggregates

Answer 3

• naturally occuring (sand)
• artificially made (crushed, sorted, washed)

Question 4

what are the different types of artificially sourced aggregates

Answer 4

• industrial waste
• reclaimed (recycled concrete)
• specially produced (lightweight)

Question 5

what are the different aggregate properties

Answer 5

• shape
• texture
• density
• size and gradation
• moisture content

Question 6

what does shape of aggregate affect

Answer 6

workability and bond characterisitcs

Question 7

what does texture affect

Answer 7

mechanical properties of concrete

Question 8

what does density affect

Answer 8

affects weight to volume relationships in mix proportioning

Question 9

what does size and gradation affect

Answer 9

affects paste performance

Question 10

what does moisture content affect

Answer 10

affects the water to cement ratio

Question 11

what are the properties of a round shape aggregate

Answer 11

• low surface area to volume ratio
• fewer edges, discontinuities, corners

Question 12

what is the influence of a round shape on concrete

Answer 12

• 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

Question 13

angular shape properties

Answer 13

• high surface area to volume ratio
• many edges, discontinuities, faces

Question 14

influence of angular shape on concrete

Answer 14

• 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

Question 15

influence of smooth surfaces on concrete

Answer 15

• provides more workability (less friction)
• less surface-cement paste interlock with smooth surfaces which lowers strength

Question 16

what are the rough surface influences on concrete

Answer 16

-less workable in fresh concrete bc more friction
- more surface-cement paste interlock with rough surfaces (higher strength)

Question 17

choice of aggregates will determine if concrete is lightweight regular or heavy. What are the different density based on?

Answer 17

• apparent specific gravity (ASG) - density based on slid material - no pores
• bulk specific gravity (BSG) - density based on solid + pore phases (including water)

Question 18

the max size of an aggregate is determine by what

Answer 18

its specific application

Question 19

the larger the maximum aggregate …

Answer 19

the smaller amount of paste required

Question 20

what do gradation curves record

Answer 20

the amount of material retained on the individual sieves

Question 21

what do you plot to create gradation curves

Answer 21

plot cumulative percent passing (y) vs sieve size (x)

Question 22

what are gradation curves

Answer 22

• putting aggregate sample through progressively smaller sieves

Question 23

what are the different gradation

Answer 23

• linear
• uniform
• gap graded

Question 24

what does gradation influence

Answer 24

• has an influence on the presence of air voids and hence the cement paste requirements

Question 25

how does water get in and affect moisutre content

Answer 25

• aggregates are not perfectly dense
• they are capable of absorbing water into tiny pores in the surface

Question 26

different types of water ratio

Answer 26

• ovendry - no potential absorption
• air dry < potential absorption
• saturated surface dry = to potential absorption
• wet - greater than absorption

Question 27

which types of water contents are variable

Answer 27

air dry and wet

oven dry and saturated surface dry only exist at one point and are constant

Question 28

what is absorption

Answer 28

amount of moisture an aggregate can absorb expressed as a percentage of the dry unit weigth

Question 29

which is the ideal water content

Answer 29

saturated surface dry aggregates (bc no excess or no lack of water)

Question 30

what does using a wet aggregate do

Answer 30

the excess water is immediately available

Question 31

what does using air dry aggregates mean

Answer 31

the amount of water absorbed is a function of time

Question 32

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?

Answer 32

• 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

Question 33

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?

Answer 33

• 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

Question 34

why do we use aggregates?

Answer 34

• inexpensive filler
• dimensional stability (shrinkage/expansion control)
• durable (Wear resistance, chemical attack)

Question 35

what is the maximum aggregate size

Answer 35

smallest sieve opening through which the entre aggregate sample will pass

Question 36

what is nominal maximum aggregate size

Answer 36

ASTM allows 5-10% retention on the largest sieve size

Question 37

how do you store aggregates

Answer 37

• avoid segregation
• do not store in tall cone-shaped piles
• wary of windy conditions
• do not let aggregates run down slope

Question 38

what is the fineness modulus

Answer 38

describes the grading curve of a fine aggregate
- usefull to check uniformity of frading between aggregate samples

Question 39

why isnt Fineness modulus not used for coarse aggregates

Answer 39

• less relevant
• high values
• low sensitivity
• fine sand increases workability

Question 40

what is effective absorption

Answer 40

amount of water needed to bring the aggregate from the AD state to the SSD state

Question 41

how to adjust moisture content

Answer 41

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

Question 42

what is the fuction of time used for air dry water content of aggregate

Answer 42

30 min absorption

Question 43

what is soundness

Answer 43

aggregate is unsound if volume changes that accompany environmental changes lead to deterioration of the concrete

Question 44

what are the two forms of freeze-thaw deterioration due to aggregate unsoundness

Answer 44

• pop-outs
• D-cracking