Aggregates

Question 1

What is an aggregate

Answer 1

A granular material used in construction

Question 2

What are the four classifications of aggregate and give approximate measurements

Answer 2

coarse (gravel): >4mm
fine (sand) : 62micrometers to 4mm
silt: 2 micrometre to 60 micrometre
clay: less than 2 micrometre

Question 3

What percentage of the concrete volume is aggregate

Answer 3

60 to 80%

Question 4

What are the two primary aggregates

Answer 4

crushed rock (often igneous, sometimes sedimentary) and sand+gravel

Question 5

What are secondary aggregates

Answer 5

Industrial by-products

Question 6

How is crushed rock made

Answer 6

Removal of unnecessary material on the surface of the rock, the rock is then drilled so an explosive can be placed inside. The exploded rock is then crushed, screened into different sizes and then pieces are then washed.

Question 7

What are marine aggregates

Answer 7

Aggregates deposited by rivers into the seabed

Question 8

What makes marine aggregates worse than land based aggregates in some scenarios

Answer 8

High chloride content which corrodes steel and high shell content which reduces workability.

Question 9

What is the maximum amount of shell percentage content in concrete that is allowed

Answer 9

10%

Question 10

Why are manufactured aggregates typically weaker

Answer 10

They are often high in porosity

Question 11

What are the benefits of manufactured aggregates

Answer 11

Low density due to high porosity which makes the concrete lighter

Question 12

Why are aggregates added to concrete

Answer 12

Add strength and durability to the concrete as well as adding volume for less price and carbon footprint, also control heat release from cement which can cause cracking. Additionally aggregates are more dimensionally stable - they don’t expand or shrink much

Question 13

What is the strength of natural aggregates and cement paste

Answer 13

200MPa, 30-50MPa

Question 14

What are the five shape types of aggregates and which two should be avoided and why

Answer 14

irregular, angular, rounded, flaky, elongated. Flaky and elongated should be avoided as they face on direction, making the concrete weak in that direction

Question 15

What are the five surface texture types

Answer 15

rough, porous, smooth, sharp, crystalline

Question 16

Why is crushed rock better than gravel for high strength concrete

Answer 16

Gravel tends to be smooth while crushed rock is angular. This makes gravel worse at adhering to the cement and reduces stress transfer between cement and aggregate

Question 17

Why is well graded aggregate better

Answer 17

Reduces void space which means more aggregate can be packed in, thus increasing strength

Question 18

What is segregation of concrete

Answer 18

Large concrete sink to the bottom, small ones rise to the top. This makes the strength vary at different parts of the concrete

Question 19

What causes alkali aggregate reaction

Answer 19

Reactive silica and carbonate react with alkaline concrete pour solution to create a gel that absorbs water and expands, causing cracking

Question 20

What is the ITZ

Answer 20

Interfacial transition zone, a 30 to 50 micrometre region adjacent to the aggregate in the cement paste which tends to have more water and less cement. This place is weak

Question 21

Why does the ITZ form

Answer 21

inefficient packing