Aggregates Flashcards
What is an aggregate
A granular material used in construction
What are the four classifications of aggregate and give approximate measurements
coarse (gravel): >4mm
fine (sand) : 62micrometers to 4mm
silt: 2 micrometre to 60 micrometre
clay: less than 2 micrometre
What percentage of the concrete volume is aggregate
60 to 80%
What are the two primary aggregates
crushed rock (often igneous, sometimes sedimentary) and sand+gravel
What are secondary aggregates
Industrial by-products
How is crushed rock made
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.
What are marine aggregates
Aggregates deposited by rivers into the seabed
What makes marine aggregates worse than land based aggregates in some scenarios
High chloride content which corrodes steel and high shell content which reduces workability.
What is the maximum amount of shell percentage content in concrete that is allowed
10%
Why are manufactured aggregates typically weaker
They are often high in porosity
What are the benefits of manufactured aggregates
Low density due to high porosity which makes the concrete lighter
Why are aggregates added to concrete
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
What is the strength of natural aggregates and cement paste
200MPa, 30-50MPa
What are the five shape types of aggregates and which two should be avoided and why
irregular, angular, rounded, flaky, elongated. Flaky and elongated should be avoided as they face on direction, making the concrete weak in that direction
What are the five surface texture types
rough, porous, smooth, sharp, crystalline
Why is crushed rock better than gravel for high strength concrete
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
Why is well graded aggregate better
Reduces void space which means more aggregate can be packed in, thus increasing strength
What is segregation of concrete
Large concrete sink to the bottom, small ones rise to the top. This makes the strength vary at different parts of the concrete
What causes alkali aggregate reaction
Reactive silica and carbonate react with alkaline concrete pour solution to create a gel that absorbs water and expands, causing cracking
What is the ITZ
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
Why does the ITZ form
inefficient packing
