Aggregates and Admixtures Flashcards
What are natural - primary aggregates made from?
Crushed rock (from hard rock formations, then physically processed, igneous makes better aggregate - sedimnetary is cheaper) Sand and gravel (comes from hard rocks by river, or eroded by glacier and wind action, they can be found on land or dredged)
What are Manufactured - secondary aggregated made from?
Industrial by-products (blastfurnace slag, fly ash, expanded clay, require thermal or other processing)
What is the definition of an aggregate?
A granular material used in construction. It can be natural, manufactured or recycled
How large is the average coarse aggregate?
> 4mm in the EU
How large is the average fine aggregate
62um - 4mm
What are the particle sizes of silt and clay
silt >60um
Clay >2um
Silt and clay are deterious to the properties of concrete
How much of a concrete mix do aggregates make up?
70-75% - therefore aggregates should be chosen to enhance the properties
How much of gravel and sand in England and Wales comes from Marine sources
~22%
What is the key difference between marine and land aggregates?
Chloride and shell content - contributes to corrosion of steel
What are the key aspects of the production of marine aggregates
Surveying, dredging, screening, washing, blending, stockpiling and quality control
What is the maximum limit of shell content in concrete
<10%
What are shells made of and how do they affect concrete?
They are made of CaCO3 and affect workability
How are manufactured aggregates created?
They are pelletized then sintered at between 1000-1200C
Gases generated cause them to expand
Spherical particles with porous cellular interior with low density
Why do we add aggregates to concrete?
To reduce cost, increase compressive strength, stiffness hardness and abrasion resistance compared to cement paste.
What aggregate properties are important to concrete?
Shape, surface texture, size and grading.
We need specific gravity (2.6-2.8)
Strength (200MPa)
Elastic Modulus (55-70 GPa)
and good dimensional stability
What do aggregate properties influence in mixes?
angular, rough and porous aggregates require more cement and water. Elongated and flaky mixes can be detremental to stength and durability, as they orientate on one plane and they trap bleed water and air voids
Angular and rough aggregates bond better with cement paste
What is the ideal range of porosity in aggregates?
< 3-10%
What is the effect of a wet porous aggregate?
It introduces free moisture to the mix (increases w/c ratio)
What is the effect of a dry porous aggregate
It can reduce the workability of the mix
What should you checn for when using aggregates?
Should be relatively clain, low number of impurities, fines, weak or unsound particles
Name some Deletrious substances in aggregates
Organic: Interfere with hydration, setting and hardening
Fine: (<63um) coat aggregate surface and weakens past aggregate bond
Weak or unsound patricles: disintegrate and develop harmful reactions
Gypsum: Iron pyrites (FeS2) and other sulphates
Chlorides: corrosion
Reactive silica and carbonate: react with alkalis in concrete pore solution to produce expansive gel
What causes the ITZ (Interfacial transition zone)?
Inefficeit packing of cement grains on large aggregate - wall effect
How thick is the ITZ and what is the w/c ratio like?
~30-50 um thick, has a higher water content and porosity.
What is the effect of the ITZ?
Creates a weaker, structurally inferior region
