Concrete constituents 2 Flashcards
Aggregate and Admixtures
What is concrete composed of?
Binding medium (cementitious binder)
Water
Coarse and fine aggregates
Chemical admixtures
What are aggregates in concrete?
Inert granular particles mixed with water and cementitious material to make concrete (70% of concrete volume) classified as:
Fine aggregate: <4.75 mm
Coarse aggregate: >4.75 mm
Name the three sources of aggregates.
Natural: Mechanically crushed minerals (e.g., quarry rock).
Manufactured: Industrial by-products (e.g., slag, fly ash).
Recycled: Processed construction waste (e.g., crushed concrete).
Why use aggregates in concrete?
Reduce cost.
Lower carbon footprint.
Improve strength, stiffness, and abrasion resistance.
Enhance dimensional stability.
What is aggregate grading?
Sieve analysis to determine particle size distribution. Types:
Uniform
Continuous
Gap-graded
No-fines
How is aggregate particle shape classified?
Rounded
Irregular
Angular
Flaky/Elongated (affects workability and strength)
What are chemical admixtures?
Chemicals added during mixing to modify concrete properties:
Dispersing (plasticizers/superplasticisers)
Air-entrainers
Accelerators
Retarders
How do plasticizers work?
Absorb onto cement particles → electrostatic repulsion.
Deflocculate agglomerations → release trapped water → improve workability.
What are the uses of air-entraining admixtures?
Improve workability (fresh concrete).
Reduce bleeding/segregation.
Enhance freeze-thaw resistance (hardened concrete).
When are accelerators and retarders used?
Accelerators: Cold weather, rapid formwork turnover.
Retarders: Hot weather, long transport times, prevent cold joints.
What is the purpose of aggregate characterization?
Assess suitability for concrete for concrete manufacture.
Compare sources.
Guide mix design.
Monitor supply uniformity.
