civ2043 pr2 Flashcards
What are aggregates?
-Inert granular particles, mixed with water and cementitious material to make concrete
-It makes up 70% of concrete volume
what is the size of a fine aggregates or course aggregate
fine = <4.75
Course aggregates = > 4.75
List sources of aggregates
- Natural aggregates
- Manufactured aggregates
- Recycled aggregated
Why aggregates are used?
-Reduce cost
-Reduce carbon and energy footprint
-Enhance dimensional stability
-Improved strength and stiffness
-Improved abrasion resistance
Why characterise aggregates
-Assess suitability of aggregates for concrete manufure
-Gather information need to design concrete mix
- Comparison of aggregates types
-Monitor uniformity of aggregates supplied
Methods to Characterise aggregates
-Grading
-Density
-Particle shape
- Particle texture
-Strength
-Shrinkage
-Rock type
What is chemical admixture
chemical added to concrete during mixing stage to modify the fresh or hardened properties of concrete produced
List 4 admixtures
-Dispersing admixtures
-Air entrainers
-Accelerators
-Retarders
why do we use dispersing admixture
-increases slump at given water content without changing w/c
-Reduces water content at a given slump to improve concrete strength and durability
-Reduce water and cement content to give equivalent strength more econommically
What is air entrainers?
-Introduces minute air bubbles into cement paste and distribute these uniformly throughout paste
why air entrainers used ?
-improves workability
-Reduction of bleeding and segrgation
-Improved resistance to damage from freeze-thaw action
What is accelerators ?
-Increases rate of hydration
-increases rate of initial strength
why do we use accelerators ?
-where rapid setting and high early strength are required
- where rapid turn-over of moulds or formwork is required
-where concrete is placed in cold conditions
What is retarders?
-Decreases rate of hydration of cement
-longer setting time and slower initial strength gain
what are retarders used for?
-where concrete is placed in hot conditions
-To prevent cold joints during placement
-Where concrete is transported long distances
