class 7 - Aggregates & admixtures Flashcards
What is the function of cement paste in concrete?
It acts as the binding medium that sets and hardens, embedding aggregate particles.
What is an aggregate?
A granular material used in construction, which can be natural, manufactured, or recycled
What are the two main classifications of aggregates in concrete?
Coarse aggregate (>4 mm in EU)
Fine aggregate (< 4 mm)
What materials are classified as silt and clay in concrete?
Silt: <60 microns
Clay: <2 microns
Why must the amount of silt and clay in aggregates be controlled?
They can harm concrete properties.
What percentage of concrete volume is typically made up of aggregates?
70-75%
What are primary and secondary aggregates?
Primary aggregates: Extracted from natural deposits (e.g., crushed rock, gravel, sand).
Secondary aggregates: Obtained from industrial by-products or recycled construction waste.
Why is the use of recycled aggregates in concrete currently low?
Ensuring performance is challenging, requiring more research and control.
What is the main concern with marine aggregates?
They contain chloride and shells, which can impact reinforcement corrosion and workability.
Why add aggregates to concrete instead of using only cement paste?
Aggregates are cheaper.
They increase strength, stiffness, and durability.
They reduce shrinkage and heat of hydration.
They improve sustainability by lowering cement content.
Why are silt and clay undesirable in concrete aggregates?
They negatively impact concrete properties, so their amounts must be controlled.
What are primary aggregates?
Naturally occurring aggregates extracted for the first time from rock formations, gravel, or sand.
What are secondary aggregates?
Aggregates obtained from industrial by-products or recycled construction & demolition waste.
What is the main difference between marine and land-based aggregates?
Marine aggregates contain chloride and shells, which can impact reinforcement corrosion and workability.
Why is desert sand not commonly used in concrete?
Its grains are too smooth and round, leading to poor bonding with cement paste.
Why are aggregates added to concrete instead of using pure cement paste?
Aggregates are cheaper, stronger, more durable, and reduce shrinkage and embodied carbon.
How do aggregate shape and surface texture affect concrete properties?
Angular and rough-textured aggregates improve strength but increase water demand.
What is the benefit of well-graded aggregates in concrete?
They reduce void space, lower cement content, and improve workability.
What happens if aggregates are poorly graded?
More cement paste is needed, and segregation & bleeding may occur.
How does aggregate porosity affect concrete?
Wet porous aggregates increase w/c ratio; dry porous aggregates absorb mix water, making concrete unworkable.
What is the impact of impurities in aggregates?
They can weaken concrete and cause issues like alkali-aggregate reaction
What is the Interfacial Transition Zone (ITZ) in concrete?
A thin (~30-50μm) weaker layer of cement paste around each aggregate particle.
Why does the ITZ have a higher water/cement ratio?
The “wall effect” prevents cement grains from packing closely, trapping water.
How do microcracks in concrete form?
They initiate at the ITZ due to drying shrinkage and stress concentration.
