Aggregates Flashcards
definition
“Inert, granular, inorganic materials, which normally consist of stone or stone-like solids.”
Usage
- Alone – road bases, fill, drainage layers
- Particulate Composites
- Portland cement concrete
- Asphalt concrete
Functions:
- Economy – inexpensive filler
- Dimensional Stability – shrinkage/expansion control
- Durability – wear resistance, chemical attack
Classification by Specific Gravity:
- Heavyweight (>3.5)
- Normal-weight (1.5 – 3.0)
- Lightweight (<1.0)
Classification by Source
Natural:
* Naturally Occurring (sand, gravel)
* Modified (crushing, washing, sieving)
Artificial:
* Industrial wastes (blast furnace slag)
* Man-made (lightweight)
* Reclaimed (recycled concrete)
desirable characteristics
Hard, strong, durable
Free of undesirable impurities
Chemical stability (or beneficial reactivity)
Important Properties:
Shape and texture
Size gradation
Moisture content
Specific gravity
Bulk unit weight
Ideal aggregate particle for workability would be:
- Spherical (no corners, low surface-to-volume ratio)
- Smooth (less friction between particles)
Aggregate shape and texture directly affect the workability of fresh concrete.
Paste acts as ______ between particles
lubricant
mechanical properties of concrete are also affected by:
Particle shape and texture.
Higher surface-to-volume ratio increases the amount of surface area available for bonding.
Extreme changes in shape can induce stress concentrations.
Rough, textured surfaces improve mechanical bond
extreme changes in shape of aggregates can induce
stress concentrations
particle size distribution determines what?
Uniform Grading and Continuous Grading
Gap Grading and No-Fines Grading
Increased Maximum Aggregate Size
WHEN NON UNIFORM SIZE, LESS WATER AND PASTE NEEDED
Maximum Aggregate Size
The smallest sieve opening through
which the entire aggregate sample will pass
Nominal Maximum Aggregate Size
ASTM allows 5-10% retention on the largest sieve size
The choice of nominal maximum aggregate size is determined by job conditions to ensure consistency:
- 1/5 Narrowest dimension between forms
- 1/3 Depth of slabs
- 3/4 Clear spacing between reinforcing and/or forms
Grading Curve:
Graphical plot of the distribution of particle sizes in an aggregate sample.
Plotted as: Cumulative % Passing Sieve vs. Sieve Size.
no more than this percent should be retained between any two consecutive sieves
45%
fuller’s maximum density curves
theory based on spherical particle packing
pi = (di/D)^(0.5)
pi= percent passing ith sieve
di= opening size of ith sieve
D= maximum particle size
to avoid segregation of aggregates, do no store in __ and be wary of ___, do not let aggregate ____
Do not store in tall cone-shaped piles
Be wary of windy conditions
Do no let aggregate run down slope
Fineness modulus
A single parameter that describes the grading curve of a fine
aggregate.
Used to check the uniformity of grading between aggregate samples.
Increasing FM = Coarser Aggregate
FM= sum of cumulative percent passing retained on standard sieves/100
FM is not normally used for coarse aggregate since: (3)
-Less relevant
-Very high values
-Low sensitivity
why is the FM of fine aggregate required for mix proportioning?
FM of the fine aggregate is required for mix proportioning since sand gradation has the largest effect on workability.
Finer sand increases workability and allows an increase in the proportion of coarse aggregate
Moisture States:
Oven Dry (OD) – All pores are empty of water. Heated in oven at 105 oC to constant weight.
Air Dry (AD) – All moisture is removed from the surface but internal pores are partially full.
Saturated Surface Dry (SSD) – All pores are filled with water but there is no film of water on aggregate surface.
Wet – All pores are filled with water and there is a film of water on aggregate surface
SSD is used as the reference state for calculating moisture contents instead of OD because:
- It is the equilibrium moisture content.
- Moisture contents in the field are much closer to SSD.
- Bulk specific gravity (BSG) more accurate using SSD.
- Moisture content can be calculated directly from apparent specific gravity (ASG)
