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)
Absorption (A):
The maximum amount of water that the aggregate can absorb:
= (Wssd- Wod)/Wod * 100%
effective absorption
amount of water needed to bring aggregate from AD to SSD
Wabs = (EA)*Wagg/ 100%
surface moisture
amount of water in excess of the SSD state
Wadd = (SM)*Wagg/ 100%
Moisture Content Adjustments:
The goal here is to adjust the mix water such that the moisture absorbed by or shedded by the aggregates is accounted for and the amount of water specified in the concrete mix design is unaffected.
Bulking of Sand:
Stockpiled coarse aggregate is generally in the AD state with an EA of less than 1%
Stockpiled fine aggregate is often in the wet state with a SM typically in the range of 2-6%
Due to its small particle size, fine aggregate can hold water in the interstices between particles.
Menisci tend to form which push the particles apart and increase the apparent volume of the aggregate
When saturated, the menisci are destroyed and bulk volume returns to normal
specific gravity
SG = density of solid/density of water
In practice, aggregates are not solid materials. They contain pores that either connect to the surface (permeable) or are sealed within the solid material (impermeable)
ASG
Apparent specific gravity (ASG) refers only to the solid material excluding permeable pores:
ASG=mass of aggregate/volume of aggregate *1/pwater
BSG
Bulk specific gravity (BSG) includes the permeable pores in the aggregate volume:
NSG=mass of aggregate (solid+pores)/volume of aggregate(solid+pores) *1/pwater
relationship between SG
ASG > BSG > BSG
unit weight
Unit weight is simply the weight of a given volume of graded aggregate. Sometimes called bulk density.
unit weight, volume includes
Volume includes both the solid
aggregate particles and the space
between them.
Affected by degree of compaction and
moisture content of aggregate
maximum unit weight typically occurs when
when fine aggregate is 35-40% of total aggregate
Of the various testing approaches available for measuring aggregate durability, preference should be given in the following order: (3)
- Field performance of concrete
- Tests that evaluate aggregates in concrete
- Tests on the aggregates themselves
soundness
Aggregates are said to be unsound if the volume changes that accompany environmental changes lead to deterioration of the concrete
Volume changes can be induced by (2):
freezing and thawing or from repeated wetting and drying.
forms of freeze-thaw deterioation
There are two forms of freeze-thaw deterioration due to aggregate unsoundness … pop-outs and D-cracking
Wear Resistance
Aggregates play an important role in determining the resistance of concrete to surface abrasion and wear.
Particles must be hard, dense, strong, and free of soft, porous, or friable particles.
Chemical Resistance
Most chemical durability problems result from a reaction between reactive silica in aggregates and alkalis contained in the cement.
Other detrimental reactions include:
Iron pyrite reacts expansively with CH
Excessive gypsum causes sulfate attack
Zinc or lead delay setting and hardening
Deleterious Substances
Iron oxide in aggregate
what can you do if an aggregate doesn’t meet astm standards
crushing, heavy-media separation, hydraulic jiggling, elastic fractioning, selective quarrying, crushing, and blending
