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Lecture #3:Aggregate Moisture and Physical Characteristics
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Particle Shape and Surface Texture
•Results from processing•Difficult to characterize•Function of:
•Angularity:•Sphericity:
•Described in terms of particle dimensions:a) angular b) rounded c) flaky d) elongated
e) flaky and elongated
•Surface texture:
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Moisture Content States
Physical Properties: Porosity & Voids Content
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Moisture Content Equations
100% xW
WWAC
OD
ODSSD
100% xW
WWMC
OD
ODAGG
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Moisture Analysis
Water 500kgcement 250kgcoarse agg 800kg (SSD) AC = 1.50%fine agg 450kg (SSD) AC = 0.75%
•If MCca = 1% and the MCfa = 1% whatis the stock water amount to maintain the same water demand?•Will the aggregate absorb or release water?
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Specific Gravity
Specific gravity is the ratio of the weight oa a unit volume of material to theWeight of the same volume of water at 20º to 25ºC.
2w H O
WtγVG= =
Wt γV
w
:
G = specific gravity
Wt = weight of material
V = volume
Wt = weight of water
where
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Archimedes Principle
2 od sw Displ H Ossd
w W w
H O2
OD Aggregate WOD
Wg. of ODAggregate under H O
WSW
2
OD
OD SW
W =
W WaG
2 2 ; od SW Displ H O agg Displ H OW W W V V
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Apparent Specific Gravity
2sV
ODa
ip H O
WG
V
Overall volume of the aggregate exclusive of the volume of the pores orCapillaries which become filled with water in 24 hrs of soaking
OD
s
ip
w
where:
= apparent specific gravity of solids (aggregate)
W oven dry weight of aggregae
V = volume of solids
V = volume of impermeable pores
γ = unit we
aG
ight of water (1 g/ml)
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Bulk Specific Gravity
2
2
ssd
s
OD
s
s
ip
pp
w
W =
V *
W =
V *
oven dry weight of aggregate
V = volume of solids
V = volume of impermeable pores
V = volume of water permeable pores
= unit
b ssdip pp H O
b odip pp H O
OD
GV V
GV V
Wt
weight of water 1 g/ml
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Specific Gravity Equations
Bulk Specific Gravity
Saturated Surface-Dry
Apparent Specific Gravity
Lab FormulaODa
pyc w OD pyc w agg
WG
W W W
OD
OD
W =
W
ODb OD
SSD SW
SSDb ssd
SSD SW
aSW
WG
W W
WG
W W
GW
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OD Aggregate Containerwith H2O
Containerwith H2Oand withAggregate
Specific Gravity Test for Sand
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Specific Gravity Relationships
Gb < Gb(ssd)< Ga < Gtrue
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Moisture Content in Field
100% xW
WWMC
OD
ODAGG
swa
aOD W
G
GW
1
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Aggregate Strength and Toughness
•Aggregate strength dependent upon:a) properties of constituent mineralsb) bonding between grainsc) porosity
•These properties are difficult to measure.•LA Abrasion test measures aggregate degradation•Affects particle breakdown
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Aggregate Surface Chemistry
•Aggregate surface chemistry affects bonding to cement
Hydrophobic: water-repellent such as limestone and dolomiteshave a positive surface charge. Work well in asphalt concrete.
Hydrophilic: water-attracting such as gravels and silicates (acidic) have a negative surface charge. Gravels may tend to create a weaker interfacial zone in concrete than lime-stone aggregates.
•Surface coating (dust of clay, silt, gypsum, etc.)tend to reduce bond strength.
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Aggregate Durability
•Physical durability: resistance to physical processes1. Unstable volume changes: due to freezing ofabsorbed water.2. Mechanical breakdown during handling,stockpiling, or placement.
•Chemical durability: resistance to chemical processes1. Alkali-silicate reaction
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Alkali-Silica Reactivity
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What is ASR?
ASR only a concern if deleterious, e.g. causes cracking.
Needs three factors to be deleterious :
- Source of alkali - Internal and external
- Reactive silica (aggregate)
- Water ( humidity) > 80 %
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What is ASR?
Onset of ASR results in formation of expansive gels which produce internal stresses which may cause cracking of concrete. Problem is often misdiagnosed.
Environmental factors such as freeze-thaw cycles, wetting/drying cycles, and traffic loading propagate cracking.
Deicing salts, marine environments, can accelerate ASR expansion and deterioration processes. ASR can accelerate corrosion deterioration.
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What is ASR?
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Deleterious Substances
•Absorbent particles: shale, leached chert, or porous flint
•Clay lumps
•Coal or wood particles
•Organic impurities
•Flat or elongated particles
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Clay Lumps