Lecture #3:Aggregate Moisture and Physical Characteristics

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:

Moisture Content States

Physical Properties: Porosity & Voids Content

Moisture Content Equations

100% xW

WWAC

OD

ODSSD

100% xW

WWMC

OD

ODAGG

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?

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

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

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)

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

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

OD Aggregate Containerwith H2O

Containerwith H2Oand withAggregate

Specific Gravity Test for Sand

Specific Gravity Relationships

Gb < Gb(ssd)< Ga < Gtrue

Moisture Content in Field

100% xW

WWMC

OD

ODAGG

swa

aOD W

G

GW

1

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

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.

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

Alkali-Silica Reactivity

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 %

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.

What is ASR?

Deleterious Substances

•Absorbent particles: shale, leached chert, or porous flint

•Clay lumps

•Coal or wood particles

•Organic impurities

•Flat or elongated particles

Clay Lumps