1. Lecture #3:
Aggregate Moisture and
Physical Characteristics

2. 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:

3. Moisture Content States
Physical Properties: Porosity & Voids Content
Moisture Content States

4. Moisture Content Equations
The aggregate stockpile is ‘wet’ if MC% > AC%. Aggregate will ‘give up’ water.
The aggregate stockpile is ‘air dry’ if MC% < AC%. Aggregate will ‘take up’ water.
Absorption controls the water demand of portland cement concrete.
Most normal weight aggregates have 0.5 to 2.0% AC%’s. Values higher than this are considered to be high-porosity and may present poor durability performance. Resistance to water penetration controls the capability of the aggregate to resist deterioration.

5. Moisture Analysis Water 500kg cement 250kg
coarse agg 800kg (SSD) AC = 1.50%
fine agg 450kg (SSD) AC = 0.75%
If MCca = 1% and the MCfa = 1% what
is the stock water amount to maintain the
same water demand?
Will the aggregate absorb or release water?

6. Specific Gravity
Specific gravity is the ratio of the weight oa a unit volume of material to the
Weight of the same volume of water at 20º to 25ºC.

7. Archimedes Principle

8. Apparent Specific Gravity
Overall volume of the aggregate exclusive of the volume of the pores or
Capillaries which become filled with water in 24 hrs of soaking

9. Bulk Specific Gravity

10. Specific Gravity Equations
Bulk Specific Gravity
Saturated Surface-Dry
Apparent Specific Gravity
Lab Formula

11. Specific Gravity Test for Sand
Container
with H2O
OD Aggregate
Container
with H2O
and with
Aggregate

12. Gb < Gb(ssd)< Ga < Gtrue
Specific Gravity Relationships
Gb < Gb(ssd)< Ga < Gtrue

13. Moisture Content in Field
The aggregate stockpile is ‘wet’ if MC% > AC%. Aggregate will ‘give up’ water.
The aggregate stockpile is ‘air dry’ if MC% < AC%. Aggregate will ‘take up’ water.
Absorption controls the water demand of portland cement concrete.
Most normal weight aggregates have 0.5 to 2.0% AC%’s. Values higher than this are considered to be high-porosity and may present poor durability performance. Resistance to water penetration controls the capability of the aggregate to resist deterioration.

14. Aggregate Strength and Toughness
Aggregate strength dependent upon:
a) properties of constituent minerals
b) bonding between grains
c) porosity
These properties are difficult to measure.
LA Abrasion test measures aggregate degradation
Affects particle breakdown
LA abrasion test measures aggregate degradation due to grinding, impact, and attrition inside a rotating steel drum with a specified number of steel spheres.
Aggregates are subjected to particle breakdown in asphalt mixtures due to manufacture, placement, and compaction during construction and under traffic.
Breakdown more severe in open graded materials rather than dense graded. Use lower strength aggregates in dense mixtures.

15. Aggregate Surface Chemistry
Aggregate surface chemistry affects bonding to cement
Hydrophobic: water-repellent such as limestone and dolomites
have 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.

16. Aggregate Durability
Physical durability: resistance to physical processes
1. Unstable volume changes: due to freezing of
absorbed water.
2. Mechanical breakdown during handling,
stockpiling, or placement.
Chemical durability: resistance to chemical processes
1. Alkali-silicate reaction

17. Alkali-Silica Reactivity

18. 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 %

19. 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.

20. What is ASR?

21. Deleterious Substances
Absorbent particles: shale, leached chert, or porous flint
Clay lumps
Coal or wood particles
Organic impurities
Flat or elongated particles

22. Clay Lumps