Aggregate Properties and Durability – Study Notes

Physical properties (aggregate unit physical characteristics):
- Shape and Texture
- Moisture States
- Specific Gravity and Unit Weight
- Particle Size Distribution
Mechanical properties:
- Compressive/Strength
Durability properties:
- Mechanical Durability
- Chemical Durability
Use all these properties to design PCC and HMAC Aggregates

Focus: Toughness and Abrasion
- Toughness or abrasion measures how soft or hard the surface of the aggregate is, or how erodible it is
- Soft aggregates are more erodible; tough aggregates are less erodible
- Example: bus stopping and starting — wheels rub on the surface; a soft road allows wheel-induced erosion and smoothing of the surface, which is undesirable for pavement
LA Abrasion Test (to assess abrasion resistance)
- Method: a metallic drum rotates with the aggregate sample and metallic spheres inside
- The drum rotates for $500$ revolutions and spheres collide with the aggregates
- After 500 revolutions, perform a particle size analysis before and after to assess change
- Less change in particle size indicates a tougher aggregate
- Acceptable abrasion values: 30 ext{--}60 ext{%}
- Abrasion performance influences design and performance of HMAC and PCC

Mechanism: D-cracking occurs when pressure from freezing cement cracks, causing aggregates to pop out of the cement matrix ("pop out"); common in sidewalks, driveways, city streets rather than interstates due to quality control differences
Cause: when aggregates freeze, trapped moisture expands, exerting pressure on the aggregate and surrounding concrete, leading to Freeze–Thaw Durability failure

Naturally accumulating water under pavements in base/subbase layers can saturate aggregates
With freezing–thawing cycles, cracking initiates in saturated bottom aggregates and progresses upward
Prevention: ensure good drainage to allow water to leave the concrete and avoid water sitting in aggregates
Porous aggregates should be avoided; use denser aggregates to reduce water ingress

Procedure: alternate cycles of wetting in a saturated solution of a sulfate salt for $16{-}18 ext{ hours}$
- If aggregates are porous, much of the solution enters pores
Post-soak: remove sample and dry at $105{-}110^ ext{ extcircled C}$ for $6{-}8 ext{ hours}$
Subject the sample to $5 ext{ cycles}$ of wetting and drying
After cycles: wash, dry and determine the loss of mass
Interpretation: mass loss greater than 18 ext{%} signals susceptibility to Freeze–Thaw damage

ASR is a chemical durability concern; moisture is required
Reactants: alkalis in cement + water + silica in aggregates
This issue exists in PCC, not in HMAC
Why PCC? Portland cement concrete uses cement powder, water, and aggregates; silica can be reactive
Alkalis in cement powder come from salts of alkali metals: two kinds present
- $Na_2O$ (sodium oxide)
- $K_2O$ (potassium oxide)
Reactive silica is found in some aggregates (not all)
- Not all silica is reactive; the presence of reactive silica drives ASR
Mechanism: hydroxyl ions from the alkaline cement solution react with reactive silica in aggregates (examples include chert, quartzite, opal, strained quartz)
- A gel forms, which swells by absorbing water and exerts expansive pressure, leading to cracking and failure of the concrete
Prevention and control:
- Use aggregates with non-reactive silica (i.e., avoid reactive forms)
- High-quality aggregate selection and quality control
- While moisture and alkalis are largely hard to control at the material source, silica content in aggregates can be managed
Summary: ASR is a marker of low-quality control in materials; poor QC increases risk of ASR-related damage

PCC requires cement powder, water, and aggregates; reactive silica in aggregates can react with alkalis to cause ASR
ASR stems from a reaction between hydroxyl ions in cement paste and reactive silica in aggregates
Preventive focus is on aggregate silica reactivity and cement alkali balance; overall material quality control is essential

Compressive strength is not a major issue here because aggregates are inherently very strong

Aggregate shapes: Angular, Rounded, Flaky, Elongated
- If all aggregates were angular: they would lock together and be stable, but flow poorly (low workability)
- If all rounded: they would flow very easily (high workability) but would be unstable (like a pile of marbles)
- Ideal: a balance of angular and rounded aggregates to achieve both flow and stability
How shapes arise:
- Rounded shape: natural rounding from water exposure (abrasion and weathering)
- Angular shape: produced by crushing processes in factories
Flaky aggregates: Very thin and brittle, similar to potato chips
- Flaky aggregates are not desirable for paving
- HMAC does not want ANY flaky and elongated aggregates
- PCC also prefers to limit flaky content, but some flaky content is less problematic for PCC than for HMAC

Texture refers to the surface roughness of aggregates
Rough textures provide better bonding between:
- Asphalt and aggregate, or
- Cement paste and aggregate
Rough surfaces are generally preferred over smooth surfaces to promote a stronger aggregate–binder bond

Material design goal: select aggregates with appropriate physical and chemical properties to ensure durability, strength, and bond with PCC or HMAC binders
Durability concerns (abrasion, freeze–thaw, ASR) directly influence maintenance strategies, service life, and durability ratings of pavements
Quality control during aggregate sourcing and processing is critical to minimize flaky/sharp shapes, porous content, and reactive silica that lead to ASR or poor durability