Aggregate Flashcards
What type of aggregates are there?
And what % of HMA does aggregate constitute?
Natural blasted from quarry
Manufactured rock usually a byproduct such as slag which is produced from processing steel tin and copper
92-96%
How is gradation measured? And how does it work
Through sieve analysis Dry aggregate passes through a series of sieves with progressively smaller openings. The weight of the particles retained on each sieve is measured and compared to total weight. Particle distribution is expressed as a % of weight on each sieve size. The test sample is always dried to a constant mass to work out sample’s dry mass.
There are also control points that give the allowable % passing for each sieve size
For e.g.
1st control point (100% passing)
2nd control point (90-80% of the mix must pass this sieve)
3rd (2.36mm No 8 sieve) controls amount of sand sized particles . Avoid alot to make an overly find mixture, but not too little to make a dense graded mix.
4th (0.075mm No 200 sieve) measures mineral dust passing through
course aggregate is a fraction of aggregate retrained on no.8 (2.36mm) sieve
Washes sieve analysis is more accurate because it’s good for 4th control point passing 200 sieve as the washing helps remove the small particles from the larger ones.
sieve analysis can be dry or washed- washed is more accurate. Dry takes 1-2 hrs and washed takes 1-3 days.
What is maximum aggregate size?
smallest sieve where 100% of the aggregate sample particles pass
Superpave mix design defines the maximum aggregate size as “one sieve larger than the nominal maximum size” (Roberts et al., 1996).
Small maximum aggregate sizes= instability/ rutting/ shoving
large max aggregate sizes= poor workability/ segregation
The largest sieve that retains some of the aggregate particles but generally not more than 10% by weight is what?
Maximum or Nominal maximum aggregate size?
Nominal maximum aggregate size
Which type of gradation contains only a small percentage of aggregate particles in the small range?
Dense
Gap
Open
Uniform
Open
this results in higher air voids as there are not enough smaller particles to fill in the voids between the larger particles.
Which type of gradation contains only a small percentage of aggregate particles in the mid size range?
Dense
Gap
Open
Uniform
Gap
Explain the following and explain their characteristics:
Dense
Gap
Open
Uniform
Size gradation changes compaction/ workability
Dense-Graded Aggregates (HRA) (most popular in america)
These aggregates have a wide range of particle sizes, from large to small, which fill the voids between larger particles.
Characteristics: Low void content, high stability, and low permeability. They are well-compacted and provide a strong, durable base.
Gap-Graded Aggregates: (SMA)
These aggregates lack certain intermediate sizes, creating gaps in the gradation.
Characteristics: Moderate void content and permeability. They can be economical and use less cement, but may have lower workability and potential for segregation.
Open-Graded Aggregates:
These aggregates have very few fine particles, resulting in high void content.
Characteristics: High permeability and stability, but low compaction and stability. They are often used for drainage layers.
Uniformly Graded Aggregates:
These aggregates consist of particles that are mostly the same size.
Characteristics: High void content, high permeability, and low stability. They are difficult to compact and often used where drainage is important.
What is LA(loss angeles) abrasion test for?
What type of aggregate is it useful for?
How is test carried out?
What about the micro-deval test?
It measures toughness and aggregate resistance to abrasion- which means it won’t lose it’s skid resistance.
Useful for open or gap graded- as these don’t benefit from cushioning effect of fine aggregate
Sample is placed in rotating drum with steel spheres. As drum rotates the aggregate degrades by abrasion and impact as it picks up the material then drops on the opposite side of the drum. Then the mass of the aggregate that has broken apart is taken as a percentage of the original mass. So lower LA loss values means that it is tougher and more resistant to abrasion.
However it is an empirical test and is not directly related to field performance of aggregates. Like limestone and slag tend to have higher LA abrasion loss but perform fine in the field.
Micro deval-uses smaller spheres and measures polishing as the smaller spheres tends to polish aggregate particles while the LA abrasion spheres tends to break them.
What does the soundness test measure?
How does test work?
It measures aggregates resistance to weathering and freeze thaw cycles. In other words it’s called durability.
The test repeatedly submerges an aggregate sample in sodium sulfate or magnesium sulfate solution. Which causes salt crystals to form in aggregate’s water permeable pores which then apply pressure on pores and tend to break the aggregate. After this is done the aggregate is dried then sieved to measure % loss of material. (around 0-15% loss with sodium sulfate test and 0-30% magnesium loss /you get different values for sodium vs magnesium).
However generally aggregate in asphalt is dried during production and then coated with waterproofing film of asphalt binder so freezing and thawing may applicable to unbound base course aggregate and may not be of concern to aggregate.
This test has been criticised for it’s inability accurately predict field performance for specific aggregates.
