HMA aggregate size and gradation design

[USCU]

GRADATION: I don't understand the differences between the "coarse" and "medium" gradations used by Caltrans (neither of which is "open").

SIZE: When would you use a 3/4" as opposed to a 1/2", what are the benefits and drawbacks? When would you specify a 1/2" course as opposed to a 1/2" medium?

None of this affects the equivalent gravel thickness of the AC?

 

[Ron]

Increasing the coarse aggregate nominal size decreases the amount of asphalt cement required to achieve comparable stability.

Conversely, decreasing the coarse aggregate nominal size increases the AC content required and makes the void parameters more difficult to control.

 

[USCU]

Thanks for the reply Ron.

There are two questions (about HMA characteristics):
1) Differences between max size 1/2" vs 3/4"
2) Differences between "coarse" and "medium" gradation.

What are the characteristics which make one worth specifying over the other, relative to the pavements in service life and its constructability.

From what I read of Ron's response, he is saying that using 3/4" instead of 1/2" uses less binder and therefore 3/4" is easier to compact. It would seem that the gradation would have more to do with the binder required than the max size of the aggregate?

This response is a start, but...Are there any pavement experts there...a true-blue civil?

 

[Ron]

USCU...unfortunately, there are not universally defined terms for what you've asked. Generally, HMA is characterized as either "Open graded" or "dense graded". Open graded mixes are generally used for friction courses and have larger top size aggregate, much higher voids, and essentially "gap" grading. Dense graded mixes have a more uniform gradation and may contain larger top size (3/4") or smaller top size aggregates (1/2" or 3/8")

Each state DOT tends to define their own based on what has worked in that state. Some states use viscosity grading for their asphalt cement, some use penetration grading (in my experience, more popular). It depends on the leanings of the state materials engineering group and the requirements of traffic, durability, material sourcing, etc.

In general, mixes with a 3/4" top size aggregate would be consider "coarse" mixes, with 1/2"-3/8" being in the "medium" gradation range, and smaller coarse aggregate being in the "fine" gradation range...below that would be sand-asphalt hot mixes.

The mix structural properties do not change significantly for dense graded mixes between "coarse" and "medium" gradations. Both can achieve significant stability and durability with proper design.

 

[USCU]

Are there any pavement experts there? Hello...? Hello...?
Anyone able to discuss pavement aggregate size and gradation, an abstract and conceptual or pragmatic perspective...Anything?

"Coarse", "medium", or "open", means gradation. I am specifically asking about aggregate variations--not cement, and am also not asking about open graded friction courses, I know what "open" gradation is used for.

It appears from Ron's post above I did not explain what I'm asking well enough since that post is off-topic.

The question is what attributes and drawbacks do aggregate variations lend to the pavement. What types of service and observed distress make one preferable over the other. There wouldn't be these multiple gradations unless they were suitable for differing circumstance. Anyways, I give up...either I can't explain the question well enough or there are no AC pavement experts participating in this form.

 

[BigH]

Might I suggest, my colleague, that you give the forum a chance to respond. It has been only 2 days since your initial post. Not every "expert" or "not" visits the site daily - as your history shows, you have visited only 8 times in the last 5 months since you've joined. Give some time and you may get some answers - show frustration so quickly and lack of courtesy, and you may not. Just some friendly advice. As, they say, "Take it or leave it."

 

[Drumchaser]

"The question is what attributes and drawbacks do aggregate variations lend to the pavement."

Variation -size-

Sorry USCU, no expert here but I will give it a shot.

The HMAC Mix Designs we specify in our pavt.s are mainly B, C, and D.
The B mix has the larger aggr. and is used for thicker "base course" type applications (6"-12" in multiple lifts)
The "C" mix aggr. is smaller than the "B" and is generally used for thin base (4"-6") courses, but mostly surface courses.
The "D" is used exclusively for thin applications where we only need a thin overlay (2" or less)

This is one small attribute/drawback of aggr.s -size- or Master Gradation Band.

The big question is will the HMAC compact to a density specified in our spec.s (which correlates to % air voids) and to longevity out on the roadway.

Speak to a lab tech that designs and tests HMAC. Call a local hot mix plant and speak to the QC person.

 

[Drumchaser]

One final note

"There are no AC pavement experts participating in this forum."

Having dealt with ACP for a number of years, the one thing I have learned: THERE ARE NO EXPERTS.

 

[BigH]

Drumchaser: - my emoticons for good snaps!

 

[Drumchaser]

BigH

If you know an ACP expert, I would greatly appreciate the name and number. I have unanswered questions of my own. The experts I have met...well I guess it's best unsaid.

 

[wmharding]

Here is my take on it. As for the expert part of it, I am not an expert in how to do it right, however; I can tell you several things that do not work well (go ahead ask me how I know). The larger open graded mixes are for draniage course and structural courses. The medium 3/4" range mixes are for grading and structural uses. The fine or tipping mix is for surface and ride. There are several factors to consider when comparing and deciding which mix to use. The geographical location is a big one. In areas that do not see frost conditions and open graded surface course can be used to help prevent hydroplaning as sand or fine agg is added to close the mix the AC content has to go up (more fines more surface area to cover) as this happens the skid coff. will come down. It is hard to discuss what you ask for without knowing where you located and the mix designs you are working with and the aggregate you are using. They all have pros and cons the other thing you never said is if the mix had any SuperPav addatives in the mix such as latex or polymer both of these change everything.

 

[LCruiser]

Are you talking about above or below the 0.45 power curve?