(gentle music) (logo whooshing)
- [Narrator] The Texas Department of Transportation
and the University of Texas at El Paso
studied the upper limits of field sands to determine
how they may impact Superpave mixtures across the state.
Field sands are used in pavements to reduce costs
and binder requirements, but their use is limited
in part to their impact on overall road performance.
- So in the black roadways that you drive on,
there is, of course, aggregate, intermediate aggregate,
there's a fine aggregate.
Typically, that fine aggregate
is gonna be either a crusher fine,
so if you're using limestone, as you're producing limestone,
in the process of crushing, you generate dust,
so that's gonna be more of a manufactured sand.
You're just naturally creating that in the crushing process.
East Texas uses a lot of river gravels and granites
and things like that, so you have less of those,
and so typically, your sands are gonna come
from natural sand sources,
which are unclean sands from things like riverbeds.
- And some of it has expensive clays or volatile clays,
which translates to expensive clays,
and clay and water doesn't mix well,
so that can lead to a lot of stripping
that happens on the pavement, and that can happen very fast.
There was several projects they start constructing.
Within six months, they were having a lot
of rutting failure, and the understanding
is why this early failure is happening.
- Historically, we had a test called sand equivalent,
and so sand equivalent tells you how dirty the sand is,
how much clay is on the sand,
and so it passed all of those tests,
and then we still had pretty bad failures rutting
and stripping in the field, and so we started looking
at another test called methylene blue,
and the methylene blue gives you the reactivity of the clay,
not just the quantity of clay.
So when we ran that sand, it was very clean,
but the small amount of clay that was on it
was very reactive, and it caused failure,
so we started this field project to look at all
of the field sands throughout the state
to see what combination of sand equivalent
versus methylene blue we could utilize
without seeing the pavement failure.
- We ended up with 25 sources of field sands
that they think they use
that they would like us to look into,
so we looked at them, and sure enough,
we start seeing that yes,
some of these field sands are very volatile
because of that amount of clay they have in them,
and they do affect early age failures.
- We did come up with a rough guideline for that,
and then we're actually taking this
into an implementation-type project
through an inter-agency contract with the university
so that we can do more field testing
on specific mix designs utilizing those materials,
and then we're gonna check for stripping
using the Hamburg wheel tracker test.
- You don't have to do any other test
that they don't do right now.
We're not coming up with a new test.
It's interpretation of the Hamburg data
that comes out of the Hamburg test.
You look at it and you can calculate a stripping value.
If that stripping value is above the limit,
then you know you're gonna have issues
in terms of early failure.
If it's not, you're gonna do well.
- One project in particular, I mean,
it was failing before it was open to the traveling public,
so we're trying to prevent that from happening again,
but we also don't want to limit available sources,
so like I mentioned,
East Texas has locally available material on site.
If we just say you can't use it
because we don't want any failures,
where's that sand coming from?
That means they have to rail it in, so all of a sudden,
projects increase pretty drastically in price.
- [Narrator] For more information
and to find the publication for this project,
please visit the TxDOT research library
at the link shown below.