(dramatic music)
(traffic whooshing)
- [Narrator] Throughout the state,
bonding hot-mix asphalt mixtures
to existing Portland cement concrete pavement
is a continuing problem.
Failure to achieve a good bond causes issues,
such as potholes.
This is worse at intersections
where a more expensive overlay is needed to fix the problem.
Researchers at the Center for Transportation Research
at the University of Texas at Austin are looking
into best practices to achieve an adequate bond
between hot-mix asphalt and concrete pavement.
- For this asphalt to work properly
with the concrete payment underlying it,
you apply a layer of tack coat, which is kind of a glue
that is bonding the top asphalt layer
with the bottom concrete layer.
Occasionally it so happens that if the surface texture
of the underlying layer is not good enough
or if there's moisture or dust or the quality
of tack coat is not appropriate, or the chemistry
of the tack coat is not appropriate
with the concrete surface,
you don't get a good adhesion
between that tack coat and the concrete
or between the tack coat and the asphalt above it.
- We've got a lot of concrete pavements throughout the state
and historically they have had an issue
bonding hot-mix layers to the top of them.
So when we do hot-mix to hot-mix bonding
we actually have a test that we can take a full depth core
and we can test the shear strength of them.
With concrete, that's difficult
because we do continually reinforce concrete.
So you would actually have to core down over a foot
into the concrete most times and go through rebar.
So that's not really a feasible test to run
on concrete pavement.
- The main goal of this project or the goals
for this project were to study what kind of processes
should be used to prepare the underlying concrete surface
and how can we evaluate the quality of the bond
between the asphalt and the concrete
through that tack coat layer
in a lab scenario and also in the field.
And so part of this was also developing a field test
that you could use for quality control purposes.
- The work done included both field
and laboratory work
where we cast concrete pavement samples
and compacted an overlay on them,
and put a tack coat in between,
and measured in the laboratory the shear strength
and the pull-off strength.
And then we also did some field work where we went out
to actual sites where there's concrete pavement
with a hot-mix overlay and we did coring
so we could bring that back to the laboratory
to do the shear testing.
- What they did is they did take some full-depth cores
for research but they also developed an in-situ test
where they could take these little one-inch cores in place,
do traffic control, not go all the way through the concrete
and do a pull-off test right there on the roadway.
And so they were able to correlate that test
to the shear test.
So we have a pretty good idea
of what we feel is good quality in terms of bonding.
- So ensuring that we have the best-quality materials,
and the best design processes to avoid premature failures,
and best design processes
that will extend the serviceable life of pavement
will ensure that we make best use of these taxpayer dollars.
- It was a good study.
It kind of reaffirmed what we thought were good practices.
I think we do have a little bit more we could build
on it in terms of what products maybe work the best
and kind of pinpoint that
but it also gave us this in-situ test to get more
of an idea of when we do place it,
did we get the performance we thought we'd get,
whereas normally we can't do that just due to the depth
that we have to core through concrete.
- [Narrator] For more information
and to find the publications
for this project, please visit the TxDOT Research Library
at the link shown below.