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(dramatic music)

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(traffic whooshing)

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- [Narrator] Throughout the state,

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bonding hot-mix asphalt mixtures

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to existing Portland
cement concrete pavement

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is a continuing problem.

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Failure to achieve a
good bond causes issues,

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such as potholes.

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This is worse at intersections

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where a more expensive overlay
is needed to fix the problem.

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Researchers at the Center
for Transportation Research

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at the University of Texas
at Austin are looking

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into best practices to
achieve an adequate bond

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between hot-mix asphalt
and concrete pavement.

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- For this asphalt to work properly

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with the concrete payment underlying it,

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you apply a layer of tack
coat, which is kind of a glue

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that is bonding the top asphalt layer

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with the bottom concrete layer.

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Occasionally it so happens
that if the surface texture

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of the underlying layer is not good enough

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or if there's moisture
or dust or the quality

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of tack coat is not
appropriate, or the chemistry

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of the tack coat is not appropriate

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with the concrete surface,

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you don't get a good adhesion

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between that tack coat and the concrete

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or between the tack coat
and the asphalt above it.

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- We've got a lot of concrete
pavements throughout the state

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and historically they have had an issue

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bonding hot-mix layers to the top of them.

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So when we do hot-mix to hot-mix bonding

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we actually have a test that
we can take a full depth core

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and we can test the
shear strength of them.

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With concrete, that's difficult

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because we do continually
reinforce concrete.

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So you would actually have
to core down over a foot

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into the concrete most
times and go through rebar.

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So that's not really
a feasible test to run

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on concrete pavement.

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- The main goal of this
project or the goals

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for this project were to
study what kind of processes

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should be used to prepare the
underlying concrete surface

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and how can we evaluate
the quality of the bond

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between the asphalt and the concrete

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through that tack coat layer

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in a lab scenario and also in the field.

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And so part of this was
also developing a field test

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that you could use for
quality control purposes.

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- The work done included both field

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and laboratory work

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where we cast concrete pavement samples

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and compacted an overlay on them,

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and put a tack coat in between,

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and measured in the
laboratory the shear strength

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and the pull-off strength.

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And then we also did some
field work where we went out

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to actual sites where
there's concrete pavement

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with a hot-mix overlay and we did coring

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so we could bring that
back to the laboratory

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to do the shear testing.

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- What they did is they did
take some full-depth cores

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for research but they also
developed an in-situ test

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where they could take these
little one-inch cores in place,

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do traffic control, not go all
the way through the concrete

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and do a pull-off test
right there on the roadway.

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And so they were able
to correlate that test

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to the shear test.

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So we have a pretty good idea

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of what we feel is good
quality in terms of bonding.

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- So ensuring that we have
the best-quality materials,

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and the best design processes
to avoid premature failures,

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and best design processes

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that will extend the
serviceable life of pavement

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will ensure that we make best
use of these taxpayer dollars.

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- It was a good study.

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It kind of reaffirmed what we
thought were good practices.

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I think we do have a little
bit more we could build

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on it in terms of what
products maybe work the best

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and kind of pinpoint that

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but it also gave us this
in-situ test to get more

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of an idea of when we do place it,

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did we get the performance
we thought we'd get,

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whereas normally we can't do
that just due to the depth

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that we have to core through concrete.

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- [Narrator] For more information

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and to find the publications

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for this project, please visit
the TxDOT Research Library

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at the link shown below.