Pavement Preservation Journal - Spring 2015 - (Page 33)
By Yetkin Yildirim, P.E.
s a result of numerous complaints about noise
from seal coats and flying stone in recent years,
TxDOT has requested the development of new
asphalt mixtures that can be built in thinner
lifts as an alternative to seal coats, and can be used to pave
thinner overlays with overall reduced life-cycle cost.
These thinner overlays must be built at or less than
0.5 in. thick to realize the economic benefits without
compromising their ability to restore the pavement surface.
The design and testing of some of the thin overlays is
mostly done using currently available equipment and
procedures e.g. Indirect Tensile Strength (ITS), Hamburg
Wheel Tracking Device (HWTD), and Overlay Tester (OT).
These tests were originally developed for mixes paved with
a thickness of at least 2 to 4 in.
The state of stress that a 4-in. overlay is subjected to
under the action of highway traffic is significantly different
from the state of stress in a 0.5-in. overlay, particularly
shear stresses under the action of braking and acceleration
forces. For this reason, it is necessary to develop mix
designs for such mixes using evaluative tools and testing
procedures that are appropriate for ultra-thin overlays.
The Texas Pavement Preservation Center at The
University of Texas at Austin - together with the University
of Texas at El Paso - have put together a team to work on
cost effective alternatives to seal coats.
The first step of this project is to develop a number of
potential mix designs for ultra-thin overlays. This process
will include developing mixes by varying factors such as
aggregate gradation, aggregate class, binder grade, and
binder content. The project will evaluate the most popular
materials used for thin overlays in Texas alongside newly
developed, polymer-based materials in order to determine
how current mix design methods should be modified to
provide optimal field performance for ultra-thin overlays.
The second step will be to identify the most suitable
set of tests to evaluate the performance of the selected
asphalt mixtures intended to be used in ultra-thin overlays.
Finally, the study will produce guidelines, curriculum for
personnel, and specifications for the design and testing of
these mixtures. The second but equally important objective
of the research includes the development of surface
preparation guidelines for ultra-thin overlays to address
underseals, micro-milling applications, and the bond
strength of these pavements.
Based on the findings of this research project, the
researchers will provide a methodology to design
ultra-thin HMA along with necessary material requirement
specifications, and construction guidelines including the
use of underseals and tack coats.
Deliverables will also include a test method to verify
the bond strength of ultra-thin overlays with existing
pavement surface and laboratory test methods to evaluate
the performance of ultra-thin layer mixes. However, it is
likely that these tests may not be needed on a routine basis
for the design and use of the ultra-thin HMA mixes but
only as a contingency to evaluate new materials or reduce
risks for higher-value projects.
The skid resistance of ultra-thin overlays also
is addressed as a part of this study. Preliminary
implementation will begin as a part of this study in the
form of a program to monitor the skid resistance on three
occasions over a one-year period.
However, this period may not be sufficient to capture the
long-term evolution of skid resistance for these overlays
and make adjustments to the design procedure and
specifications as needed.
In order to address this need, researchers will provide
TxDOT with a detailed implementation plan to track the
performance of the ultra-thin surface mixes. This will
include a plan to systematically document the materials
(e.g. aggregate class, aggregate source, and binder grade),
mix designs (e.g. JMF), and results from laboratory tests (if
any) for mixes used in different construction sites over a
period of several years.
The plan will also include details on how TxDOT can
collect and use field performance data for ultra-thin HMA
mixes (e.g. skid resistance) in order to refine the mix design
procedures and requirements.
The main goal of TxDOT's research program is applied
research that can be implemented to address the needs
identified by districts and divisions that benefit the state.
The University of Texas at Austin and El Paso recognize
this important fact, and consequently the products and
reports from this study are aimed at helping TxDOT make
informed decisions for the design and construction of
ultra-thin asphalt layers.
Yildirim is director, Texas Pavement Preservation Center at the
University of Texas-Austin
pavement preservation journal
Texas Considers Ultra-Thin HMA
Alternatives to Seal Coats
Table of Contents for the Digital Edition of Pavement Preservation Journal - Spring 2015
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Fort Collins HIR Project Provides Delegates with Up-Close Look
NCAT Reports 2012 Cycle Results, 2015 Preservation Activities
New Congress Means New Push Toward Reauthorization
FP² at TRB
Pavement Preservation in Spotlight at World of Asphalt 2015 in March
Control Potholes by Sealing Cracks, Joints in Advance
‘Thinlay’ Asphalt Overlays Next Word in Pavement Preservation
Texas Considers Ultra-Thin HMA Alternatives to Seal Coats
Thin Overlays Can Preserve Pavements as Well as Reduce Surface Noise
In California, Scrub Seals Gain Favor for Cost, Crack Sealing
IGGA Recognizes Leaders in Grooving, Grinding
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Pavement Preservation Journal - Spring 2015