2012-02TS
Published May 2012
RESEARCH
SERVICES
O F F I C E O F P O L I C Y A N A LY S I S ,
R E SE A R C H & I N N OVAT I O N
TECHNICAL
SUMMARY
Technical Liaison:
Shongtao Dai, MnDOT
Shongtao.Dai@state.mn.us
Project Coordinator:
Dan Warzala, MnDOT
Dan.Warzala@state.mn.us
Principal Investigator:
Roberto Ballarini, University of Minnesota
PROJECT COST:
$110,000
Using Computer Modeling to Improve the
Design of Unbonded Concrete Overlays
What Was the Need?
As pavements age, they are subject to cracking, rutting
and other deterioration that decreases their ride quality
and ability to sustain traffic loads. Since completely reconstructing pavements is costly, transportation agencies
sometimes extend their lives and restore their ride quality
using rehabilitation techniques that involve overlaying an
existing road with new pavement layers.
One such method, called unbonded concrete overlays, or
UBCOs, involves applying a layer of asphalt to an existing
surface followed by a layer of portland cement concrete,
or PCC. The asphalt interlayer of UBCOs helps prevent reflective cracking in which the deterioration of the original
pavement surface transfers stress and damage to the new
concrete layer. UBCOs can also be placed quickly and efficiently, making them an increasingly popular method of
pavement rehabilitation.
By using advanced
computer modeling that
employs fracture mechanics
to better predict cracking in
pavements, it may be
possible to design
thinner, more cost-effective
unbonded concrete overlays
that achieve the same
performance as more
conservative designs.
However, current UBCO design procedures for determining layer dimensions and materials use models that do not account for the effect of the asphalt interlayer on limiting
the propagation of cracks between layers. Researchers are interested in refining these
models to design pavements that are no thicker than necessary for their intended performance lives since thinner pavements are less costly and also more environmentally
friendly because they reduce the carbon footprint of construction.
What Was Our Goal?
The objective of this project was to develop a method for designing UBCOs using fracture mechanics to model crack propagation through pavement layers.
What Did We Do?
Researchers began by reviewing Minnesota procedures for UBCO design. Then they
analyzed crack propagation using finite element computer modeling and the cohesive
zone model, or CZM, which uses nonlinear fracture mechanics to predict how existing
cracks will spread into cohesive zones or uncracked areas. Linear fracture mechanics
account for only the strength, stiffness and dimensions of materials, while nonlinear
fracture mechanics also account for fracture energy, which is the energy required to
crack a given surface area and is a measure of resistance to crack propagation.
UBCOs can be placed quickly
and efficiently to restore the ride
quality and structural capacity of
aging pavements.
Researchers used CZM to relate the ultimate load capacity of a pavement—or the load
at which it will fail—to its dimensions and material properties. Then they verified the
accuracy of CZM by using ABAQUS software to model two sample structures and compare the results to available theoretical and experimental data.
Researchers used CZM to conduct failure simulations for various single-layer PCC pavement designs to derive equations relating pavement load-carrying capacity to material
properties and structural dimensions. They established design recommendations for
UBCOs, including layer thicknesses and material properties, that would yield the same
load capacity using the same equations. Then they compared the expected performance of these designs to observations of four thin-layer UBCO test sections at the
MnROAD pavement research testing facility.
continued
“As MnDOT districts make
increasing use of UBCOs,
this study is a step toward
optimizing their design.”
—Shongtao Dai,
MnROAD Research
Operations Engineer
“We developed a model
that more accurately
simulates the propagation
of cracks in pavements
than existing methods.
This is critical to
developing thinner
overlays.”
—Roberto Ballarini,
James L. Record Professor
and Head, University of
Minnesota Department
of Civil Engineering
The model developed in this study allows engineers to design a UBCO (right) with the same load
capacity as a single-layer pavement designed to current specifications (left).
What Did We Learn?
Results suggest that the fracture mechanics-based design procedure for UBCOs shows
promise. The model developed in this project can be used as an additional tool to assist
in the design of UBCOs by determining the load capacity of a new single-layer PCC
pavement designed using current procedures, and then determining thickness and
material properties for a UBCO that will yield the same load capacity.
The CZM model used in this study improves on traditional stress-based failure criteria used by other models, which are not capable of quantifying the loads required to
initiate and propagate reflection cracks in PCC pavements. Use of this model suggests
that increasing the strength of the interlayer or overlay and decreasing the stiffness of
the interlayer are effective ways of improving the load-carrying capacity of the UBCO
pavement in terms of crack propagation. However, this method does not consider the
effects of temperature, moisture and traffic loads.
The UBCO test sections observed at MnROAD only cracked near poorly designed joints
separating the sections. This suggests that thus far the design of these sections is sufficient to resist reflective cracking.
What’s Next?
Further research is needed to assess the model developed in this study by comparing its predictions to field observations of UBCO test cells. Research is also needed to
evaluate the effects of climate conditions and traffic loading. With further investigation,
three-dimensional fracture mechanics simulations could provide additional insights into
whether UBCOs can be made thinner and more cost-effective.
Produced by CTC & Associates for:
Minnesota Department
of Transportation Research Services
MS 330, First Floor
395 John Ireland Blvd.
St. Paul, MN 55155-1899
(651) 366-3780
www.research.dot.state.mn.us
This Technical Summary pertains to Report 2012-02, “Mechanistic Modeling of Unbonded Concrete
Overlay Pavements,” published January 2012. The full report can be accessed at
http://www.lrrb.org/PDF/201202.pdf.