|
HIPERPAV: A User-Friendly
Tool to Help Us "Build It Right"
by Stephen W. Forster
HIPERPAV is a user-friendly, Windows®-based computer
program that provides guidance on the design and construction of concrete
pavement. HIPERPAV enables engineers to make smart decisions about variables
that affect concrete pavement performance.
Using HIPERPAV, engineers can predict and, thus, prevent
uncontrolled cracking during the construction phase (the first 72 hours
after placement). This new capability enables the construction of concrete
pavement that requires less maintenance, saving a considerable amount
of money and reducing traffic disruption over the life of the pavement.
Background
One of the critical elements in long-term, rigid-pavement performance
is the construction of pavements that are free of uncontrolled cracks
during the early (first 72 hours) period after placement. Experience tells
us that many factors — materials, mix design, pavement design, construction
practices, and climatic conditions — influence whether a pavement will
be distress-free during this early age. These factors interact in a complex
fashion, making it difficult for the practicing engineer to adequately
consider their effect. To address this concern, the Special Projects and
Engineering Division of the Federal Highway Administration (FHWA) developed
a coordinated set of guidelines that considered these factors.
Figure 1 - Input screen for materials and mix
design parameters.
Development
Transtec Inc. was awarded a research project to address two potential
problems that occur during the first several days after placement and
that greatly affect the long-term performance of concrete paving projects:
- Uncontrolled transverse cracking in full-depth jointed concrete
pavement construction.
- Debonding of bonded concrete overlays placed on pre-existing
concrete pavements.
|
The initial objective of the project was to develop a set
of textual guidelines considering the influencing factors. However, it
soon became apparent that the number of factors and the complexity of
the interaction would not allow the development of practical guidelines
in text form. At that point, the decision was made to create a Windows®-based
computer program that would retain internally the complex models for determining
the influence and interactive results of the variable factors but would
present a simple, user-friendly interface. Thus was born HIPERPAV.
Figure 2 - Input screen for general design
parameters.
Contents
HIPERPAV contains algorithms to model the various aspects of materials,
pavement design, construction procedures, and environmental conditions
that influence the behavior of the pavement. For new (full-depth) pavement
construction, the behavior of concern is uncontrolled cracking. For bonded
overlays, the behavior of concern is debonding of the overlay. The HIPERPAV
program has two modules — one for full-depth paving and the other for
bonded overlays.
Full-Depth Paving
Concrete as a material is strong in compression and relatively weak in
tension. Thus, if concrete is going to fail by uncontrolled cracking at
an early age, it will be due to tensile stresses exceeding the tensile
strength of the concrete. Therefore, the two overall models in the HIPERPAV
module for full-depth paving must address the tensile stress and tensile
strength development in the pavement over time. For each of these two
major models, there are submodels, or segments, that deal with the influence
of materials, design, environment, and construction on the development
of tensile stresses and strength within the pavement. The model results
are then compared by the software to determine whether tensile stress
versus strength reaches a critical level at any place in the pavement
at any time during the first 72 hours.
Figure 3 - Input screen for construction parameters.
Bonded Overlays
For bonded overlays, the failure mode being evaluated is the loss of bond
at the interface between the newly placed overlay and the existing concrete
pavement. This bond is considered to fail either by tensile stress exceeding
tensile bond strength or by shear stress exceeding shear bond strength.
Therefore, in HIPERBOND, the HIPERPAV module dealing with bonded overlays,
there are four overall models that address tensile stress, tensile bond,
shear stress, and shear bond. As with the full-depth paving module, for
each of these models, there are submodels that deal with the influence
of materials, design, environment, and construction on the development
of the stresses or strengths under consideration. The model results are
then compared by the software to determine whether one or more of the
critical situations that can lead to debonding can occur during the first
72 hours.
Use of HIPERPAV
As stated above, the complexities of HIPERPAV are embedded in the software,
and the interface presented to the user is easily understood and interpreted.
Inputs
Inputs are provided by the user to a series of four input screens. The
computer module prompts the user to provide the type of data required.
If the user is uncomfortable initially in providing values or ranges of
values, the module provides reasonable default values. Data may be provided
in inch-pound or metric units, as desired, and the user can switch back
and forth between the two systems as desired for each input screen or
for individual values.
Figure 4 - Input screen for environmental parameters.
The four input screens deal with materials and mix design,
pavement design, construction procedures, and environmental conditions.
Figures 1 through 4 show each of these screens.
Analysis
Once the input is complete, the program is instructed via the menu to
"Begin Analysis," and the various models and submodels use the inputted
data to determine the relevant stresses and strengths in the pavement
over the 72-hour period of analysis.
Results
The results, which are presented in graphical form, are easy for the user
to interpret. (See figure 5.) As can be seen in the figure, a line representing
tensile stress and a second line representing tensile strength are plotted
versus time on the same graph for direct comparison. Critical conditions
are simply those situations in which the stress line crosses over (exceeds)
the tensile strength line.
Figure 5 - The results of a HIPERPAV analysis
are shown in this chart showing the stress and strength history versus
pavement performance.
Benefits
As a result of the stress versus strength graph, the engineer can immediately
see whether his project has a potential problem during the construction
phase. If so, the engineer can rerun the program, changing a suspected
input variable to see its effect on the analysis. In this way, a number
of combinations can be tried in a short period of time to determine a
combination that will work with the materials, pavement design, construction
procedures, and environmental conditions likely on the actual project.
Armed with the results of HIPERPAV, the engineer can proceed with the
project with a much higher level of confidence than was warranted using
previous procedures.
An added benefit is that HIPERPAV is a great teaching tool.
By conducting multiple runs of the program, the engineer soon learns which
variables have a significant impact on the success of the project and,
in general, how changes in parameters are likely to affect pavement performance.
And, in turn, the engineer is encouraged to consider a wider range of
factors than was done in the past. This aspect is particularly useful
for the newer, less experienced engineer.
Status of HIPERPAV
HIPERPAV has been tried by the developing researchers on several paving
jobs, and it was effective in predicting potential problems in these tests.
The next steps are to conduct and evaluate the results from a more comprehensive
set of tests on pavement sections around the country, using a wider range
of material, design, construction, and environmental inputs. This round
of testing is scheduled to be completed in 1998.
Availability of HIPERPAV
Many contractors and engineers have expressed a great deal of interest
in HIPERPAV and are eager to obtain a copy with which to experiment. FHWA's
aim is to get a reliable version of HIPERPAV into the hands of the user
community as quickly as possible. Thus, at the completion of the additional
validation testing noted above, and after any revisions needed as a result
of that testing, HIPERPAV will be distributed. This distribution should
start before the end of 1998.
Dr. Stephen W. Forster is a research geologist and
team leader in FHWA's Special Projects and Engineering Division. He holds
a bachelor's degree in geology from Union College, and a doctorate from
Syracuse University. Dr. Forster is a certified professional geologist
in Virginia, and he manages the FHWA research program on portland cement
concrete. He is a Fellow of the American Concrete Institute.
Articles & Departments
Public
Roads: 80 Years Old, But the Best is Yet to Come
Celebrate International Highway Transportation Safety
Week 1998
Wealth of Information Presented at Superpave Conference
AASHTO's SiteManager Tames Contract Documentation
Intermodal ConnectorL NHS Catches up to the 1990's
The ITS Joint Project Office: Structuring the Future
ISTEA's Tribal Technical Assistance Program Legacy
Pride and Partnership: Completing the Interstate H-3
Highway Financing
HIPERPAV: A User-Friendly Tool to Help Us "Build
It Right"
|
