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|Federal Highway Administration > Publications > Public Roads > Vol. 61· No. 6 > HIPERPAV: A User-Friendly Tool to Help Us "Build It Right"|
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.
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.
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:
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.
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.
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.
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 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.
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.
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.
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.
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.
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