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July/August 2002
The Biggest Bang for Your Buck
by John E. Naughton III and Kurt SmithThis is no pie in the sky. A simple tool is on its way to help you compare the cost of different design features of concrete pavements.
Cost and performance are an integral part of the planning, design, and construction of pavements. Although pavement design engineers may understand that a certain feature has a positive impact on performance, they may not know the full benefits of the feature or its effect on the total construction costs. Particularly with regard to concrete pavements, designers may not fully consider the cost of individual design features and their impact on performance.
To provide design engineers with the information they need to improve pavements, a research study was initiated on the incremental costs and performance benefits of various features of concrete pavements. A major component of the project is the development of a user-friendly computer software package that will enable pavement designers to compare the impact of design features on cost and performance.
The bent tie bars visible at the edge of the pavement are indicative of the use of a tied concrete shoulder, one of the many design features that this project is evaluating for cost and performance (above).
This tool will help agencies provide the best value to the traveling public. The product of this research also may enable agencies to construct or reconstruct more miles of highway each year using the same level of funds without sacrificing performance.
"Sometimes design features are added to the design for reasons other than cost," says Shin Wu, senior research engineer at Ohio University and recently retired from the North Carolina Department of Transportation. Because of this project, he adds, engineers now will have some idea how much these added features affect the overall paving cost."The findings of this project," he notes, "will provide pavement design engineers with a useful tool in making design decisions."
Literature Review and Advisory Panel
Applied Pavement Technology, Inc., which is conducting the research, completed a detailed literature review during the first phase of the project. The focus was to locate references discussing costs and benefits of design features for concrete pavements. The basis for the current reference list was a previously conducted literature review completed for a training course on "Concrete Pavement Design Details and Construction Practices" by the Federal Highway Administration's (FHWA) National Highway Institute (NHI).
A cross-section of the concrete pavement industry is serving as an advisory panel for the project, including contractors, engineers, and representatives of FHWA and State departments of transportation (DOTs). Gary Fick, chief estimator for Duit Construction of Edmond, OK, is serving on the panel. "There is a misunderstanding about what some pavement features add or take away from a roadway," says Fick.
"The results of this research will benefit our industry by giving the pavement engineering community better information about the costs and benefits of different features of portland cement concrete pavement."
Design Categories and Features
From the literature review and discussions with the project's task panel, the research team identified 10 design categories affecting pave ment performance. Each category includes one or more design features. In the load transfer category, for example, dowel bars and aggregate interlock are individual design features.
Workers place tie bars, as the wet concrete comes out of the paving machine, in preparation for adding a tied concrete shoulder, another design feature under evaluation.
Design Categories
- Pavement cross-section
- Thickness and slab size
- Concrete strength and materials
- Base and subbase
- Subgrade
- Joints and load transfer
- Joint sealing
- Drainage
- Shoulders
- Smoothness
Using these design categories and appropriate design features, the team collected expected performance information from experienced pavement designers around the country and cost information from concrete paving contractors. The objective of the data collection is to create three datasets to be used as components of the software: relative cost, relative performance, and ranking factors. A dataset represents a group of data with a common source, used as the basis for analysis. Users of the new cost and performance software will be able to modify the datasets.
Data Collection
The research team contacted pavement designers with State DOTs and contractor members of the American Concrete Pavement Association (ACPA) through letters, e-mails, and telephone calls to determine if they were interested in participating in the project. Their involvement consists of submitting cost and performance data based on a standard portland cement concrete (PCC) pavement section with varying design features and cross-sections. The team asked the DOT pavement designers to estimate the change in cost and performance, in terms of pavement life, that would be achieved by incorporating that feature.
The team determined that a minimum of 16 participants from State DOTs and 20 responses from the contractors would be the required sample size for statistical significance. A project summary and request for participation was faxed to 216 contracting companies. A total of 53 companies responded to the request, with 38 agreeing to participate. All 48 DOTs in the continental
United States were contacted, 36 DOTs responded, and 25 agreed to participate. Five of the DOTs that declined specifically stated that they currently do not use PCC or use such a small percentage in their construction that they would be unable to provide accurate data on performance.
Initial Findings
The data collected during this project are based on a standard section, and the relative performance and cost are expressed as a ratio. For example, if a tied concrete shoulder is added to the standard section, the relative performance based on the standard section might be 1.24 and the relative cost might be 1.15. This means that there would be a 24 percent increase in performance and a 15 percent increase in costs for that particular design feature.
From the contractors' responses, the initial findings indicate that selection of the drainage layer for this survey, specifically asphalt-treated permeable base with edgedrain—creates the greatest increase in cost (1.329). Initial results also show that the greatest reduction in construction cost (0.853) is the elimination of the base layer and constructing the slab directly on the subgrade. Pavement cross-section appears to have little effect on the construction costs.
The initial results from the DOTs indicate that slab thickness provides the greatest influence on relative pavement performance. Variation of the slab thickness provided both the greatest increase in performance 2.181 for 30.5-centimeter (12-inch) PCC and the greatest reduction in performance 0.541 for 20-centimeter (8-inch) PCC. These two responses, however, have high standard deviations. The responses for the use of a 30.5-centimeter PCC ranged from no increase in performance (1.0 response) to a relative increase of 3.3, showing a wide range in assumed performance. Initial results also indicate that cross-section has little effect on the performance similar to the findings on relative cost.
The research team also asked the volunteers to "force rank" the 10 design categories in order of perceived impact on performance, based on cracking, faulting, spalling, and overall ride or smoothness. These rankings are used to determine the impact of multiple design features. Project volunteers used integers between 1 and 10, and no ties were allowed. From the results received to date, the top three design categories based on pavement failure criteria were listed in order of importance.
The ranking produced during this project is subjective in nature. The intent is to capture the perceptions and expertise of the pavement design engineers. The project software is designed so that users can add more objective performance information as it becomes available.
Preliminary Ranking Results by Failure Criteria
Cracking
· Thickness
· Subgrade
· BaseSpalling
· Material
· Load transfer
· Joint sealFaulting
· Load transfer
· Drainage
· BaseSmoothness
· Load transfer
· Subgrade
· BaseData Analysis
During the proposal stages of the project, it became apparent that the performance data are not directly cumulative because of their interdependence. If dowels are expected to increase life by 40 percent, for example, and a permeable base is expected to increase life by 10 percent, the actual increase in life will not be 50 percent but somewhere between those two numbers. Cost data, on the other hand, are more or less directly additive.
To address the challenge presented by this discrepancy, the team developed an alternative method of calculating the impact of performance for combinations of multiple design features. The method uses a weighted ranking system that rates the features based on impact and importance. The team will calculate the actual ranking based on the survey results and existing performance models such as those of the American Association of State Highway Transportation Officials and the Long-Term Pavement Performance program.
Project Software
The final deliverable is an interactive computer program that enables pavement designers to compare the costs and benefits associated with various design features for concrete pavements. The team is designing the program so that the user can enter unique standard sections and then compare them using the analysis tools provided in the software. Initially, the software will be populated with data from the research project, with the expectation that new or updated information can be added as it becomes available. The user will be able to create new datasets for relative cost, relative performance, and ranking factors, and define additional design features. The user also will be able to share data with other users.
The software is being designed with both the casual user and pavement researcher in mind. The software will have the capability to compare different pavement sections and the ability to perform additional sensitivity analyses. These analyses include the ability to compare multiple cost and performance datasets, multiple ranking factor datasets, and multiple variables within a design category.
John E. Naughton III is a program director for Applied Pavement Technology, Inc., supporting airport pavement evaluation, design, and construction projects. Naughton is the principal investigator for the Task 6 Project, Incremental Costs and Performance Benefits of Various Features of Concrete Pavements. Naughton was the managing principal instructor and lead developer for NHI's Construction of Portland Cement Concrete Pavements. He was the instructor for 15 of the NHI courses and has made nearly 50 technical presentations on highway, airfield, and municipal pavements-related issues. Naughton has directed several task forces on concrete pavement specifications, including the American Concrete Pavement Association's Airfield Task Group. He has a B.S. and an M.S. in civil engineering from the University of Illinois, and he is a registered professional engineer in Illinois.
Kurt Smith is a program director with Applied Pavement Technology, Inc., and has more than 17 years of experience in pavement design, evaluation, and rehabilitation. Smith is currently serving as the co-principal investigator on two FHWA-sponsored research studies: one evaluating the development of materials-related distress in concrete pavement and the other developing guidelines for the repair and rehabilitation of concrete pavements. He also is serving as the principal investigator for an FHWA project on Portland Cement Concrete Overlays: State of the Technology Synthesis, and is overseeing an FHWA project synthesizing various pavement technology topics including pavement smoothness, surface texture and noise, and alternative dowel bars. He has a BS and an MS in civil engineering from the University of Illinois, and he is a registered professional engineer in Illinois.
For more information, please contact Kurt Smith at ksmith@pavementsolutions.com.
Other Articles in this issue:
Taking Concrete to the Next Level
Fine-Tuning Innovative Technologies
Making Roads Better and Better
Texas Tests Precast for Speed and Usability
The Biggest Bang for Your Buck
New Software Promises to Put Whitetopping on the Map
