DESIGn Of RIGID PAVEMEnTS In IOWA USInG THE MECHAnISTIC-EMPIRICAL PAVEMEnT DESIGn GUIDE

The Iowa Dept of Transportation (DOT) currently utilizes the empirically-based American Association of State Highway and Transportation Officials (AASHTO) pavement design procedures originally derived from the 1960 Road Test data. It is clear that these empirical procedures are no longer applicable to current conditions in Iowa. With the release of the new Mechanistic-Empirical Pavement Design Guide (MEPDG) in the USA, there is a big shift in pavement analysis and design and many state highway agencies undertake initiatives to implement the MEPDG. In order to effectively and efficiently transition to the MEPDG and accelerate its adoption, the Iowa DOT needs a detailed implementation and training strategy. In support of the MEPDG implementation initiatives, sensitivity studies were conducted using the MEPDG software to identify design inputs pertaining to both rigid pavements and flexible pavements that are of particular sensitivity in Iowa. This paper is the first of the two companion papers discussing the need for implementing the MEPDG in Iowa, benefits of implementing the MEPDG in Iowa, and the results of rigid pavement input parameter sensitivity analysis. The results of flexible pavement design inputs sensitivity analysis and implementation recommendations are presented in the second paper.


Introduction
With the release of the new Mechanistic-Empirical (M-E) Pavement Design Guide (MEPDG) by 2004 National Cooperative Highway Research Program (NCHRP) "Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures" Project 1-37A in the USA, pavement design has taken a leap forward. The MEPDG provides the user with an integrated set of models (climate + traffic + materials), which by a set of empirical models projects future performance (cracking, rutting, faulting, etc.).
The edition currently available for evaluation (as of Dec 2007) will change and a provisional design guide is yet to be released. Some areas of change are known even now, while others have yet to be identified and may only come to light, as they are identified during the general implementation.
In order to effectively and efficiently transition to the MEPDG, state Dept of Transportations (DOTs) needs a detailed implementation and training strategy. In addition, pavement design input parameters must be determined locally based on their effects on pavement performance.
It is suspected that it will take most states in the USA approx 3 years just to prepare to implement the MEPDG in its current form. Initiatives and strategies for implementing the MEPDG in Indiana (Nantung et al. 2005) and Texas (Uzan et al. 2005) were published recently. This paper discusses the development of a strategic plan for implementing the MEPDG in Iowa.

Objectives
The objectives of this paper: discuss the need for implementing the MEPDG in Iowa; discuss the benefits of implementing the MEPDG in Iowa; conduct sensitivity analyses to determine pavement design input parameters which have a significant effect on pavement distresses for rigid pavements in Iowa.

need for implementing the M-E Pavement Design Guide in Iowa
The on methods that have evolved from the AASHO Road Test (1958)(1959)(1960)(1961) (Carey, Irick 1962). Through a number of editions from the initial publication in 1962, the Interim Guide in 1972 and other later editions, i.e. AASHTO 1986 and AASHTO 1993, minor changes and improvements have been published (Carvalho, Schwartz 2006 The MEPDG relies on actual traffic operating at appropriate speeds and tire pressures using mathematical (not empirical) models to analyze the stress states within the pavement structures under appropriate local environmental conditions, which can change over the span of the design life of the pavement. The stress states at each time interval are used to evaluate and accumulate specific distress types using (mostly) calibrated distress models.
Even if the current AASHTO method could accurately predict the life of a pavement to yield a terminal serviceability of 2.5 after 20 years, there is no way to predict the development of the components of serviceability (rutting, cracking and patching, and roughness) over the design life.

Benefits of implementing the M-E Pavement Design Guide in Iowa
The major benefits of adopting the MEPDG are long-term. While it is possible that immediate benefits may be seen in terms of thinner pavements, or pavements with different component properties, it is more likely that the benefits will be identified in the long term. These benefits will accrue in a number of areas: More appropriate designs. Due to the inherently empirical nature of the current design methods, pavements are inherently over-designed for strength.
Other performance measures, such as thermal cracking and faulting, are not addressed. The MEP-DG method has the potential to significantly reduce the degree of uncertainty in the design process, and provide more realistic designs that are appropriate to the type of performance expected. The MEPDG approach will allow the Iowa DOT to specifically design pavement to minimize or mitigate the predominant distress types that occur in Iowa.
Better performance predictions. For the design life of pavements, the predicted occurrence of distresses will be much closer to the actual occurrence. Combined with realistic criteria for design levels of distress, this will lead to significantly less maintenance and rehabilitation activities. Currently, although pavements are designed for 20 years, it is common that major rehabilitation may be required as early as 12 years into the design life. The MEPDG will help ensure that this type of major rehabilitation activity occurs closer to the actual design life, i.e. 20 years. A saving of even 1% in maintenance and rehabilitation frequencies (which is considered conservative: estimates vary from 1% to 15%) will lead to significant savings in the long term. Iowa spends approx US $400 mln annually in maintenance and rehabilitation; therefore, a 1% savings represents a potential annual savings of approx US $4 mln.
Better materials-related research. Since the MEP-DG method is based on actual material properties, what if-type research will enable the Iowa DOT to examine the effects of specification change on ultimate performance. It is likely that over the next 5 years or so, such questions as "should richer or leaner hot mix asphalt base mixtures be promoted?" will arise. This type of question can be answered through the use of the MEPDG, reducing the need to conduct extensive, lengthy, and costly • • • field trials. Many other materials-related questions can be addressed in this manner.
Powerful forensic tool. The MEPDG software has an interesting and powerful capability as a forensic tool. By analyzing failed pavement using the actual materials properties, climate, traffic, etc., the Iowa DOT will be capable of identifying the components or factors responsible for the failure.

Sensitivity analysis -rigid pavement design inputs
Prior to the development of any implementation strategy, it is important to conduct a sensitivity analysis to determine the sensitivity of different input design parameters in the design process, which can differ from state to state depending on local conditions. Such a sensitivity study may be helpful in developing local calibration recommendations as well as aid designers in focusing on those design inputs having the most effect on desired pavement performance. For instance, a recent sensitivity study conducted on the Jointed Plain Concrete Pavement (JPCP) model in the MEPDG revealed that of the 29 inputs associated with the Portland Cement Concrete (PCC) slab only (except for edge support, a drainage path length input and an erodibility input), 11 input parameters were seen to effect cracking and 7 to effect faulting significantly (Hall, Beam 2005;Kannekanti, Harvey 2006). In support of the initiatives for implementing the MEPDG in Iowa, a study was undertaken to estimate the sensitivity of performance models used in the MEPDG (version 0.7) to various design inputs for 2 rigid pavement sections (JPCP) selected from the Iowa DOT's Pavement Management Information System (PMIS), also part of the Long Term Pavement Performance (LTPP) program (Guclu, Ceylan 2005). A history of pavement deflection testing, material testing, traffic, and other related data pertaining to these 2 sections, named PCC-1 and PCC-2, is available in the LTPP database.

Design input parameters
Two typical PCC pavement sections in Iowa, referred to as PCC-1 and PCC-2, were selected. PCC-1, located on US-218 near Johnson County, Iowa, was constructed in 1983. This section of US-218 is located in the wet-freeze environmental region. The pavement is a 24 cm thick JPCP with 4.5 m joints. The slab rests on 10 cm granular subbase course. The subgrade is an AASHTO A-7-6 material (clay).
PCC-2, located on US-20 near Hamilton County, Iowa, was constructed in 1968. The test section was westbound in the North Central LTPP SHRP (Strategic Highway Research program) region, and designated between 241 km and 247 km of US-20. This section of US-20 is also located in the wet, hard freeze-thaw environmental region of the US. The pavement is a 25 cm thick JPCP with 4.5 m joints. The slab rests on 10 cm granular subbase course. The subgrade layer is an A-6 (7) to A-6 (10) (silt-clay materials) glacial till soil. Sensitivity analyses were performed on a representative pavement section created from the 2 JPCP sections, PCC-1 and PCC-2. The standard input parameters for the representative Iowa highway pavement section were determined based on the design information for the PCC-1 and PCC-2 as well as by considering Iowa conditions (Table 1).

Analysis
A total of 30 input parameters related to design features, joint design, base properties, drainage and surface properties, climate, and PCC (general, mix, thermal and strength) properties were evaluated. Each evaluated input was varied within its recommended range to study its effect on predicted performance (faulting, transverse cracking and roughness) while assigning base case values to all other input parameters. For unknown input parameters needed to run the MEPDG software, the nationally calibrated default values were used. The varied values for the climate input were based on weather stations chosen in and around Iowa.
Several hundred sensitivity runs were conducted using the MEPDG software (version 0.7) and plots of pavement distresses were obtained over the design life. In addition, sensitivity runs were carried out to study the two-way interaction among input variables and their effect on predicted performance. A deterministic analysis (with a nominal 50% design reliability) was used.

Results
Several hundreds of graphs were created using the results of MEPDG sensitivity analysis. Selected results illustrating the effect of curl/warp effective temperature differences on predicted performance as well as the interactive effect of joint spacing and PCC thickness on JPCP performance are shown in Figs 1, 2, respectively.
The sensitivity plots were visually examined and each evaluated input parameter was categorized into one of the 5 groups: extremely sensitive (ES), very sensitive (VS), sensitive (S), moderately sensitive (MS), or not sensitive (NS). A summary of the sensitivity ratings is presented in Table 2 identifying the level of importance associated with each design input.
Since not all input factors are under the control of the designer, the parameters were categorized as follows (Note in Table 2) to aid in the better understanding of the sensitivity results: directly under the control of the designer (eg. layer thickness); may be changed, but will require committee action (eg. Specifications Committee), such as dowel diameter and spacing; may not be changed by the designer, but must be known, such as climate, traffic, coefficient of thermal expansion, etc. In this study, JPCP transverse cracking was found to be ES to curl/warp effective temperature difference, coefficient of thermal expansion, thermal conductivity, PCC  Thus, in general, the curl/warp effective temperature difference, the coefficient of thermal expansion, thermal conductivity, layer thickness, joint spacing, etc. had the greatest impact on the distresses (Table 3).

Sensitivity analyses -summary
In support of the MEPDG implementation initiatives in Iowa, sensitivity studies were conducted using the MEP-DG software to identify those input factors pertaining to rigid pavements that are of particular sensitivity in Iowa. Table 3 lists the input factors which have been identified to be of significant sensitivity for Iowa. Of these, the ES inputs merit early consideration and resolution. In addition to the factors listed in Table 3, there are some other factors that exhibit some degree of sensitivity, such as joint spacing in PCC slabs. However, this factor exhibits ES only for thin slabs; slabs within the normal range of thickness do not exhibit such high sensitivity.

Summary of observations
The Iowa DOT is expected to benefit by implementing the MEPDG. The major benefits of adopting the MEP-DG are long-term. In order to effectively and efficiently transition to the MEPDG, the Iowa DOT needs a detailed implementation and training strategy. In support of the implementation initiatives, sensitivity studies were conducted using the MEPDG to identify design inputs pertaining to rigid pavements that are of particular sensitivity in Iowa as well as those factors that are of no particular sensitivity. The ES MEPDG input parameters for transverse cracking were found to be curl/warp effective temperature difference (built-in), coefficient of thermal expansion, thermal conductivity, PCC layer thickness, PCC strength properties, and joint spacing.
Since these input parameters cannot be modified, accurate values should be input into the model. The sensitivity of the model to these parameters is extremely high; therefore, pavement performance outputs can vary significantly. Thus, extreme attention should be given to determine input data for these particular parameters. If necessary, material test(s) should be carried out to determine the magnitude of these parameters. Otherwise the accuracy of the predicted pavement distresses differs significantly.
Among the ES and S to VS input design parameters, the pavement design engineer can only modify; PCC layer thickness, properties of the dowel bar system used in transverse joints, and joint spacing. PCC strength properties are also modifiable provided that pavement design specifications are met.
Since the available field data for transverse cracking in Iowa DOT's Pavement Management Information Sys- tem (PMIS) are in different units than those used in the MPEDG, it is recommended that the units of MPEDG should be correlated to the actual field data.