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CHAPTER 6. SUMMARY AND CONCLUSIONS Overview of Report This report documents the results of a comprehensive study on Initial pavement smoothness. The overall objectives of this project are to Investigate the importance of initial pavement smoothness on pavement performance and to examine the measurement and quantification of initial pavement smoothness. Chapter 2 of this report describes the results from the SHA questionnaire survey of highway agencies and of paving contractors. Information regarding Me different types of equipment and roughness analysis methods being used are presented to provide an indication of current roughness-measur~ng practices. The questionnaire responses are summarized in appendix A, while appendix B contains an annotated bibliography of pertinent literature. The project data base that has been established for this project is described In ~. . ~. ~ . chapter 3. The data base has been developed using the Microsoft Access data base management system, and contains the results of the questionnaire surveys as well as the historical pavement roughness data obtained from State Highway Agencies and other sources. Appendix C contains historical roughness plots of the data obtained from the various sources. . c, Chapter 4 presents the results of the analyses that have been conducted on the initial pavement smoothness data obtained from State Highway Agencies. These analyses include an evaluation of the effect of initial pavement smoothness on the future pavement smoothness and on He life of the pavement. Also included is an analysis of the effect of smoothness specifications on the resulting initial pavement smoothness levels and an investigation of the most cost effective smoothness levels based on life cycle cost analyses. Appendix D contains smoo~ness-life relationships and sensitivity plots illustrating the effect of changes in crucial pavement smoothness and the corresponding changes in pavement. These relationships and plots are for specific pavement families for the States providing data to the study. Finally, chapter 5 reviews the available smoothness-measuring equipment and describes the evaluation of various summary statistics for expressing initial pavement smoothness. Appendix E contains a summary of He characteristics and capabilities of the various pieces of roughness-measuring equipment that were used in the equipment evaluated. By identifying the most suitable summary statistics for initial pavement smoothness, the accompanying requirements, properties, ancI characteristics for equipment measuring that device can be determined. 201

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Review of Significant Findings This research has produced many significant findings and conclusions pertaining to the effect of initial pavement smoothness and its uniform measurement and appraisal. The key findings of this research are listed below, with more detailed information on each finding provided in the main chapters and in the appendixes. Ouestionnaire Survey Results A questionnaire survey of State Highway Agencies was conducted to document current smoothness measuring practices and procedures. Of 53 questionnaires sent out, responses were received from 47 SHAs and 3 Federal Lands Agencies. A questionnaire survey was also submitted to paving contractors to obtain their perspective on pavement smoothness measuring practices and procedures. Highlights of the results from these questionnaire surveys include: 1. 3. For new AC pavement construction, 28 of 50 agencies report using a smoothness specification, whereas for new PCC pavement construction 40 of 50 agencies report using a smoothness specification. 2. The most common device used for measuring initial pavement smoothness is the profiIograph (all types), which is used by 41 percent of the responding agencies on AC pavements and by 64 percent of the responding agencies on PCC pavements. Common profiIograph levels established by highway agencies for corrective action are 10 to 15 in/ml (0.16 to 0.24 m/km) for AC pavements and 7 to 15 in/ml (0.~! to 0.24 m/km) for PCC pavements; common levels set for earrung Incentive pay are 3 to 7 in/ml (0.05 to 0.~! m/km) for both pavement types. 4. Regarding the use of the profiIograph, a 0.2-in (5.~-mm) blanking band is most commonly used (19 of 22 responses for AC responses and 32 of 37 responses for PCC pavements) In interpreting the profiIograms, and a 0.3 in (7.6 mm) bump size criterion is most commonly specified. 5. Most smoothness acceptance testing is performed by the highway agency (34 of 44 responses for AC pavements and 28 of 44 agencies for PCC pavements). This testing is generally performed over a 528-ft (161-m) pavement segment (23 of 31 responses for AC pavements and 36 of 44 responses for PCC pavement). Most agencies are satisfied win their smoothness specification, or felt that it needed only slight unprovements, and believe that the specification had led to an increase in the initial smoothness of the pavement. 7. Paving contractors are largely in favor of the use of smoothness specification and of the inclusion of incentive/disincentive provisions. S. General unprovements suggested by the contractors In the area of Initial smoothness testing inclucle: Standardization of specifications and test procedures. Better communication between He SHA and He contractor. Clarification regarding when incentives/disincentives apply. Trairiing of individuals performing smoothness testing and analysis. 6. 202

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Effect of Initial Smoothness on Future Smoothness Chapter 4 contains a detailed analysis of the effects of initial pavement smoothness on the future smoothness of the pavement. Key findings from this analysis are presented below. 9. Linear regression analysis conducted on Stat~furTushed roughness data revealed that initial pavement smoothness is a significant factor influencing the future smoothness of the pavement in 80 percent of the new construction projects and in 70 percent of the AC overlay construction projects (at a significance level of 10 percent). By pavement type, it was found that initial smoothness was significant In 80 percent of both new AC and new PCC pavements, in 77 percent of AC/AC overlay projects, and in 63 percent of AC/PCC overlay projects. These findings are based on over 300 projects in 10 States. 10. The percentage is lower for overlay construction projects due to Me fact that the performance of AC overlays is strongly influenced by the condition of the underlying pavement and by the development of reflection cracking. The AC/PCC category, for which reIRection cracking is perhaps most severe, was the pavement category for which the effect of initial smoothness showed the least significance (63 percent of the projects). Il. Although data for older pavements are limited, it appears that the significance of ~rutial smoothness on future smoothness decreases for older pavements. That is, the greatest percentage of new construction projects showing significance was in We O-to-IO-year age range (85 percent), and this fell off to 79 percent in Me 0-to-15-year age range and to 65 percent in the O-to-greater- than-15-year age range. 12. An examination of the al regression coefficient, which provides the relationship between initial smoothness and future smoothness at some time I, shows Cat the value averages 0.82 for new construction and 0.65 for overlay construction. This suggests that the initial smoothness does have a significant effect on the future smoothness, and that the effect of initial smoothness is more pronounced on new pavement construction. 13. Although crucial smoothness was found to have a significant effect on the future smoothness on many projects, the fact that crucial smoothness did not always show up as being significant is not unexpected. Many factors, including equipment measurement errors; variability in design, materials, and construction; subgrade sedlements/heaves; topography variation, and the presence of bridges, culverts, and other structures along the highway can obscure or eliminate the positive effects of initial smoothness on future smoothness. In other words, crucial smoothness does have an effect on the future smoothness of the pavement, but there are other factors influencing pavement performance that in some instances may overwheIrn or negate the positive effects of mitial smoothness. 14. Data from over sources, namely the AASHO Road Test and the LTPP GPS program, were also evaluated to determine Me significance of Initial pavement smoothness on the future smoothness of Me pavement. The evaluation of the 203

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AASHO Road Test data indicated that smoother sections stayed smoother over time, provided that there was a significant difference (more than 0.3 serviceability units) in initial smoothness. Me evaluation of the available LTPP GPS data showed that Initial smoothness was significant on the future smoothness over the available age range (typically less than 5 years). Ef ect of Initial Smoothness on Pavement Life An analysis of the effect of initial pavement smoothness on pavement life was conducted In chapter 4. Highlights from that analysis are summarized below. 15. Two approaches were taken in evaluating the relationship between Initial smoothness and pavement life: We use of project-specific regression models for predicting pavement life to a trigger roughness level and the development of failure curves (relating the percentage of failed [overIaicl] Elects as a function of time) for projects from two States. 16. The results of the analysis strongly indicate Mat nutial pavement smoothness has a significant effect on pavement life, using both roughness model and pavement failure analysis techniques. The analyses show that added pavement life is obtained by achieving a higher level of initial smoothness over the range of Initial smoothness values that were available for analysis. 17. The rate at which acIditional life is achieved is dependent upon, among other things, pavement type, facility type, and location. Although some indications of increasing rates of act~ed life likely due to the effects of dynamic loading were noted while conducting Me analyses, linear relationships between Crucial smoothness and pavement life generally fit the data well and were therefore used throughout the analysis. 18. ~ , ~, Sensitivity analyses, in which the percentage change in life as a function of percentage change In smoothness was determined, showed sizable increases In life for most pavement families, corresponding to nominal increases in smoothness. At least a 9 percent increase In life corresponding to a 25 percent increase in smoothness (from a target profile Index of 7 in/ml [0.~1 m/km] for concrete and 5 ~n/mi [0.08 m/km] for asphalt) was observed for the vast majority of the pavement families. A 50 percent increase In smoothness from these target levels, was found to increase life by at least 15 percent In many cases. 19. The results of this analysis are based on Me assumption that roughness is a primary factor influencing Me decision to rehabilitate a pavement. Although Mere are many other factors that can come into play, a recent survey indicated that pavement roughness is one of the top factors usec! by highway agencies In establishing the worth of candidate projects (Zimmerman 19951. ~, ~ Effect of Smoothness Specifications on Resulting Pavement Smoothness Although smoothness specifications have been gaining widespread use and acceptance, little information on Weir effectiveness in increasing the smoothness of new pavement construction is available. An analysis of data from four State 204

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Highway Agencies was provided in chapter 4 to address this issue, with the following findings: 20. The data from all four States indicate that smoothness specifications have been effective in obtaining pavements that are significantly smoother than those constructed prior to the implementation of the specification. Typically, initial smoothness values have improved between about 19 and 89 percent with reductions in the overall standard deviation of initial smoothness measurements. 21. From the available data, it appears that AC pavements are generally constructed smoother than PCC pavements, although significant reductions in initial pavement roughness were observed for all pavement types. For example, ranges in the reductions of initial pavement roughness were found to be from 22 to 60 percent for PCC pavements, from 19 to 89 percent for AC pavements, and from O to 54 percent for AC overlay pavements. 22. Prom the observations of the smoothness measurements over tone, it appears that it takes a few years for contractors to become acquainted with smoothness specifications. Immediately after Me unplementation of the specification, the initial roughness generally decreases and continues to decrease as the contractor becomes more comfortable win the specification. 23. Smoothness specifications are believed to be an effective means of improving the ~rutial smoothness of pavements. When coupled with appropriate incentive/disincentive provisions, contractors are encouraged to do those things (purchase new equipment, train personnel, improve staking operations, and so on) that are required for achieving a smooth pavement surface. Cost Effectiveness of Smoothness Specifications The final portion of chapter 4 summarizes an evaluation of the cost effectiveness of pavement smoothness specifications, including an analysis of the most cost- effective smoothness levels for new construction. A summary of Me key findings from that analysis are listed below. 24. An LCCA of several pavement families showed convincingly that the most cost-effective smoothness levels are considerably higher than what is generally accepted as the current target (i.e., PI between 5 and 10 in/ml (0.08 and 0.16 m/km). Seven of nine concrete pavement families showed the optimum cost- effectiveness (Pl) range as being between O and 5.5 in/ml (0 and 0.09 m/km). Four of five asphalt pavement families showed the optimum cost-effectiveness (Pl) range as being between O and 3.5 in/ml (0 and 0.06 m/km). And, eleven of 13 asphalt overlay families showed the optimum cost-effectiveness (Pl) range as being between O and 2 in/ml (0 and 0.03 m/km). 25. In comparison with actual current pay adjustment curves, the theoretical pay adjustment curves developed in this study showed, on the whole, much greater incentive amounts and much more punitive disincentive amounts. 26. When shifted to coincide with full-pay (Pl) smoothness levels of 5 and 3 in/ml (0.08 and 0.05 m/km) for PCC and AC pavements, respectively, 205

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recalculated theoretical pay adjustment curves still showed greater maximum incentive amounts and more punitive disincentive amounts In comparison with the current pay adjustment curves. 27. The inclusion of user costs In a comprehensive LCCA has a profound effect on Me determination of the most cost-effective smoothness level. For two selected pavement projects, a comparison of cumulative PW user costs associated with Tree distinct initial smoothness levels (MRNs of 0, 60, and 90 in/ml [0, 0.95, and 1.42 m/km]) showed significant savings on the part of the smoothest level. For both projects, the addition of user costs to total life-cycle costs (construction cost plus future overlay costs) greatly overwheLned the resulting [CC, resulting in O in/ml (O m/km) as clearly being the most cost- effective smoothness level. Smoothness Measuring Equipment and Smoothness Dices A comprehensive evaluation of various types of smoothness measuring equipment and smoothness Indices was conducted In chapter 5, with highlights from that analysis summarized below. 28. Although mechanical filter type pavement smoothness measuring equipment, such as the Mays Meter and the California ProfiIograph, have been used for measuring initial rideability for many years win generally good success, the inherent weaknesses of response-type roughness measuring systems (inadequate repeatability, poor correlation with user response, speed sensitivity, and lack of a "true" profile measurement) render them less than optimal for measuring initial pavement smoothness. In addition, currently used profiIograph equipment are relatively slow and actually measure a distortect trace of the pavement surface, thereby not allowing the concurrent measurement of other smoothness indices such as IRI, MO, RNsayers, or RN~=off. 29. Technology has advanced so Mat the currently available inertial-based and nclinometer-based equipment provide faster, more versatile, and accurate systems for measuring pavement profiles; these profiles can be used to compute all commonly used smoothness Apices, including Me Pl. In addition, Me cost of some ~nertial-based equipment is similar to the cost of computerized profilograph systems. 30. To determine Me most appropriate equipment for measuring initial pavement smoothness, this study iclentified Me pavement wavelength properties required to compute the pavement smoothness index win the best correlation to user response. Based on reported correlations and level of use, the IRI, PI, RNSayers, RQI, and RN~`off were all rated very highly. The IRI and PI (0.2 in [5.! mm] blanking band) were rated as moderate to good in correlation with user response, while Me other three indices were rated good to excellent. No information was available relating user response to PI determined using a 0.: or 0.0 In (2.5 or 0 mm) blanking band, but studies In Kansas and Michigan indicate that pavement roughness omitted by the 0.2 in (5.! mm) blanking band can be disturbing to highway users. 206

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31. Since no index stands out as the most appropriate, the Pi remains the currently recommended initial pavement smoothness statistic. To improve repeatability, increase speed, and improve Me relation with user response, obtaining measurements with inertial-based profiling equipment, computer modeling the profiIograph response, and using a 0.~- or O.O-in (2.5 or O mm) blanking band is recommended. 32. According to available information, the IRT, notwithstanding its widespread use for pavement management, does not provide an unproved user-response ~ . _~+ ~ . ~ 1 ~ O O O and comparing repeated runs. This information is summarized in table 51. 34. To optun~ze We effectiveness of initial smoothness measuring equipment, it should measure store Process and renort Profile data ranidiv using an correlation over 1~1 and me expense requlrect tor changing to the statistic does not appear justified. The RNsayers, RQI, and RNj~oa provide promise as the smoothness statistics of the future, given their excellent correlation with driver comfort. 33. To allow for measurement of these five statistics, pavement surface profiles must be measured accurately within the wavelength range of 0.9 to TIC ft (0.27 to 33.5 m). This requires a maximum sampling interval of 2 in (50.S mm) when analog antialias~ng filters are used and a 1-in (25.4 mm) maximum interval when digital filters are employed. Stationary vertical elevation accuracy must be +0.005 In (0.125 mm), and the moving precision and bias need to be ~+0.015 and _0.05 in (0.38 and 1.25 mm). Longitudinal distance measuring accuracy of 0.1 percent is necessary for locating grandma locations , , ~ , ~ ~ ~ ~ ~ automated, computerized system. For measurement of new PCC, Me equipment should be lightweight. The software should also be capable of system calibration and be able to identify and report must-gr~nd locations, as well as express roughness In terms of the most promising smoothness indices for each O.~-mi (160-m) section. Table 51. Summary of recommended properties for smoothness measur~ng equipment. Property Recommended Requirement Measured Profile Wavelengths 1.2 to 100 ft (0.37 to 30.5 m) up to 55 mi/hr (~S km/hr) Sampling Interval ~ in (25.4 mm) if digital antialiasing used 2 in (51.8 mm) if analog antialiasing used Distance Accuracy 0.1 percent Vertical Elevation Accuracy Static Precision/Bias Dynamic Precision/Bias +0.005 in/0.005 in (0.125 mm/0.125 mm) iO.015 in/0.05 in (0.38 mm/~.25 mm) 207

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Recommendations for Improvements to Smoothness Specifications The research findings listed above have significant Implications to existing specifications. In addition, through the conduct of Me research, many other suggestions for Improvements to existing smoothness specifications have become apparent. These recommendations are summarized below. 1. Continue using pavement smoothness specifications, as they are effective in obtaining improved levels of ~rutial pavement smoothness and ~rutial smoothness has been related positively to extensions to pavement life. Re-evaluate the suitability of current initial smoothness levels. Based on the results of this study, the most cost-effective smoothness levels for new pavement construction are between O and 5.5 ~n/mi (0 and 0.09 m/km) for new PCC pavement construction and between O and 3.5 in/ml (0 and 0.06 m/km) for new AC pavement construction. Highway agencies are encouraged to evaluate their own specifications in conjunction with smoothness-life relationships for families of pavements in Weir States. 3. Concurrent with Me evaluation of Axial smoothness levels, incentive and disincentive provisions need to be re-evaluated to more accurately reflect Me true benefits or disbenefits of Me achieved crucial smoothness level. In comparison with actual current pay adjustment curves, the theoretical pay adjustment curves developed In this study showed, on the whole, much greater incentive amounts and much more punitive disincentive amounts. To ensure the ability to accurately measure pavement surface profiles In the wavelength ranges necessary for cleterm~ning the recommended statistics, equipment used for measuring initial pavement smoothness should meet the requirements listed In table 51. 5. The equipment used for measuring ~rutial pavement smoothness should be maneuverable, lightweight, and measure profiles and compute indices rapidly. 6. Computerized data storage, analysis, calibration, and reporting capabilities should be strongly encouraged to improve accuracy and provide Me contractor anti State win immediate access to the smoothness information. 7. Retain the PI as the initial pavement smoothness index for current specifications, while reducing Me blanking band to 0.l and 0.0 In (2.5 and 0.0 mm), and move toward basing the index on computer-mocleled profiles from ~nertial-based profiling systems. 8. Do not move toward an ~rutial-smoothness specification based on IR] under the assumption of improver! correlation with user response. However, a transition to TR] as a means of providing correlation with Me currently used pavement management system index may be justified. 4. 208