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CHAPTER 1. INTRODUCTION Background The importance of pavement smoothness to the traveling public is undisputed. Smooth-rid~ng pavements provide a high level of comfort to users and allow for more efficient movement of vehicles over the roadway. Pavements that are exceedingly rough not only generate complaints from highway users, but they also can reduce optimum travel speeds, disrupt traffic flow, and create safety hazards. In addition, rough roads can cause vehicle damage and may increase fuel consumption, all factors that lead to Increased traveling costs to highway users. Pavement smoothness is also important to the owners of the roadway. For example, smooth-riding pavements are a positive reflection on the construction and maintenance proficiencies of the owner agency (State Highway Agencies iSHA], cities, counties, or toll road authorities). Many other purported benefits of smooth- rid~ng pavements to the owner agency are often cited, Including the following: · Smoothness is very important to providing a safe and smooth-riding surface to the public. Bumps, dips, and other changes in the surface profile create an uncomfortable ride to He traveling public, and In extreme cases may represent a potential safety hazard. · Initial smoothness is an Indicator of the overall quality of construction. Initial smoothness is often considered as one of He most Important indicators of pavement quality because achieving it requires a strong co~runitment on the part of the contractor to control all of those factors that can affect pavement smoothness (accurate grade and staking calculations, stable and smooth track- I~ne, careful consideration of embedded items in the pavement, uniformity of asphalt and concrete mixtures, efficient paver operations, and so on). Thus, if the pavement is constructec! with a very smooth surface, there is a greater likelihood that the contractor has provided good quality workmanship in many other aspects of construction. Pitiably smooth pavements last longer than initially rough pavements. It is a commonly held belief ~at, all other Wings being equal, new pavements constructed with a smoother surface profile will last longer than rougher, but otherwise similar, pavements. This concept has perhaps been most widely fostered by the AASHTO pavement design models, which suggest that a pavement with a higher initial serviceability rating (that is, pavement smoothness) will last longer than an otherwise equivalent, but ~rutially rougher, pavement (AASHTO 1993~. . Due to the importance of pavement smoothness to both He user and to the owner agency, more and more highway agencies are adopting smoothness specifications for initial pavement construction. These specifications require Hat the smoothness of a newly constructed pavement fall within a specified tolerance level, thereby ensuring 1
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that a uniform, planar surface profile is provided. In some cases, incentive or disincentive payments are tied to the level of smoothness obtained during construction. Over the years, a variety of devices have been constructed to measure pavement smoothness. One of the earliest known apparatuses for measuring pavement smoothness was a straightedge device called a Viagraph. Developed around 1900, this device was drawn over the roadway and recorded on paper a profile of the road surface (Hveem 1960~. This gave rise to the development of many other smoothness- measur~ng devices, such as the one used at the Bates Road Test (Older 1924), the original BPR Roughometer (Buchanan and Catudal 1940), Me original California profiIograph developed by Frances Hveem (Hveem 1960), the CHLOE profiler used at the AASHO Road Test (AASHO 1962), and high-speed, profile-measuring equipment, from the original GM Profilometer~ (Sp angler and Kelly 1966) to He K. J. Law ProfiIometers~ currently used under the Long-Term Pavement Performance (LTPP) program (SHRP 1994~. Although the high-speed profiling devices have gained widespread acceptance for morutoring pavement smoothness, the California-type profilograph is still commonly used for measuring ~rutial pavement smoothness. Profilographs consist of a rigid beam or frame with a system of support wheels that serve to establish a datum from which deviations can be measured (Woodstrom 1990~. The vertical movements of a profile wheel, located at the mid-po~nt of the frame, are monitored win a strip chart recorder to produce a permanent record of profile deviations. Most current smoothness specifications require the use of a specific type of profilograph to measure pavement smoothness. The smoothness specifications typically stipulate that a certain smoothness requirement be met In order for the contractor to obtain full pay; they may also include ~ncentive/disincentive provisions for the contractor. Under such provisions, He contractor is awarded a bonus for an exceedingly smooth pavement and is assessed a penally for an extremely rough pavement. Description of the Problem Although the use of smoothness specifications has grown in recent years, some concerns have been raised regarding Heir usefulness and effectiveness. While critics concede that a smooth-r~ng pavement surface is important to the public, they question He purported benefits of an extremely smooth initial pavement profile, the basis for incentive/disincentive payments, and the suitability of current profilographs for smoothness specifications. Key questions that have been raised include: · Does a high initial smoothness really lead to a longer pavement life? Although the AASHTO pavement design equations imply that high initial smoothness does lead to a longer pavement life, this supposition has never really been confirmed in a long-tam`, field Investigation of pavement 2
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performance (recall that the AASHTO design equations are based on only 2 years of performance data). It may be argued that rougher pavements increase the dynamic loading effects of truck traffic on the pavement that, In turn, induces more deterioration; but again, such a theory has never been validated. . . Is the amount of the incentive/disincentive payments for smoothness justified? The ~ncentive/dis~ncentive payments for pavement smoothness In current specifications: are based on subjective judgment. The extent to which they actually reflect cost benefits (or disbenefits) is unknown. It has been suggested that the Incentive or disincentive should be rationally based on the Increase or decrease in future costs that will be incurred by the agency and by users over the life of the pavement (Weed 1989~. Pay schedules are often based on stepped scales that raise the issue of fairness and the ability of the profiIograph to accurately discriminate between different levels of smoothness. The specified initial roughness value varies somewhat from agency to agency, but generally a maximum value of 7 in/ml (0.~1 m/km), as measured by the California profiIograph, is specifier! for a contractor to receive full pay. However, the basis for this ~rutial smoothness value is subjective and does not account for potential long-term benefits of a smoother pavement (e.g., increased life or postponement of rehabilitation). Are smoothness specifications, measurement methods, and equipment equally suited to both flexible and rigid pavements? The 1987 AAS~O rideability survey indicated that more highway agencies employ smoothness specifications for rigid pavements Man for flexible pavements (AASH-rO 1987~. Furthermore, most applications of the profiIograph have been win rigid pavements. There is some feeling that uniform smoothness specifications and standard measurement procedures should be adopted for bow rigid and flexible pavements. However, Here is some concern that As may not be possible due to differences in the construction processes of rigid and flexible pavements. It is also possible that inherent pavement traits may adversely affect the measurement of Initial smoothness. For example, jointed concrete pavements undergo cyclic thermal curling In response to changing temperature gradients. Smoothness measurements taken early In the mowing (when the jomis are curled up) are quite different from smoothness measurements taken In the afternoon (when the joints are curled down). · Which of the various pieces of roughness-measuring equipment is most appropriate for use with smoothness specifications? Several studies have recently been conducted that have compared the different types of profiIographs (and other roughness measuring equipment) for use with pavement smoothness specifications (Woodstrom 1990; Scofield 1992; Harrison and Bertrand 1991; Uddin et al. 1990~. These studies have identified limitations with all of the devices and procedures, and have also noted significant differences between the results obtained from each device. 3 .
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Project Objectives and Scope Clearly, several legitimate concerns have been raised regarding the measurement of pavement smoothness and the use of pavement smoothness specifications. To address many of these issues, NCHRP project I-31 was initiated with the following objectives: Determine the impact of initial smoothness on the ride quality of the pavement over its life and on the pavement service life. 2. Determine the effects of smoothness specifications on the initial pavement smoothness. 3. Determine the cost-effectiveness of smoothness specifications, including incentives and disincentive provisions. 4. Recommend methods to measure initial smoothness on construction projects. In simple terms, the overall objective of this research is to examine the importance of initial pavement smoothness as a quality control measure, including its impact on pavement performance, its need to be specified, its basis for incentive/dis~ncentive payments, and its uniform appraisal and measurement. Work Approach In order to achieve the project objectives, a comprehensive literature review on pavement smoothness, smoothness specifications, and smoo~ness-measur~g equipment was conducted. This was followed with a survey of SHAs to obtain information on current smoo~ness-measuring practices, specification requirements, and overall effectiveness of and satisfaction with nutial smoothness specifications. With this background Information, two parallel research efforts were initiated. First, tune-series pavement smoothness data and pavement smoothness data measured before and after the Implementation of a smoothness specification were collected from selected SHAs. The analysis of these data provided insight as to the effect of crucial smoothness on overall rideability and on pavement life, and also indicated the relative effectiveness of pavement smoothness specifications in achieving initial smoothness levels. Armed with data on the cost of constructing smooth pavements, analyses were also conducted on the relative cost-effectiveness of initial smoothness levels. The second course of research focused on an evaluation of both the devices that are used to measure initial pavement smoothness and the associated indices that are used to express the initial smoothness level. This evaluation was performed using Information available from existing literature only, and did not involve the actual field testing of the equipment. This evaluation led to an assessment of He overall suitability of various equipment types and smoothness indices, and also to the development of generic equipment specifications for the measurement of Initial pavement smoothness. 4
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Overview of Report This report describes the findings and results obtained under this project. It contains five chapters (in addition to this one) that summarize various aspects of the work. Chapter 2 summarizes the current smoothness-measur~ng practices and specifications used by SHAs. Chapter 3 of this report describes the data collection activities and the associated development of the project data base. Chapter 4 provides the results of several analyses conducted on the effect of ~rutial smoothness and on initial smoothness specifications. Chapter 5 describes the evaluation of smoothness measurement statistics and equipment characteristics for measuring initial smoothness and presents various recommendations for future initial smoothness testing. Finally, chapter 6 provides a brief summary of the report and proposes recommendations for Proving existing smoothness specifications. Five appendixes are provided in support of this report. Appendix A provides a summary of pavement smoothness-measuring practices by State, while appendix B presents an annotated bibliography of pertinent literature identified In the literature search. Appendix C contains a summary of historical roughness plots and graphs collected from the participating SHAs and appendix D presents relationships between initial smoothness and pavement life for selected SHA projects. Finally, appendix E contains a brief description of the characteristics anc! capabilities of various roughness-measur~ng devices. 5
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LIST OF FIGURES (continued) Figure 23 Figure 24 Figure 25 Figure 26. Figure 27 Figure 23 Figure 29e Figure 30e Figure 31e Figure 32. Figure 33. Figure 34. Figure 35e Figure 36. Figure 37e Figure 38. Figure 39e Figure 40. Figure 22 Effect of al coefficient in the relationship between time and future roughness . e~eeeeeeee~ Wee e Regression analysis of a Georgia PCC project . . . . . . . . . . . . . . . . . Percentage of projects showing significance of initial pavement smoothness (by State and pavement types). . . . . . . . . . e.. Percentage of projects showing significance of initial pavement smoothness (by State and type of construction). . . . . . . . . . ee. . e Overall percentage of projects showing significance of initial pavement smoothness (by pavement typed . . . . ee Overall percentage of projects showing significance of initial pavement smoothness (by type of construction). Percentage of projects showing significance of initial pavement smoothness (by age range and pavement type). Percentage of projects showing significance of initial pavement smoothness (by age range and type of construction) . . . . . . . . . . Average al values by pavement type for all projects and age ranges Average al values by type of construction for all projects and age ranges . eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee Average al values by pavement type for all projects and age ranges Breakout of overall significance by pavement type . . eeeeee Percentage of new construction projects (by State) for which initial pavement smoothness is significant oooooeeeeeeeee~eee~e o Percentage of overlay projects (by State) for which initial pavement smoothness is significant . . eee~eeeeeee~ eve e Historical roughness plots for LTPP GP~5 experiment (Kohn et al. 1996) . . e. ee. e Example three-d~mensional plot of linear regression model e. Example three-dimensional plot of linear regression model with initial smoothness-time interaction term aeeeeeeeeeeeee~ Example three-dunensional plot of multiple nonlinear regression model . . . . . . ewe eleven e Conceptual illustration of roughness model procedure for determining smoothness-life relationships Figure 41. Conceptual illustration of pavement life estimation using regression failure curves en eeee~ Figure 42. Example plot of pavement life versus initial smoothness using the roughness model approach . . . . . .. . . . . . . . . .. . . . . . ewe Figure 43. Example sensitivity plot showing percentage change in life versus percentage change In roughness Figure 44. Pavement failure curves for PCC interstate pavements in Kentucky (all traffic levels) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I 1V 51 52 66 66 67 67 70 ~71 73 73 74 76 77 78 84 87 88 89 92 94 95 . 100
Representative terms from entire chapter: