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OCR for page 201
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.
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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
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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,
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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)
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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
Representative terms from entire chapter:
initial pavement
