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Statistical Analysis of LTPP SPS-3 Experiment on Preventive Maintenance of

Flexible Pavements

Hamid Shirazi Applied Research Associates, Elkridge, Maryland, United States

Regis L. CarvalhoApplied Research Associates, Elkridge, Maryland, United States

Manuel Ayres JrApplied Research Associates, Elkridge, Maryland, United States

Olga SeleznevaApplied Research Associates, Elkridge, Maryland, United States

A B S T R A C T

This paper describes the evaluation of preventive treatments in mitigating the rate of distress propagation inflexible pavements. The analysis was based on data from preventive maintenance treatments data collected inthe Long Term Pavement Performance (LTPP) program. Data were obtained from 81 sites across the UnitedStates and Canada which were part of the specific pavement experiments (SPS-3). SPS-3 was designed tomonitor the performance of four treatments: thin overlay, chip seal, crack seal and slurry seal under differentdesign conditions. Design conditions considered were precipitation, temperature, traffic, subgrade materialsand pavement condition prior to applying preventive treatment. Fatigue cracking, rutting and longitudinalroughness data collected during the LTPP program were used to compare the overall performance of differ-ent treatments. A weighted average index was defined to represent the overall performance of the sections overthe years. Statistical techniques were used to compare the effectiveness of each treatment in relation to oth-ers and the control section, which did not receive any treatment. Conclusions from the analyses indicated thatthin overlay and chip seal are effective treatment options for most design conditions with respect to fatiguecracking. Thin overlay outperforms other treatments in most design conditions with respect to rutting andin some cases with respect to roughness. The difference between the performance of crack seal, slurry seal andcontrol section was not found to be statistically significant with respect to any distress type and design factor.

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B A C K G R O U N D

Rehabilitation and pavement preservation represent the majority of pavement maintenance activity in theU.S. The importance of pavement network preservation cannot be overemphasized. Preventive maintenanceincludes treatments that are applied to pavements primarily to prevent development of distress or to reducethe rate of damage development. Preventive maintenance activities are focused primarily on improving pave-ment functional performance and prolonging pavement life through the following: • Waterproofing the pavement by sealing fine cracks or fissures.• Restoring proper characteristics of the pavement surface (e.g., to counteract weathering, aging, harden-

ing, or raveling of asphalt).• Preventing further deterioration of cracks from contamination with incompressible materials that lead to

crack enlargement, and from hardening and raveling of the exposed crack surfaces.Preventive maintenance activities are not intended to have a direct improvement of pavement structural

capacity. Selection of the appropriate type and timing of treatment application forms the basis for a soundpreventive maintenance practice. The selection of the preventive treatment is often based on the experienceand local practice specific to a region or district within a public highway agency.

To help highway agencies to make the most informative and cost effective decision regarding preventivemaintenance options, the Strategic Highway Research Program (SHRP) initiated project H-101, PavementMaintenance Effectiveness in late 1980s. The purpose of project H-101 was to develop a database that wouldpermit increased understanding of selected maintenance treatments in extending pavement service life or re-ducing the development of pavement distress, including an evaluation of the cost effectiveness of pavementmaintenance treatments.

Flexible pavement sites selected for SHRP H-101 continued to be monitored under FHWA LTPP pro-gram as Special Pavement Studies-3 (SPS-3) experiment. The LTPP SPS-3 experiment was designed in 1990to evaluate the effectiveness of maintenance alternatives and to determine the optimum timing for applyingtreatments for flexible pavements. In addition, local highway agencies were encouraged to integrate supple-mental experimental sections into collection and analysis associated with the SPS-3 experiment. The purposeof supplemental sites was to compare differences in performance of treatments due to different material char-acteristics and construction techniques employed locally by the agencies.

This research focused on evaluation of the results of the LTPP SPS-3 experiment. The objective of thisstudy was to assess the effect of the above mentioned maintenance treatments on pavement performance andto develop quantitative information on performance and optimum timing of preventative maintenance treat-ments. The assessment of treatments included identification of important pavement performance indica-tors, analysis of the impact of each treatment on pavement performance, and analysis of the impact oftreatment timing on pavement performance.

Over the years, numerous SHRP, FHWA and NCHRP sponsored studies were conducted to assess SPSexperiments status, key data element availability (e.g. traffic, subgrade, materials, monitoring), as well as con-duct preliminary analyses of the collected data. Smith, Freeman and Pendleton (1993) implemented damagemodeling approach with an index varying from 0 to 1. The index is dependent on accumulated traffic/age,expected traffic/age to failure and the shape of the performance trend. This model became the base forremaining life analysis used in developing the AASHTO 1993 design guide.

Morian, Epps and Gibson (1997) concluded that thin asphalt concrete overlay treatments performed bestafter 5 years as compared to other treatments. Chip seal performance was best in the southern region with pre-dominantly wet-no freeze environment. They also concluded that crack seal treatment performed very well

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in wet-freeze environments where the wide shallow sealant reservoir was routed. Crack seal performance inthe other two regions was not as successful. The study indicated that the question of timing could not be to-tally resolved from the visual observation of the SPS-3 sites, but indications were that earlier applications ofthe preventative maintenance treatments provided greater benefits than later application.

A year later in another study, Morian, Gibson and Epps (1998) concluded that structural adequacy did nothave a significant effect on the performance of SPS-3 treatments. Thin overlay had significant effect in rut-ting and roughness reduction while other treatment options were either slightly effective or non-effective.They developed multiple regression models for cracking, rutting, ride quality, and friction predictions and anindex called Pavement Rating Score (PRS).

Eltahan and Von Quintus (2001) conducted survival analysis of SPS-3 sites in the Southern LTPP regionin 1999 to obtain life expectancy of each treatment, effect of timing and the benefit of treatment to the life span.The study revealed that, after six years of service, sections which received maintenance when in poor condi-tion had a probability of failure twice as much as sections initially in fair or good conditions. Also, sectionsin fair and good condition had about the same probability of failure. The overall median survival times forthin overlay, slurry seal, and crack seal were 7, 5.5, and 5.1 years, respectively. A median survival time for chipseal could not be determined because fewer than 50 percent of these sections had failed at the time of theanalysis. The study concludes that chip seals outperformed thin overlay, slurry seal, and crack seal treatmentswith respect to controlling the reappearance of distress.

Hall, Correa and Simpson (2002) showed that more than 40 percent of SPS-3 sites had problems in theapplication of maintenance treatments, mostly chip seal. Analysis showed that the most effective maintenancetreatments was thin overlay treatment, followed by chip seal treatment and slurry seal in terms of roughness,rutting, and fatigue cracking. Thin overlay treatment was the only one of the four SPS-3 maintenance treat-ments to produce an initial small reduction in roughness, and the only one of the four to have a significanteffect on long-term roughness, relative to the control section. For rougher pavements, however, there wassome evidence that chip seals and slurry seals also had some effect on long-term roughness, rutting, and crack-ing, relative to the control section. Crack seals did not have any significance on long-term roughness, rutting,or fatigue cracking.

D E S I G N E X P E R I M E N T

SPS-3 experiment was initiated in the United States and Canada in 1990 and 1991. An experimental designwas developed to help determine the impact of important factors on flexible pavements. Design factors selectedinclude the following:

1. Moisture (2 categories: wet, dry)2. Temperature (2 categories: freeze, no-freeze)3. Subgrade type (2 categories: fine grained, coarse grained)4. Traffic loading (2 categories: low, high)5. Pavement condition (3 categories: good, fair, and poor)Therefore, the design experiment contained 11 categories. This created a total of 48 combinations (classes)

in the design experiment. 33 States and Canadian provinces participated in the experiment. Some States as-signed few sites to the experiment bringing the total number of experiment sites to 81. In practice however,there were some deviations from design. The number of sites in each design category is presented in Figure 1.As shown in the figure, the distribution of the sites is not uniformly based on actual experiment design. Forexample, there are more sites in freeze zone as compared to no-freeze zone; so are pavements in initial poorconditions compared to fair and good conditions.

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Figure 1. SPS-3 Experiment Breakdown Based on Major Design Factors

Table 1 illustrates the SPS-3 sites based on actual design factors. As an example, site labeled KE-A in Table1 is site A in Kentucky which is located in wet-freeze zone with fine subgrade material and low traffic. The sitewas initially in good condition. Although based on the experiment design, at least one site should have beenassigned to each of the 48 classes, many classes were left without a designated site in the actual experiment asshown in Table 1.

M A I N T E N A N C E T R E A T M E N T S

Four preventive maintenance treatments were considered in the experiment. These include thin HMA over-lay, slurry seal, crack seal and chip seal. The effectiveness of combinations of treatments was not considered.Test sites were constructed with five consecutive sections each receiving one of the treatments and a controlsection with no treatment. All sections of a site were subject to the same traffic loading and environmental con-ditions. In many cases these sites were linked to GPS sections serving as control sections. The following is abrief description of the treatments used in the LTPP SPS-3 study: • Thin Hot-Mix Asphalt Overlay – The primary purpose of thin HMA overlays was to improve pavement

surface condition, protect the pavement structure, reduce the rate of pavement deterioration, correct sur-face deficiencies, reduce permeability and improve the ride quality of the pavement, particularly when ac-companied by surface milling. For SPS-3 experiment, thin HMA overlay thicknesses ranged from 0.5 into 1.6 in.

• Slurry Seal – The primary purpose of slurry sealing was to seal low-severity surface cracks, waterproof thepavement surface, and improve skid resistance at speed 30 mph. Slurry seal is a mixture of well-graded ag-gregate (fine sand and mineral filler) and asphalt emulsion that is spread over the entire pavement sur-face with a squeegee or a spreader box attached to the back of a truck. Thickness is generally less than 0.4in.

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• Crack Seal – The primary purpose of crack sealing was to prevent the intrusion of moisture throughexisting cracks. Crack sealing is applied to “working” cracks ( i.e., those that open and close with changesin temperature). Sealants are typically thermo-plastic (bituminous) materials that soften upon heating andharden upon cooling.

• Chip Seal – The primary purpose of chip sealing was to seal pavement surface and improve friction. Anasphalt emulsion is directly applied to the pavement surface (0.35 to 0.50 gal/yd2) followed by the appli-cation of aggregate chips (15 to 50 lb/yd2), which are then immediately rolled to imbed chips.

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Table 1. Actual SPS-3 Categories

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A S S E S S M E N T O F S P S - 3 D A T A A V A I L A B I L I T Y

An important aspect of this study is in-depth knowledge of the contents of the LTPP database. The infor-mation in the LTPP database is divided into the following modules:• Inventory: section location, characteristics, and material characteristics.• Materials Testing: material properties and characteristics.• Climatic: temperature, humidity, precipitation, cloud cover, and wind statistics.• Maintenance: activities performed since inclusion in the LTPP program.• Rehabilitation: major improvements since inclusion in the LTPP program.• Traffic: annual traffic summary statistics since it was opened to traffic.

Each of these modules contains tables that provide information on the various design features and per-formance measurements of a particular pavement section. Figure 2 provides a summary of SPS-3 data avail-ability. Figure 2a presents the histogram of the number of times the sites were surveyed. As shown in theFigure 2a, the sites were surveyed between 2 and 9 times during the experiment. Most sites were surveyed 6times. Figure 2b presents the number of years the sites were monitored. As shown, 88% of the sites were mon-itored for at least 4 years and about 22% of the sites were surveyed for 10 years or more. Figure 2c illustratesthe number of times the treatments were applied to the sites. In 74% of the sites the treatment was appliedonly once as expected, however, 22% of the sites have received the treatment twice and 4% for 3 times. Treat-ment application more than 1 time might be due to the long time some sites were monitored. By March 1 2006,all SPS-3 sites were de-assigned from the experiment.

P E R F O R M A N C E I N D I C A T O R

To evaluate the effectiveness of preventive maintenance treatments regarding the performance of pavementsections, the alternatives at each site were compared among each other and the control section. Previous stud-ies have used many different performance indicators. The most common were: most recent survey measuresof distress (Hall et al, 2002), expert Task Group field reviews of distress (Morian et al, 1997), evaluation of dis-tress trends (Morian et al, 1998; Von Quintus, Simpson, and Eltahan, 2001; Ambroz and Darter, 2001), distressregression models (Morian et al, 1998), area under/above condition indicators (Chatti et al, 2004), distress levelimmediately after rehab/treatment (Ambroz et al, 2001), and average distress over survey period (Ambroz etal, 2001; Von Quintus, 2001)

Different alternatives for performance indicators were evaluated in this study. The objective was to findan indicator that could represent the pavement performance over the monitoring period and that provided asimple, stable and comparable parameter that could minimize the effect of survey measurement errors. Theparameter selected was the weighted average of the distress normalized over the survey period, as calculatedusing the following equation:

Where• WD is the weighted distress value (e.g. area of fatigue cracking) over the total survey period• i is the survey number (i=0 is the initial distress level immediately after the treatment)• Di is the distress value measured at the ith survey• Pi+1 is the period (in years) between survey i and survey i+1• n is the total number of surveys for the section

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Figure 2. Assessment of SPS-3 data availability

Compendium of Papers from the First International Conference on Pavement Preservation

The weighted average, in reality, represents the total area under the distress versus time curve normalizedby the total time period between first and last surveys. As such it is a measure of pavement performancerelative to the specific distress over the entire survey period. Moreover, the normalized value allows for com-parisons of pavement sections at different survey periods. The advantages of using the weighted distress valueare the following:• The measured variable is related to the pavement performance over the whole analysis period;• The analysis is repeatable and not subjective;• The concept is similar to the relationship between performance and serviceability;• The effect of variability from measurements by different surveyors is reduced;It offers a parameter that can be used to compare sections with different survey periods.

The effectiveness of treatments in prolonging the pavement life was evaluated using a couple of load-as-sociated distresses and ride quality surveyed in the LTPP program for the SPS-3 experiment. Performance wasevaluated as the deterioration measured by fatigue cracking, rutting and roughness (using the internationalroughness index, IRI).

Figures 3 to 5 illustrate Box-Whisker plot of weighted distress index for fatigue cracking, rutting and IRIof all SPS-3 sites. The boundaries of the box present lower and upper quartiles and the middle line is the me-dian. The whisker marks the minimum and maximum limits of the distress. Only the surveys that containeddistress measurements for all treatments were used to draw the graphs, and some outliers were excluded.

As shown in Figure 3, thin overlay and chip seal exhibited lower fatigue cracking quantities as comparedto slurry seal, crack seal and control section. The median of chip seal was slightly lower than thin overlay. Chipseal also presented the smallest min-max range. Sections that were treated with crack seal had lower weightedaverage fatigue cracking as compared to slurry seal and control section. Slurry seal seems to be less effectivein mitigating the progression of fatigue cracking over the monitored period.

Figure 4 illustrates a significant difference between sections that were treated with thin overlay over othersections. Thin overlay was effective to reduce rutting immediately after the treatment and, as consequence,presented a lower level of rutting over the analysis period. Although chip seal marginally outperformed therest of treatments, the difference has little advantage over the remaining treatments and it is not statisticallymeaningful.

Similar conclusion is found in terms of IRI performance, as show in Figure 5. Thin overlay was the mosteffective maintenance option in mitigating IRI over the years. Possibly, this type of treatment was the mosteffective to reduce initial roughness immediately after the treatment, rather than providing a significant struc-tural improvement of the pavement section However, inferring more conclusion from other treatments requiremore rigorous statistical analysis which is presented in the next section.

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Figure 3. Weighted average fatigue cracking; median min-max box chart

Figure 4. Weighted average rutting; median min-max chart

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Figure 5. Weighted average IRI; median min-max chart

S T A T I S T I C A L A N A LY S I S A P P R O A C H

The statistical test selected for the analysis, was the Friedman Test. This is a non-parametric test (distribution-free) used to compare paired observations on a subject. It is also called a non-parametric randomized blockanalysis of variance. Unlike the parametric repeated measures ANOVA or paired t-test, this test makes noassumptions about the distribution of the data (e.g., normality). In addition, unlike the t-test, Friedman testcan be used for multiple comparisons, as is the case for the SPS-3 experiment with 4 different types of treat-ment, in addition to the control section. Friedman test, like many non-parametric tests, uses the ranks of thedata rather than their raw values. The test statistic for the Friedman’s test is a Chi-square with n-1 degrees offreedom, where n is the number of repeated measures.

The performance of the pavement sections with the preventive maintenance treatment was comparedwith the performance of similar pavements without the treatment in the control sections, as well as betweenthe different treatment types. Friedman test was applied for all 11 categories of the design factors for each dis-tress type. The values used were the weighted average distresses normalized for the analysis period. The resultsindicate whether a statistically significant difference exists between any pair of treatments.

As an example, Table 2 presents the results for wet climate. A total of 41 sites were located in wet climaticregion with fatigue cracking measurements available for all treatments. Based on test results, thin overlay andchip seal had the lowest sum of ranks, and slurry seal and control section presented the highest. A lower sumof ranks indicates better performance.

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Table 2. Friedman Results of Fatigue Cracking for Treatments in Wet Climate

Analysis results indicated that the difference in performance of thin overlay and chip seal compared to slurryseal and the control sections was significant. Also, chip seal performed significantly better than crack seal.The difference between other treatments was not significant.

The results for all design conditions are summarized in Table 3. To compare the treatments, a design cat-egory from the left side of the table and a distress from the top of the table are chosen. One of the treatmentsfrom the left and another one from the top are selected. If the intersecting cell is empty, there is no signifi-cant difference identified; otherwise, the cell is filled with treatment performing better. In the table, TH, SL,CR, CH and CO are acronyms for thin overlay, slurry seal, crack seal, chip seal and control section respectively.

As shown in the table, chip seal and thin overlay performed better with respect to fatigue cracking. Thinoverlay was the preferred treatment with respect to rutting. No treatment was found prevalent in treating IRI;however, thin overlay outperformed other treatments in some cases.

Although conclusions such as these are valuable, the main contention of the SPS-3 is to identify theeffects of the major design factors on treatments performance. In other words, the experiment was intendedto identify if different climate conditions, subgrade material, traffic level or initial pavement condition affectsthe choice of a preferred treatment. Therefore, the results from different categories of design factors were com-pared for each distress, as summarized in the following sections.

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Fatigue Cracking

Temperature makes a significant difference when comparing thin overlay with slurry seal and the control sec-tion. Thin overlay outperforms both treatments in freeze zone. Temperature does not affect the performanceof other treatments since chip seal outperforms slurry seal, crack seal and control section in both freeze andno freeze zones. The same conclusion is valid for precipitation. Thin overlay outperforms slurry seal and con-trol section only in wet regions. There are no other significant differences in the performance of other treat-ments with respect to temperature.

It was found that the subgrade type affects the performance of the thin overlays. Thin overlay generallyperforms better than slurry seal if the subgrade is coarse material. Traffic affects the performance of the thinoverlay as compared to slurry seal and control section. Under high traffic condition, the performance of thinoverlays prevails over both the control and slurry seal sections.

The performance of the maintenance treatments with respect to initial pavement condition is regardedas application timing. Timing of treatment application influences its performance. If the pavement is in poorconditions, thin overlay performs better than slurry seal, crack seal and the control section. Also, chip seal out-performs crack seal with a significant statistical difference under such pavement conditions. It is importantto note that the comparisons were based on fatigue cracking surveys and chip seals, in general, may only maskthe cracks, rather than correcting the distress.

Rutting

In most cases thin overlay performs better for rutting when compared to the other treatments and the con-trol section. Temperature plays a significant role when comparing chip seal and slurry seal performance.Under freezing condition, chip seal outperforms slurry seal. Only precipitation was statistically influential inrutting when comparing chip seal with thin overlay and slurry seal. In wet conditions, thin overlay outperformschip seal, and in dry condition chip seal performs better than slurry seal.

Under higher traffic roads, thin overlay performs better than the control section while in low traffic thereis not a significant difference. Also surprisingly, slurry seal performs worse than control section under low traf-fic condition with respect to rutting. No other design factor affects the performance of the pavement in a sta-tistically significant way with respect to rutting

International Roughness Index

Temperature is an important design factor to define maintenance activities in regard to surface smoothness.Thin overlay performed significantly better than slurry seal, crack seal and chip seal in freezing condition. Inno-freeze condition, there were not a significant differences among treatments with respect to IRI. Precipita-tion does not affect the performance of the treatments. There is no significant difference among treatmentsin dry and wet conditions.

In pavements with coarse subgrade, thin overlay was superior to chip seal. In pavements with fine sub-grade, thin overlay outperforms control section. Subgrade material does not influence the performance of thetreatments in any other combination.

Traffic also affects the performance of maintenance treatments when roughness is considered. Thin over-lay outperforms crack seal, control section and chip seal in roads under higher traffic. In low traffic roads therewere no significant differences.

Pavement initial condition affects the performance of the treatments. Thin overlay performs better thancrack seal, control section and chip seal if the condition of the pavement is poor. There is no significance dif-ference in other combinations.

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Table 3. Summary of Friedman tests results

*TH, SL, CR, CO and CH are the acronyms for thin overlay, slurry seal, crack seal, control section and chip seal, respectively.

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C O N C L U S I O N S

Based on the analysis of SPS-3 sites of LTPP program, the following could be concluded:• Fatigue Cracking: Thin overlays and chips seal were more effective than slurry seal and crack seal treat-

ments in mitigating fatigue cracking;• Rutting: Thin overlay was the prevalent option with respect to rutting. Chip seal was more effective than

slurry seal in freeze zones and in wet regions. There were no significant differences between slurry seal,crack seal and the “do nothing” scenario with respect to rutting.

• If the pavement is in freeze zone or with high traffic or initially in poor condition, thin overlay performsbetter than when the pavement is in non-freeze zone, with low traffic and in fair or good condition withrespect to fatigue cracking and IRI;

• Thin overlay was also more effective in wet regions only with respect to fatigue cracking;• Chip seal performance was not affected by any of the design factors with respect to fatigue cracking;• Design factors have very little or no influence on treatments with respect to rutting. Chip seal is only

marginally more effective in freeze zone and in dry climate.

A C K N O W L E D G M E N T

The authors would like to acknowledge FHWA and LTPP program for sponsoring this study conducted undera LTPP research project.

R E F E R E N C E S

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October 2005.

Chatti, K., Buch, N., Haider, S. W., Pulipaka, A., Lyles, R. W., and Gilliland D. (2004). LTPP Data Analysis: Influence of Design and Con-

struction Features on the Response and Performance of New Flexible and Rigid Pavements, Draft Final Report. National Cooperative

Highway Research Program, NCHRP Project 20-50(10/16), Transportation Research Board, National Research Council. 2004.

Eltahan, A. A., Von Quintus, H. L., (2001). LTPP Maintenance and Rehabilitation Data Review. Federal Highway Administration,

Office of Infrastructure Research and Development, Washington, D.C., Report Number FHWA-RD-01-019. 2001.

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Cooperative Highway Research Program, NCHRP Web Document 47 (Project 20-50[3/4]): Contractor’s Final Report. June 2002.

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Federal Highway Administration, Tech Brief, Report No. FHWA-RD-96-208. 1996.

Morian, D. A., Gibson, S. D., and Epps, J. A., (1998). Maintaining Flexible Pavements – The Long-Term Pavement Performance

Experiment SPS-3 5-Year Data Analysis. Federal Highway Administration, Office of Infrastructure Research and Development,

Washington, D.C., Report No. FHWA-RD-97-102. 1998.

Smith, R. E., Freeman, T. J., and Pendleton, O., (1993). Pavement Maintenance Effectiveness. Strategic Highway Research Program,

Report SHRP-H-358. 1993.

Von Quintus, H. L., Simpson, A. L., and Eltahan, A. A., (2001). Rehabilitation of Asphalt Concrete Pavements – Initial Evaluation of

the SPS-5 Experiment. Federal Highway Administration, Office of Engineering Research and Development, Report No. FHWA-RD-

01-168. 2001.

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