Anticipation is Sweet
|<< Previous||Contents||Next >>|
Research Examines Results of Preventive Maintenance on Pavements After 14 Years in Service
by Larry Galehouse, P.E., P.S., and John O'Doherty, P.E.
When applied early, preventive maintenance treatments used as part of a sound pavement preservation strategy will cost less than the reconstruction and rehabilitation of highways that are allowed to deteriorate. Michigan, which has been active in implementing pavement preservation programs, has experienced as much as a 10:1 return on preventive maintenance investments.
Network pavement quality improvements also have rewarded states with preservation programs. Since instituting a pavement preservation program in the early 1980s, the Kansas Department of Transportation has seen its good pavements increase from less than 50% in 1983 to approximately 95% in 2003. Yet, despite advances in materials and clear benefits gained over the past 14 years, we still need to gain a better understanding of the optimal timings for the application of the various preventive maintenance treatments.
While we all want highways to improve using available funding, it takes time for a preservation strategy to produce readily observable results. Despite the plethora of professional courses, training workshops, seminars, CDs and video programs promoting pavement preservation for agencies and industry, a lack of good, real-world data tends to inhibit the understanding and general acceptance of the concept. Here, we want to further document the effectiveness of pavement preventive maintenance treatments, some of which have far exceeded their expected service lives.
Sections to the test
The Specific Pavement Study-3 (SPS-3) project titled, "Preventive Maintenance Effectiveness of Flexible Pavements," part of the Strategic Highway Research Program's (SHRP) Long Term Pavement Performance (LTPP), compared the effectiveness and mechanisms for selecting maintenance treatments to preserve and extend flexible pavement service life, safety and ride quality. The overall goal was to assess the benefits of treating test sections rather than scoring the relative performances of various treatments. Study factors included climatic zone, subgrade type, traffic, initial condition and structural adequacy. Treatments tested were slurry seal, chip seal, crack seal and thin hot-mix asphalt (HMA) overlay.
SHRP administered the LTPP project for its first five years, after which it was managed by the Federal Highway Administration (FHWA). The FHWA's DataPave Internet site contains performance data collected on individual projects. The FHWA's Pavement Division formed Regional Expert Task Groups to conduct periodic field performance reviews of the treatments.
Although some of the SPS-3 projects were later overlaid, 48% were still operating in 2004 when the seven projects had been visited, photographed and analyzed for performance after 13 or 14 years of service - long past their expected design lives. DataPave data also were analyzed.
The SPS-3 research sought to define the most effective treatment application timings, evaluate treatment effectiveness for prolonging pavement lives and share information and experience among highway agencies and industry. SPS-3 projects comprised five test sections: an untreated control, a thin HMA overlay, an emulsion slurry seal, a crack-seal section and an emulsion chip seal. Some states also tested other sections. Eighty-one SPS-3 sites with 486 test sections were placed in the U.S. and Canada in 1990 and 1991. All sections were monitored for performance by LTPP.
The Expert Task Groups developed site-specific construction specifications and coordinated construction. To reduce variability, the same placement crews and supervision were used throughout each of the four LTPP regions for slurry and chip seals. The same slurry seal emulsion was used for all projects, as was a single chip seal emulsion. Aggregates varied by state. The crack sealing materials and crews were the same in each region. Thin HMA overlays were provided by local agencies, with a different HMA and placement crew at each site. For the seven projects described here, the HMA overlay lift thickness varied from 0.7 in. in Texas to 1.8 in. in Missouri.
LTPP and the regional Expert Task Groups evaluated section performances through the first five years. Some deteriorated sections were overlaid and abandoned. State monitoring continued on several of the remaining sections. Evaluation tools included the following:
- Distress surveys using the SHRP P-338 Identification Manual;
- Deflection using a falling-weight deflectometer;
- Ride quality, or longitudinal profile, using the K.J. Law-type profilometer; and
- Rut depth using PASCO data and Dipstick.
After the planned five-year evaluation period, several reports were written, and the Expert Task Groups formed the following general conclusions:
- Preventive maintenance treatments generally outperformed control sections;
- Treatments applied to good pavements gave good performance;
- Traffic and structural adequacy did not appear to affect performance;
- Crack seals performed best when cracks were pre-routed and provided the most benefit when applied to good pavements;
- Slurry seals were best in no-freeze climates, outperformed controls and were most effective on good pavements;
- Chip seals performed well except when placed over pavements in poor condition in wet-freeze zones;
- Thin overlays consistently outperformed untreated controls; and
- Generally, preventive maintenance treatments exceeded expectations.
Although the North Atlantic region sections were abandoned by the end of the five-year experiment, after 14 years, 39 of the original 81 SPS-3 test sections were still operational. The seven projects, representing a broad cross section of environmental zones, were constructed in 1990 and evaluated in 2003 and 2004. The selected projects represent wet, no-freeze and wet, freeze zones; southern, north-central and western regions; and 1990 annual equivalent single-axle loads between 11,000 and 307,000.
The LTPP DataPave analysis for the seven projects revealed:
- The International Roughness Index showed no observable trend;
- Thin HMA overlay sections had slightly better overall rut performance than other treatments. (This may have been affected by pre-existing rutting in the wheel paths.);
- Chip seals provided the best overall cracking performance. This observation is consistent with a Texas DOT report of a study of 14 SPS-3 Texas projects;
- Initial cracking tended to be reflective, caused by underlying cracks or by thermal forces or both, and later by fatigue;
- Initially, cracking in the slurry and chip seals diminished, only to resume when the slurry seals began to wear off;
- Except for thin overlays, the treatments increased initial roughness; and
- Thin overlays produced the smallest changes in roughness, while the control and crack seal sections produced the largest increase in roughness.
Pay more later
Pavement preservation is a long-term strategy enhancing functional pavement performance using integrated, cost-effective practices that extend pavement life, improve safety and motorist satisfaction while achieving sustainable, manageable condition levels for pavement networks. Pavement preservation includes routine and preventive maintenance and minor rehabilitation.
Reconstruction replaces an existing pavement structure with an equivalent new pavement structure. Rehabilitation, including restoration treatments and structural overlays, enhances an existing pavement's structure to restore its load capacity. Resurfacing applies additional pavement material (typically more than 1.5 in.) to provide more structural integrity or improved rideability. The reconstruction, rehabilitation and resurfacing costs in Figure 1 are from FHWA's "Highway Statistics 2001" (the most recent available) publication and do not include adding lanes to increase capacity. The annualized numbers assume a 20- to 30-year (average 25-year) life for reconstruction, 10- to 20-year (average 15-year) life for rehabilitation and 10- to 15-year (average 12.5-year) life for resurfacing. Costs vary regionally.
SPS-3 project construction costs were atypical. Figure 1 exhibits typical preventive maintenance costs averaged from 66 projects of the same type placed in Michigan in 2001. They include mobilization, traffic control and required warranty bonds. They have been annualized for the average life extension to the underlying pavement, not the treatment life. Michigan assumed an average 7.5-year life extension from thin HMA overlays, 5 years from slurry seals, 2 years from crack seals and 4.5 years from chip seals.
Agencies should determine treatment performances for their specific conditions. Figure 1 shows potential savings in a program emphasizing preventive maintenance. The annualized cost could be $2,600 for a crack seal or $2,800 for a chip seal to keep a good road in good condition (distinct from extending its useful life) at a mere fraction of the reconstruction cost. In fact, a simplified analysis shows that lane-mile costs for four slurry seal treatments applied over a 25-year period would be approximately $68,000, while without maintenance, the reconstruction cost would be $330,000. With performance similar to that of the projects examined here, even greater savings could be expected.
LTPP DataPave website data were used to evaluate the seven SPS-3 projects after 13 or 14 years of service. These conclusions are only for the seven projects and may not apply to the 52% of SPS-3 projects no longer in service.
Significantly, after 14 years, chip seal sections are generally performing longer than expected, reducing longitudinal, transverse and fatigue cracking, and have been especially effective in sealing and protecting center-line joints. DataPave cracking results generally mirror the visual surveys. Slurry seals are showing signs of wear, but did provide sealing protection for most of their service lives, and their underlying pavements were generally in better condition than the crack sealed and control sections. Crack sealing results were mixed, confirming earlier conclusions by Expert Task Group observers that routed seals perform better, and performance also may depend on pretreatment conditions. Conventional thin HMA overlays, ranging in thickness from 0.7 to 1.8 in., had the lowest International Roughness Indexes and rutting, but were then exhibiting fatigue (and other) cracking and some potholes. Some state-specific special sections did well (most notably the Bonifiber overlay in California), while others did not.
Despite being in the most severe climate, Michigan's SPS-3 project performed well, although it was in good condition before treatment. The Illinois, Michigan and Missouri original pavements were in the best condition of the studied sections, and inspections confirmed the efficacy of preventive treatments. The excellent conditions of the Michigan and other test sections clearly illustrate that preventive maintenance treatments in all climates provide protection and extend service lives when appropriately applied. Selection of the best treatment depends upon many factors including initial pavement condition, local availability of quality materials and construction, local user-delay costs, climate and traffic.
Annualized costs show preventive maintenance treatments applied early in a pavement's life are more cost-effective than allowing the pavement to deteriorate until it needs resurfacing, rehabilitation or reconstruction. A preventive maintenance strategy costs less and raises the overall quality of the road network, keeping good roads good. Preventive maintenance treatments reviewed here have performed well beyond their design lives, indicating even lower possible annualized costs and higher pavement quality. Since 1990, when these projects were initiated, we have seen many improvements in materials, construction techniques, equipment and performance-related specifications, and these improvements should extend pavement life and lower costs even more.
Galehouse is director of the National Center for Pavement Preservation (NCPP), Okemos, Mich. O'Doherty is in charge of education and training at the NCPP.
Reprinted from Roads & Bridges, June 2005.
|<< Previous||Contents||Next >>|