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16 chapter four PERFORMANCE, MAINTENANCE, AND REHABILITATION OF THIN OVERLAYS This chapter provides detailed findings on how transportation agencies assess performance of thin overlays, expectations for service life, and factors used to determine whether thin overlays are considered successful. Some diversity in the DOT responses was expected because the definition of thin asphalt overlay is applied differently from state to state. A thin asphalt surface can range from a simple small NMAS dense-graded mix to a specially formulated OGFC for high traffic (Shatnawi and Toepfer 2003). As such, the expec- tation of the DOT may vary with the type of application. For example, North Carolina DOT evaluated the use of ultrathin wearing course for jointed plain concrete pave- ments and used reflective cracking as the measure of per- formance (Corley-Lay and Mastin 2007). This summary of the performance of thin overlays does not attempt to separate the range of mixes and applications because some of the responses are not specific and could not be classified by mix type. This section of the report divides the agency responses into the four basic AASHTO climate regions to see if there are any patterns related to climate. The four climate regions were developed during the Strategic Highway Research Pro- gram in the early 1990s based on weather station data. The climate regions define the relative degree of precipitation (dry or wet) and low temperature (freeze or no freeze). States that experience more than one climate type were placed in the predominant category. Other reports noted that climate is a factor in the selection of a thin asphalt surface (Irfan et al. 2009; Wang et al. 2012a; Liu and Gharaibeh 2013). The analysis of the survey data by climate region is lim- ited by the lower number of responses from the agencies in three climate regions. At the same time, an analysis of all the data in a single group would be dominated by the large response from the wet-freeze climate region. The analysis is based on the percent of responses within each climate zone. The responders were asked how they monitored or verified the service life of thin asphalt overlays. Half of the responses indicated they used a single method, mostly manual condi- tion surveys. The other half responded with multiple meth- ods. Figure 10 combines all responses and expresses the tally based on the percent of responses in each category for each climate region. The use of video records was noted in all four climate regions. The use of deterioration curves to monitor pavement service life had the lowest response. The responses of the industry group expressed a large emphasis on man- ual condition surveys and no use of deterioration curves or threshold values. Just as the definition of a thin overlay varies, an agencyâs measure of service life will depend on the various types of surface (Brewer and Williams 2005). Liu and Gharaibeh (2013) compiled 341 thin overlay test sections from the LTPP GPS and SPS data representing 40 states and eight Canadian provinces. The data were divided into subsets according to the four climate zones based on precipitation and a freezing index. Liu and Gharaibeh observed that cli- mate was a factor in the performance of thin asphalt over- lays. Dry-freeze and wet-freeze climates achieved shorter performance life. Moisture had a lesser impact on service life. The analysis determined that the median life expec- tancy for the four climate groups was 7 years (wet-freeze) to 9.5 years (wet-no freeze). The responses from agencies that participated in this synthesis survey give a similar time frame (Figure 11). PERFORMANCE STANDARDS AND THRESHOLD VALUES In Indiana, a Purdue University study (Irfan et al. 2009) dis- cussed three performance measures used to determine overlay treatment lifeâInternational Roughness Index (IRI), PCR, and rut depth. It was found that overlay treatments have different service lives depending on the criteria used for the evaluation. The thresholds used and expected service life for thin asphalt overlays is given in Table 7. MAINTENANCE One of the options for extending the life of thin overlays is to apply a fog seal. This may be helpful to retard raveling as the pavement ages. The responders were asked how the optimal rate for fog seals was determined. Figure 12 displays the dis- tribution of the responses. The predominant response was no fog seals or rejuvenators are used. There were slightly more responses for the agency standard rate over an application rate based on level of distress.
17 FIGURE 11 Reported service life of thin asphalt concrete overlays. (Source: Survey responses.) (Chart shows percent of responses from each climatic zone; table shows number of responses.) Legend Key >12 yrs. 10-12 yrs. 8-10 yrs. 5-8 yrs. <5 yrs. Dry-Freeze Climate Dry-No Freeze Climate Wet-Freeze Climate Wet-No Freeze Climate Industry > 12 yrs 1 0 2 1 1 10-12 yrs 1 1 6 4 0 8-10 yrs 0 2 10 2 2 5-8 yrs 4 2 11 3 2 < 5 yrs 1 2 5 0 0 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Su rv ey R es po ns e FIGURE 10 How pavement service life is monitored/verified and number of responses for each category. (Source: Survey responses.) (Chart shows percent of responses from each climatic zone; table shows number of responses.) Legend Key Threshold Curves Video Manual Dry-Freeze Climate Dry-No Freeze Climate Wet-Freeze Climate Wet-No Freeze Climate Industry Threshold 1 0 9 1 0 Curves 1 0 4 1 0 Video 3 3 8 4 2 Manual 0 3 11 4 5 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Su rv ey R es po ns e
18 Performance Indicator Roughness (IRI) Condition (PCR) Rut Depth Threshold used 110 in./mi (1.74 m/km) 85 0.25 in. (6 mm) Expected life (years) 7â10 7â11 8â11 Source: Irfan et al. (2009). TABLE 7 EXPECTED SERVICE LIFE OF THIN ASPHALT OVERLAYS (Chart shows percent of responses from each climatic zone; table shows number of responses.) Legend Key Distress based Standard rate Not used Dry-Freeze Climate Dry-No Freeze Climate Wet-Freeze Climate Wet-No Freeze Climate Industry Distress based 1 1 3 0 1 Standard rate 2 1 4 1 2 Not used 2 2 12 5 3 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Su rv ey R es po ns e FIGURE 12 Method used to determine fog seal or rejuvenator application rate. (Source: Survey responses.) The responders were asked how they determine when special procedures are used to maintain surface condition (Figure 13). As examples, special procedures could include texturing, grinding, or fog seals. The key part of this survey question is âwhenâ the special procedure was needed, not what special procedure was needed. Using the response cat- egories provided in the survey, the results show two catego- ries split a majority of the response. Approximately 35% of the responses indicated no special activities are used, and 35% stated the special activity was based on type and level of distress. Pavement condition rating and smoothness split the remaining 30% of responses. There were only two responses that indicated the agency automatically scheduled intervals. The âno activityâ response (35%) for this survey question was noticeably smaller than the response for fog seals (more than 50%). One could conclude that most agen- cies are monitoring surface condition of their thin asphalt overlays, but aging of the surface (need for a fog seal) is not a priority or that agencies question the effectiveness of fog seals. COST/BENEFIT OF THIN OVERLAYS A 2012 study (Wang et al. 2012b) based on survey results from 29 state DOTs compared the extension of pavement life for several preservation treatments for flexible pave- ments. The study found that thin asphalt overlays were the most expensive initially, but they also extended pavement life the longest. Based on the survey responses, thin overlays extended pavement life an average of 5.4 years; chip seal, 1.9 years; crack seal, 1.7 years; and slurry seal, 1.1 years. An Oregon study evaluated 87 sites in the state that were treated with different types of materials for preservation treat- ment (Parker 1993). The study concluded that thin, dense- graded asphalt overlays appeared to be the more cost-effective treatment on a life-cycle basis, particularly in heavy traffic conditions. Based on the variability in service life reported by agen- cies, as shown in Figure 14, responders were asked to explain
19 FIGURE 13 Basis for using special procedures to maintain surface condition. (Source: Survey responses.) (Chart shows percent of responses from each climatic zone; table shows number of responses.) Legend Key Automatic Smoothness Rating Distress No special Dry-Freeze Climate Dry-No Freeze Climate Wet-Freeze Climate Wet-No Freeze Climate Industry Automac 0 0 2 0 0 Smoothness 0 1 3 0 1 Rang 1 1 5 2 1 Distress 1 2 10 3 2 No special 4 2 9 3 2 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Su rv ey R es po ns e FIGURE 14 Possible explanations for large range in service life. (Source: Survey responses.) (Chart shows percent of responses from each climatic zone; table shows number of responses.) Legend Key Construction Quality Traffic Volume Surface Preparation Existing Surface Dry-Freeze Climate Dry-No Freeze Climate Wet-Freeze Climate Wet-No Freeze Climate Industry Construct. Quality 1 2 6 2 1 Traffic Volume 3 1 4 1 2 Surface Prep. 1 0 4 3 0 ExisÂng Surf. 2 1 10 4 2 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Su rv ey R es po ns e
20 FIGURE 15 Mileage versus service life distribution of thin overlay projects on the general system in Ohio. (Source: Chou et al. 2008.) whereas Minnesota and Louisiana described an ultrathin bonded wearing course. Several agencies reported that they had thin asphalt overlay projects that significantly failed to perform as expected. Similar to the answers for the previous question, none of the DOTs in the dry-freeze climate region stated they had experienced a sig- nificant failure. In the two wet climate regions and in the dry-no freeze climate region agencies stated they had a significant fail- ure. In all, seven DOTS responded that they had experienced a failure: Arizona, Kansas, Missouri, Ohio, Rhode Island, Geor- gia, and Massachusetts. Three of these DOTs provided insight to the reason for early failure. Ohio attributed the failures to pavement rehabilitation selection, construction, or traffic load. Massachusetts also identified pavement rehabilitation selection as a critical factor along with surface preparation and adequate tack coat. Georgiaâs comment implies that reflective cracking was a significant problem. Tennessee has recently made comparisons in bid prices based on cost per square yard for both microsurfacing and thin lift 4.75-mm NMAS asphalt mixtures in their pavement preservation treatments. A bid comparison (Table 8) shows that thin overlays are competitive in price with the micro- surfacing preservation alternative. why there is a large range in service life. Figure 15 summa- rizes the responses. This question did not attempt to classify types of thin asphalt overlays. It does provide four categor- ical responses that are common for most paving sections. Most responders identified multiple categories. Similar to the survey response for service life, the response to âwhyâ differs by climate region. Traffic volume and condition of the existing surface are the predominant response in the dry-freeze climate. Construction quality is half of the dry- no freeze response but is based on only two responses. The wet-freeze response noted that condition of the existing surface was important and the other categories were gener- ally equally weighed. The responses in the wet-no freeze climate were similar to the wet-freeze climate, but more con- cern was expressed about the amount of surface prepara- tion (including selection of the right treatment for the right project). The industry response is similar to that of the DOTs in that variation in construction quality is only 20% of the total. Other comments given by respondents as to why there may be a large variation in service life were as follows: ⢠Environmental conditions ⢠Variations in construction quality standards (interstate versus secondary roads) ⢠Regional variations in material and construction quality ⢠Roads needing rehabilitation were only mill and fill because of cost or other factors. Seven DOTs responded that they had thin asphalt overlay projects that exceeded their expectations. Those agencies were Texas, Kansas, Ohio, Rhode Island, Minnesota, Georgia, and Louisiana. Three DOTs added a comment relative to their response that included the type of thin asphalt overlay. Texas referred to the performance of permeable friction course, Year Microsurfacing ($/yd2) 4.75-mm NMAS ($/yd2) 2013 2.02 2.24 2011 2.41 1.88 2009 2.15 2.09 Source: Woods (2010). TABLE 8 COMPARISON OF TENNESSEE DOT BID PRICES FOR PRESERVATION TREATMENTS
