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MICROSURFACING SUMMARY Microsurfacing is a widely used tool for both pavement preservation and preventative maintenance. It is generally considered to be a highly specialized process, and public highway agencies often depend on the experience of the microsurfacing contractor and its emulsion supplier for both design and construction. This report documents the state of the practice of this pavement preservation and maintenance tool and identifies critical knowledge gaps that could be filled by additional research. The objective of this synthesis is to identify current practices in microsurfacing that highway maintenance practitioners report as effective in addressing specific pavement preservation and maintenance problems. To accomplish the stated objective, the consultant used four instruments to collect and process data: 1. A comprehensive review of the literature. 2. A survey distributed to maintenance engineers at all state departments of transportation (DOTs) and Canadian provincial transportation agencies. The DOT survey achieved an 88% response rate; the Canadian survey response rate was 93%. 3. Evaluation of all 50 state DOT microsurfacing specifications as well as the one used by the U.S. Federal Lands Highway Division (FLHD). 4. Case studies of six microsurfacing projects from five U.S. states and one Canadian province. Although microsurfacing is the specific subject of the report, the review of literature and state DOT specifications found that microsurfacing and slurry sealing were often included together in the same specifications section, many times with little or no differentiation between the two treatments. Only 18 of 51 standard specifications included a section specifically titled "microsurfacing." Other agencies had sections with titles such as "Cold-Laid Latex Modified Emulsion Pavement Course" (Pennsylvania) and "Paver-laid Surface Treatment" (Alabama). An example of this microsurfacing terminology issue is the FLHD specification, which states in Section 410--Slurry Seal: "This work consists of applying an asphalt slurry seal or a poly- mer modified microsurfacing mix on an existing pavement surface" (2003, italics added). The word "microsurfacing" is found only in this sentence and the remainder of the section does not differentiate between the two treatments, which gives the two the same set of specifications. Therefore, an effort to standardize the terminology in this area would be beneficial. The Inter- national Slurry Surfacing Association advocates categorizing both as "Slurry Systems" while maintaining the following distinction: microsurfacing always contains a polymer-modified emulsion that is designed to break chemically and, as a result, can be turned over to traffic within a short period of time (usually about an hour after application). Second, the survey found that few agencies have a formalized approach to their micro- surfacing program that evaluates the potential impact to the environment. This may be because pavement preservation is inherently green based on its focus on keeping good roads in a condi- tion where they do not need to consume more energy and raw materials to restore their ser- viceability. Microsurfacing has a smaller environmental footprint than other treatments, as described by Takamura et al. in 2001 and Chelovitz and Galehouse in 2010. Its rapid curing times provide a means to minimize work zone delays and, as a result, accrue the benefits of enhanced safety. Nevertheless, with the current focus on environmental responsibility in trans-

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2 portation, a more robust policy toward assessing the environmental footprint of pavement preservation and maintenance during the planning and treatment selection process is warranted. Chapter nine provides a number of conclusions that are supported and documented in the report. The most significant of these are: Microsurfacing is best suited to address rutting, raveling, oxidation, bleeding, and loss of surface friction. Microsurfacing does not perform well if it is applied to structurally deficient pavements. This makes project selection the most important step in the micro- surfacing design process with regard to impact on the final performance of the micro- surfacing itself. The majority of the survey respondents assign the contractor the responsibility for developing the job mix formula (i.e., the mix design). The majority of the same popu- lation rated their microsurfacing project performance as satisfactory. Most of the U.S. and Canadian agencies believe they do not have an adequate level of competition among qualified microsurfacing contractors for their programs. This may be because many agency microsurfacing programs do not consistently advertise a suf- ficient amount of work each year for interested contractors to invest in the technical capacity and equipment necessary to competitively bid on these contracts. Most agencies do not prequalify microsurfacing bidders. This may be because the pool of competent and qualified contractors is inherently shallow. Contractor experience was also cited as the most important factor affecting microsurfacing quality. This indicates that there is a need to develop a training and certification program at the national level. Microsurfacing has a smaller environmental footprint than most pavement preservation and maintenance treatments. Additionally, its ability to return the road to full-speed traf- fic in roughly 1 h minimizes user work zone delays and enhances traffic safety. These two intangible benefits may justify its use over other practical options that are of mar- ginally lower cost. This report documents a number of effective practices identified in the course of the study. An effective practice is defined as a practice found in the literature and then confirmed as in use and effective by survey responses or the specification content analysis. The synthesis iden- tified two effective practices in microsurfacing project selection, three in design, four in con- tracting, and six in construction, as well as three more that came from the case study analyses. The most significant are: Agencies in northern climates can mitigate potential quality issues induced by a short microsurfacing season by requiring a warranty. Making microsurfacing contract packages as large as is practical reduces the unit price and increases the number of lane-miles that can be treated each year. Requiring that a test strip of 500 to 1,000 ft (152.4 to 304.8 m) in length be constructed and inspected will allow the agency and the contractor to ensure that microsurfacing equipment is properly calibrated and that any workmanship issues are resolved before full-scale microsurfacing production. If the microsurfacing is scheduled to occur after dark, it is important that the test strip be constructed after dark. Microsurfacing can be effectively employed on roads where routine winter snow removal is a factor if the pavement upon which it is placed is structurally sound. Eight future research projects are suggested to fill critical knowledge gaps. The three most significant research needs are as follows: Evaluate the differences in microsurfacing costs and performance in the eight U.S. states that require a warranty versus a similar number and geographical distribution of those that do not.

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3 Investigate and quantify the environmental benefits of microsurfacing inside the larger set of pavement preservation and maintenance tools. This project would provide hard, factual justification for selecting microsurfacing over a lower-priced option if all other technical considerations were equal. The lack of rigorous field tests based on a rational quantification of measurable micro- surfacing properties leads to a suggestion for research to develop a suite of field tests that allow an inspector to test the microsurfacing mix after it has been laid as well as tests to identify when the mix has cured to a sufficient degree to open it to traffic with- out fear of damaging it. In conclusion, the synthesis found that microsurfacing is being used successfully on a rou- tine basis across North America. It found that agencies were consistently satisfied with its performance and that it is effective in a pavement preservation role if the underlying pave- ment is structurally sound. To summarize, microsurfacing works well to address rutting, rav- eling, oxidation, bleeding, and loss of surface friction. If it is applied to the right road at the right time, most agencies expect an effective service life extension of six to seven years.