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5 CHAPTER ONE INTRODUCTION INTRODUCTION of transportation (MOT). A content analysis of microsurfac- ing specifications from 18 U.S. states was also undertaken. Microsurfacing treatments are widely used for both pavement Finally, six case studies from five U.S. states and one Canadian preservation and preventative maintenance. Although there province were conducted to furnish specific information on has been extensive research on hot-mix asphalt design and chip agency-level microsurfacing successes. seal surface treatments, microsurfacing design and construc- tion continues to depend more on the specialized experience of the microsurfacing contractor and its emulsion supplier BACKGROUND than the information developed through agency research and Microsurfacing consists of a mixture of polymer-modified experimentation. Therefore, the need to benchmark the state asphalt emulsion, graded aggregates, mineral filler, water, and of the practice and identify those areas where further research other additives. The mixture is made by a specialized machine will add value to this important pavement preservation and and placed on a continuous basis by mixing the materials maintenance tool is timely. "Microsurfacing mixtures are simultaneously in a pug mill. Figure 1 shows the process in made of high-quality aggregate and asphalt emulsion com- the microsurfacing machine, which results in a free flowing ponents" (Johnson et al. 2007) as well as mineral filler, water, composite material, spread on the underlying pavement using and polymer modifiers. It is applied cold by means of a spe- a spreader box. The mixture's consistency permits it to be cial purpose mixing and laying machine. The International evenly spread over the pavement, forming an adhesive bond Slurry Seal Association (ISSA) categorizes it as a "slurry to the pavement. "The mixture contains asphalt emulsion that system," not be confused with slurry seal (ISSA 2010a). breaks onto the pavement surface through heterogeneous or homogenous flocculation. Particles of asphalt coalesce into SYNTHESIS OBJECTIVE films, creating a cohesive mixture. The mixture then cures, by loss of water, into a hardwearing, dense-graded asphalt/ The objective of this synthesis is to identify and synthesize aggregate mixture that is bonded to the existing pavement" current commonly accepted practices for using microsurfacing (National Highway Institute 2007). Microsurfacing does not in highway pavement preservation and maintenance pro- enhance the structural capacity of the existing pavement (Smith grams. Its focus is on finding commonalities among micro- and Beatty 1999). Hence, it is used as a pavement preservation surfacing practices from separate sources of information that and maintenance treatment to improve the functional charac- have reported good performance and may ultimately be classi- teristics of the pavement surface and extend its service life. fied as effective practices. Although microsurfacing is specif- ically the subject of this report, the review of literature and History state department of transportation (DOT) specifications found a distinct lack of uniformity in the terminologies used to Slurry surfacing originated in the 1930s in Germany, where describe microsurfacing and slurry sealing. Thus, this report it was called "micro-asphalt concrete" (ISSA 2010a). It con- will discuss the differences between the two systems and their sisted of a mixture of very fine aggregate, asphalt emulsion, applications to furnish a better understanding to the reader. and water. This technique for maintaining road surfaces started Finally, the synthesis seeks to find microsurfacing programs slurry surfacing development in the rest of the world. In the that have been effectively implemented and that document 1960s, the introduction of improved emulsifiers, continuous microsurfacing's unique ability to address specific pavement flow machines, and set control additives created a technical preservation and maintenance problems. environment in which the promise for slurry surfacing was realized. "In the mid-1970s, Screg Route, a French company, In addition to a literature review, the synthesis is based designed Seal-Gum, a micro-asphalt concrete that was sub- on data from a recent survey, six case studies, and the con- sequently improved by the German firm Raschig, and mar- tent analysis of state DOT microsurfacing specifications. A keted in the United States under the trade name `Ralumac' survey on microsurfacing practices distributed to state and during the early 1980s" (ISSA 2010a). provincial maintenance engineers achieved an 89% overall response rate, which corresponds to responses from 44 U.S. Since being introduced into North America, microsurfacing state DOTs and 12 Canadian provincial/territorial ministries has become a routine tool in the highway pavement manager's

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6 Common Applications Microsurfacing's flexibility to be used across three categories of maintenance is because it is a thin surfacing that can be laid at a thickness of two to three times the size of the largest stone in the mix. The emulsion in the system is polymer-modified with additives that create a chemical break that is largely inde- pendent of weather conditions. The emulsion forces water from the aggregate surface during breaking, which allows the newly surfaced road to be opened to traffic within 1 h or less of its application under a range of conditions (Price 2010). Micro- surfacing specifications call for high-quality aggregates, fast MICRO- setting/curing, and stiff emulsion to allow thicker layers to surfacing be placed. As a result of the layer thickness, the following Mix extended performance characteristics and applications are possible: FIGURE 1 Schematic of microsurfacing machine (ISSA 2010a). Correction of minor surface profile irregularities (Bae and Stoffels 2008; Olsen 2008). pavement preservation and maintenance toolbox. Table 1 Rut filling (Labi et al. 2007; Olsen 2008). shows the FHWA matrix that defines the suite of pavement Higher durability (Labi et al. 2007; ISSA 2010a). management planning tasks and where pavement preser- Night work or cooler temperatures (Olsen 2008; Caltrans vation treatments fall within the universe of pavement pres- 2009). ervation and maintenance guidelines. One can see that the Restoring surface friction to concrete bridge decks (Olsen major feature that separates pavement preservation treat- 2008). ments from the others is that pavement preservation does not increase the strength or capacity of the road. The bold box Although it has been reported to seal small surface cracks in Table 1 shows that microsurfacing can be used in three of resulting from thermal changes (Bae and Stoffels 2008), micro- the activity types: preventive maintenance, routine mainte- surfacing is usually not intended as a crack treatment and will nance, and corrective maintenance. The first two fall within the not prevent cracks in the underlying pavement from reflecting pavement preservation realm. through to the surface (Johnson et al. 2007). Research in Min- nesota, which is case studied in chapter eight, has shown that 1. An example of preventive maintenance would be the using a softer binder shows the potential to reduce the level of use of microsurfacing to cover oxidized or raveled reflective cracking (Johnson et al. 2007). Therefore, this option is primarily a preservation treatment to keep good roads in good pavement to prevent further deterioration (Labi et al. condition and not an appropriate tool to use on pavements 2007). whose structural condition has been compromised. 2. Using microsurfacing as a rut filler would be routine maintenance (Jahren et al. 1999). 3. Using it to restore surface friction on a road where Environmental Impact skid numbers have fallen below safety minimums is an example of corrective maintenance (Hicks et al. Pavement preservation is inherently green owing to its focus on 2000). conserving energy and raw materials, and reducing greenhouse TABLE 1 MICROSURFACING'S RELATIONSHIP TO PAVEMENT PRESERVATION GUIDELINES Pavement Preservation Guidelines Micro- Increase Increase Reduce Restore surfacing Type of Activity Capacity Strength Aging Serviceability New Construction X X X X Reconstruction X X X X Major Rehabilitation X X X Structural Overlay X X X Minor Rehabilitation X X Pavement Preservation Preventive Maintenance X X X Routine Maintenance X X Corrective (reactive) X X Maintenance Catastrophic Maintenance X Source: after Geiger (2005).