National Academies Press: OpenBook

Microsurfacing (2010)

Chapter: Chapter One - Introduction

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Suggested Citation:"Chapter One - Introduction." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
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Suggested Citation:"Chapter One - Introduction." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
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Suggested Citation:"Chapter One - Introduction." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
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Suggested Citation:"Chapter One - Introduction." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
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Suggested Citation:"Chapter One - Introduction." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
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Suggested Citation:"Chapter One - Introduction." National Academies of Sciences, Engineering, and Medicine. 2010. Microsurfacing. Washington, DC: The National Academies Press. doi: 10.17226/14464.
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5INTRODUCTION Microsurfacing treatments are widely used for both pavement preservation and preventative maintenance. Although there has been extensive research on hot-mix asphalt design and chip seal surface treatments, microsurfacing design and construc- tion continues to depend more on the specialized experience of the microsurfacing contractor and its emulsion supplier than the information developed through agency research and experimentation. Therefore, the need to benchmark the state of the practice and identify those areas where further research will add value to this important pavement preservation and maintenance tool is timely. “Microsurfacing mixtures are made of high-quality aggregate and asphalt emulsion com- ponents” (Johnson et al. 2007) as well as mineral filler, water, and polymer modifiers. It is applied cold by means of a spe- cial purpose mixing and laying machine. The International Slurry Seal Association (ISSA) categorizes it as a “slurry system,” not be confused with slurry seal (ISSA 2010a). SYNTHESIS OBJECTIVE The objective of this synthesis is to identify and synthesize current commonly accepted practices for using microsurfacing in highway pavement preservation and maintenance pro- grams. Its focus is on finding commonalities among micro- surfacing practices from separate sources of information that have reported good performance and may ultimately be classi- fied as effective practices. Although microsurfacing is specif- ically the subject of this report, the review of literature and state department of transportation (DOT) specifications found a distinct lack of uniformity in the terminologies used to describe microsurfacing and slurry sealing. Thus, this report will discuss the differences between the two systems and their applications to furnish a better understanding to the reader. Finally, the synthesis seeks to find microsurfacing programs that have been effectively implemented and that document microsurfacing’s unique ability to address specific pavement preservation and maintenance problems. In addition to a literature review, the synthesis is based on data from a recent survey, six case studies, and the con- tent analysis of state DOT microsurfacing specifications. A survey on microsurfacing practices distributed to state and provincial maintenance engineers achieved an 89% overall response rate, which corresponds to responses from 44 U.S. state DOTs and 12 Canadian provincial/territorial ministries of transportation (MOT). A content analysis of microsurfac- ing specifications from 18 U.S. states was also undertaken. Finally, six case studies from five U.S. states and one Canadian province were conducted to furnish specific information on agency-level microsurfacing successes. BACKGROUND Microsurfacing consists of a mixture of polymer-modified asphalt emulsion, graded aggregates, mineral filler, water, and other additives. The mixture is made by a specialized machine and placed on a continuous basis by mixing the materials simultaneously in a pug mill. Figure 1 shows the process in the microsurfacing machine, which results in a free flowing composite material, spread on the underlying pavement using a spreader box. The mixture’s consistency permits it to be evenly spread over the pavement, forming an adhesive bond to the pavement. “The mixture contains asphalt emulsion that breaks onto the pavement surface through heterogeneous or homogenous flocculation. Particles of asphalt coalesce into films, creating a cohesive mixture. The mixture then cures, by loss of water, into a hardwearing, dense-graded asphalt/ aggregate mixture that is bonded to the existing pavement” (National Highway Institute 2007). Microsurfacing does not enhance the structural capacity of the existing pavement (Smith and Beatty 1999). Hence, it is used as a pavement preservation and maintenance treatment to improve the functional charac- teristics of the pavement surface and extend its service life. History Slurry surfacing originated in the 1930s in Germany, where it was called “micro-asphalt concrete” (ISSA 2010a). It con- sisted of a mixture of very fine aggregate, asphalt emulsion, and water. This technique for maintaining road surfaces started slurry surfacing development in the rest of the world. In the 1960s, the introduction of improved emulsifiers, continuous flow machines, and set control additives created a technical environment in which the promise for slurry surfacing was realized. “In the mid-1970s, Screg Route, a French company, designed Seal-Gum, a micro-asphalt concrete that was sub- sequently improved by the German firm Raschig, and mar- keted in the United States under the trade name ‘Ralumac’ during the early 1980s” (ISSA 2010a). Since being introduced into North America, microsurfacing has become a routine tool in the highway pavement manager’s CHAPTER ONE INTRODUCTION

pavement preservation and maintenance toolbox. Table 1 shows the FHWA matrix that defines the suite of pavement management planning tasks and where pavement preser- vation treatments fall within the universe of pavement pres- ervation and maintenance guidelines. One can see that the major feature that separates pavement preservation treat- ments from the others is that pavement preservation does not increase the strength or capacity of the road. The bold box in Table 1 shows that microsurfacing can be used in three of the activity types: preventive maintenance, routine mainte- nance, and corrective maintenance. The first two fall within the pavement preservation realm. 1. An example of preventive maintenance would be the use of microsurfacing to cover oxidized or raveled pavement to prevent further deterioration (Labi et al. 2007). 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 skid numbers have fallen below safety minimums is an example of corrective maintenance (Hicks et al. 2000). 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 setting/curing, and stiff emulsion to allow thicker layers to be placed. As a result of the layer thickness, the following extended performance characteristics and applications are possible: • Correction of minor surface profile irregularities (Bae and Stoffels 2008; Olsen 2008). • Rut filling (Labi et al. 2007; Olsen 2008). • Higher durability (Labi et al. 2007; ISSA 2010a). • Night work or cooler temperatures (Olsen 2008; Caltrans 2009). • Restoring surface friction to concrete bridge decks (Olsen 2008). Although it has been reported to seal small surface cracks resulting from thermal changes (Bae and Stoffels 2008), micro- surfacing is usually not intended as a crack treatment and will not prevent cracks in the underlying pavement from reflecting through to the surface (Johnson et al. 2007). Research in Min- nesota, which is case studied in chapter eight, has shown that using a softer binder shows the potential to reduce the level of reflective cracking (Johnson et al. 2007). Therefore, this option is primarily a preservation treatment to keep good roads in good condition and not an appropriate tool to use on pavements whose structural condition has been compromised. Environmental Impact Pavement preservation is inherently green owing to its focus on conserving energy and raw materials, and reducing greenhouse MICRO- surfacing Mix FIGURE 1 Schematic of microsurfacing machine (ISSA 2010a). Pavement Preservation Guidelines Type of Activity Increase Capacity Increase Strength Reduce Aging Restore Serviceability Micro- surfacing 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 Preventive Maintenance X X X Pavement Preservation Routine Maintenance X X Corrective (reactive) Maintenance X X Catastrophic Maintenance X Source: after Geiger (2005). TABLE 1 MICROSURFACING’S RELATIONSHIP TO PAVEMENT PRESERVATION GUIDELINES

7gases by keeping good roads good (Chehovitz and Galehouse 2010). Microsurfacing’s environmental footprint is lower than most common pavement preservation and maintenance treat- ments (Takamura et al. 2001). Figure 2 is from a study on the environmental impact of several commonly used pavement preservation and maintenance treatments. The study devel- oped “eco-efficiency” indices for the five categories shown in the figure and found that microsurfacing had a substan- tially lower environmental footprint than the other options (Takamura et al. 2001). This study does not include the reduced greenhouse gas emissions resulting from microsurfacing’s ability to greatly reduce traffic delays in work zones (Johnson et al. 2007). Additionally, the “risk potential” and “health effects” categories did not include the reduction in work zone accident risk inherent to microsurfacing (Erwin and Tighe 2008). Therefore, microsurfacing’s “true” footprint may be even smaller in relation to hot-mix asphalt options for pavement preservation and maintenance programs. When looking at options to address pavement preservation and maintenance issues, the engineer can use the environmen- tal and safety benefits of microsurfacing as possible justi- fication to offset any marginal increase in construction cost versus other alternatives. KEY DEFINITIONS The report uses a number of technical terms in a precise sense that is important for the reader to understand. The technical vocabulary of this field has undergone a radical transformation as new products and new concepts have been developed. The major terminology challenges found in the literature revolve around two major areas. The first is the terms applied to vari- ous activities associated with pavement maintenance. The sec- ond is the fundamental definition of microsurfacing versus slurry sealing. The lack of uniformity of microsurfacing ter- minology exists not only in the technical literature but also in the standard specifications of state DOTs in the United States. The specific definitions have been controversial and oftentimes rooted in local construction jargon. The terminology discussed here will be used throughout the report and a Glossary is included following the References. Pavement Preservation and Maintenance Terminology The first set of terms for pavement maintenance terminology that apply to microsurfacing are defined by the FHWA (Geiger 2005) as follows: • Pavement preservation is “a program employing a net- work level, long-term strategy that enhances pavement performance by using an integrated, cost-effective set of practices that extend pavement life, improve safety, and meet motorist expectations.” • Preventive maintenance is “a planned strategy of cost- effective treatments to an existing roadway system and its appurtenances that preserves the system, retards future deterioration, and maintains or improves the functional condition of the system (without significantly increasing the structural capacity).” • Routine maintenance “consists of work that is planned and performed on a routine basis to maintain and pre- serve the condition of the highway system or to respond to specific conditions and events that restore the high- way system to an adequate level of service.” • Corrective maintenance “activities are performed in response to the development of a deficiency or defi- ciencies that negatively impact the safe, efficient oper- ations of the facility and future integrity of the pavement section. Corrective maintenance activities are generally reactive, not proactive, and performed to restore a pave- ment to an acceptable level of service due to unforeseen conditions.” The report will refer to various specific applications of microsurfacing using these terms where appropriate and as a “pavement preservation and maintenance program” when discussing it in general terms. Microsurfacing Versus Slurry Seal Terminology The issue of whether or not microsurfacing is merely a polymer-modified slurry seal has yet to be settled. ISSA’s Inspector’s Manual for Slurry Systems (2010a) differenti- ates each technology in the following manner: • Slurry seal—A mixture of aggregate, emulsified asphalt, water, and additives properly proportioned, mixed, and spread over a properly prepared surface. Slurry seal is applied in a mono-layer. A mono-layer is considered one stone thickness (based on the largest stone in the grada- tion) spread on the pavement surface. • Polymer-modified slurry seal—A slurry seal designed with an asphalt emulsion that has been modified with a polymer or other special purpose additive to enhance one or more properties of the slurry to better meet a par- ticular project requirement. Modifying emulsions may improve the bond between the asphalt and the aggregate and may improve durability and toughness of the seal. FIGURE 2 Microsurfacing environmental footprint compared with two types of pavement preservation overlays (Takamura et al. 2001).

• Microsurfacing—A mixture of cationic polymer- modified asphalt emulsion, 100% crushed aggregate, water, and other additives properly proportioned and spread over a prepared surface. The special purpose polymers and additives used in microsurfacing allow higher than normal rates of application and multi-layer applications for projects such as rut filling and high- way leveling and resurfacing. A multi-layer application allows material depths to exceed the normal one stone thickness rule. Figure 3 shows the difference between microsurfacing and the two slurry seals and Table 2 consolidates the ISSA defin- itions and other technical definitions found in the literature. Microsurfacing appears to have three features that differentiate it from slurry seals: 1. It always contains polymers. 2. It cures rapidly through chemical reaction, which per- mits traffic to be returned in a shorter time. 3. It can be placed in layers thicker than one stone deep. The California DOT (Caltrans) Maintenance Technical Advisory Guide (Caltrans 2009) goes further in its discussion of the differences in the two technologies. Table 3 was taken from that guide and adds additional information to assist the reader in understanding the fundamental differences between microsurfacing and slurry seal. The Asphalt Emulsion Man- ufacturers Association (AEMA) states that microsurfacing is an alternative to hot mix for rut filling (AEMA n.d.). Finally, a pooled fund study devoted to updating microsurfacing and slurry seal design procedures also faced this terminology issue and concluded that because “constructability issues are the same for both,” the two might be categorized as “Slurry Surfacing Systems” (Fugro-BRE/Fugro South 2004). This 8 agrees with the approach used in Australia and New Zealand, where “Slurry Surfacing [is] a general term for Slurry Seal and Microsurfacing” (Austroads 2003b). With one exception, this synthesis will concentrate the remainder of its discussion strictly on microsurfacing. In chap- ter three, the topic of project selection will include an analysis of those situations in which microsurfacing is uniquely appro- priate and those where a slurry seal is a better option. PROTOCOL TO DEVELOP CONCLUSIONS, EFFECTIVE PRACTICES, AND FUTURE RESEARCH NEEDS The major factor in developing a conclusion was the occur- rence of similar trends found in two or more research instru- ments. Additionally, greater weight was given to information developed from the survey of highway agencies. The litera- ture review and specification content analysis were consid- ered to be supporting sources. Finally, the case studies were used to validate the conclusions as appropriate because they are examples of how U.S. and Canadian highway agencies have actually implemented microsurfacing. Effective practices were identified in the same manner as conclusions, with one exception. An effective practice is specific to a single factor in microsurfacing practice and may only apply to a specific set of circumstances, such as agencies in northern climates; whereas, conclusions can be generalized. Future research needs were developed based on practices that were described in the literature and con- firmed as effective by one of the research instruments but generally not widely used. Gaps in the body of knowledge found in this study were also used to define the areas where more research would be valuable. FIGURE 3 Difference between slurry seals and microsurfacing (adapted from Bickford 2008).

Source Microsurfacing Slurry Seal ISSA (2010a) ìA mixt ure of cationic polymer modified asphalt emulsion, 100% crushed aggregate, water and other additives…multi-layer applications for projects such as rut filling…” “A mixture of aggregate, emulsified asphalt, water, and additives … applied in a mono-layer…one stone thickness …” AEMA (undated) “…a polymer modified quick traffic slurry seal system…can be placed in much greater thicknesses than conventional slurry seal…” “…a blend of emulsion, aggregate, water, and additive.” Hicks et al. (1999) “… a polymer-modified cold slurry system…uses aggregate, which normally passes the 9-mm (3/8-in.) sieve. [It will] cure and develop strength faster than conventional slurry seals and can be placed in thicker layers …” “…a slow- or quick-set emulsion and aggregates that typically pass the 6-mm (1/4-in.) sieve… used to seal minor surface cracks and voids, retard surface raveling, fill minor ruts, and improve surface friction.” Hicks et al. (2000) “A mixture of polymer modified asphalt emulsion, mineral aggregate, mineral filler, water, and other additives, properly proportioned, mixed, and spread on a paved surface.” “A mixture of slow setting emulsified asphalt, well graded fine aggregate, mineral filler, and water. It is used to fill cracks and seal areas of old pavements, to restore a uniform surface texture, to seal the surface to prevent moisture and air intrusion into the pavement, and to provide skid resistance.” FLHD (2003) “Microsurfacing emulsions break and cure much more quickly than unmodified slurry seals, allowing faster return to traffic and less traffic damage ... can be placed in thicker lifts for rut-filling. By definition, microsurfacing contains polymers… microsurfacing can be thickly applied in multiple layers… [it] breaks chemically…[which] permits the microsurfacing to gain cohesive strength rapidly.” “…slurry seals may or may not contain polymers. Slurry seals are generally laid at thicknesses of 0.40 to 0.60 in. (1 to 1.5 cm), whereas slower breaking slurry seals cure on the surface, “skinning over” and preventing thorough breaking and curing when they are applied at greater thicknesses.” Austroads (2003b) “A Bituminous Slurry Surfacing, usually containing polymer, which is capable of being spread in variably thick layers for rut-filling and correction courses, and for wearing course applications requiring good surface texture.” “A thin layer of Bituminous Slurry Surfacing, usually without a polymer modifier.” AEMA = Asphalt Emulsion Manufacturer’s Association. 9 TABLE 2 CURRENT DEFINITIONS FOR MICROSURFACING AND SLURRY SEAL Differences In: Microsurfacing Slurry Seal Asphalt Emulsion Always polymer modified, quick set Could be polymer modified Aggregate Quality/Gradation Stricter specifications for sand equivalent; use only Type II and Type III Can use Type I, II, or III Additives/Break Chemical break largely independent of weather conditions Breaking and curing dependent on weather conditions Mix Stiffness/Equipment Stiffer mix, use augers in the spreader box and secondary strike-off Softer mix, use drag box Applications Same as slurry seal + rut filling, night work, correction of minor surface profile irregularities Correct raveling, seal oxidized pavements, restore skid resistance Source: Caltrans (2009). TABLE 3 CALIFORNIA DOT MAINTENANCE TECHNICAL ADVISORY GUIDE ON MICROSURFACING VERSUS SLURRY SEAL

ORGANIZATION OF THE REPORT The information collected in this study will be presented as follows: • Chapter two—Summary of Information Collected: The research methodology and key characteristics of the agen- cies that responded to the survey are covered in detail. • Chapter three—Design Practices: Microsurfacing proj- ect selection, material specifications, job mix formula, and other activities related to the development of micro- surfacing construction documents are presented. • Chapter four—Contracting Procedures: Microsurfacing contract types, administration issues, and warranties are addressed in this chapter. • Chapter five—Construction Practices: The various as- pects of microsurfacing construction are reviewed. 10 • Chapter six—Microsurfacing Equipment Practices: The various components of the microsurfacing equipment train are discussed. • Chapter seven—Quality Control and Quality Assur- ance and Performance Measures: This chapter reviews the salient aspects of quality management as well as commonly used metrics to measure microsurfacing performance. • Chapter eight—Case Studies: Six case studies from five states and one Canadian province are presented, high- lighting specific aspects of microsurfacing success at the agency level. • Chapter nine—Conclusions: This chapter synopsizes the conclusions and effective practices found in the report and makes recommendations for future research to fill gaps in the body of knowledge.

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 411: Microsurfacing explores highway microsurfacing project selection, design, contracting, equipment, construction, and performance measurement processes used by transportation agencies in the United States and Canada.

Microsurfacing is a polymer-modified cold-mix surface treatment that has the potential to address a broad range of problems on today’s highways.

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