National Academies Press: OpenBook

Manual for Emulsion-Based Chip Seals for Pavement Preservation (2011)

Chapter: Chapter 2- Factors Affecting Chip-Seal Performance

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Suggested Citation:"Chapter 2- Factors Affecting Chip-Seal Performance." National Academies of Sciences, Engineering, and Medicine. 2011. Manual for Emulsion-Based Chip Seals for Pavement Preservation. Washington, DC: The National Academies Press. doi: 10.17226/14421.
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Suggested Citation:"Chapter 2- Factors Affecting Chip-Seal Performance." National Academies of Sciences, Engineering, and Medicine. 2011. Manual for Emulsion-Based Chip Seals for Pavement Preservation. Washington, DC: The National Academies Press. doi: 10.17226/14421.
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Suggested Citation:"Chapter 2- Factors Affecting Chip-Seal Performance." National Academies of Sciences, Engineering, and Medicine. 2011. Manual for Emulsion-Based Chip Seals for Pavement Preservation. Washington, DC: The National Academies Press. doi: 10.17226/14421.
×
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Suggested Citation:"Chapter 2- Factors Affecting Chip-Seal Performance." National Academies of Sciences, Engineering, and Medicine. 2011. Manual for Emulsion-Based Chip Seals for Pavement Preservation. Washington, DC: The National Academies Press. doi: 10.17226/14421.
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3The performance of a chip seal depends on many factors, including the condition of the pavement to which the chip seal is to be applied, pavement geometry, traffic volume and type, materials, and construction practices. The following discussion describes these factors and their effects on the per- formance of the chip seal. 2.1 Pavement Behavior and Condition 2.1.1 Deflection of Substrate The amount of deflection is an indication of whether fu- ture fatigue can be expected. If deflection is significant, a chip seal may be inappropriate. The maximum level of deflection will vary depending on the traffic volume; however, if fatigue cracking is already present, chip-seal performance may be reduced. However, a chip seal may reduce moisture infil- tration into the subgrade, thus reducing the potential for future fatigue. Therefore, the decision to chip seal over exist- ing fatigue cracking requires judgment depending on the performance expectation of the existing pavement. 2.1.2 Cracking Severity Chip seals are most effective as a pavement preservation technique before cracks are ranked as high severity (Peshkin et al. 2004), defined as a crack width of 3⁄8 in. Although the chip- seal binder has the ability to seal cracks greater than this width, as crack width increases, the emulsion residue is less effective at bridging the gap across the crack and sealants should be used to fill these cracks prior to chip sealing. 2.1.3 Flushing/Bleeding Chip seals may be applied to remedy friction loss, but pen- etration of chips into flushed pavement surfaces may limit effectiveness unless chips can be retained with lower emulsion application rates. Flushing and bleeding of the existing surface often occurs in wheel paths. If this occurs, the emulsion appli- cation rate must be reduced in the wheel paths to prevent future flushing and bleeding. This can only be accomplished with variable spray rate distributors or by varying the size of the nozzles in the distributor spray bar. 2.1.4 Texture Surface roughness affects the amount of emulsion needed to hold aggregate chips in place. The texture of the substrate pavement should be evaluated using the sand patch test or CT meter prior to chip sealing to determine whether an adjust- ment to the design emulsion application rate is appropriate and to what level. 2.1.5 Soft Substrate Surface A soft substrate surface can allow chips to be embedded in the surface after trafficking, resulting in possible flushing. The ball penetration test has been shown to be an effective tool for measuring this potential. 2.1.6 Uniformity The amount of emulsion applied to the substrate may need to be varied if the substrate surface does not have the same tex- ture and compliance along the alignment. Uniformity should be mapped prior to construction to identify locations where emulsion application rates should vary from design. Unifor- mity can also vary transverse to the centerline, which often occurs when wheel paths are flushed. In this case the emul- sion application rate should be reduced in the wheel paths by using a distributor equipped with a variable application spray bar or placing smaller nozzles in the conventional distributor spray bar in locations that will affect the wheel paths (Shuler 1991, Martin 1989). C H A P T E R 2 Factors Affecting Chip-Seal Performance

2.2 Traffic Characteristics The traffic volume and type of traffic affect the selection of materials used on chip seals. Generally, higher traffic volume and a higher percentage of heavy trucks on an undivided road- way present a greater likelihood for vehicle damage if traffic is not adequately controlled during construction. In addition, the volume and type of the traffic are directly related to the poten- tial for chip embedment in the substrate. Also, traffic accelera- tion affects chip-seal performance as chips are more likely to be dislodged under these loads than at constant speeds, which can lead to flushing and bleeding of the surface. This section dis- cusses the factors that influence the material’s selection process depending on traffic volume, type of traffic, and speeds. 2.2.1 Chip Selection Larger chips provide more tolerance for emulsion applica- tion variance and are less likely to become totally embedded by traffic if the substrate is resistant to embedment. Larger chips require higher emulsion application rates for proper embedment, thus increasing the sealing ability of the chip seal. However, large chips are noisier and provide higher risk of vehicular damage during construction. 2.2.2 Emulsion Selection Emulsions modified with elastomeric polymers provide higher adhesion for aggregate chips, often both during con- struction and later in the life of the seal. Also, research (Shuler 1991) indicates that on high traffic facilities (i.e., greater than 7,500 vehicles per day per lane), modified asphalt emulsions are required to hold chips in place due to reduced emulsion application rates that are necessary to reduce the potential for embedment in the substrate and consequent flushing. 2.2.3 Fog Seal The application of a fog seal over a fresh chip seal provides high color contrast that improves visibility of striping, and short-term performance improvement (Shuler 2007). Care should be taken whenever applying a fog seal since pavement friction could be reduced if the fog seal is applied at too high an application rate, the fog seal emulsion has a high residue content, or the fog seal has not broken sufficiently to support uncontrolled traffic. 2.3 Geometry 2.3.1 Divided/Undivided Divided alignments generally reduce the possibility for vehicle damage caused by loose, flying chips because of the separation of opposing traffic. 2.3.2 Gradient/Curves Steep inclines and curves may adversely affect performance due to tractive forces and slower moving vehicles. Therefore, traffic control may need to remain in place until the emulsion has cured sufficiently to retain the chips. 2.3.3 Intersections Turning, acceleration, and deceleration can cause chip loss and flushing. Therefore, traffic control may need to remain in place until the emulsion has cured sufficiently to retain the chips. 2.3.4 Width Vehicle movement tends to be more concentrated on nar- row, secondary roads than on wider primary facilities. This results in a greater tendency for flushing in the wheel paths on these types of pavements. 2.4 Highway, Residential, Urban, or Rural 2.4.1 Highway Chip seals can be successfully constructed on highways with over 7,500 vehicles per day per lane with little or no consequences with respect to vehicle damage (Shuler 1991) if important principles are followed. However, other fac- tors should be considered regarding the use of chip seals on high traffic highway pavements. Because noise increases with increasing traffic volume and chip size, smaller aggregates are often desired for high traffic facilities. However, more accuracy relative to emulsion spray rate is needed when using smaller aggregates because the smaller embedment depth increases the potential for chip loss. 2.4.2 Residential Chip seals constructed with larger aggregates are rough textured. This rough surface texture is often unpopular among residential users such as roller skaters and skateboarders or individuals who need to lie on the pavement to repair vehicles. 2.4.3 Rural Rural settings are the most appropriate for chip seals. Traf- fic tends to move more consistently with less stopping and starting, and volumes are often lower, creating wider vehicle separation providing less possibility for vehicle damage. 2.4.4 Urban Urban environments are often the most challenging envi- ronment for chip seals because of the higher traffic volumes 4

and frequent turning, stopping, and starting. Although chip seals can be constructed in such environments with success, the time required for emulsions to gain sufficient strength to resist the turning, acceleration, and deceleration of vehicles in large volumes is often long enough to preclude their use. 2.5 Materials 2.5.1 Aggregate Chips Aggregate properties, including size, shape, and gradation; cleanliness; moisture content; toughness and durability; and porosity influence chip-seal performance. 2.5.1.1 Size, Shape, and Gradation Aggregates that interlock after construction rolling and early trafficking provide higher stability under loads. This interlocked aggregate surface is more resistant to displace- ment and thus has a lower potential for dislodgement of chips and vehicle damage and flushing. Larger aggregates require higher emulsion application rates in order to provide an equivalent embedment percentage to smaller aggregates. This higher application rate allows slightly more tolerance during construction with respect to depth of chip embedment in the binder. Also, the higher binder appli- cation provides greater sealing ability. Aggregates retained between two adjacent sieve sizes provide the best interlock, followed by aggregates that occupy the space between three adjacent sieve sizes; these are often described as one- and two-sized aggregate chips, respectively. The perfor- mance of the one- and two-sized chips is related to the manner by which the chips are embedded in the emulsion. If well- graded aggregates are used, the fine aggregate often enters the emulsion before the coarse aggregate, causing the coarse aggre- gate to have less binder available for adhesion and resulting in loss of the coarse fraction, vehicle damage, and flushing. 2.5.1.2 Cleanliness Emulsified asphalts can be produced with the ability to coat aggregate chips containing small quantities of minus no. 200 aggregate. The maximum amount of this fine aggregate is dependent on the emulsion. For example, medium setting emulsions can tolerate a higher percentage than most rapid setting emulsions; this is often related to the demulsibility of the emulsion. The higher the demulsibility, the less minus no. 200 can be tolerated before setting occurs and there is loss of adhesion to the coarse chips. 2.5.1.3 Moisture Content Aggregates in the saturated surface dry condition provide better resistance to sweeping than dry aggregates when asphalt emulsions are used. Therefore, construction aggregates should be moistened by spraying water on the stockpile and mixing with a front-end loader before chip-seal operations begin. 2.5.1.4 Toughness and Durability Aggregates must have enough strength to resist crushing during construction and trafficking. Breakdown of the aggre- gate during construction and trafficking could lead to inunda- tion and flushing if the coarse particles are reduced to fine particles. 2.5.1.5 Porosity Porous aggregates will absorb more asphalt than nonporous aggregates. Therefore, the amount of asphalt absorbed must be accounted for during the design stage. 2.5.2 Asphalt Emulsion The performance of a chip seal is largely dependent on the asphalt emulsion. Performance is related to the adhesive abil- ity of the binder for the aggregate chips and the underlying pavement, the durability and flexibility of the binder, and the ability of the binder to maintain these properties over a wide range of environmental conditions and snowplow action. 2.5.2.2 Emulsion Type Emulsified asphalts should be rapid setting types. Although rapid setting emulsions allow faster removal of traffic control, they can set too quickly in very hot weather or even form a skin on the surface after application, creating a barrier to chip embedment. Medium setting emulsions require more time to set, requiring traffic control to remain in place longer and increasing the possibility for vehicle damage. However, while rapid setting emulsions are the most desirable as a chip-seal binder, medium setting emulsions have been used success- fully on low volume roads when strict traffic control is main- tained. Medium setting emulsions have also been successful when chips are more uniformly graded or contain higher quantities of minus no. 200 particles. 2.5.2.3 Emulsion Class Emulsions are classified based on the particle charge of the asphalt droplets within the water phase of the suspension. Anionic emulsions have a negative charge and cationic emul- sions a positive charge. There are also a limited number of emulsions classified as nonionic with no appreciable charge. These emulsions can be considered interchangeable as both classifications should provide equal performance with respect to adhesion to aggregate chips. Research has shown no 5

difference in the adhesion of different classes of emulsions to aggregates (Shuler and Lord 2009). However, the selec- tion of emulsion should take into consideration the environ- mental conditions present during construction. For example, anionic emulsions break by evaporation so humid site con- ditions could lead to longer setting times for anionic com- pared with cationic materials. High float emulsions are a class of emulsion that can be either anionic or cationic and are for- mulated with a gel structure to produce a thicker asphalt coating on aggregate chips. Some high float emulsions con- tain oil distillates that increase setting time, so increased traf- fic control may be needed for these products. 2.5.2.4 Viscosity Grade Emulsions are produced in two viscosity categories, desig- nated “−1” and “−2” for low and high viscosity, respectively. Because aggregate chips require approximately 40% initial embedment during construction, the high viscosity emul- sions should always be used since the low viscosity emulsions would flow off the substrate. The viscosity of the emulsion during construction is an important factor. Emulsions with too low viscosity could flow off the pavement before the aggregate chips are embed- ded, resulting in a loss of chips under traffic and potential environmental issues. Emulsions with too high viscosity may not provide adequate coating of the aggregate chips, leading to a loss of chips. 2.5.2.5 Application Rate The binder application rate must be correct during con- struction to achieve optimum performance of the chip seal. Too little emulsion will not retain chips in place under traf- fic, and too much emulsion will lead to flushing and loss of friction. The optimal application rate is a function of the vol- ume of voids in the compacted aggregate chip layer, the volume and type of traffic, the pavement gradient, and the condition of the substrate pavement. 2.5.2.6 Emulsion Application Temperature Emulsion application temperature should be within a range to provide uniform transverse and longitudinal spraying but should not exceed 185°F (85°C). 2.6 Construction Preparation Preparation of the pavement surface prior to chip-seal operations can influence performance of the chip seal. This preparation varies depending on the condition of the exist- ing pavement but at a minimum should include sweeping the surface to remove loose debris, dust, or other contaminants. 2.6.1 Fog Seal Pre-Treatment of Substrate If the pavement surface is extremely dry or porous or there is possible loss of some of the chip-seal binder to the pavement, a light fog seal application should be considered prior to chip sealing. Sometimes hot mix asphalt patches are needed to re- pair the existing pavement prior to chip sealing. These patched areas should be sprayed with a light fog seal prior to applying the chip seal since fresh hot mix asphalt can absorb the emul- sion after chip sealing, resulting in significant loss of chips. 2.6.2 Repairs Alligator cracking, potholes, failing patches, and active cracks greater than 1⁄4 in. in width should be repaired to pro- vide a stable surface for the new chip seal. 2.7 Maintenance Fog seals are often applied to a new chip seal to provide additional binder and higher contrast for pavement strip- ing paint. Care should be taken whenever applying a fog seal since pavement friction could be reduced if the fog seal is applied at too high an application rate, the fog seal emulsion has a high residue content, or the fog seal has not broken sufficiently to support uncontrolled traffic. 6

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Manual for Emulsion-Based Chip Seals for Pavement Preservation Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 680: Manual for Emulsion-Based Chip Seals for Pavement Preservation examines factors affecting chip performance, highlights design and construction considerations, and explores procedures for selecting the appropriate chip seal materials. The report also contains suggested test methods for use in the design and quality control of chip seals.

Appendices A to J of NCHRP Report 680 provide further elaboration on the work performed in this project.

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