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26 CHAPTER FIVE MATERIAL SELECTION INTRODUCTION sions, because they routinely come in either anionic or cationic forms. Chip seal material selection is generally dependent on climatic conditions, binder and aggregate quality, product availability, and an organization's experience with particular practices. Size and Gradation Bituminous binders and cover aggregate make up the finished product. The bituminous binder's functions are to seal the The aggregate gradation plays a key role in the design, con- existing surface from water intrusion, provide an interfacial struction, and performance of chip seals. Aggregate size, bond between the aggregate, and provide the adhesive that typically referred to as nominal top size, is the smallest bonds the aggregate to the existing flexible pavement surface. sieve through which all of the aggregate passes. The aver- The aggregates in a chip seal provide a number of functions. age of the smallest dimension of the aggregate is referred to Cover aggregate should provide a good skid-resistant surface as the ALD (Hanson 1934/35). The nominal size of aggre- while being resistant to polishing, durable against abrasion gate is selected based on traffic, surface condition, and type effects, and resistant to the disintegration caused by weath- of chip seal. Larger aggregate particle sizes are generally ering (Seal Coat . . . 1993). Material selection is becoming more durable and less sensitive to variations in binder appli- cation rate (Gransberg et al. 1998). Additionally, as the more complicated as technology enhancements are contin- binder material is meant to seal the surface, a larger-sized ually developing adhesion agents, polymer modifiers, and aggregate will result in a thicker binder layer, enhancing the geotextiles marketed for chip seal use. quality of the chip seal. However, if not properly embedded and swept, larger aggregate can cause more damage to vehi- AGGREGATE SELECTION cles immediately after application. Its coarser texture also results in a chip seal with higher noise emissions. The sur- Aggregate selection is critical to determining which type of vey results shown in Figure 24 indicate that the most com- chip seal to use, which type of binder to design for, and which mon size for a single-course chip seal is usually a 3/8-in. type of construction procedures to specify. The quality of (10-mm) chip. In addition, survey respondents commonly aggregate is important to the overall success of the chip seal indicated that double-course seals usually have a 1/2-in. program, and quality involves a number of constructability (12.5-mm) initial aggregate application, followed by a sec- issues about using aggregates that are clean, durable, and ond aggregate application of approximately one-half that abrasion resistant. The cover aggregate is expected to transfer nominal size. the load to the underlying surface. It should provide adequate skid resistance and should be durable against climatic effects The specified gradation should be such that the texture of and traffic wear. In North America, aggregate selection is a the chip seal is consistent. Tight gradation bands, which ensure function of geography where availability and transportation a uniformly graded aggregate, with minimal fines and dust, are distance essentially define the aggregate selection process. necessary for a high-quality project. The literature review and Local availability often constrains the quality of the aggre- survey responses show a consensus that single-sized aggregate gate, causing agencies to select lower-quality local aggregates with less than 2% passing the No. 200 sieve is considered ideal based on cost and availability. This situation conflicts with (Wegman 1991). The amount of fines in the gradation affects philosophies in New Zealand and Australia, where aggregate the binder's ability to adhere to the aggregate. Because the is transported up to 500 mi to ensure the performance and amount of fines increases every time the material is handled, longevity of their treatments (Beatty et al. 2002). They justify Minnesota requires a tighter specification of less than 1% the added expense of using higher-quality aggregate with the passing the No. 200 sieve to allow for degradation during benefits accrued in extended service life. The cost implica- material movement and installation (Janisch and Galliard tions of using the higher-grade aggregate in conjunction with 1998). The ideal grading for an aggregate used in chip seals the appropriate binder type should be carefully assessed using is one in which all the particles of stone are very close to one life-cycle cost analysis. Another consideration is the ionic size, which helps ensure that the chip seal is only one-stone compatibility of the aggregate with the selected binder to thick. Single-sized aggregate produces a constant embed- ensure that good adhesion is developed between the aggregate ment depth, which is a critical factor for the success of a chip and the binder. This is especially critical when using emul- seal. A uniformly graded aggregate provides a more consis-

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27 45 side and become covered within the binder. As a result, flat- 40 ter aggregate is more susceptible to bleeding in the wheel- paths. Because the orientation of cubical aggregate is not as No. of Survey Respondents 35 susceptible to displacement by traffic, the opportunity for 30 bleeding is reduced. 25 The angularity of the aggregate, a characteristic that can 20 be measured by testing for percent fracture, determines a 15 chip seal's propensity to damage by stopping or turning traf- fic (Wade et al. 2001). Australian practice requires that 75% 10 of the aggregate have at least two fractured faces (Sprayed 5 Sealing Guide 2004). Rounded aggregates, as indicated by 0 low percent fracture, are susceptible to displacement by traf- 5/8 in. (16.0 mm) 1/2 in. (12.5 mm) 3/8 in. (10 mm) fic because they provide the least interfacial area between the aggregate and binder. The roundness of the aggregate will FIGURE 24 Single application chip seal size. determine how resistant the chip seal will be to turning and stopping movements. tent embedment that results in improved aggregate reten- Aggregate Cleanliness tion, surface friction, and drainage capabilities of the seal (McHattie 2001). Table 5 lists typical chip seal gradations Dust on the aggregate surface is one of the major causes of taken from various state DOT manuals in the United States. aggregate retention problems. Dust is defined as the per- The reader's attention is directed to the Minnesota gradation centage of fine material that passes the No. 200 sieve. To for choke stone. This was the only U.S. reference that fur- improve the quality of the material, the percentage of fines nished the specifications for this type of aggregate that passing the No. 200 sieve should be specified as a maximum would be used with the racked-in seals described in chapter of 1% at the time of manufacture (Janisch and Gaillard 1998). three. Dusty and dirty aggregate ultimately lead to problems with aggregate retention. Asphalt binders have difficulty bonding Aggregate Shape to dirty or dusty aggregate, causing the aggregate to be dis- lodged on opening to traffic (McLeod 1969). The shape of cover aggregate is crucial to the successful per- formance of a chip seal. Aggregate shape is typically char- It is recommended that the aggregate be sprayed with water acterized by angularity. As the orientation of the embedded several days before the start of the project (Maintenance Chip chip is important, cubical aggregate shapes are preferred Seal Manual 2000). Washing chip seal aggregate with clean, because traffic does not have a significant effect on the final potable water before application may assist in removing fine orientation of aggregate (Janisch and Galliard 1998). Cubi- particles that will prevent adhesion with the binder. In addi- cal materials tend to lock together and provide better long- tion, damp chips will assist the binder in wetting the rock, term retention and stability. The quantity of flat particles in thus increasing embedment (Maintenance Chip Seal Manual the aggregate can be determined by the Flakiness Index test 2000). In addition to washing with water, petroleum materials (Seal Coat . . . 2003). A low Flakiness Index indicates that are sometimes used to clean the aggregate before application. all the particles are near to having a cubical shape. Under Petroleum-based materials such as diesel fuel are commonly traffic, elongated and flat particles will lie on their flattest used to wash aggregate in Australia and New Zealand (Sprayed TABLE 5 TYPICAL GRADATIONS FOR CHIP SEAL AGGREGATE (% passing) State and Gradations South Arizona Arizona Minnesota Dakota South Sieve Alaska Low High Minnesota Choke Montana Type Dakota Size E Chip Traffic Traffic Aggregate Stone Grade 4A 1A Type 1B 1/2 in. 100 100 100 100 100 -- 100 100 3/8 in. 90100 100 7090 90100 100 100 4070 100 1/4 in. -- 7090 010 4070 100 -- -- -- No. 4 1030 110 -- 015 85100 030 015 1090 No. 8 08 05 05 05 1040 015 05 030 No. 40 -- -- -- -- 05 -- -- 04 No. 200 01 01 01 01 01 02 01 --

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28 Sealing Guide 2004). Such practice is not likely to be found in aggregates carry the additional benefit of a significant reduc- North America owing to environmental restrictions. tion in windshield breakage claims, because their specific gravity is approximately 25% of that of natural stone aggre- gate (Gallaway and Harper 1966b). However, lightweight Aggregate Toughness and Soundness aggregates are generally more expensive than natural aggre- gate and may have high water absorption. Figure 25 illustrates Resistance to abrasion, degradation, and polishing will ensure the proportion of respondents using synthetic aggregate. Cana- that the selected aggregate remains functional for the expected dian responses were excluded because none of the provinces life span of the chip seal. It is desirable to use aggregates with responded that they regularly use synthetic aggregates. resistance to polishing, as indicated through tests such as the British Wheel test (AASHTO T279, ASTM D3319). The results of this test indicate the polished stone value of the Precoated Aggregates aggregate, and the Australians recommend a polished stone value in the range of 44 to 48 (Sprayed Sealing Guide 2004). Precoated aggregate can be used to increase the performance Resistance to degradation and abrasion is also an important of the chip seal as well as to expedite the construction process characteristic of suitable aggregate. The survey results indi- (Harris 1955). The use of precoated aggregate improves aggre- cated that testing for those characteristics is quite common and gate binding properties, reduces dust in the aggregate, and usually measured by the Los Angeles abrasion test (AASHTO results in better contrast between the pavement and its mark- T96, ASTM C131). Resistance to weathering and freeze-thaw ings. Precoating generally involves applying either a film degradation is generally measured by either magnesium sulfate of paving grade asphalt or a specially formulated precoating loss or sodium sulfate loss (AASHTO T104, ASTM C88). bitumen to the aggregate. Precoated aggregates considerably shorten the required curing time by minimizing the problems associated with aggregate dust and moisture. Reduced dust Aggregate Type enhances the bonding between the aggregate and binder and reduces vehicle damage resulting from loose chips. The literature review and survey responses revealed that aggregate selection is usually based on the availability and Precoating the aggregate chips with asphalt before place- cost of aggregates within proximity to the project. Igneous, ment has been found to decrease the initial amount of chip metamorphic, sedimentary, and manufactured aggregates have loss (Kandhal and Motter 1991). In that same study, chips that all been successfully used for chip sealing (Sprayed Sealing were 90% precoated were found to have up to an 80% lower Guide 2004). Table 6 illustrates the varieties of aggregate initial loss than uncoated aggregates. The amount of pre- used for chip seal projects, both domestically and abroad. coating asphalt is typically 0.8 to 2.4 gal/yd3 (4 to 12 L/m3) Limestone, granite, and natural gravels are most widely used (Sprayed Sealing Guide 2004). The application rate depends in North America. on the size and absorptive properties of the aggregate, amount of moisture and dust present, and type of precoating material. A comprehensive report studied the suitability of light- Precoated aggregate is typically used with asphalt cement weight aggregate as cover stone for chip seals (Gallaway and binders. When emulsion binders are used, the aggregate is Harper 1966b). That report indicated that lightweight aggre- usually not precoated because the precoating inhibits the gate proved to be a highly successful cover aggregate for chip breaking of the emulsion (Seal Coat . . . 2003). The rough seals. A more recent study showed that lightweight synthetic surface of the aggregate provides the interface necessary for aggregate furnished a superior ability to retain its skid resis- the emulsion to cure. tance (Gransberg and Zaman 2002). Such a phenomenon was highlighted by Australian and United Kingdom responses that The survey indicated that most U.S. and Canadian agencies stressed the use of calcined bauxite, a synthetic aggregate, in do not precoat chip seal aggregates. The states in which pre- high-stress areas where chip polishing is an issue. Lightweight coating aggregate was used with asphalt cement binders were TABLE 6 43% NATURAL AGGREGATE USED FOR CHIP SEALS 29% Australia, New Zealand, North America United Kingdom, South Africa Type (%) (%) Limestone 37 13 Quartzite 13 38 Granite 35 38 Trap Rock 13 25 United States AU, NZ, UK, SA Sandstone 10 25 Natural Gravels 58 25 FIGURE 25 Proportion of agencies using Greywacke, Basalt 4 88 synthetic aggregate.