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8 must be considered when making decisions regarding chip- Gradation seal selection. The gradation of the aggregate used in the chip seal is critical to performance. Generally, the more one-sized the aggregate, 3.5 Evaluating the Pavement the better the performance potential. One-sized aggregates Some aspects of the pavement surface can have an effect on include those materials retained within two consecutive sieve the performance of the chip seal. For example, texture of the sizes. Two-sized aggregates are materials retained between surface, resistance of the pavement surface to penetration three consecutive sizes. Good performance should be expected of the chips under traffic, variability of the pavement surface for any chip seal with up to two-sized gradations. However, along the alignment, and pavement gradient all affect chip- as the gradation becomes less uniform (a wider variety of sizes), seal performance. These factors must be considered during obtaining good performance will be more difficult to achieve. the chip-seal design process. Uniformity can be quantified by using the coefficient of uni- formity (Cu) used for soil and aggregate classification (ASTM D 2487). 3.5.1 Surface Texture Cu is defined as the ratio of the size for which 60% passes The texture of the pavement surface must be known prior to divided by the size for which 10% passes. Thus, a more one- chip sealing so that an adjustment can be made for the design sized material will have a smaller Cu. emulsion spray rate. Texture of the surface can be measured An example of how the Cu value can be used to judge uni- using either sand patch (ASTM E 965) or CT meter (ASTM E formity is provided in Table 1 for hypothetical aggregate 2157) to obtain the texture depth. The correlation between gradations. Aggregates 1, 2, and 3 are examples of one-sized these two tests has been established. An adjustment to emul- materials, and Aggregates 4, 5, and 6 are two-sized. Aggre- sion spray rate needs to be applied to account for the texture. gate 7 has many sizes. A Cu value less than 4.0 is defined as uniformly graded (ASTM D 2487). The first six aggregates all have Cu values of less than 2.0 and, therefore, would be 3.5.2 Penetration of Chips into Surface defined as uniformly graded. The seventh aggregate, with Cu The pavement surface should be tested using the ball pen- of 7.2, would be considered well graded. etration test to determine if chips are likely to penetrate the Another approach has been proposed which evaluates the substrate pavement after trafficking and to what level. If pen- ratio of material passing 70% of the median size to that pass- etration is possible, adjustment to the emulsion application ing 140% of the median size, termed the performance-based rate will be required. uniformity coefficient (PUC) (Lee and Kim 2008). It is ex- pected that particles at 70% of the median size will be sub- merged in asphalt when fully compacted, and particles at 3.5.3 Variability of Surface 140% of the median size will not have enough binder to hold along Alignment them in place. Thus, the closer the PUC ratio is to zero, the The surface of the pavement affects the emulsion appli- more one-sized the gradation. For aggregates in Table 1, the cation rate. Therefore, if the surface varies along the alignment, PUC ratios for the first six aggregates range from 0 to 0.21, the application rate must change to match these conditions. with 0.58 for the seventh aggregate. A thorough map should be made indicating where material Aggregate gradations A, B, and C in Table 2 are recom- application rates should change in accordance with the chang- mended for chip-seal aggregates. While they are not as uni- ing surface conditions. These changes can be communicated form as those for Aggregates 1 through 6 in the hypothetical to equipment operators by painting on the pavement surface example, they present practical materials suitable for chip- in front of the distributor truck. seal construction. These aggregates have PUC ratios of 0.04 to 0.31 and Cu values of 1.18 to 3.28. 3.6 Materials Toughness The suitability of all materials to be used in the chip seal should be evaluated before construction begins. Toughness is defined as the ability of an aggregate to resist crushing forces. Toughness is an important consideration dur- ing aggregate processing to ensure that the gradation produced 3.6.1 Aggregates does not change during handling by loaders or other construc- Aggregate properties relevant for chip seals include grada- tion equipment while stockpiling, loading, or spreading on tion, toughness, soundness, cleanliness, fracture, and polish the pavement during construction. Crushing resistance is also resistance. important during service when vehicles travel over the chip-

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9 Table 1. Coefficient of uniformity for selected chip-seal aggregates. Hypothetical Aggregate Gradations 1 2 3 4 5 6 7 Sieve, mm Sieve Passing, % 19 3/4 100 100 12.5 1/2 0 60 100 100 100 9.5 3/8 0 0 60 100 100 75 4.75 4 0 0 60 45 2.38 8 0 25 1.19 16 12 0.60 30 0.30 50 0.075 200 2 D60 > 16.33 12.5 11.2 9.5 7.5 4.75 7.2 D10 > 13.1 9.9 9.8 5.5 5.1 2.7 1 Cu > 1.25 1.26 1.14 1.73 1.47 1.76 7.20 Median (M) > 15.75 12 11 8.7 7.2 4.3 5.5 % Passing @ 0.7M > 0 0 0 18 7.5 15 37 % Passing @ 1.4M > 100 88 100 95 100 71 64 PUC > 0.00 0.00 0.00 0.19 0.08 0.21 0.58 seal aggregate. Crushing resistance historically was judged Soundness using the Los Angeles Abrasion test (AASHTO T 96), but more Freezethaw resistance and weathering are not common per- recently the Micro-Deval test (AASHTO T 327) has been in- cluded as an indicator of toughness. Specifications often limit formance issues for chip-seal aggregates because water should the abrasion loss to 35%, but this value should vary depend- drain freely from the surface of chip seals. However, magne- ing on traffic level. Several state specifications have stipulated sium and sodium sulfate loss tests (AASHTO T 104) are rou- the values shown in Table 3. tinely used to evaluate soundness of concrete and hot mix Lightweight aggregates manufactured from expanded asphalt aggregates and, therefore, should be included for shale, clay, and other materials have been used in chip seals evaluating chip-seal aggregate suitability. A limit of 10% loss on low-traffic facilities for many years (Gallaway 1966). Al- is considered appropriate for chip-seal aggregates. though Los Angeles Abrasion test results indicate the ma- terials are acceptable, on high-traffic facilities they degrade Cleanliness prematurely (Shuler 1991). This suggests the Los Angeles Abrasion test is not an appropriate measure of toughness Although asphalt emulsions have the ability to coat dusty for such materials. aggregates, the fraction passing the no. 200 (0.075 mm) sieve Table 2. Recommended aggregate gradations. Passing, % Sieve, mm Sieve Gradation A Gradation B Gradation C 19 3/4 100 100 12.5 1/2 90 100 100 100 9.5 3/8 5 30 90 100 100 100 4.75 4 0 10 5 30 90 100 2.38 8 0 10 5 30 1.19 16 0 2 0 10 0.60 30 0 2 0.30 50 0 2 0.075 200 0 1 0 1 0 1 D60 > 11.4 10.8 7.8 6.8 3.9 3.4 D10 > 9.65 4.75 2.38 2.38 2.5 1.19 Cu > 1.18 2.27 3.28 2.86 1.56 2.86 Median (M) > 11.1 10.3 7.3 6.15 3.7 3.1 % Passing @ 0.7M > 3.5 20 11 27 11 26 % Passing @ 1.4M > 95 100 92 86 91 89 PUC > 0.04 0.20 0.12 0.31 0.12 0.29

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10 Table 3. Los Angeles Abrasion and evaluated if polishing is suspected. The polished stone value Micro-Deval loss versus traffic level. obtained using the British Wheel (AASHTO T 279) is the most common test used for this purpose. A limit of 31 is recom- Traffic, L.A. Abrasion Micro-Deval veh/day/lane Loss, % max Loss, % max mended (Utah DOT 2008). <500 40 15 5001,500 35 13 >1,500 30 12 3.6.2 Aggregate Properties for Design The size and shape of the aggregates used for chip seals should be limited to 1% or less. However, higher values can determine the spread rate of the aggregate and spray rate of be tolerated by many emulsions, especially if medium setting the emulsion. Aggregate gradation is determined during sieve materials are used. Adhesive ability of the emulsion should be analysis to ensure that materials meet the specifications for evaluated in the laboratory using the sweep test; the aggregate the roadway to be sealed. The shape of the particles is needed should be considered suitable for use when 90% retention of to make sure they are not too flat or elongated and for input the aggregate can be achieved. Sieve analyses should be con- into the design process. These properties are discussed below. ducted using washed samples since the material passing the no. 200 sieve often adheres to coarse aggregates. Sieve analysis Flakiness (AASHTO T 27) must be done in conjunction with the wash- ing procedure (AASHTO T 11). The flakiness index, a measure of the percentage of par- ticles that are long and slender in comparison to the width, is used in many designs of chip seals. A low flakiness index Angularity is desired because it indicates cubical-shaped aggregates. The amount of interlock present in a chip-seal aggregate Aggregates with a high flakiness index tend to lie flat and surface is directly related to the amount of angularity of become submerged in the binder during construction and the individual aggregate particles. The higher the interlock, later under traffic, resulting in flushing. Limits on flakiness the greater the resistance to dislodgement of particles and the index recommended in Table 5 (Austroads 2006, South African potential for vehicle damage and flushing of the surface. The Roads Agency 2007, Wood et al. 2006) are based on the expe- forces present at the chip-seal surface are directly related rience of several agencies. to the amount and type of traffic expected. Therefore, a greater percentage of mechanically fractured particles should Average Least Dimension be used for roads with high traffic volumes and truck per- centages. The literature provides some guidance, summarized It is assumed that the aggregates will orient to the flattest in Table 4, regarding fracture requirements for chip-seal direction after construction and trafficking. Therefore, to be aggregates. sure the aggregate chips are not submerged in binder dur- ing service, the average of the least dimension of the aggre- gates is used to determine aggregate spread rate and emul- Polish Resistance sion spray rate. The median aggregate size determined from Because vehicular traffic may cause aggregates to polish and sieve analysis and the flakiness index is needed to determine reduce friction, the aggregates used for chip seals should be the average least dimension (ALD). Although ALD can be Table 4. Mechanically fractured requirements for chip-seal aggregates. Vehicles per Day per Lane Parameter Test Method 1,500 One Fractured Face ASTM D 5821 90 95 100 Two Fractured Faces ASTM D 5821 85 90 90 Table 5. Flakiness index requirements for chip-seal aggregates. Vehicles per Day per Lane Parameter Test Method 1,500 Flakiness Index Tex 224-F 35 30 25 (Tx DOT 2004) FLH T508 (Mn/DOT 2005)

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Table 6. Asphalt emulsions used for chip seals. Polymer Modified Polymer Modified Polymer Modified RS-2 CRS-2 HFRS-2 RS-2 CRS-2 HFRS-2 Min. Max. Min. Max. Min. Max. Min. Max. Min. Max. Min. Max. Viscosity SSF,@ 122F AASHTO T 59 100 300 100 300 100 400 100 400 100 300 100 300 Storage Stability, 1 day, % AASHTO T 59 1 1 1 1 1 1 Sieve Test, % AASHTO T 59 0.1 0.1 0.1 0.1 0.1 0.1 Demulsibility, % AASHTO T 59 60 95 60 95 60 95 60 95 60 95 60 95 Particle Charge AASHTO T 59 Positive Positive Oil distillate by volume of emulsion, % AASHTO T 59 3 3 Residue by Evaporation, % Appendix D 63 63 65 65 63 63 Float Test, 140F, s AASHTO T50 1200 1200 Penetration, 77F, 100g, 5s AASHTO T49 100 200 100 200 100 250 100 250 100 200 100 200 Ductility, 77F, 5cm/min, cm AASHTO T5 40 40 40 40 40 40 Torsional Recovery, % CT-332* 18 18 18 Toughness, in-lbs CPL-2210** 70 70 70 Tenacity, in-lbs CPL-2210** 45 45 45 Elastic Recovery, % CPL-2211** 58 58 58 * California Test Method ** Colorado Test Methods

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12 measured directly (Austroads 2005), the following relationship same time as specific gravity measurements. A correction can be used to estimate ALD more quickly (Jackson 1963): in the residue application rate of 0.02 gallons per square yard has been suggested (McLeod 1969) if absorption is ALD = [ M 1.139285 + ( 0.011506 ) FI ] 1%. However, for cubical aggregates embedded to 50% initially, a correction of approximately 0.014 gallons per Where square yard is estimated for absorption of 1%. Therefore, M = median particle size from sieve analysis and an adjustment of 0.01 to 0.02 seems reasonable for each FI = flakiness index. percent of aggregate absorption. An alternative method has been proposed (Dumas 2004) that uses five sieves in the aggregate gradation to obtain ALD 3.6.3 Emulsion Properties instead of just one, the median. When this method was used, the calculated ALD values were within the 98% confidence lim- The properties of the emulsion used for chip seals are im- its for ALD determined using the equation above. portant and should be checked to ensure compliance with requirements. Desirable emulsion properties are provided in Table 6. Loose Unit Weight, Specific Gravity, These properties are based on current state specifications for and Absorption both conventional and polymer modified emulsions, includ- Loose unit weight (AASHTO T 19) and specific gravity ing anionic, cationic, and high-float emulsions. Because of (AASHTO T 85) are used to estimate the volume of voids the wide array of emulsions available in the United States, not in the loose aggregate chips and to determine how much as- every combination of conventional and modified emulsion phalt to apply to the pavement surface so that the appropri- could be included. However, those included in Table 6 have ate embedment of chips occurs during construction. Ag- been successfully used in chip-seal construction over a range gregate absorption is also needed and is obtained at the of environments and traffic conditions.