National Academies Press: OpenBook

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

Chapter: Chapter 3 - Design and Construction Considerations

« Previous: Chapter 2- Factors Affecting Chip-Seal Performance
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Suggested Citation:"Chapter 3 - Design and Construction Considerations." 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 3 - Design and Construction Considerations." 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.
×
Page 8
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Suggested Citation:"Chapter 3 - Design and Construction Considerations." 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.
×
Page 9
Page 10
Suggested Citation:"Chapter 3 - Design and Construction Considerations." 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.
×
Page 10
Page 11
Suggested Citation:"Chapter 3 - Design and Construction Considerations." 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.
×
Page 11
Page 12
Suggested Citation:"Chapter 3 - Design and Construction Considerations." 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.
×
Page 12

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7Before a chip seal can be designed, there are certain factors that must first be known about the project. The first part of this process involves selecting the pavement to chip seal. The second part is selection of the type of seal to be placed. 3.1 Identifying Appropriate Pavements to Chip Seal Chip seals are most effective for pavement preservation when applied to pavements with limited or no distress (i.e., cracking has not begun or is less than 1⁄8-in. wide, rutting is less than 3⁄8 in., and structural distress is isolated with low- severity fatigue). Pavements with severe cracking and rutting may require multiple patches and crack sealing prior to chip sealing. In general, pavements in poor condition will achieve shorter service lives than pavements in good condition. 3.2 Type of Seal The types of chip seal include single, double, and single chip seal with choke stone; any of these chip seals may have a fog seal applied afterward. 3.2.1 Single Chip Seal A single chip seal consists of a spray application of asphalt emulsion followed by an application of aggregate chips, prefer- ably one stone layer thick. 3.2.2 Double Chip Seal A double chip seal is two applications of a single chip seal. The first chip seal is constructed with aggregate one sieve size larger than the second chip seal. 3.2.3 Single Chip Seal with Choke Stone This type of seal is a single chip seal but with crushed fine aggregate applied to the surface of the chip seal prior to rolling. 3.2.4 Fog Seal Applied to Chip Seals A fog seal may be applied to a fresh chip seal to provide slightly more asphalt to account for possible deficiencies in emulsion application rate, to provide a higher contrast between pavement markings and the surrounding surface, and to pro- vide possible improved cracking performance. Care should be taken whenever applying a fog seal since pavement fric- tion could be reduced if the fog seal is applied at too high an application rate, the fog seal emulsion has a high residue con- tent, or the fog seal has not broken sufficiently to support uncontrolled traffic. 3.3 Chip Seal Selection The type of chip seal used depends on many factors. Under high traffic volumes, double seals or seals with choke stone have lower potential for chip loss and vehicle damage. Dou- ble and choke stone seals should provide less tire–pavement noise due to smaller aggregate size. The life expectancy of dou- ble seals and seals with choke stone should be higher and the sealing ability should also be greater than single seals under the same conditions. However, the cost of double seals is obvi- ously higher. 3.4 Selecting the Aggregate Size The chip seals described in this manual are single chip seals consisting of one application of asphalt emulsion and one layer of aggregate chips. The gradation of the chips should consist of one or two consecutive sieve sizes with little or no material passing the 0.075-mm sieve (no. 200). The maximum aggregate size in the chip seal influences the performance of the chip seal. Larger aggregates provide greater sealing per- formance because of the higher asphalt volume needed to retain the chips in place. However, larger aggregates produce more traffic noise, have rougher texture, and have greater po- tential to damage vehicles. These advantages and disadvantages C H A P T E R 3 Design and Construction Considerations

must be considered when making decisions regarding chip- seal selection. 3.5 Evaluating the Pavement Some aspects of the pavement surface can have an effect on the performance of the chip seal. For example, texture of the surface, resistance of the pavement surface to penetration of the chips under traffic, variability of the pavement surface along the alignment, and pavement gradient all affect chip- seal performance. These factors must be considered during the chip-seal design process. 3.5.1 Surface Texture The texture of the pavement surface must be known prior to chip sealing so that an adjustment can be made for the design emulsion spray rate. Texture of the surface can be measured using either sand patch (ASTM E 965) or CT meter (ASTM E 2157) to obtain the texture depth. The correlation between these two tests has been established. An adjustment to emul- sion spray rate needs to be applied to account for the texture. 3.5.2 Penetration of Chips into Surface The pavement surface should be tested using the ball pen- etration test to determine if chips are likely to penetrate the substrate pavement after trafficking and to what level. If pen- etration is possible, adjustment to the emulsion application rate will be required. 3.5.3 Variability of Surface along Alignment The surface of the pavement affects the emulsion appli- cation rate. Therefore, if the surface varies along the alignment, the application rate must change to match these conditions. A thorough map should be made indicating where material application rates should change in accordance with the chang- ing surface conditions. These changes can be communicated to equipment operators by painting on the pavement surface in front of the distributor truck. 3.6 Materials The suitability of all materials to be used in the chip seal should be evaluated before construction begins. 3.6.1 Aggregates Aggregate properties relevant for chip seals include grada- tion, toughness, soundness, cleanliness, fracture, and polish resistance. 3.6.1.1 Gradation The gradation of the aggregate used in the chip seal is critical to performance. Generally, the more one-sized the aggregate, the better the performance potential. One-sized aggregates include those materials retained within two consecutive sieve sizes. Two-sized aggregates are materials retained between three consecutive sizes. Good performance should be expected for any chip seal with up to two-sized gradations. However, as the gradation becomes less uniform (a wider variety of sizes), obtaining good performance will be more difficult to achieve. Uniformity can be quantified by using the coefficient of uni- formity (Cu) used for soil and aggregate classification (ASTM D 2487). Cu is defined as the ratio of the size for which 60% passes divided by the size for which 10% passes. Thus, a more one- sized material will have a smaller Cu. An example of how the Cu value can be used to judge uni- formity is provided in Table 1 for hypothetical aggregate gradations. Aggregates 1, 2, and 3 are examples of one-sized materials, and Aggregates 4, 5, and 6 are two-sized. Aggre- 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 defined as uniformly graded. The seventh aggregate, with Cu of 7.2, would be considered well graded. Another approach has been proposed which evaluates the ratio of material passing 70% of the median size to that pass- ing 140% of the median size, termed the performance-based 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 140% of the median size will not have enough binder to hold them in place. Thus, the closer the PUC ratio is to zero, the more one-sized the gradation. For aggregates in Table 1, the PUC ratios for the first six aggregates range from 0 to 0.21, with 0.58 for the seventh aggregate. Aggregate gradations A, B, and C in Table 2 are recom- mended for chip-seal aggregates. While they are not as uni- form as those for Aggregates 1 through 6 in the hypothetical example, they present practical materials suitable for chip- seal construction. These aggregates have PUC ratios of 0.04 to 0.31 and Cu values of 1.18 to 3.28. 3.6.1.2 Toughness 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 does not change during handling by loaders or other construc- tion equipment while stockpiling, loading, or spreading on the pavement during construction. Crushing resistance is also important during service when vehicles travel over the chip- 8

seal aggregate. Crushing resistance historically was judged using the Los Angeles Abrasion test (AASHTO T 96), but more recently the Micro-Deval test (AASHTO T 327) has been in- cluded as an indicator of toughness. Specifications often limit the abrasion loss to 35%, but this value should vary depend- ing on traffic level. Several state specifications have stipulated the values shown in Table 3. Lightweight aggregates manufactured from expanded shale, clay, and other materials have been used in chip seals on low-traffic facilities for many years (Gallaway 1966). Al- though Los Angeles Abrasion test results indicate the ma- terials are acceptable, on high-traffic facilities they degrade prematurely (Shuler 1991). This suggests the Los Angeles Abrasion test is not an appropriate measure of toughness for such materials. 3.6.1.3 Soundness Freeze–thaw resistance and weathering are not common per- formance issues for chip-seal aggregates because water should drain freely from the surface of chip seals. However, magne- sium and sodium sulfate loss tests (AASHTO T 104) are rou- tinely used to evaluate soundness of concrete and hot mix asphalt aggregates and, therefore, should be included for evaluating chip-seal aggregate suitability. A limit of 10% loss is considered appropriate for chip-seal aggregates. 3.6.1.4 Cleanliness Although asphalt emulsions have the ability to coat dusty aggregates, the fraction passing the no. 200 (0.075 mm) sieve 9 1 2 3 4 5 76 Sieve, mm Sieve 19 3/4 100 100 12.5 1/2 0 0 0 0 60 60 100 100 100 9.5 3/8 0 60 100 100 75 4.75 4 0 45 2.38 8 25 1.19 16 12 0.60 30 0.30 50 0.075 200 2 16.33 12.5 11.2 9.5 7.5 4.75 7.2 13.1 9.9 9.8 5.5 5.1 2.7 1 1.25 1.26 1.14 1.73 1.47 1.76 7.20 15.75 12 11 8.7 7.2 4.3 5.5 0 0 0 18 7.5 15 37 100 88 100 95 100 71 64 0.00 0.00 0.00 0.19 0.08 0.21 0.58 % Passing @ 0.7M > % Passing @ 1.4M > PUC > D60 > D10 > Cu > Median (M) > Hypothetical Aggregate Gradations Passing, % Table 1. Coefficient of uniformity for selected chip-seal aggregates. Sieve, mm Sieve 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 0 0 2 20.30 50 0 0.075 200 0 1 1 1 11.4 10.8 7.8 6.8 3.9 3.4 9.65 4.75 2.38 2.38 2.5 1.19 1.18 2.27 3.28 2.86 1.56 2.86 11.1 10.3 7.3 6.15 3.7 3.1 3.5 20 11 27 11 26 95 100 92 86 91 89 0.04 0.20 0.12 0.31 0.12 0.29 Passing, % Gradation A Gradation B Gradation C % Passing @ 0.7M > % Passing @ 1.4M > PUC > D60 > D10 > Cu > Median (M) > Table 2. Recommended aggregate gradations.

should be limited to 1% or less. However, higher values can be tolerated by many emulsions, especially if medium setting materials are used. Adhesive ability of the emulsion should be evaluated in the laboratory using the sweep test; the aggregate should be considered suitable for use when 90% retention of the aggregate can be achieved. Sieve analyses should be con- ducted using washed samples since the material passing the no. 200 sieve often adheres to coarse aggregates. Sieve analysis (AASHTO T 27) must be done in conjunction with the wash- ing procedure (AASHTO T 11). 3.6.1.5 Angularity The amount of interlock present in a chip-seal aggregate surface is directly related to the amount of angularity of the individual aggregate particles. The higher the interlock, the greater the resistance to dislodgement of particles and the potential for vehicle damage and flushing of the surface. The forces present at the chip-seal surface are directly related to the amount and type of traffic expected. Therefore, a greater percentage of mechanically fractured particles should be used for roads with high traffic volumes and truck per- centages. The literature provides some guidance, summarized in Table 4, regarding fracture requirements for chip-seal aggregates. 3.6.1.6 Polish Resistance Because vehicular traffic may cause aggregates to polish and reduce friction, the aggregates used for chip seals should be evaluated if polishing is suspected. The polished stone value obtained using the British Wheel (AASHTO T 279) is the most common test used for this purpose. A limit of 31 is recom- mended (Utah DOT 2008). 3.6.2 Aggregate Properties for Design The size and shape of the aggregates used for chip seals determine the spread rate of the aggregate and spray rate of the emulsion. Aggregate gradation is determined during sieve analysis to ensure that materials meet the specifications for the roadway to be sealed. The shape of the particles is needed to make sure they are not too flat or elongated and for input into the design process. These properties are discussed below. 3.6.2.1 Flakiness 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 is desired because it indicates cubical-shaped aggregates. Aggregates with a high flakiness index tend to lie flat and become submerged in the binder during construction and later under traffic, resulting in flushing. Limits on flakiness index recommended in Table 5 (Austroads 2006, South African Roads Agency 2007, Wood et al. 2006) are based on the expe- rience of several agencies. 3.6.2.2 Average Least Dimension It is assumed that the aggregates will orient to the flattest direction after construction and trafficking. Therefore, to be 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- sion spray rate. The median aggregate size determined from sieve analysis and the flakiness index is needed to determine the average least dimension (ALD). Although ALD can be 10 Traffic, veh/day/lane L.A. Abrasion Loss, % max Micro-Deval Loss, % max <500 40 15 500–1,500 35 13 >1,500 30 12 Table 3. Los Angeles Abrasion and Micro-Deval loss versus traffic level. Vehicles per Day per Lane Parameter Test Method <500 500–1,500 >1,500 One Fractured Face ASTM D 5821 90 95 100 Two Fractured Faces ASTM D 5821 85 90 90 Table 4. Mechanically fractured requirements for chip-seal aggregates. Vehicles per Day per Lane Parameter Test Method <500 500–1,500 >1,500 Flakiness Index Tex 224-F (Tx DOT 2004) FLH T508 (Mn/DOT 2005) 35 30 25 Table 5. Flakiness index requirements for chip-seal aggregates.

Min. Max. Min. Max. Min. Max. Min. Max. Min. Max. Min. Max. Viscosity SSF,@ 122°F 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 Polymer Modified HFRS-2RS-2 Polymer Modified RS-2 CRS-2 HFRS-2 Polymer Modified CRS-2 Table 6. Asphalt emulsions used for chip seals.

measured directly (Austroads 2005), the following relationship can be used to estimate ALD more quickly (Jackson 1963): Where M = median particle size from sieve analysis and FI = flakiness index. An alternative method has been proposed (Dumas 2004) that uses five sieves in the aggregate gradation to obtain ALD instead of just one, the median. When this method was used, the calculated ALD values were within the 98% confidence lim- its for ALD determined using the equation above. 3.6.2.3 Loose Unit Weight, Specific Gravity, and Absorption Loose unit weight (AASHTO T 19) and specific gravity (AASHTO T 85) are used to estimate the volume of voids in the loose aggregate chips and to determine how much as- phalt to apply to the pavement surface so that the appropri- ate embedment of chips occurs during construction. Ag- gregate absorption is also needed and is obtained at the ALD M FI= +( )×[ ]1 139285 0 011506. . same time as specific gravity measurements. A correction in the residue application rate of 0.02 gallons per square yard has been suggested (McLeod 1969) if absorption is 1%. However, for cubical aggregates embedded to 50% initially, a correction of approximately 0.014 gallons per square yard is estimated for absorption of 1%. Therefore, an adjustment of 0.01 to 0.02 seems reasonable for each percent of aggregate absorption. 3.6.3 Emulsion Properties The properties of the emulsion used for chip seals are im- portant and should be checked to ensure compliance with requirements. Desirable emulsion properties are provided in Table 6. These properties are based on current state specifications for both conventional and polymer modified emulsions, includ- ing anionic, cationic, and high-float emulsions. Because of the wide array of emulsions available in the United States, not every combination of conventional and modified emulsion could be included. However, those included in Table 6 have been successfully used in chip-seal construction over a range of environments and traffic conditions. 12

Next: Chapter 4 - Selecting the Appropriate Chip Seal »
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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