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15 CHAPTER 6 Chip-Seal Design The basic chip-seal design methods proposed by Hanson Where (19341935) and by Kearby (1953) provided the basis for future Bd = design binder application rate, gal/yd2 design methods. From the original Hanson concepts evolved (L/m2); the McLeod procedure (McLeod 1960, 1969) that was later Bb = basic binder application rate, gal/yd2 (L/m2); adopted by the Asphalt Institute (Asphalt Institute MS19) and EF = emulsion factor; the Austroads and South African methods (South African PF = polymer factor (for polymer modified Roads Agency 2007). The Kearby method was later improved emulsions, only); and (Benson and Gallaway 1953, Epps et al. 1981) and adopted by As, Ae, Aas, Aaa = adjustments for substrate texture, embed- Texas. The United Kingdom designs "surface dressings" or ment, absorption into substrate, and chip seals using some of the Hanson concepts combined with absorption into cover aggregate, gal/yd2 ideas of Jackson (1963). (L/m2). Several methods have been used for the design of chip seals. Where The following discussion describes a design method based on Bb = VF ALD; the Austroads method. VF = design voids factor, gal/yd2/in (L/m2/mm); and The purpose of chip-seal design is to select aggregate and ALD = average least dimension of cover aggregate. asphalt emulsion application rates that will result in a durable pavement seal. The quantity of binder required depends on Where the size, shape, and orientation of the aggregate particles; VF = Vf + Va + Vt, embedment of aggregate into the substrate; texture of the Vf = basic voids factor, substrate; and absorption of binder into either the substrate Va = aggregate shape adjustment factor, and or aggregate. Vt = traffic effects adjustment factor. This design method is based on the following assumptions Thus the design binder application rate is: for aggregate, traffic, and embedment: Bd = {[(Vf + Va + Vt ) ALD ] EF PF } + As + Ae + Aas + Aaa Aggregate: one-sized aggregates with a flakiness index of 15% to 25% Each of these parameters is discussed below. Traffic: 10%, or less, heavy vehicles Embedment: 50% to 65% chip embedment after two years 6.1.1 Basic Voids Factor, Vf The basic voids factor depends on traffic level because traf- 6.1 Emulsion Application Rate fic determines how much of the aggregate is embedded in the The emulsion application rate is the spray quantity of asphalt binder. Figures 1a and 1b use English and SI units, respec- emulsion applied during construction; it is determined as tively, for traffic of less than 500 vehicles per day per lane. Fig- follows: ures 2a and 2b use English and SI units, respectively, for traf- fic of greater than 500 vehicles per day per lane. The three Bd = [ Bb EF PF ] + As + Ae + Aas + Aaa curves in each figure represent a range of basic voids factors

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16 1.80 Basic Voids Factor, Vf, gal/yd2/in. 1.70 1.60 1.50 1.40 Bleeding Limit 1.30 1.20 1.10 1.00 Target 0.90 Raveling Limit 0.80 0 50 100 150 200 250 300 350 400 450 500 Traffic, Veh/day/lane (a) 0.29 Basic Voids Factor, Vf, L/m2/mm 0.27 0.25 Bleeding Limit 0.23 0.21 0.19 Target 0.17 Raveling Limit 0.15 0 50 100 150 200 250 300 350 400 450 500 Traffic, Veh/day/lane (b) Figure 1. Basic voids factor versus traffic for 0 to 500 vehicles/lane/day (Austroads 2006). from a low binder content (raveling limit) to a high binder adjustment, Vt, needs to be made. Table 9 shows recom- content (bleeding limit), which should not be exceeded. mended adjustments. 6.1.2 Adjustment for Aggregate Shape, Va 6.1.4 Average Least Dimension The design method assumes the flakiness index will be The volumetric design of a chip seal is based on the assump- between 15 and 25; an adjustment must be made for aggre- tion that aggregate particles tend to lie with the least dimension gates outside this range. Table 8 provides suggested adjust- vertical. The least dimension is defined as the smallest dimen- ment factors. sion of a particle when placed on a horizontal surface, the par- ticle being most stable when lying with the least dimension 6.1.3 Adjustment for Traffic, Vt vertical. Thus in a chip seal, the final orientation of most par- ticles is such that the least dimension is near vertical, providing The basic voids factor, Vf, was developed for an average mix there is sufficient room for the particles to realign. This aver- of light and heavy vehicles in a free traffic flow situation. When age least dimension, ALD, is as follows: this is not the case due to composition; non-trafficked areas; overtaking lanes with few heavy vehicles or for large propor- ALD, mm = M , mm [1.139285 + ( 0.011506 FI )] or, tions of heavy vehicles; channelization and slow-moving, heavy vehicles in climbing lanes; or stop/start conditions, an ALD, in. = ALD, mm 25.4

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17 1.10 Basic Voids Factor, Vf, gal/yd2/in. 1.00 Bleeding Limit 0.90 0.80 0.70 Target 0.60 Raveling Limit 0.50 500 1500 2500 3500 4500 5500 6500 7500 8500 9500 Traffic, Veh/day/lane (a) 0.25 Basic Voids Factor, Vf, L/m2/mm 0.2 Bleeding Limit 0.15 Target 0.1 Raveling Limit 0.05 0 500 1500 2500 3500 4500 5500 6500 7500 8500 9500 Traffic, Veh/day/lane (b) Figure 2. Basic voids factor versus traffic for 500 to 10,000 vehicles/lane/day (Austroads 2006). Where factor allows a greater volume of binder around the aggregate M = median size of the aggregate (mm) and particles to compensate for reduced aggregate reorientation FI = flakiness index. as a result of rapid increase in binder stiffness after the initial breaking of the emulsion. 6.1.5 Emulsion Factor The basic binder application rate for emulsions, Bbe, is cal- culated as follows: An emulsion factor is applied to the basic binder application rate (before allowances) when using asphalt emulsions. This Bbe = Bb EF Table 8. Suggested adjustment for aggregate shape, Where Va (after Austroads 2006). Bbe = basic binder emulsion application rate rounded to the nearest 0.2 gal/yd2 [0.1 L/m2]; Aggregate Aggregate Flakiness Va, Type Shape Index, FI, % gal/yd2/in. [L/m2/mm] Bb = basic binder application rate, gal/yd2 (L/m2); and Too flaky, not EF = emulsion factor = 1.0 for emulsions with less than Very Flaky >35 recommended 67% residue and 1.1 to 1.2 for emulsions with residues Flaky 2635 0 to 0.056 [0 to 0.01] Crushed Angular 1525 0 greater than 67%. Cubic <15 +0.056 [+0.01] Rounded 0 to +0.056 [0 to +0.01] Binder application rates are for residual binder and do not Uncrushed Rounded +0.056 [+0.01] include the water content of emulsion.

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18 Table 9. Traffic adjustment, Vt (after Austroads 2006). Traffic Adjustment, Vt, gal/yd2/in. [L/m2/mm] Traffic Flat or Downhill Slow-Moving Climbing Lanes Normal Channelized Normal Channelized Overtaking lanes of multilane rural roads where traffic +0.056 [+0.01] 0 0 0 is mainly cars with HV <=10% Non-traffic areas such as shoulders, +0.112 [+0.02] 0 0 0 medians, and parking 015 0 0.056 [0.01] 0.056 [0.01] 0.112 [0.02] EHV*, 1625 0.056 [0.01] 0.112 [0.02] 0.112 [0.02] 0.168 [0.03] % 2645 0.112 [0.02] 0.168 [0.03] 0.168 [0.03] 0.224 [0.04]** >45 0.168 [0.03] 0.224 [0.04]** 0.224 [0.04]** 0.281 [0.05]** * Equivalent heavy vehicles, EHV, % = HV% + LHV% x 3 Where HV = vehicles over 3.5 tons and LHV = vehicles with seven or more axles ** If adjustments for aggregate shape and traffic effects result in a reduction in basic voids factor, Vf, of 0.224 gal/yd2/in [0.4 L/m2/mm] or more, special consideration should be given to the suitability of the treatment and the selection of alternative treatments. Note that a minimum design voids factor, Vf, of 0.56 gal/yd2/in [0.10 L/m2/mm] is recommended for any situation. 6.1.6 Polymer Modified Emulsion Factor a) Texture of Existing Surface, As When polymer modified emulsions are used, the appli- The surface texture of the existing substrate may have cation rate should be adjusted using the factor PF listed in some demand for emulsion and should be accounted for. Table 10. This depends on the texture depth of the substrate, the type The basic binder polymer modified emulsion application of substrate (existing chip seal, hot mix asphalt, or slurry rate is calculated as follows: seal), and the size of cover aggregate to be applied. The cor- rection ranges from 0 gal/yd2 (L/m2) for chip seals over hot Bbpme = Bb EF PF mix asphalt with texture depth no more than 0.1 mm to +0.11 gal/yd2 (L/m2) for 1/4-in. to 3/8-in. (5 to 7 mm) chip seals Binder application rates are for residual binder and do not over a surface with texture greater than 2.9 mm. A guide for include the water content of emulsion. estimating this correction is shown in Figures 3a and 3b for U.S. customary and SI units, respectively. The pavement texture is commonly measured by the 6.1.7 Correction Factors sand patch test (ASTM E 965). The test is accurate but Corrections should be considered to account for the fol- is slow, exposes personnel to traffic, and wind effects can lowing factors: affect results. The sand patch test was correlated to the circular track meter (ASTM E 1845) test method that is a) Texture of existing surface, faster, less susceptible to variation, and poses fewer safety b) Aggregate embedment into substrate, concerns. c) Binder absorption into the substrate, and The sand patch test is a volumetric method for determining d) Binder absorption into the chip-seal aggregate. the average depth of pavement surface macrotexture. A known volume of small particles (either sieved sand or small glass beads) is poured onto the pavement surface and spread evenly Table 10. Polymer modified into a circle using a spreading tool. Four diameters of the cir- emulsion factors. cle are measured, and an average profile depth is calculated from the known material volume and the averaged circle area. Traffic, This depth is reported as the MTD in millimeters. The method PF veh/day/lane is designed to provide an average depth value and is considered <500 1.0 insensitive to pavement microtexture characteristics. 500 to 2,500 1.1 The CT meter test method (ASTM E 2157) is used to mea- >2,500 1.2 sure and analyze pavement macrotexture profiles with a laser

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19 0.12 0.10 Correction, gal/yd2 0.08 0.06 0.04 0.02 0.00 0 5 10 15 20 25 Sand Patch Diameter (based on 1.5 in3 sand volume) (a) 0.60 0.50 Correction, As, L/m2 0.40 0.30 0.20 0.10 0.00 0 100 200 300 400 500 600 Sand Patch Diameter (based on 25,000 mm3 sand volume) (b) Figure 3. Surface texture correction factor, As versus sand patch diameter. displacement sensor. The laser sensor is mounted on an arm using the results from the ball penetration test method. In this which follows a circular track that has a diameter of 284 mm method, a 3/4-in. (19-mm) ball bearing is driven into the sub- (11.2 in.). Depth profiles are measured at a sample spacing strate surface with one blow of a Marshall compaction ham- of 0.87 mm, and the data are segmented into eight 111.5-mm mer and several tests are conducted and averaged. When ball (4.39-in.) arcs of 128 samples each. A MPD is calculated for penetration exceeds 3 mm, the pavement is considered too each segment, and an average MPD is then calculated for the soft to chip seal; alternative preventive maintenance tech- entire circular profile. niques should be considered. Recent research under NCHRP Project 14-17 developed the following relationship between sand patch texture depth and c) Absorption of Emulsion into Substrate, Aas CT meter texture depth: The correction for potential loss of emulsion to the substrate Sand patch texture, mm = ( 0.9559 CT meter texture ) + 0.1401 by absorption is applied primarily to chip seals constructed over surfaces other than hot mix asphalt pavements or previ- ous chip seals. The following corrections are suggested: b) Embedment into Substrate, Ae Granular unbound pavements +0.04 to +0.06 gal/yd2 The embedment correction factor compensates for loss of (+0.2 to +0.3 L/m2) voids in the chip seal under traffic due to chips being forced Pavements using +0.02 to +0.04 gal/yd2 into the surface of the substrate. The depth of embedment cementitious binders (+0.1 to +0.2 L/m2) depends on the volume and type of traffic and resistance of the Asphalt stabilized surfaces -0.04 to 0 gal/yd2 substrate. The corrections shown in Figure 4 are recommended (-0.2 to 0.0 L/m2)