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

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15 The basic chip-seal design methods proposed by Hanson (1934–1935) and by Kearby (1953) provided the basis for future design methods. From the original Hanson concepts evolved the McLeod procedure (McLeod 1960, 1969) that was later adopted by the Asphalt Institute (Asphalt Institute MS19) and the Austroads and South African methods (South African Roads Agency 2007). The Kearby method was later improved (Benson and Gallaway 1953, Epps et al. 1981) and adopted by Texas. The United Kingdom designs “surface dressings” or chip seals using some of the Hanson concepts combined with ideas of Jackson (1963). Several methods have been used for the design of chip seals. The following discussion describes a design method based on the Austroads method. The purpose of chip-seal design is to select aggregate and asphalt emulsion application rates that will result in a durable pavement seal. The quantity of binder required depends on the size, shape, and orientation of the aggregate particles; embedment of aggregate into the substrate; texture of the substrate; and absorption of binder into either the substrate or aggregate. This design method is based on the following assumptions for aggregate, traffic, and embedment: • Aggregate: one-sized aggregates with a flakiness index of 15% to 25% • Traffic: 10%, or less, heavy vehicles • Embedment: 50% to 65% chip embedment after two years 6.1 Emulsion Application Rate The emulsion application rate is the spray quantity of asphalt emulsion applied during construction; it is determined as follows: B B EF PF A A A Ad b s e as aa= [ ]+ + + +  Where Bd = design binder application rate, gal/yd2 (L/m2); Bb = basic binder application rate, gal/yd2 (L/m2); EF = emulsion factor; PF = polymer factor (for polymer modified emulsions, only); and As, Ae, Aas, Aaa = adjustments for substrate texture, embed- ment, absorption into substrate, and absorption into cover aggregate, gal/yd2 (L/m2). Where Bb = VF × ALD; VF = design voids factor, gal/yd2/in (L/m2/mm); and ALD = average least dimension of cover aggregate. Where VF = Vf + Va + Vt, Vf = basic voids factor, Va = aggregate shape adjustment factor, and Vt = traffic effects adjustment factor. Thus the design binder application rate is: Each of these parameters is discussed below. 6.1.1 Basic Voids Factor, Vf The basic voids factor depends on traffic level because traf- fic determines how much of the aggregate is embedded in the binder. Figures 1a and 1b use English and SI units, respec- tively, for traffic of less than 500 vehicles per day per lane. Fig- ures 2a and 2b use English and SI units, respectively, for traf- fic of greater than 500 vehicles per day per lane. The three curves in each figure represent a range of basic voids factors B Vf Va Vt ALD EF PF A A A Ad s e as aa= + +( )[ ]{ }+ + + +   C H A P T E R 6 Chip-Seal Design

16 0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50 1.60 1.70 1.80 0 50 100 150 200 250 300 350 400 450 500 Traffic, Veh/day/lane B as ic V oi ds F ac to r, Vf , g al /y d2 /in . Bleeding Limit Raveling Limit Target (a) 0 50 100 150 200 250 300 350 400 450 500 Traffic, Veh/day/lane 0.15 0.17 0.19 0.21 0.23 0.25 0.27 0.29 B as ic V oi ds F ac to r, Vf , L /m 2 /m m Bleeding Limit Raveling Limit Target (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 content (bleeding limit), which should not be exceeded. 6.1.2 Adjustment for Aggregate Shape, Va The design method assumes the flakiness index will be between 15 and 25; an adjustment must be made for aggre- gates outside this range. Table 8 provides suggested adjust- ment factors. 6.1.3 Adjustment for Traffic, Vt The basic voids factor, Vf, was developed for an average mix of light and heavy vehicles in a free traffic flow situation. When this is not the case due to composition; non-trafficked areas; overtaking lanes with few heavy vehicles or for large propor- tions of heavy vehicles; channelization and slow-moving, heavy vehicles in climbing lanes; or stop/start conditions, an adjustment, Vt, needs to be made. Table 9 shows recom- mended adjustments. 6.1.4 Average Least Dimension The volumetric design of a chip seal is based on the assump- tion that aggregate particles tend to lie with the least dimension vertical. The least dimension is defined as the smallest dimen- sion of a particle when placed on a horizontal surface, the par- ticle being most stable when lying with the least dimension vertical. Thus in a chip seal, the final orientation of most par- ticles is such that the least dimension is near vertical, providing there is sufficient room for the particles to realign. This aver- age least dimension, ALD, is as follows: ALD mm M mm FI ALD , , or, , = + ×( )[ ]1 139285 0 011506. . in ALD mm. .= , 25 4

17 (a) 0.50 0.60 0.70 0.80 0.90 1.00 1.10 500 1500 2500 3500 4500 5500 6500 7500 8500 9500 Traffic, Veh/day/lane B as ic V oi ds F ac to r, Vf , g al /y d2 /in . Bleeding Limit Raveling Limit Target (b) 0 0.05 0.1 0.15 0.2 0.25 500 1500 2500 3500 4500 5500 6500 7500 8500 9500 Traffic, Veh/day/lane B as ic V oi ds F ac to r, Vf , L /m 2 /m m Bleeding Limit Raveling Limit Target Figure 2. Basic voids factor versus traffic for 500 to 10,000 vehicles/lane/day (Austroads 2006). Where M = median size of the aggregate (mm) and FI = flakiness index. 6.1.5 Emulsion Factor An emulsion factor is applied to the basic binder application rate (before allowances) when using asphalt emulsions. This factor allows a greater volume of binder around the aggregate particles to compensate for reduced aggregate reorientation as a result of rapid increase in binder stiffness after the initial breaking of the emulsion. The basic binder application rate for emulsions, Bbe, is cal- culated as follows: Where Bbe = basic binder emulsion application rate rounded to the nearest 0.2 gal/yd2 [0.1 L/m2]; Bb = basic binder application rate, gal/yd2 (L/m2); and EF = emulsion factor = 1.0 for emulsions with less than 67% residue and 1.1 to 1.2 for emulsions with residues greater than 67%. Binder application rates are for residual binder and do not include the water content of emulsion. B B EFbe b= × Aggregate Type Aggregate Shape Flakiness Index, FI, % Va, gal/yd2/in. [L/m2/mm] Very Flaky >35 Too flaky, not recommended Flaky 26–35 0 to –0.056 [0 to –0.01] Angular 15–25 0 Cubic <15 +0.056 [+0.01] Crushed Rounded – 0 to +0.056 [0 to +0.01] Uncrushed Rounded – +0.056 [+0.01] Table 8. Suggested adjustment for aggregate shape, Va (after Austroads 2006).

6.1.6 Polymer Modified Emulsion Factor When polymer modified emulsions are used, the appli- cation rate should be adjusted using the factor PF listed in Table 10. The basic binder polymer modified emulsion application rate is calculated as follows: Binder application rates are for residual binder and do not include the water content of emulsion. 6.1.7 Correction Factors Corrections should be considered to account for the fol- lowing factors: a) Texture of existing surface, b) Aggregate embedment into substrate, c) Binder absorption into the substrate, and d) Binder absorption into the chip-seal aggregate. B B EF PFbpme b= × × a) Texture of Existing Surface, As The surface texture of the existing substrate may have some demand for emulsion and should be accounted for. This depends on the texture depth of the substrate, the type of substrate (existing chip seal, hot mix asphalt, or slurry 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 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 over a surface with texture greater than 2.9 mm. A guide for 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 sand patch test (ASTM E 965). The test is accurate but is slow, exposes personnel to traffic, and wind effects can affect results. The sand patch test was correlated to the circular track meter (ASTM E 1845) test method that is faster, less susceptible to variation, and poses fewer safety concerns. The sand patch test is a volumetric method for determining 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 into a circle using a spreading tool. Four diameters of the cir- cle are measured, and an average profile depth is calculated from the known material volume and the averaged circle area. This depth is reported as the MTD in millimeters. The method is designed to provide an average depth value and is considered insensitive to pavement microtexture characteristics. The CT meter test method (ASTM E 2157) is used to mea- sure and analyze pavement macrotexture profiles with a laser 18 Traffic Adjustment, Vt, gal/yd2/in. [L/m2/mm] Flat or Downhill Slow-Moving Climbing Lanes Traffic Normal Channelized Normal Channelized Overtaking lanes of multilane rural roads where traffic is mainly cars with HV <=10% +0.056 [+0.01] 0 0 0 Non-traffic areas such as shoulders, medians, and parking +0.112 [+0.02] 0 0 0 0–15 0 –0.056 [–0.01] –0.056 [–0.01] –0.112 [–0.02] 16–25 –0.056 [–0.01] –0.112 [–0.02] –0.112 [–0.02] –0.168 [–0.03] 26–45 –0.112 [–0.02] –0.168 [–0.03] –0.168 [–0.03] –0.224 [–0.04]** EHV*, % >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. Table 9. Traffic adjustment, Vt (after Austroads 2006). Traffic, veh/day/lane PF <500 1.0 500 to 2,500 1.1 >2,500 1.2 Table 10. Polymer modified emulsion factors.

19 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0 5 10 15 20 25 Sand Patch Diameter (based on 1.5 in3 sand volume) Co rr ec tio n, g al /y d2 (a) 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0 100 200 300 400 500 600 Sand Patch Diameter (based on 25,000 mm3 sand volume) Co rr ec tio n, A s, L /m 2 (b) Figure 3. Surface texture correction factor, As versus sand patch diameter. displacement sensor. The laser sensor is mounted on an arm which follows a circular track that has a diameter of 284 mm (11.2 in.). Depth profiles are measured at a sample spacing of 0.87 mm, and the data are segmented into eight 111.5-mm (4.39-in.) arcs of 128 samples each. A MPD is calculated for each segment, and an average MPD is then calculated for the entire circular profile. Recent research under NCHRP Project 14-17 developed the following relationship between sand patch texture depth and CT meter texture depth: b) Embedment into Substrate, Ae The embedment correction factor compensates for loss of voids in the chip seal under traffic due to chips being forced into the surface of the substrate. The depth of embedment depends on the volume and type of traffic and resistance of the substrate. The corrections shown in Figure 4 are recommended Sand patch texture, mm CT meter textu= 0 9559.  re( )+ 0 1401. using the results from the ball penetration test method. In this method, a 3⁄4-in. (19-mm) ball bearing is driven into the sub- strate surface with one blow of a Marshall compaction ham- mer and several tests are conducted and averaged. When ball penetration exceeds 3 mm, the pavement is considered too soft to chip seal; alternative preventive maintenance tech- niques should be considered. c) Absorption of Emulsion into Substrate, Aas The correction for potential loss of emulsion to the substrate 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: • Granular unbound pavements +0.04 to +0.06 gal/yd2 (+0.2 to +0.3 L/m2) • Pavements using +0.02 to +0.04 gal/yd2 cementitious binders (+0.1 to +0.2 L/m2) • Asphalt stabilized surfaces −0.04 to 0 gal/yd2 (−0.2 to 0.0 L/m2)

20 d) Absorption of Emulsion into Aggregate Chips, Aaa Absorption of emulsion into the chips requires a cor- rection of +0.02 gal/yd2 (+0.1 L/m2) for each 1% of water absorption. 6.2 Aggregate Application Rate The aggregate application rate is determined based on ALD, traffic volume, and chip size. The aggregate spread rate for 3⁄8-in. (10-mm) and larger chips depends on the traffic. a) For pavements with less than 200 vehicles/day/lane: Where W is loose unit weight, lb/yd3. b) For pavements with more than 200 vehicles/day/lane: The range of spread rates for 3⁄8-in. (9-mm) and smaller chips depends on whether there are one or two layers of chips placed. It ranges from 0.104W to 0.093W (290 to 260 m2/m3) for a single layer to 0.089 W to 0.072W (250 to 200 m2/m3) for two layers. Aggregate spread rate, lbs yd ALD, in. W2 = [ ] 25.27 700Aggregate spread rate, m m ALD, mm2 3 = . Aggregate spread rate, lbs yd ALD, in. W2 = [ ] 27.08 750Aggregate spread rate, m m ALD, mm2 3 = 6.3 Time Until Sweeping and Traffic The time required before sweeping or before traffic can be al- lowed on the fresh chip seal is related to the moisture content in the chip seal (Shuler 2009). The laboratory test method in NCHRP Project 14-17 may be used to determine when the chip seal can withstand sweeping and traffic stresses. In this method, test specimens of the emulsion and chips are fabricated in the laboratory and tested at three moisture contents. The moisture at which less than 10% of the chips are dislodged during the test is the target moisture content to be achieved in the field before sweeping or traffic operations should commence. 6.4 Other Considerations 6.4.1 Chips Required to Avoid Roller Pickup Additional aggregates than are actually estimated to pro- duce a one-stone layer should be spread during chip-seal con- struction to aid in reducing the potential for embedded chips to be picked up by pneumatic rollers. The amount of addi- tional material will vary, but generally is between 5% and 10%. 6.5 Example Design An example of how to use the design method to determine the binder and aggregate spread rates follows. If Maximum aggregate size = 1⁄2 in.; Median aggregate size = 3⁄8 in.; Flakiness index = 30%; 0 0.5 1 1.5 2 2.5 3 3.5 4 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Traffic, Veh/day/lane B al l P en et ra tio n, m m B CA Above Line C Correction = -0.06 gal/yd2 (-0.30 L/m2)From Line B to CCorrection = -0.04 gal/yd2 (-0.20 L/m2)From Line A to B Correction = -0.02 gal/yd2 (-0.1 L/m2)No Correction Needed this side of Line A Surface Too Soft for Chip Seal Above this Line Figure 4. Correction factors for chip penetration into substrate (Austroads 2006).

Loose unit weight, W = 110 lbs/ft3; Traffic = 1,500 veh/day/lane, channelized, with equivalent heavy vehicles, EHV%=16; Polymer modified emulsion binder with 70% residue; Sand patch diameter for texture = 18 in.; Ball penetration = 0.5 mm; Substrate is old chip seal with no expected absorption poten- tial; and Chip-seal aggregate has 1% water absorption Design binder application rate (from Section 6.1) is: Vf = 0.85 gal/yd2/in., from Figure 2b; Va = −0.028 gal/yd2/in., from Table 8; Vt = −0.112 gal/yd2/in., from Table 9; B Vf Va Vt ALD EF PF A A A Ad s e as aa= + +( )[ ]+ + + +   ALD = 0.328 in.; EF = 1.2, from Section 6.1.5; PF = 1.1, from Table 10; As = 0.02 gal/yd2, from Figure 3a; Ae = 0, from Figure 4; Aas = 0, from Section 6.1.7c; and Aaa = +0.02 gal/yd2, from Section 6.1.7d. Aggregate spread rate (from Section 6.2) AL= D, in W, lbs yd 25.27 3   [ ] = = 25 27 0 328 110 27 38 . . .5 lbs yd2. Bd = − −( )[ ] + + 0 85 0 028 0 112 0 328 1 2 1 1 0 02 . . . . . . .    0 0 0 02 0 35 2 + + =. . gal yd 21