Chip Seal Best Practices (2005)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

101 INTRODUCTION The very early practitioners of chip seals appear to have used a purely empirical approach to their designs. Sealing a pavement was considered then, as it is now in many circles, an art. The design of a chip seal involves the calculation of correct quantities of a bituminous binder and a cover aggre- gate to be applied over a unit area of the pavement. Several design approaches outlined in the available literature are briefly described in this appendix. The details of the various design methods in use in the United States, Canada, and overseas are reported here. An effort has been made to report the salient details of each method without describing the entire method in detail. Repre- sentative examples of design charts and tables are presented to illustrate the level of design detail that is involved in each method. The reader should refer to the literature for details. HANSON METHOD The first recorded effort at developing a design procedure for seal coats appears to have been made by a New Zealander, F.M. Hanson (1934/35). His design method was developed primarily for liquid asphalt, particularly cutback asphalt, and it was based on the average least dimension (ALD) of the cover aggregate spread on the pavement. Hanson calculated ALD by manually calipering a representative aggregate sam- ple to obtain the smallest value for ALD that represents the rolled cover aggregate layer. He observed that when cover aggregate is dropped from a chip spreader on to a bituminous binder, the void between aggregate particles is approximately 50%. He theorized that when the layer is rolled, this value is reduced to 30% and it is further reduced to 20% when the cover aggregate is compacted by traffic. Hanson’s design method involved the calculation of bituminous binder and aggregate spread rates to be applied to fill a certain percent- age of the voids between aggregate particles. Hanson speci- fied the percentage of the void space to be filled by residual binder to be between 60% and 75%, depending on the type of aggregate and traffic level. McLEOD METHOD Throughout the 1960s, N. McLeod developed a design pro- cedure based partially on Hanson’s previous work (McLeod 1969). McLeod’s design determines the aggregate application rate based on gradation, specific gravity, shape, and a wastage factor. McLeod provided a correction factor owing to the frac- tion of voids. The binder application rate is determined by the aggregate gradation, pavement condition, traffic volume, and type of asphalt. McLeod made it apparent that correction fac- tors for the quantity of binder lost by absorption of aggregate and texture of existing surface are recommended. McLeod’s work also gives guidelines on the appropriate type and grade of asphalt for the selected aggregate and surface temperature at time of application. The Asphalt Emulsion Manufacturers Association and the Asphalt Institute have gone on to adapt this method in the form of recommendations for binder types and grades for various aggregate gradations, and correction factors to the binder application rate based on existing surface condition (Seal Coat . . . 2003). KEARBY METHOD In 1953, J.P. Kearby, an engineer with the Texas Highway Department, made one of the first efforts at designing chip seal material application rates in the United States. Kearby was quick to point out that “computations alone cannot produce satisfactory results and that certain existing field conditions require visual inspection and the use of judgment in the choice of quantities of asphalt and aggregate” (Kearby 1953). Kearby developed a method to determine the amounts and types of asphalt and aggregate rates for one-course surface treatments and chip seals. Kearby’s work resulted in the development of a nomograph that provided an asphalt cement application rate in gallons per square yard for the input data of average thickness, percent aggregate embedment, and percent voids (Kearby 1953). The design methodology requires the knowl- edge of some physical characteristics of the aggregate, such as unit weight, bulk specific gravity, and quantity of aggregate needed to cover 1 yd2 of roadway. The unit weight test, bulk- specific gravity test, is done for calculating unit weight and bulk-specific gravity. Figure C1 is the nomograph developed by Kearby for use in chip seal design. In addition to developing the nomograph, Kearby recom- mended the use of a uniformly graded aggregate by outlining eight grades of aggregate based on gradation and associated average spread ratios. Each gradation was based on three sieve sizes. He also recommended that combined flat and elongated particle content not exceed 10% of any aggregate gradation requirement. Flat particles were defined as those with thick- ness less than half the average width of the particle, and elon- gated particles were defined as those with length greater than twice that of the other minimum dimension. Kearby suggested that when surface treatments are applied over existing hard- paved surfaces or tightly bonded hard base courses, the per- centage of embedment should be increased for hard aggre- gates and reduced for soft aggregates. He also mentioned that APPENDIX C Chip Seal Design Details

102 some allowance should be made for highway traffic. It was suggested that for highways with high counts of heavy traf- fic, the percent embedment should be reduced, along with the use of larger-sized aggregates, and for those with low traffic, it should be increased with the use of medium-sized aggre- gates. However, Kearby did not recommend any numerical corrections. Kearby also elaborated on the following construction aspects of surface treatments and seal coats based on his experience at the Texas Highway Department: • Chip seals had been used satisfactorily on both high- volume traffic primary highways and low-volume traffic farm roads, with the degree of success largely depending on the structural strength of the pavement rather than on the surface treatment itself. • Thickness of the surface treatment ranges from 1⁄4 in. to 1 in., with the higher thickness being preferred. How- ever, lighter treatments have, in general, proven satisfac- tory when the pavement has adequate structural capacity and drainage. • In general, most specification requirements for aggre- gate gradation are very broad, resulting in considerable variations in particle shape and size as well as in percent voids in the aggregate. • It is better to err on the side of a slight deficiency of asphalt to avoid a fat, slick surface. • Considerable excess of aggregate is often more detri- mental than is a slight shortage. • Aggregate particles passing the No. 10 sieve act as filler, thereby raising the level of asphalt appreciably, and cannot be relied on as cover material for the riding surface. • Suitable conditions for applying surface treatments are controlled by factors such as ambient, aggregate, and surface temperatures and general weather and surface conditions. • Rolling with both steel-wheeled and pneumatic rollers is virtually essential. During the same period, two researchers from the Texas Highway Department (Hank and Brown 1949) published a paper about their aggregate retention studies on seal coats. They conducted tests to determine the aggregate retention under a variety of conditions, including source of asphalt cement, penetration grade of asphalt, number of roller passes, binder type (asphalt cement versus cutback), aggregate gradation, and binder application temperature. All of their tests were conducted under the same condi- tions, with only the test parameter being variable. Those FIGURE C1 Nomograph to determine asphalt cement application rate in seal coats and one- course surface treatments (Kearby 1953).

103 authors concluded that aggregate retention was not signifi- cantly different from that of asphalt cements picked from five different sources commonly used by the Texas Highway Department at the time. In the same study, the effect of aggregate gradation on the performance of chip seals was investigated. An OA- 135 asphalt cement (close to an AC-5) applied at a rate of 0.32 gal/yd2 was used under different aggregate treat- ments. The corresponding aggregate loss values are repro- duced in Table C1. These results highlight the authors’ con- tention that increased No. 10-sized aggregate content poses aggregate retention problems in seal coats. In addition, those researchers showed that a smaller portion of aggre- gate, less than 1⁄4 in. in size, results in better performance of the seal coat. MODIFIED KEARBY METHOD (TEXAS) In 1974, Epps and associates proposed a further change to the design curve developed by Kearby for use in seal coats by using synthetic aggregates (Epps et al. 1974). On the basis of high porosity in synthetic aggregates, a curve showing approximately 30% more embedment than with the Benson– Gallaway curve was proposed. The rationale for this increase was that high-friction, lightweight aggregate may overturn and subsequently ravel under the action of traffic. In a separate research effort, the Epps team (1980) con- tinued the work done in Texas by Kearby (1953) and Benson and Gallaway (1953), by undertaking a research program to conduct a field validation of Kearby’s design method. Data from before and after construction of 80 different projects were gathered and analyzed for this purpose (Holmgreen et al. 1985). It was observed that the Kearby design method predicts lower asphalt rates than what was used in Texas practice, and the study proposed two changes to the design procedures. The first one is a correction to the asphalt appli- cation rates based on level of traffic and existing pavement condition. The second is the justification of the shift of the original design curve proposed by the Kearby and Benson– Gallaway methods, as suggested for lightweight aggregates (Epps et al. 1974). Eq. C1 was used to calculate the asphalt application rate (in gallons per square yard), which included two correction factors determined for traffic level and existing surface condition. The modified Kearby method also recommends a labora- tory “board test” method to find the quantity of aggregate needed to cover 1 yd2 of roadway. The board test is performed by placing an adequate number of rocks on an area of 1 yd2. The weight of aggregates that cover this area is determined and converted into a unit of pounds per square yard. Epps and associates developed correction factors for the Kearby method, based on what seemed to be working well in practice (Epps et al. 1980). The binder application rate cor- rection factors corresponded to traffic level and surface con- dition. Epps also suggested that consideration be given to varying the asphalt rate both longitudinally and transversely, as reflected by the pavement surface condition (Epps et al. 1980). Since that time, this design approach has been labeled as the modified Kearby method by both practitioners and researchers. Since the publication of that design procedure, the Texas Department of Transportation’s Brownwood District has expanded on the asphalt application correction factors to include adjustments for truck traffic and existing surface condition. Table C2 shows the design output that was used in a research study documenting chip seal performance on high- volume roads in Tulsa, Oklahoma, in 1989 (Shuler 1991). It reveals the differences in design binder and aggregate appli- cation rates when using the two different methods with the same design input parameters. One can see that there are con- siderable differences in the resultant rates calculated by each of the two methods. One must remember that both these meth- ods are being used by agencies that then expect experienced field personnel to adjust the design rates to match the chang- ing surface conditions found in the actual project. It must also be noted that the project carried an estimated 38,000 average daily traffic (Shuler 1991) and, therefore, these rates will probably appear higher than expected. However, most expe- rienced chip seal personnel are used to seeing rates for low- to moderate-volume roads. ROAD NOTE 39 The United Kingdom’s Transport Research Laboratory has published several editions of a comprehensive design proce- dure for “surface dressing” roads in the United Kingdom (Design Guide . . . 1996). The technology that makes this design procedure so advanced is the extensive use of a com- puter design program based on decision trees (Colwill et al. 1995). Known as Road Note 39, this design procedure is A E d W G T V= −{ } +5 61 1 62 5 1. . ( )C TABLE C1 EFFECT OF AGGREGATE GRADATION AND AGGREGATE TREATMENT ON RETENTION Test Condition for Aggregate Aggregate Loss as a Percentage of Original 12.6% passing No. 10 sieve 72.0 6.7% passing No. 10 sieve 57.4 0% passing No. 10 sieve 30.5 12.6% passing No. 10 sieve and rock preheated to 250°F 17.7 12.6% passing No. 10 sieve and rock precoated with MC-1 33.6 Source: Hank and Brown 1949.

104 highly advanced and uses a multitude of input parameters. Traffic level, road hardness, surface conditions, and site geom- etry are critical input factors. Skid-resistance requirements and likely weather conditions are secondary inputs into the pro- gram (Design Guide . . . 1996). This procedure includes the following five steps: 1. Selection of the type of dressing—The selection of sur- face dressing (surface treatment) is made from five treatments: single dressing, pad coat plus single dress- ing, racked-in dressing, double dressing, and sandwich dressing. 2. Selection of binder—Binders are selected from either emulsion or cutback asphalt, specified based on viscos- ity. Modified binders such as polymer-modified binders are also recommended if their need and additional cost can be justified. The grade of binder is selected based on the road traffic category and construction season. 3. Selection of aggregate—The nominal size of aggregate is selected based on traffic and hardness of existing surface. Specified are 20-, 14-, 10-, 6-, and 3-mm nominal-size aggregates. However, the 20-mm size is not commonly used, owing to the risk of windshield damage. 4. Binder spread rate—The required rate of binder spread depends on the size and shape of aggregates, nature of existing road surface, and degree of embedment of aggregate by traffic. The rate of binder spread should not vary by more than 10% from the target figure. 5. Rate of aggregate spread—The aggregate spread rate is determined based on a “tray test” and depends on the size, shape, and relative density of the aggregate. The basic inputs into the decision trees include selection of the type of treatment and selection of grade and type of binder based on traffic and construction season. Table C3 is taken from the Road Note 34 design manual and lists the design inputs used in the chip seal design software. The aggregate type and size are selected based on skid and friction requirements, likely weather conditions, and hard- ness of existing surface. The resulting design application rate of binder is determined by the size and shape of aggregates, nature of existing road surface, and degree of embedment of aggregate by traffic. The resulting design application rate of aggregate spread rate depends on the size, shape, and relative density of the aggregate (Design Guide . . . 1996). AUSTROADS SPRAYED SEAL DESIGN METHOD The 2004 Austroads’ Sprayed Seal Design Manual provide a performance-based design method that uses an extensive list of input parameters for determining aggregate and binder application rates. Aggregate angularity, traffic volume, road geometry, ALD of aggregate, aggregate absorption, pavement absorption, and texture depth are the input variables for this TABLE C2 COMPARATIVE DESIGN OUTPUT FOR THE MODIFIED KEARBY AND McLEOD CHIP SEAL DESIGN METHODS Existing Surface Condition Slight Bleeding Normal Slight Raveling Design Method Nominal Aggregate Size Modified Kearby McLeod Modified Kearby McLeod Modified Kearby McLeod Emulsion Rate (gal/yd2) Emulsion Rate (gal/yd2) Emulsion Rate (gal/yd2) Emulsion Rate (gal/yd2) 0.25 0.18 0.29 0.22 0.33 0.27 3/8 in. Natural Aggregate Aggregate Rate (lb/yd2) (lb/yd2) (lb/yd2) (lb/yd2) 21.2 17.1 21.2 17.1 21.2 17.1 0.29 0.30 0.33 0.34 0.37 0.39 5/8 in. Natural Aggregate Aggregate Rate 24.6 25.6 24.6 25.6 24.6 25.6 0.54 0.27 0.58 0.32 0.62 0.36 3/8 in. Synthetic Aggregate Aggregate Rate 17.1 14.0 17.1 14.0 17.1 14.0 0.51 0.30 0.55 0.35 0.59 0.39 5/8 in. Synthetic Aggregate Aggregate Rate 14.3 18.3 14.3 18.3 14.3 18.3 Source: Shuler 1991.

105 method. The main assumption of this design model is that the aggregate in a seal is orientated approximately one layer thick and contains a percentage of air voids. Thus, filling a percent- age of the voids with binder determines the binder application rate. The minimum binder application rate is determined by the percentage of voids to be filled, the total available voids, and the thickness of the seal. The first step in the Austroads procedure is to determine a basic voids factor. Adjustments for aggregate characteristics and anticipated traffic levels are added to derive a design voids factor. That factor is then multiplied by the ALD of the aggre- gate to determine the basic binder application rate. This base binder application rate is then modified with allowances to cater to the texture and absorption of the pavement surface and the aggregate. Some aggregates are susceptible to absorbing binder, resulting in the decrease of effective binder and a possible loss of aggregate from the seal under traffic. Adding allowances to the basic binder application rate compensates for this characteristic. The amount of binder required depends on the size, shape and orientation of the aggregate particles, embedment of aggregate into the base, texture of surface onto which the seal is being applied, and absorption of binder into either the pavement or aggregate. The geometry of the road can affect the design of a seal, and it is necessary to make adjustments to the binder application rate. Geometric factors include narrow lanes, climbing lanes, and turning locations. Where traffic is channeled into confined wheelpaths, such as TABLE C3 ROAD NOTE 34 OPERATIONS IN DESIGNING SURFACE DRESSING Operation Task Section* Selection Section* Concept Decide to surface dress 2.1 6.2 Latitude 6.2.1 Altitude 6.2.2 Type selection and Stage 1 binder-spread category parameters Road hardness 6.2.3 Traffic category 6.2.4 Traffic speed 6.2.5 General surface condition 6.2.6 Highway layout 6.2.7 Material selection 6.3 Skid-resistance requirements 6.3.1 Site Season and weather conditions 6.3.2 Consider existing condition of site 7.1 Divide up site 7.1 Select type of surface dressing 7.3 Single surface dressing 2.2.1 Racked-in surface dressing 2.2.2 Double surface dressing 2.2.3 Inverted double surface dressing 2.2.4 Sandwich surface dressing 2.2.5 High-friction surface dressing 2.2.6 Site Rationalize types of surface dressing 7.4 Select type of chippings 8.1 Uncoated chippings 8.1.2 Lightly coated chippings 8.1.3 Artificial aggregate chippings 8.1.7 Select size of chippings 8.2 6 mm, 10 mm, 14 mm, or combinations Select type of binder 9.1 Unmodified bitumen emulsion, cutback bitumen 9.1.1 Modified binder 9.1.2 Material Selection Resin binders 9.1.4 Unmodified bituminous binders Stage 1 binder-spread category 9.2.2 Stage 2 binder-spread category (from aggregate properties) 9.2.4 Chipping shape Type of chipping 6.4.1 6.4.2 9.2.5 Surface condition 6.5.1 Gradient 6.5.2 Stage 3 adjustment factors (from site conditions) Shade 6.5.3 Local traffic 6.5.4 Target rate of spread of binder 9.2.5 Modified bituminous binders 9.2.2 Rate of Spread of Binder Resin binders 9.3 *Refers to paragraph in design manual that governs the specific aspect of chip seal design in that row of the table. Source: Design Guide for Road Surface Dressings 1996.

106 on single-lane bridges, tight radius curves, or pavements with confined lane widths, a traffic adjustment factor is necessary. The design binder application rate is calculated by adding all the allowances to the basic binder application rate. It should be noted that some of the allowances may be negative, and thus the design binder application rate may be lower than the base binder application rate. For multiple course chip seals, the Austroads design methodology distinguishes between whether the additional courses are applied immediately or later. When it is planned that all courses of the chip seal will be placed on the same day, the design is essentially the same as for a single-course treat- ment, with a reduction in the design voids factor. Adjustments are made for designing as a reseal, but adjustments for surface texture and embedment are not performed. When it is planned to stage a delay in the application of the courses, the binder application rates for the additional courses are generally set at a minimum, and aggregate application rates are commonly reduced to 70% of conventional design. Figure C2 illustrates the Austroads Design Procedure for Single/Single (single course) Sprayed Seals. SOUTH AFRICAN METHOD, TRH3 South Africa has an extensive and well-developed chip seal program on routes with up to 50,000 equivalent vehicle units (Beatty et al. 2002). The South African design process for chip seals is based on a number of input parameters. Traffic vol- ume, preferred texture depth, and surface hardness are the pri- mary inputs in the design process. Practical adjustments for FIGURE C2 Austroads sprayed seal design procedure, 2000.

107 climate, gradients, existing coarse texture, hot applications, preferred aggregate matrix, and use of polymer-modified binders are common. The approach taken by the South African design method, TRH3, is a hybrid of the United Kingdom and Australian design methodologies. The selection of surfacing is made between single seal with modified binders, double seals, Cape seals, and sand seals. The decision is primarily based on the traffic level and pavement condition. Of particular interest is that this method measures and evaluates surface hardness by using a ball penetration test, corrected for temperature. The grade of binder is selected based on traffic level, road surface temperature, climatic region, and aggregate condition. The required rate of binder spread is determined by using charts that incorporate aggregate spread rate, traffic level, and ALD. Embedment (mm) Embedment (mm) Tr af fic (v eh /la ne /da y) N et C ol d Bi nd er (L /m 2 ) 0 0,5 1,0 1,5 2,0 4 3 2,5 2 0 5000 10000 15000 20000 25000 30000 35000 40000 3 2.5 1.5 0.5 1,23 2 1 0 0.5 1 1.5 2 Corrected Ball Penetration 0 - 1 mm 1,5 9mm ALD Minimum Text. 1mm Text. 0.7mm Text. 0.5mm max. min. FIGURE C3 Example of South African chip seal design charts. The aggregate spread rate is extrapolated from design charts based on the ALD of the aggregate and the required texture depth. The South Africans have eliminated single seal design without modified binder, because they do not construct any single-coarse seals without the use of a modified binder. Another important assumption of this design method is includ- ing correction factors to adjust binder application rates when using modified binders. Polymer-modified binder application rates are adjusted, because the South Africans have found that aggregate orientation is different in comparison with conven- tional seals. The design charts shown in Figure C3 are exam- ples of typical TRH3 charts, and Figure C4 is a sample design spreadsheet illustrating the application of the TRH3 chip seal design method.

108 Road / Pad (Street / Straat) : Carriageway / Padbaan : Lane / Laan : From / Van : Seal Type / Seëltipe : Design method / Ontwerpmetode Binder Type / Bindmiddel Tipe : General / Algemeen Traffic Counts / Verkeerstellings Heavy Vehicles per day / Swaarvoertuie per dag Light Vehicles per day / Ligtevoertuie per dag (LV) Equivalent LV per day / Ekwivalente LV per dag Climatic Zone / Klimaatstreek Ave Ball Penetration / Gem. Balpenetrasie (corrected) (mm) Texture Depth / Tekstuur Diepte (Existing) (mm) Aggregate / Aggregaat Particulars of Aggregate / Besonderhede van Aggregaat Source / Bron Type / Tipe Nominal Size / Nominale Grootte ALD / GKA (Meas. / Gemeet) (mm) Flakiness / Platheid (%) AIV / AIW (Dry/Droog // Wet/Nat )* (%) ACV/AVW (Dry/Droog // Wet/Nat) * ( %) Basic double seal design 19,0/6,7 mm N1 Dark grey dolorite Petra quarry, Bloemfontein 29 920 11,7% dry : 16,8% wet Class S-E1 modified binder (SBR type of modifier) + 30% anionic emulsion diluted 50:50 7 480 2/3 boundary 2,57 slow lane fast lane Total / Totaal 170 850680 0,40 37 400 3 400 11,7% dry : 16,8% wet 2 720 680 24.0 7,1 19,0/6,7mm double seal Ch. 28 500 Ch. 62 800 to / na : TRH 3 1998 Northbound and Southbound Slow lane Approved by/date: Checked by/date: Prepared by/date: dolorite, precoated @ 14 l/m3 with sacrasote 70 or similar approved 19,0mm 6,7mm 12,2mm 4,5mm FIGURE C4 South African TRH3 chip seal design method sample. (Continued on next page.)

109 Method used: TRH3 (Incorporating latest amendments) Ball penetration: 1,53mm Road description: N1 Section 15: Sydenham to Glen Lyon Corrected ball penetration: T0 = 1,53 - 0,04(17-43) (km 28,5 to 62,8) - Bloemfontein W. Bypass = 2,57mm (TMH6) Binder: Class S-E1 (SBR modified) Sand patch length: 1,96m Aggregate (Bottom): Precoated 19,0mm Grade 1 aggregate with Existing texture depth T = 250/(1000x0,3x1,96) ALD = 12,2mm and Flakiness Index = 24% = 0,40mm (Top): Precoated 6,7mm Grade 1 aggregate with ALD = 4,5mm and Flakiness Index = 8,6% Traffic conditions: 4 250 vehicles per day (of which 20% are heavy vehicles) i.e. 3 400 light vehicles + 850 heavy vehicles Assume slow lane 80% traffic, including 80% of heavy vehicles. On slow lane: (0,8 x 3 400) + (40 x 680) = 29 920 elv/lane/day (Design for slow lane) Design texture depth: 0,7mm (Desired final texture depth) ALD of aggregate: ALD of bottom layer + 50% of ALD of top layer = 12,2mm + 2,25mm = 14,45mm Adjustment for modified binder: 2,03 x 0,035 = 0,07 l/m2(Fig.9,TRH3) Embedment (from charts): 2,32mm Adjustment for existing texture: 0,14 litre/m2 (Fig.7,TRH3) Modified embedment: 0,5 x 2,32 = 1,16mm Adjustment for climate: 2,03 x 0,01 = 0,02 l/m2 (Fig.2, TRH3) Nett cold binder (from charts): 2,03 l/m2 * Adjustment for new asphalt: - 0,10 l/m2 (discresionary) Adjustment for grade: Nil Nett cold binder (after adjustments): 2,03 + 0,07 + 0,10 + 0,02 - 0.10 = 2,12 l/m2 Design Data PAWC: 0,172 x 13,55 = 2,33 l/m2 F.S. Concept seal 1976 : 2,24 l/m2 Adjustment for hot application: 1,08 x 2,12 = 2,30 l/m2 Tack coat (hot applied): 1,15 l/m2 Penetration/Tack coat (hot applied): 1,00 l/m2 Fog spray (60% anionic, diluted 50:50) 1,00 l/m2 (Effective = 1.0*0.3*50% = 0.15l/m2) Aggregate spread rate: 19,0mm aggregate: 70 m2/m3 (Fig.F-1, TRH3) 6,7mm aggregate (Applied in two layers): 110 m2/m3 Layer 1 : 450 m2/m3 as choke layer Layer 2 : + 155 m2/m3 as top layer on double seal and as single seal on sides (Fig.F-1, TRH3) Control check (Alternative design Methods) Spray rates FIGURE C4 (Continued).