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101 APPENDIX C Chip Seal Design Details INTRODUCTION aggregate gradation, pavement condition, traffic volume, and type of asphalt. McLeod made it apparent that correction fac- The very early practitioners of chip seals appear to have tors for the quantity of binder lost by absorption of aggregate used a purely empirical approach to their designs. Sealing a and texture of existing surface are recommended. McLeod's pavement was considered then, as it is now in many circles, work also gives guidelines on the appropriate type and grade an art. The design of a chip seal involves the calculation of of asphalt for the selected aggregate and surface temperature correct quantities of a bituminous binder and a cover aggre- at time of application. The Asphalt Emulsion Manufacturers gate to be applied over a unit area of the pavement. Several Association and the Asphalt Institute have gone on to adapt design approaches outlined in the available literature are this method in the form of recommendations for binder types briefly described in this appendix. and grades for various aggregate gradations, and correction factors to the binder application rate based on existing surface The details of the various design methods in use in the condition (Seal Coat . . . 2003). 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- KEARBY METHOD sentative examples of design charts and tables are presented to illustrate the level of design detail that is involved in each In 1953, J.P. Kearby, an engineer with the Texas Highway method. The reader should refer to the literature for details. 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 HANSON METHOD satisfactory results and that certain existing field conditions require visual inspection and the use of judgment in the choice The first recorded effort at developing a design procedure for of quantities of asphalt and aggregate" (Kearby 1953). Kearby seal coats appears to have been made by a New Zealander, developed a method to determine the amounts and types of F.M. Hanson (1934/35). His design method was developed asphalt and aggregate rates for one-course surface treatments primarily for liquid asphalt, particularly cutback asphalt, and and chip seals. Kearby's work resulted in the development of it was based on the average least dimension (ALD) of the a nomograph that provided an asphalt cement application cover aggregate spread on the pavement. Hanson calculated rate in gallons per square yard for the input data of average ALD by manually calipering a representative aggregate sam- thickness, percent aggregate embedment, and percent voids ple to obtain the smallest value for ALD that represents the (Kearby 1953). The design methodology requires the knowl- rolled cover aggregate layer. He observed that when cover edge of some physical characteristics of the aggregate, such as aggregate is dropped from a chip spreader on to a bituminous unit weight, bulk specific gravity, and quantity of aggregate binder, the void between aggregate particles is approximately needed to cover 1 yd2 of roadway. The unit weight test, bulk- 50%. He theorized that when the layer is rolled, this value is specific gravity test, is done for calculating unit weight and reduced to 30% and it is further reduced to 20% when the bulk-specific gravity. Figure C1 is the nomograph developed cover aggregate is compacted by traffic. Hanson's design by Kearby for use in chip seal design. method involved the calculation of bituminous binder and aggregate spread rates to be applied to fill a certain percent- In addition to developing the nomograph, Kearby recom- age of the voids between aggregate particles. Hanson speci- mended the use of a uniformly graded aggregate by outlining fied the percentage of the void space to be filled by residual eight grades of aggregate based on gradation and associated binder to be between 60% and 75%, depending on the type of average spread ratios. Each gradation was based on three sieve aggregate and traffic level. sizes. He also recommended that combined flat and elongated particle content not exceed 10% of any aggregate gradation McLEOD METHOD requirement. Flat particles were defined as those with thick- ness less than half the average width of the particle, and elon- Throughout the 1960s, N. McLeod developed a design pro- gated particles were defined as those with length greater than cedure based partially on Hanson's previous work (McLeod twice that of the other minimum dimension. Kearby suggested 1969). McLeod's design determines the aggregate application that when surface treatments are applied over existing hard- rate based on gradation, specific gravity, shape, and a wastage paved surfaces or tightly bonded hard base courses, the per- factor. McLeod provided a correction factor owing to the frac- centage of embedment should be increased for hard aggre- tion of voids. The binder application rate is determined by the gates and reduced for soft aggregates. He also mentioned that

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

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103 TABLE C1 Eq. C1 was used to calculate the asphalt application rate EFFECT OF AGGREGATE GRADATION AND AGGREGATE (in gallons per square yard), which included two correction TREATMENT ON RETENTION factors determined for traffic level and existing surface Aggregate Loss condition. Test Condition for Aggregate as a Percentage of Original 12.6% passing No. 10 sieve 6.7% passing No. 10 sieve 0% passing No. 10 sieve 72.0 57.4 30.5 A = 5.61 E d { 1- W 62.5G T +V} (C1) 12.6% passing No. 10 sieve and rock preheated to 250F 17.7 12.6% passing No. 10 sieve and rock precoated with MC-1 33.6 The modified Kearby method also recommends a labora- tory "board test" method to find the quantity of aggregate Source: Hank and Brown 1949. 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 authors concluded that aggregate retention was not signifi- and converted into a unit of pounds per square yard. cantly different from that of asphalt cements picked from five different sources commonly used by the Texas Highway Epps and associates developed correction factors for the Department at the time. Kearby method, based on what seemed to be working well in practice (Epps et al. 1980). The binder application rate cor- In the same study, the effect of aggregate gradation on rection factors corresponded to traffic level and surface con- the performance of chip seals was investigated. An OA- dition. Epps also suggested that consideration be given to 135 asphalt cement (close to an AC-5) applied at a rate varying the asphalt rate both longitudinally and transversely, of 0.32 gal/yd2 was used under different aggregate treat- as reflected by the pavement surface condition (Epps et al. ments. The corresponding aggregate loss values are repro- 1980). Since that time, this design approach has been labeled duced in Table C1. These results highlight the authors' con- as the modified Kearby method by both practitioners and tention that increased No. 10-sized aggregate content poses researchers. Since the publication of that design procedure, aggregate retention problems in seal coats. In addition, the Texas Department of Transportation's Brownwood those researchers showed that a smaller portion of aggre- District has expanded on the asphalt application correction gate, less than 1/4 in. in size, results in better performance of factors to include adjustments for truck traffic and existing the seal coat. surface condition. Table C2 shows the design output that was used in a MODIFIED KEARBY METHOD (TEXAS) research study documenting chip seal performance on high- volume roads in Tulsa, Oklahoma, in 1989 (Shuler 1991). It In 1974, Epps and associates proposed a further change to the reveals the differences in design binder and aggregate appli- design curve developed by Kearby for use in seal coats by cation rates when using the two different methods with the using synthetic aggregates (Epps et al. 1974). On the basis of same design input parameters. One can see that there are con- high porosity in synthetic aggregates, a curve showing siderable differences in the resultant rates calculated by each approximately 30% more embedment than with the Benson of the two methods. One must remember that both these meth- Gallaway curve was proposed. The rationale for this increase ods are being used by agencies that then expect experienced was that high-friction, lightweight aggregate may overturn field personnel to adjust the design rates to match the chang- and subsequently ravel under the action of traffic. ing surface conditions found in the actual project. It must also be noted that the project carried an estimated 38,000 average In a separate research effort, the Epps team (1980) con- daily traffic (Shuler 1991) and, therefore, these rates will tinued the work done in Texas by Kearby (1953) and Benson probably appear higher than expected. However, most expe- and Gallaway (1953), by undertaking a research program to rienced chip seal personnel are used to seeing rates for low- to conduct a field validation of Kearby's design method. Data moderate-volume roads. 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 ROAD NOTE 39 predicts lower asphalt rates than what was used in Texas practice, and the study proposed two changes to the design The United Kingdom's Transport Research Laboratory has procedures. The first one is a correction to the asphalt appli- published several editions of a comprehensive design proce- cation rates based on level of traffic and existing pavement dure for "surface dressing" roads in the United Kingdom condition. The second is the justification of the shift of the (Design Guide . . . 1996). The technology that makes this original design curve proposed by the Kearby and Benson design procedure so advanced is the extensive use of a com- Gallaway methods, as suggested for lightweight aggregates puter design program based on decision trees (Colwill et al. (Epps et al. 1974). 1995). Known as Road Note 39, this design procedure is

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

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

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

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107 climate, gradients, existing coarse texture, hot applications, The aggregate spread rate is extrapolated from design charts preferred aggregate matrix, and use of polymer-modified based on the ALD of the aggregate and the required texture binders are common. The approach taken by the South African depth. The South Africans have eliminated single seal design design method, TRH3, is a hybrid of the United Kingdom and without modified binder, because they do not construct any Australian design methodologies. The selection of surfacing is single-coarse seals without the use of a modified binder. made between single seal with modified binders, double seals, Another important assumption of this design method is includ- Cape seals, and sand seals. The decision is primarily based on ing correction factors to adjust binder application rates when the traffic level and pavement condition. Of particular interest using modified binders. Polymer-modified binder application is that this method measures and evaluates surface hardness by rates are adjusted, because the South Africans have found that using a ball penetration test, corrected for temperature. The aggregate orientation is different in comparison with conven- grade of binder is selected based on traffic level, road surface tional seals. The design charts shown in Figure C3 are exam- temperature, climatic region, and aggregate condition. The ples of typical TRH3 charts, and Figure C4 is a sample design required rate of binder spread is determined by using charts spreadsheet illustrating the application of the TRH3 chip seal that incorporate aggregate spread rate, traffic level, and ALD. design method. Embedment (mm) 0 0,5 1,0 1,5 2,0 0 5000 Traffic (veh/lane/day) 10000 4 15000 3 2,5 20000 25000 2 30000 35000 40000 Corrected Ball Penetration 0 - 1 mm 1,5 9mm ALD 3 Net Cold Binder (L/m2) 2.5 2 1.5 1,23 max. 1 min. 0.5 0 0.5 1 1.5 2 Embedment (mm) Minimum Text. 1mm Text. 0.7mm Text. 0.5mm FIGURE C3 Example of South African chip seal design charts.

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108 Prepared by/date: Basic Checked by/date: 19,0/6,7 mm Approved by/date: double seal design Road / Pad (Street / Straat) : N1 Carriageway / Padbaan : Northbound and Southbound Lane / Laan : Slow lane From / Van : Ch. 28 500 to / na : Ch. 62 800 Seal Type / Seltipe : 19,0/6,7mm double seal Design method / Ontwerpmetode TRH 3 1998 Binder Type / Bindmiddel Tipe : Class S-E1 modified binder (SBR type of modifier) + 30% anionic emulsion diluted 50:50 General / Algemeen Traffic Counts / Verkeerstellings slow lane fast lane Total / Totaal Heavy Vehicles per day / Swaarvoertuie per dag 680 170 850 Light Vehicles per day / Ligtevoertuie per dag (LV) 2 720 680 3 400 Equivalent LV per day / Ekwivalente LV per dag 29 920 7 480 37 400 Climatic Zone / Klimaatstreek 2/3 boundary Ave Ball Penetration / Gem. Balpenetrasie (corrected) (mm) 2,57 Texture Depth / Tekstuur Diepte (Existing) (mm) 0,40 Aggregate / Aggregaat Particulars of Aggregate / Besonderhede van Aggregaat Dark grey dolorite Source / Bron Petra quarry, Bloemfontein Type / Tipe dolorite, precoated @ 14 l/m3 with sacrasote 70 or similar approved Nominal Size / Nominale Grootte 19,0mm 6,7mm ALD / GKA (Meas. / Gemeet) (mm) 12,2mm 4,5mm Flakiness / Platheid (%) 24.0 7,1 AIV / AIW (Dry/Droog // Wet/Nat )* (%) ACV/AVW (Dry/Droog // Wet/Nat) * ( %) 11,7% dry : 16,8% wet 11,7% dry : 16,8% wet FIGURE C4 South African TRH3 chip seal design method sample. (Continued on next page.)

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109 Data 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 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 2 Nett cold binder (after adjustments): 2,03 + 0,07 + 0,10 + 0,02 - 0.10 = 2,12 l/m Control check (Alternative design Methods) PAWC: 0,172 x 13,55 = 2,33 l/m2 F.S. Concept seal 1976 : 2,24 l/m2 Spray rates 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 2 3 Layer 1 : 450 m /m as choke layer Layer 2 : + 155 m2/m3 as top layer on double seal and as single seal on sides (Fig.F-1, TRH3) FIGURE C4 (Continued).