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15 their design. Chip sealing a pavement was considered then, surface conditions on the surfaces they are planning to chip as it is now in many circles, an art. Experience-based design seal. None of the North American agencies quantitatively is performed by starting with a base rate for the binder and characterize surface texture, whereas 75% of the inter- aggregate determined after years of experience in the field. national respondents characterized surface texture by using The main reason for this approach is the variable nature of the sand patch method. Also of importance is that all of the existing surfaces. Factors such as transverse and longitudinal non-North American respondents characterize surface hard- texture differences affect the ultimate performance of a given ness during the design of their chip seals. The significance of chip seal and are independent of the design parameters, thus characterizing surface hardness is that the chip seal's aggre- creating a controversy as to whether a formal design proce- gate can be selected based on its expected embedment depth dure is really an exercise in pointless computation. Agencies into the underlying pavement. that predominantly use empirical methods are basing their design on the assumption that the chip seal contract merely Characterization of the pavement's surface texture is a specifies a base rate for binder and aggregate. Therefore, the critical step in the design process because nonuniform sur- design is used primarily to estimate the quantities of each to face textures in both the transverse and longitudinal direc- be used during the bidding phase. tions make it difficult to design a binder application rate. In Australia and New Zealand, it is a priority to perform corrective measures to restore the pavement's surface CHIP SEAL DESIGN PRACTICES before a chip seal application. It is a common practice to treat flushing surfaces with a high-pressure water treat- To accomplish the chip seal design in accordance with the ment to remove the excess binder and obtain a sufficient formal methods, the engineer must first determine the input and uniform texture depth. Another technique for correct- characteristics for project design. These characteristics basi- ing surface texture, known as prespraying, involves the cally involve the following stages of design: application of binder to select portions of the traffic lane and shoulders, while making sure not to apply any binder Evaluate surface texture; to the wheelpaths. A number of North American agencies Evaluate traffic conditions: volume, speed, percentage indicated that they require the use of variable spray noz- of trucks, etc.; zles on the asphalt distributor to account for the transverse Evaluate climatic and seasonal characteristics; texture differential. Evaluate and select type of chip seal; Evaluate aggregate selection; Determine binder application rate; and Sand Patch Method Determine how many hours per day are available for construction operations. A suitable test procedure for determining the texture depth is the sand patch method, also known as the sand circle test. This method is a procedure for determining pavement Evaluate Surface Texture surface macrotexture through the spreading of a prede- termined volume of sand or glass bead material on the Surface texture refers to the surface properties of the pave- pavement surface of a given area (ASTM E965). Ensuing ment surface (Sprayed Sealing Guide 2004). It is a measure- calculations of the volume of material that fills the surface ment that influences the nominal size of aggregate used for voids determine the surface texture. The principle of this the chip seal and thus ultimately determines material appli- method is fairly straightforward; the greater the texture cation rates, skid resistance, and road noise. Figure 10 illus- depth, the greater the quantity of material lost in the sur- trates how the survey respondents typically characterize the face voids. 7 Sand Patch 7 Visual 5 8 AU, NZ, UK, SA 3 Qualitative Factors 5 Canada 15 United States 1 Level of Oxidation 2 11 7 Surface Hardness 0 5 10 15 20 FIGURE 10 Typical surface characterization methods.

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16 TABLE 2 ments also deserve special consideration. These movements CORRECTION FACTOR FOR EXISTING SURFACE CONDITION all exert forces on the aggregate that cause it to roll, chang- Asphalt Application Rate ing its position in the binder and often exposing the previ- Correction ously embedded surface that is covered in asphalt. This con- Surface Texture [gal/yd2 (L/m2)] dition reduces the road's skid resistance and makes it prone Flushed asphalt surface 0.06 ( 0.27) to bleeding. Therefore, specifying a different type of chip Smooth, nonporous surface 0.03 ( 0.14) seal such as the racked-in seal (discussed later in this chap- Slightly porous, slightly oxidized surface 0.00 (0.00) ter) may be in order. Slightly pocked, porous, oxidized surface +0.03 (+0.14) Badly pocked, porous, oxidized surface +0.06 (+0.27) Source: Epps et al. 1980. Evaluate Climatic and Seasonal Characteristics As previously stated, emulsions are thought to be more Visual Texture Analysis appropriate than asphalt cements during cool weather con- struction when ambient temperatures are low, and in areas Visual assessment of the existing pavement surface can also where the aggregate may be damp (Griffith and Hunt 2000). be used in determining binder application rates. Surface Thus, the designer must select a binder whose inherent char- characterization using visual assessment is quite subjective, acteristics match the environment in which the chip seal will because surface characterization terminology is not consistent be placed. The existence of high pavement surface tempera- within agencies, let alone between them. Despite that issue, tures would indicate the use of a hot asphalt cement binder. visual correction factors are essential correction factors for The length of daily window in which traffic control can be both the Kearby and McLeod design methods. Table 2 dis- employed could influence the designer to select a chip seal plays a range of correction factors developed for the Kearby design that can allow the road to be opened to traffic as method, the foundation of which has become known as the quickly as possible. Locations where there are a large num- modified Kearby method. Table 3 provides a similar range of ber of turning movements could cause the designer to spec- correction factors developed for the McLeod method. ify racked-in chip seals to protect the aggregate from rolling and bleeding. The designer must also specify the temperature ranges and weather conditions in which chip seal construc- Evaluate Traffic Conditions tion is permitted. Finally, the need to apply all types of chip seals in the warmest, driest weather possible using dry aggre- The traffic volume on the pavement surface, in regard to gates cannot be overemphasized. ADT, plays a role in determining the amount of binder needed to sufficiently embed the chips. Having a fundamen- tal knowledge of local traffic volumes and considerations is Evaluate and Select Type of Seal essential for determining the appropriate binder design rate. When traffic is used as a chip seal design criterion, the per- Essential to the design methodologies of Australia, South centage of heavy vehicles should be considered. This may be Africa, and the United Kingdom is a contention that differ- done by calculating ADT and then using an adjustment fac- ent types of seals require different design methodologies. tor for the heavy vehicles. Typically, higher traffic volumes Critical differences based on the construction sequence, reduce binder application rates (Seal Coat and Surface Treat- number of courses sealed, and variations in aggregate nomi- ment Manual 2003). This is because the heavy traffic will nal size generally distinguish between the different types of continue to embed that aggregate into the underlying surface chip seals. The basic divergence with double chip seal design after the road is opened to traffic. Additionally, areas where is that the total design binder application rates are less than there are substantial starting, stopping, and turning move- for a conventional single-course chip seal (McLeod 1969). TABLE 3 CORRECTION FACTOR FOR EXISTING SURFACE CONDITION Asphalt Application Rate Correction Surface Texture [gal/yd2 (L/m2)] Black, flushed asphalt 0.01 to 0.06 (0.04 to 0.27) Smooth, nonporous 0.00 (0.00) Absorbent--slightly porous, oxidized +0.03 (+0.14) Absorbent--slightly pocked, porous, oxidized +0.06 (+0.27) Absorbent--badly pocked, porous, oxidized +0.09 (+0.40) Source: Asphalt Surface Treatments--Construction Techniques 1988.

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17 Single Chip Seal Racked-in Seal A single-course chip seal is the most common type of chip A racked-in seal is a special seal in which a single-course seal. It is constructed from a single application of binder chip seal is temporarily protected from damage through the followed by a single application of uniformly graded aggre- application of choke stone that becomes locked in the voids gate, as shown in Figure 11. These seals are selected for of the seal. The choke stone provides an interlock between normal situations where no special considerations would the aggregate particles of the chip seal (see Figure 13). The indicate that a special type of chip seal is warranted. It choke stone is used to prevent aggregate particles from dis- should be noted that the following figures are conceptual lodging before the binder is fully cured. These chip seals are diagrams and that other variations on these designs are used in order in areas where there are large numbers of turning in the field. movements to lock in the larger pieces of aggregate with the smaller aggregate and prevent the aggregate from being dis- lodged before the seal is fully cured. Double Chip Seal A double chip seal is constructed with two consecutive appli- Cape Seal cations of both the bituminous binder and the uniformly graded aggregate, as shown in Figure 12. The aggregate in the Cape seals, named after the area in South Africa where they second application is typically about half the nominal size of were invented, are basically a single chip seal followed by a the first application. Double chip seals have less noise from slurry seal (see Figure 14). The original South African tech- traffic, provide additional waterproofing, and are a more nique was to use a larger than normal base stone (up to 3/4 in.). robust seal in comparison with a single chip seal (Sprayed However, their application in North America and other coun- Sealing Guide 2004). Therefore, double chip seals are used tries revolves around the use of a smaller nominal-sized aggre- in high-stress situations, such as areas that have a high per- gate. Cape seals are very robust and provide a shear resistance centage of truck traffic or on steep grades. comparable to that of asphalt (Sprayed Sealing Guide 2004). FIGURE 11 Single chip seal. FIGURE 12 Double chip seal. FIGURE 13 Racked-in seal. FIGURE 14 Cape seal.

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18 Inverted Seal treatment is intended to be literally one stone thick. Most agencies use a nominal size that ranges from 3/8 in. (9.5 mm) Figure 15 shows how an inverted seal is constructed. It is to 1/2 in. (12.7 mm). As the nominal aggregate size increases, called an inverted seal because the larger-sized aggregate the surface texture becomes coarser, with a resultant increase goes on top of the smaller-sized aggregate and is therefore an in road noise and ride roughness. Additionally, the potential inverted double seal. These seals are commonly used to for windshield damage owing to dislodged and projected repair or correct an existing surface that is bleeding. The pieces of aggregate increases as the size of the aggregate Australians have successfully used these seals on bleeding increases. The Montana DOT (MDT) Maintenance Chip surfaces with 30,000 ADT. Also, the seals are used for restor- Seal Manual (2000) provides a comprehensive discussion on ing uniformity to surfaces with variation in transverse sur- desirable aggregate characteristics. It states that the charac- face texture (Sprayed Sealing Guide 2004). teristics of a "good aggregate" are as follows: Sandwich Seal Maximum particle size--gradation shows 3/8 in. maxi- mum; The sandwich seal, as shown in Figure 16, is a chip sealing Overall gradation--one-size, uniformly graded; technique that involves one binder application sandwiched Particle shape--cubical or pyramidal and angular (one between two separate aggregate applications. Sandwich seals fractured face of 70%); are particularly useful for restoring surface texture on raveled Cleanliness--less than 2% passing the No. 200 sieve; surfaces. and Toughness to abrasion--abrasion not to exceed 30%. Geotextile-Reinforced Seal The final aggregate design consideration has to do with the type of stone that will be used to produce the chip seal aggre- Reinforcing a chip seal with geotextile products can enhance gate. Both natural stone and synthetic aggregates are available the performance of a conventional chip seal over extremely and will be discussed in detail in chapter five. It suffices to say oxidized or thermal cracked surfaces. The geotextile is care- at this point that the cost of transporting acceptable aggregates fully rolled over a tack coat, followed by a single chip seal often limits the chip seal designer's options. However, as the being placed on top, as shown in Figure 17. aggregate essentially protects the binder that is forming the barrier to water intrusion, the designer should use life-cycle Evaluate Aggregate Selection cost analysis rather than simple comparative pricing to de- termine if a high-quality aggregate is economically viable The selection of the specific aggregate essentially establishes (Maintenance Chip Seal Manual 2000). Once the aggregate the thickness of the chip seal, because this type of surface is selected, the designer can move on to designing the binder. FIGURE 15 Inverted seal. FIGURE 16 Sandwich seal (dry matting). FIGURE 17 Geotextile-reinforced seal.