National Academies Press: OpenBook

Chip Seal Best Practices (2005)

Chapter: Chapter Three - Chip Seal Design

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Suggested Citation:"Chapter Three - Chip Seal Design." National Academies of Sciences, Engineering, and Medicine. 2005. Chip Seal Best Practices. Washington, DC: The National Academies Press. doi: 10.17226/13814.
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Suggested Citation:"Chapter Three - Chip Seal Design." National Academies of Sciences, Engineering, and Medicine. 2005. Chip Seal Best Practices. Washington, DC: The National Academies Press. doi: 10.17226/13814.
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Suggested Citation:"Chapter Three - Chip Seal Design." National Academies of Sciences, Engineering, and Medicine. 2005. Chip Seal Best Practices. Washington, DC: The National Academies Press. doi: 10.17226/13814.
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Suggested Citation:"Chapter Three - Chip Seal Design." National Academies of Sciences, Engineering, and Medicine. 2005. Chip Seal Best Practices. Washington, DC: The National Academies Press. doi: 10.17226/13814.
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Suggested Citation:"Chapter Three - Chip Seal Design." National Academies of Sciences, Engineering, and Medicine. 2005. Chip Seal Best Practices. Washington, DC: The National Academies Press. doi: 10.17226/13814.
×
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Suggested Citation:"Chapter Three - Chip Seal Design." National Academies of Sciences, Engineering, and Medicine. 2005. Chip Seal Best Practices. Washington, DC: The National Academies Press. doi: 10.17226/13814.
×
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Suggested Citation:"Chapter Three - Chip Seal Design." National Academies of Sciences, Engineering, and Medicine. 2005. Chip Seal Best Practices. Washington, DC: The National Academies Press. doi: 10.17226/13814.
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13 INTRODUCTION Before there is any consideration of design methodology, it must be understood that the selection of those roads that will benefit from the pavement preservation technology inherent with chip sealing is the first and most fundamental step in the design process. Chip seals are not meant to enhance the structural capacity of the pavement section and therefore should not be applied to roads that exhibit severe distress (Moulthrop 2003). The formula for chip seal success is elo- quently framed by the following quotation: “Succinctly stated, the correct approach to preventive maintenance is to place the right treatment on the right road at the right time” (Galehouse et al. 2003). There are basically only two types of materials used in chip seals: binder and aggregate. Aggregate selection is a function of geography, where availability and transporta- tion distance essentially define the aggregate cost function. Aggregate selection is not only a function of seeking opti- mum gradation; it is also a function of selecting the most appropriate chip seal for the project (Moulthrop 2003). The Long-Term Pavement Program included the Specific Pave- ment Study 3 (SPS-3), which looked specifically at the tim- ing of pavement maintenance actions. It found that roads that were in poor condition (i.e., exhibited high levels of distress) when a chip seal was applied had a probability of failure that was two to four times greater than those that were in good condition. It also found that “chip seals appear to outperform the other treatments . . . in delaying the reappearance of dis- tress” (Eltahan et al. 1999). The binder selection process is a function of the pave- ment’s surface, size and gradation of aggregate, compatibil- ity with local aggregate, and local climatic considerations (Gransberg et al. 1998). One of the major difficulties in the design of material application rates is the nonuniformity of the existing pavement surface. The engineer must remember that variation in the existing pavement occurs both in the transverse and longitudinal directions. The transverse varia- tion is usually defined as the difference in the surface texture on the wheelpaths and outside and between the wheelpaths, including rutting. Longitudinal variation occurs as the surface condition varies along the road from areas where the under- lying surface is oxidized to other areas where the surface may be smooth or bleeding. Particular attention should be given when determining binder application rates on pavements dis- playing varying surface textures. Such conditions necessitate alterations to the binder application rate as the underlying sur- face changes, making the specification of a single material application rate impossible. As a result, careful characterizing of the existing surface throughout the length of the chip seal project is vital to producing a successful end product. CHIP SEAL PROGRAMMING At the heart of a successful chip seal program is commitment to selecting the most appropriate PM treatment for the situa- tion. PM programming that identifies the optimum timing of a chip seal cycle will maximize the economic benefits (Weg- man 1991). Figure 7 is a flow chart showing the chip seal pro- gram cycle. Chip seals will enhance pavement condition and extend pavement service life when applied on pavements showing minimal distress (Moulthrop 2003). Again, chip seals are not expected to improve structural capacity to the pavement. However, it appears to be common practice to apply chip seals to pavements that have structural distresses as a stop- gap measure. Survey respondents indicated that determining when to use a chip seal could result from a combination of factors, ranging from formula-driven algorithms to birthday sealing or visual evaluation of the pavement surface. Some agencies rely on their internal pavement management system data as the trigger for deciding when to place a chip seal. By identifying the triggers that initiate selection of a chip seal, the survey responses identified a difference of philoso- phies in chip seal use between the North American and the international respondents, as shown in Figure 8. In North America, the most common conditions that would trigger a chip seal are evidence of distress and prevention of water infiltration. The international respondents identified the loss of skid resistance and the need to provide a wearing surface as major reasons for chip sealing. CHIP SEAL DESIGN METHODS Chip seal design methods largely fall into two fundamental categories: empirical design based on past experience and design based on some form of engineering algorithm. A large body of research is available on formal chip seal design practices. A contemporary chip seal design process involves the determination of grade, type, and application rate for a bituminous binder when given the aggregate size and type, CHAPTER THREE CHIP SEAL DESIGN

14 methodologies in practice today. The literature review and survey results revealed the use of two generally accepted chip seal design methods in use in North America: the Kearby method and the McLeod method. Although a few North American agencies have also developed their own formal design procedures that are not based on either method, most use either an empirical design method or no formal method at all. Overseas, there are four additional chip seal design meth- ods in use. The United Kingdom’s Transport Research Labo- ratory (1996) has published several editions of a comprehen- sive design procedure for chip seals (called “surface dressing” in the United Kingdom). Commonly known as Road Note 39, this design method is based on a computer software program. A variation of Road Note 39, Road Note 3, has been developed for surface dressing design in tropical regions. Australia, New Zealand, and South Africa have also developed engineering- based chip seal design methods for use in their respective countries. Australia’s is called Austroads Provisional Sprayed Seal Design Method (2001). New Zealand uses this method with its own regional variation, and South Africa’s method is called TRH3 (i.e., Technical Recommendations for Highways, Surfacing Seals for Rural and Urban Roads). The primary formal design methodologies in practice today in North America and overseas are reviewed and analyzed in Appen- dix C. Table 1 shows the percentage of North American respondents using the various design methods. Past Experience in Empirical Methods The very early practitioners of surface treatments or seal coats appear to have used a purely empirical approach to surface condition of existing pavement, traffic volume, and actual type of chip seal being used. Figure 9 illustrates the proportion of agencies that formally design their chip seal application rates before construction. The earliest recorded effort at developing a design proce- dure for chip seals was made by Hanson (1934/35). Traces of Hanson’s design can be found in all major chip seal design Design and Preparation of Plans Call for Bids and Award of Contract Identify Potential Chip Seal Projects Planning and Selection of Chip Seal Projects Pre-construction Activity • Patching • Crack Sealing Equipment Inspection • Calibration • Production Chip Seal Operation Performance Monitoring Post-Contract Evaluation Feedback FIGURE 7 Chip seal program cycle [after Senadheera and Khan (2001)]. 10 9 5 2 9 1 2 2 0 2 4 6 8 10 12 Distress Water Infiltration Oxidation Skid Resistance Wearing Surface AU, NZ, UK, SA North America FIGURE 8 Reasons for chip sealing. 38 10 5 6 8 0 0 5 10 15 20 25 30 35 40 US Canada AU, NZ, UK, SA Yes No FIGURE 9 Proportion of agencies formally designing chip seal application rates. Chip Seal Design Method United States (%) Canada (%) Kearby/Modified Kearby 7 0 McLeod/Asphalt Institute 11 45 Empirical/past experience 37 33 Own formal method 19 0 No formal method 26 22 TABLE 1 CHIP SEAL DESIGN METHODS IN NORTH AMERICA

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

Visual Texture Analysis Visual assessment of the existing pavement surface can also be used in determining binder application rates. Surface characterization using visual assessment is quite subjective, because surface characterization terminology is not consistent within agencies, let alone between them. Despite that issue, visual correction factors are essential correction factors for both the Kearby and McLeod design methods. Table 2 dis- plays a range of correction factors developed for the Kearby method, the foundation of which has become known as the modified Kearby method. Table 3 provides a similar range of correction factors developed for the McLeod method. Evaluate Traffic Conditions The traffic volume on the pavement surface, in regard to 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 essential for determining the appropriate binder design rate. When traffic is used as a chip seal design criterion, the per- centage of heavy vehicles should be considered. This may be done by calculating ADT and then using an adjustment fac- tor for the heavy vehicles. Typically, higher traffic volumes reduce binder application rates (Seal Coat and Surface Treat- ment Manual 2003). This is because the heavy traffic will continue to embed that aggregate into the underlying surface after the road is opened to traffic. Additionally, areas where there are substantial starting, stopping, and turning move- 16 ments also deserve special consideration. These movements all exert forces on the aggregate that cause it to roll, chang- ing its position in the binder and often exposing the previ- ously embedded surface that is covered in asphalt. This con- dition reduces the road’s skid resistance and makes it prone to bleeding. Therefore, specifying a different type of chip seal such as the racked-in seal (discussed later in this chap- ter) may be in order. Evaluate Climatic and Seasonal Characteristics As previously stated, emulsions are thought to be more appropriate than asphalt cements during cool weather con- struction when ambient temperatures are low, and in areas where the aggregate may be damp (Griffith and Hunt 2000). Thus, the designer must select a binder whose inherent char- acteristics match the environment in which the chip seal will be placed. The existence of high pavement surface tempera- tures would indicate the use of a hot asphalt cement binder. The length of daily window in which traffic control can be employed could influence the designer to select a chip seal design that can allow the road to be opened to traffic as quickly as possible. Locations where there are a large num- ber of turning movements could cause the designer to spec- 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- tion is permitted. Finally, the need to apply all types of chip seals in the warmest, driest weather possible using dry aggre- gates cannot be overemphasized. Evaluate and Select Type of Seal Essential to the design methodologies of Australia, South Africa, and the United Kingdom is a contention that differ- ent types of seals require different design methodologies. Critical differences based on the construction sequence, number of courses sealed, and variations in aggregate nomi- nal size generally distinguish between the different types of chip seals. The basic divergence with double chip seal design is that the total design binder application rates are less than for a conventional single-course chip seal (McLeod 1969). Surface Texture Asphalt Application Rate Correction [gal/yd2 (L/m2)] Flushed asphalt surface – 0.06 (– 0.27) Smooth, nonporous surface –0.03 (– 0.1 4) Slightly porous, slightly oxidized surface 0.00 (0.00) Slightly pocked, porous, oxidized surface +0.03 (+0.14) Badly pocked, porous, oxidized surface +0.06 (+0.27) Source: Epps et al. 1980. TABLE 2 CORRECTION FACTOR FOR EXISTING SURFACE CONDITION Surface Texture Asphalt Application Rate Correction [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. TABLE 3 CORRECTION FACTOR FOR EXISTING SURFACE CONDITION

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

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

19 Determine Binder Application Rate The previously outlined designed methodologies all deter- mine a basic binder application rate that typically depends on the average least dimension (ALD) of the aggregate and type of chip seal being used. Intuitively, larger-sized aggregates require additional binder to achieve the optimum embedment. There are different schools of thought with regard to embed- ment. One approach is to seek to achieve approximately 50% embedment after rolling and thus leave room for traffic to fin- ish the process by further embedding the aggregate after the newly chip sealed road is opened. This approach strives to avoid bleeding in the wheelpaths by leaving room for the additional embedment during the chip seal’s service life. The major disadvantage of this approach is that it leaves the aggre- gate that is not on the wheelpaths prone to being dislodged by traffic movements across the lane’s width. The other school of thought is to achieve an embedment of up to 70% during construction across the entire road width. This approach will adjust the binder application rate based on the measured or perceived surface hardness and account for hardness in the design. The latter school of thought means being on guard against aggregate loss, and it may mean leaving the road in a condition in which it is prone to bleeding if the design calcula- tions do not exactly match the existing surface. The design binder application rate is calculated after con- sidering a number of correction features or allowances to the basic binder application rate. Typical adjustments are based on traffic characteristics, surface texture, aggregate absorp- tion characteristics, and surface hardness. Typically, binder application rates are reduced where large traffic volumes are expected to considerably reorient and embed the aggregate after final rolling. The binder application rate may also be adjusted depending on the existing surface texture. It is nec- essary to increase the application rate on pocked, porous, or oxidized surfaces, because such textures will absorb more binder. In contrast, it is necessary to decrease the binder appli- cation rate on surfaces that exhibit susceptibility to bleeding. Surface hardness, as measured by the ball penetration test or a penetrometer, characterizes the likely depth of aggregate embedment into the underlying pavement. CHIP SEAL DESIGN CONCLUSIONS AND BEST PRACTICES Unquestionably, all of the design methods can effectively guide inexperienced personnel through the process of chip seal design. The following best practices can be drawn from a comparison of the chip seal design methodologies. To begin, the selection of the binder is a very important decision and should be made after considering all the factors under which the chip seal is expected to perform. After all, the pri- mary purpose of a chip seal is to prevent water intrusion into the underlying pavement structure, and the asphalt layer formed by the binder is the mechanism that performs this vital function. The previously explained design methods are all based on the assumption that single-course chip seal design requires the use of uniformly graded aggregate spread one stone thick in a uniform manner. The application rates of all methods appear to be based on residual binder, and each method has a proce- dure for dealing with adjustments owing to factoring the loss of binder to absorption by the underlying pavement surface and the aggregate being used. Contemporary design practices need to determine binder application rates based on surface characterization, absorption factors, traffic conditions, climate considerations, aggregate selection, and type of chip seal being constructed. Another important discovery is that all methods have a design objective for embedment to be between 50% and 70% of the seal’s depth. A detailed discussion of formal design methods is contained in Appendix C. Best practices for chip seal design are difficult to isolate, because there appears to be such a large variation in practices from agency to agency. However, the following can be iden- tified as meeting this project’s definition for best practices: • Chip seals perform best only on roads with low under- lying surface distress that will benefit from this tech- nology. • The international practice is to characterize the under- lying road’s texture and surface hardness and use that as a basis for developing the subsequent formal chip seal design. U.S. and Canadian agencies obviously recognize the need to factor in the underlying surface into the design, as shown in Figure 10, where the majority of North American responses indicated a rou- tine use of qualitative characterization in the design process. Thus, the next logical enhancement would be to incorporate international methods to quantitatively characterize the underlying surface in the chip seal design process. • One of those enhancements would be to try using the racked-in seal as the corrective measure for bleeding instead of the North American practice of spreading fine aggregate and sand on the bleeding surface.

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 342: Chip Seal Best Practices examines ways to assist in the development and implementation of pavement preservation programs by identifying the benefits of using chip seal as part of a preventive maintenance program and by highlighting advanced chip seal programs in use around the world. The report includes approximately 40 best practices in the areas of chip seal design methods, contract administration, equipment practices, construction practices, and performance measures. According to the report, the increased use of chip seals for maintenance can be a successful, cost-effective way of using preventive maintenance to preserve both low-volume and higher-volume pavements.

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