National Academies Press: OpenBook

Evaluation of Bonded Concrete Overlays on Asphalt Pavements (2022)

Chapter: Chapter 3 - State of the Practice

« Previous: Chapter 2 - Literature Review
Page 22
Suggested Citation:"Chapter 3 - State of the Practice." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluation of Bonded Concrete Overlays on Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26760.
×
Page 22
Page 23
Suggested Citation:"Chapter 3 - State of the Practice." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluation of Bonded Concrete Overlays on Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26760.
×
Page 23
Page 24
Suggested Citation:"Chapter 3 - State of the Practice." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluation of Bonded Concrete Overlays on Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26760.
×
Page 24
Page 25
Suggested Citation:"Chapter 3 - State of the Practice." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluation of Bonded Concrete Overlays on Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26760.
×
Page 25
Page 26
Suggested Citation:"Chapter 3 - State of the Practice." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluation of Bonded Concrete Overlays on Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26760.
×
Page 26
Page 27
Suggested Citation:"Chapter 3 - State of the Practice." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluation of Bonded Concrete Overlays on Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26760.
×
Page 27
Page 28
Suggested Citation:"Chapter 3 - State of the Practice." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluation of Bonded Concrete Overlays on Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26760.
×
Page 28
Page 29
Suggested Citation:"Chapter 3 - State of the Practice." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluation of Bonded Concrete Overlays on Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26760.
×
Page 29
Page 30
Suggested Citation:"Chapter 3 - State of the Practice." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluation of Bonded Concrete Overlays on Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26760.
×
Page 30
Page 31
Suggested Citation:"Chapter 3 - State of the Practice." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluation of Bonded Concrete Overlays on Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26760.
×
Page 31
Page 32
Suggested Citation:"Chapter 3 - State of the Practice." National Academies of Sciences, Engineering, and Medicine. 2022. Evaluation of Bonded Concrete Overlays on Asphalt Pavements. Washington, DC: The National Academies Press. doi: 10.17226/26760.
×
Page 32

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.

22 To supplement the literature review, a survey was developed and distributed online to 28 SHAs identified as having constructed at least one BCOA project. The survey objectives included determining the level of BCOA implementation and obtaining relevant information regarding the design, materials, construction, maintenance and rehabilitation, and performance of BCOAs. The survey (see Appendix A) consisted of 12 mostly closed-ended questions covering the following BCOA topics: • Experience and use, • Project selection and evaluation, • Design, • Materials, • Construction, • Maintenance and repair techniques, • Availability of performance data, and • Willingness to participate in Phase II of the study. Figure 8 illustrates the locations of the 27 responding agencies. Experience and Use Agency experience with and use of BCOA pavements is summarized in Table 8. Before the 1990s, seven SHAs indicated constructing BCOA projects. By the 1990s, more than 75% of the responding agencies indicated constructing at least one BCOA project per decade. Figure 9 illustrates the 17 agencies that indicated considering BCOA projects from 2018 to 2020. Project Selection and Evaluation Table 9 and Figure 10 summarize the agencies’ response to considerations for BCOA use. A high percentage of the respondents considered using BCOAs on U.S. and state highways and on facilities with low to moderate traffic volumes. In contrast, fewer respondents considered using them on interstate routes or on high-traffic facilities. BCOAs are more likely to be con- sidered for use in rural areas than in urban areas, and their most common applications are on mainline roads and at intersections. All responding agencies relied on pavement cores to assess the existing asphalt-surfaced pavement, and a high percentage of agencies used the results of distress surveys, FWD testing, and backcalculation. About one-quarter of the agencies used dynamic cone penetrometer (DCP) and laboratory testing results to evaluate subsurface material layers. Two agencies indicated conducting GPR testing. C H A P T E R 3 State of the Practice

State of the Practice 23   WA OR CA MT ID NV AZ UT WY CO NM TX OK KS NE SD ND MN IA MO AR LA MS AL GA FL SC TN NC IL WI MI OH IN KY WV VA PA NY ME VT NH NJ DE MD MA CT RI State Respondents Figure 8. SHAs responding to survey on BCOA project construction. How many BCOA projects has your agency constructed in the past, by decade? Decade Number of Projects Total Agencies 0 <5 5–15 >15 1960–1970 14 1 0 0 15 1970–1980 12 3 0 0 15 1980–1990 12 3 0 0 15 1990–2000 5 14 1 1 21 2000–2010 6 9 7 1 23 2010–present 4 14 5 1 24 Table 8. Number of responding SHAs reporting BCOA projects (N = 27).

24 Evaluation of Bonded Concrete Overlays on Asphalt Pavements WA OR CA MT ID NV AZ UT WY CO NM TX OK KS NE SD ND MN IA MO AR LA MS AL GA FL SC TN NC IL WI MI OH IN KY WV VA PA NY ME VT NH NJ DE MD MA CT RI State Respondents Figure 9. SHAs considering construction of BCOA projects during 2018–2020. In what capacities are BCOAs primarily considered for use in your agency? Consideration Responses Total Agencies Roadway facility type Interstate routes 8 U.S. routes 19 State routes 22 County, city, or other local roads 14 Traffic volume, average annual daily traffic Low: <5,000 16 Moderate: 5,000–20,000 22 High: >20,000 9 Environmental setting Rural 26 Urban 17 Application type Mainline 22 Intersection 18 Turning lane 10 Interchange ramp 6 Frontage road 2 Underpass 1 Parking lot/truck climbing lane 1/1 Table 9. Number of responding SHAs considering BCOA use (N = 27).

State of the Practice 25   Design Figure 11 illustrates that the AASHTO Design Guide (1993) and BCOA-ME are the most commonly used structural design methods. With the incorporation of the short-jointed plain concrete pavement module into AASHTOWare® Pavement ME Design software (PaveME), this method has also seen wide use. The most common design configuration (cited by 17 respondents) was a 6-in. thick slab with a 6- × 6-ft slab size. Other frequently used configurations included a 5-in. thick slab and 6- × 6-ft slab size and a 4-in. thick slab and 4- × 4-ft slab size (Table 10). Few agencies reported using other sizes (Table 11). Agencies hesitated to use BCOAs with thicknesses less than 3 in. because of potential structural issues and were somewhat hesitant to use 3-ft wide slabs, which tend to place longitudinal joints in the wheelpaths (in accordance with the literature review). 2 4 7 9 19 24 27 0 5 10 15 20 25 30 GPR Testing Lab Testing of Existing AC DCP Testing Lab Testing of Unbound Layers FWD and Backcalculation Distress Surveys Cores NUMBER OF AGENCIES Figure 10. BCOA evaluation techniques (N = 27). 1 1 2 2 2 4 5 7 8 0 2 4 6 8 10 Illinois DOT Texas DOT Colorado DOT ACPA StreetPave None ACPA BCOA AASHTO PaveME BCOA-ME AASHTO 1993 NUMBER OF AGENCIES Figure 11. BCOA design methods (N = 26).

26 Evaluation of Bonded Concrete Overlays on Asphalt Pavements Materials Table 12 provides insight into the materials used in BCOA construction. As can be seen, almost half the respondents reported having a specialized paving mixture. Of the 13 with a specialized mixture, 11 used fibers in their mixtures, with the most common fiber type being polypropylene. Moreover, eight respondents used rapid early-strength mixtures for early-opening- to-traffic situations. Construction Figure 12 illustrates responding agencies’ methods of preoverlay repair and surface prepa- ration work. In addition to patching potholes in the existing asphalt surface layer, 10 of the responding agencies perform partial- and full-depth patching, and 7 conduct crack sealing. Twenty-four agencies indicated milling the existing asphalt surface to prepare it for the BCOA (Figure 13). Most agencies clean the surface using power brooms, air blasting, or other equip- ment. Four agencies reported performing no surface preparation before placement of the BCOA. What BCOA thickness and slab size does your agency use (or has used)? BCOA Thickness (in.) Slab Dimensions Total Agencies3- x 3-ft 4- x 4-ft 6- x 6-ft 2 2 1 0 3 3 6 2 1 9 4 3 13 5 21 5 0 2 10 12 6 0 2 17 19 7 0 0 2 2 Table 10. Common BCOA design thicknesses and slab sizes (N = 24). BCOA Thick- ness (in.) Slab Sizes 5.5- x 5.5-ft 5.5- x 6.0-ft 4- x 4-ft to 8- x 8-ft 10- x 10-ft 11- x 11-ft 12- x 12-ft 12- x 10-ft 12- x 15-ft 14- x 15-ft 4.5 0 1 0 0 0 0 0 0 0 5.0 1 0 1 0 1 0 0 0 0 6.0 1 0 1 1 1 4 1 2 0 7.0 0 0 0 0 0 0 0 1 1 8.0 0 0 0 0 0 0 0 2 0 Table 11. Other BCOA design thicknesses and slab sizes (N = 6). Does your agency utilize a specialized paving mixture for BCOA applications? Specialized Paving Mixture Yes No Total Agencies Specialized paving mixture used for BCOA applications? 13 14 27 Fibers used in BCOA concrete mixtures? If yes, what types of fibers are used? Polypropylene/synthetic macrofibers/macro polyolefin Structural/microfibers (monofilament)/unspecified 11 (5/1/1) (1/1/2) 2 13 Early-opening-to-traffic BCOA concrete mixture available? 8 5 13 Specific BCOA concrete mixture requirement for aggregate? 5 7 12 Specific BCOA concrete mixture requirement for cementitious materials? 2 9 11 Specific BCOA concrete mixture requirement for chemical admixtures? 1 10 11 Table 12. SHAs using specialized BCOA paving mixtures (N = 27).

State of the Practice 27   Maintenance and Repair Techniques Table 13 summarizes the maintenance and repair techniques used to address BCOA distress. Corner breaks and slab cracking are frequently mitigated through slab replacements and crack sealing, joint spalling is often addressed through spall repairs, and joint and crack faulting is often treated through diamond grinding. Also, slab debonding or delamination issues are usually addressed by full slab replacement. For agencies that conduct joint sealing, deficient or damaged joint seals are generally repaired by resealing. Availability of Performance Data Thirteen agencies reported having detailed condition data on their BCOA projects (Table 14). This proportion might have been expected to be higher, but some projects were likely constructed and subsequently overlaid or replaced many years ago, making any collected performance data difficult to locate. Among the responding agencies, most indicated that data are available for 1 6 7 10 10 17 0 5 10 15 20 25 30 New AC Layer None Crack Filling or Sealing Partial-Depth Patching Full-Depth Patching Pothole Patching NUMBER OF AGENCIES Figure 12. Preoverlay repair methods (N = 27). 1 2 4 8 18 24 0 5 10 15 20 25 30 Vacuum Truck Water Blasting None Air Blasting Power Sweeping Milling NUMBER OF AGENCIES Figure 13. Surface preparation methods (N = 26).

28 Evaluation of Bonded Concrete Overlays on Asphalt Pavements smoothness, joint and crack faulting, and transverse and longitudinal cracking. Eight agencies reported having an overall condition indicator, and eight conducted FWD testing for each BCOA project. Willingness to Participate in Phase II To close out the survey, participants were asked about their willingness to provide BCOA documentation and actively participate in Phase II of this study. As shown in Figure 14, 19 agencies indicated a willingness to support the evaluation and testing of selected in-service BCOA projects by providing more detailed information and access and traffic control. Site Selection for Phase II The 19 agencies willing to participate in Phase II of the study were asked to provide the follow- ing for three to five BCOA projects: • Project selection: – BCOA projects with “typical” performance. – Mainline projects, tangent sections (ideally with BCOA intersections). • Parameters for each project: – Location: route, direction, milepost limits, and nearest town or cross street. – In-service age: 5 or more years preferred. – Design elements: thickness, slab size, special mixture, synthetic macrofibers. – Features: sawcut width, with or without tie and dowel bars, with or without joint sealing. – Traffic: AADT < 5,000; AADT 5,000–20,000; and AADT > 20,000. Indicate the maintenance/repair techniques used to address BCOA distress (no. of agencies). Distress Type Crack Seal Joint Reseal Spall Repair Partial Slab Replacement Full Slab Replacement Diamond Grind Corner breaks 5 1 1 9 7 1 Transverse cracks 9 1 1 3 7 0 Longitudinal cracks 9 0 1 2 9 0 Transverse spallinga 2 5 12 2 3 0 Longitudinal spallinga 2 4 10 1 4 0 Faultinga 1 2 0 3 4 11 Debonding 0 0 0 2 12 0 Joint seal damage 2 6 0 0 0 0 a Joints and cracks. Table 13. BCOA maintenance and repair techniques (N = 19). What types of condition/performance data are collected and available for analysis? Condition and Performance Data Total Agencies Smoothness/IRI 12 Joint and crack faulting 12 Longitudinal cracking 12 Transverse cracking 11 Overall condition indicator 8 FWD deflection data 8 Corner breaks 6 Joint spalling 5 Shattered slabs 4 Backcalculated layer moduli 3 Concrete–asphalt bond quality measurements 2 Other data: GPR/ride/wear 1/1/2 Table 14. Types of condition and performance data available (N = 26).

State of the Practice 29   In total, the research team received information on 52 BCOA projects from 14 SHAs (Arkansas, Colorado, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Maryland, Minnesota, Missouri, Montana, Ohio, and Pennsylvania). Projects shorter than 0.5 mi and with less than 5 years of service were removed from con- sideration. The research team’s initial selection criteria were based on a length adequate for randomly identifying and testing up to three 0.1-mi segments. Further, BCOA pavements with short in-service lives would likely not provide the needed “end of life” performance for evaluating existing or developing prediction equations. After these initial criteria were applied, 29 BCOA projects remained. Features of the 29 projects are summarized in Table 15. The majority of BCOA projects were constructed in the North Central and Western regions (14 and 12 projects, respectively). Slightly more than half the projects (16 projects) included joint sealing, and most (24 projects) did not use synthetic macrofibers. The majority of projects (20 projects) included 6- × 6-ft slabs ranging in BCOA design layer thickness from 4.8 to 6.0 in. To evaluate a range of project features (slab size, AADT, joint sealing, and synthetic macro- fibers), the research team reviewed the 29 projects on the basis of site knowledge and informa- tion obtained from responding agencies. The research team then arranged projects by region and AADT category, and selected projects to represent a range of slab sizes, as well as projects with and without joint sealing and synthetic macrofibers. Ideally, recommended projects would represent all regions, all AADT categories, all slab sizes, with and without joint sealant, and with and without synthetic macrofibers; however, this would require more than 140 projects. Therefore, the selected projects best represent the range of BCOA conditions in agencies that responded to the research team’s request for information. Twenty projects were selected for WA OR CA MT ID NV AZ UT WY CO NM TX OK KS NE SD ND MN IA MO AR LA MS AL GA FL SC TN NC IL WI MI OH IN KY WV VA PA NY ME VT NH NJ DE MD MA CT RI State Respondents Figure 14. SHAs willing to participate in Phase II.

30 Evaluation of Bonded Concrete Overlays on Asphalt Pavements detailed site evaluations; however, because of traffic control restrictions, one project was removed from the site evaluations. • Region: – North Atlantic: one project (Pennsylvania) – North Central: 10 projects (One each in Iowa, Kansas, and Missouri; three in Illinois; and four in Minnesota) – South: two projects (Louisiana) – West: seven projects (six in Colorado and one in Montana) • Includes synthetic macrofibers: – Yes: six projects (only five of the 29 originally submitted projects included fibers) – No: 14 projects • Includes joint sealant: – Yes: 12 projects – No: eight projects • Traffic: – <5,000 AADT: five projects – 5,000–20,000: 10 projects – >20,000: five projects The selected projects are shown in Table 16 and Figure 15. Available agency BCOA standard specifications are included in Appendix B. Summary Using an online survey, the research team asked agencies that had known experience with BCOAs to respond to questions related to BCOA experience and use, project selection and evaluation, design, materials, construction, maintenance and repair, performance data, and will- ingness to participate in Phase II of the study. The following summarizes key findings from the agency survey: BCOA Thick- ness (in.) Slab Size Joint Seal Macro- fibers No. of Projects by Region Total AADT ≤ 5,000 5,000–20,000 ≥ 20,000 NA NC S W NA NC S W NA NC S W <6 4- x 4-ft Yes Yes 0 0 0 0 0 0 0 0 0 0 0 0 0 No 0 0 0 0 0 0 2 0 0 0 0 0 2 No Yes 0 0 0 0 0 2 0 0 0 0 0 0 2 No 0 0 0 0 0 0 0 1 0 0 0 0 1 <6 5.5- x 5.5-ft Yes Yes 0 0 0 0 0 0 0 0 0 0 0 0 0 No 0 0 0 0 0 0 0 0 0 0 0 0 0 No Yes 0 0 0 0 0 0 0 0 0 0 0 0 0 No 0 3 0 0 0 0 0 0 0 0 0 0 3 <6 6- x 6-ft Yes Yes 0 0 0 0 0 0 0 0 0 0 0 1 1 No 0 0 0 1 1 1 0 0 0 0 0 2 5 No Yes 0 1 0 0 0 0 0 0 0 0 0 0 1 No 0 1 0 0 0 0 0 0 0 0 0 1 2 6 6- x 6-ft Yes Yes 0 0 0 0 0 0 0 0 0 0 0 1 1 No 0 0 0 0 0 0 0 1 0 1 0 4 6 No Yes 0 0 0 0 0 0 0 0 0 0 0 0 0 No 0 0 0 0 0 4 0 0 0 0 0 0 4 6 12- x 12-ft Yes Yes 0 0 0 0 0 0 0 0 0 0 0 0 0 No 0 1 0 0 0 0 0 0 0 0 0 0 1 No Yes 0 0 0 0 0 0 0 0 0 0 0 0 0 No 0 0 0 0 0 0 0 0 0 0 0 0 0 Total Number of Projects 0 6 0 1 1 7 2 2 0 1 0 9 29 NOTE: NA = North Atlantic; NC = North Central; S = Southern; W = Western. Table 15. Features of BCOA projects selected for Phase II.

State of the Practice 31   • The number of BCOA projects constructed over the last six decades has steadily increased, and even more so over the last 30 years. Over the last two decades, 14 of the responding agencies have constructed one to five BCOA projects, five agencies have constructed five to 15 BCOA projects, and one agency has constructed more than 15 BCOA projects. • BCOA projects are constructed predominantly on state and U.S. routes with low to moderate traffic volumes. For rural routes, BCOAs are placed at mainline and intersection locations. • The primary methods to evaluate site locations for BCOA application are coring, distress surveys, and FWD testing and backcalculation. • BCOA structural design is most often accomplished using the AASHTO 1993, BCOA-ME, and PaveME v2.3 methods. • The most common combinations of overlay thickness and slab size are 4-in. BCOA with 4- × 4-ft slab size, 5-in. BCOA with 6- × 6-ft slab size, and 6-in. BCOA with 6- × 6-ft slab size. • Half the responding agencies use a specialized concrete mixture, with 11 specifying the use of fibers (polypropylene being the most common). • The predominant preoverlay repair is patching. The most common surface preparation activi- ties are milling and power sweeping. • The most common BCOA maintenance and repair activities, with nine or more responses each, are transverse and longitudinal crack sealing, transverse and longitudinal joint and crack spall repair, diamond grinding, partial slab replacement, and full slab replacement. • Approximately half the responding agencies have detailed condition and performance data for their BCOA projects, including data on IRI, faulting, and transverse and longitudinal cracking. • Nineteen agencies indicated a willingness to provide the research team with BCOA documen- tation on selected projects and provide access and traffic control in support of the detailed site investigation. • On the basis of an evaluation of detailed data, 20 BCOA projects were selected for inclusion in the detailed site investigations; however, one project was removed from the detailed site evaluation because of traffic control restrictions. a Originally indicated as 6- x 6-ft slabs; however, field evaluation confirmed 12- x 12-ft slabs. b This project included a variable joint spacing (nominal slab size measured approximately 6 ft wide by 7 ft long) to align BCOA joints to working cracks in the asphalt pavement. This design resulted in early faulting and is no longer recommended by the Minnesota DOT. State Route 2019 Age (years) BCOA Design Thick- ness (in.) Slab Size Joint Sealant? Tie Bars? Macro- fibers? AADT CO I-70 7 6.0 6- x 6-ft Yes Yes No 5,000–20,000 CO SH-83A 19 5.0 6- x 6-ft Yes Yes No >20,000 CO SH-83B 14 6.0 6- x 6-ft Yes Yes No >20,000 CO SH-121A 19 6.0 6- x 6-ft Yes Yes Yes >20,000 CO SH-121B 8 6.0 6- x 6-ft Yes Yes No >20,000 CO US-6 21 5.5 12- x12-fta Yes Yes No <5,000 IA US-71 7 6.0 6- x 6-ft No No No 5,000–20,000 IL CH-10 10 5.25 6- x 6-ft No No Yes <5,000 IL CH-27 16 5.25 5.5- x 5.5-ft No No No <5,000 IL SR-53 7 4.0 4- x 4-ft No No Yes 5,000–20,000 KS I-70 8 6.0 6- x 6-ft No Yes No 5,000–20,000 LA US-167 21 4.0 4- x 4-ft Yes No No 5,000–20,000 LA US-425 16 4.0 4- x 4-ft Yes No Yes 5,000–20,000 MN CSAH-7 10 5.0 6- x 6-ftb No No No <5,000 MN CSAH-22 8 6.0 6- x 6-ft Yes No No 5,000–20,000 MN I-35 10 6.0 6- x 6-ft Yes Yes No >20,000 MN TH-30 26 6.0 12- x 12-ft Yes Yes No <5,000 MO US-60 20 4.0 4- x 4-ft No No Yes 5,000–20,000 MT SR-16 18 4.0 4- x 4-ft No No Yes 5,000–20,000 PA SR-119 9 6.0 6- x 6-ft Yes Yes No 5,000–20,000 Table 16. Features of BCOA projects selected for site evaluation.

NOTE: Four project sites were clustered very closely in Denver, Colorado. Figure 15. Locations of BCOA projects selected for site evaluation.

Next: Chapter 4 - Field Performance of Selected Projects »
Evaluation of Bonded Concrete Overlays on Asphalt Pavements Get This Book
×
 Evaluation of Bonded Concrete Overlays on Asphalt Pavements
Buy Paperback | $91.00
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

The use of thin bonded concrete overlays on asphalt (BCOAs) as a rehabilitation treatment first gained momentum in the 1990s. Since the first documented thin BCOA application in the United States, in Louisville, Kentucky, in 1991, BCOAs have seen a dramatic increase in popularity.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 1007: Evaluation of Bonded Concrete Overlays on Asphalt Pavements documents BCOA practices through a literature review and agency survey; documents performance through site investigations that assessed in-service design, construction, performance, preservation, and rehabilitation; and compares the results of current design methods with actual performance.

Supplemental to the report is NCHRP Web-Only Document 329: Bonded Concrete Overlays on Asphalt Pavements: Resources for Evaluation, which provides Appendices A through G of the contractor’s final report.

READ FREE ONLINE

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!