Pavement Management Systems _ Reading #1

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Unformatted text preview: “We flux—Ha...“ wt 5.- E.J. Yoder & MM. Witczak CHAPTER] Pavement types, wheel loads, and design factors The field of pavement design is dynamic in that concepts are continually changing as new data become available. There are many methods of design available, since opinions regarding suitability of designs vary from locale to locale. In particular, materials that are available for construction of pavements have a major influence on design. There are, however, principles of design that are common to all problems irrespective of other extenuating circumstances. The design of airport and highway pavements involves a study of soils and paving materials, their behavior under load, and the design of a pavement to carry that load under all climatic conditions. All pavements derive their ultimate support from the underlying subgrade; therefore. a knowledge of basic soil mechanics is essential. In the early stages of development, design consisted of rule«of—thumb proce- dures based on past experiences. During the period 1920 to 1940, engineers made a concerted effort to evaluate the structural properties of soil, principally for foundations for buildings and bridges. During this time a vast amount of basic data was accumulated, which enabled the engineer to design foundations on a rational basis. At that time, soil mechanics as applied to pavements dealt primarily with classification of soils, which in itself was a big step, however inadequate. Highway engineers were aware that performance of pavements was dependent to a large extent upon the types of soils over which the highway was constructed. As a result, correlations of pavement performance with subgrade types were established. In general, the studies showed that highways constructed over plastic soils showed higher degrees of distress than those constructed over granular de- posits. Frost action and adverse drainage conditions were recognized early as two of the primary causes of pavement failure. a1 4 PAVEMENT TYPES AND DESIGN FACTORS Nevertheless, many highway departments utilized standard cross sections for most highways. This meant that a road, even though it crossed several soil types, was constructed using a constant thickness. The practive was often justified on the basis of economics. Beginning in the 1950s, gear loads imposed by heavy aircraft necessitated a more rational approach. Also, at about the same time, truck traffic increased immeasurably with the result that severe breakup was common on some highways. It is the purpose of this book to summarize the basic fundamentals involved in the design of pavements and to set forth the techniques that will enable an engineer to design a pavement to fit a variety of situations. TEST ROADS IN THE UNITED STATES The Bureau of Public Roads in the United States“ and the AASI—IO have been responsible for several test roads constructed in the United States. In addition, several state highway departments have constructed test pavements for the purpose of evaluating the effect of load and materials on pavement design. The first major test road was known as the Bates Experimental Road and was constructed in Illinois in 1920. This test road was constructed using various materials, including brick, asphaltic concrete, and Portland cement concrete. The results of this test road gave basic data that were used by design engineers for many years. The next major test road was designated the Maryland Test Road, and the tests were made on a 1.1-mile section of concrete pavement constructed in 1941. Major conclusions were drawn from the research project relative to the effect of loads on pumping of rigid pavements. The WASHO road test was constructed in Idaho for the purpose of evaluating the design of flexible pavements. This test road was conStructed under controlled conditions; four different axle loadings were used during the testing program. This project brought out forcibly the fact that major distress was confined largely to the spring seasons of the year, and illustrated the effect of the thickness of » wearing course on pavement performance. In 1951. a major road test was planned for Ottawa, Illinois. This road test has been designated the AASHO Road Test, and included both concrete and asphal- tic pavements. Major findings dealt with concepts of serviceability, as well as the effect of relative pavement thickness on performance. In addition to the major road tests, the Federal Highway Administration has sponsored research programs throughout the country wherein pavements have been evaluated under varying soil, climatic, and loading conditions. The Corps of Engineers has for the past 20 years conducted extensive research programs on prototype pavements as well as pavement test sections. There is little doubt that the results of these field test programs have had major influence on present—day design concepts. In addition, performance of prototype pavements in service has had significant influence on design. This is not surprising, if one considers that it is difficult if not impossible to evaluate ‘The Bureau of Public Roads now carries the name Federal Highway Administration, shortened FHWA. DEFINITION 05 fully design C( time that use} Subsequent pavements m literature. De out the text. DEFINITION I Historicall: 1.1). The cla simplificatior subject to th The fiexib over a base ( grade. In con may or may The essent is the marine ment, becau: the load over capacity is 31 of rigid pav minor variai capacity of t It should ments inclut Figure 1.1. ‘ pavements at are shown in extended thr DEFINITION OF PAVEMENT TYPES 5 {ully design concepts in the laboratory. Furthermore, it has been known for some time that user opinion in the final analysis dictates the adequacy of the design. Subsequent chapters of this text will rely heavily on the results of the test pavements mentioned above, as well as performance data published in the literature. Detailed findings of the various field projects will be discussed through- outthetexc DEFINITION OF PAVEMENT TYPES Historically, pavements have been divided into two broad categories (see Figure 1.1). The classical definitions of pavements, in some cases, represent an over— simplification, as will be discussed in later paragraphs. Pavement classification is subject to the limitations inherent to all classification techniques. The flexible pavement may consist of a relatively thin wearing surface built over a base course and subbase course, and they rest upon the compacted sub— grade. In contrast. rigid pavements are made up of Portland cement concrete and may or may not have a base course between the pavement and subgrade. The essential difference between the two types of pavements, flexible and rigid, is the manner in which they distribute the load over the subgrade. The rigid pave- ment, because of its rigidity and high modulus of elasticity, tends to distribute the load over a relatively wide area of soil; thus, a major portion of the structure capacity is supplied by the slab itself. The major factor considered in the design of rigid pavements is the structural strength of the concrete. For this reason, minor variations in subgrade strength have little influence upon the structural capacity of the pavement. It should be noted at this point that the classical definition of flexible pave» ments includes primarily those pavements that have an asphalt concrete surface. Binder Sudace i Salazar”! course Prime __ /_,._Aa—_LL _ | Base |course Subbase course I Compacted subgrade __________ _.__,___________.__ Natural subgrade (a) Portland-cement concrete 1' Base course may or may not be used I I. .L .1 _________ -ul___-________ a) l I L .1 $35.". 1.1. Components of (a) flexible and (b) rigid pavements. Base courses under rigid pavements are often called subbase'courses. For these illustrations the base and subbase courses are shown in a “trench” section. .See Figure 1.2 for designs wherein the base is either drained or extended through the shoulder for drainage. 6 PAVEMENT TYPE AND DESIGN FACTORS In contrast, the classical rigid pavement is made up of Portland cement concrete. It should be obvious that the definitions “flexible and rigid” are arbitrary and were established in an attempt to distinguish between asphalt and Portland cement concrete pavements. Asphalt pavements may possess stiffness much as Portland cement concrete pavements. This is true when stabilized materials are used in any of the pave- ment components or if, for example, relatively thick asphaltic concrete layers are used. At the other extreme, if very thin surfaces are used (for example, surface treatments), the pavement can be considered to be flexible. Hence, the reader must bear in mind that the definitions are arbitrary and may or may not be strictly true. Base courses are used under rigid pavements for various reasons, including (1) control of pumping, (2) control of frost action, (3) drainage, (4) control of shrink and swell of the subgrade, and (5) expedition of construction. The base course (often called a subbase course) lends some structural capacity to the pave» ment. However, its contribution to the loadcarrying capacity may be rela tively minor. The loadcarrying capacity of a truly flexible pavement is brought about by the load-distributing characteristics of the layered system. Flexible pavements consist of a series of layers with the highest-quality materials at or near the surface. Hence, the strength of a flexible pavement is the result of building up thick layers and, thereby, distributing the load over the subgrade, rather than by the bending action of the slab. The thickness design of the pavement is influenced by the strength of the subgrade. If an asphalt pavement has high stiffness, it may behave essentially as a rigid pavement and fatigue of the surface or of any pavement component may become critical. In these cases, concepts underlying design approach those historically adopted for concrete pavement design. For example, full—depth asphalt pavements are used in certain situations. This type of pavement undoubtedly approaches the rigid condition and the classical methods for designing flexible pavements no longer apply. The same is true if a cementing agent is used as a stabilizing additive in the base or subbase. Base courses are constructed some distance beyond the edge of the wearing surface. This is done to make certain that loads applied at the edge of the pave- ment will be supported by the underlying layers. If the layers are built with an abrupt face, loads applied at the surface are likely to cause failure due to the lack of support at the pavement edge, Base courses generally are extended about 1 foot beyond the edge of the pavement, although in special situations they may be extended for greater distances. Roadway and Airport Cross Section. Figures 1.2 and 1.3 show typical cross sections of a highway and of a runway and taxiway. The standard width of high- ways that carry large volumes of traffic (i.e., high-type highways) is generally 24 feet, although for highways that carry lesser amounts of traffic the width may be somewhat less, The shoulders adjacent to the traffic lane again are of variable width, generally about 10 feet. Base courses and subbase courses under highway pavements may be constructed using one of several techniques. If the material is pervious, it may extend through the shoulder to permit drainage at the point it intersects the side slope. DEFINITION C Ditch Figure 1.2. 1 slope % [0 l; E Figure 1.3. type of airpc- In some cs are built i: tion drain: of the pat given to course ma chapters. In cont feet. The feet, dept Greater v bers. Rut may not ‘1 merits til construcz distance the other Taxiw upon the Many for high tervals. Thicke in some The par 9-6-9 inc pavemei DEFINITlON OF PAVEMENT TYPES 7 Shoulder Medial Shoulder varies 24' navement varies 24' pavement varies l l . Ditch . Subbase Storm drain. through shoulder drain variable spate Figuyg 1.2, Typical cross section of a highway. Pavement slope l to k inch per foot. Shoulder slope % to 1% inches per foot. Cut and fill slope variable. A Taxiway AI 13 Runway B Storm drain Figure 1.3. Typical runway and taxiway cross section. Widths are variable depending on the type of airport. Distances A, A', and B are determined by clearance standards. In some cases, particularly in cuts, subbase drains will be used. Many highways are built utilizing trench construction (see Figure 1.1). In this type of construc- tion drainage is not attempted. Performance surveys have shown that many miles of the pavements have functioned satisfactorily, as long as proper attention is given to the gradation and the compaction of the base~course and subbase- course materials. These factors will be discussed in great detail in subsequent chapters. In contrast to highways, airfield runways are constructed in widths up to 500 feet. The widths of civilian airfields are variable, ranging between 50 and 200 feet, depending upon the type of airfield. Typical runways are 150 feet wide. Greater widths are used on some military airfields to accommodate heavy bom- bers. Runways are nearly always crowned, whereas highway pavements may or may not be crowned. In some cases it is more economical to build highway pave- ments tilted downward toward the outside lane with no crown. This type of construction, however, is not justified on major airfields, because of the long distance the water must travel to drain from one edge of the pavement to the other. Taxiway widths are variable, ranging between 20 and 100 feet, depending upon'the class of airport and are typically 75 feet wide. Many airfields have been built with subbase drainage similar to that indicated for highways. However, to be effective, the drains must be spaced at closer in- tervals. Thickened Pavement Sections. Pavements with thickened edges are used in some situations to accommodate high stresses that exist at the pavement edge. The pavement sections are designated, for example, as 9-8-9 inch, 9.79 inch, or 9-6-9 inch (Figure 1.4). Thickened-edge pavements are more costly than uniform pavements, because of the grading operations that are required at the thickened PAVEMENT TYPES AND DESIGN FACTORS I” ‘I i (a) ” Runway 10% (light aircraft) 1000' (heavy aircraft) (b) Thickened center width varies I (c) h load concentration. (a) Transverse section of 9-8-9 Figure 1.4. Thickened pavements for big (normal traffic, no channelization); inch highway pavement; (b) longitudinal section of runway end (c) "keel" section for high load concentration on runway. ened-edge highway pavement was popular ere in the neighborhood of 18 to 20 feet avement edge. On wider pavements, how- 3 and 4 feet from the pavement edge, edge. In addition, the use of the thick at the time when pavement widths w and traffic traveled very close to the p ever, traffic concentration is between alleviating the necessity for using a thickened edge. Taxiways and runway ends should always be constructed using a heavier section than the central portion of the runway because of high concentration of traffic (Figure 1.5). Touchdown at the end of the runway may not be critical because the airplane is partially airborne. The distance from the end of the runway for which’a thickened section is used ranges between 10 percent of the total runway length and 1000 feet. A “keel” seCtion is a thickened center used on airport pavements (Figure 1.4c). HIGHWAY AND AIRPORT PAVEMENTS COMPARED ents and the performance of airport pave- Rigid highway pavements that carry high The performance of highway pavem ments are for the most part different. HIGHWAY AN Figure LS. T ends, taxiways taxiways may volumes of l clay subgra plastic soils serious dist factors that are the sarn to each fac truck, but on airport: 9000 pount to 2000 tr' excess of 1 for the lift Tire pn inch), whe 90 psi. L2 traffic tra\ airfield is center. As feet of the Moderr condition 75 percer. ' One co one time b HIGHWAY AND AIRPORT PAVEMENTS COMPARED 9 Jf a;le Building area Ru nway Figure 1.5. Triangular runway system showing location of strengthened pavements. Runway ends, taxiways, and aprons are designed for greater thickness than interior of runways. Exit taxiways may be classified as noncritiml. volumes of heavy traffic nearly always result in pumping distress if built directly on clay subgrades. On the other hand, many airfield pavements built directly over plastic soils have shown little or no pumping. Flexible highway pavements show serious distress at pavement edges, whereas airfield pavements do not. The chief factors that must be considered in the design of highway and airfield pavements are the same; however, diflerences exist regarding the quantitative values assigned to each factor. The total weight of an airplane is usually greater than that of a truck, but the number of repetition of loads is much greater on highways than on airports. The design load for a major highway is ordinarily in the vicinity of 9000 pounds on dual tires, and the expected repetition may be as much as 1000 to 2000 trucks per day. In contrast, a heavy airplane may have wheel loads in excess of 100,000 pounds, but only 20,000 to 40,000 coverages' may be considered for the life of the pavement. Tire pressures on jet aircraft may be as high as 400 psi (pounds per square inch), whereas for conventional truck tires, pressures are in the vicinity of 60 to 90 psi. Lateral placement of traffic on highways is such that nearly all truck traflic travels within 3 to 4 feet of the pavement edge. In contrast, traffic on an airfield is such that the distribution of traffic is concentrated primarily in the center. As a general rule, the traffic on a runway is distributed over about 60 feet of the pavement. Modern aircraft have steerable nose wheels which have resulted in channelized conditions on airfield taxiways. Results of recent studies have shown that 75 percent of this type of traffic will occur on about 7.5 feet of pavement. ' One coverage results when each point on the traffic area of the pavement has been traversed one time by a wheel. 10 PAVEMENT TYPES AND DESIGN FACTORS WHEEL LOADS xtremely important. The most severe distress e traffic follows a designated line along the Little distress is generally found on The geometry of the pavement is e to an airfield pavement occurs wher aprons and taxiways and at runway ends. or in the center portion of the runways. From the above discussion, it is seen that the major differences between high- way and airfield pavements are repetition of load. distribution of traffic, and geometry of the pavement. In turn, each of these is affected by pavement width and type of aircraft. For a given wheel load the aprons and a given tire pressure, highway pavements are n a highway is much thicker than airfield pavements, because repetition of load 0 higher and also because the loads are applied closer to the pavement edge. This does not mean to imply, however, that airfield pavements are generally thinner than highway pavements; gross loads on airfields are much higher with the result that in actual practice these pavements are thicker. -- - -- - - Trailer Tractor -- - Single axle v T with single tires Tandem axle Single axle with dual tires with dual tires Fig“. L7. B0 (0) Aircrait Co.) Main sin le- - ‘ WHEEL LOA tire geagr - “— Nose wheels 7 Types of (by basic catego axles, and ( wheels may -= For high‘ Twin-tandem =‘\_ N h as p 20,000 pom 058 W e l gear - half the ax -- tandem axll (C) Large ml the case of r- twin-tande: - - - on many la Double -- In the d twin -tandem <- - =*—* Nose wheels gears - - largest plat -. —- craft. How i ' . this book : l (d) dition oi l E - I . . i Figure 1.6. Plan View of several basic types of wheel configuration. (a) Single trailer-truck Cond‘tlon in~tandem landing gear, (cl) double twin- ‘ Althoug unit, (12) tricycle landing gear with single tires, (C) [W tandem gear. (Note: Not to scale”) Pr “ices of a l WHEEL LOADS l '| Figure 1.7, Boeing 707 gear (a) Nose wheel. (b) twin-tandem main gear. (Courtesy Boeing Aircraft Co.) WHEEL LOADS Types of airplane- and truck-wheel arrangements can be divided into several basic categories, including (1) single and dual wheels, (2) single and tandem axles, and (3) nose wheel, tricycle, and bicycle landing gears. Truck and airplane wheels may be arranged in several combinations of these listed above. For highways the legal axle load in most states ranges betwen 18,000 and 20,000 pounds, which implies that a load on one set of dual tires will be one- half the axle load. Thus, if greater loads are required, it is common to add a tandem axle.’ Large modernday aircraft utilize either bicycle or tricycle landing gears. In the case of tricycle landing gears, the main gear load can be of single, dual, or twin-tandem type (Figure 1.6). Figure 1.7 illustrates the twin-tandem gear used on many large aircraft. In the design of airport pavements, the design wheel load may be that of the largest plane which will use the field. Table 1.1 shows typical data for several air- craft. However, design procedure that will be presented in subsequent chapters of this book account for mixed traflic of varying loads and types of gears. The con- dition of takeoff governs thickness design of airport pavements since under this condition the load is greatest due to fuel weight. ‘ Although the most common multiple—axle trucks have two axles in groups, present-day practices often include as many as three or four axles in a group. i t .31 -. J.--la . 12 PAVEMENT TYPES AND DESIGN FACTORS TABLE 1.1. Dal: for Several Typical Aircraft“ Max Max Load Gross Main Gear Each Main Tire Weight Dimension Assembly Pressure Type of Plane (lb X 10’) Type 01' Gear (in) (lb X 10‘) (psi) Boeing 707-320C 336.0 Twin-tandem 56 X 34.5 157.0 180 Boeing 707-12013 258.0 Twin-tandem 56 X 34 120.0 170 Boeing 737 111.0 Twin 30.5 25 .8 148 Boeing 727-100 1700 Twin 34.0 76.9 166 Boeing 74-7 713.0 Double twin- tandem 58 X 44 166.5 204 Convair Cv 880 185.0 Twin-tandem 45 X 21.5 87.0 150 Lockheed L101l-1 411.0 Twin—tandem 70 X 52 195.0 175 McDonnel— Douglas DClO-lO 413.0 Twin-tandem 54 X 64 194.0 175 McDonnel- Douglas DC 8—43 318.0 Twin-tandem 55 X 30 148.0 177 McDonnel— Douglas DC 9-15 91 .5 Twin 24 42.4 127 Concorde 388.0 Twin-tandem 66 X 26.4 184.3 184 BAC 1-11-500 100.0 Twin 21 47.5 174 " From FAA (Reference 5) and the Asphalt Institute (Reference 3). See Reference 3 {or a complete summary of aircraft data. On the other hand, the length of runways may or may not be determined on the basis of takeoff conditions depending on a number of factors. Runway lengths are determined on the basis of aircraft characteristics as well as temperature, altitude, and so on, at the site. Table 1.2 shows typical lengths for several air- craft. These values are for illustrative purposes only, since each site must be analyzed on an individual basis. Allowable axle loads for highways vary from state to state as indicated in Table 1.3. The majority of the states permit single—axle loads of 18,000 pounds and maximum tandem-axle loads of 32,000 pounds. Tandem spacings range be» tween 40 and 48 inches. Tire pressures are controlled generally by allowable load per inch of width oi tire. Gross weights are quite variable from state to state and may be calculated utilizing a formula as indicated in the extreme right-hand column of Table 1.3. TIRE PRESSURES, CONTACT PRESSURES, AND TIRE IMPRINT If the effect of the tire wall is ignored, the contact pressure between the tire and pavement must be equal to the tire pressure. For low-pressure tires, how- ever, contact pressures under the tire wall may be greater than at the center of the tire. For high-pressure tires the reverse is true. For most problems, however, the assumption is made that contact pressures are uniform over the imprint area. DESIGN FACT Plant Boeing Boeing Boeing Boeing Boeing’ Dougi, Conva BAG T “ Dat ° The only ' efiect runw. plant“ To the n radius of ( DESIGN 1 Pavem‘ mixtures design st: The 5: design 0 upon thc tors. Lik and it b a design affected SETUCIUI‘f DESIGN FACTORS 13 TABLE l.2. Typical Runway Lengths hr Several Aircraft and Conditions“ Normal Max Temp. of Hottest Month Elevation Length 5 Plane Type (°F) (ft) (ft) Boeing 707~lOO . 100 Sea level 10,500 Boeing 707-l00 75 3000 l l ,500 Boeing 707-100 75 1000 10 ,500 Boeing 727 75 1000 7 .800 Boeing 747 75 1000 10,500 Douglas DC 9 75 1000 8 ,000 Convair Cv 880 75 1000 10,500 BAC l—ll 75 1000 7,500 “ Data from charts in FAA publication (ref. 4). “The lengths shown in the table are relative and are for illustrative purposes only since the required lengths are dependent upon many factors, including effective grade of the runway, setting of the wing flaps, and takeoll weight. Each runway must be analyzed for its own particular conditions and the critical plane using the runway. In the majority of the problems, circular tire imprints are assumed. Hence the radius of contact is as follows: P a = \1— (L1) [971’ where a = radius of contact P = total load on the tire p = tire pressure (assumed to be equal to contact pressure) For some cases tire imprints as illustrated on Figure 1.8 are used. The rela- tionship between pressure and the geometry of the imprint is as shown on the figure. DESIGN FACTORS Pavement design consists of two broad categories: (1) design of the paving mixtures, and (2) structural design of the pavement components. These two design steps must go hand in hand. The structural design of pavements is basically different from the structural design of bridges and buildings in that the pavement structure lies exposed upon the ground surface and, hence, is greatly influenced by environmental fac- tors. Likewise, a highway, for example, will cross many different soil deposits and it becomes necessary for the design engineer to select in a rational manner a design value representative of the area under question. The strength of soil is affected by many factors, including density, moisture content, soil texture, soil structure, rate of load application, and degree of confinement. In addition, soils TABLE 1.3. Truck Axle Sputinu and Woighl Lil-ilk“ Gross Weight Limits (In thousands at poundsi g (Pneumatic TirPs Only 5 2 § '2 E ‘ .,: - ~' .g ..c .2 g :5 § ~: 2 €51 “:5; a v- s»— C 4 6 ~33 I: g E g 5 3.- SE 53 .2 E: 5 S m”: i—fi I—fi 5-23“ Grass am e g g a: £3 .23 2.2 .22 3.2 E B Weights 22—: as . “3.: :22 :g 2.: :2 :2 :éfi by State :< a: r: if... é‘v‘; 9.2;: .54. .43 .aa'. 2<5 Formulae Allbama 40 NS 18 36 36 54 54 72 73.2 73.2 E E E E E Alaska. 42 500 20 34 20 34 40 54 GS 100 Arizona 40 NS 18 32 36 50 54 68 76.5 76. 8 Arkansas 40 NS 18 32 30 44 4B 62 73.2 73. 2 Calilornia NS NS 18 32 36 50 54 68 76. 8 76.8 800 Colorado 40 NS 18 36 30 46 54 66 73. 6 73.6 (L; + 40) Connecticut NS 600 22. 4 36 36 53.4 53. 8 67 .4 73 73 Delaware 48 700 20 36 30 65 48 66 73. 2 73. 2 District of Columbia 40 NS 22 38 44 60 66 70 70 70 Florida. 40 NS 20 40 40 60 60 66. 6 66 . S 66 . 6 Georgia 40 NR 20.3 40.6 40.6 61 61 73.2 73.2 73.2 300 Hawaii 42 — 24 32 48 54 54 65 73.2 72. 2 (L: + 40) 20 34 40 54 50 80 85.5 105.5 Idaho NS 800 18L 32L 36L 5014 ML 68L 76. BL 76. 8 Illinois 40 NS 18 32 3G 50 50 64 73.2 73. 2 Indiana 40 800 18 32 36 50 54 65 73.2 73.2 low: 40 NR 18 32 36 50 54 68 72.6 72.6 Kansas 40 NR 18 32 36 50 54 68 73.2 73.2 . N N N N N N N Kentucky 42 600 18 32 36 50 54 68 73.2 73.2 I E E E E E E Louisiana. 40 450 18 32 18 32 36 50 64 65 32L Maine 48 600 22 36 32 51.8 51.8 66.3 73.2 73.2 Mxryland NS NS 22.4 40 44. S 55 55 65 73.2 73.2 Massachusetts NS 800 22.4 36 44. 8 58.4 60 73 73 73 N N N N Michigan 42 - 18 32 36 50 54 68 73.2 M Minnesom 40 NR 18 32 35 54 54 54 73 . 2 73. 2 _ v _ ‘ N N N N N N N Mismssmpi 40 Table 18 32 36 50 46 60 73.2 73.2 Missouri 40 NR 15 32 36 50 54 68 73.2 73.2 Monuna 40 NS 18 32 36 50 54 68 76. 8 76.3 211T 40 30 50 76.5 80.5 95 Nebraska 40 NR 18L 32 36L 50L 54L 681. 71.11. 71 . 1L Nevada 42 NS 18 32 36 50 54 68 76.8 76.8 47.5L New Hampshire NS 600 22.4 36 33.4 55 52.8 66.4 73.2 73.2 E E E New Jersey 40 800 22.4 32 22.4 32 44. 8 73.2 73.2 73. 2 New Mexico 40 600 2l.6 34.3 43.2 55.9 64.8 77.5 86.4 86.4 1000 New Yark 46 800 22.4 3E 44.8 58.4 67.2 71 71 71 (Li + 34) (Cunlinuedl é 3%! .1 :3 TYPES OF Di‘ Sate North Clrolina Ohio ML. Oregon Pennsylvania Rhode Island South Carolina South Dak at: Tennessec Te xn Utah Varmonl. Virginia Wishington 7 West Virginia _ Wisconsin _ Wyoming _ Legend N S Not spec ;‘ NR. Not restri E Plus Wei: L Limit or. N On desig‘ S Includin: T Limited 1 V 0n prim; L1 Dismnc: ' Vehicles 1 ‘1 Fram Nation vary from: nature 05 TYPES OF Distinc structural of one 01 payment Classified a- . yvyver-usli , W. TYPES OF DISTRESS, STRUCTURAL AND FUNCTIONAL 15 TABLE 1.3. (confirmed) Gross Weight Limits (1n thousands of pounds) E Pneumatic Tires Only 5 3 § 15 f. — a: -lg .; - Pi = .E D D or (is :s E "is g s g: as g :2 s s :53 Hi i=5; is; an... 553 “3.: 5 as :1 2: 2.2 2.0 2.2 fig, Weights 2—: Se a g; :5“ :2“ i: :éé‘ E? E=°§ by State 24: :23 s 5.- ms .55 ii; a"; 55.4. z<c Formulae North Carolina 48 600 18 3e 30 47.5 47.5 54 7a 73 2 750 North Dakota 40 550 18 32 36 5D 54 64 54 64 (Li + 40)‘ 900 Ohio N3 650 19 32 as 51 57 70 75.8 73 (Li + 422%) 20 34 20E 34E 40E 54E 55.5 90 Oklahoma 40 650 18L 32L mint 3an afiEL bOEL 73.21. 7 3. 2L 20 34 E E N N Oregon 40 550 18L 32L as 50 36 50 76 76 Pennsylvania 36 800 22. 4 36 33 47 50 60 73 . 2 73. 2 Rhode Island 40 ns 22.4 NS 36 44 538 57.4 73.2 73.2 32L South Carolina 40 NR 20 36 35 46 50 65 73. 2 73. 2 South an0m 40 500 18 32 as 50 54 68 73 . 2 73. 2 Tennessee 40 NS 18 32 36 50 45 62 73.2 73.2 run 40 650 18 32 36 50 54 as 72 72 Utah 40 NS 15 as as 51 54 69 79.9 79.9 301.. 401. 50L 50L 501. 601. Vermont 45 600 22.4 36 44.8 55 57.2 73.2 73.2 73.2 Virginia 40 650 18 a2 36 50 54 68 7o 70 Wuhington 42 550 18 32 224 as 45 so as 72 x N N West Virginia 40 N R 18 32 35 5o 54 70 70 70 s s s Wisconsin 42 NS 19.5 32 39 51 54 65 73 73 20v 35v 60V sov 92V 101v Wyoming 40 NS 131. 32!. 36 50 54 as 73. 9L 73. 9L Legend: NS Not specified NR Not restricted Plus weight on front axle Limit on Interstate only On designated highways Including tolerance and on designated highways Limited to state highways On primary and secondary highways i Distance in feet between first and last axles of group Vehicles with axles over 18 feet apart. {actor is 650 for lower axle spacings. ‘E‘CHMZL‘M I From National Highway Users Conference. vary from point to point along a roadway; this fact coupled with the random nature of the traffic input makes the pavement design process a complex one. TYPES OF DISTRESS, STRUCTUR'AI. AND FUNCTIONAL Distinction will be made here between two different types of failure. The first, structural failure, includes a collapse of the pavement structure or a breakdown of one or more of the pavement components of such magnitude to make the payment incapable of sustaining the loads imposed upon its surface. The second, classified functional failure, may or may not be accompanied by structural 1.4% d L. .- 16 SERVICEAB'IUTY PAVEMENT TYPES AND DESlGN FACTORS some eflect. For examplt permits the accumulatic after the construction IS nd inadequ 0.6L that may can a tight we ewise, sealii f shoulc will insure L water. Lik Maintenance 0 L: V6323 performance. . It 15 to be recogm: design is but one of Provision can be mad climatic variables, as the close tie-in of recognized by the rea Figure 1.8. Tire imprint assuming rectangle and semicircles. s intended function t is such that the pavement will not carry out it high stresses in the assengers or without causing 1:, due to its roughness. failure bu without causing discomfort to p plane or vehicle that passes over i Obviously the degree of distress for both catagories is gra severity of distress of any pavement is largely a matter of opinion of the person observing the distress. However, the difference between the two types of failures \ is important, and the engineer must be able to distinguish between them. As an example, consider a rigid highway pavement that has been resurfaced with an develop rough spots as a result of breakup tural breakdown of SERVICEABILITY Perhaps the big: acceptable pavemer upon the opinion is dependent, at le example, less roug secondary roads th dational, and the asphaltic overlay. The surface may in the bituminous overlay (functional failure) without struc On the other hand, the same avement ma crack and . . ., p y e for able condition h sur 5 . other when applie the overall structure. break up as a result of overload (structural failure). Maintenance mea the first situation may consist of resurfacing to restore smooth—riding qualities to e may require complete the pavement, However, the structural type of failur rebuilding. The cause for eit \ Design concept and these conce} l ay be three- l dmre floads, and The Present S PSI) concept was her of the aforementioned distress conditions m 55 loads, high repetition 0 fold. Firstg’pverload including excessive gro high tire pressures can cause either structural or functional failure. Second. climatic conditions as well as environmental conditions may cause surface irregu- \ iceability Index larities and structural weaknesses to develop. For example, frost heaving, volume [33“?ant at am. etting and drying, breakup resulting from freezing and \ Whereas a ratini The present 1 change of soil due to w thawing, or improper drainage may Many of the climatic variables can be est ditions may, at best, be poor. he paving materials. due to freezing 1 A third cause may be disintegration of t pavements, for example, 3‘ . l and thawing and [or wetting and drying. Scaling of rigid may result from nondurable aggregates and can be caused by or aggravated by the application of salts for ice removal. Base-course materials may breakdown, thus generating fines which may cause an unstable mix to develop. Subgrades also are susceptible to climatic conditions. Construction practices may have ement distress. ‘ 1 rate the pavemi be the prime cause of pay climatic con- imated, but prediction of . l Objective measr ments include “ The reader is ' serviceability ind i methods of maki \ Eestation. Referei and the role of SERVICEABILITY 17 some effect. For example, rutting of the subgrade during construction, which permits the accumulation of water and subsequent softening of the subgrade after the con5truction is completed, may cause pavement distress. Use of dirty aggregates and inadequate inspection during construction are obvious factors that may cause pavement deterioration. Design procedures must be accompained by stringent inspection and field control in order to provide adequate pavement structures. Many types of pavement distress are a function of maintenance, or, more correctly, lack of maintenance. Sealing of cracks and joints at proper intervals Will insure a tight wearing surface, as provision against surface infiltration of water. Likewise, sealing of flexible—pavement surfaces is extremely beneficial. Maintenance of shoulders will be discussed in great detail as it affects pavement performance. It is to be recognized from the above discussion that inadequate structural design is but one of many diflerent factors that may cause pavement distress, Provision can be made during the design phase to take into account many of the climatic variables, as well as construction and maintenance techniques. However, the close tie-in of the factors with pavement performance should be fully recognized by the reader. SERVICEABILITY Perhaps the biggest question the paving engineer must answer is “What is an acceptable pavement?” The answer to this, of course, is qualitative and depends upon the opinion of the individual rating the pavement. Further, the answer is dependent, at least in part, upon the intended use of the pavement. As an example, less roughness can be tolerated on high-speed expressways than on secondary roads that carry low volumes of traflic. Also, it is obvious that "accept- able condition” has one meaning when applied to highway pavements and an- other when applied to airport pavements. Design concepts must account in some way for serviceability of the structure and these concepts must also distinguish between structural and functional distress. The Presenl Serviceability index. The serviceability index (designated the PSI) concept was developed during the AASHO Road Test.‘ The Present Serv- iceability Index is based upon a rating scale that designates the condition of the pavement at any instant of time. A rating of 5.0 indicates a "perfect" pavement, whereas a rating of 0 indicates an “impassible” pavement. The present serviceability index is determined by a panel of individuals who rate the pavement on a rating scale from 0 to 5.0. The index is correlated with objective measurements made on the pavement surface. These objective measure» ments include a measure of roughness index, extent of cracking and patching, ' The reader is referred to the material in Chapter 19 for a detailed discussion of the present serviceability index concept. Special reference is made to the factors that afiect the index and methods of making objective measurements. Data in Chapter l8 illustrate various distress mani- festation. Reference will be made throughout the remainder of this text to serviceabilitv trends and the role of user opinions in design. 18 PAVEMENT TYPES AND DESIGN FACTORS and for flexible pavements, the average rut depth in the wheel tracks. The bility can be made by important point here is that an estimation of servicea making the objective measurements, and then, through correlation equations, calculation of the index can be made. The primary factor that determines the PSI is longitudinal roughness of the pavement. In fact, many engineers drop the other terms (cracking, patching, etc.) from the correlation equations. Serviceability can, thus, be determined solely through the use of pavement roughness measurements with a high degree of accuracy. THE DESIGN PROCESS, DESIGN STRATEGIES As was mentioned in previous paragraphs, it is necessary in the design process to distinguish between functional and structural failures. At least in the case of highways, the primary factor overriding most design decisions is that of func— tional failure, although it is necessary to build into the pavement structure resistance against structural failure to insure that the pavement will carry out its intended function. Figure 1.9 shows a generalized relationship between serviceability and age. Starting at year 0, it is to be noted that the pavement will have initial high serviceability, although this rarely approaches the PSI value of 5.0. As traffic is applied to the pavement the serviceability will decrease; the rate of decrease depends upon the amount of routine maintenance placed into the pavement. At year y}, the road may have major maintenance applied to it, such as resurfac- ing, and the serviceability then is again at its initial value. As traffic progresses the serviceability again drops to year 322, and this process is continued throughout the life of the pavement. Figure 1.10 illustrates that the design process for pavements is not an exact one, and is dependent upon many factors. Figure 1.10a shows the generalized relationship between accumulated 18,000—pound single-axle loads (EAL) and required thickness. In this case, the accumulated axle loads would be those Major maintenance With routine (Resunace, etc.) maintenance Serviceability ——v- Little routine maintenance Figure 1.9. Generalized serviceability versus age. Required thickness ———5— THE DESIGN PRC Accumuiated 18.00’. 2n' lst resurfacew Required thickness ———— Figure 1.10. Pri versus equivalen- alternate design: interval of resur anticipated 0 as shown in 1" During th relative to ti Referring to initial desigr design thick] applied that would be a; alternate th; thickness eq: tenance wou strate that t1 can select fo plan major r It must b to the life 1 wherein the costs, depen ‘S j .J I THE DESIGN PROCESS, DESIGN STRATEGIES 19 Required thickness ——>— Years of traffic -—+ (11} 2nd resurface lst resurface Totai cost Maintenance cost __— Average annual cost —v- (d) versus equivalent load repetition; ((7) thickness requirements as a function of time; (c) several alternate designs as a function of years: (d) initial cost, maintenance cost, and total cost versus interval of resurfacing. (All curves apply to flexible pavements.) initial design would carry the pavement through year 3:1 and that the initial design thickness would be :1. At this interval in time, a resurface would be applied that would carry the road to y2, at which time a second resurface would be applied to take the road to interval of time equal to ya. Another alternate that the design engineer might select would be to make the initial thickness equal to 22, which would take the road to ya years before major main- can select for initial design, depending upon the year of life at which he might plan major maintenance of the facility. 20 PAVEMENT TYPES AND DESIGN FACTORS costing the pavement is demonstrated in diagramatic form in Figure 1.10:1. If an initial design is to be minimal (perhaps thin pavement section) the maintenance cost increases, since the road will wear out at a fairly rapid rate. However, if the designer chooses to increase the initial cost by building a substantially stronger pavement, the maintenance costs decrease accordingly. Hence, it is seen that the 1 decision-making process includes, in part, balancing the total cost as illustrated in the upper curve of Figure 1.10d against inconvenience to the pavement user and many other factors. The total cost of the pavement structure should include not only the actual maintenance cost applied to the pavement surface itself, but added road user costs that are caused by the shutdown of the facility during the time that surface maintenance is supplied. '1 Checking process Design as a Trade-off Process. It should be recognized from the above dis- cussion that the decision-making process that is at the heart of the design must i rely heavily on trade-off of inconvenience and maintenance cost against the initial cost. It becomes necessary for the design engineer to make a decision relative to the serviceability that he wishes his pavement to achieve, and from this, make an estimate of the life of the pavement that might be expected. This decision-making process is not a simple one, and is dependent upon many factors, including the type of facility itself. To illustrate the above point, it is feasible to maintain low-volume roads at frequent intervals. On the other hand, expressways are difficult to maintain and , the road user cost resulting from shutdown of the facility may be so high as to ! preclude practically any maintenance at all. The same can be said for airport I pavements, where the structure must be a revenue-earning facility, and this fact 1 coupled with the need for safety may be the governing factor regarding the initial design that is adopted. Figure 1.1] shows a flow diagram illustrating the principles involved in the design proeess, On the left‘hand side of the figure are shown the input variables, including loads and environment as well as the various material properties that must be evaluated either in the laboratory or the field. It is important to note that these are stochastic in nature and the variability of any given input factor may be extremely high. . The decision-making process, as shown in the second part of the diagram, requires that the engineer put together the variable factors listed in the input column and from these select design values that he considers to be applicable to the particular design problem. From this the pavement section is selected and in the ideal case, the pavement is evaluated and the evaluation is then checked Design .-v Decision process on 1.‘ A... '7, E "E n: Input variables against the original assumptions as shown in the lower portion of the diagram. i It is important to note that the design process includes essentially a decision» ‘ making process in which the engineer attempts to predict the eventual perform- ‘ ance of the pavement structure without going through the long-term process of 1 . i waiting until traffic has been applied to the road. An essential part of the design process is the cost analysis. although this is not necessarily the only nor best factor to consider for any individual case. Neverthe~ , . less, routinely it is desirable to minimize the total cost of the pavement structure \ including initial cost plus maintenance cost. _____._.-—_..___4 S n5 6r; CJr C a: . wmwuofi 9.235 5 8m .36» 3:93.382 new 8:23:52 2: .333 5:335 :38 3588th v5 $055 2.3.6 ucm mcozaEamma WEBB umcfimm xomco «2 Q E .3 .2 ago. 222:5 EoEw>ma ‘0 :oauw_¢w .mmqum :Emo—u 3: E 1939:: “gaugim .:.— 95mm“ .3 .550» £329.: mumtzw A: in.er 3323 new mummm G 56» flab—Em «moo .5sz 8m 33 van :3 2.4 Q E .m ESE SENSBQm 8m um: €525 m 5:5» mo_.__m> cymmu B cozuflwm 8 Q n .558 $3.398“ ESE—2 anion 3 £55. £063 EEEEEE 5 3:9. £333 .v is» 532m 2: S. van two; 8 w w .QE‘S warm—E EEG .383 55535“. $805 :2 .68 :lrf 333...? 59: sleuamw :3 n U D p. w 9 u av E s 21 22 PAVEMENT TYPES AND DESIGN FACTORS SELECTED REFE 1.3. For a g? \Nhy? 1.4. List and flexible pavemr SYSTEMS ANALYSIS The optimization of the decision-making process in light of minimizing the total cost of the structure is known as the “systems analysis” approach. All of the factors listed in Figure 1.11 are interrelated, and it is difficult at best to isolate the variables on a general basis. The engineer must make a reasonable estimate of all of these variables, and from these select a design to fit the conditions, and SELECTED RE 1. American A proceed from there to construct a pavement that will carry out its intended func- Highways," tion. Thus, the problem becomes a statistical one in which estimates are made 2‘ American A and reasoned judgments are made on the basis of these estimates. Weights an The chapters in this book that contain detailed discussions of each of the states," w; factors outlined in Figure 1.11 are given in the figure. Throughout the book 3. Asphalt In continual reference will be made to the matter of variability and how it may “Bum”: 1" be handled in design, and methods of design accounting for total cost will be ‘ ‘1 Eda“ A‘ . Advisor C ~ discussed. Y l. j 5, Federal A‘ u and 5A, 1 ‘ PAVEMENT PERFORMANCE AND THEORY 5- “mm” l Proceeding National l ton, D.C. k, Historically, pavement design has been approached from two broad, differing 1‘1. points of view. First, the practicing engineer often approaches the problem if solely from the standpoint of pavement performance. In contrast, researchers ‘ ‘ _ and educators approach the problem largely from theoretical concepts. 1 Neither of the above approaches is satisfactory within itself. Complete reliance 'l upon pavement performance represents a static condition wherein one must wait a relatively long period of time before new concepts can be proven out. On the other hand, theoretical equations are generally based upon simplified assumptions and many times do not apply to conditions as they exist in the field. Ideally, the engineer must rely upon both approaches to take best advantage of design in— formation and to be able to use materials at hand in a wise manner. It will be the authors’ intent to discuss both the theoretical and practical points of view throughout the text, although this may lead to some confusion for the reader in putting the major factors into perspective. However, it will be a principal theme of this text that, although theory has much application, the final design must be influenced largely by performance of existing pavements. To be of most use, theory must not conflict with performance. This conflict will not arise if the engineer treats both theory and performance data in a logical manner. These points will be discussed in great detail in later chapters of this text. PROBLEMS AND QUESTIONS 1.1 . Draw a complete typical cross section of a flexible (a) high-type four-lane, 24—foot wide i highway, and (b) 75-foot airport taxiway. Include in the sketch one side of roadway or taxiway l in 41-foot cut, the other on 5-foot fill. Indicate on the sketch all dimensions, slopes, and other i pertinent data. Exclude actual dimensions of thickness of paving components. Side slopes for the highway are 4: 1, and for the airport are 1% percent maximum. 1.2. Discuss the basic design diflerences between an airport and highway pavement. SELECTED REFERENCES 23 1.3. For a given wheel load, which will be thicker, a highway or an airport pavement? Why? 1.4. List and discuss briefly five factors that will affect the performance of both a rigid and flexible pavement and that are difficult to evaluate during the design phase. SELECTED REFERENCES 1. American Association of State Highway Officials, “A Policy on Geometric Design of Rural Highways," Washington, DC. 1954. 2, American Association of State Highway Officials, "A Policy Concerning Maximum Dimension, Weights and Speeds of Motor Vehicles to be Operated over the Highways of the United States," \Vashington, D.C., 1964. 3. Asphalt Institute, “Full-Depth Asphalt Pavements for Air Carrier Airports," The Asphalt Institute Manual Series No. 11(MS-ll), 1975. 4». Federal Aviation Administration, "Runway Length Requirements for Airport Design," FAA Advisory Circular A C l50/5325—4 (including changes 1 through 7), 1965. 5. Federal Aviation Administration, “Aircraft Data,” FAA Advisory Circular A C 150/5325-5 and 5A, 1968. 6. Lawton, “"arren L., “Static Load Contact Pressure Patterns Under Airplane Tires." Proceedings, Highway Resaarch Board, 1957. 7. National Highway Users Conference, "State Motor Vehicle Size and Weight Laws," Washing- ton, D.C. (published annually). ...
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Pavement Management Systems _ Reading #1 - “We...

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