Pavement & Foundations

Lec-1 about Pavement & Foundations

1. CT-407 Pavements & Foundations

2. Course Outline Pavements Types of pavement, wheel loads, load distribution characteristics. Design considerations. Methods of design of pavements, group index method, CBR method, Westergaurd method, ORN-31 method, AASHTO design guide method. Construction and maintenance. Pavement evaluation and rehabilitation.

3. Books Principles of Pavement Design (Second Edition) by E.J. Yoder, M.W. Witczak Pavement Analysis and Design (Second Edition) by Yang H. Huang Highway Engineering by Martin Rogers The Handbook of Highway Engineering by T.T. Fwa Highway Engineering Handbook, Second Edition by Roger L. Brockenbrough & Kenneth J. Boedecker

4. Types of Pavement Flexible Pavement may consist of a relatively thin wearing surface built over a base course and subbase course and they rest upon a compacted subgrade. Rigid Pavement are made up of Portland Cement Concrete and may or may not have a base course between the pavement and subgrade.

5. Types of Pavement

6. Types of Pavement Difference 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 pavement because of its rigidity and high modulus of Elasticity, tends to distribute the load over a relatively wide area of soil; thus 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 variation in the subgrade strength have little influence upon the structural capacity of the pavement.

7. Types of Pavement Base course are used under rigid pavements for various reasons, including Control of pumping Control of frost action Drainage Control of shrink and swell of the subgrade Expedition of construction The base course (often called a subbase course) lends some structural capacity to the pavement. However, its contribution to the load carrying capacity may be relatively minor.

8. Types of Pavement The load carrying capacity of 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 the flexible pavement is the result of building up thick layers and thereby, distributing the load over the subgrade, rather than by bending action of slab. The thickness design of the pavement is influenced by the strength of the subgrade.

9. Types of Pavement Width of Base courses for flexible & rigid pavements 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 pavement 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 ft beyond the edge of the pavement, although in special situations they may be extended for greater distances.

10. Roadway and Airport Cross Section Roadway Cross Section

11. Roadway and Airport Cross Section Roadway Cross Section The standard width of highways that carry large volumes of traffic is generally 24 feet, although for highways that carry lesser amount of traffic the width may be somewhat less. The shoulders adjacent to the traffic lane again are of variable width, generally about 10 feet.

12. Roadway and Airport Cross Section Airport Cross Section

13. Roadway and Airport Cross Section Airport Cross Section Airfield runways are constructed in widths up to 500 feet. The width 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 bombers. Taxiway widths are variable, ranging between 20 and 100 feet, depending upon the class of airport and are typically 75 feet wide.

14. Roadway and Airport Cross Section Crown in Roadway and Airport Cross Section Runways are nearly always crowned, whereas highways pavements may or may not be crowned. In some cases it is more economical to build highway pavements 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.

15. Thickened Pavement Sections Pavements with thickened edges are used in some situations to accommodate high stresses that exist at the pavement edge. Thickened edge pavements are more costly than uniform pavements, because of the grading operations that are required at the thickened edge.

16. Thickened Pavement Sections Thickened Highway Pavement Section

17. Thickened Pavement Sections Thickened Highway Pavements The use of the thickened edge highway pavement was popular at the time when pavement widths were in neighborhood of 18 to 20 feet and traffic traveled very close to the pavement edge. On wider pavements, however, traffic concentrations is between 3 and 4 feet from the pavement edge, alleviating the necessity for using a thickened edge.

18. Thickened Pavement Sections Thickened Runway End Pavement Section (Longitudinal)

19. Thickened Pavement Sections Thickened Runway Pavement Section (keel section)

20. Thickened Pavement Sections Thickened Runway Pavements 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. 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 ft. A “keel” section is a thickened center used on airport pavements.

21. Thickened Pavement Sections Triangular runway system showing location of strengthened pavements runway ends, taxiways, and aprons are designed for greater thickness than interior of runways.

22. Highway and Airport Pavements Compared The major differences between highway and airfield pavements are repetition of load, distribution of traffic, and geometry of the pavement. In turn, each of the these is effected by pavement width and type of aircraft.

23. Highway and Airport Pavements Compared For a given wheel load and a given tire pressure, highway pavements are thicker than airfield pavements, because repetition of load on highway is much 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.

24. Wheel Loads Types of airplane and truck-wheel arrangements can be divided into several basic categories, including Single and dual wheels Single and tandem axles Nose wheels, tricycle, and bicycle landing gears. Truck and airplane wheels may be arranged in several combinations of these listed above.

25. Wheel Loads Plan view of several basic types of wheel configuration (single trailer-truck unit)

26. Wheel Loads Plan view of several basic types of wheel configuration (tricycle landing gear with single tires)

27. Wheel Loads Plan view of several basic types of wheel configuration (twin tandem landing gear)

28. Wheel Loads Plan view of several basic types of wheel configuration (double twin-tandem gear)

29. Wheel Loads For highways the legal axle load in most states ranges between 18,000 and 20,000 pounds, which implies that a load on one set of dual tires will be one half of the axle load. Thus, if greater loads are required it is common to add a tandem axle.

30. Wheel Loads Large modern-day 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.

31. Wheel Loads Twin Tandem gear used on many large aircraft. Boeing gear(a) Nose wheel (b) twin-tandem main gear

32. Wheel Loads In the design of airport pavements, the design wheel load may be that of the largest plane which will use the field. The condition of takeoff governs thickness design of airport pavements since under this condition the load is greatest due to fuel weight.

33. Wheel Loads On the other hand, the length of runways may or may not be determined on the basis of takeoff conditions depending on number of factors. Runway lengths are determined on the basis of aircraft characteristics as well as temperature, altitude, and so on, at the site.

34. Wheel Loads Allowable axle loads for highways vary from state to state. The majority of the states permit single axle loads of 18,000 lbs and maximum tandem-axle loads of 32,000 lbs. Tandem spacing's ranges between 40 and 48 inches. Tire pressures are controlled generally by allowable load per inch of width of tire. Gross weights are quite variable from state to state and may be calculated utilizing the formula.

35. Tire Pressure, 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, however, 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.

36. Tire Pressure, Contact Pressures and Tire Imprint In the majority of the problems, circular tire imprints are assumed. Hence the radius of contact is as follows: a= radius of contact P= total load on the tire p= tire pressure (assumed to be equal to contact pressure)

37. Tire Pressure, Contact Pressures and Tire Imprint For some cases tire imprints as illustrated on the figure are used. The relationship between pressure and geometry of the imprint is as shown in the figure.

38. Design Factors Pavement design consists of two broad categories Design of the paving mixtures Structural design of pavement components

39. Types of Distress, Structural and Functional Two different types of failures: Structural Failure: includes a collapse of the pavement structure or a breakdown of the one or more pavement components of such magnitude to make the pavement incapable of sustaining the loads imposed upon its surface. Functional Failure: may or may not be accompanied by structural failure but it such that the pavement will not carry out its intended function without causing discomfort to passengers or without causing high stresses in the plane or vehicle that passes over it, due to roughness.

40. Serviceability The Present Serviceability Index (PSI): The PSI 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.

41. Serviceability The Present Serviceability Index (PSI): These objective measurements include a measure of roughness index, extent of cracking and patching, and for the flexible pavements, the average rut depth in the wheel tracks. The important point here is that an estimation of serviceability can be made by making the objective measurements, and then, through correlation equations, calculations 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.

42. Serviceability The Present Serviceability Index (PSI): Serviceability can, thus, be determined solely through the use of pavement roughness measurements with a high degree of accuracy.

43. The Design Process, Design Strategies

44. The Design Process, Design Strategies Figure 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.

45. The Design Process, Design Strategies At year y 1 , the road may have major maintenance applied to it, such as resurfacing and the serviceability then is again at its initial value. As traffic progresses the serviceability again drops to year y 2 and this process is continued throughout the life of the pavement.

46. The Design Process, Design Strategies

47. The Design Process, Design Strategies Figure illustrates that the design process for pavements is not an exact one, and is dependent upon many factors. Figure a 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 anticipated over the design period. If the data are converted to years of traffic, as shown in Figure b, the lines take the general shape indicated on the graph.

48. The Design Process, Design Strategies During the design process, the designer has open to him several options relative to the initial design he might propose as suggested in Figure c. Referring to the upper curve of this graph, the dashed lines indicate that the initial design would carry the pavement through year y 1 and that the initial design thickness would be t 1 .

49. The Design Process, Design Strategies At this interval in time, a resurface would be applied that would carry the road to y 2 , at which a second resurface would be applied to take the road to interval of time equal to y 3 . Another alternate that the design engineer might select would be to make the initial thickness equal to t 2 , which would take the road to y 2 years before major maintenance would be required.

50. The Design Process, Design Strategies It must be clearly understood at the outset that the design decision relative to the life that might be expected from the pavement is a tradeoff decision, wherein the engineer balances increased maintenance costs with increased initial costs, depending upon the staging he might select for his design. The matter of costing the pavement is demonstrated in diagrammatic form in Figure d.

51. The Design Process, Design Strategies If an initial design is to be minimal (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.

52. The Design Process, Design Strategies Hence it is seen that the decision-making process includes, in part, balancing the total cost as illustrated in the upper curve of Figure d 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 applied.

53. Pavement Performance and Theory Historically, pavement design has been approached from two broad, differing points of view. First, the practicing engineer often approaches the problem solely from the standpoint of pavement performance. In contrast, researchers and educators approach the problem largely from theoretical concepts. Neither of the above approaches is satisfactory within itself. Complete reliance upon pavement performance represents a static condition wherein one must wait a relatively long period of time before new concepts can be proven out.

54. Pavement Performance and Theory 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 information and to be able to use materials at hand in a wise manner.