This document provides an outline for a course on pavements and foundations. It discusses various topics related to pavement design including types of pavements, design methods, construction, evaluation and rehabilitation. It also lists several reference books on highway and pavement engineering. Different types of pavement structures and materials are described. Methods for designing flexible and rigid pavements considering factors like load distribution and subgrade strength are explained. Design of airport pavements is also addressed.

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
1. Design of the paving mixtures
2. Structural design of pavement components

39. Types of Distress, Structural and
Functional
• Two different types of failures:
1. 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.
2. 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):
1. The PSI is based upon a rating scale that
designates the condition of the pavement at any
instant of time.
2. A rating of 5.0 indicates a “perfect” pavement,
whereas a rating of 0 indicates an “impassible”
pavement.
3. The present serviceability index is determined by a
panel of individuals who rate the pavement on a
rating scale from 0 to 5.0.
4. The index is correlated with objective measurements
made on the pavement surface.

41. Serviceability
• The Present Serviceability Index (PSI):
4. 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.
5. 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.
6. 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):
7. 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 y1, 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 y2 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 y1 and
that the initial design thickness would be t1.

49. The Design Process, Design
Strategies
• At this interval in time, a resurface would be
applied that would carry the road to y2, at which
a second resurface would be applied to take the
road to interval of time equal to y3.
• Another alternate that the design engineer might
select would be to make the initial thickness
equal to t2, which would take the road to y2
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.