Lec 17 Highway Engineering - Rigid Pavemen Design

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Highway Eng. Design of Rigid Pavements 14 15

Dr. Firas Asad

In this lecture;

A-General

B-Types of Rigid Pavements

C-Joints in PCC pavements

D-Design of PCC Pavements

- AASHTO 1993 Method.

Structural Design of Rigid Pavements

Information listed in this lecture is mainly taken from AASHTO Guide for Design of

Pavement Structures (AASHTO, 1993), Traffic and Highway Engineering (Garber,

2009), Asphalt Pavements (Lavin, 2003), Pavement Analysis and Design (Huang,

2004), http://www.pavementinteractive.org (Accessed on 2015) and Highways

(OFlaherty, 2007).

A- General

Rigid highway pavements are normally constructed of Portland cement concrete

(PPC) and may or may not have a base course between the subgrade and the

concrete surface. When a base course is used in rigid pavement construction, it is

usually referred to as a subbase course.
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B- Types of Rigid Pavements

Rigid highway pavements can be divided into three general types: plain concrete

pavements, simply reinforced concrete pavements, and continuously reinforced

concrete pavements. The definition of each pavement type is related to the amount

of reinforcement used.

B-1 Jointed Plain Concrete Pavement (JPCP)

Plain concrete pavement has no temperature steel or dowels for load transfer.

However, steel tie bars often are used to provide a hinge effect at longitudinal joints

and to prevent the opening of these joints. Plain concrete pavements are used

mainly on low-volume highways or when cement-stabilized soils are used as

subbase. Joints are placed at relatively shorter distances (10 to 20 ft) than with

other types of concrete pavements to reduce the amount of cracking.

B-2 Simply Reinforced Concrete Pavement

Simply reinforced concrete pavements have dowels for the transfer of traffic loadsacross joints, with these joints spaced at larger distances, ranging from 30 to 100 ft.

Temperature steel is used throughout the slab, with the amount dependent on the

length of the slab. Tie bars also are used commonly at longitudinal joints.

B-3 Continuously Reinforced Concrete Pavement (CRCP)

Continuously reinforced concrete pavements have no transverse joints, except

construction joints or expansion joints when they are necessary at specific positions,

such as at bridges. These pavements have a relatively high percentage of steel, with

the minimum usually at 0.6 percent of the cross section of the slab. They also

contain tie bars across the longitudinal joints.


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C- Joints in Concrete Pavements

Different types of joints are placed in concrete pavements to limit the stresses

induced by temperature changes and to facilitate proper bonding of two adjacent

sections of pavement. These joints can be divided into four basic categories:


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D-Structural Design of PCC Pavements

The main objective in rigid pavement design is to determine the thickness of the

concrete slab that will be adequate to carry the projected traffic load for the design

period.

D-1 Methods of Design

Several design methods have been developed over the years, some of which are

based on the results of full-scale road tests, others on theoretical development of

stresses on layered systems, and others on the combination of the results of tests

and theoretical development. However, two methods are used extensively: the

AASHTO and American Concrete Pavement (PCA) methods. In this lecture only the

1993 AASHTO method will be adopted.

1993 AASHTO Empirical Design Method for Rigid Pavement

The AASHTO design procedure provides for the determination of the pavement

thickness and the amount of steel reinforcement when used, as well as the design ofjoints. It is suitable for plain concrete, simply reinforced concrete, and continuously

reinforced concrete pavements.

Design Considerations

The factors considered in the AASHTO procedure for the design of rigid pavements

as presented in the 1993 guide are

Pavement performance

Subgrade strength

Subbase strength

Traffic

Concrete properties


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Drainage

Reliability

Pavement Performance.Pavement performance is considered in the same way as

for flexible pavement. The initial serviceability index (Pi) may be taken as 4.5, and

the terminal serviceability index may also be selected by the designer.

Subbase Strength. Subbase thickness is usually not less than 6 in. and should be

extended 1 to 3 ft outside the edge of the pavement structure.

Subgrade Strength. The strength of the subgrade is given in terms of the

Westergaard modulus of subgrade reaction k, which is defined as the load in lb/in2

on a loaded area, divided by the deformation in inches. Values of k can be obtained

by conducting a plate-bearing test in accordance with the AASHTO Test Designation

T222 using a 30 in. diameter plate. Estimates of k values can also be made either

from experience or by correlating with other tests.

Traffic. The treatment of traffic load is similar to that presented for flexible

pavements, in that the traffic load application is given in terms of the number of

18,000 lb equivalent single-axle loads (ESALs). ESAL factors depend on the slab

thickness and the terminal serviceability index of the pavement.

Tables below give ESAL factors for rigid pavements with a terminal serviceability

index of 2.5. Since the ESAL factor depends on the thickness of the slab, it is

therefore necessary to assume the thickness of the slab at the start of the

computation. This assumed value is used to compute the number of accumulated

ESALs, which in turn is used to compute the required thickness. If the computed

thickness is significantly different from the assumed thickness, the accumulated

ESAL should be recomputed. This procedure should be repeated until the assumed

and computed thicknesses are approximately the same.


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Reinforcing Steel

Steel reinforcing may be used in concrete pavements to reduce the amount of

cracking that occurs, as a load transfer mechanism at joints, or as a means of tying

two slabs together. Steel reinforcement used to control cracking is usually referred

to as temperature steel, whereas steel rods used as load transfer mechanisms are

known as dowel bars, and those used to connect two slabs together are known as

tie bars.


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Concrete Properties. The concrete property is given in terms of its flexural strength

(modulus of rupture) at 28 days. The flexural strength at 28 days of the concrete to

be used in construction should be determined by conducting a three-point loading

test as specified in AASHTO Designation T97.

Drainage. The drainage quality of the pavement is considered by introducing a

factor (Cd) into the performance equation. This factor depends on the quality of the

drainage - as described in the design of Flexible Pavement- and the percent of time

the pavement structure is exposed to moisture levels approaching saturation. The

Table below gives AASHTO-recommended values for Cd.

Reliability. Reliability considerations for rigid pavement are similar to those for

flexible pavement as presented in the previous lecture. Reliability levels, R%, and

the overall standard deviation, So, are incorporated directly in the design charts.


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The design equation above can be solved for the thickness of the pavement (D) in

inches by using either a computer program or the two design charts in the next

pages. The use of a computer program facilitates the iteration necessary, since D has

to be assumed to determine the effective modulus of subgrade reaction and the

ESAL factors used in the design.


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Lec 17 Highway Engineering - Rigid Pavemen Design

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