RIGID HIGHWAY PAVEMENT

In concrete rigid highway pavements, the strength of the pavement is contributed mainly by the concrete slab, however, in flexible pavements; successive layers of the pavement contribute cumulatively.

Since the modulus of elasticity of the concrete slab is much greater than that of the foundation material, a major portion of the load carrying capacity is derived from the slab it self. the slab has often been referred to as beam action.

Types of rigid pavements

Jointed unreinforced concrete pavements (JUCP)

The pavement consists in a succession of cast in place unreinforced concrete slabs separated by joints to prevent expansion from developing stresses and to control cracks.
The slabs are linked together by tie bars or dowels to transmit the vertical stresses.

Jointed reinforced concrete pavements (JRCP)

The pavement consists in a succession of cast in place reinforced concrete slabs separated by joints to control cracks.
The slabs are linked together by tie bars or dowels to transmit the vertical stresses.
JRCP are used where a probability exists for transverse cracking during pavement life due to such factors as soil movement and/ or
temperature/ moisture change stresses.
The longitudinal reinforcement is the main reinforcement. A transverse reinforcement though not absolutely necessary in most
cases is usually added to facilitate the placing of longitudinal bars.

Continuously reinforced concrete pavements (CRCP):

Are used for rather highly trafficked roads where a good level of comfort is expected.
The principal reinforcement , in the form of prefabricated mesh or reinforcing bars installed at mid-depth of the slab, is again the longitudinal steel which is essentially continuous throughout the length of the pavement.
This longitudinal reinforcement is used to control cracks which form in the pavement due to volume change in the concrete.

Basically, the use of the different types of rigid pavement is as follows:

JUCP is suitable for all levels of traffic, whenever the risk of subgrade movement is low and an uncontrolled cracking not
very prejudicial.
JRCP is suit able for all levels of traffic is used when the risk of settlement s of
and the subgrade can not be neglected.
CRCP shall basically be considered only for rather high design traffic (>30 msa).

The structural cross-section of a rigid pavement

Typical Rigid Pavement structure

The load bearing capacity of the soil subgrade is not important in the design of concrete pavements.
The prime requirement is to provide a foundation on which construction traffic can operate without impairing the shape to
which the surface trimmed.
The capping layer consist s of selected fill and is provided in cases of low strength roadbed
material.

It protects the underlying subgrade from construction traffic loading and provides a stronger platform for the subbase layer,
which is placed on top of the capping layer.
Base courses are often called subbase courses when used with rigid pavements.

These terms are often used interchangeably and it should be noted that either of the terms refers to a prepared layer of material immediately between the concrete slab and subgrade.
The base courses (sub-bases) are utilized under these pavements for several reasons:
■ Control of pumping, at joint s and slab edges.
■ Control of frost action
■ Drainage
■ Control of shrink and swell of the subgrade due to volume changes in road bed soils susceptible to such phenomena
■ To expedite construction.
■ To provide a stable “working platform” for the construction equipment
■ To provide a uniform concrete slab support

If water accumulates in the pavement under a joint, mud pumping is likely to occur as heavy vehicles pass from one slab to the next.
If the subgrade soil is not free draining it is necessary provide to a subbase. should be free from draining and should continue through the road shoulder.

Reinforcement in rigid highway pavements

Dowel bars:

■ Are load transfer devices. They transfer load across transverse joints while checking on movement of the slabs.
■ It is thus necessary that adequate load transfer across joints limits slab deflections to reduce faulting, spalling and corner breaks.
■ Since they are load transfer devices, they must be fairly heavy and spaced at close intervals to provide resistance to bending, shear, and bearing on the concrete.
■ Dowel bars are used at joints on long slabs or where load transfer by “aggregate interlock” is suspect. Joints without dowels are generally
satisfactory if the joint opening is 1 mm or less.

Tie Bars

■ Are used to tie adjacent slabs together along the longitudinal joint tightly so that load transfer across is ensured.
■ Tie-bars at longitudinal joints are provided to hold these joints tightly closed and therefore perform a similar function to steel between transverse joints.
■ The purpose of a tie-bar is to hold reinforcement a joint tightly closed to allow load transfer by aggregate interlock.

Distributed fabric steel reinforcement of wire
■ They are also called bar matts are and are used to control temperature cracking without increasing structural capacity.
■ It is usually placed at the mid depth of the slab and discontinued at joints.
■ The purpose of distributed steel or temperature steel is basically one of crack control. Temperature steel will not prohibit the formation
of cracks but act s as a tie member, which controls the width of the crack opening.
■ It holds tightly closed any cracks that may form, thus maintaining the pavement as integral
structural unit.

Joints in rigid highway pavements

Joint s are placed in concrete pavements, whether reinforced or not to permit expansion and contraction pavement, thereby relieving stresses due to environmental changes (temperature and moisture), friction, and to facilitate construction.

Contraction joints

■ Provided to relieve tensile stresses due to temperature, moisture and friction, therefore the controlling cracking.

Expansion joints

They are transverse and are used to relieve compressive stresses from the expansion of concrete.
They are expensive to maintain and are susceptible to pumping.
The primary function of an expansion joint is to provide space for the expansion of the pavement, thereby preventing the development
of compressive stresses, which cause the pavement to buckle.

Warping joints

Allow a slight relative rotation of the slab portions delimited by joints and reduce the strains due to warping.
Warping joints are also contraction joints.
Longitudinal joints are always warping joints but they can be found also as transverse joints in some cases.

Construction joints

Are required to facilitate construction, especially when concreting is stopped.
For JUCP and JRCP, they shall be coupled with other joints and additional reinforcement shall be placed when dealing with transverse construction joints for CRCP

Failure in rigid highway pavements

These result from:
Traffic loading
Temperature differentials
Moisture
Subgrade movement (volumetric changes)

The principal structural requirements for pavements

The subgrade should be able to sustain traffic loading without excessive deformation; this is controlled by the vertical compressive stress or strain at this level.
Bituminous materials and cement-bound materials used in road-base design should not crack under the influence of traffic; this is controlled by the horizontal tensile stress or strain at the bottom of the bound layer.
The road-base is often the main structural layer of the pavement, required to distribute the applied traffic loading so that the underlying materials are not overstressed. It must be able to sustain the stress and strain generated within itself without excessive or rapid deterioration of any kind. To control fatigue cracking caused by load repetitions.
In pavements containing bituminous materials, the internal deformation of these materials must be limited.
The load spreading ability of granular sub-base and formation layers must be adequate to provide a satisfactory construction platform.