PAVEMENTS

                                          PAVEMENTS

Pavement is a durable surface material laid down on an area intended to sustain vehicular or foot traffic, such as road or walkway. The two types of pavements are flexible and rigid pavements. Flexible pavements are pavements whose top layers is a bituminous surface usually underlain with a layer of granular material and a layer of suitable mixture of coarse and fine materials. This pavement consists of the sub-grade, sub-base, road base and surfacing. Rigid pavements are pavements in which the deformation of the sub grade does not reflect on the surface, this is mainly due to the fact that the load is distributed over a wide area of sub grade soil.

 

INTRODUCTION

Pavement Types

There are two (2) broad categories of road way pavements namely flexible and rigid pavements

All the pavements for earth, dry bond macadam, WBM, stabilized soil and bituminous surface roads fall under the category of flexible pavements. While cement concrete roads constructed to behave as a slab or cantilever are known as rigid pavement roads. These might be reinforced or mass concrete pavement.

PAVEMENT STRUCTURE AND FUNCTION

FLEXIBLE

 This type of pavement function by gradual dispersal of traffic load over a considerably large area thus the strength requirement of the sub­­-grade is less than that of the upper layers. Flexible pavement is composed of a series of granular layers topped by a relatively thin high quality bituminous wearing surface. It is a structure that maintains and distribute load to the sub-grade and it depends on aggregate interlock, particle friction and cohesion for stability. Layers of flexible pavement comprises of;

a.     Wearing surface: This ranges from 25mm bituminous surface treatment for low-cost, light traffic road to asphalt concrete for heavily travelled roads. Provides smooth riding surface, resist wear and abrasion due to moving vehicles. Provide water proof surface and protect the base and sub-grade from surface water.

b.     Base: Layer of very high stability and density. It distributes stresses due to wheel load on wearing surface and transmits mineral stresses on sub-grade in order to prevent excessive deformation of foundation. Must be capable of resisting effect of capillary water and/or frost action.

c.      Sub-base: Used in cases of weak sub-grade or as a construction table. It is used to build up the pavement strength economically above that provided by the sub-grade soil.

d.     Sub-grade: Serves as foundation of the road, support all load applied on pavement structure. Usually natural earth surface, compacted soil from cut section or upper of embankment.

RIGID PAVEMENTS

These have sufficient rigidity and have high modulus of elasticity. Thus a small thickness to distribute the wheel loads upon the sub-grade and equalizes (bridge) minor irregularities in the sub­-grade support due to flexural strength and load transfer capacity in sheer. It is normally laid directly on the sub-grades.

The fundamental difference between flexible and rigid pavement is in structural composition.

Preparation and construction of a road base

The natural soil upon which the road would be built serves as the foundation of the road. This is the typical sub-grade of the pavement structure.

Specifically, for flexible and rigid pavements the road base upon which the surface course is built comprises the base and sub-base layer over a well prepared sub-grade.

In flexible pavement construction, the different layers that made up the base are constructed stages. The sub-grade is compacted for strength or stabilized using admixtures where necessary. The base is built using material hauled to site usually granular, compacted properly in layers to the specified thickness.

In cases where the sub-grade soil is entirely suited as a road base, only proper compaction and drainage are necessary before building the concrete wearing surface. The base constructed should provide uniform foundation support for the roadway and free from deleterious substances.

Soil of low shearing strength, high volume change, organic and alkali soils should not be used for foundation of road pavements. The road base is finally brought to shape and dimension by the use of suitable fine grading machines.

Borrow Pits

Materials that are needed for the formation of roadway alignment are not usually obtained from the excavation within the limits of  R.O.W of the highway. Thus additional full materials are required to complete the full operation and are therefore obtained on mass off site. The sources of such materials are called Borrow pits. Standard specification for the kinds of borrow materials and the source (borrow pits) for major highway construction are indicated in design. This is to regulate excavation of materials and their incorporation into the road structure.

Joints in rigid pavement

The cross-section of a rigid roadway compress a pavement superimposed upon the sub-grade and most usually this pavement is composed of cement concrete slab on top of sub-grade. This slab is composed of pavement quality concrete with considerable rigidity and high modulus of elasticity which enables it to act as a beam and bridge over any localized or minor irregularities in the surface of the layers beneath cement concrete pavement is ideal as traffic surface with relatively small thickness, it distributes wheel load upon sub-grade. It provides a good riding surface. Maintenance costs are comparatively low and it is easy to clean and it is dust free. The vehicle operation cost on cement concrete road is minimum and wear and tear and mechanical breakdown of vehicle is comparatively less. It has a high salvage value as a base for anew wearing course.

 Pavements are exposed to severe temperature and moisture conditions as well as wheel load influence which produce severe stresses on cement concrete pavement. Among these is bending or deflection under wheel loads, warping of the slab due to difference of the top and bottom of the pavement, which lead to the slab being raised off the sub-grade which the self weight of the slab might not be able to take.

Warping of the slab due to difference in moisture content at top and bottom of the slab and tension produced as contraction during falling of temperature or while drying out is restricted by sliding friction between the slab and the sub- grade. Compression failures and buckling or blow up also occur because of the expansion of slab.

To prevent progressive cracking up of the pavement and for control of the expansion and contraction of the slab , longitudinal and transverse joints are provided..

These joints are;-

·        Expansion joints

·        Contraction joints

·        Construction joints

Expansion joints; - These are usually placed transversely at regular intervals to provide adequate space for slab to expand due to temperature changes. The approximate width for this type of joint is provided from 2 to 2.5cm. These joints are placed across the full width of the slab and are 19mm to 25mm wide. The joint space is filled with a compressible filler material that permits the slab to expand. Filler material can be cork, rubber or bitumen material. A means of transferring the load across the joint space is provided as there are no aggregate that will develop as interlocking mechanism. This means is usually dowel bars which are embedded and kept fixed in concrete at one end and the other is kept free to expand. At the free end a metal cap is provided to offer a space of about 2.5cm for movement during expansion.

Contraction joints; - these are the most commonly used transverse joint in jointed concrete pavement. They are placed transversely at regular intervals (but spaced closer than the expansion depending on the type aggregate use end type of sub grade) across the width of the pavement to release some of the tensile stresses that are induced due to contraction of the slab when its temperature falls. The load transfer is affected through the physical interlocking of the aggregate projecting out the joints. It may be necessary to install a load transfer mechanism, where there is doubt about the ability of the interlocking gains to transfer the load. When a load transfer mechanism is used, no receiving cap is provided; each bar is covered with a plastic sheath for two-third of its length so that when a slab contracts, the free end of the dowel leaves a gap into which it can return when the slab expands to its original length.

Construction joints; - These are those other than expansion and contraction joints that are formed when construction work is unexpectedly interrupted e.g. by mechanical breakdown or bad weather, at points where joints are not normally required by the design. When the full length of a pavement is not laid in one concreting operation, a formal constitutional construction joints has to be established between the two abutting slabs. These slabs are tied with tie bars in order to develop proper bond between the new and old concrete.

 

REFERENCE

Naiman S.P. & P.A. Witherspoon, highway construction, civil engineering construction III,    V, 8, pp. 1284 – 1298, 1972

UNESCO Nigeria technical and vocational education. Revitalisation project

phase II, Nigeria, 2006