CHAPTER!

INTRODUCTION

1.1. Back Ground

A major portion of road network in Sri Lanka is in rural areas. It is about 60% of the

total road network in Sri Lanka. Most of the rural roads are macadam bases (R.

Shanthini, 2006). Design life of a well-constructed macadam base is about 10 - 15

years (Fwa, 2006). Poor design, construction and maintenance can lead to many

distresses in macadam roads during the predicted design life. Quality of the

aggregate, bitumen and construction procedure also affect the reduction of the life of

macadam roads in rural areas.

Bitumen is a byproduct of petroleum refineries. Therefore the cost of bitumen has

also increased together with the escalation of crude oil. It is one of the indications

that the bitumen may not be the main construction material for roads in future.

Common practice in bitumen road construction is to heat up the necessary amounts

of bitumen at the site, which leads to heavy air pollution due to its high emission of

blackish smoke. Therefore, the use of bitumen for road construction may leads

environmental pollution, thus it is convenient to use an environment friendly road

construction material with higher life time and lessermaintenance cost.

During the last decennium has seen that an intensive process of road construction in

Sri Lanka. The government encourages construction of concrete roads in rural areas.

Major reason for this is that the concrete roads demands less equipments to

construction and those can be met within the rural areas itself. It does not need

skilled labours for concreting. Therefore, the local contractors in the village can

handle the projects with the help of villagers. However, this has lead to poor

performance of the recreantly constructed concrete roads. The main reason for poor

quality construction is the lack of basic knowledge of the construction on concrete

roads.

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Concrete cannot be deformed as flexible road paving materials. Concrete has lower

tensile and flexural strengths. Therefore, concrete roads have to be designed in such a

way that the induced flexural stresses would not exceed the threshold limits for

designed traffic. Rigid pavements should have proper base support to reduce the

flexural stresses, induced due to vehicular movements. A uniform layer of concrete

throughout the road trace is essential to reduce the distresses. This regularity cannot

be achieved at the joint locations. Therefore, the rigid pavements can be identified as

a set of slab panels. When the connectivity fails at a joint location, load of the wheel

at the joint has to be carried by one of the slabs. This leads to many irregularities and

distresses in the pavements. Therefore, a good load transfer should be maintained at

the joints. This timber piece will not contribute to the load transfer between the slab

panels which can leads to edge failures.

In Sri Lanka, rural road contractors are used to pave the concrete as panels along the

road trace. They are used to separate the slabs by a piece of timber in transverse

direction, which is generally fixed as the form work and let that timber piece to

remain embedded in concrete when casting the adjoining slab.

Structural strength requirements of low volume rural roads can be satisfied by grade

25 concrete layers with thickness lesser than 6 inches (Gayani, 2010). Dowel bars

insertion is not an efficient way to transfer the loads in such thin pavements.

Aggregate interlock in the joint between adjacent slabs is the ideal mechanism of

load transferring for thin concrete slabs in rural roads (ACPA, IS405.01P).

Interlocking of aggregates exits at a cracked concrete section when the two parts are

close to each other. Concrete will crack due to drying shrinkage and thermal

movements and it s possible to form a crack a predetermined location by making the

concrete section weaker at that location. Shrinkage is the main disadvantage of

concrete pavement over the flexible pavements. Shrinkage cannot be completely

prevented in any concrete construction. It will govern the performance of rigid

pavements as they expose to severe environmental effects throughout its life span.

Therefore, the shrinkage cracks can occur transversely through the slab width.

Although these transverse cracks cannot be eliminated, they can be effectively used

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to transfer load among slabs. In order to achieve this, the width of the propagated

crack should be limited to accommodate proper aggregate interlock.

Ultimate shrinkage strain, daily length variation and the curling of the concrete slabs

due to temperature variation affect the crack width. Daily variation of the concrete

temperature governs the expansion and curling of concrete slab. In tropical countries,

there are seasonal differences in monthly rainfall but temperatures mostly stay the

same. Therefore, daily temperature variation is severe than seasonal variation of the

climate (Brynn, 2010). Therefore, the concrete pavements are subjected to

continuous curling and warping during 24 hour period as a fatigue load. It reduces

the connectivity of adjacent slab panels. Concrete expands with higher temperatures

and contracts with lower temperatures (Neville, 1996). Therefore, width of a

transverse crack will be wider at the maximum curling time and maximum

contraction. These factors should be considered with the design parameters of rigid

pavement joints for rural roads in Sri Lanka.

There is no specification for rigid pavement design for Sri Lanka. Therefore, an

alternative load transfer mechanism such as aggregate interlock has not been

considered in concrete road construction. Procedure of making a saw cut, proper time

for the saw cut and alternative methods of making a weaker plane has to be described

and adapted by local designers and contractors, in order to introduce the aggregate

interlocking mechanism.

1.2. Objectives

The main objective of the research to determine the optimum joint spacing for the

rigid pavements suitable for rural roads in Sri Lanka

Following methodology was adopted to achieve the main objective of this study.

1. Literature survey of rigid pavement design and construction

2. An experimental investigation to study the temperature variation in a concrete

slab to obtain the daily contraction of a pavement slab due to daily

temperature variation

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3. Development of finite element model to incorporate the thermal variation in a

concrete slab to obtain daily thermal deformation

4. An experimental investigation on variation of load transfer efficiency with

the crack width

5. Estimation of the ultimate shrinkage strain in a concrete pavement based on

available literature

6. Determination of the joint spacing based on the experimental investigation on

load transfer efficiency and thermal and shrinkage effects of concrete

1.3. Significance of the Research

Sri Lankan road construction industry has not adopted good practice in rigid

pavement construction and the constructed roads are in a poor state due to many

structural issues (Priyantha, 2010).Technical issues lead to early propagation of

distresses and gradually reduce the life span of the pavements.

Local contractors in many areas are used timber pieces in-between adjacent slab

panels of the pavements to separate them. This makes full separation and total loss of

load transfer. The timber piece will decay in 1 - 2 years and then the joint will

become a gap of about 10 - 15 mm width. Such a wide gap will obviously cause the

joint faulting. Sand and base material will pump into the surface through the joint

with repeated loading at rainy weather and make a void underneath the joint. This

has been one of the reasons of having distresses in rigid pavements in rural areas.

There are no proven guideline and specification for the joint construction in rigid

pavements in Sri Lanka. This research will produce a reliable guideline to design

joints to reduce deterioration against the traffic load. The research describes how to

initiate the transverse crack at a required location while providing a method to ,"

predict the location of crack according to the slab thickness. This will help to use

shrinkage cracks effectively for the load transfer requirement.

Finite element model using nonlinear analysis software ANSYS, was used to

simulate the thermal behavior of concrete exposed to solar radiation. The FEM was

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used to estimate the effect of curling and warping in concrete pavements throughout

the day.

There are very few researches carried out about joint parameters of rigid pavements.

There was no such research found in Sri Lanka about load transfer efficiency through

the induced crack or a joint. This research describes the relationship between crack

width and load transfer efficiency. The outcome of the research is to determine

suitable joint spacing incorporating the requirements of load transfer efficiency and

contraction of concrete due to moisture movement and temperature variation.

1.4. Scope of the Report

This thesis includes all the work performed under this study.

Fist chapter provides an introduction including importance of rigid pavements, load

transfer technique and the objectives of the research.

Second chapter covers literature survey on historical background of rigid pavements,

the stresses developed due to rigidity, thermal behavior due to diurnal and seasonal

climate variations and load transfer mechanisms in rigid pavements.

Third chapter explains the development oflaboratory scale experiment and 3-D finite

element model to simulate the diurnal thermal behavior ofconcrete pavement.

The fourth chapter describes the laboratory scale experimental to obtain the

relationship between the crack width and the load transfer efficiency.

Fifth chapter presents estimation of the shrinkage of concrete and estimation of

suitable joint spacing.

Chapter six presents the conclusion and recommendations.

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