08-02-2010, 06:38 AM
IMPROVEMENT OF MECHANICAL PROPERTIES OF RIGID PAVEMENTS
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INTRODUCTION
Transportation-basic infrastructure-development
Rigid and flexible
Basic neccesity-stable against loads
Rigid offers more strength “load
But change in temp. “warping up and warping down
To relieve stress-joint
Joint from the weaker plane and most of defects occur there
Increase the mechanical properties such as load carrying capacity and cracking resistance
Methods for improvement
Rigid Pavement
Consist of subgrade, (base course) and Cement concrete slab
It resists minor deformations
Got more strength and life than flexible pavements
JPCP
most common type
control cracks by dividing pavement into individual slabs seperated by contraction joints
3.7m wide and 6.1m long
Use of dowel bar
JRCP
Control cracks by dividing pavement into individual slabs seperated by contraction joints but longer than JPCP
Uses reinforcing steel
CRCP
Does not require any contraction joints
Use reinforcing steel for crack control
JOINTS IN RIGIDPAVEMENTS
TRANSEVERSE JOINTS
Expansion joints
Contraction joints
Warping joints
Construction joints
EXPANSION JOINTS
Provided to allow for the expansion of the slabs
Also permit the contraction of slabs
Provide at interval of 50-60m in winter and 90-120m in summer
In rough interface spacing-140m
The approximate gap width of joints 20-25mm
CONTRACTION JOINTS
Provided to permit the contraction of the slabs
Load transference at the joint is provided through the physical interlocking by aggregates projecting out at the joint faces
Max.Spacing for unreinforced concrete slab-4.5m and for reinforced concrete slab of thick-20cm-14m
LONGITUDINAL JOINTS
Provide in cement concrete roads
Provide to prevent the longitudinal cracking
Act as hinge to maintain the two slabs together-at the same level
WARPING CRACKS
If joints are not well designed-accommodate warping of slabs at edges - results “ development of excessive stresses and cracking at the edges in an irregular pattern
No structural defect due to the warping cracks if proper reinforcement is provided at the longitudinal & transverse joint as it take care the structural adequacy
BLOWUP/BUCKLING
Occur in hot weather
At a transverse crack or joint
Insufficient width
Caused by infiltration of incompressible material into the joint space
A localized upward movement of the slab edges(buckling) or shattering will occur in the vicinity of the joint
DOWEL BARS
Short mild steel bars
Provides mechanical connection between slabs without resisting horizontal joint movement
Half length is bounded in one cement concrete slab and remaining in other.
Keep free for the movement during expansion and contraction of the slabs
32 -38 mm diameter, 460 mm long and spaced 305 mm apart
REINFORCING STEEL
Used to provide load transfer
Transverse and Contraction joints are often omitted
Since there are no joints the PCC cracks on its own and the reinforcing steel provides load transfer across these cracks
Unlike Dowel Bars- it is bounded to the PCC on either side of the crack in order to hold the crack tightly together
TIE BAR
It is used across the longitudinal joint of cement concrete pavement
It ensure two adjacent slab to remain firmly together
These are not designed to act as load transfer device
Max. tensile force equal to the force required to over come frictional force between the bottom of the adjoining pavement slab and the soil sub grade
Force is considered from joint location to the subsequent joint or free edge
AGGREGATE SELECTION
Research was conducted-three different aggregate types of lime stone, river gravel and trap rock and two different sizes of 25mm and 38mm
Fracture energy represents the cracking resistance, Power Spectral Area Parameter(PSAP)
represents concrete surface roughness and indicates its ability to transfer load across a cra
USES OF FIBERS IN CONCRETE PAVEMENTS
Fibers added in sufficient volume to plain concrete improve properties, fracture toughness and crack width control
Fiber used improve cracking performance of concrete pavements, reduce required slab thickness and increase the allowable joint spacing
CONCLUSION
Concrete pavements are preferred now a days due to its durability and good riding surface.
Inorder to improve the load transfer capacity several methods can be adopted. Conclusion drawn from present study are:
Load transfer across transverse joint can be carried out through dowel bars .
Tie bars help to hold the adjacent slabs together in the case of longitudinal joint.