Design of Rigid Pavements

In this blog we are going to read about the design of rigid pavements. This include the satisfactory definition, advantages, disadvantages and failure in case of rigid pavements.

What Is Pavement?

A road surface, or pavement, is the durable surface material laid down on an area intended to sustain vehicular traffic and loads. Such a pavement has enough plasticity to absorb shock. Generally there are two types of pavements namely, flexible pavement and rigid pavement. In this we are going to read about the rigid pavement. So let’s get ahead of learning.

Rigid Pavements:-

Rigid pavements are those which possess noteworthy flexural strength or flexural rigidity. The rigid pavements are made of Portland cement concrete- either plain, reinforced or pre-stressed concrete. The stresses are not transferred from grain to grain to the lower layers as in the case of flexible pavement layers.

The plain cement concrete slabs are expected to take up about 40kg/cm^2 flexural stress. The main point of difference in the structural behavior of rigid pavement as compared to the flexible pavement is that the critical condition of stress in the rigid pavement is the maximum flexural stress occurring in the slab due to wheel load and the temperature changes whereas in the flexible pavement it is the distribution of compressive stress.

The rigid pavement has the slab action and is capable of transmitting the wheel load stresses through a wider area below. As the rigid pavement slab has tensile strength, tensile stresses are developed due to the bending of the slab under wheel load and temperature variations. The rigid pavement does not get deformed to the shape of the lower surface as it can bridge the minor variations of lower layer.

The cement concrete pavement slab can very well serve as a wearing surface as well as an effective base course. Therefore usually the rigid pavement structure consists of a cement concrete slab, below which a granular base or sub-base course may be provided. Though the cement concrete slab can also be laid directly over the soil sub-grade, this is not preferred particularly when the sub-grade consists of fine grained soil. The rigid pavements are usually designed and the stresses are analysed using the elastic theory, assuming the pavement as an elastic plate resting over an elastic or viscous foundation.

Advantages Of Rigid Pavements:-

Advantages of rigid pavement are as follows-

  1. When the surface fails due to various reasons the damaged road forms a very economical base for a new coat.
  2. Wear and tear of tyres as well as mechanical breakdown is less thereby reducing the operational cost.
  3. Smooth riding surface and low maintenance cost. Being dust free, cleaning is easy.
  4. Low tractive resistance along with high night visibility along with factors mentioned in 2 and 3, above results in low transportation cost.

Disadvantages of rigid pavements:-

Disadvantages of rigid pavements are as follows, which restricts its use in only important road.

  1. Providing and maintenance of the joints is a great disadvantages as compared to other pavements.
  2. Though the smooth surface of the concrete highway is a definite advantage so far as the operating cost is concerned the glare on the eye of the driver is objectionable.
  3. As compared to other pavements the initial cost of construction long with skilled Labour and supervisors result in comparatively high cost of construction.
  4. Comparatively long period of construction along with the curing time of about four weeks during which the road has to be closed to traffic, considerably delay is caused in the opening of the road.

In the case of initial construction of the road the question of closing the road for traffic however, does not arise.

Failure of cement concrete pavements: –

Failure of concrete pavements are recognized mainly by the formation of structural cracking. The failures are mainly due to two factors –

  1. Structural inadequacy of pavement system
  2. Deficiency of pavement materials.

Along with the above mentioned failures there are some general causes of failures, which we will going to read after the above mentioned reasons.

So let us now see the reasons –

1.     Structural inadequacy of pavement system-

Inadequate sub-grade support or pavement thickness would be a major cause of developing structural cracking in pavements. Following are the causes and types of failure which develop-

  • Inadequate sub-grade support and poor sub-grade soil
  • Inadequate pavement thickness
  • Incorrect spacing of joints.

Above give rise to the failures of the following types-

  • Widening of joints
  • Cracking of slab corners
  • Mud pumping
  • Cracking of pavement longitudinally
  • Settlements of slabs.

2.     Deficiency of pavement materials –

Following are the chief causes which would give rise to the different defects or failures of cement concrete pavements-

  • Poor surface finish
  • Poor workmanship in joint construction
  • Soft aggregates
  • Improper and insufficient curing
  • Poor joint filler and sealer material.

The various defects that creep in due to the above are-

  • Spalling of joints
  • Slippery surface
  • Ingress of surface water and further progressive failures
  • Formation of shrinkage cracks
  • Disintegration of cement concrete
  • Poor riding surface
  • Formation of cracking.

Various typical and basic types of failure in rigid pavements are as follows-

  • Warping Cracks- if the joints are not well designed to accommodate the warping of slabs at edges, this results in development of excessive stresses due to warping and the slab develops cracking at the edges in an irregular pattern. There is no structural defect due to warping cracks if proper reinforcement is provided at the longitudinal and transverse joints as it takes care of the structural adequacy. Generally, hinge joints are provided for relieving the slabs of warping stresses.
  • Shrinkage Cracks – the shrinkage cracks are normally developed during the curing operation of cement concrete pavement immediately after the construction. The placement of cracks are in longitudinal as well as in the transverse directions.
  • Spalling of Joints- sometimes when pre formed filler materials are placed during casting of pavement slabs, the placement is somehow dislocated and filler is thus placed at an angle. The concreting is completed without noticing this faulty alignment of the material. Thus this forms an overhang of a concrete layer on the top side and the joint later on shows excessive cracking and subsidence.
  • Scaling of Cement Concrete- in cement concrete pavement scaling is observed by showing overall deterioration of the concrete. The scaling is mainly attributed due to the deficiency in the mix or presence of some chemical impurities which damage the mix. Further due to excessive vibration given to mix, the cement mortar comes to the top during construction and thus with the use, the cement mortar gets abraded exposing the aggregate of the mix. This makes the pavement surface rough and shabby in appearance.
  • Mud Pumping- when the soil slurry ejects out through the joints and cracks of cement concrete pavement caused during the downward movement of slab under the heavy wheel loads then mud pumping is recognized.

That’s all about the rigid pavements and the facts related to it. Hope you find it useful.

Do share in the comments what you think of this, and any advises are welcomed.

Happy reading, happy learning!

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