Friday, February 28, 2014

Critical Stress Combination for Rigid Pavements

Hi, How you doing?
As we know that the design of a pavement is governed by the analysis so, this is very important to know the various combinations of stresses that will be induced in the slab. There are two kinds of loading:

  1. Traffic Loading
  2. Temperature variation.
Traffic loading will induce the compressive or tensile stresses in the pavement and temperature variation results in to kinds of stresses. One is known as warping stress and another is frictional stress.

That makes total of three kinds of stresses that will be induced in the pavement slab. Now, we have to find out the critical combination of these three.

  • In Summer Season:
In summer season the average temperature is higher and it results in the overall expansion of the slab. This induces the frictional stresses which will be compressive so as to restrict the expansion. Remember friction will always act in the opposite direction of the movement of the bottom of the slab. That means when it tries to expand, friction will try to contract it or restrict the expansion. That makes it to induce the compressive stress.

In the after mid-noon hours in summer season, the temperature at the top layer will be higher as compare to the bottom layer. That will make the slab to tend to warp in downward direction. Under the self weight of the slab warping will be restricted and therefore will result in the tensile stress in the bottom layer.

 Downward warping results in contraction of the bottom layer which is restricted through the induction of the tensile warping stress. So, warping stress of tensile nature is induced.
Now, at the edge region the traffic load stress of tensile nature is also induced and that is where the critical load combination is formed.

Critical stress combination(At the edge)  = Traffic stress + Warping stress - frictional stress.


  • In Winter Season:
In the after mid-noon hours in winter season again, traffic load stress and the warping stresses will be tensile in the bottom layer. Also, the frictional stresses will be tensile because, there is overall contraction of the pavement due to lower average temperature and therefore friction will act along the tensile direction.
So, the Critical stress combination(At the edge)  = Traffic stress + Warping stress + frictional stress.

At the corners the critical load combination is that of the traffic load and the warping stress at the top layer at the mid-night hours.
See, in the night the temperature at the bottom layers will be higher as compare to the temperature at the top layers so, there will be upward warping. That will induce tensile warping stress at the top layer.
 So, Critical stress combination(At the corner)  = Traffic stress + Warping stress 

Thank you for your time!

Sanjay.



Sand Cone Method to Determine Density - A solved Example

Hi, This problem deals with the Sand Cone Test used to determine the density of the compacted soil. Problem: A sand cone density test w...