Thursday, July 26, 2012

Is excess cover good in insitu RC structures?


Many times, during my site inspections, I have found fault with the Contractors for providing an inadequate cover for the reinforcements in R.C structures.  Obviously inadequate cover for the R.F has to be rectified immediately as less cover causes the larger crack widths in concrete and thereby it exposes the reinforcement to surrounding environment to get corroded. This is an important factor to consider in which bridge structures as it withstand severe environmental conditions.

 Furthermore, an inadequate cover leads to unsatisfactory fire resistant as well as the under developed bond strengths between concrete and R.F also.   Hence there should not be any argument whether inadequate cover is acceptable or not. The clear and loud answer is NO!!!!!
Then what about the other side of the story? In other words,  is provision of  an  extra cover than the specified limits  acceptable? Oki doki…take a little time and come up with an answer………………..
Many distinguished concrete specifications are very clear on this matter. The tolerance for the concrete cover is between +10 mm to -10 mm. However some inexperienced contractors, especially small scale subcontractors, do posses a misconception that increased concrete cover is acceptable and allowed to go unnoticed. This is wrong…….
What is the underlying theory behind the upper tolerance limit of the concrete cover? The upper limit of the concrete cover governs by its effect on Crack widths. Generally the Crack width of the concrete is governed by three factors namely 1.)  Tensile strain of the point  2.) Distance to the R.F bar from the concerned point &  3.) Depth of the tensile zone
In this case, the second factor is significant and it governs the upper limit of the concrete cover.  The increased concrete cover leads to longer distances of R.F bar from the cracking point.  Hence with an increased concrete cover, the crack width also becomes larger.
So cover for concrete should be in its specific limits whether it is plus or minus…keep checking for that….
Oki guys…that’s it for today…Looking forward to see your comments on this topic…. Cheers!!!!!