Maximum Allowable Concrete Cover

[humanengr]

Can someone shed some light on what should be used for a MAXIMUM allowable concrete cover.
This question is NOT from the point of view of dimensional tolerances to meet effective
design width or depth.

The situation is a large column foundation pedestal. I need to maintain minimum anchor bolt
edge distance. However, I also must develop the strength of the anchor bolts with re-bar (longitudinal bars and ties). The anchor bolts dia. is 56 mm (2.2 in.), so I need
approx. 13 in. edge distance.

Placing the rebar around the anchor bolts, to develop the strength of anchor bolts, results in
"excessive" cover (about 5-1/4 in.). Is it acceptable to have that much cover, or must it
be reduced to minimize crack width?.

To make matters worse, the project is in India, so I have to cross-reference against Indian Codes (not an easy read). The Indian Codes, explicitly state the concrete cover "should not exceed 75 mm".

I appreciate any feedback.

 

[jayrod12]

The only crack control I can think of off the top of my head would be for flexure. Specifically clause 10.6.1 of CSA A23.3-04, I don't know the clause in the ACI code.

I guess you could apply that formula to your column if it is acting a flexural member however I've never really come across this problem. Wouldn't you develop the anchor bolt as long as the steel was outside of the anchor bolt? Put your reinforcement at the 75mm mark and put your anchor bolts wherever they need to be inside your column cage.

Besides, if your column is that big you'll probably have intermediate ties for your vertical rebar (assuming you have multiple bars per face) so that should help develop your anchors.

 

[msquared48]

Nothing wrong with the rebar cage containing the anchor bolts being away from the anchor bolts.

Just make cover for the stirrups for the cage 75mm and place the verts just inside the stirrups. No worries. The shear cone for the anchor bolts should engage the cage.

Mike McCann
MMC Engineering

 

[TXStructural]

There is no maximum cover, but the combination of inherent shrinkage and other factors may make excessive cover break away from underlying concrete (at the cage, typically.) I prefer to avoid using cover as part of my critical embedment. While testing shows that ACI 318 Appendix D is valid, I prefer to be certain that structural connections, with a potential for tension, develop a positive connection with reinforcement. If you are properly using confinement reinforcement (or supplemental reinforcement), you shouldn't need that kind of cover.

 

[LOKSTR]

humanengr,

When you develop vertical rebar for anchor bolt tension, make sure the c/c distance between anchor bolt and rebar is maximum of heff/2 for the rebar to be effective.