is a closer bar spacing beneficial?

[CTW]

I'm designing a precast wall (which actually performs more like a beam) as protection for equipment from a crane magnet. My steel on the exterior face will be #7 @ 10" c/c horizontally for negative moment and #4 @ 18" vertically for shrinkage. But I was wondering whether or not there would be much benefit from spacing either or both of these bars closer together. For instance, is 6" going to offer much benefit over spacing the bars at 10"? My thinking is that the closer spacing will help keep large chunks of concrete from breaking loose. I know the panels will get beat up and need replacing, but I want to make sure they last as long as possible.

 

[stx1]

It does not sound like you need additional rebar reinforcement. Try steel fiber reinforcement. This will add better resisitance to cracking and impact resistance than regular concrete.

 

[RDK]

Remember over reinforcing concrete will result in the concrete failing first. This will result in a sudden catastrophic failure of the wall instead of a gradual failure with the steel yielding slowly and the concrete taking more of the load. If you place the bars closer then use smaller bars. Also you need some space between the bars for the concrete to get into (surprising how often this gets missed especially for radial steel near the centre)

It sounds like what you are looking for is local impact resistance. Then I’d go with the steel (or glass) fiber idea.

Another idea you might want to consider is a sacrificial layer of concrete (or wood or other material) that is easy to change and protects the structural core.
Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[glennd]

RDK is correct on the issue with smaller bars at closer spacing as the placement of the concrete around the bars becomes difficult. The other issue is the difficulty in tieing the bars together when placed closer together. If a small spacing is required a welded mesh reinforcement would assist (not sure if you guys have it like we do in Oz).