Maximum Reinforcement Percentage in Walls

Too much reinforcement is not desirable because the concrete cannot easily be placed around the bars. So far as I am aware, there is no other reason for limiting the maximum amount of reinforcement. In your situation, if you do not need all of the reinforcement provided, simply cut off the starter bars not required structurally so as to provide easy placement of the concrete.

But, laughable as it may be, please post your photos to help us understand the exact situation. Faced with the pictorial evidence, I may change my mind.

Caveats. I have been retired for a while now and terminology changes; further, I worked with ACI codes.

I think the maximum steel requirement in walls has to do with having enough concrete to carry loads from the loading surface to transfer to and develop the bars. I note that you have steel at EF in a thin wall. This suggests a flexural loading. If so, the larger bars would change the geometry for the flexural calculation. The wall may be a shearwall too.

I seem to remember that over reinforcing was permitted as long as the loading in the actual bar provided did not exceed the load that would be in the correct bar. If I am correct, then cutting back to T25@200 would be okay, you would have a T25 bar, but it would only carry the load of the T20.

This argument would also work for the wall below, or is it a footing, that is a big jump for a wall.

You will need to dig through the calculations and appropriate codes to see if my thinking does apply to your problem.

I don't know what size aggregate you are using, but that provides one of the minimums for the spacing of bars. Are the 2T25 placed against each other? if so, the development length changes.

If you are sure that the T20@200EF is adequate, then I would remove all of the unnecessary bars until I had T25@200EF, if you don't, you will find that you have to use pencil vibrators to get between the bars to compact the concrete, because there won't be room for the normal sized ones, and you may still end up with voids.

Here are two photos. It's a bit messy.

It is a vertical concrete shaft and so acts as a tubular wall. One side in compression and the other in tension under wind load.

You can see the very worst case here where upto 6 bars are in contact which is also not allowed. I believe the maximum is 4 at laps but can't find the clause in the code.

Thanks for the comments so far. Very helpful.

As a precaster and contractor we find that there is usually always another way to skin the cat. ( no offense Del ) Since the photos are somewhat oblique it is still hard to tell which are laps and which are simply too many bars. The previous comments about aggregate size are right on track. ACI standards will tell you what is allowed for clearance and CRSI standards will tell you what you need for the laps. Simple volume calcs will give you the tale of what is superfluous rebar. The engineer of record will need to sign off on what ever you decide to do. Going to a self-consolidating ( SCC ) type of concrete can eliminate the bughole or voiding issues and will often give you the strengths you need with with less experienced workers. If you get a cage that simply won't allow even a pencil vibrator in ( which often doesn't have sufficient force in a column that big ) you can get a rebar vibrator that clamps onto the bar and turns the rebar into the vibrator. They really work. I don't see anything in the photos that can't be dealt with. Onward and upward.

That is true of flexural reinforcement. ACI only allows 75% of "balanced" ratio of steel in flexural members because the failure of the steel is slow and the failure of the concrete is, as you say, catastrophic. "Balanced" is when the steel ratio is the exact amount to bring both the steel and the concrete to full load.

I am glad there's a voice of reason in this topic, cant believe the guy at start who said he cant think of any other reason to not over-reinforce concrete !

jo,

The main reason to limit reinforcement percentage in walls is to facilitate the pouring of concrete. If you don't believe me, take a look at the picture in Post #3 and consider how well concrete will bond to the bars in that project.

Too much tensile reinforcement in a concrete member reinforced only or primarily on the tension side can cause sudden, brittle failure when the compression block reaches its failure strength. However, a wall is normally reinforced equally on each face. That means the amount of tensile reinforcement is normally equal to the amount of compression reinforcement. When that is the case, sudden, brittle failure is not an issue.

Which brings us back to the conclusion that the primary reason to limit the maximum reinforcement percentage in walls is to permit concrete to flow around and between the reinforcement.