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Re: Slender Masonry Walls

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In a message dated 4/23/99 12:02:13 PM EST, BSmithSE(--nospam--at)aol.com writes:

<< I don't feel comfortable splicing vertical bars in tall masonry walls (h/t 
 exceeding 30) and specify no splices in our projects.  Even with pencil 
 vibrator I never am certain if there is proper grout consolidation where the 
 bars are spliced.  Splices create that much more congestion particularly if 
 8" CMU is used.  I operated a vibrator when placing 8" thick concrete wall 
 for my house and was frustrated to find later that I still had some rock 
 pockets ... and I thought I was doing a good job - not too much and not too 
 little.  I've gotten some complaints from contractors about this but I 
 usually insist.  Open end units must be used of course.  I once called Jim 
 Amrhein to see if he allows vertical bars splices and he said yes.  But 
still 
 my prejudice is hard to kill.
 
 Brad Smith  
  >>


I can appreciate the concern about wanting to specify a continuous bar.  But, 
from the construction viewpoint, this can be difficult when you have 25-30 
feet of bar flapping in the breeze as you try to place the lower courses.  I 
realize it should be tied off, but on most jobs it usually isn't done very 
well to prevent movement. 

I would also be concerned that as the grout is placed in individual lifts, 
that the rebar projecting up above the wall will move in the breeze breaking 
the interlock with the grout before the grout sets (if I remember correctly, 
the code does not allow you to re-vibrate the individual cell after the grout 
has set for a minimum time period). 

When really concerned we will detail for the splices to occur at the third 
points of the wall height, cutting down on the height of rebar projecting 
above the wall.  We are also using grout aid now in the grout mix to help 
achieve better bond with the CMU units to help reduce shrinkage and 
separation from the CMU shell walls.  My understanding is that the grout aid 
helps with placement of the grout besides shrinkage.

One area of concern for me is the length of the starter bars coming out of 
the footings and location of lap splice.  If  walls of short length are 
suppose to fail by flexure for in-plane shear, obviously we don't want the 
splice at the top of the footing, but most jobs are detailed this way (can't 
easily change common construction methods though).  See Priestly's book on 
concrete and masonry for a furthur explanation if interested.

Michael Cochran