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RE: Reinforced Concrete Blockwork Construction

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In Reply.

-----Original Message-----
From: Harold Sprague [mailto:harold.sprague(--nospam--at)]
Sent: Tuesday, July 27, 1999 10:16 AM
To: 'seaint(--nospam--at)'
Subject: RE: Reinforced Concrete Blockwork Construction

    "But in the US the knock-out units are very common."

Yes. This is very true.  But it is also true that CMU fabricators prefer
to produce U-block.  First of all it contains less overall concrete so
the manufacturing/curing process is shorter.  There is less wear and
tear (friction) on the molds when they are withdrawn.  This results in
lower maintenance costs (for mold replacement and downtime to replace
molds).  The original purpose of knockout block was to allow the
installation of materials such as conduit, piping, other such items in
partially grouted walls.  A knockout block tends to be more  expensive
(a couple cents per block does add up) than a U-block.  I'm sure all the
contractors would prefer to pay the extra money just to convert them to
U-block  All these factors basically means that the U-block is also very
common and will be for a long time to come.  In fact, I had the
opportunity to work with the designers/inventors of the "Ice-Block"
system which uses U-shaped polystyrene blocks with horizontal and
vertically reinforced/grouted cells at 16" o.c.  The system was
originally developed on the west coast back in the early 1990's and was
based conceptually on the CMU "U" style block.  

    "For many years the NCMA and the USACE have had details showing the
use of
     the knock out bond beams  in partially grouted walls. The NCMA and
     standard details do not show "U" shaped bond beams except at
lintels.  The
     NCMA even has a video tape showing the proper installation and
     using the knock out bond beams."

I wouldn't know why these organizations ignore the existence of bond
beam U-block installation.  Possibly that it is the same procedure as
using knockout block.  But even ACI 530 Commentary shows examples of
U-block being used in a normal bond beam application.    

   "I have seen many jobs where the "U" shaped bond beams are shown on
    details, but the masons routinely replace them with the knock out
    without calling the architect or engineer."

There is no real structural difference between a "knock-out" block and
"U-block with the same material properties.  So I frequently see the
substitution of for "U-block" also.   

     "Another advantage is that the
      masons can replace regular CMU with a knock-out unit, but they can
      replace them with a "U" shaped unit."

Advantage?  U-block is only used to install horizontal reinforcement
with solid grouting.  It would never be expected (even by the masons) to
be used in a normal condition.

     "The only bond beams used on the west coast are the lintel "U"
shaped bond
      beams, but the only reason is the predominance of fully grouted
      walls.  Where partially grouted walls are the norm, you will see
the use of
      the knock-out bond beams."

The ONLY?  Come on now.  That's a pretty broad reaching statement.  With
all of your experience I'm sure you realize that many methods of
construction are used in all areas of the country and world.  Just
because the predominant method of construction in your area is what you
have explained does not mean it is the "only" method.