Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Masonry reinf size

[Subject Prev][Subject Next][Thread Prev][Thread Next]

Partrick, Ted wrote:

> This is a little late to bring up in this thread, but here goes:
>
> In a seminar put on by our local concrete association on concrete masonry,
> an masonry consultant with the portland cement association recommended that
> two re-bar never be used in horizontal applications in an 8" cmu wall unless
> they are stacked vertically (one above the other in the block). His
> explanation was that in many applications the bars create congestion and the
> risk is high that the block will not be fully grouted. In the u-block used
> for many lintels, there is not a lot of space in the bottom of the block.
>
> Do those of you with inspection experience in cmu construction find that
> 2-#6 bars placed horizontally can be reliably grouted?
>
> Ted
>

I have designed and inspected hundreds of buildings with CMU and when
constructed in accordance with the requirements of the ACI specifications, they
can be constructed with up to two # 7 rebar either vertically or horizontally in
an eight inch wall.  Of course you will get a lot of whining from the mason and
hear how he has never put so much reinforcing in a wall in the thirty years he
has been doing masonry work and sure, a maximum of two # 5 bars are much easier
to work with.  I usually start checking one #4 bar at max spacing and increase
the bar size up to one #7.  If this doesn't work then reduce the spacing and
check the single bar.  Vertical grout pours must be limited to a maximum of 5
feet (4'-8" for block modules).  Mechanical consolidation using a vibrator with
a maximum one inch diameter head is usually required to be sure there are no
grout voids in the wall.  You should always try to get single bars to work when
you can because it does get congested when you lap larger bars and have 4 of
them in the same place at a lap splice.  If I have to use two bars, I try
reducing the spacing when 2 #5's don't work.  Horizontal reinforcing is much
easier and grout must be placed at the bond beam elevation.  The horizontal bars
should be placed side by side to get grout cover.  Also don't forget that you
can use the area of the joint reinforcing for meeting the minimum steel area.
Grout stop can be placed in the course below the bond beam and knock-out block
should be used.  The bond beam grout must be consolidated but can be done using
a trowel or piece of clean wood.  I do not like using lintel or "trough" block
for bond beams because they have thicker bottoms and are tapered making it
harder to support the reinforcing and getting a minimum of 1/2 inch of grout
cover on the reinforcing.  The grout mix design is very important.  To meet code
or spec requirements it must have a minimum 28 day compressive strength of 2000
psi.  Grout should be placed at a minimum of 8 inches and a maximum of 10 inches
slump (usually a 6 1/2 to 8 sack mix).  This will produce a very fluid mix that
will flow through the block cells around the reinforcing with minimal effort.
With this much water in the mix, it will also carry the fines into the block
pores for good bond.  Grout should set for at least 15 minutes prior to
mechanical consolidation.  The vibrator should be placed in the cell, turned on
and removed in approximately 2 to 4 seconds to remove the air left by the grout
wetting the pourous block.  This will typically cause the grout to settle 4 to 6
inches and additional grout will need to be placed.  I prefer that the grout and
mortar be mixed with portland cement or a portland cement blend with lime
normally called "mortar cement".  Masonry cement is a proprietary mix and not
allowed in high seismic areas, is more susceptible to water seepage and
efferesence, and who knows what is in the mix.  Just try getting a supplier to
tell you what is in it.  Most codes require full time inspection for masonry
work on major projects.

Joe Ferguson, P.E.
fergy(--nospam--at)intellex.com