CMU center webs 1

[boffintech]

Do CMUs have a top and a bottom?

The center web of a concrete masonry unit is thicker (wider)on one side than the other (top vs bottom). I deal with one project manager who insists that block is supposed to placed with that center web widened side down. He claims that although the center web widened side up makes "for a convenient handle" it must be flipped over to be placed correctly.

Can't find anything is ACI530 or any of my inspector's handbooks on this. Is he on crack? Which is correct or does it really matter?

On another note, why isn't there a masonry forum on ET?

 

[Ron]

boffintech....the difference in top and bottom sizes is due to the tapered core necessary for molding and demolding. It has nothing to do with strength and the block may be placed up or down.

 

[concretemasonry]

The taper needed for maufacturing and stripping is very small and is usually not even noticeable. The much more noticeable, obvious widened webs and face shells are incorporated to reduce labor costs. the terms for these are "mortar beds" and "hand holds" and are in addition the the minimum web and face shell dimensions in ASTM C90.

The proper placement of the widened webs and face shells is at the top. You project manger is not wrong, but he definitely is not correct and does not have a strong background in masonry.

The widened elements serve two purposes:

1. A widened area to make it easier to handle and pick up.

2. A wider bed to place the mortar if it is required on that portion of the unit.

It is difficult to see why you would be applying mortar to the center web unless you laying the block in a full mortar bed. - Very, very unusual. Normally, the only block laid in a full mortar bed are those in the first course on a footing. Even in high rise load bearing masonry full mortar beds are usually used only to contain the grout is a cell to be grouted. The grout is used to bond reinforcement and not to increase the compressive strength of the wall. Even in 15 to 20 story 6" loadbearing block buildings, full bedding is use to contain the grout in the cells to be filled.

With 2 core block, the webs do not align perfectly. If smooth end block (non-stretchers) are used, the webs align better, but not good enough to have a major effect on the strength. For maximum strength, specially comfigured one core block provide perfect core alignment and load distribution.

Dick

 

[concretemasonry]

I am new to this forum, so I can only offer a guess about the lack of a masonry forum on ET.

In the U.S., masonry is not used as frequently as it is used in engineered projects in the rest of the developed world.

The U.S. has good masonry standards, codes and specifications, but since masonry is taught in few universities, few engineers are confortable with it. I learned the hard way when my first design project was for a concrete block rocket test stand instrumentation center that had to be designed for both blast and seismic. It scared me to death since I though block were only for basements.

There is some very advanced use of masonry in many countries and it based on U.S. codes, research and standards beginning with Jim Amhriens work in the 1960's in California. I was on a project in Brazil that had about 20 to 25 - 15 to 21 story loadbearning masonry structures under construction at the same time. The loadbearing walls were 6" thick partially reinforced block walls built using 4 different strengths of masonry. When I asked the engineer what codes and standards he used, he said "We use yours, but we use and understand them better".

Because many of the international designers are employed by the owner/contractors there is an emphasis on reducing costs by using better materials (higher strengths), engineering and more co-ordination of the design, selection/manufacture of materials and construction practices. Can you immagine one site supply yard containing block for 20 buildings in 4 or 5 strengths and coordinating the use of the correct units at the correct location on the correct floor of all the buildings? The contractors and suppliers have worked out efficient systems that have virtually eliminated the problems of mis-identifications and unnessary site testing and inspections.

Masonry is the most common residential building material in the developed countries, so it has greater familiarity internationally that in the U.S. Perhaps more international engineers will contibute to this site to justify a forum on the subject.

Dick

 

[dik]

Regarding the cores being grouted for reinforcing only... prior to codes incorporating filled CMU's I used to treat the 5" core as concrete with the 7-1/2" shell for stability and have done several 18-20 storey buildings using 8" CMU's and 8" HC slabs... I thought it was a reasonable approach back then...

Dik

 

[henri2]

boffintech, look at figure 4.1 and Figure 6.1 of the MIA Reinforced Concrete Masonry construction Inspector's Handbook-4th edition. Even though this shows a cross section of face shells, it clearly shows that the thicker end is the top end. In addition to making handling easier, this creates a larger surface area for mortar to be placed on.

Where is mortar applied when hollow units are placed? If the 2003 IBC governs, then Section 2104.1.2.2 applies. Also refer to figure 4.19 of the MIA handbook.