4" Reinforced CMU (too tight for rebar?)

[RareBugTX]

Hello All:

I recall in a project before a plan examiner did not approve a proposed SFR (3000 sf two story) with 4" reinforced CMU. This was a local city limitation. On a similar project now and would like to know what your experience is using 4" cmu and if you had issues placing steel and grouting cells. It is a little tight to get a #5 or a #4, i am left off with only 3/4" for grout. What is your take on this?. To me is fine and the numbers check for in plane loading, shear and moment.

 

[JAE]

We don't use reinforced 4" blocks at all for anything structural.

 

[structSU10]

In the same boat as JAE - 8" block is the smallest I've used for structural purposes.

 

[msquared48]

4" is listed in Amrhein with h/t ratios over 25, but I have never used it as you are really limited to #3 vertical bars.

I would never use it for a bearing wall. Even for solid concrete, the minimum thickness for a bearing wall is 5.5".

Unless this is a screen wall, as in for a dumpster, I would not use 4". Heck, with the truck drivers I have seen in our area, I would use3 nothing less than 24" block for the dumpster walls!



Mike McCann
MMC Engineering

 

[AELLC]

We use 4" cmu only for fences around houses here, and the block has zero reinforcing in it - however there are H-pilasters spaced about 12' oc with 2-#4 verts.

Many old homes were built with 4" CMU but we would use at 6" CMU now.

 

[JedClampett]

ACI 530, Table 1.15.1 gives limits for reinforced CMU to grout lifts vs. space. I'm not sure if there's any way to meet it with 4 inch blocks.

 

[hokie66]

6" blocks are the smallest which are practical to reinforce.

 

[concretemasonry]

hokie has it right from a practicality standpoint.

Block 4" thick(nominal) CAN be reinforced if they are the right units and meet the local code/standards AND are made to be filled.

Most that 4" CMUs are not made for reinforcement, but can be. The question is driven by the customer/designer and local building practices. If you go by the normal standards the units may work for reinforcement, but units are made with tapered core and a face shell wider at the bottom. In many areas, contractors will desire an almost closed core for ease of laying and mortar spreading (the weight is not a factor). Unfortunately, this does not work well with grout filling and rebar placement. Also, the myriad of core shapes/numbers/spacings and configurations are not acceptable to people not familiar with the local units and practices. What works on paper is not acceptable. The reason is that a mold cost for a specialty low volume unit could be $5000 to $7000, so it is not common for a specific locality.

The 6" (150mm) units are very common in many areas in the world using the normal unit dimensions for engineered units because of the demand, especially when higher strengths (f'm) may be required. The down side to a 6" wall thickness is it is not exactly on a standard masonry module (4"/90mm to 100mm), but they can easily be detailed for corners and openings.

The 4" from the Jim Amrhein book is an example of what may be done with 4" block and the international designers may have studied from him or attended seminars and caused the local use of masonry. They did use 6" on many highly loaded walls and switched to 4" for some interior bearing walls to replace the 6" poured walls on high rises. Because of the number of load bearing walls, the cost was not a factor and the engineers were usually employed by developers with an ear on square footage of floors and compatibility with floor plans starting in the 1970's.

The 6" units are so practical that they are the most common for high performance engineered construction in many areas.

Just a little bit of design/construction trivial.

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[a2mfk]

I am surprised ACI 530 allows 4" block for load bearing walls, but I have only used it for bathroom partitions... Where are you located where this even calcs out for a two story building? Are you including moment from your floor beams with your wind/seismic calcs?

To add to concretemasonry's thoughts, even if you could get it to calc out, my experience with partially grouted CMU is that sometimes even 8" CMU is a challenge when it comes to quality control of proper rebar location and good grout consolidation. Also, I think it would impossible to have a lap splice. I don't see it being very practical to have a hooked bar in your footing that extends up 8 to 10 feet, and then how do you lap it with the second story walls?

 

[concretemasonry]

a2mfk -

If you have a problem with partially grouted CMU walls, you are considering the wrong SHAPE of units. The number of webs can be 1, 2, 3 or 4. The location of the webs can vary.

For normal running bond, a block with 2 webs spaced at 8" (for 16" long units) is ideal. These are fairly common in most regions for 6", 8" and 12" wall thicknesses.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[a2mfk]

Dick- maybe I am misunderstanding your comment, but I have no issue with partially grouted CMU, it is the norm in Florida and all my career has been designing with this type of construction. But always 8" CMU with two cells. Sometimes 12" if needed structurally. My point was that even with 8" CMU you can have rebar placement and consolidation problems, imagine if you use 4". I still find it hard to believe it is allowed for anything other than decoration.

 

[concretemasonry]

That availability is what I was referring to.

The ASTM C90 standards are written to allow specialized units since the shape configurations are market dependent. That one of the reasons the dimensions of the parts (face shells, webs, and equivalent web thickness/ft) of a block are given as minimums web thicknesses can be reduced for use in designs that have grouted cores. Shapes with widened "mortar beds" (top surface as laid) are commonly used to prevent rebar from being too close the the inner far face shell.

Unfortunately, Florida has had a history of limited selection of CMU shapes, because of the common casual use based on the mainly on residential work and the old construction methods have just been followed since there was little demand for something different.

In an earlier life, I worked for high production company that made block making equipment and molds for domestic and international use. That company had about 1000 different mold designs just for "standard" 8" & 200mm wall units that became common, since every area responds to the local markets. On top of that there were the one-time-only designs.

Even earlier, I worked for CMU producer (just 1 plant) that normally had about 1000-1200 different CMUs on the inventory. For real loadbearing construction, we made "double H block" (2 webs 8" o.c.) in 6", 8", 10" and 12". In one case, we made the same configuration in a 14" thick unit. - All really for partial grouting.

If you have a large project, just check with suppliers near the site.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[msrrr77]

Never used reinforced CMU below 6" block thickness. In most application I have used 4" block as a veneer, not as a primary structural element.

 

[concretemasonry]

RareBug -

Is this an architectural requirement for the wall thickness?

Often a contractor can charge/bid more for a 4" wall than a 6" walls because of the problems laying a unit with an h/2 or 1 - little or no material savings and more labor to create a smooth wall.

With a 4" CMU, you can get the steel in, but it is difficult to get the grout in properly. You might even end up with mortar forced into the cores rather than a proper grout(8" to 11" slump), which results in poor continuity/bond of materials.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[RareBugTX]

Guys thanks for all your valuable posts. The reasoning behind the selection of 4" CMU is that this is mostly a one story SFR and, that there will be 3.5"-4" natural stone used as a finish on 3/4 of all four elevations. No cavity, full collar joint and figured that I did not need more cause i have other considerations like the need for insulation batts that are going to kill at least another 4" on the inside. Am thinking on the stone as being part of the load bearing supporting assembly. Did not want to make a very chubby wall for nothing. The house is in the San Antonio area where there are very good masons. Also, the house is not a shoebox, the part that is a second floor is 1/3 of the first floor (in the center, whew!) so I need to do a steel frame and wanted a masonry wall to provide a good substrate for the stone (I do not believe on stone supported on termite and mold prone wood framing). Plus IRC does not allow frame two stories with veneer heavier than 50 psf supported on wood. Anyway. Additional feedback is appreciated.

 

[concretemasonry]

RareBug -

If that wall is built wet as a bonded wall it is considered an 8" masonry composite wall and not a 4" wall. If the stone is applied after the 4" wall is laid, it is always a challenge to get composite action for the stone and block to work together.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[madmantrapper]

challenge to get composite action for the stone and block to work together

Challenge is an understatement!

 

[concretemasonry]

Definitely a challenge if the block and stone are laid at separate times.

If the 4" and the stone are laid at the same time to build a wall, then it very possible and I have seen it approved. The reason is that the mortar is a thin portion and mortar strength has a minor effect on the strength of a wall (even in flexure). Composite 8" brick and block walls are often built with joint reinforcement with projecting tabs since 6" joint reinforcement is not practical for irregular natural stone. The importance of the joint reinforcement is to tie the veneer to the block back-up that provides vertical alignment until the mortar cures.

That is a situation where the codes and testing cannot anticipate every construction possibility and an engineering background and understanding of the loads and mechanics of a residential wall to sign a design.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[RareBugTX]

Theres been composite action around for masonry multiwythe walls lasting thousands of years long before ACI-530 Right?. Just kidding! This post is enlighting. Thanks guys