Most common fÆm (masonry) 1

[JoeBaseplate]

I have seen number ranging from 1500-2500 psi depending on the engineer and the firm creating the drawings. I am used to 1800 psi but was recently suggested by a more seasoned engineer that this may be too conservative and I should use 2000 psi minimum as typically thats a minimum number the cmu supplier can provide.

So my obvious question is, whats the economical and readily available f'm.

 

[concretemasonry]

Depending on the block producer, the number you mentioned may meet the "cook-book" code standards. If a higher f'm is required, it can be met easily. There have been some tests done that will eventually change the relationship between the unit strength and prism strength, but the gears move very slowly when it comes to codes and standards. An actual prism test "trumps" everything, even with conservative codes.

I have made 8" hollow prisms that tested 4800 psi with 2500 psi mortar using 8500 psi unit strength hollow block. I could never find an engineer that could find an excuse to use the combination that provided the strength.

Unfortunately, the ASTM specs for block are so low no producers bother to test for compressive strength because the present manufacturing processes, there is no cost advantage to make the old low strength, because the cost of a minimal block is higher. There is nothing wrong to specify a higher f'm if it can be documented.

If it is just a minimal,routine use, just stick to the older concepts, but if you need high strength, it can be obtained easily. Others use our codes and standards, but others build 20 story partially reinforced loadbearing masonry by using the same codes, but by using them better. - Very few clean-outs and all products color coded for strength and approved before delivery.

Dick

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

 

[JAE]

Most of our designs (if not all) are 1500 psi.

 

[msquared48]

I agree with JAE.

Mike McCann
MMC Engineering

 

[concretemasonry]

JAE -

That is very conservative and can cost the owner a bundle if it is a large job.

If it is a small part of a project, it is much easier to use the low values for design since cost is not a major factor, especially if it is heavily reinforced and over-grouted for a quick warm and "fuzzy" feeling.

The 1500 psi will be gone when the code cycles have an effect. I can't think of when I ever saw one as low as 2000 psi.

Dick

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

 

[JAE]

Dick,
In most wall designs in our area, raising f'm helps a little - but not that much. With basic IBC rules you can specify an ASTM C90 standard block, use type M or S mortar and get f'm = 1,500 psi. This has been a standard f'm used in most handbooks, textbooks, Masonry guides, etc. that I've seen for almost 30 years. I can't think of but one project that I've EVER seen using an f'm higher than 1500.

 

[RFreund]

Is there a location/table where you can find combinations of Block/mortar/grout for certain f'm values?

EIT

 

[hawkaz]

1500 is the standard here. I called out 2500 psi once (due to site requirements). They had to custom make these.

 

[SteelPE]

RFreund

The table you are looking for is in part 1 of the specification (ACI 530.1). I think it is either table 1 or table 2.

 

[ron9876]

I'm with JAE. Added cost here to go above 1,500.

 

[JoeBaseplate]

Wow, 1500 psi seems kinda low. BTW I am in IL so seismic forces usually don't govern. I wonder if the lower numbers quoted above have to do with seismic design vs wind for lateral.

I guess I'll stick to 1800 psi. Obviously we call out these numbers on the drawings and haven't had an issues with supplier coming up with extra cost because of this particular f'm.

 

[dcarr82775]

I stick with 1,500 most of the time.

 

[concretemasonry]

If you just request a prism test of the standard units using Type S mortar, you could save a lot of worrying and provide a more economic building for the owner.

The block controls the prism strength and very few block producers bother to run compressive strengths or prisms because it is just plain boring and is not really necessary (most block are about 40%+ over ASTM minimums). The basic compressive strength has not been changed for decades, but the automated equipment, curing and controls have raised the strength. It costs more to make a minimum strength weak block than a good, higher strength block.

Dick

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

 

[ron9876]

All I know for sure is that i just called for 2,000 psi on a project and it was an add. I am in South Florida so I don't know if that makes a diference.

 

[JLNJ]

I always spec at least 2000 psi. The basic CMUs around here break over 4000 psi (net section). I think this is due to the quality of the limestone readily available locally and the dry mix they work with in the CMU manufacturing process.

It's a no-brainer to get to 2000 psi with type N mortar. I think you can even get 2500 with type N if you have 4000 psi block. Anything less for f'm and you're fighting shear problems at your lintels and overly conservative flexural strengths for tall-ish walls.

The f'm = 1500 psi originates from the absolute minimum ASTM C90 spec of 1900 psi unit strength and type M or S mortar. The trouble can come when you spec C90 block but ask for a higher f'm. The contractor will claim it costs him more to get units better than the 1900 psi in the C90 spec. Fact is, around here they cost the same and you don't want to go with the manufacturer who makes "cinderblocks" which break at just 1900 psi.

 

[PUEngineer]

From a recent seminar with a masonry coalition...yes, location has a great deal to do with it. Florida and Texas were mentioned as two specific states, along with the 'south' in general, where calling for a higher strength block could have cost impacts. Something to do with what aggregates are locally available.

From my understanding, I'd agree with concretemasonry, that you're more than likely going to get the higher strength block regardless. We were told by this masonry coalition that specifying f'm of 2000 to 2500, at least in the midwest, wouldn't have any cost impact.

 

[concretemasonry]

A lot of the strength requirements has to do with the locally available aggregates and the local quality demands. For the most part, the manufacturing equipment is essentially the same with varying degrees of automation and sophistication. The curing is a variable and the further north, the better the curing and in the south the water and natural heat/humidity can give variable results.

I think the old, archaic 1500 psi as an assumed strength will be increased as soon as the recent tests are correlated with the historic testing.

Dick

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

 

[JoeBaseplate]

PUEng,

Thanks for the post. I think that explains the widely different opinions by some very experienced folks on the topic (very confusing to novices like me). I guess it would be a good idea to call 2 or 3 local suppliers and ask for their opinion as their is no universal number that can be used for the whole country.

 

[Bobber1]

In Southern California, CMU construction is very common. We conservatively use 1500 psi for most work but have no problem with up to 3250 psi when necessary. We always talk with CMU suppliers when going to the higher strengths. Today, just about all masonry in SoCal requires special inspection so that is generally not an issue.

 

[concretemasonry]

If the producer does not have file of past prism tests, he should not even be considered as a supplier.

If the producer is good they have the tests and production records, they can easily dial in a block that meets the minimums within 100-200 psi (and usually higher) five or ten years later (depending on the consistency of the technician making the prism). The making of higher strength block is just child's play if you have records and consistency.

A professional friend (classmate)of mine designed a 7 story hotel with a requirement of 3000 psi for a hollow 8x8x16 prism. The average strength was 3200 psi. Four or five years later, his own firm did an adjoining 7 story addition and the average was over 3300 psi.

It is very important to screen possible future suppliers to minimize the costs and construction snags since what is arbitrarily specified may not be adequate or possibly troublesome and expensive.

Dick

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