Concrete Block Strength (CMU)

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Old 07-19-10, 07:27 PM
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Concrete Block Strength (CMU)

We used to call these cinder blocks but I guess the proper term is Concrete Masonary Unit. Anyway, I am wondering what the typical strength of these blocks is supposed to be.

The reason I ask is because I am an American (retired Air Force) living in the Philippines and block makers here are a dime a dozen. I just walked my dog about 4 kilometers and we probably passed by 4 or 5 block makers. The problem is the block they make is worthless by US standards. Much of it is hand packed into forms and stacked in the sun to dry. There is so little cement in the blocks it has to be covered to protect it from the rain. Masons can actually cut the block with a trowel. After a wall is built here they smear it with concrete in a stucco like fashion or the rain would soon leave it a pile of sand. There are some machine made block here which are better but still not very strong. I did find one place that makes a 6X8X16 block that they say is rated at 750psi and a friend used it to build his wall. He said that when they dropped one from 10 ft up it didn't break. I know US block must be stronger than that though. I believe it is due to kiln drying. Is that correct? I have a rather large wall and eventually house project coming up. The wall will require around 3000 block and no idea on the house yet. So I am considering buying my own block maker which will make 6X8X16 blocks 3 at a time for about $800. I would then be able to know what went into the block first hand. Would I be able to make stronger block than 750psi or would that require additional equipment?

Thanks and God bless,
TJ
 
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Old 07-19-10, 08:19 PM
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Judging from your description the block you saw being made were cheap units made to local use requirements and not a written specification, but they are cheap to buy.

There are good block made in the Philippines that exceed the U.S. standards. In fact in the U.S., it costs more to make a weak block because of the equipment used and usually they are 25% to 50% over the minimums, but could be 4 times over the minimum if desired.

It is possible to make good block on a hand tamper or small manual machine and many of the old, durable block made in the U.S. in the 1905-1935 period that are still perfectly good and functioning well after 80 to 100 years. Sears sold many of these machines through the years.

The keys to making acceptable block are good, clean aggregate of the proper gradation, a proper amount of cement (not too much or too little), the right amount of mixing water and enough compaction to get good density and be able to handle the block since it will only be in the mold a short time (the new automated equipment cycles 10 times per minute). - It takes knowledge to do it right.

Historically, the weak block have performed well through the years in that climate, even during cyclones, although the wood roof will be gone. That could be a reason the local block for 1 story homes and privacy walls being unchanged in your local area.

Proper construction and details are really key and it just takes a minimal amount of steel reinforcement at critical locations (around openings and on the top course) to create a structure much, much stronger than wood.

6" block can carry very great loads and are used in many countries. I have seen groups of 10-15 - 6" block loadbearing structures in Brazil that were 12 to 22 stories high with NO concrete columns or or steel columns. - Made to U.S. codes, but the codes were used better, as the local engineers state.

Dick
 
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Old 07-20-10, 05:30 AM
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Originally Posted by Concretemasonry View Post
Judging from your description the block you saw being made were cheap units made to local use requirements and not a written specification, but they are cheap to buy.
Oh, yes, they are and you get what you pay for for sure. The 3 inch hand made blocks are 4 pesos or about 9 cents each. The more quality 750psi 6X8X16 blocks are about 40 cents.
Originally Posted by Concretemasonry View Post
There are good block made in the Philippines that exceed the U.S. standards.
I'd like to find that.
Originally Posted by Concretemasonry View Post
In fact in the U.S., it costs more to make a weak block because of the equipment used and usually they are 25% to 50% over the minimums, but could be 4 times over the minimum if desired.
Why would you want to make a weaker block?
Originally Posted by Concretemasonry View Post
It is possible to make good block on a hand tamper or small manual machine and many of the old, durable block made in the U.S. in the 1905-1935 period that are still perfectly good and functioning well after 80 to 100 years. Sears sold many of these machines through the years.

The keys to making acceptable block are good, clean aggregate of the proper gradation, a proper amount of cement (not too much or too little), the right amount of mixing water and enough compaction to get good density and be able to handle the block since it will only be in the mold a short time (the new automated equipment cycles 10 times per minute). - It takes knowledge to do it right.
What is the proper mix of cement/sand/gravel for strong block?
Originally Posted by Concretemasonry View Post
Historically, the weak block have performed well through the years in that climate, even during cyclones, although the wood roof will be gone. That could be a reason the local block for 1 story homes and privacy walls being unchanged in your local area.
You mean typhoons? Yes, the block structures do alright but if it weren't for termites, everything would be out of wood due to cost.
Originally Posted by Concretemasonry View Post
Proper construction and details are really key and it just takes a minimal amount of steel reinforcement at critical locations (around openings and on the top course) to create a structure much, much stronger than wood.

6" block can carry very great loads and are used in many countries. I have seen groups of 10-15 - 6" block loadbearing structures in Brazil that were 12 to 22 stories high with NO concrete columns or or steel columns. - Made to U.S. codes, but the codes were used better, as the local engineers state.

Dick
Everything built here has reinforced concrete columns. Security walls have a column every 3 meters or so with a bond beam every meter. I am hoping that with better block I can go at least to 5 meters between columns.
 
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