Aggrevating Variation in Block Size

[Campmaki]

Re: Aggrevating Variation in Block Size

Look at it like this: You will be mortaring the brick, so you may as well get a head start on figuring it out with the block.

I hear an echo!!!!!!Get after it and grab the trowel, at least lay them in a "bed" of mortar. You do not have to plumb them if they are square, just level across and they will be plumb. Them straight edge them from one end to the other.And for gods sake measure a lot.

[NCMan]

Re: Aggrevating Variation in Block Size

Years ago on a remote island in the South Pacific I watched local natives making CMUs. The islands were/are remote and the cost of importing and transporting the block was not economical. Transporting cement, however, was. The CMU machine was simple and hand operated. One simply filled a mold thru the open top. The compacting consisted of ramming from the top and vibration was a few raps on the side with a mallet. Then a short wait. A lever on the side was pulled down, this extracted the center cores. Then a bail on the backside was popped up and the side walls hinged away from the block. Another lever was pulled down and the finished block rotated to be sitting on its side in front of the operator.

Now the rotated block was sitting on a short board which in the mold created the fourth side. The board with the block was carefully lifted and moved away to cure. Another board was placed where the first was and the mold re-assembled, the sides and back hinged up and the bails pushed down to hold them. The cores were elevated and the whole process repeated.

Slow, yes but time was plentiful and in the course of a month an amazing amount of block could be made. I was impressed and vowed that if ever I found such a machine for sale I would buy it. This of course made no sense as at the time I lived and was traveling on a small sailboat.

Fast forward to twenty five years ago. I was looking for a small cement mixer and placed an advert in our local newspaper for one. An older fellow responded that he had just the machine I was looking for and that I should come see it. Well you can guess the rest. Along with the small cement mixer he had this CMU block maker. Of course I had to have it and so bought it and the mixer for $50. It is an "eastern block" maker in that the blocks have three holes. Western blocks have two holes. It has a divider so that one can make two half blocks.

So the units are out there although uncommon. They are very cool in that one can alter the ends of the mold so that trapezoidal block can be made. This would allow one to build in a circle so cylindrical tanks can be built. By flipping a block halfway thru a circle one could create an "S" shaped structure which would be a two roomed building with but one wall. If one was so inclined one could pattern the board side of the mold and so create blocks with relief. I was always going to build a undulating retaining wall with fish in relief swimming along it.

Of course none of this was done, and the block maker sits under cover. It's a bit rusty, was when I bought it but "protected" by a coat of oil and made of fairly heavy steel.

Bests,
Wiley

Wiley......interesting story and pics. Thanks for sharing.

[Tscarborough]

Re: Aggrevating Variation in Block Size

In the late 70s I watched the crew work one of those in Cozumel, Mexico. They bought a mechanized system by the late 1980s, which was of such good quality that I brought a 4" unit back to show around the plant. That is a 21 pound chunk of concrete. The inspectors were like, "what are you trying to hide?".

[Wiley]

Re: Aggrevating Variation in Block Size

Years ago on a remote island in the South Pacific I watched local natives making CMUs. The islands were/are remote and the cost of importing and transporting the block was not economical. Transporting cement, however, was. The CMU machine was simple and hand operated. One simply filled a mold thru the open top. The compacting consisted of ramming from the top and vibration was a few raps on the side with a mallet. Then a short wait. A lever on the side was pulled down, this extracted the center cores. Then a bail on the backside was popped up and the side walls hinged away from the block. Another lever was pulled down and the finished block rotated to be sitting on its side in front of the operator.

Now the rotated block was sitting on a short board which in the mold created the fourth side. The board with the block was carefully lifted and moved away to cure. Another board was placed where the first was and the mold re-assembled, the sides and back hinged up and the bails pushed down to hold them. The cores were elevated and the whole process repeated.

Slow, yes but time was plentiful and in the course of a month an amazing amount of block could be made. I was impressed and vowed that if ever I found such a machine for sale I would buy it. This of course made no sense as at the time I lived and was traveling on a small sailboat.

Fast forward to twenty five years ago. I was looking for a small cement mixer and placed an advert in our local newspaper for one. An older fellow responded that he had just the machine I was looking for and that I should come see it. Well you can guess the rest. Along with the small cement mixer he had this CMU block maker. Of course I had to have it and so bought it and the mixer for $50. It is an "eastern block" maker in that the blocks have three holes. Western blocks have two holes. It has a divider so that one can make two half blocks.

So the units are out there although uncommon. They are very cool in that one can alter the ends of the mold so that trapezoidal block can be made. This would allow one to build in a circle so cylindrical tanks can be built. By flipping a block halfway thru a circle one could create an "S" shaped structure which would be a two roomed building with but one wall. If one was so inclined one could pattern the board side of the mold and so create blocks with relief. I was always going to build a undulating retaining wall with fish in relief swimming along it.

Of course none of this was done, and the block maker sits under cover. It's a bit rusty, was when I bought it but "protected" by a coat of oil and made of fairly heavy steel.

Bests,
Wiley

[Greenman]

Re: Aggrevating Variation in Block Size

It works fine if you have a quality, consistent product. Any gaps that are left because of the fact that these were designed to be laid with inter-block mortar can be filled before render is applied. Worked ok for me.

I do think that blocks laid to a professional standard do look better.

[edsiajb]

Re: Aggrevating Variation in Block Size

I did resolve my block sizing issues by culling the worst offenders and grinding the others. I got a diamond cup for my angle grinder - wow! It will really grind quickly....

[edsiajb]

Re: Aggrevating Variation in Block Size

Here is a quick primer on how CMU are made so that you can understand why it is unpossible to produce perfect units:

A mix of cement and aggregate with just enough water to allow hydration is funneled into a mold. It is then subjected to high pressure and vibration which consolidates the material. The mold is then slipped off the unit, it moves down the line to a rack and is moved to a steam kiln. It is kilned for 24-48 hours and moved to the yard.

If there is too much water, the CMU may slump after it is removed from the mold, it may also slump if there is too much vibration during movement to the racks and kiln.

Molds are very expensive, and the aggregates used in CMU are very abrasive, so the mold profile changes throughout the life of the mold. For different sizes and types of block the mold is reconfigured, and size discrepancies can be made here (these are bad block and are normally crushed and reused).

The ASTM certifications recognize these manufacturing realities and give allowances for CMU.

It is the same for brick, although the challenges are a little different:

Brick shrink irregularly during kilning, and the extruding process itself leads to defects that are allowable.

Brick, block, and cast stone are all meant to be used with mortar and the joints are designed to work within the allowances of the manufacturing method.

Thank you for this info - I am always fascinated by manufacturing processes (I am a process engineer). I can sure understand that there are many variables involved. The worst block I had were purchased directly from a local manufacturer. Seven years ago on a different project, my best block came from this same manufacturer. Your explanation really helps understand the possibilities that could cause this.

And, I've seen them open the kiln doors before - very cool to see. I wish I could see the entire process.

[Tscarborough]

Re: Aggrevating Variation in Block Size

Look at it like this: You will be mortaring the brick, so you may as well get a head start on figuring it out with the block.

[NCMan]

Re: Aggrevating Variation in Block Size

This all reminds me of a Habitat for Humanity project I worked on years ago. Some genius (a Board Member) figured it would be more "volunteer friendly" to dry stack an entire three bedroom house (w/9' high basement walls as well as a volunteer-poured footing). It turned into such a fiasco, after all was said and done, lots of donated money was spent, as well as hundreds of volunteer hours in labor, it was all torn down. For what it's worth, although I understand why some folks dry stack their block, I recommend mortaring the block, especially the bed joints of the first course.

[cobblerdave]

Re: Aggrevating Variation in Block Size

G'day
I had a bit of fun with my stand. All the full block was free but the 1/2 blocks were brought for the job. The half blocks were all a few ml higher.
Such is life..... You just have to suck it up and trim them down.
Of course like bricks they are shorter in length than twice there width to account for the mortar gape to allow the corners so your always got a gape at the corners.
All in all its an easy construction method for the novice, I"d be more worried about how level the base slab and the hearth slab turn out.
Probably accounts for all the questions on leveling The hearth on ceramic board!
You couldn't get a more uniform products,CF board or fire brick.
Regards dave

[david s]

Re: Aggrevating Variation in Block Size

The blocks are not that difficult to lay with mortar once you've had some practice, but first timers will find it hard and slow. In this case it is easier to dry stack them, although I prefer to use masonry adhesive if doing it this way. I think it looks better if mortar joints are used, but if the thing is going to be rendered or covered in some other way it then it doesn't matter. If you are building a modular oven then you won't be cutting and laying bricks either, so in this case the no mortar method is preferable for those without bricklaying skills.

[Tscarborough]

Re: Aggrevating Variation in Block Size

Here is a quick primer on how CMU are made so that you can understand why it is unpossible to produce perfect units:

A mix of cement and aggregate with just enough water to allow hydration is funneled into a mold. It is then subjected to high pressure and vibration which consolidates the material. The mold is then slipped off the unit, it moves down the line to a rack and is moved to a steam kiln. It is kilned for 24-48 hours and moved to the yard.

If there is too much water, the CMU may slump after it is removed from the mold, it may also slump if there is too much vibration during movement to the racks and kiln.

Molds are very expensive, and the aggregates used in CMU are very abrasive, so the mold profile changes throughout the life of the mold. For different sizes and types of block the mold is reconfigured, and size discrepancies can be made here (these are bad block and are normally crushed and reused).

The ASTM certifications recognize these manufacturing realities and give allowances for CMU.

It is the same for brick, although the challenges are a little different:

Brick shrink irregularly during kilning, and the extruding process itself leads to defects that are allowable.

Brick, block, and cast stone are all meant to be used with mortar and the joints are designed to work within the allowances of the manufacturing method.

[stonecutter]

Re: Aggrevating Variation in Block Size

The pdf suggests dry stacking because anyone can build the base that way...it's a suggestion to make building a large cmu base user friendly, but it's not proper technique. As stated, best practice need not be followed, unless you are building contractually. I built one stand dry, just to try it. It worked out fine, the cmu I had were decent. But there were a couple I had to adjust here and there. My current oven is on a drystone base, but the next cmu stand will have mortared courses...no more dry stacking blocks.

You have learned about another function of mortar, it is a leveler of units....in addition to being a gasket, and an interconnecting web of masonry.

[Tscarborough]

Re: Aggrevating Variation in Block Size

I have supplied at least 5 projects over the years, big ones, 10 of thousands of blocks each where they decided to do surface bonded cement over dry laid block. For some reason they all involved animals, either dog kennels or horse barns. They all did it to save money, and each and every one lost their ass on the job. They would call me out to the job and demand that I either supply "good" block or pay them for the labor they were spending trying to make them plumb and level. I would simply show them the ASTM specifications and remind them that I told them it was a bad idea. CMU are designed to be laid in a mortar bed and the tolerances are designed for a nominal 3/8" joint.

It is faster and easier to lay them in mortar, period.

[wotavidone]

Re: Aggrevating Variation in Block Size

I realise it'll be a PITA, but can you sort them before you start, so each course contains blocks of roughly the same height?