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Aggrevating Variation in Block Size - Forno Bravo Forum: The Wood-Fired Oven Community



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Aggrevating Variation in Block Size

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  • Aggrevating Variation in Block Size

    Several days ago, I began dry stacking my block. All of a sudden I realized that my block had a good deal of variation in height. They should be 7 5/8" tall, but some of them are from 1/4" to 3/8" taller than they are supposed to be!

    There is also surprising (to me anyway) variation in the other dimensions. Some of the block are not square top to bottom - and sort of lean out or lean in.

    Needless to say, the stacked block look bad...with big gaps here and there. I always thought concrete block were more uniform in size.

    Is this a common issue?

    Should I try to fill the gaps with mortar before I pour the cores?

  • #2
    Re: Aggrevating Variation in Block Size

    Now you know the purpose of mortar. Those dimensions are well within standards for CMU, although the slumps units sound pretty crappy.


    • #3
      Re: Aggrevating Variation in Block Size

      If you can live w/what you have, you can always adjust your formwork for your hearth slab to be square and level. Once the slab is poured and is square and level, the block work won't matter. Just be sure to fix it at that point and not let it go. Unless the block is real bad, it should just be a few minor adjustments to your form work to get it right, in my opinion.
      Last edited by NCMan; 06-04-2014, 09:35 AM. Reason: Typo
      My Build:

      "Believe that you can and you're halfway there".


      • #4
        Re: Aggrevating Variation in Block Size

        If you chip off the little dags on the bottom of the blocks you will get a more uniform size. If you are not mortaring between the bricks an alternative is to use a masonry adhesive and some 5/16" washers placed at strategic places will keep the work level. The first row of bricks should have a bed of mortar between them and the foundation slab.
        Last edited by david s; 06-04-2014, 11:26 AM.
        Kindled with zeal and fired with passion.


        • #5
          Re: Aggrevating Variation in Block Size

          In the past, when I've used block before, I believe they were much more uniform in size (and squareness). Yes, I understand that is one of the purposes of mortar.

          But if I have a block 1/8" short and the next block is 3/8" tall, that is a 1/2" variation causing a 1/2" gap, and would look rather bad even if mortar were being used between the block.

          As all of this will be covered in a yet-to-be-decided finish, I'm not really that worried about it...just aggravated.


          • #6
            Re: Aggrevating Variation in Block Size

            I didn't say they were GOOD block, just within tolerance. If I put those CMU on a job, I would not be selling many block.


            • #7
              Re: Aggrevating Variation in Block Size

              Why are you people shying away from laying the block with mortar? You are going to lay the most difficult part of the build with fireclay. Laying the base with mortar would get builders used to handling the tools of the trade.


              • #8
                Re: Aggrevating Variation in Block Size

                Originally posted by Campmaki View Post
                Why are you people shying away from laying the block with mortar? You are going to lay the most difficult part of the build with fireclay. Laying the base with mortar would get builders used to handling the tools of the trade.
                Maybe because the plans recommend dry stacking and filling every other core with re-bar and concrete?


                • #9
                  Re: Aggrevating Variation in Block Size

                  And because my height works out better without the mortar joints...


                  • #10
                    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?


                    • #11
                      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.
                      Last edited by Tscarborough; 06-04-2014, 08:34 PM.


                      • #12
                        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.
                        Old World Stone & Garden

                        Current WFO build - Dry Stone Base & Gothic Vault

                        When we build, let us think that we build for ever.
                        John Ruskin


                        • #13
                          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.
                          Last edited by Tscarborough; 06-04-2014, 08:51 PM.


                          • #14
                            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.
                            Kindled with zeal and fired with passion.


                            • #15
                              Re: Aggrevating Variation in Block Size

                              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
                              Last edited by cobblerdave; 06-05-2014, 02:40 AM. Reason: Speeling
                              Measure twice
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