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Re: [multimachine] size of pebbles in concrete

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  • Adam Simmons
    As far as rebar - you wouldn t need much, some #3 or #4 bars bent in a rough frame would be plenty sufficient. Adding aggregate to a quickcrete is a simple
    Message 1 of 9 , May 28 10:06 AM
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      As far as rebar - you wouldn't need much, some #3 or #4 bars bent in a rough frame would be plenty sufficient.  Adding aggregate to a quickcrete is a simple way to increase his volume, i concur with the washed 3/4 crushed.  If you want to get fancy, sift out everything under 1/4" or so.

      The biggest I've put in a wall has been 1-1/2", any bigger and you have some serious issues pumping it.


      On Tue, May 28, 2013 at 10:03 AM, Adam Simmons <xyrthx@...> wrote:
      The first thing I have to add to this conversation is vibration.  Rocks/Aggregates/chunks stronger than concrete are great to add to your mix in order to strengthen it up -- but proper vibration is key.  You don't want air pockets around the rocks, it makes it weaker not stronger.  You can rent/buy/build your own, and you should look into it.

      I'm a project manager / estimator for a heavy civil contractor in California -- we build water/waste water plants, energy plants, reservoirs, etc.  On every job we have significant structural concrete, sometimes 3' thick walls w/7,000 PSI concrete.  Everything is designed to meet CA's rigorous seismic requirements. 

      I cannot stress enough how important vibration is to consistency of the finished product -- slump/water content also play heavily into this.  I do have some mix designs I can share if anybody is interested.

      That said, there are 2 primary types of strength people discuss with concrete, compression and tension.  Compression is your 3,000, 4,000 or 7,000 PSI number -- tension is weak and concrete and why we have to add reinforcing steel.  There are a couple other ways people have been trying to add tensile strength to concrete, but really, adding rebar is very simple and effective.

       - Adam


      On Tue, May 28, 2013 at 8:23 AM, Nick Andrews <nickjandrews@...> wrote:
       

      A very good point.  Around here we get what is called crusher fines for landscaping, basically the waste from blasting granite for rock walls and such or aggregate crushing for concrete and asphalt.  You could sift out the fines and use the 1/4"+/- portion for this work with very good results.  Lots of fractured faces.  And make damn sure there is no clay in the large or small aggregates!  There are plasticizers you can add to stiff mix to make it easier to fill smallerish forms but still use less water for higher strength, too.


      On Tue, May 28, 2013 at 8:49 AM, BRIAN GLACKIN <glackin.brian@...> wrote:
       

      Back to aggregate.
       
      For best results, use crushed stone.  Pebbles are smooth sided and while they pack well, the smooth surfaces provide less than ideal bonding locations for the concrete.  Break a piece of concrete with pebbles and it will break along the pebble surfaces.  If you break a piece of concrete with crushed stone, it is likely to break through the aggregate and not along the contact surfaces.
       
      The resulting aggregate should be more cubical in nature not slabby.  It can be produced using modern equipment or for small runs of concrete such as with a MM, it can be hand crushed and sorted using either a larger rock or iron implement and a small hand screen.  Excess fine dust can be washed out of the aggregate.  If you do this, consider the retianed moisture when calculating the amount of water to add to the mix.

      On Tue, May 28, 2013 at 10:10 AM, Nick Andrews <nickjandrews@...> wrote:
       

      Adding fiber to the concrete does not increase its strength or take the place of steel reinforcement.  It is added to reduce shrinkage cracking.  To make stronger concrete you can use more cement, but one of the best ways is to use less water.


      On Mon, May 27, 2013 at 11:03 PM, chris green <hraefn_2@...> wrote:
       

      For 6" and 8" foundation walls, we generally use 3/4-minus gravel,  about 20mm, so the quoted sizes seem about right.

      A supplier of gravel for concrete, or readymix concrete supplier, will be able to make a sand/gravel mix and recommend the amount of cement mix for any given strength the builder requires.

      To greatly increase the strength of concrete, one can mix in more portland cement and/or add the new Polyvinyl Alcohol fibers (PVA) which chemical bind with the concrete, unlike metal or other plastic fibers which only have a mechanical bond to the concrete.
      This is an inexpensive way to increase strength and greatly reduce cracking (about an 80% reduction). Add about 2 lbs/ 1 kilo to a cubic meter of concrete, which increases the cost around $2 or 1.5 Euros, judging by the last price I had for the fibers.

      Generally speaking, PVA fibers are supplied by the Kuraray company. Their European home page is:
      http://www.kuraray.eu/en/produkte/product-ranges/kuralon/

      Adding magnetite powder to the concrete also greatly increases the strength and toughness, and we sometimes use it for the top layer or floor pads in repair shops which will see tracked vehicles like Cats. It also makes the concrete noticeably heavier...

      Cheers,

      Chris Green.





      From: Pat <rigmatch@...>
      To: multimachine@yahoogroups.com
      Sent: Monday, May 27, 2013 8:06:52 PM
      Subject: [multimachine] size of pebbles in concrete

       
      Oliver Scholz, a German engineer is building a 2/3 scale lathe and sent me this yesterday:

      "You were right about the size of pebbles. I downloaded a brochure that
      describes the production of concrete for small construction sites. The
      paper comes from an Austrian civil engineer from Insbruck named Manfred
      Krulis.

      For the size it says the following:

      largest piece
      8 mm: For parts up to 80 mm thickness
      16 mm : For components of 80 to 120 mm thickness
      22 mm: For components that are more than 120 mm thick
      32/63 mm: For heavy pieces of 200 (300) mm without reinforcement or with
      large intervals between reinforcements

      So i think, i'll make a cut-off at 16 mm and cast the first 2/3 version
      with this."

      Pat






      --
      Nick A

      "You know what I wish?  I wish that all the scum of the world had but a single throat, and I had my hands about it..."  Rorschach, 1975

      "They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."- Benjamin Franklin, Historical Review of Pennsylvania, 1759

      "Suburbia is where the developer bulldozes out the trees, then names the streets after them." Bill Vaughan

      "The price of apathy towards public affairs is to be ruled by evil men." Plato





      --
      Nick A

      "You know what I wish?  I wish that all the scum of the world had but a single throat, and I had my hands about it..."  Rorschach, 1975

      "They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."- Benjamin Franklin, Historical Review of Pennsylvania, 1759

      "Suburbia is where the developer bulldozes out the trees, then names the streets after them." Bill Vaughan

      "The price of apathy towards public affairs is to be ruled by evil men." Plato



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