CONCRETE SLABS/FOUNDATIONS ;)

Discussion in 'Planting, growing, nurturing Plants' started by tomintaz., Oct 22, 2005.

  1. tomintaz.

    tomintaz. Junior Member

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    Hiya.

    Consider this: A massive saving in future heating & energy cost needed for HOUSE foundation [slab] can be achieved by forming a CONCRETE & STRAW BALE slab.

    1. First pour a thin layer of concrete/mixed with straw to form the initial cover slab [2-3 inches] over whole area.

    2. Immediately after first pour; place whole straw bales on top of slab so the gap between each bale [every side] has a gap of about 2-3 inches free around each bale, maybe 4-5 inches near the wooden boundry edges. The next part of the pour is the touchiest part of the entire operation. Straw is again mixed with the concrete *IN THE TRUCK*, but this time 25% more concrete will be needed than the first pour. Both pours can be done in the same day.

    The straw concrete is poured around the bales, creating a honeycomblike web. The concrete should be poured carefully [on top] of the bales, with overflow to the sides. Otherwise, the bales will want to move & slip sideways. You can also use small sheets of plywood on top of the bales on which to pour the concrete. take your time & pour the concrete slowly. This part is *CRITICAL* & needs to be done well, since this is where the slab gets it's strength & ability to provide even insulation. The pour is stopped when the concrete is just about level with the top of the bales, an inch or 2 below is ideal.

    3. The next day, or when the first pour has set firmly, the cover coat is poured. *NO straw is added to the mix this time, because the straw will make it difficult to float the slab [make it smooth] as the strands will stick out, especially if the surface is finished mechanically. A high density, 4,000 psi mix works best for a well floated & finished slab. Regular concrete without bales should be used wherever a chimney is to be raised or anywhere a strong structual base is required.

    This slab will take a good 20-30% more concrete than a regular 6-inch pour, but the result is a strong, super insulated slab that will work for you twelve months a year. It's a cheap investment for what it gives you in return; & since the straw is fully coated by the concrete, they will not rot.

    When all parts of the house are equally well insulatedthe slab will NEVER be a humidity sink or source of discomfort.

    :wink: NOTE: When pouring the slab, the forms have to be a good 20 inches high, since the bales are roughly 14 inches when laid sideways. You need to allow for 2-3 inches of poured concrete over & under the bales.

    Be prepared for funny looks when you tell the drivers of the concrete trucks that you want to add chopped straw for rienforcing to the concrete mix in the truck. Since the truck is already on site, you will want to take advantage of it's capacity to easily mix the the straw thoughly with the concrete. We add one [1] bale for every three cubic metres of concrete.

    A good way to chop the straw is to make two [2] chain saw runs along the strings of the bales & then cut the strings. The added straw will eat up a lot of water, so you will have to have the driver add water as the mix thickens. Add just enough water to be able to pour; remember that too much water weakens the concrete.
     
  2. Tamandco

    Tamandco Junior Member

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    Have you actually tried this technique Tomintaz?

    With the bottom layer, 'mixed with straw', is there a possibility of the concrete becoming affected by cancer as the straw exposed to the outside surface of the slab, provides a medium for water to enter the concrete and as it itself decomposes?

    Tam
     
  3. gardenlen

    gardenlen Group for banned users

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    mmm dunno,

    there is no mention of reinforcing mesh? and most slabs need beams for tensile strength around the parameter and through the middle in the standard contruction requirements of local councils, these beams are formed with trenches.

    also have a concern with straw/straw bales and termites doesn't take much of a crack for them to get in and they would love a feed of straw. also need to consider that the driver just wants to unload and go get another load so there also could be extra costs in delaying him on site along with the extra cost of the cement. can't remember but i think it was then about $140AUD a cubic meter for 40 mpa.

    also sounds like a lot more work on digging foundations may be needed so there could be other extra costs apart from extra cement, not to mention getting council approvals.

    another tip is was told was not to float off too much as that brings too much water to the surface and causes a lot of the cracking seen in slabs.

    my thinking whatever strengths you don't build into your slab can't be done later hey! and if a mistake is made rectifying it can be almost out of the question.

    len :?
     
  4. SueinWA

    SueinWA Junior Member

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    I have a problem with the straw being added to the concrete.

    Regular concrete is not all that water-resistant unless it is quite low in water, which you can't do with a truck mixer (it would have to be close to a low- or no-slump consistency). The pieces of straw that are mixed in with the concrete will eventually rot, forming channels that will wick water through to the solid bales. Then you will have rotting bales of concrete in a concrete box, losing the original support of the baled straw.

    I would strongly suggest that you replace the straw with alkaline-resistant fibers, usually available where bulk concrete is sold. One bag of fibers costs about $10USD, and will provide strength to one cubic meter of concrete. I would use these fibers in the bottom (initial pour), sides, and also the top. It will help prevent the formation of cracks.

    I would also be careful about covering all the pours with plastic, to avoid premature drying of the concrete. Remember that concrete curing is a chemical process, NOT a drying process. It takes 28 days to cure concrete, and there's no way to rush it. You have to keep it moist for at least a week, ideally, and prevent exposure to hot sun and wind.

    I would also have wide eaves on the house, to protect the surrounding soil from as much moisture as possible.

    Sue
     

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