Ratios And What they Affect

  • I have my foamer down well and have numerous blocks, planters etc made. To me they are fragile, not brittle really, just fragile. I am using .64 gal water and 9.4lbs cement in 5 gallon bucket and fill it till full with 88g / qt foam made w Drexel and water. I see that stucco, when applied to the outside, makes a tremendous difference.

    My question is in the ratios.

    1. what does heavier foam density (100 or 120) vs lighter foam density (with all other ratios the same) affect? Does heavier foam / qt create stronger or less strong bricks?

    2. what does water ratio affect? Going to 5 gal water per 94lb bag or going to 9 gallon per 94lb bag. Is the 5gal stronger or the 9 gal stronger?

    This site (https://naturalbuildingcollective.wordpress.com/tag/compressive-strength/) provides field tests to identify MPa for compression and tensile on adobe bricks. I would guess these field tests would work on these block types too!? They say 1.3Mpa or 190psi compession and 1.8 tensile is strong enough to build with.

    I realize I could keep doing tests to figure out my answers above as to what affects what but thought I would ask and see if someone has already done that.


  • I have tied batches with less water and found them to create weaker blocks. I do not remember the exact measurements of those tests. I have not tried testing more water yet though. I would imagine that after you dissolve out the cement you would only dilute the cement solution thus leaving more water to evaporate out and maybe even a weaker brick.

  • Water/cement ratio has a HUGE effect on strength - the higher the ratio (the more water), the weaker the concrete. Your .64 gallons of water = 5.34# , so your water/cement ratio is 5.34/9.4 which, if Siri did the math right, is a w/c ratio of .568 = pretty high in the world of regular concrete. You can read all about water/cement ratio at various civil engineering sites on the internet. Here's what is on Wikipedia - it is accurate info:

    The water–cement ratio is the ratio of the weight of water to the weight of cement used in a concrete mix. A lower ratio leads to higher strength and durability, but may make the mix difficult to work with and form. Workability can be resolved with the use of plasticizers or super-plasticizers.
    Often, the ratio refers to the ratio of water to cement plus pozzolan ratio, w/(c+p). The pozzolan is typically a fly ash, or blast furnace slag. It can include a number of other materials, such as silica fume, rice husk ash or natural pozzolans. Pozzolans can be added to strengthen concrete.
    The notion of water–cement ratio was first developed by Duff A. Abrams and published in 1918. Refer to concrete slump test.
    Concrete hardens as a result of the chemical reaction between cement and water (known as hydration, this produces heat and is called the heat of hydration). For every pound (or kilogram or any unit of weight) of cement, about 0.35 pounds (or 0.35 kg or corresponding unit) of water is needed to fully complete hydration reactions.[1]
    However, a mix with a ratio of 0.35 may not mix thoroughly, and may not flow well enough to be placed. More water is therefore used than is technically necessary to react with cement. Water–cement ratios of 0.45 to 0.60 are more typically used. For higher-strength concrete, lower ratios are used, along with a plasticizer to increase flowability.
    Too much water will result in segregation of the sand and aggregate components from the cement paste. Also, water that is not consumed by the hydration reaction may leave concrete as it hardens, resulting in microscopic pores (bleeding) that will reduce final strength of concrete. A mix with too much water will experience more shrinkage as excess water leaves, resulting in internal cracks and visible fractures (particularly around inside corners), which again will reduce the final strength.
    The 1997 Uniform Building Code specifies a maximum of 0.5 ratio when concrete is exposed to freezing and thawing in a moist condition or to de-icing chemicals, and a maximum of 0.45 ratio for concrete in a severe or very severe sulfate condition.

  • Anyone tested with foam density?

  • Major Contributor

    Sorry to revive an oldish thread.. I have been really experimenting with the foam density and water ratios.. I’m finding that foam density is more a function of water required for a specific volume of foam.. at the specified density I have been using 4-4.5 gallons of water per 47 gallons of aircrete.. This with 7gallons to begin makes for a very thin and runny mix that cures very slow. I have much better early strength by reducing the initial water content to 5 gallons. I don’t have scientific measurements, but the 2 day cure strength is noticeably better than batches with more water after a week. Your mileage may very with water, concrete quality and weather :)

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