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  • Easy Single-Handed Concrete Mixing with a Tarpaulin

    A good idea but unnecessary for fixing posts. A 'dry mix' rammed in is far better. Advantages, apart from less mixing, are support for the post positioning and no shrinkage of the concrete due to surplus water in the mix.

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  • oliver90owner commented on jessyratfink's instructable Unusual uses for ice cubes3 years ago
    Unusual uses for ice cubes

    Cooling in large masses of concrete is simply to prevent cracking during te curing process. The exothermic reaction is particularly pronounced in the early part of the strength gain - from the tri-calcium silicate content reactions. The di-calcium silicates are slower in curing, but all the heat of reaction will expand the curing mass and cause cracks - outer skin would cool and crack shortly after the initial setting process. Most concrete continues to cure over a period of years, but for building purposes, the structure needs early strength (removal of shuttering, placement of more concrete above, etc). Modern day dam structures would be using fly ash (coal ash from electricity generation) as part of the cementitious mix, to slow the strength gain, while giving more than adequate...

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    Cooling in large masses of concrete is simply to prevent cracking during te curing process. The exothermic reaction is particularly pronounced in the early part of the strength gain - from the tri-calcium silicate content reactions. The di-calcium silicates are slower in curing, but all the heat of reaction will expand the curing mass and cause cracks - outer skin would cool and crack shortly after the initial setting process. Most concrete continues to cure over a period of years, but for building purposes, the structure needs early strength (removal of shuttering, placement of more concrete above, etc). Modern day dam structures would be using fly ash (coal ash from electricity generation) as part of the cementitious mix, to slow the strength gain, while giving more than adequate final strength. Of course, there is also the extra problem that chemical reaction rates increase with temperature, so the expansion problem is further exacerbated in huge structures such as dams.

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