Usable life of bagged, premixed concrete

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wally22

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#2348120 4-Nov-2019 14:36

neb:

I've seen a lot of instructions that go something like "Dig hole / Wet sides of hole (to avoid excessive water loss in next step) / Pour in water / Chuck in premix". Seems like a somewhat suboptimal way to get the right mixture.

I think this has led to a general thinking in some circles that premixes are substandard. Poor instructions attempting to make it easier. Perhaps assuming that complicated instructions won't be followed?

neb

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#2348129 4-Nov-2019 14:53

wally22:
I think this has led to a general thinking in some circles that premixes are substandard. Poor instructions attempting to make it easier. Perhaps assuming that complicated instructions won't be followed?

Good point! A lot of the instructions are just "slap it all together and walk away". The silly thing is you can get one of those ~$3 flexible plastic buckets from the Warehouse that's the perfect size for mixing 40kg (and by extension 20kg) bags of premix, then you just grab the handles and pour it down the hole or wherever it's supposed to go. Then wait for it to set a bit, wet it down, and seal it in with Glad Wrap - which should be in pretty much every kitchen - to avoid it drying out. Ok, it's not an optimal solution but the cost is close to zero and you'll get reasonable results compared to the "slap it together and walk away" instructions.

raytaylor

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#2348388 5-Nov-2019 09:04

For a fence, its fine.

The posts are supported by each other, linked by the fence rails and so the water poured in to the top of the hole will set it enough to stand upright.

Groundwater will then be absorbed and set some more underground. It sets so quickly that I dont think excess groundwater over time would be an issue.

However we have found that quick set is not as strong as normal concrete. You can tell from the MPA rating.
In my expierence, it crumbles really fast if there is any movement in the post.

I have used quick-set to make several above-ground concrete blocks for small posts holding solar panels where we couldnt dig.
And I have used standard concrete to make a similar above-ground blocks for the same.
The quick-set blocks have crumbled over time. The standard is still strong and solid.

So personally i think quick-set is only good for fence posts where its a large structure with many posts supporting each other to reduce movement, and since its in the ground, if it crumbles, its not going to matter.
Though when we build basic fences, we just ram the soil in around the posts and dont use concrete at all.

Ray Taylor

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neb

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#2348479 5-Nov-2019 15:06

From what I've read about the various acceleration methods, the more you accelerate, the worse the concrete. Chemically-assisted curing seems to be the worst of all, with things like thermal blankets being bad but not quite as bad as additives. Leaving out the moist-curing step doesn't help either. Rule of thumb seems to be, the more you try and rush it, the worse it ends up.

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