Usable life of bagged, premixed concrete

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neb

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#259915 30-Oct-2019 14:35

I've inherited a bunch of 40kg bags of premix concrete which have been stored under someone's house for 12-18 months. Surprisingly the contents are still in powdered form and haven't turned to, well, concrete. I mixed up a test batch and cast it into a 25mm thick layer (it just happened to be the amount I scooped out of the bag) in an old plastic container. After three days curing it wasn't too hard to break it in half, see the photo.

So I'm wondering, is this still usable and I was a bit too hasty in trying to test its strength (I realise it's only supposed to be at 40% strength after 3 days, and it dried out for about 24 hours when I was away), or do I now have to dispose of a few hundred kg of not-really-hardfill and not-really-premix?

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#2346332 30-Oct-2019 14:48

it depends on what you are planning on using it for, It certainly won't ever be full strength, but if its for general jobbing with nil or light loads its still probably usable,but I wouldn't pour a carpad with the stuff..

the official skinny from over the ditch is 6 months....

https://www.cementaustralia.com.au/faqs

Does cement have a use by date?

The recommended shelf life of grey cement products is approximately 6 months from the date of manufacture. It is recommended that Off White Cement is used within 3 months of manufacture. These timeframes are based on the understanding that the product has been stored unopened and off the ground away from moisture. Cement is recommended to be stored indoors out of the weather and with the pallet wrap always maintained around the product. It is essential to ensure that stock of this product is rotated with the oldest pallet always used first.

wally22

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#2346339 30-Oct-2019 15:10

It is possible that you put too much water in. You could try another test and ensure you only use enough water to mix, not to make is easy to use. Make sure the mix is dry and hard to trowel smooth and it will be a valid test. The main issue with concrete strength is too much water. But maybe you know this already.

neb

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#2346344 30-Oct-2019 15:14

Yeah, I know about the six-month figure, but figured it was more a liability-avoidance margin than anything else, i.e. it's the minimum OK time no matter how badly you store it.

I'm not planning to use it for anything serious, concrete in a new letterbox, that sort of thing. In that specific case the existing letterbox needs replacing soonish, so I haven't got too much time to experiment with different mixes and setting times, which is why I asked here.

neb

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#2346354 30-Oct-2019 15:28

wally22:
It is possible that you put too much water in. You could try another test and ensure you only use enough water to mix, not to make is easy to use. Make sure the mix is dry and hard to trowel smooth and it will be a valid test. The main issue with concrete strength is too much water. But maybe you know this already.

Ahh, good point! In this case just sorta kludged it since I didn't know if it would set at all, so just added a quantity of water till it looked right. Almost certainly too much, I think for premix it's supposed to be 10:1 (so 4L water for a 40kg bag), and I'm sure I used a lot more. When I do the letterbox with the full 40kg I'll use the right ratio.

richms

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#2346360 30-Oct-2019 15:39

I would use it for letterbox, or a pole for a hose tap. Not a birdhouse tho. They really do go weak.

Richard rich.ms

wally22

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#2346383 30-Oct-2019 17:27

neb:

When I do the letterbox with the full 40kg I'll use the right ratio.

If it is not a rapid set mix, then best to mix it in a barrow or on a board or path to the correct consistency only, not to the full ratio. You don't know how much moisture is already in it. It won't be much, as it hasn't gone off, but the less water you use, the stronger it will be, up to its specification.

20MPa is quite strong enough for most things around the home.

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nickb800

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#2346386 30-Oct-2019 17:36

If it's for something more important then you could add straight cement to it in the usual ratios (treating the premix as the gravel/sand component). So your certainly shouldn't have to throw it out

raytaylor

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#2346958 31-Oct-2019 20:53

Quick set is very much not as strong as standard

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neb

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#2347629 2-Nov-2019 16:03

Thanks for all the above advice. In particular, @wally22: Yep, definitely too much water, since I was using maybe a handfull of mix and just added a dash of water to it, it ended up way too runny. Mixing the full batch with a more readily estimable amount of water produced the proper thick porridgey consistency. @wally22 again, I also mixed it in a separate container as suggested and moved the result into the form I was casting it in. Finally, @nickb800, yep, good point. This stuff still seems OK, it set at the expected rate (unlike the earlier watery batch), will report back in a day or two.

I've also done a bit more digging and apparently it's at least 18 months, possibly two years old. So premix is still fine after at least 1 1/2 years provided it's been stored in the dry, the only thing I had to do was break up a few clumps near the edges of the bag, but that may have been like that since the day or manufacture.

netspanner

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#2347967 4-Nov-2019 10:04

As a non concrete person.I knew a farmer who just poured the dry mix directly into the hole for fence posts and said the water in the soil was enough to harden it. It appeared to work, and I did it once for a mailbox and gave it a sprinkle with the water can. BTW he also said he used to pee on it.

Is this not good practice?

geoffwnz

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#2348011 4-Nov-2019 10:14

netspanner:

As a non concrete person.I knew a farmer who just poured the dry mix directly into the hole for fence posts and said the water in the soil was enough to harden it. It appeared to work, and I did it once for a mailbox and gave it a sprinkle with the water can. BTW he also said he used to pee on it.

Is this not good practice?

I would suggest that he might need to see his Dr if he can pee enough for a line of fence posts. :-p

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wally22

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#2348013 4-Nov-2019 10:15

The standard practice for bags of Rapid set is to line the hole with plastic to prevent groundwater adding to the mix and therefore weakening it. Then place product and required water into the hole and mix it there. It sets so quick if you mix it first then you may have ready set concrete by the time you finish decanting the last of the mix into the hole.

In wet areas there is often enough, or even too much, groundwater present.

I have no information about urine, except it is mostly water.

netspanner

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#2348065 4-Nov-2019 12:35

geoffwnz:

I would suggest that he might need to see his Dr if he can pee enough for a line of fence posts. :-p

It might be a good way to spend the afternoon, Put in a fence post, drink some beer, lay the concrete, pee in the hole. Next post ....

neb

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#2348106 4-Nov-2019 14:24

netspanner:
geoffwnz:

I would suggest that he might need to see his Dr if he can pee enough for a line of fence posts. :-p

It might be a good way to spend the afternoon, Put in a fence post, drink some beer, lay the concrete, pee in the hole. Next post ....

I think you've got the ordering wrong, it more likely went something like a dozen cans on a Friday afternoon, followed by, "hey, you know what would be a good idea?...". Then "OK, so we couldn't find a chainsaw and 6000 feet of tram cable, but you know what else would be a good idea?".

neb

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#2348109 4-Nov-2019 14:29

wally22:
The standard practice for bags of Rapid set is to line the hole with plastic to prevent groundwater adding to the mix and therefore weakening it. Then place product and required water into the hole and mix it there.

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.

The old-premix experiment seems to be going fine, just moist-curing it for a few days under the house before I place it in its final position in direct sunlight out on the street.

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