OT--Concrete Mix

When I do a small job, that requires only a small amount of concrete, I will sometimes purchase one or two bags of premix at the BBstores. I have used many bags over the years with no problems.
The last two times--one small patch job and a mailbox post--resulted in the stuff not setting up for a few days to a week!!!
Just yesterday evening, I mounted my mailbox post and this evening the stuff is still SOFT! And, by the way, there was no signs of it being partually set up--like having small lumps!--
Has the mixture been changed??

I don't know if they changed the mix, but I think they use the absolute minimum amount of cement needed. Don't use any more water than what it takes to make it damp and give it about a month to get solid.

I used some last fall that I thought I'd have to tear out and re-do. I got up next to it about 2 months later and it had finally cured into very solid concrete. I'm glad I didn't tear it out.

Tim

Cold weather will slow it down quite a bit, use hot water to mix it in winter for a faster set.

Cold weather will slow it down quite a bit, use hot water to mix it in winter for a faster set.

This happens with premixed cement, such as Quickrete etc. The formular is probably the same, but they likely didn't mix all their batch to a great consistancy. Your bay may have far more sand than concrete, which it should have only twice as much. My guess is that the bags you got weren't mixed well, at least not consistantly.

Ive heard that the cement cant settle to the bottom of the bag during trucking and handling.I would mix some samples.The soil may be taking up all the water from the mix on the mail box post.Put a plastic bag in the hole next time.I roll premix bags around a few times be for opening the bag.A block foundation had to be torn down when premix was used on a job years ago here.Never set up in places.Freezing can ruin fresh concrete. I put a plastic sheet under concrete slabs.

Always add a little cement to the mix to get good strength and quick set up.

Did you use the cheap store brand, or real Sakrete?

I don't really buy that the plant messed up the mix design, its on the bag, the compressive strength, how much water to use, etc. Its possible, but does not seem likely, that the batching plant mixed and bagged faulty material.

Storage, bags are compromised with moisture, that is more likely. End user screwing up the water cement ratio (too much water)seems more likely, not saying that is the case here though.


Sounds like it never hydrated properly and reached its compressive strength.


My experience with bags, at least from jobs I've managed, is a good one. There is a place south of here in Stormville that pre mixes for contractors, and I think name brands like Quikcrete.

Many large masonry contractors like ones I've awarded contracts to, (multi million dollar contracts to give some scale), would buy tractor trailer loads of pre-mixed material from this supplier and every day to adhere to NYC D.O.B. rules a controlled inspection took place, they filled mortar cubes for compressive strength testing, none ever failed, was 3000 psi. I've used the same material, was always good.

Seems odd, well at least if you have to take any of it up, its soft !


The last time I had any experience with a material that did that, it was a 1000 psi lightweight concrete fill specified on the plans and specs for the job by the A/E, ordered 20 something yards, for various areas on the building, elevator machine room roof deck infill slab in same machine room, and similar, hoisted up in a concrete bucket with crane, took longer to place, none of it set up right, was warm out too, total crap, I'd never use that weak of a mix again and would have the A/E, approve different, was a big waste of time, had an after hours crew, cash, everything set up to go smooth, 'cept that material was not right, even the next morning, you could tell the difference between regular concrete and this material.

It is supposed to meet the specs on the bag, but like someone said, it can separate in shipping or even before bagging. I think the minimum is 3500 lb, which should be fine. I have a small important job coming up this spring and I plan on adding portland. I have a couple of bags left over from last summer and they are somewhat hard so I will not use them for anything important.

Maybe somebody accidently put some salt peter in it?

Sorry, I couldn't resist it. Goes back to an old joke I heard in the military.

I think they have quick setting mix in bags, I think I used that
for my wood shed posts.