Add cement to coarse concrete?

[hallonst]

I have a question about the pre-made bags you buy at the hardware store, like coarse concrete, fine concrete. I feel that there is mostly sand in those bags, that the finished concrete has quite poor strength and corrosion resistance. In many cases, it probably doesn't matter and works just fine.

So now I'm wondering if you can simply mix in extra cement in the bags of coarse concrete to get "stronger" concrete? Anyone know or tried it?

 

[jonaserik]

I have a question about pre-mixed bags you buy at the builder's store, like coarse concrete, fine concrete.
I feel that there's mostly sand in those bags, that the finished concrete has quite poor strength and corrosion resistance.
In many cases, it surely doesn't matter and it's good enough.

So now I'm wondering if you can simply mix in extra cement into the bags of coarse concrete to get "stronger" concrete?
Anyone know or tried?

It becomes denser, normal mix 1 cement 3 concrete gravel, and as little water as possible. Read a bit here
https://www.cement.heidelbergmaterials.se/sv/betongarbeten

 

[cpalm]

I feel that there is mostly sand in those bags, that the finished concrete has quite poor strength and corrosion resistance.

The strength class is stated on the bags. Either C32/40 or C20/25. I don't think it's as simple as adding more cement. The cement content is indeed slightly higher in the stronger concrete, but if I've understood correctly, there's also a different composition of the aggregate to match the cement content.

 

[AG A]

It won't have much impact on strength. However, it can become significantly nicer to work with when it comes to plastering or sanding. Both coarse concrete and especially fine concrete are hopeless to sand. As you said, it mostly feels like gravel. So when small amounts are to be cast, and a fine surface is desired, I always mix in a little extra.

 

[Vante]

I have a question about the ready-made bags you buy at the hardware store, like coarse concrete, fine concrete.
I feel like there's mostly sand in those bags, that the finished concrete has rather poor strength and corrosion resistance.
In many cases, it probably doesn't matter and is good enough.

So now I'm wondering if you can simply mix in extra cement into the bags of coarse concrete to get "stronger" concrete?
Anyone who knows or has tried it?

What is being cast?

 

[hallonst]

Yes, exactly. That's kind of what I think too. I've understood that the strength largely depends on the ballast. And regular coarse concrete doesn't contain any "stones" (8-20) but only gravel/sand. So really, you should be able to add a "reasonable" amount of stones and also cement to get "better" concrete. Maybe not something I would do for casting something with requirements, but for post bases and other hobby casting, it should be better than just using the contents of the bags.

Thanks for the good answer!

 

[hallonst]

What's going to be cast?

Accidentally built a bathroom in the wrong end of the house...
So I'm going to put down some drainage here:

 

[cpalm]

Accidentally built a bathroom at the wrong end of the house…

You don't need any "bank vault concrete" for that, buy regular C32/40 if you don't want to skimp.

 

[jonaserik]

Place a small sand bed at the bottom and lower the pipes (consider the slope) then sand around to about 2 cm above the pipe, what remains to the top floor. Pour in concrete and you can take a stone and rub it down. This will fill it up, so less concrete is needed. It also works great to push down large stones with, layer at the top minimum about 3 cm. You should prime the edges twice so the new concrete binds. If you're worried, it is possible to drill into the sides and insert rebar, this gives a bit more bearing capacity to the casting.

 

[kniv]

Yes, exactly. That's kind of what I'm thinking too. I've understood that the strength largely depends on the ballast. And regular coarse concrete doesn't contain any "stone" (8-20) but just gravel/sand.
So, in theory, one should be able to add an "appropriate" amount of stone and also cement to get "better" concrete.
Maybe it's not something I'd do for casting something with specific requirements. But for footings and other hobby casting, it should be better than just using what's in the bags.

Thanks for the good answer!

Why would footings and other hobby casting need "better" concrete?

 

[hallonst]

You don't need any "bank vault concrete" for that, just buy regular C32/40 if you don't want to skimp.

Hehe, exactly, I agree.
But the question was probably not specifically for this project, but more in general.

I've mixed concrete before for many different projects and often picked up sand and gravel with a trailer.
I've also picked up wet concrete with a trailer, which is a bit stressful, but the end result is really good.

But now I thought I would make it easy for myself and buy ready-made bags. Mostly because I don't have that much to cast.

 

[cpalm]

But now I thought I'd make it easy for myself and buy pre-made bags. Mostly because it's not that much that I need to cast.

It might take more than one might spontaneously think for a trench like that. Depending on how thick you cast it, of course. I probably used up a pallet during my pipe replacement in the basement just for the trenches in the floor.