Concrete Mix ratio

[fxpose]

My local Home Depot and Lowes both sell bulk sand/gravel pre-mixed (in 9 cubic ft sacks) for $29. A yards' worth of this aggregate mix would run $87. The gravel in this mix are 3/4" in size, if that matters. But I don't know what this sand to gravel ratio is. I assume it's roughly 2 parts sand and 3 parts gravel?

At what ratio should I mix Portland cement? One shovelful for every 5 shovelfuls of agrregate?
I have bags of cement which I picked up real cheap. I'm mainly using this to pour footings and to fill block cores.

[Les]

Re: Concrete Mix ratio

I've always used 3 sand, 2 gravel, 1 portland.

Les...

[fxpose]

Re: Concrete Mix ratio

I guess it's really not that critical to have a perfect mix ratio for most general DIY concrete work. I've used your ratio as well as 3 gravel, 2 sand, and 1 portland and didn't seem to make that much of a diff. I've also thrown in shovelfuls of extra portland and gravel I had on hand to standard concrete mix to increase volume.

[david s]

Re: Concrete Mix ratio

For quality high strength concrete the mix is 2:2:1 by weight of aggregate,sand, cement and just enough water to make it fluid (add some plasticiser to aid this)

[Neil2]

Re: Concrete Mix ratio

" One shovelful for every 5 shovelfuls of agrregate?"

And then mix well. Lots of concrete problems can be attributed to under mixing. If using a concrete mixer, let it mix for at least 5 min.

[nissanneill]

Re: Concrete Mix ratio

fxpose's
MIxing concrete is a little scientific to get it right, id to get the strongest and most durble mix without having to increase the Portland proportions.
When you have the sand and gravel (agregate) mixed together already, then you have little option to mix it as required.
You need to consider what the 4 constituents of concrete do individually in the mix, the water obviously makes the mix liquid, too much and it is weak, too little also weak because it is not mixed properly. The Portland is the 'glue' that bonds and holds all of the ingredients, the gravel (agregate) simply bulks out the mix and adds the toughness and durability or hardness and the sand simply fills in the gaps between the gravel.
Using a cement mixer, you should mix concrete properly by starting with the water (a good estimation initially and after a couple of mixes you will have established almost exactly how much you need for the quantity of ingredients used), then your portland to get the 'glue' properly wetted and ready for the gravel. It is important to have this ingredient well covered and thoroughly wet with the portland for maximum strength and then add the sand. Make sure it is thoroughly mixed and add a little more water if the 'slump' of the concrete is not quite right or a little more Portland and maybe sand if it is a little overwet (or too runny).
The strength of your concrete is also determined by it's proportion in the mix. If you want it to be water proof, (eg if rendering or lining a tank or making a pool base) then increase the portland percentage, but keep the mixing correct.

Neill

[dmun]

Re: Concrete Mix ratio

Just a little plug for my

Concrete mixer tutorial.

Worth a look, imho.

[david s]

Re: Concrete Mix ratio

Don't forget the plasticiser. If the batch is mixed with too much water it will not be as strong and the shrinkage will be increased. If you don't have the proprietary plasticiser then ordinary dishwashing detergent will suffice. Only a small amount is required- about 10ml for a batch weighing around 30Kg.

[fxpose]

Re: Concrete Mix ratio

Thanks everyone!

As mentioned in dmun's mixer thread I've been starting with enough water and then adding gravel (prior to my sand/gravel mix purchase) which helps self clean the mixer before each batch. And of course, starting with water first reduces the chance of creating clumps of mix in the mixer.