Concrete Questions

[n74tg]

Two questions:

1. I've seen recommended proportions of cement, sand, gravel expressed as the ratio 1:2:3.  I have seen the ratio expressed both as by weight and by volume.  In fact the instructions on the bag of Portland specify by volume.  However, if by volume (probably the easier way to mix it) doesn't that assume that cement, sand and gravel all weigh the same amount per cubic foot (ie have the same density).  That's the only way the by volume and by weight methods would be equal.  What experiences and or recommendations have you all had mixing either by weight or by volume?

2. I have also seen other ratios, such as 1:2:4.  This concrete will fill the cells in my dry-stack walls and be used in the bond beam, so I'd expect I don't need anything more than maybe 2000 psi concrete.

Any thoughts....thanks

[glenn kangiser]

Only one thing these values can be is that they are rule of thumb measurements that work pretty well for non-critical purposes and generally fall within an acceptable strength range.

If no one is there asking for cylinder tests and this is a recommended ratio then I don't see a problem.  I recently did a pour using a pump and pea gravel.  The concrete plant charged more for the mix because to meet the engineers specs with pea gravel, it required more cement to glue the smaller rocks together.  Larger rocks take less cement to achieve the same hardness.

The mixes on the bag do not specify what size rock so therefore cannot predict what the ultimate strength of your mix will really be.  Just generally that one ratio is good for one thing and a weaker ratio is good for something else.

Critical mixes require cylinder crush test and then you don't know if you made it for 28 days.  Generally the concrete company will design the mix so it will pass the test for sure.  They don't want problems.

[desdawg]

To me in my unsophisticated mentality it means one shovel full of this, two shovels full of that etc. Then you can adjust it to suit you and your specific purpose.

[glenn kangiser]

That's it exactly, desdawg.  What could be easier.

Now --why'd I write all that other crap. :-?

[JRR]

When doing my own on-site mixing, concrete or mortar, I like to make sure the water-to-cement ratio is in the ball park.  

(For professional work, there is actually a "slump" measurement ... and concrete suppliers will talk of "slumps" [measured in "inchs"] as well as strengths.  A typical slump might be 2" [very tight], to 6" [much looser].  "Slump" is the distance the top of a test cone (looks somewhat like a megaphone) of wet concrete falls after the metal cone is lifted.)

But for homemade batches of concrete or mortar, a lot can be detemined by dragging a pointed blade, such as a trowel, thru the wet mixture.  If the furrow made keeps its shape and doesn't fall in or gather water ... the cement/water ratio is pretty good.  Also, for mortar, the wet mixture should cling to a clean metal trowel even as the trowel is turned over.