what would be a good ratio for portland cement, sand, and rock to make concrete for a slab for a storage shed ? The slab will be 12x18, and 4 -4 1/2" thick, with rebar. About 3 yds of mix.

 

I don't know that you need rock in the mix. 3 sand to 2 portland should be fine.

 

Buzzman Hold on there. The cost to mix up concrete to cover that area is more or equal getting a truck out. I do this for a living and my cutoff on hand mixing is 1.5 yards using Portland cement sand and YES you do need stone in it without a doubt "Shrinkage/ Slump" I have the mixers and dump trucks and still will order the Crete out.

Just so you get a scope on this a yard of concrete is approx 4,000 lb wet or dry. So you need 12,000 lb of material delivered and hand mix it. !!!

In my area the concrete companies bought 3 yd trucks for exactly projects like this.

A mix of hand Crete goes like this. A 92 lb bag of Portland cement will have 16 to 24 full shovels of sand to it. 16 shovels full equals about 200lbs. the stone is a 1/3 deal more or less in ratio to sand.

 

"A shovel" is not a valid unit of measure.
The correct formulation is 1-2-3 by Volume, portland cement/graded sand/graded gravel.

One 94 pound bag of portland cement is one cubic foot of material, so for 2800 PSI concrete, you would need 2 cubic feet of sand and 3 cubic feet of gravel, giving you a finished volume of about 5 and half cubic feet of mixed concrete. For a cubic yard of concrete, you would need 5 bags portland, 10 CuFt sand and 15 CuFt Gravel.

More or less.

 

Your 1-2-3 theory sounded extremely accurate until the last line, more or less. We always counted shovels & never had a problem.

 

So tell me how you gauge a "shovel". I can see the amount of a shovel varying by +/- 100% per shovel, which is not conducive to an accurate mix.

I can also assure you that no ASTM standard exists for the "shovel" measurement, nor will you be allowed to use it on a commercial project (That is to say, you can use it at your own liability when the tests are performed).

 

There is no way that it can vary by +/- 100%, at least not if the shovel man can see. I'm not saying it's an exact science but it will work for buzz's slab. We aren't paving a federal highway.

 

I bet you also say quite often, "Cain't see that from my house", or "Close enough for the wimmens I go out with".

 

Thanks for the replies. The reason I need to mix is deliverd concrete is 70 miles away, and cost is very high for small load. I also work for a road building contractor, and been working 6 days a week. No concrete trucks on Sunday.
The rock and sand is no problem my employer dump a truck load each in my yard. I have 15 yds of each, maybe more like 40 yds. Quad axle dump trucks.

 

Tscarborough
Are you trying to make me out to be a redneck or someone somone from the hills, with a lesser understanding? I was born in Brooklyn, wise ass.

 

Those are common jobsite soliloquies, Pulpo. I am not a fan of your type of measuring, that is all.

 

Is Brooklyn a pedigree of quality and concrete knowledge?

I recognize it is just a shed in NE MN, but people can get an bad idea and understanding of how to make even minimal concrete. This can creep into all concrete ideas that are more critical.

A slab that size and mixing it is very ambitious even with a few friends. You can be guaranteed of getting different batches and colors of concrete. When using sand, the moisture can cause bulking and throw off the weight of the actual sand in a shovel off by 10% even if you are absolutely careful.

Dick

 

Tscarborough
You don't have to be a fan & that's fine but you were trying to make me sound like some uneducated person who can't speak properly.

 

Sorry, Pulpo, I didn't mean to do so. Those are just common sayings that I hear too often on jobsites meaning, "this may not be right, but it is close enough", and that is not a sentiment that I share.

 

Ok, I guess the sayings change from place to place. Here we say, close enough for government work.