Concrete

[zzvyb6]

I ordered my driveway at 4000 psi strength (6-1/2 bag mix) low slump. I suggest the other 'concrete cooks' list their recipe in terms of bag fraction, shovels/buckets of aggregate and bucket(s) of water if you will be hand shoveling it into a mixer. That would be really helpful !

 

[Cat_Driver]

Always add fibers in concrete

 

[FTG-05]

The actual weights used to make 4.5 yards of 4000 psi concrete are:

1. Cement = 2810 pounds

2. Stone = 8250 pounds

3. Sand = 5620 pounds

THIS GIVES YOU A BASIC 4000 PSI CONCRETE MIX RATIO OF:

1 PART CEMENT
2.93 PARTS STONE
2 PARTS SAND

Rounded off it's basically a 1 : 3 : 2 mix ratio

If I break the weights down per cubic yard of concrete, it comes to:

1. Cement = 624 pounds

2. Stone = 1833 pounds

3. Sand = 1249 pounds

There are 94 pounds of cement in a "sack or bag" of cement. That means 4000 psi concrete is also called a 6.5 sack or bag mix.

Dave M7040

Good stuff, thanks for posting!!!

 

[lilranch2001]

As Piney alluded to, type of aggregate is important, along with cement strength, and sand content (type/size/contamination)

 

[Tractor Seabee]

Rebar goes a long ways towards strength as well. It depends on the application of course. A bridge beam has far more tensile forces acting upon the bottom of the beam then a concrete slab.

I always go by "a 5 bag mix", meaning I use (5) 94 lb bags of Portland cement when making a cubic yard of concrete from the gravel from my gravel pit. For high strength concrete, I use a "6 bag mix". But that is just knowing the aggregate source, and adding the Portland cement to it to get what I need..

Gravel from a site pit adds a strong variable. Batch plants and other certified sources of concrete use washed aggregate, gravel and sand. Other clays and organic materials degrade strength rapidly. Unless you do test cylinders and have them tested by a certified lab you have no idea even what your theoretical strength is.

Rebar is necessary for tensile strength. Slab on sub-grade grade that is properly compacted are dependent on the underlying sub-grade for tensile strength which is a minimal issue unless bridging bad soils. Dynamic loads are a big part of the strength requirement. Current concrete industry engineering is going a lot toward low slump and fiber only in slabs on grade. I have had excellent results; finishers hate me. If pumping use water reducing additives to increase flow rate.

Admittedly a lot of old concrete using site materials, lots of water and cement did hold up pretty well. A lot didn't.

Ron

 

[Tractor Seabee]

I ordered my driveway at 4000 psi strength (6-1/2 bag mix) low slump. I suggest the other 'concrete cooks' list their recipe in terms of bag fraction, shovels/buckets of aggregate and bucket(s) of water if you will be hand shoveling it into a mixer. That would be really helpful !

If every thing is done properly you will wind up far in excess of 4K#. In today's economy and at my age I would never bust my butt to make concrete by hand with a shovel. I have a small HF mixer for bag concrete I use for fence post and other 1-4 ready mix bags at a time. I put a ring bolt on the tongue so I can move it around with tractor using my FEL bucket trailer ball hitch.

Ron'

 

[BrokenTrack]

Gravel from a site pit adds a strong variable. Batch plants and other certified sources of concrete use washed aggregate, gravel and sand. Other clays and organic materials degrade strength rapidly. Unless you do test cylinders and have them tested by a certified lab you have no idea even what your theoretical strength is.

Rebar is necessary for tensile strength. Slab on sub-grade grade that is properly compacted are dependent on the underlying sub-grade for tensile strength which is a minimal issue unless bridging bad soils. Dynamic loads are a big part of the strength requirement. Current concrete industry engineering is going a lot toward low slump and fiber only in slabs on grade. I have had excellent results; finishers hate me. If pumping use water reducing additives to increase flow rate.

Admittedly a lot of old concrete using site materials, lots of water and cement did hold up pretty well. A lot didn't.

Ron

That is true, but at what point does it really matter? I am not pouring a concrete bridge deck, I am just pouring a concrete slab for a barn, mudroom, or garage. I know from experience that our gravel pit takes 5 bags of Portland cement to make a cubic yard.

It just seems today everything has to be a science, but my question is why?

I got a barn floor that does not have entrained concrete, fiber, or rebar, and yet I clean it out all winter, every winter since 2015, using my bulldozer. To me that is good enough, and there is two ways to get that:

(1) Hire an engineer to spec out the mix
(2) Use your experience on what has worked in the past

Neither is wrong, because neither is a guarantee for success.

 

[BrokenTrack]

Most of my concrete work is in pouring concrete slabs, and if that is the case, I forgo my cement mixer because it is too much work, and very time consuming. For concrete slabs I turn to my rototiller.

It is pretty easy, you just haul in your gravel and put it in over your pad area. Then you figure out how many bags of Portland cement that you need, and then spread it out evenly over your concrete slab location. Then set the depth of your tines to the depth of your slab thickness, and take your rototiller and thoroughly mix up your gravel/cement.

After that, you add in the water, and mix up your concrete again.

You cannot have rebar, mesh or pex before you mix, but you can add it in afterwards. Then you just finish the concrete as always.

I can make my own concrete for about $50 a cubic yard which is about half of having it delivered.