Concrete Strength vs Thickness

[buzz]

I'm just getting ready to pour a new garage pad this coming weekend and have had a couple different opinions from the guy placing the concrete versus the supplier in regards to my needs. Looking for some quick answers.

Bit of background, 1021 sq ft garage, Grade Beam is already poured. For the pad base there is just under 2ft of a clay / rock mix used for parking lots and then another 6-7" of 3/4 down. It's also been watered and compacted. It should be pretty solid. Only real issue was I ran out of 3/4 to level it perfectly so it's a little short in a few spots, but I'll make those up with concrete. On to the concrete question...

Concrete pad will be roughly 6" throughout the center area, however gets as thick as 8" on the outside. The concrete supplier and is saying to go with a 30 MPa mix, while the placer wants to use a 32 MPa. Is the 32 even needed with such a thick pad? Is there an advantage to using it? I would assume the thicker you go on the pad, the weaker the mix you could use (within reason) however I could be totally wrong and that's why I'd like some input.

 

[dxdexter]

The advantage of a 32Mpa (4700psi) mix is cold weather durability. There is not a great deal of difference between the two mixes strength-wise (290psi), but the Canadian Standards (CSA) recommends 32Mpa (air entrained) as a minimum strength for exterior concretes exposed to freeze-thaw and deicing chemicals. That being said ,if you decide on the 30Mpa then I think you will have no problems since the concrete is inside a structure and not exposed on a permanent basis. The concrete is however exposed to freeze-thaw (if unheated) and deicing chemicals dripping from your vehicle.

I assume you are in Canada if you are using metric strengths, so ensure you ask for 6% air entrainment for the cold winter climate .

The slab for a residential garage should be a minimum of 4", but 5" or 6" is better as it improves the flexural strength of the slab. Make sure the base and sub grade materials are well compact as they are the support for the slab. Any saw cuts should be done ASAP when concrete has sufficiently hardened or gained enough strength to prevent tearing as it is sawed (raveling) and should be a minimum of 25% of the slab depth.

 

[buzz]

Thanks for the info Dex.

You assumed correct, I'm in Winnipeg, Canada. I also talked some more with the sales guy at LaFarge and got some further info from him as well. I'm ending up going with the 30 MPa w/o air as it will be in a heated garage so it won't have to cope with the freeze-thaw cycles.

 

[MDH9252]

Concrete finishers always want to use higher strength concrete because it's easier to place and finish (less rock and more concrete per volume). If you're pouring 6-8 inches deep, save the money and use the cheaper concrete. In the U.S. most use 3000PSI for driveways, 3500PSI for floors (easier to make a smooth finish), and 4000PSI or higher on poured foundation walls (depending on code and engineering specs).

 

[dxdexter]

Thanks for the info Dex.

You assumed correct, I'm in Winnipeg, Canada. I also talked some more with the sales guy at LaFarge and got some further info from him as well. I'm ending up going with the 30 MPa w/o air as it will be in a heated garage so it won't have to cope with the freeze-thaw cycles.

Unless there is a specific reason for use of non-air entrained concrete (i.e. surface hardeners), then I would advise against its use. Even if the garage is heated any dripping of salts from your vehicle can cause spalling or scaling of the surface. The process is osmosis and is described in the excerpt from the Cement Association of Canada:

"Osmotic pressures develop from differential concentrations of alkali solutions in the paste. As ice develops, it creates an adjacent high-alkali solution. The high-alkali solution, through the mechanism of osmosis, draws water from lower alkali solutions in the pores. This osmotic transfer of water continues until equilibrium in the fluids’ alkali concentration is achieved. Osmotic pressure is considered a minor factor, if present at all, in aggregate frost action, whereas it may be dominant in certain pastes. Osmotic pressures, as described above, are considered to be a major factor in “salt scaling.”

The salt can also crystallize. As the salt crystals grow they expand within the paste. Air voids allow for this expansion to take place without affecting the concrete.

Air entrained concrete should be the same price, but will be slightly more difficult to finish.