Increased Concrete Strength: Added Thickness vs: More Cement

[LifeLongWNYer]

Hi folks. I'm planning a new garage and want to provide for the installation of a lift at some point in the future. In conducting a little research, it appears that most lift manufacturers want to see at least 4" of 3,000 lb concrete under their lifts.

I believe in an extra margin of safety and am wondering whether increasing the thickness of the slab is more effective in gaining strength than adding cement to the mixture ( to get stronger concrete ).

About 30 years ago I had a garage slab poured that was 6" of 4,500 lb concrete. To this day, it doesn't have a chip or a crack and very little spalling from road salt runoff. I'm not sure I can afford to build another floor that way, but wonder about the difference between added thickness and a richer mix.




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[A**holes Garage]

Dig a little deeper at the proposed lift loading locations and pour thicker concrete, more rebar, stronger mix etc. Pour the rest of your slab at the normal thickness and strength.

 

[rlitman]

More cement in the mixture does not necessarily mean it is stronger.
Less water does, as does a sharper aggregate. Overworking can also reduce strength.

4" of 3000PSI concrete is pretty much just a minimum for a car. I'd expect a lift to want better than that. But if it were me, I'd just make it 6" for at least a foot or two around the lift bases.

 

[LifeLongWNYer]

Thanks, A**hole, I get what you are saying, on everything except the stronger mix. The concrete comes in a truck with 10 yards of wet stuff in it. How do I tell them to make a stronger mix where the lift will be?



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[LX-Markham]

Dig a little deeper at the proposed lift loading locations and pour thicker concrete, more rebar, stronger mix etc. Pour the rest of your slab at the normal thickness and strength.

This.

Slab thickening where you think the posts are going to go. Just use the same concrete strength everywhere.

 

[Architorture]

Depends on the loads...for a lift a thickened slab would be effective...for impact type loads or rolling loads you want the stronger mix to achieve more surface hardness

 

[Davefr]

You better keep the slab moist for several weeks following the pour. A slow cure with plenty of moisture creates a much stronger slab.

The curing of concrete is a chemical process and water/moisture is needed to be absorbed to aid the process. Try and pick cool moist weather for the pour.

That will help increase the strength and it's easy to do.

 

[jack stand]

Thanks, A**hole, I get what you are saying, on everything except the stronger mix. The concrete comes in a truck with 10 yards of wet stuff in it. How do I tell them to make a stronger mix where the lift will be?



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I'm pretty sure that 3000# mix is the minimum that you want in the rest of the slab anyway, so if you make a couple of 4'x4'x6" "pads" at your lift location, (that is 2" extra depth at these lift pads) you'll be fine.

 

[A**holes Garage]

Thanks, A**hole, I get what you are saying, on everything except the stronger mix. The concrete comes in a truck with 10 yards of wet stuff in it. How do I tell them to make a stronger mix where the lift will be?



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You may not have to have a stronger psi mix. Just thicker, wider area with a double or triple mat of rebar at the load bearing post locations.

Not sure what the concentrated loads are per post, but a boxed or pier area 12" deep x 8" or so wider that the post base footprint?

The lift provider should have recommendations as well.

Plus, you can set J-bolt anchors if pouring new instead of drill in.

 

[brownbagg]

look i test concrete everyday for 31 years, here the down and out

the answer is both or should i say either, but there also one more, less water, or a super plastizers.

by thicking up the slab you are creating more resistance to bending by load factors so less cracking.

adding more cement increase the psi and its more resistance to bending by load factors, less cracking.

controlling the water or using a supra p mix, increase psi ( see above) less bending

ok what is bending, even with a compacted sub soil when a load is applied to a slab the weight cause a bending motion, this is what causes the cracking. cracking is a failure.

oh all concrete crack. sorry bub, that an excuse concrete finishers use for sloppy workmanship. I can place a slab anytime of week and guarantee no cracking, the problem is, if everybody not on board, it will crack, one small stupid move by a finisher, producer or time and heat, and it will crack. but its easy to prevent. time temperature and slump

 

[kc10a]

I lived in Germany for about 9 years. Their concrete doesn't crack. When they pour it, you would swear that it needs more water. It takes them a long time to work it, but it sure lasts. In the late 90's, I flew on a base that was built by the Germans in the late 30s. It had never been resurfaced and there were no cracks at all. The bunkers were the same way. It was amazing.

 

[WNYflyer]

A majority of concrete slabs-on-grade are designed as un-reinforced concrete. In otherwords any steel reinforcing installed is assumed to add no structural strength to the slab. The steel that is often added is for crack/shrinkage control, etc. rather than strength.

Thus for plain un- reinforced concrete slabs the strength in bending varies with the square root of the concrete mix strength and the square of the thickness. Thus for example:

strength = 3000 psi...............square root of 3000 psi = 55 psi

thickness = 4"............... 4" squared = 16

So if you change the concrete to 4000 psi concrete the square root becomes 63 psi. Therefore 63 psi/55 psi = 1.15 Thus a 15 % increase in concrete slab strength in bending by going to 4000 psi concrete.

But say you keep the 3000 psi concrete but go to 5 inches thick. Then 5" squared = 25. Thus 25/16= 1.56 or a 56 % increase in slab strength in bending when changing the thickness to 5".

In general you gain bending strength in your slab quicker by increasing the thickness rather than increasing the actual concrete strength itself. Of course the cost implications of any changes would need to be looked at to see what gets you the biggest bang for your $$$'s

Theses are basic generalities and there may other reasons to also increase the concrete strength or increasing the thickness (i.e extra weight to act as a counter balance etc.)

Hope it helps some.

 

[RivennHewn]

Call your concrete supplier, ask for a salesman, tell them what you need.

If you give them specifics, they'll match you with the right mix.

 

[NUTTSGT]

I don't have a lift nor do I plan on putting one in. If I were to do a pour with the expectations of a future lift, I'd err on the side of overkill. Not only would I make the pad thicker, I'd dig a hole for a pillar to sit on under the pad. Something similar to this, except no sonotube.


and add a little reinforcment


Personally, I like overkill and it probably won't cost you more than an extra $100 for the wire,rebar and concrete if you do it yourself.

 

[ConCretin]

I believe in an extra margin of safety and am wondering whether increasing the thickness of the slab is more effective in gaining strength than adding cement to the mixture ( to get stronger concrete ).

For a lift, i agree with WNYflyer - added depth around the posts will give you more bang for negligible bucks. The added depth will also increase resistance to bending and other loads such as punching shear and allow for deeper anchors, which would provide more resistance to pull-out. Of course, this is all overkill and a 4" slab will work fine.

With that said, I'd never place a garage slab with less than a 4000 psi mix. The additional compressive strength is completely unnecessary but the higher water/cement ratio will provide a much more durable surface.

Finally, don't neglect the base under the slab - this is the most important part. Unless you're going to place a much thicker structural slab, the sub grade is going to support the loads you place on the slab.

 

[C96]

You sound a little iffy about weather or not a lift will ever be installed. That said, don’t worry about it then. Just pour your slab as you normally would.

If / when the time comes for a lift and you finally decide which one to buy (2 poles, 4 poles etc) lay it out exactly where it will go and saw cut the slab. Dig your footings accordingly, install rebar and anchors etc, otherwise plan and install the footings NOW.

I realize what you say the manufacture suggests, but in my opinion, having separate footings dug with steel reinforced concrete for the posts is much better than simply having the lift bolted to a 4” slab. You might get away with doing that with a 4 post lift, but a 2 post lift, forget about it. No way would I install a 2 post lift without steel reinforced concrete footings.

What ever you decide, Good Luck with your build, sounds fun!

 

[T-Mac]

I did my 30x50 in two pours-back slab where the lift is-6 in thick 4000 psi with rebar and fiber frt slab 4000 psi-fiber only-over a year now-no cracks-site prep is crucial.

 

[LX-Markham]

I don't have a lift nor do I plan on putting one in. If I were to do a pour with the expectations of a future lift, I'd err on the side of overkill. Not only would I make the pad thicker, I'd dig a hole for a pillar to sit on under the pad. Something similar to this, except no sonotube.

sorry, but I disagree with this idea. The pillar (or caisson/pile) will be great for supporting the lift, but will create a "support" in what otherwise is a floating slab on grade. As the slab settles (or heaves) the surrounding concrete will crack.

As mentioned, the other option is simply pour the slab and worry about a lift when/if the time ever comes. In my case, I was dealing with an existing garage slab. We just cut the slab, excavated and poured a concrete footing for each of the 4 post locations.

...and yes, that^^^ is a void under the existing slab on grade. the existing grade settled a couple of inches.

 

[joes169]

With that said, I'd never place a garage slab with less than a 4000 psi mix. The additional compressive strength is completely unnecessary but the higher water/cement ratio will provide a much more durable surface.

I agree completely. Not only is 4000 psi more durable, but it often gives the concrete crew a better finish in less time, meaning labor savings. More often than not, in my professional experiences, the cost difference for 4000 psi + concrete is negligable............

 

[joes169]

...and yes, that^^^ is a void under the existing slab on grade. the existing grade settled a couple of inches.

Believe it or not, that's extremely common, especially in our region where floor ledges are used. I'd say 90%+ of new homes will have a substantial of "suspended" floor with the first 10 years. This is where steel re-enforcement is worth every penny in extending the floors useable life IMO.........

 

[LX-Markham]

oh I know it's common.
so far I'm 2 for 2. Both houses I have owned have had voids under the garage slab.

 

[theoldwizard1]

Dig a little deeper at the proposed lift loading locations and pour thicker concrete, more rebar, stronger mix etc. Pour the rest of your slab at the normal thickness and strength.

+1 !

It may actually be easier to just pour a "normal" floor and after it cures and you are certain of EXACTLY where you want your lift footing located, cut a 2'x2' square, dig down 18 (24?) inches, install rebar and pour high strength concrete,

 

[wssix99]

it appears that most lift manufacturers want to see at least 4" of 3,000 lb concrete under their lifts.

I believe in an extra margin of safety and am wondering whether increasing the thickness of the slab is more effective in gaining strength than adding cement to the mixture ( to get stronger concrete )

Most of the finishing advice I've received personally and seen on this site recommends 4000 psi, so you should be set there. I also think you are well advised to go a little thicker - and watch closely! I specified 5" on my slab and was shocked to find it was being poured at 3.5". Fortunately, we were able to fix that during the pour. (My issue was due to challenges around the foam insulation installation and the fact that the floors were being poured to marks on the wall vs. an actual thickness.) I'm told that a good finisher should be able to get within 1/4" of spec, but how many people go back and confirm this in a residential setting? (Until its too late.)

You can gradually taper the slab to be thicker around the posts, but you want to avoid any sharp thickness changes. Sharp thickness changes, along with some other advice above, can differential stresses in the slab - leading to major cracking. You should avoid changes in rebar/reinforcing across the slab, mixing different concretes, and sharp thickness changes unless the different sections are isolated by a full expansion joint. Adding in foundations or piles under the lift leg would also require the resulting "pads" to be isolated with an expansion joint. (Otherwise the general pad could settle around those piles and cause cracking.) Piles would also cause tipping concern for a 2 post lift.

If you want to go all out where your lift will be, you can section off the slab with an expansion joint, go all-out in the lift section, and go normal in the other area.

Putting down a nice flat (or gradually tapered) slab is going to be the strongest and lower maintenance option. Adding reinforcing will give you insurance against unwanted/unplanned cracking and may also allow you to space your saw cut joints further away from each other.


It's natural to think that thicker/stronger "foundations" or "pads" under lift posts are better. In reality, it gains nothing. (Unless its in a retrofit application.) It's possible to "key" pads in to the larger slab for the bending (in the slab) strength that the two post lift needs, but you then have cuts in the slab where fluids can easily infiltrate, etc.

In a very basic sense, there are two types of forces your slab will see from the two post lift: The gravitational force from the weight of the lift and vehicle and then the bending force in the slab created by off-centered loads on the lift.

The vertical bearing force is minimal due to the size of the metal pads on the lift. Depending on the size of those pads, the weight of the lift could create less stress on the slab than the car just sitting there on the slab by itself. It all depends on the contact patch area of your tires and the surface area of the lift plates. Either way - the bearing pressure of your lift will be a few 10's of psi. Since your slab concrete is several 1000's of psi in compression strength, your slab won't even know the lift is there. (assuming the load is perfectly balanced) This is why you don't need foundations or pads and why you won't find lift installation instructions calling for them.

If you look at a cross section of your slab with a lift post sticking out of it, you'll see the shape of an inverted "T." That shape resists bending at the base of the lift and counteracts any tipping forces caused by off-center loads. Having a nice contiguous slab makes a bigger and stronger "T."

That all being said, there are some folks who have used a very deep (and engineered) footing to counteract bending forces, but that's a really expensive compared to a nice, flat slab.

 

[Zeke]

I'm not an engineer but I can vouch for concrete being of better quality when it's placed at optimum mixture ratios. Overwatering is to me the single most abused procedure at any site. It's a tricky business and of all the concrete I have seen placed I have only seen a half dozen lead finishers that REALLY knew what they were doing. By that I mean they had total control over the finishing process all the way to loading up with their tools cleaned up.