Concrete slab

[jpig]

Hi every one I'm putting in a lift, my floor had cracks in it so I cut out a 13x10' section. Pouring a 6" thick pad with 1/2" rebar. Question is should I put a in a saw cut and should I pin it to the existing 4" to 3" slab. I'm thinking it should be a separate slab due to the different thicknesses. Any ideas?

 

[Falcon67]

I would pin to keep them from moving in relation to each other. I'll let others talk about saw cuts - I put in 960 sq/ft of floor and there's not a saw cut one in it.

 

[Leaflessshadetree]

I'd pin it to the existing slab. Also wet the existing concrete before you pour. I would finish the edge where it meets the existing slab. I wouldn't put any saw cut (or tooled) joints in the 10x13 section.

 

[cva1993]

Sawcuts are only useful in forcing any concrete cracks to occur in the sawcut thus making the cracks less visible. Regardless of difference in slab thickness it is a good idea to 'tie' the patch into the rest of the slab. How far apart did you space the rebar?

 

[Cyberbear]

Along with everything else, it may be a good idea to allow for a proper size 12" deep concrete pad under each post of the lift you are going to use. Better too much than not enough, and don't skimp on the steel. Another good thought is to pour a 12" footing around the new slab perimeter, or at the very least a "false footing" if money is real tight.

 

[soj]

Along with everything else, it may be a good idea to allow for a proper size 12" deep concrete pad under each post of the lift you are going to use. Better too much than not enough, and don't skimp on the steel. Another good thought is to pour a 12" footing around the new slab perimeter, or at the very least a "false footing" if money is real tight.

Can you explain the difference between "footing" and "false footing"?

Thanks, jp

 

[bczygan]

Pin it!

 

[wssix99]

First, I would make sure you follow the manufacturer's instructions. They should have specs for this kind of thing.

Question is should I put a in a saw cut

That depends. How much rebar are you putting in? With the proper amount of rebar, you should be able to forgo any saw cuts. (The rebar will control any shrinkage cracking that the saw cut joint would typically be used for.)

should I pin it to the existing 4" to 3" slab. I'm thinking it should be a separate slab due to the different thicknesses. Any ideas?

I'd first check with the manufacturer. You will probably need to pin or key the slab to keep the lift and new pad from tipping over as easily. If you can do both, (and your old slab gives you enough to pin to) then great.

 

[Kevin54]

Another vote here for pinning it.

 

[matt_i]

Also pin, it would **** to pour a new section and have it sink lower than the rest of it.

Part of the problem is its really difficult to get the subbase compaction perfect right up to a sawed shoulder, especially if there is a step as you have, so the steel rods help to bridge across that little inconsistency.

 

[NUTTSGT]

I'm in the "pin it" crowd also myself.

I'll also through this into the thought process as well.

Cyberbear mentioned 12" where the posts will sit, not a bad idea in my book. Your plan calls for a 10'x13' pad 6" thick. That comes out to just under 2 1/2 yards of concrete. Most places have a minimum yard order, if not, you get assessed a surcharge. You might as well take full advantage of the minimum yard order for your money. If you were to pour that section 12" thick, it comes out to 4.81 yards (just under 5 yds).

Now if you follow along and get my drift, make the pour as thick as you can for your money. If the locals have a minimum yardage of three, make it 7" thick. If their minimum is 4 yards, that is just under 10" thick. But if it's 4 yards, why not spend about another hundred bucks and make the entire pour 12" thick ?

I use this link to do some quick concrete calculations.

http://www.concretenetwork.com/concrete/howmuch/calculator.htm



I know it's a little bit of a run on but find out what the local guys minimum order is before you set your depth of the pour.

 

[jpig]

Thanks for all of the thoughts on my questions. The rebar is spaced 24" both ways. I'm not sure what the minimum yards are. I will find out. Sounds like everyone is in agreement about pinning.

 

[Cyberbear]

A "false footing" is a term used in my area of the country to describe a footing trench that is narrower at the bottom than at the top. It's used to save time and materials = cost saving, although its not that much in a job your size. The steel is what really counts, I'd use 12" o/c most of the time with #4 bar.
Opinions vary about the need for pinning one slab to another. based on all the possible variables in such a pour as yours, I'd go ahead and pin it just for the peace of mind knowing you did all you could do. If you live in an expansive soil area, pinning is a very real consideration. I've seen things change and move where the adobe/clay content of the soil is high and it rained heavy.

 

[wssix99]

Thanks for all of the thoughts on my questions. The rebar is spaced 24" both ways. I'm not sure what the minimum yards are. I will find out. Sounds like everyone is in agreement about pinning.

My rebar calculator says you'd need 6" spacing with #4 rebar to eliminate the need for a saw cut joint. (You can also use larger bar if you want to space things out more, but the last time I checked #4 was way more economical per lb. than any other size.)

The need for rebar is also proportional to the thickness of your slab. (More concrete = more shrinkage force as it cures = need more steel to control)

 

[Lootenny]

Have you checked to see what the lift manufacturer says you need underneath the lift's posts? I was surprised when I checked on the Rotary site that their 2 post lift rated at 10,000 lbs only needs 4" of 3000psi concrete under it. No, more won't hurt anything, but at some point you have to say "enough."

 

[jpig]

I would like to thank every one for the info. A wealth of info on this site

 

[over40pirate]

You could make the slab 4-5" thick, and make footings for the lift.
How much weight will ever be placed inside the lift area?

 

[tcianci]

You specified an area and a thickness. Hopefully, that information was obtained from the manufacturer of the lift. The comments on here that advocate absurd thickness of concrete either 1: own a concrete company, 2: have no freakin clue or 3: both. While sufficient concrete thickness, which can only be properly specified by the product manufacturer, is important, it's equally important to place the concrete on a properly drained and compacted base. If the location of the pad is well inside a conditioned space, you probably won't need to address drainage issues, but any concrete you place is only as good as the base you put it on. The thickness and the compressive strength of the concrete only allows the the pad to distribute the applied weight to the base.

If you think about it from a strictly mathematical sense, the weight you're planning on bearing is the combined weight of the vehicle and the hoist. Whenever the vehicle is parked in the existing space, you're distributing the weight of the vehicle over the 4 contact points of the tires. With a 4000-5000 lb vehicle, that's about 1200 lbs per tire. If you knew the contact area of the tire bearing the weight, you could arrive at a pounds per square inch number. Considering that the concrete specified by the lift manufacturer is specified at 3000 PSI compressive strength, and you may now be bearing about 1200 lbs over a contact patch of maybe 25 sq inches per tire that gives a pressure of less than 50 PSI. Then you add the load applied to the pad by the weight of the hoist itself, and you still have a pretty small number.
Considering that you already have to deal with hauling away the concrete from your cut out area, It wouldn't make any sense haul out and dispose of much more material for a gain in load bearing capability the device will never see. To place much more than the manufacturer-recommended pad for the installation of your lift doesn't make financial or mechanical sense
Depending on how you removed the existing pad, you may have sufficiently disturbed the underlying soil. The most important thing is to be sure that soil is properly compacted for the placement of the new pad.

 

[Trey T]

...

The most important thing is to be sure that soil is properly compacted for the placement of the new pad.

Not just properly compacted but that gravel (aka foundation) under the slab (aka concrete structure) is properly sized.

To make it simple, I highly suggest looking at the manufacturer's concrete slab design and implement it properly according to your existing foundation and slab.

 

[Justanoldguy]

(The rebar will control any shrinkage cracking that the saw cut joint would typically be used for..

Seriously?

Learn something new every day.

Even if it is totally false.

 

[wssix99]

Seriously?

Learn something new every day.

Yea, there is a technical guide put out by the American Concrete Institute (ACI) that gives specs for this: ACI 360R-06 If enough of the floor's cross section is steel, the shrinkage cracks are kept to small you can't notice them easily. (Hence, the control joints aren't needed to control the unsightly cracks.)

We just built a house and our ground floor is a heated, epoxy coated slab. The expansion and control joints can be filled, but the epoxy never hides them 100%. So, for the section of the slab in our foyer, we over-reinforced the slab so we didn't have any joints. Even before the epoxy, the floor was perfect. (Except for one crack coming out of a re-entrant corner, which the epoxy filled in and hid.)

I used the same guide in the garage to allow us to space the control joints further out to accommodate placement of the lifts.

Even if it is totally false.

If you don't believe the guide, (and are still awake after reading it...) I have pictures to prove it!