Detach 48x28 Garage - Footings

[Pettrix]

Did my pour yesterday. All went well. 35 yards of concrete (4,500 psi winter mix - achieves 1,300 psi within 24 hours). Got kind of crazy with 3 concrete trucks all up on the property at the same time (plant messed up the truck spacing - was supposed to be 4 trucks total with 1 truck every 30 minutes).

Total concrete cost = $6,800 (that also includes house footing - as I am building both at the same time)

Temps in day at 43F and night at 22F. Concrete stayed at 40-42F even at night.

 

[ovrszd]

Great start. Thanks for the pics.

 

[Pettrix]

I am doing a 5" concrete slab for the garage. What do you recommend for rebar spacing?

I got #4 rebar and some say to go 48" oc (on center) and others say to go 24" oc.

 

[ROUSTABOUT]

About to break ground on my 48x28 detached garage and on my engineering footing schedule it doesn't show any footings where the two 18' wide garage doors are. I know they are not "required" as there is nothing being supported wall wise there but someone told me that it's "better" to pour the footing under those doors as to keep the whole garage footing as a rectangle, making it stronger. One would run the horizontal footing bars but no vertical bars would be run since there is no wall being stacked on top of that area.

Is that true? Should I run a continuous footing underneath the garage doors or stop at the door area, and then restart again?

My engineering calls for a footing that is 18" below grade, 24" wide, 12" tall, with three #4 rebars at the bottom of the footing. Garage will be ICF (Insulated Concrete Forms) so I will be stacking ICF (6" concrete core) for the garage walls. I am using 4,500 psi concrete footing pour.

Ifn' ize you, I'd wet set them blocks in the footing. Lay them until just above top finish grade of slab. Then I'd pour the first blocks full of Crete directly out of truck. Pop chalk line around inside and pour away. You can tie wall to slab with rebar. Tie block to footer with rebar. I have been building with ICF since early 1990's.

 

[Pettrix]

So on a garage 5" slab with 4,500 psi concrete. I will put in a car lift. For me rebar schedule, some say to go 48" oc (on center) and others say to go 24" oc.

 

[ovrszd]

I've always been told 24".

2 post or 4 post lift?

 

[ponytug]

My question is why are you using rebar?

To make up for poor soil, or poor base rock?
(I would find a way to get the base compacted out of good material.
Much cheaper than the amount of rebar and concrete needed.)

To hold the pieces together when they crack?
(I would use wire mesh panels)

Or are you wanting to add strength?
For strength, I would go with spacing in the 1-3 times the thickness of the pour, I.e. spacing 5-15", #3-#4 rebar on "chairs" or "dobies" 1.5" high. You want the rebar about a third of the way up from the bottom, I.e. about an inch and a half. I would go for the 5" spacing under where you plan to have the floor lift pads, if you aren't pouring a deeper footing for them. (I would)
Lots more here, if you want it; Concrete Driveway Construction - Thickness, Rebar & More - Concrete Network

Or just go for 10ga wire mesh panels and call it good. Rebar isn't cheap, it is labor intensive, and if you aren't driving big trucks or combines on the floor, you probably don't need it.

Don't use fiber for strength. That is a myth. It does slow early cracking.

Once you do your pour, keep it wet and covered for at least a month to develop strength. You will be glad that you did.

Notice that concrete continuously gains strength the longer it is wet for more than six months. Letting concrete dry early is wasting concrete strength.

All the best,

Peter

 

[dirttoys]

Guys sorry for the dumb question, why do you use block instead of just forming and pouring the concrete to the desired height?

Thanks,

ed

 

[ovrszd]

It's always been a mystery to me how a pour crew can elevate mesh to the middle of the wet slab while they are walking around on it?

 

[ovrszd]

It's always been a mystery to me how a pour crew can elevate mesh to the middle of the wet slab while they are walking around on it?

 

[ponytug]

It's always been a mystery to me how a pour crew can elevate mesh to the middle of the wet slab while they are walking around on it?

Yes, well the answer is that they don't generally do a good job as there is no way to control the rebar height accurately. Lots of testing has shown that it just doesn't reproducibly get the rebar into optimal position. I cringe every time I see someone do it.

Using dobies, or chairs, or stone/bricks are much to be preferred.

All the best,

Peter

 

[ovrszd]

Yes, well the answer is that they don't generally do a good job as there is no way to control the rebar height accurately. Lots of testing has shown that it just doesn't reproducibly get the rebar into optimal position. I cringe every time I see someone do it.

Using dobies, or chairs, or stone/bricks are much to be preferred.

All the best,

Peter

I think I agree. Although you are quoting "rebar" rather than wire mesh. Rebar is usually set on plastic chairs. You can step on them and they will rebound and put the rebar back in the middle of the slab. "Wire mesh" is simply laid on the pad.

I've saw local DIYs use bricks as chairs. I really like that idea.

I've never heard them called "dobies". Why I love Internet forums.

 

[m1garand762]

We dug the footings as a complete rectangle so there will be a footing under the doors. We will run horiztonal rebar (three #4 bars in a 24" wide footing). We won't run any vertical bars under the door area but will of course have verticals at 32" and 16" oc depending on the wall area that was called out for.

I'm doing a 5" slab on grade pour after the ICF stem wall is done. Entire garage walls will be ICF (6" concrete core with 3" foam on each side).

All the footings and slabs will be 4,500 psi hydramix mix (water resistant). $168 per yard for that mix.

What brand of ICF block are you using

 

[etpm]

Yes, well the answer is that they don't generally do a good job as there is no way to control the rebar height accurately. Lots of testing has shown that it just doesn't reproducibly get the rebar into optimal position. I cringe every time I see someone do it.

Using dobies, or chairs, or stone/bricks are much to be preferred.

All the best,

Peter

When I had the slab poured for my shop I had rebar and wire in the slab. I had grade beams too. Like you said I was dubious about the ability of the workers to pull the wire and rebar to the proper position. I ended up positioning it myself using stone. When the guys came to do the work they were surprised I did all that "extra work" that they said was me just wasting my time. As I watched them sloppily work I was so glad I did the extra work. They did not make the slab flat nor did they slope it. Instead they put a big dip in the slab. 1 inch dip over 15 feet. Since I was building a machine shop the floor was thicker than normal, 7 inches minimum. It's a good thing I went so thick after discovering the large dip.
Eric

 

[Pettrix]

What brand of ICF block are you using

Nudura with a 6" concrete core

 

[Pettrix]

I've always been told 24".

2 post or 4 post lift?

Both, a 2 post and 4 post

 

[Pettrix]

Guys sorry for the dumb question, why do you use block instead of just forming and pouring the concrete to the desired height?

Thanks,

ed

You mean the ICF stemwall?

 

[ponytug]

I think I agree. Although you are quoting "rebar" rather than wire mesh. Rebar is usually set on plastic chairs. You can step on them and they will rebound and put the rebar back in the middle of the slab. "Wire mesh" is simply laid on the pad.

I've saw local DIYs use bricks as chairs. I really like that idea.

I've never heard them called "dobies". Why I love Internet forums.

I figure to do concrete pours once. If it cracks on me, or has some other defect, I know myself and it would annoy me every time I walked past it.

I use chairs or dobies when using mesh as well. The sweet spot for steel reinforcement is about a third of the way up the slab, regardless of whether it is rebar or mesh. I would say if the bricks are the right height for your pour, run with it!

All the best,

Peter

 

[m1garand762]

Nudura with a 6" concrete core

Ok thanks, that’s what we are use for the house

 

[ovrszd]

Both, a 2 post and 4 post

For the 4 post, no big deal. They stress the concrete very little.

For the 2 post you need to thicken the concrete at the post locations. Easiest way is to simply create a bowl at the post locations to thicken the concrete. 10-12". Sometimes sonotube is used.

When drilling the concrete for the wedge bolts, drill all the way thru if you can.