Concrete Help

[kmacht]

Just found this forum recently and the advice in the archives is great. I got the go ahead from my wife to replace our falling down attached two car garage with something better. I have found a place that makes prefab garages. We are looking to remove the existing attached garage and put in a 24x24 two story detached garage. The builder recommends a 4 to 6" concrete slab. I checked with my local building department and they are ok with it on a slab as long as it is under 600 sf. They told me I do not need footers as long as I don't go over the square foot limit. I plan on doing the concrete work my self and hiring a ready mix truck to deliver the material. I have done a 12x12 patio before but never anything this big. My problem comes in what to do with the existing 20x20 pad. I want to move the garage 4' away from the house so the pad has to be extended 9'x25' on one side and 5'x16' in the rear. I believe that the existing pad is a monolitic pour 4" thick with the edges extending about 12" down. I have in other posts here that I can tie a new slab into the old one by drilling holes and epoxying in some rebar. Do I have to continue the rebar all the way through the new slab or can I switch it over to a wire mesh? Another question is if there should be a gap between the two slabs. I saw a post where someone mentioned putting some foam inbetwen the two slabs. A third question I have is should I pour another 2" on top of the existing slab to bring it up to 4" thick and make the garage floor more uniform? I would prefer to do this in order to get the garage further up off the ground but am worried about the new concrete bonding to the old stuff properly. The old 20x20 pad does not have any expansion joints in it. There is only one long but thin crack that has formed over the 30 years since it was orginally poured. If anybody has any suggestions or comments on how to proceed it would be appreciated. I am located up in Connecticut if it makes any difference.

Thanks,

Keith

 

[Torque1st]

Welcome Keith!

FWIW- A 2" slab over the old slab will crack all the way around. Rebar is much better than wire mesh. If supported properly it will not end up on the bottom of the slab like wire mesh.

Drill some holes and epoxy in some rebar and anchor bolts around the perimeter of the old slab where the walls will go. Cast in some rebar and anchor bolts in the new sections. Run a course or two of concrete block around the perimeter of the entire building to get it up higher. Fill the cores of the blocks around the rebar and bolts. Use the anchor bolts to hold down the building.

 

[akdiesel]

Nothing against your ability, but you have a lot of good questions for someone that has not done that much concrete work and I would offer the idea of contracting it out so you do not end up with cracks or worse.
A 20' x 30' pad 4"-6" deep is going to take a lot of time and patience not to mention a lot of help if you do plan on doing this your self.
I have always had good advice from www.concretenetwork.com
Goodluck.

 

[GarageEnvy]

I am going through a similar scenario right now. I even posted a question on here about it but here is the scenario and what I was told so far. I just saw cut away my attached garage to make way for another attached garage. Long story that is irrelevant to your story but I ended up with a tie-in problem. Designer, county and concrete workers wanted 5-6" of material to tie into and after the saw cut there wasn't that much left so if you have a 4" slab check with your concrete guys to make sure they're OK with it. Yes I've now been told by 3 concrete contractors, 1 designer and 2 engineers that rebar is better than mesh for various reasons. A common problem is concrete workers not pulling the mesh up into the slab. As far as going over the top, that is what I was faced with but the concrete contractor insisted on a minimum of 4" on top otherwise he could almost guarantee a crack. I am in California and our footings are only 12" deep. My house is older and I found that our footing was a shovel style angled toward the wall. I am now welding and fabricating a brace to support the wall while the existing footing is completely removed. Good call on the detached structure to avoid this situation.

 

[kmacht]

Ok, thanks for the advice. It sounds like pouring over the old slab isn't a good idea. From the advice above I am probably best just pouring two extensions to the existing slab and tying it all together with rebar. I would have prefered to build it on a stem wall or raised block wall as suggested above but unfortunatly the garage is pre-fab construction (comes pre-built in two sections) and it has to be placed on a pad. They can't place it on a wall due to the delivery method. The only other option is to re-grade the yard which is probably what I will end up doing. The front and side already slopes away from the garage, I would just have to re-grade the rear of the garage and probably put in some drainage. I'll check out the concrete website listed above to get some more of the specifics on what is best practice for pouring a concrete slab. I do realize that this is a big project but it isn't my first big project. I have done a 10x40' porch, a 12x16 deck, a 12x12 concrete pad for a hottub, a brick walkway, installed a above ground swimming pool (including all the site work), built a 10x12 shed, am restoring a 55 ford pickup from the ground up, and am about 3/4 of the way through building a full size two seat kitplane. Ten years ago I had no clue on how to do any of this stuff. I tend to spend alot of time researching and planning before actualy taking on a project. I have yet to have to hire someone to finish or fix something that I have started. I figure that with enough information and a few family helpers the concrete should be pretty easy compared to the other projects I have done.

Keith