Cost for a cement slab foundation

Builder · May 7, 2006

We junked the wire mesh rolls & mats back in the early 90's.

Try to think of it this way: Let's say you pour a garage floor over somewhat disturbed ground. You don't want the slab to break, but despite your best efforts of compacting the ground, the ground might/will settle a 1/2". If you have a rebar grid system drilled into the sides of the block walls, areas of settling are less likely to develop into large cracks, or crack at all!!!

You can't drill wire mesh into side walls, therefore you can't "hang" the slab off the vertical strength of the wall and the footing below the wall. When you DO drill bar into the walls, it will actually suspend the pad "in the air" above the settled ground, something mesh could never do.

I've demo'd lots of rebar slabs that had rebar drilled into supporting side walls and the ground under the slab had settled several inches, yet the slab wasn't cracked!!!!

In fact, we had one garage floor where you could hear the hollow spots under the slab, yet the owner had a CAT backhoe (16,000lbs) parked on it and NO cracks!

I know I'm just a sleazy builder /forums/images/graemlins/grin.gif but I've been around this stuff a while. I know what engineers spec on pretty little sets of blueprints and what really works. /forums/images/graemlins/smile.gif

Bar does what I described above. Add fiber to the crete for a couple bucks per yard and you've eliminated the need for wire mesh. /forums/images/graemlins/wink.gif

Take it from an old pro.

EddieWalker · May 7, 2006

Hi L39Builder,

One of the things I love about this site is the ability to learn from others. You touched on a few points here I'm not familiar with.

( the ground might/will settle a 1/2". (

I've seen ground settle, move and erode under a foundation. It's a huge problem here in Texas with our clay soils and extreme weather conditions.

( If you have a rebar grid system drilled into the sides of the block walls, areas of settling are less likely to develop into large cracks, or crack at all!!!(

I do this when transitioning from building slabs to exterior slabes. I put an expansion block in between, then tie the two slabs together with rebar. I've done it to allow the greater movement of the concrete due to exposure from the elements. When the concrete that is exposed gets hot, it moves, and the same when it gets cold, so there is a certain level of movement. I understand this and realize you need to put a spacer between the two pads to stop cracking, but if the ground settles under it, the concrete usualy settles with it and the expansion block allows this movement to a certain degree also with the rebar holding the two slabs together.

This makes sense to me and I've never had a problem with it. But your tecnique sounds allot better.

( You can't drill wire mesh into side walls, therefore you can't "hang" the slab off the vertical strength of the wall and the footing below the wall. When you DO drill bar into the walls, it will actually suspend the pad "in the air" above the settled ground, something mesh could never do.(

This really caught my eye. How far apart do you space the rebar to support a concrete slab when you drill into the walls? How do you engineer this? This is totally new to me and you've lost me on how to design it. Obviously the thickness of the slab and size of rebar must be a huge factor in determining how to do this. Is there a source on this?

( I've demo'd lots of rebar slabs that had rebar drilled into supporting side walls and the ground under the slab had settled several inches, yet the slab wasn't cracked!!!!(

I've never seen this either. How did the ground settle if there was no weight or preasure on it. I'd love to be able to pour a slabe that doesn't rely on the ground below it for support, but lack the experiece and knowledge. I rely on getting the ground as compacted as possible to eliminate any settleing.

The only time I've seen something like what you descripe is from erosion caused by water getting under the slab. But that's totally different than the ground settling under a slab.

( In fact, we had one garage floor where you could hear the hollow spots under the slab, yet the owner had a CAT backhoe (16,000lbs) parked on it and NO cracks!(

This is just amazing!!!! How big was the slab? How thick? What size rebar and how far was it spaced? This sounds like a bridge or even a suspended parking garage to some degree. That's an aweful heavy load to be able to park on a suspended pad. That's very impressive!!

Thank you for sharing your experience and knowledge.

Eddie

CurlyDave · May 8, 2006

Wire is actually stronger than rebar...

Are you sure about that one, Eddie?

Comparing 6-6-10 wire, with a piece of 10 gauge wire every 6 inches, to #4 rebar on 2' centers gives me 4 #10 wires for each piece of rebar. The total cross-sectional area of the four wires together will be 0.031 sq in.

A #4 bar will be ~0.5 inches in diameter for an area of 0.196 sq in -- over six times the area of the wire.

The wire will probably have higher tensile strength than the bar, but it would have to very special wire to be 6x as strong.

I do agree that the wire is nearly impossible to get in the center of the slab, while the bar is dead easy to get centered.

I know I will never again use wire, once was enough...

CurlyDave · May 8, 2006

In fact, we had one garage floor where you could hear the hollow spots under the slab, yet the owner had a CAT backhoe (16,000lbs) parked on it and NO cracks!

I helped my neighbor pour a slab actually designed this way from the start.

He had very expansive soil and the engineer was concerned that the earth would expand and push the slab up, breaking it.

We had a special type of corrugated cardboard, folded in a pattern, which was strong enough to hold the concrete during the pour, but which would rapidly decompose afterward, leaving the concrete free floating. The generic name for this is "void boxes".

I think the real secret of your hollow garage floors is that the concrete actually rested on the footings, which held it up.

Making the footing part of the same pour as the slab allows the footing to act as a grade beam at the edge of the slab, and greatly increases strength.

EddieWalker · May 8, 2006

( Wire is actually stronger than rebar...

Are you sure about that one, Eddie? )

Dave,

I'm not on solid ground here to the point of being able to support this claim. It's what I've heard over and over again by old timers on jobs back in California.

Codes would change so fast there that a home builder I know had given up on foundations all together and just subed it out. He said it cost him too much money to deal with the inpsectors and all the changes that kept coming up. He lost tens of thousands on a project because it was done to code before a change that nobody seemed to know about except the inspector.

Anyway, I'm with L39Builder on wire mesh. I haven't touched in in over 20 years and can't imagine any reason to ever use it again. Too much effort only to end up with a substandard result.

Eddie

Builder · May 8, 2006

Eddie,

Try to think of it this way: not all of the ground under the garage will ultimately settle, but small "pockets" of it will, either from lack of compaction, bad soil, water under slab, or just a slight settling. Usually I can tell just by walking on a floor where it has settled underneath.

Of course, the 8" or 12" blocks walls of a normal garage aren't strong enough to support the entire slab of a 2,3 or 4 car garage, but they'll support some pockets that may develop next to the walls, while firm compacted soil will still be under a large percentage of the garage slab.

If there's really soft spots, say where there was a tree stump, I'll auger a 18-24" post hole tube down to undisturbed and fill it to the top of the height of the stone with concrete creating a pier for the concrete to rest on. It makes almost like a table leg under the slab in the soft areas.

To lay out the grid, I hammer drill my block walls down say 4-8" from the top of the solid block course (depending on slab slope) with a 5/8" masonry bit about 2" deep, holes being 2-3' apart (depending on your grid pattern). Then I insert the tips of the bar into their holes while laying out the grid pattern. Then I put chairs under the bar to support it over my stone & vapor barrier. Then I order truck loads of fiber crete and have at it.

reason for all this is when you dig footings, you are likely to have what is called "overdig", which is the space you need to lay block on top of your footings. Overdig is backfilled, of course. But typically, we don't like to run the jumping jack to rough & hard right next to the walls when they're still "green" or they might crack at the mortar joints. So often times the soil just inside of a new block wall isn't compacted as well as in the middle of the slab. Therefore, we drill the rebar grid directly into the wall a 2' or 3' o/c to keep the slab from settling.

Many "armchair experts" will tell you how things are done in perfect "laboratory conditions" or on a set of blueprints, but I'll tell you 90% of my jobs are done in far less ideal conditions. I WISH I had the time to make sure everything was perfect and every square inch of soil would pass a compaction test, but that's not reality.

Hope that helps.

Ender · May 8, 2006

We put down a 6" slab with a monolythic base on all 4 sides, rebar in the trenches and in the center slab last October. Did all the prep work except putting up the forms and the actual concrete work. Cost 3990.00 here in central Oregon.

We took bids from 4 different guys. All gave us about the same cost. We chose the guy who said he would come out and look over the job, before we gave it to him.

Builder · May 9, 2006

( We put down a 6" slab with a monolythic base on all 4 sides, rebar in the trenches and in the center slab last October. Did all the prep work except putting up the forms and the actual concrete work. Cost 3990.00 here in central Oregon.

We took bids from 4 different guys. All gave us about the same cost. We chose the guy who said he would come out and look over the job, before we gave it to him. )

Nuthin wrong with that. You just have to compare the cost of using 100% concrete and labor versus concrete, block & labor to lay block & pour crete.

inveresk · May 13, 2006

L39Builder

See attached detail. What you describe is the underbuilding arrangement in the new workshop/office going up on my place at the present time. There's 10M rebar throughout the foundation, kerb and slab that's tied together to a grid of 10M rebar at 24" in the slab.

This detail was devised mainly to comply with the seismic codes here. The foundation, kerb and slab are designed to act like a large box frame so any subsidence due to seismic movement might see the whole structure tilt but the risk of building collapse due to part of it giving way should be reduced.

As it happens, there's mesh in the floor as well but it's purpose is to act principally as a shrinkage crack inhibitor. The slab is 70' x 26' and crack free.

PineRidge · May 13, 2006

Don't think it's been mentioned yet so I'll throw this into the concrete mix. Make sure that whomever you hire to do your concrete work can guarantee that the concrete trucks will be there as promised for any large pour.

When we were putting in a large concrete apron & drive at our last place it was 90 degree weather the boys were really working with the crete, everything was going as planned, then all of a sudden the trucks quit coming. By the time the next truck arrived we ended up with an ugly cold joint where the rest of the concrete was poured. It could have easily been avoided had the concrete company done what they said they would do. The very next day the concrete cracked right at the cold joint. /forums/images/graemlins/frown.gif UGLY!