pouring a new slab over old?

[scotty t]

i've an old garage i want to fix up. it's got a crack free but very wavy floor. it's not even close to flat. my concrete guy says he can pour over it with a barrier between the two slabs. the new slab will be a minimum of 4" thick. does this sound right? the guy has many years of concrete experience. any body experience with a re-pour like this?

 

[bgarrett]

My father did it. Use lots of steel

 

[Kevin54]

What the barrier may be is a latex bonderizer to insure both slabs will be as one. If your concrete is stable now, you may be able to get by with more. It sounds like you'll have a good base for it. A lot of concrete guys want to tear out and start fresh, but if you have the room to do it, and by that I mean that you have enough foundation wall height to do it, and you don't already have a low headroom garage door now, there's no reason it won't work. When my dads kitchen was getting rebuilt from termite damage, I never knew how many times my dad had poured a back porch on the house, then built the kitchen on top of the slab. The contractor stopped tearing out at 4 or 5 layers. He also told me that in the old days, a lot of places would pour over a previous pour and the deepest he tore out was 32" of concrete.

Just make sure they don't pour up against any drywall or wood where you will end up with water damage or termite damage. And make sure your garage door and garage door entrance will accommodate the additional 4" which will probably actually be 3 1/2" if they frame it in the front with a 2x4.

Post up some pics

 

[boiler7904]

First question is are you looking for flat or level? It can be flat without being level.

If the existing slab is stable and crack free it can be poured over. Make sure they use a bonding agent on top of the old slab and don't allow the new concrete to be poured against any wood or drywall. I would think that the control joint pattern should match the pattern in the existing slab so that you can control cracks in both slabs in the same places. No guarantee that the new slab will be any flatter than what you have now.

With that being said, have you thought about what it will do to the overhead door(s)? What about any walkway doors? Are you going to re-frame these openings to make them taller and accommodate existing doors or buy new custom doors?

Without seeing the slab, adding 4" seems like it might be overkill. How deep are the low spots compared to the average elevation? How high are the high spots compared to the average elevation? What's the end goal with this pour? A flat, exposed concrete slab or a solid flat base for a future covering like vinyl tile, porcelain tile, epoxy, etc.? If you're going for something like vinyl or porcelain tile, you can probably grind the highs and use a commercial leveling compound to fill the lows before covering the entire slab with a finishing compound. Ardex is the first manufacturer that comes to mind for leveling and patching compounds.

 

[NUTTSGT]

You can see pictures of my floor that I poured over in my thread (link in sig). At the back wall it was 3 1/2" thick and the deepest was 6 1/2" at a front corner. I used no bonding agent but I did tear out part of it at the front footer and used some rebar. I also had a foundation to pour up against and used sill seal for the expansion joint.

Why no bonding agent ? The way I looked at it, the floor was going nowhere with foundation or block wall all the way around it. When a footer is poured and a concrete stem wall is poured over that, I've never seen a boding agent used, rebar yes not not any chemicals, unless I missed that. Also part of my line of thinking is guys that pour over XPS insulation for in floor heating aren't using a boding agent and that is also a smooth surface.

The floor was cut for control joints and it's been done for almost 3 years with no issues. You can also see in my thread how I raised the door openings also.

 

[scotty t]

let me explain further, the existing slab slopes to the rear of the garage, has very noticeable high and low spots. there is a 2' tall foundation wall surrounding a floating slab. the increased elevation will help with water issues, the doors are not an issue (man door is high enough and the over head is going to be raised anyway). i want flat but with an appropriate slope to front of the garage. i think he wanted a 4" minimum thickness to avoid cracking later. the new floor will not have any covering, just car parts and the usual garage clutter. oh yea the barrier he wants to use is not a bonding agent but something to keep the two slabs from bonding. this more of a slab on top of a slab than a thin leveling top coat.

 

[scotty t]

You can see pictures of my floor that I poured over in my thread (link in sig). At the back wall it was 3 1/2" thick and the deepest was 6 1/2" at a front corner. I used no bonding agent but I did tear out part of it at the front footer and used some rebar. I also had a foundation to pour up against and used sill seal for the expansion joint.

Why no bonding agent ? The way I looked at it, the floor was going nowhere with foundation or block wall all the way around it. When a footer is poured and a concrete stem wall is poured over that, I've never seen a boding agent used, rebar yes not not any chemicals, unless I missed that. Also part of my line of thinking is guys that pour over XPS insulation for in floor heating aren't using a boding agent and that is also a smooth surface.

The floor was cut for control joints and it's been done for almost 3 years with no issues. You can also see in my thread how I raised the door openings also.

i remember reading your thread awhile ago, you did nice work.

 

[wssix99]

Will you be left with enough height on your gas curb? How are you going to adjust your garage doors and driveway?

 

[bgarrett]

what is a 'gas curb'?

 

[scotty t]

the man door is is about 8" above the floor so no prob there, the over head is getting re arranged anyway. the garage is detached and about 100' from the house so gas curb is not an issue.

 

[NUTTSGT]

let me explain further, the existing slab slopes to the rear of the garage, has very noticeable high and low spots. there is a 2' tall foundation wall surrounding a floating slab. the increased elevation will help with water issues, the doors are not an issue (man door is high enough and the over head is going to be raised anyway). i want flat but with an appropriate slope to front of the garage. i think he wanted a 4" minimum thickness to avoid cracking later. the new floor will not have any covering, just car parts and the usual garage clutter. oh yea the barrier he wants to use is not a bonding agent but something to keep the two slabs from bonding. this more of a slab on top of a slab than a thin leveling top coat.

I think if you have 2 feet of clearance, it would not be a problem adding the 4" of concrete. I can totally understand wanting the extra elevation for the floor, I was there at one time. The one place I would break it out though is at the overhead doors. You need to keep the water out from between the two slabs. You read my thread, so I believe you know where I'm coming from.

Once the floor is poured, pour a new apron out front, deep enough that it covers the face on both sections of concrete to help keep the water out.

i remember reading your thread awhile ago, you did nice work.

Thank you.

 

[theoldwizard1]

I would go with a self leveling top.

Types of Overlay Systems ConcreteNetwork.com

 

[scotty t]

I would go with a self leveling top.

Types of Overlay Systems ConcreteNetwork.com

it's beyond that, there's probably as much as 4" difference between the high spots and the lows, so by the time time you put enough over the highs it'll be quite thick. all things considered tear out and re-pour would be the optimal way to do it, but that's not gonna happen. this floor was terrible when it was first poured, it hasn't moved or cracked or spalled or anything like that. my concrete guy is the one who put the plan together, i just wanted to see if the guys here had some thing different to suggest. this job is going to be a lot like NUTTSGT's floor.
thanks for the input guys
scotty t

 

[Kevin54]

what is a 'gas curb'?

A gas curb is what you see in a lot of garages where they have a 4" pad on top of the floor. It's normally large enough that most will put their toolboxes up on it and still have room to maneuver around walking on it. As far as calling it a "gas curb", I don't know why it is named that.

I did a quick search and found this....just a site explaining a code and a couple of pics. It may have gotten it's name because gas pumps used to set on a raised area. I think the reason some homes have them today is so you don't drive into a living area of the house, that the wheels of the car will contact it first and it gives a reference point for stopping instead of nosing the vehicle into the wall

http://en.allexperts.com/q/Building-Codes-Inspections-3546/2009/1/garage-gas-curb-contacting.htm

http://www.iccsafe.org/iccforums/Pages/default.aspx?action=ViewPosts&fid=11&tid=23880

 

[wssix99]

As far as calling it a "gas curb", I don't know why it is named that.

I did a quick search and found this....just a site explaining a code and a couple of pics. It may have gotten it's name because gas pumps used to set on a raised area. I think the reason some homes have them today is so you don't drive into a living area of the house, that the wheels of the car will contact it first and it gives a reference point for stopping instead of nosing the vehicle into the wall

Its called a gas curb because gas fumes are explosive, heavier than air, and pool on the floors of garages. (Just like they do around gas pumps.) Its more dangerous around the pumps themselves and in confined spaces (ie: garages) because of this effect.

The gas curb is there to keep the fumes from getting in to the living areas, and down in to basements, sources of ignition, etc. Garage floors should also be sloped towards the garage doors to help these fumes flow outside the garage.

Even more reason for the OP to fix the problem, if there are valleys in the wavy floor where fumes could collect.

 

[Kevin54]

Its called a gas curb because gas fumes are explosive, heavier than air, and pool on the floors of garages. (Just like they do around gas pumps.) Its more dangerous around the pumps themselves and in confined spaces (ie: garages) because of this effect.

The gas curb is there to keep the fumes from getting in to the living areas, and down in to basements, sources of ignition, etc. Garage floors should also be sloped towards the garage doors to help these fumes flow outside the garage.

Even more reason for the OP to fix the problem, if there are valleys in the wavy floor where fumes could collect.

I'm not going to disagree about the gas being heavier than air, but I really don't think a 4" curb in a garage is going to prevent any of that. Gas fumes, depending on how much gas may have been spilled, and the temperatures of the room may very well reach above any 4" curb. I've personally saw gas fumes ignite in mid air about 5-6' above a pile of brush before.

 

[DangerousDan55]

I was thinking that the 4" raised curb was to stop flammable liquids spilled or leaks from fuel tanks from flowing into the house. The entry door should have a seal to prevent gas/fumes from entering the house. Thinking agout the 18" above floor requirement for gas water heaters. What would 4" curb stop? Also the garage floor being 4 inches lower than the house door & sloped to drain ANY liquid out through the vehicle entrance way.

In my fire service, I've been to a gas leak/explosion in a garage. A propane bottle removed from a BBQ grill, stored in the garage. They did not close the valve tightly & overnight the very slow leak alowed the vapors build above the 4" "gas curb". The vapor did reach the pilot of the gas water heater which was at the 18" above floor requirement. BOOM!
Luckyly only the sheetrock was cracked & garage doors bent & took off thier tracks. No stucture fire.
The propane bottle was intack with a small candle size flame at the valve opening.
We just closed the valve tightly, & check things out and left it with the Fire Marshal.

Anyhow, here is a 2006 Doc on "fire curbs" and such....

 

[DangerousDan55]

Sorry, forgot to attach....
http://www.nachi.org/attached-garage-fire-hazards.htm

 

[wssix99]

I'm not going to disagree about the gas being heavier than air, but I really don't think a 4" curb in a garage is going to prevent any of that. Gas fumes, depending on how much gas may have been spilled, and the temperatures of the room may very well reach above any 4" curb. I've personally saw gas fumes ignite in mid air about 5-6' above a pile of brush before.

I recall hearing that the National Building Code is 8". I assume its that way because if one goes higher, its more than one step and we'd need staircases to get over the curb. That's probably where the risk/reward equation tips.

 

[Kevin54]

I was thinking that the 4" raised curb was to stop flammable liquids spilled or leaks from fuel tanks from flowing into the house. The entry door should have a seal to prevent gas/fumes from entering the house. Thinking agout the 18" above floor requirement for gas water heaters. What would 4" curb stop? Also the garage floor being 4 inches lower than the house door & sloped to drain ANY liquid out through the vehicle entrance way.

In my fire service, I've been to a gas leak/explosion in a garage. A propane bottle removed from a BBQ grill, stored in the garage. They did not close the valve tightly & overnight the very slow leak alowed the vapors build above the 4" "gas curb". The vapor did reach the pilot of the gas water heater which was at the 18" above floor requirement. BOOM!
Luckyly only the sheetrock was cracked & garage doors bent & took off thier tracks. No stucture fire.
The propane bottle was intack with a small candle size flame at the valve opening.
We just closed the valve tightly, & check things out and left it with the Fire Marshal.

Anyhow, here is a 2006 Doc on "fire curbs" and such....

I know that propane is heavier than air, which is one thing I don't like about our furnace. I'm on a crawlspace, and when I first moved here 20 years ago, I inquired as to why there was a heat duct beside my furnace. But the heat duct was actually a register and not hooked to anything. I was told that it was the way they did the furnaces in the house because if there was ever a propane leak, being that propane is heavier than air, it would let the gas go down into the crawlspace and not buildup in the house. There is a gap under the door, so I don't really imagine all of my propane dropping down a 4" x 11" register.

 

[Twiggss]

I know that propane is heavier than air, which is one thing I don't like about our furnace. I'm on a crawlspace, and when I first moved here 20 years ago, I inquired as to why there was a heat duct beside my furnace. But the heat duct was actually a register and not hooked to anything. I was told that it was the way they did the furnaces in the house because if there was ever a propane leak, being that propane is heavier than air, it would let the gas go down into the crawlspace and not buildup in the house. There is a gap under the door, so I don't really imagine all of my propane dropping down a 4" x 11" register.

So the gas would build up under the whole house and blow up?

 

[Kevin54]

So the gas would build up under the whole house and blow up?

Pretty much that was the idea behind it? I guess it's safer to have something blow up underneath you than to blow up on the same level .

 

[wssix99]

So the gas would build up under the whole house and blow up?

I would hope the gas would escape from the crawl space vents and then dilute. It would be a lot safer down there, away from sources of ignition.

 

[Kevin54]

I would hope the gas would escape from the crawl space vents and then dilute. It would be a lot safer down there, away from sources of ignition.

It can't escape from the vents as the furnace is in the middle of the house, plus the fact that the vents are at the top of the foundation and my crawlspace is almost 4' deep.

 

[Big-Foot]

I had a 40 year old 2-1/2 car garage with a concrete slab that was in terrible condition with spalling and cracks. I could not afford to rip out the whole floor and a local concrete company told me that the floor was so thick, old and settled, that they could pour another floor on top and it would be fine. They cut out part of the old slab where the large entry door was and poured a new floor with rebar on top of the old one. The new floor was 4-6" thick (old floor was not level either).
They ramped up three feet into the new slab at the entry point.
It all looked good for about 3 months. Then the new floor started cracking and not just a little. The concrete guy just shrugged his shoulders and said "well it shoulda worked" and left it at that.

As it turned out - yes the old slab had settled over the years and it had probably finished settling - UNTIL - the weight of the new slab on top of it started the settling process all over again.

The only way to fix it right is to remove the old floor in my opinion. I'll never do that again..

Good for me is that I sold that house and now it is someone else's problem.