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Why can you pour concrete over concrete?

Posted by clyde6n (My Page) on
Fri, Nov 2, 07 at 13:42

We have a 12 X 35 concrete patio in the back it was poured in several sections over several years. The original portion is rock solid. The later sections have sank about an inch and were poured over the top of a septic tank. We are going to remove septic and repair/replace the patio with something uniform. The price quotes for removing the tank and concrete have been crazy. I have been told by several people that I can not just pour concrete over the top of the existing patio after the tank has been filled in. I want to know why not. I have been told can not pour concrete over concrete but i want to know why. I have told it will not bond but I want to know ehre the heck a slab that big would go. Why can't you pour concrete over concrete?


Follow-Up Postings:

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RE: Why can you pour concrete over concrete?

there are bonding agents that you can use. but even then they won't help much. new concrete just does not stick to old. if this will end up being a thin layer, it will start to chip and crack away in no time. if it is going to be a couple inches or more thick, you MAY can drill the old slab and install some rebar sticking up to stop lateral movement, but you really want a good 3-4 inches of concrete to stop cracks.

rent a jack hammer and bust it out yourselves, that should save a good bit of money.

why the heck did someone pour over a septic? you cannot do that.


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RE: Why can you pour concrete over concrete?

Thanks We were planning on pouring the new slab at least 3 inches thick. So that should work OK then. They were able to pave over the concrete because they did not tell any of the regulatory agencies they were doing it. Now we get to fix it.


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RE: Why can you pour concrete over concrete?

i did not think about this earlier.

back in '91 my parents added on to their back porch and made a sunroom. the floor of the porch was a washed concrete and they were installing tile. the crew put in short pieces of rebar, then poured a 2" layer on top of and out beyond the existing porch about 6 inches. they have not had any problems in since then, and that is what 16 years now!

so yes, build your forms out past the existing that will stay, then secure it with some rebar in a few places and you should be fine.

get a permit and ask the inspector how much of a footing is required. if you are up north where frost heave is an issue, you may have to dig a deep footing. this will help to secure the overlay, so it will work out in the end. at 3" thick, on grade, i don't think an inspector will make you remove the old first.


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RE: Why can you pour concrete over concrete?

I once added new concrete over old in a carport that had poor drainage. Added an apron out front to ease entry into the carport. Since the apron was not over old concrete, I used the reinforcing wire mesh in that area. The area with mesh was perfect, the area over the old concrete was cracked rather badly. Concrete shrinks as it cures. The mesh helps control cracking. If you add more to your existing concrete, use both bonding agent and mesh.


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RE: Why can you pour concrete over concrete?

I'm really in a pickle also. The former owner built walls onto a double carport, converting it into a room. The bottom plates are on the slab-- and we get water seepage underneath the end wall when it rains hard. I have no where to go with a drain tile, so I have considered either laying a concrete support wall, or an entire concrete floor 4 to 5 inches in height. Will I need to remove all the old slab, or can I go on top of the concrete. It will be a supporting wall/floor, so I'm a little concerned.


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RE: Why can you pour concrete over concrete?

I have an old garage slab that is uneven and cracked. I want to put 12" of gravel on top and pour a new floor. Do i need to break up the old slab or is the gravel enough


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RE: Why can you pour concrete over concrete?

you're putting a foot of gravel over an exisitng garage floor plus a new slab (that should be 3-4in. thick) for a total of ~16 inches? Either that's a typing error, or you are raising the floor quite a bit.


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RE: Why can you pour concrete over concrete?

I have an uneven basement. And I was either going to pour a concrete stub wall around the perimeter or pour over the existing slab. I have never heard not to pour over an existing slab until I started researching my pour. I just don't listen, cause as you explained in your questions the mass of the concrete will not slide off especially if you vertical rebar attached to the old floor. I just lifted my house and added 2 feet to the basement wall. We put vertical rebar in and tied with horizontal pieces. contractor lifted the house and I set up the forms and poured. We didn't add any kind of adhesive to the bottom of the old foundation.I paid the fee for the egineer idea. It has been half a year now and it looks great. No cracks. Made it very strong. Like putting a cap on old box. My main point is that I don't see why people worry about concrete so much. Just put some d@mn rebar in it and pour. And of course have your proper thickness. It is all logical. The thinner you go the weaker it is. And pay attention to where it is going to be poured at. Meaning if the temperatures fluctuates a lot then insulation may need to be added. I poured sound barrier walls for the highway for three years. We would pour 30 by 30 pieces in one day and lift them with a heister the next.Concrete is very strong. And one last thing.I was reading of a guy in a post(engineer) saying to cut expansion joints which I have done too. Great idea if the slab is not for keeping the water out such as garages driveways but for my interior floor I am sure as h3ll not going to cut expansion joints when I am going to put drywall in my basement.


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RE: Why can you pour concrete over concrete?

The quote is high partially because of the brute-force time-consuming work required. A "professional" can't push a wheelbarrow full of concrete pieces any faster than you. But you can do it. Rent a jackhammer, get some good ear protection, and find another person or two to switch off on the jackhammer and bust it up - if you do it all yourself in one shot you will be sorry the next day! Then find some teenagers in need of "character development" and a bunch of wheelbarrows, along with a rented dumpster - the going rate can be very affordable if the teens happen to be your own, otherwise $5 or $10 an hour would make for a good spending money opportunity for some teens in your neighborhood. Or find the neighbor/friend/relative that always borrows your stuff/time that "owes you" to help out - assure them that you can be flexible on the date. The tank, you may need to have removed by the pros however.


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RE: Why can you pour concrete over concrete?

I have a old cement garage floor, around 40 years old. It has many deep cracks and the cracks are raised. Also the floor has sunk in at least 4" and when we get a heavy rain the water comes in the garage because it is lower then the ground out side. What I want to do is pour 4" of new concrete on top, will it hold up?


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RE: Why can you pour concrete over concrete?

You'd be better off pulling the old crete, dig down and bring in some compactible fill, vapor barrier if required in your area, lay down some mesh, and re-pour your floor.


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RE: Why can you pour concrete over concrete?

Nice response drywall_diy_guy. I agree with it completely.


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RE: Why can you pour concrete over concrete?

I am wondering why they are removing the old septic tank? There is a code procedure for abandoning a septic tank in place.

First you pump the tank, then pressure wash the interior and pump out the residue, then punch a hole through the bottom of the tank and fill the tank with sand.


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RE: Why can you pour concrete over concrete?

I want to put hydronic heating pex lines on top of my concrete slab in a Florida room. Can I fasten the pex and then pour a 2 1/2 inch real soupy concrete mix that will self-level over the pex then tile over that when it cures. Is this possible ? and How should I go about it ? thank you. Dan


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RE: Why can you pour concrete over concrete?

"Can I fasten the pex and then pour a 2 1/2 inch real soupy concrete mix that will self-level over the pex then tile over that when it cures. Is this possible ?"

It is possible, but their are lighter concrete products like gyp-crete that would avoid loading the slab as much and is preferred for covering radiant.


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RE: Why can you pour concrete over concrete?

I have a slab on grade that was done as a multi pour (not by me). It is in good condition, but a very poor pours.
Some pours are out of level up to 1 1/2".
It will be inside a building when I am through.
I want to pour over the multi slabs and get it all level and up overall 1 1/2" thickness higher, making the low part @ 3" thicker.
It will have a wall and footer all the way around it. In addition there will be an 6-8" collar of new concrete around three sides and a 6 foot slab on the 4th. (slab is @ 23X30 feet and the new slab will be 30X32 feet, with the 23' being the new 30' and the 30' being the new 32'. The "extensions will have rebar and mesh to code.
Code guy has approved to build with the above rebar/mesh and original slabs.
I plan to pour the extensions with regular concrete and a footing but need to know can I just pour regular concrete over the old slab and will it work? Should I pour deeper over the old and install mesh, or can I pour the 1 1/2" overlay and put mesh near the bottom of it.
Would I need to do the short rebar sticking out?
If so, how deep do the holes need to be in the old slab and should I epoxy them into it?
The old slabs have been there for @ 2 yrs and show no signs of cracking or sinking. There are joints where the multi pours are joined. How should I treat these?
I know the best would be to remove and completely redo, but trying to save some money if possible.
What are your suggestions? I am new here and found your thread and feel you are all pretty knowledgeable in this area.
Thanks for any help. If you have a question on this, just ask and I'll try to explain clearer.


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RE: Why can you pour concrete over concrete?

Most concrete shrinks as it cures. It may crack badly even if added over an existing slab with no cracks. Uneven thickness increases the chances of cracking. So you will need the mesh for sure, a 4 x 4 mesh might be good.


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RE: Why can you pour concrete over concrete?

I have an old concrete porch 10 x 30 that someone has closed in to make a sun-room. It is a dirt floor with 4" concrete. The house was built about 1970. The concrete has cracked about 1.5 " deep along one wall in the corner and along a portion of the wall. The foundation was moving. We have corrected that problem. Now I have to put a new concrete floor down. A tile man said just to put a mud bed down. Another said to pour 3" concrete on top. Cant get the old concrete out. I was told by a friend to make sure they clean the exiting concrete and put a mess wire in the new concrete. and add a bonding agent in the new concrete. Also they said to put in 2 expansion joints in the floor so the concrete could move without cracking.
Would a mud bed work if its about 1.5" thick then a self leveling floor on top of the mud bed. Someone told me that would all crack eventually.
What is your opinion?


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RE: Why can you pour concrete over concrete?

My contractor had concrete left over from the inside pour, so he put it in the form for the outside entrance slab. He's planning to top the pour with lime stone mix about 3 inches thick tomorrow ! Will the top adhere to the bottom layer ? and will it compromise the strength of the slab ?

Dan


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RE: Why can you pour concrete over concrete?

My contractor had concrete left over from the inside pour, so he put it in the form for the outside entrance slab. He's planning to top the pour with lime stone mix about 3 inches thick tomorrow ! Will the top adhere to the bottom layer ? and will it compromise the strength of the slab ?

Dan


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RE: Why can you pour concrete over concrete?

Get a price from a slab jacking contractor. They pump grout under the existing slab to level it. No demo at all.


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RE: Why can you pour concrete over concrete?

It's a good idea to do, but anyways here is a good link that explains the answer to your question:
http://answers.yahoo.com/question/index?qid=20130330121607AATWkr7


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