garage slab over slab

huangyhg

garage slab over slab
have 2 car garage, 24x24, foundation settled and takes water
on one corner that has dropped and cracked, about 4x4 section. would like to pour concrete over exisiting slab, without tearing down structure. only use it to park one car. can it be done? if so, any suggestions for preparation and framework?  
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maverrickep,
it may be wise to investigate the reason for the failure prior to doing anything. pouring concrete over the existing concrete could cause larger problems due to increased load.
i agree with erv.  the smart thing to do is have someone investigate the cause of the problem.  you have an area  4x4 that has basically failed (cracked and dropped)........ something is going on in the sub-surface.  hire a local geotechnical engineer or structural engineer.  they will most likely request a boring done.  with the information provided by the boring, they can properly determine the cause of the "drop" and recommend proper solutions.
the geotech may also recommend a test pit instead of a boring.  that way, everyone can see what's happening -
besides, it's damn near impossible to get a truck-mounted rig "close enough" to the corner of a garage (or any structure.)  roof overhangs, telephone and electrical lines, etc. are a real problem in raising the boom.  sometimes a bobcat with a little backhoe is all that's needed -
i totally agree with the above posts to investigate the cause of the failure and correct it. this is essential.
as to the part of your question about placing new concrete on top of existing concrete - it is often refered to as "white topping" and it can work, if done properly and the existing slab is in good conditon. after you have corrected the stated subsurface problems, consider placing new concrete in the repaired area only to match the elevation of the existing slab. after proper curing, place the new slab (uniform thickness) over the entire 24 x 24 area, match all joints in the existing slab. if joints do not match, over time the new slab may develope "reflective cracking" at locations more or less above joints (and crakcs) in the existing slab.
there are two schools of thought about the contact area between the old & new concrete. some say to isolate the two slabs from each other, perhaps with polyethylene sheet or sand layer. other say to attempt to get a bond between the the old and new concrete. my expereince has been to just make sure the old concrete is very clean, then place (and cure) the new slab.
recently had a job with mysterious and severe cracking in an interior masonry partition. there was a noticible belly in the slab. did a few borings to find a 2 foot space between the slab and the soil. we realized that a storm drain pipe that went under the building was leaking and caused the settlement. we blocked the drain and pressure injected grout into the void (slab jacking). worked like a charm.
as far as pouring atop an existing slab, it's very difficult to get a proper bond and prevent cracking and spaulling off. it might be better to cut out a section, see the problem and backfill with compacted gravel and pour a new portion of slab on sound bearing.
i would agree with what stress02 said.  you could probably remove the 4x4 setion of old concrete for what it would cost to have the boring or test pit done and then you might be able to figure out what's causing the settlement.  
pouring the 4x4 section back would probably be the same cost (not including the cost to fix the settlement issue) as pouring a concrete top.