How To Pour A New Concrete Slab Over An Older Damage Slab

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    A friend asks about their garage floor that they are using as an exercise room and is wondering what is the best method they can use to level the floor with a new slab placed on top of it. The existing job was performed maybe 40 years ago along with deterioration there are many cracks and unlevel areas. They are also wondering about a smooth vs brushed finish for the floor. They have talked with a contractor and the small 12 x 18 area has been quoted at $2,700 for approximately 2 to 4 inches of concrete.


    Ok so the first thing you should know when considering a job like this is that any slab that you pour over an existing one will have problems with separation. There are precautions you can take such as tying the existing slab into the new surface with anchors and rebar but if you have an existing floor that is of a decent depth that is cracking and one that has movement throughout the seasons you are looking for problems. What happens is the slab below moves and then this results in what is called a reflection.. what you will consider just the same crack coming through where it was before. The worst part is that when you are leveling a floor like this there will be thin and thick areas so you are not  pouring a full 4 inch slab.

    The best choice since the garage is small is to break out the floor with a jack hammer which will probably take a couple days for the home owner or at least half a day for a crew of workers and then set new forms where necessary and add compacted gravel for a good firm base. Once the base is established you can pour the slab as it was new and you shouldn’t have a problem for another 30 years if conditions are good.

    Unfortunately most people try to shortcut this process and just have the floor leveled because they are tired of looking at a cracked floor and in a garage converted for an exercise room there will be a lot of time spent in it.

    If you are considering doing this there is a method that you should follow. The first thing is that you will need to anchor new rebar into the existing slab and then use extra rebar around any cracks. It is a difficult process but you are basically stitching the old slab together and also to the new slab.

    However most contractor will simply come out and place a rebar mesh on small pedestals above the existing floor and then pour enough concrete to level the area and cover the rebar mesh. They will consider the existing slab structurally sound and just pour the new slab as if they were pouring over a gravel base.

    It might work depending on all the conditions because the slab would be better than just the gravel but if there are voids under the slab that allow the original slab to heave in wet or cold conditions then you are basically on your own as any type of warranty would be worthless.

    To complete this job the contractor will need to order 2 to 3 yards of concrete at an approximate cost of $75 per yard depending on your area costs can be different. When subtracted from the original quot there is quite a bit of labor in this quote even if you count the use of rebar. I probably would pay the original quote for a contractor to remove the old slab and replace it with a 4 inch slab that has center piers and rebar but not just for a leveling of the existing slab.

    Final Note

    So, I hope that helps you understand how you might approach this problem. The slab in this situation is over 40 years old and if it is stable then good results might occur on the other hand you can’t really tell by looking at it.

    One way is to tap on the slab around any cracks and then in a good area. The slab should not sound like it is hollow under it or that it is thin at that point.

    Textures are really up to you but it is best to have a floated slab that is smooth in an interior area. Brush finishes are best for walkways where slipping could occur.

    Cost is always a factor in making this type of decision but maybe you might be able to work with the contractor to reduce the price if you were to remove the slab yourself and apply a stone base. Its an exercise room anyway so breaking out the concrete could just be good exercise and a money saver.



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