ACI 117-9, Level Alignment

[a2mfk]

In section 4.3.1.1, for tops of slabs-on-grade a number of 3/4" is given.

I read this to say that for a slab-on-grade, if the plans say to place this at 100'-0" elevation above sea level, and you also cite ACI 117 tolerances, you could be at 99'-1/4" or up to 100'-3/4", and the slab level alignment would still be OK.

Not a specific problem, but something I have to deal with.

If you interpret this section to meet something different please explain, maybe could it also be suggesting 3/4" out of level within the slab surface, ie, relative to the slab surface you can be 3/4" out of level? But over what distance? I think that is a separate issue that is dealt with in floor finish tolerances with the flatness scale (4.5.6).

Thanks in advance.

 

[dcarr82775]

I have always interpreted the levelness requirement the way you did in the 2nd sentence.

Flatness is a different animal

 

[Ron]

Agree. But if you try to work levelness, flatness and thickness tolerances together, you get an extremely tight requirement.

 

[a2mfk]

Thanks dcarr.

Ron- I think this is up your alley and was hoping you'd chime in.

Lets just say that I am being forced to use this section for residential slabs-on-grade and leave it at that.

To me this means that anywhere on the slab that you measure, it must be within that 3/4" limit. Regardless of the set elevation requirement (say there is no actual number, like 100'-0" above sea level), essentially this interpretation would restrict you to 1-1/2" of elevation variation between any two points on a slab regardless of the distance between the two points.

Part two of this issue I am dealing with addresses local variations in the floor slope or flatness, versus global differences which are given above by "levelness".

I think I got it....

 

[dcarr82775]

I don't think you are correct with respect to the distance between the high and low points, it does matter. The distance between the high and low point matters because if they are too close together you violate the flatness criteria. Ignoring the FF/FL numbers and using the 10' straightedge method, if your high and low points are too close together you can pretty easily locate the straightedge such that the gap is too large (1/8. 1/4, 3/8 whatever the maximum gap was specified to be). I think there is another restriction on flatness that further restricts how quickly you can go from high to low, but I do not have 117 handy.

 

[a2mfk]

Dcarr- you have to reread it, its a bit confusing unless you have it all right in front of you.

In the "level" section, there is no mention of horizontal distance criteria. This is why I originally assumed the numbers apply to tolerance within a stated elevation on the plan, such as Finished Floor El = 100'-0".

The flatness is addressed in another section. And agree fully there, it is absolutely a function of horizontal distance.

 

[dcarr82775]

a2mfk, I agree they are different. I was just saying that you can violate the flatness requirements by being too far out of level over a small distance, essentially limiting how quickly the top of slab elevation can change.

The annoying bit is the measurements normally have to be taken within a few days of casting the slab.