SLOPING THE GRADE?

[Ibeam]

Slab on Ground - 46'x134', 4 interior drain points (not at evenly spaced intervals) with the slab elevation level around perimeter & at intermediate joints. 4 inch difference between drain elev. and perimeter, slab sloping to drain points.

Does the concrete thickness vary in order to create the slope or should the base be graded to give a constant 6" slab thickness? I have 6" slab, 6" compacted aggr. base, 1 ft. select fill. It seems like it would be difficult to accurately grade the base considering the contours. My boss says to vary the concrete thickness, with 6" being the minimum at the drain points, but 4" seems like a big difference in thickness. Any sage advice out there?

 

[JAE]

You have to first decide what your minimum slab thickness is (6" ??) and then it is just a matter of where you spend the cash. Sloping the grade is not that difficult, but it does take time and contractor labor to do. Varying the slab thickness doesn't take the construction effort, but you pay for more concrete. Either way, the rest of the project labor and materials stays about the same.

 

[boo1]

When we have installed sloping slabs, the thickness was mainained a constant. The loads should dictate the thickness. Why waste the extra $$$$$, your slab will require an additional 38 cubic yards of concrete. (if you take an average if 2 inches) Lay out grade stakes in a grid pattern.

 

[Ibeam]

Yes, my slab design thickness is 6", which I had intended to leave constant. I'm not a slab on ground expert by any means, so the varying grade I'm faced with left me wondering if varying the slab thickness might be the correct method for construction. Also, I figured my compacted aggr. base (help spread pt. loads)didn't exactly ease the grading process.

From your posts I'm feeling better about standing firm with my design, but I'll need to convince the bossman. He's been on the site for quite a few of these jobs and claims this is the way to do it (he's a mechanical but has seen alot of construction). I don't see the added grading labor being worth the extra concrete bucks as boo1 points out.

 

[markls8]

Not my area of expertise, but a uniform slab thickness also contributes to a more uniform, less stressed (= less cracking) cure. - Brian.

 

[Ibeam]

Just a follow up.. the grade will be flat, concrete thickness will go from 10" at the perimeters down to 6" at the low points (drains).

I had 2" clr. cover on top originally, I guess it will stay and will end up as 6 at the 10" thk. areas. I've been calling out the "soff-cut" method for joints recently but haven't seen it performed yet. Maybe that will help.

 

[Ron]

Your varying thickness of concrete creates a more challenging jointing process for shrinkage control joints. Make sure your control joints are spaced closer than you would normally make them (I suggest 20xslab thickness, max., for this application(this will give you the joint spacing in inches)). Saw the joints as soon after placement as you can without raveling the aggregate. Don't wait until the next day.

The varying thickness of the slab creates varying restraint, thus a greater tendency to cause "random" cracking.

For the voting, I would keep the concrete thickness uniform and slope the subgrade.

 

[raghav]

TO MY UNDERSTANDING IT IS ADVISIBLE TO PROVIDE SLOPE IN FILL(PREFERABLE BEING MORE ECONOMICAL) OR IN THE AGGREGATE BASE AND IT IS NOT DIFFICUULT.MAINTAIN CONSTANT THICKNESS.
THERE IS NO TECKNICAL PROBLEM IN PROVIDING SLOPE IN CONCRETE.BUT YOU SHOULD ALSO LOOK FOR ECONOMICAL SOLUTION .

RAGHAV