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Slab on Grade-Confusion?

Slab on Grade-Confusion?

Slab on Grade-Confusion?


Design of slab on grade has gone through so much discrete sets of research and the whole thing appears very confusing to me:

-Why is a mat only a fraction thicker than a garage slab not designed for no-crack condition and provided with joints?
-Why cut reinforcement at every construction joint?
-Why slabs in expansive soils have to be reinforced with beams? Why not reinforced flat slabs?
-Why same equations for a slab resting directly on expansive soils and the slab under which expansive soil has been removed and drainage provided? Doesnt drainage matter.
-Should slabs on outdoor(eg car park) be designed and detailed the same way as indoor slabs?

As long as the soil is fine and loading is not critical, detailing them old-fashioned way seems straightforward, but not when things change.

A few lines from you will be highly appreciated.


RE: Slab on Grade-Confusion?

That is too complicated a subject to answer here very comprehensively, but I'll try to hit the high points.

In expansive soils, the slab is designed to act as both a cantilever (drying/shrinking soil), and a simple beam (wet/expanding soil). This most effectively done with beams. The use of a perimeter beam also reduces the movement of moisture at the slab edge, and minimizes erosion of soil from beneath the slab.

Slabs supported on soil typically only transfer loads to the soil. The mechanics of this transfer affect the slab serviceability (deflection, cracking), but ultimately the slab distributes loads so they don't sink into the dirt.

Saw cuts or joints reduce cracking by reducing shrinkage strain seen in longer slabs. This can result in curling of slabs at joints, as the slab dries at the top while staying moist at the bottom. Cutting reinforcement allows the release of strain at joints, but reduces continuity across joints, which may allow faulting or raveling.

Parking slabs are usually acting as mud slabs, just keeping the tires out of the dirt and keeping the underlying soil relatively dry.

RE: Slab on Grade-Confusion?

Thanks TX

I could only visit here today(eng-tips forums was down in my area for months)


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