Return to index: [Subject] [Thread] [Date] [Author]

Re: Structural Slab on Grade

[Subject Prev][Subject Next][Thread Prev][Thread Next]
We design for bad soil conditions here in the prairies all the time.  A structural slab on grade is a bit of an oxymoron.  You are buildiing either a slab on grade, a structural slab, or a mat foundation.

Slabs on grade are typically 6" slabs with 10m@16" o/c ea. way top on compacted fill and poly as per geotech report.  This is essentially a way to cover the dirt, it is a glorified wood floor.  There is little to no structural capacity in these slabs for vertical loads, but I have used them to transmit the horizontal reactions resulting from a rigid frame.  So it literally is a slab on grade.  These typically experience a lot of cracking and a lot of movements, esepecially in frost susceptible soils.

Structural slabs have two levels of reinfoorcing, or one level of bottom reinforcing with top reinforcing mats over piles.  Depending on the loading, structural slabs range from 5"-8" with thickened poritons around piles or grade beams to transfer the shear to the piles or pile caps.  These slabs are built on a shearmat (plywood) sitting on 6" cardboard void form.  Once the slab is poured, the voidform eventually rots away leaving you with the void.

A mat foundation in the prairies is used for massive loads, or where a strucutral slab is more expensive due to soil conditions (longer friction piles).  I can think of only two jobs where we recommended using them.  Typically 2'-3' thick slabs are poured on prepared surfaces with levels of C and A base and geotextiles as per geotechnical report.  These would support slip formed grain silos or grain storage facilities and or potoatoe or other food product storage.

To your question on crack control for strucutral slabs.

Depending on your pours, you can put joints in where the pour stops.  Look for areas where there are "bottlenecks" in the concrete and add 10m bars at 12-18" o/c by about 4'-6" long perpendicular to the anticpated line of the crack - works great.

(bottle necks = slabs around corners, pour joints, slab getting wider or narrower (in plan) etc)

On a rectangular space the top layer of steel should be able to transfer the loads and handle all the cracking due to shrinkage.  I've never recommended or seen sawcutting on structural slabs.

Short answer - use top reinforcing steel to handle the cracks. 

Hope this helps,