I regularly hear the question “Where should the reinforcement mesh go in slabs?”
Well here is some advice. . . .
Understanding The Materials
Concrete – Strong in Compression, but weak in Tension.
Steel Reinforcement– Strong in Tension, and strong in Compression (As long as the concrete can stop it bowing)
Lots of concretors will tell you “We put steel the reinforcement close to the bottom of a driveway slab because when you load a slab, the bottom is in tension,which needs the steel, and the top compresses.” . . . . . THIS IS WRONG!
Here Is The Explanation Why
For a simple beam that is supported at the edges like the sketch below the bottom part, coloured pink is in tension as the load tends to push the centre downwards.
For this type of beam, or slab, reinforcement in the tension zone (the bottom) is the way to go. . . . . BUT THIS DOESN’T REPRESENT A SLAB ON THE GROUND!
The sketch below does represent your typical slab on ground with the slab supported fairly evenly by the ground.
Typically the loading can come close to the edge of the slab; car tyres if it’s a driveway, brick walls for a house slab.
With the heavy loads on the edges but the support across the whole area the loads are trying to bend the ends down . . . so the tension area is the top.
Finally here is the case where there is a load towards the middle of a slab over a soft spot such as a badly backfilled trench.
In this case at various points of the slab; the bottom, the top, or even the whole slab can be in tension.
What Is The Answer?
Well for a slab on ground the bending stresses (which cause tension) are actually quite low, as the concrete actually takes transfers the load to the ground by spreading it out over a wider area see sketch.
The main purpose of the steel reinforcement is to hold the concrete together and keep any shrinkage cracks as narrow as possible.
If you are concerned about the surface appearance, reinforcement is better closer to the top where it will better control surface cracks.
The best place to put steel mesh reinforcement in a concrete slab, cast on the ground, is about 1/3rd of the depth from the top. . . . . Unless you have an engineering drawing that says otherwise!!
Make sure the reinforcement is in the right position by insisting on Bar Chairs.