R.C. slab top cover

[Pierre Vivier]

How can the contractors ensure that they achieve the desired top cover thickness on a large slab with reinforcing on both faces?
For example - a large reinforced concrete reservoir roof slab, lightly sloped for drainage. At areas the concrete will be to far from end shutters to judge the required height of concrete.
I was thinking of an inverted cover block (spacer block) to achieve the desired slab thickness & top cover when floating, but this just seems like a bad idea.

 

[BridgeSmith]

I would think a laser level and a tape measure would do the trick, unless they're using a screed machine, then it's just a matter of putting a spacer block on it for a 'dry run'. If it's not absolutely huge in both directions, a taut string line also should work.

 

[SlideRuleEra]

How can the contractors ensure that they achieve the desired top cover thickness on a large slab with reinforcing on both faces?

Place the reinforcement in the right place, use plenty of rebar supports (chairs), and tie the rebar securely.
The time for a Contractor to be concerned about rebar location and cover is during concrete forming, not during concrete placement.

On thing that helps... large, heavily reinforced slabs often use larger rebar sizes (say #6, or bigger). Large bars have good structural properties themselves; when properly placed, supported and tied they don't move a lot.

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[WARose]

I worked with one who used wires (sticking up vertically off the chairs or re-bar) to be sure he got the right height. (I.e. if the wire was covered, you were ok.)

 

[EngineeringEric]

I've seen strings and I've seen contractors with a stick push it into wet concrete, re-vibrate, and move on. I think most systems that are more complicated than a string, stick, rod, tape, or similar are just too complicated