A reader's Concrete and Concreting question: Bar chairs?

Shane M,   from   Cork : Ireland   had this bar chairs question.

Bill's answer

Bar chairs - Plastic bar chairs, supporting the reinforcing mesh on a concrete slab.

• In the photo above, the small black things in rows under the reinforcing mesh are the bar chairs.
• I guess they get the name chairs because the steel mesh or rods sit in them, that is, there are moulded in grooves to hold the bars in position.  (otherwise the bars could move when the vibration hits).
• There are different types, plastic is by far the most common, as the ones above, but there are still make plastic tipped steel ones etc.
• The plastic ones shown have a wide circular base to stop them being pressed through the orange "Fortecon" water proof membrane.
• They perform the job of keeping the rebar in the correct design position.
• They have to be capable of not only carrying the weight of the steel, but the men and their gear and then the weight of the concrete.
• So a drawing, but more likely a specification sheet might say "35 top cover"
• Meaning that there has to be 35mm of concrete covering the top of the steel during the construction pour.
• So depending on the thickness of the slab you buy bar chairs to suit, that will hold the steel in the right place.
• Typically you might get 55-65.   The recesses on the top of the chair at right angles to each other, are at different depths, to give you two choices of heights.
• In the photo the chairs are set at about 600 centers. (better a few too many , rather than too few, as they can collapse).

Also in the photo, you can see the joints in the sheets of mesh.  The mesh has to overlap at the joints at least two bars or one full bar spacing.  The edge bars on full sheets where they overlap are usually thinner.

Engineers like to see the rebar on slabs like this close to the top, and concretors want it lower down to stop the risk of it getting too high.  So they have to get the chairs right, and then tie the steel at the joints now and again to stop it popping up.

OK Shane, that's it, who woulda guessed that little things like bars chairs would be so important.