Reinforcement spacers come in many shapes and sizes. Presumably be "dés" they are referring to those used to hold rebars a predetermined distance from the formwork (and thus from the outside face of the finished concrete). I don't know of a terminological distinction between this kind and those used to separate rebars deeper inside concrete.

Maybe "external spacers" "outside face spacers", etc. Whatever, such spacers - when used - are made of concrete so that they bond effectively with the poured concrete, thus making water ingress more difficult (than if plastics spacers, say, are used).

Presumably your text would allow spacers of the non "dés" type set inside the concrete. Typically, in the case of round columns, these would be flat disks with indents round the edges in which the rebars sit.

For details see http://www.halfen-frimeda.no/doc/fib-kat.PDF for example.

--------------------------------------------------
Note added at 1 hr (2007-07-27 10:16:00 GMT)
--------------------------------------------------

"Point-supporting spacer" as used in the above ref. would be a possibility.