concentrated

Yes.

Thank you.

John is concentrating.
John is concentrating on his work.

To clarify, what Roberta posted is correct, however the original form ("is concentrated") is also correct.

No.
In your sentences, 'concentrated' is being used as an adjective. When used to mean 'showing a lot of effort or determination', it can only be used directly with a noun.
e.g. He made a concentrated effort to improve his work.

Either use 'concentrating', or use 'is focussed'.

What's your first language? UK or US English?

This seems to be misleading; there are plenty of examples that contradict you.

He was concentrated on getting his fuel probe in the basket

He was concentrated on his thoughts.

He was concentrated on getting away from everything as fast as possible. In the background, he could hear Ot's voice say to turn right on 153rel street

On this morning, though he was concentrated on the outside shrubbery as he leaned against a log fence, he became aware of a tension gripping him

He was concentrated on the CARES project, a capital asset realignment of VHA;

The only thing he was concentrated on was Crue, that moment and his love for him.

Johnson said he was concentrated on improving his fitness

(etc)

https://www.google.co.uk/webhp?hl=all&gws_rd=cr,ssl&ei=hx59VbrGDsKnygOo_4CIDQ#safe=off&hl=all&q=%22he+was+concentrated+on%22

also...

https://www.google.co.uk/webhp?hl=all&gws_rd=cr,ssl&ei=hx59VbrGDsKnygOo_4CIDQ#safe=off&hl=all&q=%22she+was+concentrated+on%22

Bazza is not a native-speaker of English. Bazza consistently copies and pastes bad advice about English grammar and usage which he finds online.

Thank you.

John is concentrated. Just add water.