Help for chicken newbie

Hiya. Im going to inherit 3 chickens on Monday. I have fenced off half my garden so my dear dogs cannot get to them but the half that is fenced off is concrete. I want to put something on top of this so the hens are a bit more comfortable and able to scratch around. Im new to chicken keeping so unsure as what is ok, my first though was just ordinary bark chippings but I've read this is not so good. anyone with any suggestions?
Also how would I go about making them a dust bath is a large plastic/wooden bowl filled with dust enough?

Not advisable to use sawdust this is bad for their respiratory tracts.

Our girls run in on concrete slabs and it has worked very well for us - it has a three inch layer of hardwood chippings which aren't cheap but are lasting really well - they scratch about in the chippings to find grit that I've scattered about and the odd dried worm which I've placed for them.  They have two dust baths one is an old tin and the other is a wooden box both of which I fill with soil and diatom - they love them both!!! and it's great to watch them  something I've been doing a lot of since we had them 2 months ago!!!  Good luck with your girls.