Rebar in a concrete slab in contact with the ground will form an electrode to a degree, but if this is to be relied upon, then there are a number of parameters in  the casting of the slab to be considered and need to be communicated to the builders.

1) (obvious I hope) there should be nothing between the slab and the earth, so it has to be on the 'wet' side of any bitumastic or plastic membrane

2) The conductivity of the mixture used for the concrete must be known to be good - there are additives (marconite is one example ) that can be used with the ballast to give a well defined conductivity if the concrete supplier is not used to this sort of question.

3) joints in the rebar become more important - things need to be twisted and tied firmly, and in many parts of the world, more than one contact point must be brought to the surface so that a 'ring-round' test can be performed to verify that the terminals are connected by a low resistance path, to demonstrate that the rebar/mesh is actually well connected, not electrically isolated patches.

4) Where the contact points come to the surface provision must be made for a good mechanical connection that is in the right place and accessible.

It is an excellent idea to connect the rebar to the earthing network, but unless the steps above are ensured, the results may be hit and miss from site to site and just calculating the effect as if  total rebar length was a direct burial is unlikely to be an accurate estimate.
Mike

Great answer thank you!