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A 2"  full depth pour concrete counter top will weigh about 25# per sq' so you are designing a 900# top.  I don't know of anyone that offers up a reccomendation on spans like this. At 2" you can weld up some 6 guage or 4 guage hiway mesh with #3 or #4 rebar welded on 6" spacing.  The reinforcement should be in the lower half of the top as the majority of the stress will be on the bottom of the piece. I have even seen carbon fiber strips epoxied on the bottom of countertops for additional support.   I hope this helps at least a little, i know it is probably more vague than you would like.  It's all fun and games until my fat ass brother in law shows up, opens up a beer and confuses your countertop with a bus stop park bench and plants his 42x30 wrangler boot cuts in the middle of it with a loud SNAP!  Good Luck.