[Greenbuilding] Type of concrete for radiant floor

[Richard Hakim]

Hello.
 
We are building a house on the coast of the Pacific North West.  We are
putting in a hydronic radiant floor in 1.5" of concrete.  Being remote, I
need to have the gravel and cement for this concrete trucked in and then
hand mix and pour on site.  
 
I am trying to figure out the type of concrete to order for my radiant
floor.  I've read that the concrete for a radiant floor is a "lighter mix".
Surely that can't be all I need to tell the gravel yard?  Is there a
technical term for the particular concrete mix to use?
 
Another question I have is around the amount of concrete to use.  We planned
to use 1.5" of concrete in the floor.  Our intent around using 1.5" was to
strike a reasonable balance between time for the floor to heat versus
thermal mass, and also this gives us the ability to use a 2x2 for a sleeper.
However some people are telling me stories of concrete cracking with 1.5"
and are suggesting going with 2" of concrete.  Because we have to hand mix,
this is a big difference - plus we would have to cut our own sleepers out of
2x4.   If anyone has thoughts on this dilemma I would love your advice.
 
Thank you for any help you could give.
 
 
Richard