It is not so much only the strength that needs to be specified, but the class of the concrete as per CSA A23.1. It's been a long time since I looked at S413, but it used to require a C-1 concrete, which, amongst other items, requires 35 MPa concrete.
I've been told by someone in the know, that to meet the C-1 or C-xl class the W/C ration is max of 0.45 or something like that which ends up giving you much more concrete strength than you ask for. 35MPa is easy to get with a low W/C ratio.
Minimum is 35 Mpa. It is not easy to make a C1 concrete (35 MPa) that is useable with a low w/c ratio (0.40 as per CSA23.1). Super-plasticizers will be needed so the strength is not compromised by the finishers wanting to add more water for workability. If the suspended parking slab is designed with CSA S413 code then an integral corrosion inhibitor must be used in the mix design to protect the steel from chlorides.
