Mud Floor over 24"OC joists? Weight an issue?

Anyone successfully put down a mud floor without eventual cracking with 24" OC joists? I'm a little concerned about the weight.

This house has 24"OC floor joists. The planned mud floor would be 3' x 5.5' and span 2-3 (2x10) joists and over 3/4" OSB. My mud floors usually go 1.5" thick.

I can probably sister 2 of the joists, but there's no room to add separate joists in between due to HVAC and plumbing.