Concrete can radiate cold. Put cardboard down first as it will be a buffer. Tarp and wood chips too. That is how we made our broader in the garage and mine did okay
I would put cardboard down on the concrete floor. Have some type of safe heat in place. If using a heat lamp make sure it is securely mounted several ways and keep it clean of dust.
We have a heat lamp that is chained to a hook and then wired to another hook. And the electric cord is also...