I don't know why you would not use pvc pipe. Various workman have left piles of it here, I use it for all kinds of things including hanging my clothes on it. re-purposing is great.
An alternative to using cardboard is solarization as described in this article. (scroll down to solarization -- the article is really about killing nematodes.
http://edis.ifas.ufl.edu/ng047
Some people also use black plastic (viscuine) because it serves as a heat trap.
Either of these should work on cold soil. The only problem Ive heard about is that if left in place, the plastic will crack and make a big non-degradable mess.
I haven't tried putting a sheet of black plastic over cardboard, but theoretically it would seem a good way to both warm the soil, speed up the disintegration process, and kill unwanted vegetation by excluding sunlight. Just remove the plastic when its done its job and save it for another project.
If you use clear plastic you are also killing any soil bacteria--it really is for sterilizing the soil. Some soil bacteria you want to keep. Its sort of like taking antibiotics for a urinary tract infection--it kills all of your intestinal flora.
