You could cast concrete in a cardboard box... except for one thing - it's not water tight. The cardboard will suck the moisture out of the concrete, leaving a chalky finish at best. At worst the box will blow out or fall apart before the concrete has cured.
The solution to this problem is, of course, Duct Tape. Taping the inside of the box will make it water tight, and also give the concrete some texture. Taping cardboard edging to the corners will help strengthen the box and minimize the bowing under pressure that happens when casting.
Materials + Supplies:
- Cardboard Boxes (2)
- Cardboard Edge Protectors (90 degree bends)
- Duct Tape (1/3 Roll)
- CHENG Pro-Formula Concrete Mix (Tahoe Color Shown)
- Sakrete 5000+ Concrete Mix
- Weights / Rocks / Sand / Something Heavy
- 5 Gallon Bucket
- Shovel / Trowel
- Particle Mask
- Rubber Gloves