I've never worked with 12 inch PVC, so I can't speak from experience on that size of pipe. I've done allot of work on 6 inch and smaller PVC pipe and have some knowledge of how it works.
First, primer is a must. Purple primer is the standard and I don't even know if there is any other kind. Calling it primer is sort of misleading. It's not a primer in comparison to paint, but is actually a conditioner that starts a chemical reaction in the PVC for the glue. In joints that I've seen fail, there is usually just a hint of primer on them. I'm no scientist, but I've never seen a joint fail with plenty of purple primer on it.
Next, of the glues available to use on PVC pipe, the clear glue is the best. Each color has a specialty that it does, but none are as strong as the clear. In my area, it's the only glue allowed for water lines.
The glue is also misnamed. It doesn't really work as a glue when compared to wood glue. It doesn't actually bond the two pieces together, but instead, it sort or welds the plastic together.
The primer started the process and actually softens the PVC. Then the glue breaks down the PVC and bonds the two pieces together into one.
If you ever manage to pull apart or break off a properly glued PVC connection, you will see that the glued area doesn't fail. It will be the PVC itself that breaks off inside the pipe wall.
The only reason your pipe failed was that it wasn't primed and glued properly.
Eddie
