I am in the concrete formwork business (among several other things) and my forms are made from Structural Foam - 45% Recycled Content - HDPE (High Density PolyEthylene). We do up to $20m business per year and have sent-out feelers in all directions in search of an adhesive. If we could find an adhesive for HDPE that could hold-up to the jobsite abuse in the concrete formwork arena, it could be worth a fortune to me. There are adhesives that will stick them together - but not good enough for my business that we have located as of today. My [consultant] polymer chemist has been working on this for years, but nothing yet has held-up to our testing. With concrete forms, plastic forms must be stronger than the Hydrostatic Loads would dictate - due to the heat of hydration of the concrete - up to 180F - which fatigues the matrix. My forms do incorporate heat dissipation fins. Check with US Plastics on the web, they will have it if anyone does - but don't expect it to be as strong as the plastic itself.
On another related subject, plastic can be fusion welded; I've utilized fusion welding in tests and got good results, but it is more expensive than the bolted connections we are currently using. The very best fusion welding does not use "Prescriptive Rods", but rather uses melted shavings from the exact plastic being used - this was the only way we found to get a super-strong bond capacity.
OPG3 Tweak everything!