In reply to Steve Crowe:
Hi Steve, tricky to say why they've come loose. A good glue in, placed correctly shouldn't come loose. Repeat loading tests have been done, and this technology is used for building bridges etc. It's designed to last and to withstand high dynamic loads.
This is the first I've heard of this issue, so perhaps it's isolated to a specific area and bolt/resin combination? Hard to tell without investigating further. If you want to mail me with some examples I'll get the ball rolling on that.
If we're talking about lifespan, it depends again on the specific materials used and the bolt model. The large majority of the original Pen Trwyn Eco-bolts have recently passed their inspection and are 20 years old and going strong. Not bad eh?
As far as knowing when to replace a glue in; is the visible part of the anchor showing evidence of cracking, excessive wear or corrosion? Does the anchor rotate or can it be pulled out? If yes to any of these, then replace it.