Way back in 1992, Duane Raleigh did an excellent study of different types of climbing bolts for Climbing Magazine. What I particularly like about Duane Raleigh's methodology is that he tried to simulate different hardnesses of rock in order to see how different bolts would perform in different rock types. The verdict that Duane Raleigh rendered is simply stunning. It turns out that even the biggest, strongest bolts are only as good as the rock that you put them in.
"Anchors Away - the Nuts of Bolts." by Duane Raleigh. Climbing Magazine. October, 1992.
After reading Duane Raleigh's study several years ago, the thought occurred to me - if the numbers that were recorded in the study are to be believed, then I should be able to walk over to almost any crag in the Santa Barbara and Ojai areas and simply and easily pull bolts out of the wall. The first bolt that I looked at was a bolt at "the Foot," in Ojai. On a cool winter day in 2004, I placed the nail-remover of a small household hammer beneath the hanger of a bolt atop that cliff. I never put more than moderate pressure on the hammer...the results sickened me. Five years and dozens of bolts later, I continue to be amazed at how poorly most bolts hold in the walls of our local crags.
More about bolts from Duane Raleigh: "Mechanical Bolts"