huangyhg

pretensioning bolts
has anyone pretensioned a 3/4" bolt themselves and later removed it? physically, how difficult was it? would you recommend air tools to help with removal?
i'm looking at pretensioning bolts myself (don't ask why) and am trying to figure out how big of a pain this would be...

a properly pre-tensioned 3/4" high strength bolt (astm a325) will have an applied torque somewhere between 375 and 425 ft.-lbs. that's what you'll have in reverse to remove it, assuming it hasn't been exposed to weathering. if that's the case, the torque required to break it loose might exceed 500 ft.-lbs.
an air impact wrench would help, but to develop that kind of torque, you'll need an industrial model, commonly called a yo-yo. otherwise, use a spud wrench with a "cheater bar" or pipe on it to get the required torque with less applied effort on your part.
i know of lab tests that used pretensioned 1" bolts. they had to get the bolts off somehow, so i know it's possible. i'm pretty sure they just take a 3-4' long piece of pipe and use that as a cheater bar on a spud wrench.
if you are tensioning bolts grouted with adhesive, manufactures', such as hilti, representative will love to give you on-the-mark advices. it is highly dangerous if you break the bolt, it flies like a rocket (saw it once in field testing, the bolt was torqued to fail).
sorry, retrieve my statements on the "dangerous". the bolt was torqued, then pulled to fail. just a story, irrelevant to your question.
hammer wrench... if you can secure one, would be cheapest way to go.
what is the base material the bolt is tightened to? what is the base set-up to develope and hold the pre-tension?
other than wrench, you will need tension gauge for installation.
if grouted in concrete using quick set adhesive (epoxy), you might have a tough time to retrieve the bolt (see "danger" above).
b16a2,
looking back at some of my rocaps it looks like my 3/4" bolts tensioned at 200 to 320 ft lb. not to difficult. as others have mentioned, a cheater bar works good or you can get a multiplier. both will help, but 200-300 pounds is not that much.

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