re-snug Bolts on pier

Discussion in 'Off Topic' started by Tim Povlick, Feb 24, 2021.

  1. Tim Povlick

    Tim Povlick Cyanogen Customer

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    Slightly off topic?

    I have a pier mounted to a concrete slab via eight 1/2" diameter stainless steel stud-bolts epoxied into the concrete. This is a high grade epoxy for this type of anchorage and should hold at least 1000# / stud (8000# total). The installation was 5 months ago. Checking the torque, surprisingly, was able to spin the capnut about 1/20th a turn. Is this due to the stud bolts stretching a bit?

    Thanks
    Tim
     
  2. William B

    William B Cyanogen Customer

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    Unlikely to be 1/2' bolts stretching Tim otherwise half the structures in the world would be falling down!

    Chemical bolts are actually much harder to install correctly than most folks realise with a multitude of ways that they can fail.

    A few of the more common failures:

    Exceeding the recommended torque settings.

    Torquing the bolts before the resin has had sufficient time to reach full strength, mechanical full strength does not occur together with apparent 'setting' of the resin anchors, for some resin mixes full strength is not reached for three to four days after initial curing, depends very much on temperature, how well the initial mix of hardener and resin was carried out and which mixture of chemicals is used for the anchor.

    Weak concrete that had too much water in the mix will fracture and allow the bolt to pull out of the main concrete body as a V shaped plug of solid resin still attached to the surrounding fractured concrete.

    A concrete mix that was too wet will also be very weak in the top few inches as the aggregate and sand separates and sinks to the bottom of the pour while the mix sets, the bolts have to be long enough to reach down into the heart of the slab to reach the stronger concrete.

    A concrete mix that was too dry will also be weak and often contains voids which reduce the anchor surface area.

    Installing the chemical anchors before the concrete was dry and up to full strength, six to eight weeks minimum after concrete pouring in the summer, much longer in the winter.

    Drilling the hole over size so that the thickness of resin between stud and wall is too great, allowing the resin to stretch and 'flow' under stress.

    Accidentally drilling straight through the slab and out the back so that for chemical capsule bolts part of the capsule is pushed into the earth below the slab and then the hardener and resin do not mix with the correct ratio, some of the resin or some of the hardener leaks into the soil and the resulting resin mix is weak. Further problems can occur if the slab is accidentally drilled right through the back and the underlaying ground is damp as any moisture will wick back up the wall of the hole in a matter of minutes and form a barrier to the resin.

    Failure to clean the anchor hole sufficiently, simple brushing is never enough and we would normally use a combination of compressed air and vacuuming to clean the holes adequately, any dust that remains forms a barrier to the resin, more of an issue with injection resins rather than mix-in-the-hole capsule resins which churn the mixture in the hole and blend any dust into the mix.

    Failure to de-grease the studs with a suitable solvent prior to fitting which results in a weakened bond between the stud and the resin.
    Many anchor stud bolts are supplied still coated with a thin film of lubricant from the manufacturing process and require solvent or detergent de-greasing before use.

    There are several more failure scenarios for chemical bolts but the above are the 'highlights'.

    All you can do for now is to mark the failed bolt with a dab of coloured lacquer, re-torque the bolts to the correct torque settings with calibrated torque wrench, according to the manufacturers specification, and recheck the torque settings in a few months time. If the marked bolt has lost it's torque setting again during that time that would indicate a faulty installation.

    In such a case you may be able to spin the stud out, cold, or after heating with a blow torch, and then re-drill the hole for a larger diameter anchor, or exchange the anchor for an expansion bolt, though eight bolts sounds a lot for a typical amateur class instrument so unless you are throwing a metre+ class instrument around at huge slew rates then seven good fixings would probably be more than enough!

    William.
     
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  3. Tim Povlick

    Tim Povlick Cyanogen Customer

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    Hi William

    Thanks for the detailed information as well as failure points. Should have stated the concrete is 20 years old and is in good condition. Drilling the holes took a large rotary hammer drill.

    The stud bolts are all-threaded SS but I didn't think to clean them. The other pitfalls you mention were dodged, hopefully.
    The datasheet (Henkel EA-7363) shows for a 1/2" stud 4" deep in concrete, the strength is 22,000#.
    Hopefully eight attachments (> 100,000#) would hold up 150# of astro-gear.
    Will snug the capnuts and hopefully they hold tight.

    Greatly appreciate your time and expertise.

    Tim
     

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  4. Doug

    Doug Staff Member

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    Tim,

    I have four 1/2" bolts holding my pier in place, and I have more weight on it My pier alone is the better part of 100 lbs. It's as solid as a rock.

    The only thing that moves is the ground underneath. (It's all clay in my back yard... despite the pier going 9 feet deep, it tilts an arc-minute or two northward every spring thaw!)

    Doug
     
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  5. Tim Povlick

    Tim Povlick Cyanogen Customer

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    Hi Doug,

    I wonder if you're seeing Earth's Precession; 50.3 arc-seconds / year according to:

    https://www.universetoday.com/77640...a motion is called,a full precession to occur.

    At 3 meters deep (hopefully) the pier is below the frost line. :)


    The pier here is on a concrete slab on a bed of sand as the local landscape looks like Mars.


    BTW, the missing anchor was for figuring out grounding and has been added.

    Thanks,
    Tim

    PS : Pix: A local sand dune
     

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  6. Doug

    Doug Staff Member

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    It's below the frost line, but it still shifts. Much of Ottawa is built on Leda clay, laid down by the Champlain Sea at the end of the last ice age. It's horrible stuff - it turns to sticky mush when wet, and if you step in it your shoe comes up looking like an elephant's foot. Ask me how I know LOL. At our particular location it's clay all the way down to the bedrock (I know because the builders hit the bedrock when they excavated our neighbour's basement).

    Interesting thought about precession but I think it's in the wrong direction, and not quite large enough. It's probably 3-4 arc-minutes, just judging by how much I have to move the adjusters. It's not something that I go out of my way to measure, I just fix it!

    Anyway I don't think one problem bolt out of eight is going to have an impact on your pier. Even if it is apparently installed in a Martian desert.
     
  7. Tim Povlick

    Tim Povlick Cyanogen Customer

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    That clay is dreadful stuff.
     

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