Jim Lux jimlux at earthlink.net
Wed Feb 1 15:15:57 EST 2006

At 10:08 AM 2/1/2006, Pat Barthelow wrote:
>Steve, K7LXC Said:
> >Hiya, Ted --
> >
> >     I wouldn't worry about the bolt pattern. The  existing bolts can be
> >chopped off with a hand grinder and then you can  rent a drill with a
> >concrete bit or a hammer drill to drill new holes.  Epoxy in the new anchor
> >bolts and you're good to go. Construction guys do this  all the time and it
> >works great. When you're done, the epoxy is typically  stronger than the
> >concrete.
> >
> >Cheers,
> >Steve   K7LXC
> >Professional tower services for amateurs
> >Cell: 206-890-4188
>I am curious about the Epoxied anchor bolts.  Do the instructions have you
>drill the holes for the bolts and epoxy in a way that the hole in the
>concrete,  is lined up in the direction of the pull?  Or is the axis of the
>user drilled hole, NOT  collinear with the force exerted?

The instructions, which are quite detailed, cover a lot of this..
The adhesive isn't always epoxy, by the way.  Acrylic is also used.

But, in general, you drill at right angles to the concrete surface, 
independent of the direction of pull.  The adhesive/steel is stronger than 
the concrete, typically, so it's really no different than a "bolt cast in 
concrete", except stronger and more reliable.  For shear loads, it's quite 
strong (heck, for shear loads, you wouldn't even need the adhesive), and 
for tension loads, the concrete/adhesive/steel bond is stronger than the 
concrete is.

>   Can the bond of
>the epoxy and its smooth walled hole in the concrete withstand the stress?

Yes.  The miracle of epoxy (and many other adhesives) is that they do NOT 
rely on mechanical "tooth" for their holding (unlike, say, white glue or 
wallpaper paste).  It's a chemical bond with the surface, and, so, you can 
adhere things that are mirror smooth (steel, aluminum, glass).  You need to 
pick an adhesive chemistry that's appropriate to the materials, but the 
actual bond is at a molecular level (which is why oil films are a problem 
with epoxy..)

One thing that doesn't work well with adhesives is polyethylene (or teflon, 
for that matter).  They don't have many sites that allow bonds to form, so 
you do have to do some sort of mechanical interlocking, or, change the 
surface properties chemically.

>Actually,  I think the bolts themselves have some sort of a sliding wedge
>expansion feature too, which would also share a piece (maybe most?)  of the
>force exerted...  Just curious...

That's a different fastener technology: the expansion bolt.  At least two 
flavors of that exist:
1) The soft metal insert that you drive the bolt into so it expands (you 
drive either by pushing, or by screwing the (tapered) bolt in) (A more 
structural version of the plastic inserts used to put screws into plaster 
2) The pair of opposing wedges approach, where, after bottoming the bolt 
head, you keep tightening, and it draws up the base (towards the surface) 
crushing or wedging the fastener. (The structural version of the Molly Bolt)

Personally, I'd trust the chemical anchor more than the mechanical 
expanders, but both are plenty strong, if properly(!) installed.  Neither 
are something you can casually whip on out and do.  Chemical anchors are 
NOT just a matter of getting out the old masonry bit, drilling a hole, 
slathering some 5min epoxy on any old bolt, and shoving it in the 
hole.  You need to get the proper installation kit and follow the 
instructions (especially things like cleaning the hole and checking the 

Mfrs include: Hilti, Ramset, Sumitomo, Simpson, etc.


More information about the TowerTalk mailing list