You need to go to the required lengths to ensure the new work is well
attached to the old concrete. Drilling holes and doweling is a common
practice. Use a large powerful hammer drill and carbide masonry bits. I
prefer to drill into the existing concrete at say 10-15 degrees or more from
square and in different directions of lean for the different dowels. This
makes it harder for the dowels to pull out than if they were all parallel to
each other.
Thin caps (thin slab of concrete poured over the original concrete) are to
be avoided as they often do not bond well and under stress will detach. An
electric demo hammer (small electric jack hammer) can ne used to take off
several inches of the original concrete surface. "Real" jack hammers are
quite a handful weighing in at about 100 lbs but if you are young, spry,
strong, and not terribly bright you can get the job done faster (or hire it
done.) You then form up the new concrete pour. Weld plates sufficiently
heavy duty for the loads expected can be placed in the new pour and then
later welded to to attach tower mounting hardware. I'd consider a single
large HD steel weld plate considerably larger in width and length than the
footprint of the tower with several "J" bolts ("J" shaped rebar works well.)
Drilling the added concrete and epoxying in bolts would not be my personal
first choice. My personal first choice due to my experience (and lack
thereof in some situations) is to lay out my ideas with sketches and submit
them to a close personal friend who has a masters degree in mechanical
engineering from UCLA and over 35 years hands-on experience. He sanity
checks my ideas and sometimes suggests alternatives.
I don't loan him out so I recommend you run any ideas you intend to
incorporate by a PE. The most dangerous time for us is when we don't know
that we don't know. Sometimes I resemble that remark but with luck I am
usually smart enough to get appropriate help. I also have a friend with 35
plus years in antenna design. They both consult with me when I do non
standard things such as putting a Hy-Gain Hy-Tower vertical on top of my
metal barn with no radials and no concrete base.
Any $ saved by not getting some pro advice will pale in comparison to the
potential losses if you have a mechanical failure.
Best of luck to you and share with us what your approach turns out to be.
Patrick AF5CK
-----Original Message-----
From: Jack Berry
Sent: Thursday, January 31, 2013 11:43 AM
To: towertalk@contesting.com
Subject: [TowerTalk] Refitting existing concrete base
Question: Does anyone have knowledge or experience adding concrete to an
existing base to increase the mass and adding new anchor bolts? Bolts
would have to be drilled and epoxied in the resulting base??
I have a substantial base for an existing tower, ~ 2.7 yds of concrete. It's
in use with a guyed 60' GM aluminum tower.
I will replace this tower with a USTower 72' crankup that needs a 5x5x8 ft
cube of concrete with different & more anchor placements.
Current base is 5ft x 5ft x 6" pad on top of a 42"x6' deep cylinder of
concrete. All is tied together with rebar, one pour.
God Bless & 73!
Jack - WE5ST
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