[TowerTalk] Refitting existing concrete base

[Patrick Greenlee]

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 at 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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