May 18, 2017 —
It’s possible to use epoxy to mount cell antennas to water tanks with great success. The requirements: Select the right epoxy. Prepare the surfaces correctly. And take advantage of supervision by factory-trained technicians.
In the Pacific Northwest, installers have bonded cell antenna mounts to water tanks and buildings using epoxy for the past 16 years. The first application was made in 2000. I have been involved with such installations for the past four years. Photo 1 shows a work crew atop a water tank.
The Many Forms of Epoxy
The Oxford Dictionary defines epoxy as an adhesive, plastic, paint or other material made from a class of synthetic thermosetting polymers containing epoxide groups. It includes thousands of products from cheap auto body filler to bonding materials used on the Space Shuttle. Cured epoxy also comes in many forms, ranging from soft to very hard. In one form, it can be soft and rubbery. In another form, it can be so hard that hitting it with a sledge hammer will barely leave a mark.
Choosing the best epoxy for the job is not difficult. To bond steel plates to water tanks so you can mount a cell antenna or radio transmitter, you need an epoxy with several characteristics.
1. Chose epoxy that makes make a high-strength bond to steel, measured as both shear strength and pull-off strength.
2. Use epoxy that doesn’t shrink when it cures. (Epoxy that shrinks starts to pull away as it cures.)
3. Choose epoxy that won’t allow rust to creep under it.
4. Use epoxy that won’t weaken or deteriorate as it ages, and epoxy that is resistant to ultraviolet light (UV resistant).
5. Chose epoxy that expands and contracts with the steel during large temperature swings.
6. For the health and safety of the installers, select epoxy that contains no solvents. (In addition, epoxy that contains solvents will shrink when it cures.)
Once you find an epoxy with all of these characteristics, engineers can use the pull off and shear strength to calculate the size and number of plates needed to handle the weight of the antenna, radio transmitter or other equipment to be hung from the tank. Some engineers we work with will use a 10-to-1 safety factor. That is, if the equipment to be hung weighs 300 pounds, the size and number of plates will be designed to hold 3,000 pounds. The higher the bond area, the higher the weight the system can safely hold.
Once you have the best material for the job and the engineer has designed the safest design possible, the next choice is selecting the contractor.
Cell tower contractors go through extensive ongoing training to learn to climb to extreme heights, install complicated electronic equipment, wire it together and adjust all the settings so everything works flawlessly. But nowhere in the training are there the necessary instructions for correctly bonding antenna mounting plates with epoxy. So how does a contractor know what is required to do the job right?
No wonder there reported failures of epoxy mounts. I have inspected many failed epoxy mounted plates, and everything I have found indicates four leading errors: application error, improper surface preparation, unmixed epoxy and only partial coverage (contact).
Photo 2 shows a failed plate with rust. It shows two colors of exposed surfaces. The plate oxidized before mounting, and the epoxy was not completely mixed.
|Photo 3 shows a plate with rust, paint and partial contact. The plate oxidized before it was mounted. It made less than 25 percent contact with the tank. Paint ran down the inside surface after the plate was bonded.|
|Photo 4 shows a plate with oxidation before mounting. Its shiny epoxy results from the surface having been prepared with a wire wheel that polished the steel, leaving an unsatisfactory surface.|
Oxidation and Grinding
I have spoken with many cell tower contractors to learn what process they used when they mounted plates with epoxy. Almost all of them told me similar stories. They would arrive at the site and spend the first day laying out and preparing the mount locations. The next day, they would mix the epoxy and mount the plates. Not as simple as it sounds.
One contractor said he showed up on site to have the tank owner inform him that they were not allowed to grind on the tank. Many tank owners will not allow grinding the tank surface because the heat that grinding generates will damage the interior coating as much as welding does.
The contractor did as instructed, using epoxy to mount 60 plates without grinding. The plates began falling off two months later. Fortunately, the area was fenced, and no one was injured. This site has been redone with proper supervision, and the tank owner said he is happy with the outcome.
The contractor who ground the surfaces one day and applied the epoxy the next day was allowing the steel to oxidize, reducing the bond strength to near zero. The contractor who was not allowed to grind installed the plates after wire brushing, polishing the metal surface, had again reduced the bond strength to almost zero. It is difficult to blame contractors when they have no training in the correct procedures to apply epoxy.
Read any epoxy instructions. They all say almost exactly the same thing for surface preparation: Apply to a clean, dry, well-roughened surface.
If grinding is not allowed to prepare the surface, what can you do? The other big problem, at least here on the West Coast, is rain and humidity. If we wait for a dry day, we will never get anything done.
Why have some epoxy mounts been in place for over 16 years without failure?
When the cell companies in our area began using epoxy to mount antennas, they always specified that a factory-trained technical representative must be on the site to instruct the contractors and oversee the application. After several years of successful applications, someone, somewhere, decided to drop the factory-trained representative from the equation. It was then that they began to see failures, with plates falling off.
The solution is simple.
1. We need an epoxy with a very high bond strength that does not shrink as it cures, does not contain solvent and works in the rain.
2. We need to devise a method of preparation that does not involve grinding or heat.
3. Finally, we need an epoxy manufacturer that provides on-site training and technical assistance for the contractors.
Belzona, an epoxy industry leader with more than 65 years of experience, has a solution for each of the problems with epoxy use. Its epoxy products are not only solvent-free, nonshrinking and able to be applied on a wet surface, but the company also provides factory-trained technicians to instruct contractors on site and supervise every step of an epoxy application.
Over the past five years in Washington alone, we have completed work on more than 120 cell sites, bonding hundreds of plates with epoxy. Many of the jobs have involved a pull test requested by tank owners or specified by design engineers.
Our involvement in supervising applications has led to the development of several tools and accessories that are now used to make epoxy application easier and safer. For instance, we use industrial suction cups (see Photo 5) to provide the installer with a handle to aid in pushing the plates down to squeeze out excess epoxy while elevated to height in a crane basket. Magnets hold plates in place until initial cure takes place (see Photo 6). A magnetic vice allows us to use up to 6,000 pounds of downward pressure to squeeze out excess epoxy.
Preparing the surface without grinding is one of the trade secrets that we share with contractors (see Photo 7). We take steps to hold the plates in place while the epoxy cures (see Photo 8). The resulting plate installation supports cell antennas and cable runs with long-lasting strength (see Photo 9).
It is possible to use epoxy to mount cell antennas to water tanks with great success when you select the right epoxy, when you prepare the surfaces correctly and when you take advantage of supervision by factory-trained technicians.
Grady Knight is a distributor with Belzona Technology in Woodinville, Washington.