Sunday, November 13, 2016

Reinforced Concrete: Rebar for Towers

Concrete is basically man-made rock. Rock is strong in compression and weak in tension. That is, it makes strong columns and weak beams. This is why, for example, ancient Greek and Roman architecture had so many columns filling open spaces in their buildings: it was only those short spans that allowed the beams to support roof and floor loads, including their own massive weight.

The integration of steel into manufactured concrete building components corrected this limitation. The steel enables concrete to remarkably increase its tension capacity. We see this in bridge spans, office building floor slabs and even in our garage floors and house footings. Reinforced concrete is a major component in all but the smallest towers we use in amateur radio.

There is substantial tension present in self-supporting towers when faced with a strong wind and in the guy anchors and bases of guyed towers. So when we dream of big towers and big antennas we need to reinforce concrete.


Steel rebar (reinforcement bar) is specified in the foundations of towers and guy anchors by the tower manufacturers to meet the engineering requirements for their products to support themselves, the loads (antennas) and couple those loads to ground. Rebar is not an option, so we need to do it right.

I ordered over 1,000' (300 meters) for rebar of various sizes to include in my Trylon self-supporting tower, the LR20 guyed tower and extra to get me started on a third tower. For these big orders go with an industrial supplier since their prices are far lower than the retail outlets, including the big box stores, and delivery is often free or they charge a nominal amount.

The amount required can be surprisingly large. For the Trylon #12 base section foundation I needed 200' of rebar: 20x5' verticals and 5x20' ties. The weight is substantial, coming in at ~200 lb (90 kg).

These are convenient lengths since rebar in Canada and the US the standard length is 20', even for the metric sizes we use in Canada. Where US rebar sizes are specified the tower manufacturers recommend rounding up to the next largest metric size.

Cutting rebar

Small diameter rebar used for ties can be cut to length with bolt cutters. Since the largest bolt cutters I have have a ⅜" capacity (up to #3 rebar) that was not an option. A hacksaw is too slow and tedious for the amount of cutting I needed to do. This is a job for power equipment. My tool of choice for cutting rebar is a circular saw with a 7" steel cut-off (abrasive) disk. A cut-off saw with an abrasive disk is easier and more accurate if you happen to have one.

Cutting steel this way can be dangerous when not done properly. The steel that is thrown off by the tool is a fast, dense stream of hot particles accompanied by metal vapour. Skin, eye and breathing protection is required. If you are feeling lucky the last can be omitted if done outdoors and the wind is at your back. The rebar will be very hot following cutting so do not touch it anywhere near the cut and use thick gloves of natural fibre such as cotton. Toss it aside to cool while you move on to the next cut.

I lay out several scraps of lumber on the ground to support the full length of the rebar and both sides of the cut. The end of the measured length of rebar butts against an immovable object to avoid errors due to inadvertent movement. My booted foot presses down on the other side of the cut as the tool does its work. Cut straight and keep some pressure on the steel for maximum safety and speed.

To reduce waste I did not cut each 20' length of 20M rebar into 5' lengths. I cut one 5' length for a Trylon vertical and cut the remaining 15' into two 7.5' lengths for the LR20 anchors. Thus there is no waste. I need 21 of those for the 3 anchors. I now have 19 in storage for a future guyed tower of similar size. If you do have waste it can be used for surveying stakes, supporting tree saplings or other jobs around the house.

Tools to bend rebar

Readily available tools to bend rebar include hand tools, mechanical benders and hydraulic benders. The latter two are expensive for the modest amount of rebar bending that the typical ham will need. Hand tools are far cheaper but are not well suited to making bends of the required radius, angle and position, and their capacity is typically limited to #5 rebar. Searching the internet for advice turns up far too many useless techniques that are either too ad hoc or require expensive equipment. I needed a better way.

I have never bent rebar before. This is perhaps surprising since I've put up so many towers over the years. In the past, where rebar was involved, it was done by someone else and all I had to do was make sure it was in position and guide the pouring of concrete. So this project was also a learning experience for me.

I purchased a "hickey" bending tool by mail order. As you can see it is basically a snipe with hardened steel pegs to hold and bend the rebar. The typical application of a hickey is hold the rebar against a hard surface with your foot (stiff boot) and pull up. I did an experiment to see how this would work out. I wrapped the bend area with masking tape, marked the pins with a pencil to see where the rebar would bend in relation to the tool.

It was not a happy experience. The leverage was enough to lift me off the ground and the resulting bend radius was far too large. The ties for the Trylon cage need a much sharper bend (small bend radius). For the time being I tossed the hickey aside to reconsider how I'd bend the ties and turned to attack the 20M verticals. These are too large for a hickey in any case so I designed my own tool.

Two trees and 3 pipes

20M rebar (slightly larger than #6) is not easy to bend without mechanical assistance. The substantial leverage required is difficult to achieve with hand tools and readily available supports. But where there's a will there's a way, with a little ingenuity.

The 20 verticals in the Trylon rebar cage require a hook to match the flare at the bottom of the foundation (see Trylon drawing above). Although perfection isn't mandatory we want to get close so that the flare area doesn't crack when the tower is subjected to extreme load. So don't skip this task because it seems too difficult.

The simple jig I constructed required two small trees near to each other and enough room to swing a 10' pipe acting as a snipe. The picture shows the large picture subsequent to bending one of the 20M verticals. If you study the picture I expect you can discern how the jig works. A close up of the bend zone is in the next picture.

Both pipes are 1.5" schedule 40 water pipe. Graded steel isn't necessary. Inside one pipe is a 2' length of ¾" pipe whose inner diameter is slightly greater than the 20M rebar (19.5 mm). That inner sleeve ensures that above the desired bend zone the rebar doesn't get bent. To get the 12" from the bend zone to the bottom I measured 13" (leaves partially hides the rebar) and carefully position the movable pipe just less than 1" from the fixed pipe.

In action the movable pipe is moved right until the rebar is locked by friction and gripped by the trees. Then the movable pipe is pulled, hard, until it taps the barrel. By happenstance the barrel is perfectly positioned to achieve the desired bend angle.

Once I got the jig built and tested the bending of the verticals was very quick. All 20 20M verticals were done in 35 minutes. Stacked together they make a pretty sight. They are almost exactly identical.

With the verticals done I returned to the problem of forming the ties. Unhappy with the action of the hickey alone I built a jig to hold to hold the smaller rebar and provide a hard edge for the hickey to work against. As it turned out there was a piece of steel mounted to one of the LR20 sections that appeared ideal for my purpose.

The angle steel is ¼" thick and attached to a tower girt with ⅜" bolts. It served as a bracket for a side-mounted yagi (and may yet again). To grip the rebar I installed ⅜" bolts with two ½" nuts serving as the surface for holding the rebar. All hardware is grade 5. The tower section is sandwiched between other sections to hold the jig during the bending. Even at 120 lb per section I periodically had to reassemble the sandwich. That's a lot of torque!

As in my previous attempt I covered the bend area with masking tape and marked all the key points. The forward (closest to the jig) pin on the jig was positioned about ½" from the edge of the angle steel. The test bend turned out perfectly. I measured the distance from the jig to the outside of the bent rebar to determine how to position the rebar for the tie formation.

I made two bends in this fashion then flipped over the rebar and did the final two bends from the other end. This way most of the rebar's weight and length does not have to be supported while bending. It's important to get the sides the correct length, square and in a plane. Get it wrong and it'll be difficult to correct, as I discovered. Don't waste 20' of rebar due to carelessness.

The result is quite good. The tie sides are 53" (outside edges), which is 7" less than the hole's sides. Rebar should be no closer than 3" from the sides of the concrete form or there is risk of moisture incursion which can crack the concrete and corrode the rebar after several years. I added an extra 1" margin.

Building the rebar "cage"

Professionals build the rebar cage above ground, in a jig, then transport it to the tower site and place it in the hole. I considered doing it that way until I realized that the rebar cage would weigh close to 200 lb (90 kg). That's a two man job and I didn't want to inconvenience anyone else. Besides which I don't have a jig and didn't want to build one.

Since the hole is, of course, almost exactly the precise shape required I used that instead of a jig. By building it in place I avoided having to carry it and, somehow, lowering it into the hole. But it did mean several hours spent sitting in a hole in the ground!

To begin I dropped all 5 ties and 4 verticals into the hole. In the hole I lifted the top one, resting one side on a ladder (the one I used to get in and out of the hole). With that as support I wired the opposite corners to two verticals leaning against the sides of the hole. Then I did the ones by the ladder. The ladder had to be removed since it wasn't quite the correct height. I did the remaining ties much the same way, working from the top to the bottom. After the second tie was in place the 4 verticals were stable enough to hold the partially-complete cage.

For my tower (see the #12 foundation detail above) the ties are ~12" apart. The verticals are ~10.5" apart on average. Precision isn't critical, but utter sloppiness must be avoided. Don't fret over minor inequalities in the spacing.

All the crossings are wired for the first 4 verticals and the ties. The remaining 16 verticals do not require 100% coverage. I wired all of them at the top and bottom ties. For the remaining 3 ties I alternated one wired crossing (middle tie) and two wired crossings. Never skip two crossings in a row.

My wiring technique was sloppy at first. It got better. I used steel rebar wire (very cheap and widely available). Using pliers I followed the common wiring techniques I discovered on the internet. It is important to ensure the crossing rebars are pulled tight against each other so that the corrugations lock them together. These connections only need to survive the pouring of the concrete since they are not mechanically needed after the concrete has set.

Since some of my bends were imperfect a few of the verticals needed to be repositioned so that all the crossings had good contact. Squeezing them together by hand isn't always possible or advisable.

When I was done I was pleased to find that I achieved the requisite 3" spacing from all 4 walls of the hole. There were a few spots where I had to shave some dirt off the walls to get it right.

There are lots of crossings so take the time to check them all. Don't skip this step or the foundation can develop cracks in a few years due to water incursion and corrosion.

As you can see in the picture the rebar cage is not perfectly rigid. Each side has some play since the wired crossings don't fully lock the rebar. For that you'd need to spot weld the rebar. I lifted the rebar verticals from the bottom to move the cage in small steps until the bottom tie was the correct distance from all the hole walls. Scrap lumber wedged between the tops of the verticals and the walls temporarily squared the cage. In this way I confirmed the dimensions were correct for the entire cage. The props were removed for the next step.

Rebar chairs

Rebar cannot sit on the ground. Like the rest of the rebar they must be separated at least 3". This requires what are called rebar chairs. These can be as simple as concrete blocks, although there are commercially available chairs made of plastic and concrete. Concrete chairs have protruding wires so they can be attached to the rebar. Do not use stone, wood or other random material since they will decay or fail to bond with the concrete.

Since plastic is a poor choice for a cage of this weight, are difficult to get the concrete into during the pour, and I couldn't easily find proper concrete chairs, I went with concrete bricks. I found a suitable product at a local brick factory I was directed to by a contractor working on my house. Placed on their sides they are 3.5" tall, which is perfect. They were also unbelievably cheap. Do not use clay brick!

Not every vertical needs a chair. Use enough to ensure the chairs can hold the cage without sag or shifting of the cage. I used less than 10 bricks. Add or shave dirt under the bricks as needed to keep the cage level and all them taking their share of the weight.

Securing the cage

If you look closely at the previous picture you can see a tower stub sneaking into the frame. Clearly my pictures are not quite in chronological order. However it is useful for this next step.

The #12 section base stubs clear the inside of the rebar cage by only a few inches. The sides of the cage are ~53" on the outside and less than 51" on the inside. The stubs on the #12 tower section flare out to almost 46", leaving a gap as small as 2.5" (8 cm). It was therefore necessary to reinstall the temporary lumber blocks at the top of the cage when the base section and stubs were manoeuvred over the hole.

With the tower and stubs in position per the manufacturer's drawings I wired the stubs to the rebar cage. The tower section with stubs weighs close to 200 lb (90 kg) so pulling the wire tight did not shift it. The lumber blocks kept the rebar cage from shifting while I tightened the wires. I made sure there was good tension in all directions and removed the blocks. Some adjustment of the wires ensured that the tower and cage were in position and stable against moderate force.

In the next article I'll back up a step to show how I suspended the tower base section over the hole. That deserves an article of its own.

Cleaning up

When you're all done there is one more small task to perform. It is a good idea to use a steel wire brush to remove surface rust on the rebar and to remove any mud and dirt clinging to the rebar. Even newly delivered rebar will have some rust on it since steel readily oxidizes. Rust will weaken the bond with the concrete. Although a little bit of rust is not a problem cleaning takes only a few minutes. Not all the rust will come off and it needn't.

Mud and dirt will block concrete from reaching the rebar and can be a greater problem than rust. Dirt from the walls will find its way into the gaps where rebar crosses, whether tied or not, from banging and dragging against the sides of the hole during construction and positioning of the rebar cage. If the wire brush can't get into these small spaces use something smaller.

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