CONCRETE FENCE POST FORMS - Making Posts

THIS TWO-FORM GATE POST SET IS REALLY THE SECOND USE OF
THESE TWO FORMS.  I BUILT A SIMPLE SPREADER BAR TO USE FOR
LIFTING THE PAIR

          I selected a place with easy access for the first try at pouring fence posts in place.  We are building a fence so we can better control access to a creek bottom that runs through part of our property.  There will be several access points along the fence.  The site chosen meets the criteria of being easy to get to.  It is also in the open and immediately adjacent to a roadway, fairly flat, and has no sticky clay in case it rains while we are experimenting with the post forms.

THIS PHOTO IS OBVIOUSLY SIDEWAYS.
IT SHOWS THE BASE RING FORM AND
THE RE-BAR ASSEMBLY TO STRENGTHEN
THE TRANSITION BETWEEN THE BASE
AND THE POST.

          Long ago, when I was young and strong, I didn't think twice about digging a hole just the right size with a shovel.  Now, I am either lazier or smarter.  We use the backhoe to dig holes for anything that involves two or more posts tied together.  These post forms are designed to be poured in conjunction with a 24" x 6" base ring.  The bottom of the base ring is set at 30".  We have no deep frost line to worry about.  We do have high shrink/swell clay (obviously not at this site) but the concrete posts with bases like this haven't so far been affected by that problem.

          I set up the laser level to get the bottoms as close to even as I can.  They are dug about three inches deeper than needed and then backfilled and compacted as much as possible to form a hard footing for the base.  The re-bar is a new addition.  Previously there didn't seem to be a need for re-bar since the entire outside of the post was steel.

(NOTE: This Blogspot system has a mind of its own when it comes to photos.  I didn't intend for the photo to be sideways.)

          As the post forms are lowered, we have the re-bar set loosely inside the base ring and we simply lower the form over the re-bar.

THE FIRST USE OF THE POST FORMS.  NEARLY LEVEL
AND ALMOST PLUMB.  YOU CAN SEE THE IDENTIFYING
NUMBERS ON THESE TWO FORMS

          It takes about 6.5, 80lb. sacks of pre-mix concrete to fill each base and post assembly, or about 13 sacks for this post pair.  I calculate an 80 lb. sack of pre-mix will yield 0.6 cubic feet of concrete.  The simple calculation is, total cubic feet divided by 0.6 = the number of sacks of concrete needed.  This number will depend upon how you mix the concrete and how you get it free of voids.  I mix one sack at a time in my little mixer (you can see it behind the top rail in the photo) with as little water as it takes to make it not granular.  We don't have a vibrator so we do much taptaptapping.  For these posts the concrete needs to flow more than if you are doing flat work but only enough to get the voids filled.  We also add a half-pint of dry cement to each sack of pre-mix.  It takes about 45, 80 lb. sacks to equal one cubic yard of concrete.  A half pint of cement weighs just less than two pounds so the mix is increased by about one sack per yard.  If you mix the concrete wetter and you don't do much tapping, a sack will probably go a little farther but you will have an inferior product in the end.

FORMS FILLED AND EXPOSED CONCRETE COVERED.

          It doesn't take long to mix up and dump and tap 13 sacks of pre-mix concrete.  You might notice here that I used four sets of vice grips to set the spacing on this two post set.  The rails go completely through the post and extend about an inch on the off side.  The vice grips simply clamp the rail to the socket.  Surprisingly, setting the rails to extend to the height of the flanges at each end with clamps on the ends of the top and bottom rails made the unit almost square and plumb.

         
There are two more things to point out in this photo of the filled forms.  First, the 2x4xVicegrip system to plumb and hold the unit is no good.  It takes forever to set the unit in place on the fence line and parallel and level and plumb when you have to adjust this stuff every time you move the forms a half inch.  The blocks to support the height and level of the unit work okay because absolute height and level accuracy are not necessary and you can slide the whole thing or each end on the blocks.  The next shop project is to build some simple screw adjusters to take the place of the 2x4xVicegrip setup so the plumb can be dialed in without any effort.  Secondly, these forms are heavy enough that the bottom rail is bowed up by their weight hanging on the rail's ends.  I have some heavy 2x4" channel iron that I am going to set under the rail, with the flat side down, to support the rail better but still allowing the unit to be moved.  The other idea is to make a tripod like setup with universal adjusters to attach to the top and bottom rail of each end of the unit or three to use for a three post corner or T.  With such a setup, the chore would be to get the unit set up with the tripods.  The adjusting would be easy.

REMOVING THE SECOND FORM

I had no good idea how long to leave the forms in place.  The local concrete merchant thought two days would be sufficient.  I gave this first set four days.  A small impact wrench makes quick work of removing the bolts.  The warp in the forms also became our friend.  Even with form release of some kind (diesel in our operation) these forms are intricate enough to want to stay where they are.  The warp helped get them started away from the green concrete.

GREEN BASE AND POST BOTTOM

          The base forms are easy to remove unless you forget to clean up around them when you finish pouring.  I use a little release on the bolts and I use some on the outside of these forms also. I would use form release on the outside of the post forms too but it would end up in the mix when you scrape concrete from the ring stiffener

TWO FORMS READY TO BE CLEANED

          It takes only a few minutes to wire wheel the excess stuff from the forms.  A coat of form release applied when you clean the forms and before you bolt them back together will save the time and effort of trying to coat them when they are together or of having to take them back apart to coat them.

TRAGIC ENDING TO OUR FIRST ATTEMPT AT CONCRETE POSTS

          Sometime on the fifth day, after the forms were removed and before the concrete had a chance to cure much, this first set apparently got bumped hard enough to crack the posts right at the bottom rail.  The set was still standing when I noticed the crack line.  I pushed the set over and broke it apart before it had a chance to cure farther and make it harder to salvage the rails.

          Of course, those of you who know me will know that I am now worried that the volume of concrete between the rails and the form wall isn't enough to form a strong post at those points.  The second set is now about two weeks old and it seems to be very strong.  I haven't asked it to hold tension or hold a gate yet but that is the next test.  We'll see.

GREEN SECOND SET JUST OUT OF THE FORMS
I LEFT THE FORMS ON THIS SET FOR SIX DAYS

THERE WILL BE A 16' GATE BETWEEN THESE TWO SETS

 

THE SUPERVISOR AND HIS ASSISTANT

          These forms have been a fun project.  The posts will be fairly expensive but they should outlast me and if the cost of alternative materials continues to rise they may become very competitive.  The cost of these posts compared to other options is not exactly straight forward.  If you constructed a two post set like these and used the same materials except used concrete for the posts, the concrete costs about $18.40 per post and base.  A number 1 tie is over $25.00 in this area and a 6"x6"x8' treated post is about $16.00.  If you used wood posts you likely wouldn't use the tubing rails, or any rails.  Hem/Fir 2"x6"x8' cost about as much as I paid for the 2 1/2" tubing, but I bought a truck load and got a good price.  If the posts turn out to be strong enough, I plan to make some more traditional braces with an X or a traditional H between the posts which will save three or four rails.  I'll let you know.

JCE