Sustainable Concrete Post Construction for Fencing and Trellising of Organic Crops
Concrete Post Design:
The project’s main objective is to try different designs for fabricating concrete posts intended to substitute for wood or steel posts used for fencing and trellising. To meet this objective, any new design in concrete is required to have a form factor similar to a standard 8-foot long wooden post, and it must function in similar respects including being able to attach wire at suitable heights, withstand 2,000 lbs wire tension and at least 80 mph wind load, face outdoor weatherability, match or exceed longevity, and be cost competitive. Moreover, it should be doable by small family farmer/ranchers.
Many different designs were conceived and tested. Techniques that proved troublesome were rejected. Not too surprisingly it was the simplest design that proved to adequately meet and even exceed all criteria. Overall this project turned out to be a complete success.
Concrete Post Fabrication:
A wooden form was constructed using a 4’x8’ sheet of exterior plywood serving as a stiff (23/32” thick) base for 8-foot long 2”x4” studs affixed horizontally upright to make multiple compartments for the hardening of ready-mix cement. By this means, concrete posts were fabricated according to various designs with the intention of demonstrating that a sustainable trellis system could be built at a cost comparable to one utilizing conventional, non-sustainable wooden posts.
As a final design, line posts were formed with dimensions 3-1/2”x3-1/2”x 8’, and both corner and end posts with dimensions of 3-1/2”x5-1/4”x 8’. This meant that one bag of ready mix concrete was used for each line post and 1-1/2 bags were required to form end and corner posts. At one end of each post compartment a notch in the form allowed positioning of a ½-inch rebar in the center along the length of each post. Ten foot lengths of rebar were used so that each post had 24-inches of steel protruding from the bottom. The weight of line posts so constructed averaged 92 lbs, while the larger end posts averaged 134 lbs. in weight.
Added to the basic post construction, v-grooves were formed along the sides by various means while the concrete began to set. (A commercial manufacturer of concrete posts would want to have this provision built into the form as an essential part of an automated process, perhaps with silicone spacers). For a single person operation it was found to be simple enough for slanted v-grooves to be added using a screwdriver pushed into the hardening cement at the appropriate levels along the sides to create voids.
Because the project’s further focus is to produce a trellis system for grapevine support, and it was desired to meet the needs of all trellising systems including those of more complicated split canopies (such as the Geneva Double Curtain), additional features were added to the basic construction including perpendicular 1-inch holes cast into the top of the post to provide attachment points for transverse supports. These holes were formed using PVC tubing held in place by the wooden form (albeit not used by this project). It was found that PVC-removal was best accomplished after a minimum of 3 days hardening to avoid cracking open the concrete. Moreover, by applying a thin coating of silicone grease to the PVC before pouring the concrete, the PVC could be readily removed using a hammer to drive a second piece of PVC until the form could be readily disassembled.
A smooth rounded top gave each post a more “professional” appearance. This was accomplished by adding to the top of the form a cut-in-half 3-1/2-inch length of 4-inch PVC tubing which readily detached after the concrete dried. An even more finished look was achieved by using a concrete edging tool to make smooth, curved sides on the face of each post. (If similar curved edges are desired on the other side of the post, the form compartments would have to include suitable curves).
Another feature added to the face of end posts was the stenciling of the initials of the grapevine variety to be planted. It was found that suitable writing in concrete mix was impossible because of the large quantity of aggregate rock used as strengthener. To overcome this obstacle, cement mix (normally used for brick-or block-laying) was added to the surface where letters were to be impressed.
A further embellishment distinguishes one block of grapevine variety from the next by having affixed a 4”x4” tile with a grapevine design. This likewise was more easily done by displacing concrete mix with some cement mix to better bind the ceramic tile.
Concrete Post Installation:
Whereas an attempt was made to drive 28-day hardened concrete posts into heavy clay soil, it turned out that holes for post-setting had to be augured. Too many post tops were broken, perhaps because the entire force of the pile driver was applied to a single line at the curved top. A flat formed top may have helped, but the pile driver was rented for one day only and no flat-topped posts had been made.
To install a post into an augured 2-foot deep hole, a 5-foot length of half inch rebar was first driven 2-foot into the center of the bottom of the hole, and then removed using a pipe wrench. Next, a 3-foot long two inch aluminum angle was upright-positioned within the hole so that the vertex of one end of the angle rested at the half-inch hole opening while the upper part of the aluminum angle leaned against the side of the augured hole. It was then possible for one person to lift a post with the extending rebar in the groove of the angle. By further tilting the post upright the extending rebar followed the groove into the pre-punched hole, and the post rested upright without support. Next, clay was tamped into the hole surrounding the two bottom feet of a line post. A spirit level was used to plumb the posts. End posts were at this point embedded in concrete after several failures occurred in tamped clay (even in one instance with the end post anchor installed). It was possible that 2,000 pound tensioning of the attached wires was tried too early after installing, but it was deemed safer to use a concrete base (at extra cost).
Next, hi-tensile wire was attached to the end posts using two 5-cent saddle clamps per post. The vine support wires were then simply laid into corresponding v-grooves of each line post. Before tensioning, anchors were installed at each end post, first using buried concrete and rebar, but as time drew short, standard screw disc anchors were used. After first tensioning the anchor wires, the trellis wires were tensioned to 1 ton. It is important to note that whereas normal post spacing in vineyards does not exceed 18 feet, the extra strong concrete posts were successfully installed at 30 foot intervals. This further reduced the overall cost of the trellis.
Finally, vines were affixed to the fruiting wires as per standard practice. As the growing season drew to a close, it was decided to wait for vine dormancy before fully load testing the trellis in order to avoid damaging vine shoots before winter hardening.
This project was made possible by the generous NCR SARE grant, student participation (sometimes voluntary), and cost overruns funded by the Ohio River Vista Vineyard and Research Station. The one fault was in the project application which underestimated costs.
A cost-competitive trellis system was constructed using hand-made concrete posts. In every way (with the exception of failed pile-drive-installation), the concrete post has proved a suitable substitute for the wooden post. Pending extreme load testing to be conducted after winter hardening of the vines, it may be concluded that this project is a complete success as of this date. It may be stated that:
• Cost-competitive concrete posts can be fabricated using a simple wooden form (with or without PVC inserts).
• Hi-tensile wire may be affixed to end posts with saddle clamps and simply placed (without fasteners) in pre-formed v-grooves of line posts, saving both labor and material.
• Extra-strength concrete post utilization allows spacing to be increased from the usual 18 feet to 30 feet, meaning fewer posts are required per acre of trellising.
• Concrete posts can be made and installed by one person capable of leveraging 134 pounds (i.e. lifting one end – about 67 pounds – at a time).
• It is important to allow concrete curing for a minimum of 3 days (preferably 28 days for maximum hardening) before installing.
WORK PLAN FOR 2009
After winter hardening of the vines, the trellis system will be extreme load-tested using concrete weights. The project will be considered a success if the breaking point (margin of safety) is greater than 4 times the tensioning (or 8,000 pounds). This will be measured in two ways, first with a 4-ton co-linear load and then together with a 2-ton side load producing a 10-inch deflection at the center of a 1-ton tensioned trellis.
• Whereas it is anticipated that weather, including winter freezes will not adversely affect the posts, this remains to be tested.
• A documentary slideshow and/or video will be produced to demonstrate the complete fabrication and installation of the concrete post trellis system.
• To date, six tours have been conducted for curious neighbors (8) as well as regional growers (4) of vines.
• Postings at local feed stores and a newspaper article will next year offer the opportunity for farmers in the area to take a look at the concrete post trellis system to hopefully realize its applicability to sustainable fencing.
• Kinkead Ridge Vineyard along with the Brown County Extension Office is hosting a seminar during the coming winter for the dissemination of information pertinent to regional winegrowers. I have agreed to present this project as a seminar speaker. If there is sufficient interest, a field day will be scheduled.
• Also next year, in addition to conducting ad hoc tours, a web site presentation will be prepared to document all aspects of the project.
• Finally, a poster session presentation held at the Kentucky Academy of Science meeting at Northern Kentucky University next fall will be part of the Materials Section of the NKU Technology Department.