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Spray-in-Place Concrete Fencing

Construction How-To, Fences, Fencing, Outdoor Living January 4, 2007 admin



 

 

Extreme do-it-yourselfers looking to build a really effective fence should consider the option of a spray-in-place concrete fence. These concrete partitions are durable, economic to construct and cost almost nothing to maintain. And while some springy interlopers might be able to vault over the top of these tough-as-nails fences, they will practically need a bulldozer to plow through one.

 

 

Building a spray-in-place fence is not an uncomplicated undertaking, but it’s no major step for an extreme DIY’er.

Planning and laying out where the fence will run is a straightforward proposition, but all spray-in-place fences are not created equally. You may elect to build the more standard straight-line fence, or the slightly more sturdy zigzag design. Other variables and options include what type of top your fence will feature, how tall it will be, and whether you want square, rounded or angled corners.

 

The rest of the fence construction process somewhat mirrors other concrete projects, with the big exception being the application process that creates the actual fence. Obviously, in a conventional concrete job, the concrete is poured out into forms. But for a spray-in-place fence, the forms and rebar stand up vertically and the concrete is applied with a Shotcrete applicator. As the name implies, a specialized concrete is actually “shot” in place with a pressurized gun. And yes, it does stick.

 

So, if you’re in the mood for a little adventure – not to mention a learning experience – it’s time to get started. And as you learn the process, keep something else in mind. Barring catastrophic natural disasters, a properly constructed spray-in-place fence could theoretically last for centuries. And that is not just another fence story. 

There are several different colors and forms of concrete fencing to choose from 

Special concrete forms and color additives can create a stone or stucco appearance for a decorative touch.

First Things First – Before You Begin Building Your Fence

Murphy’s Law need not apply to your fence-building project, if you plan well. Obviously, you will need to diagram where the fence will run. This usually involves establishing the location of the property lines. Consulting a survey is extremely useful at this stage of the proceedings. If you are in doubt about property lines, getting a current survey might be a real cost-saving investment. Build the fence on your neighbor’s property, and they can force you to tear it down – an expensive and embarrassing proposition, especially if the fence is made of concrete.

 

Also, before you begin, a cautionary word about the height of your fence. Cities, municipalities, and rural county authorities have all manner of rules, regulations and zoning restrictions related to fences. Typically, a fence in a residential area can be much higher in the back yard than in the front. Suffice to say, find out in advance what size fence is permitted where you plan to build.

A portable concrete mixer, a concrete pump, a shotcrete gun, and hoses all being demonstrated in use

A portable concrete mixer, a concrete pump, a shotcrete gun and hoses will be required for the preoject.

 

Equipment You Will Need

The required materials for the fence include sand, water, cement, ad mixture and a colorant for the final coat of the fence. Hardware and equipment needs are slightly more complicated, but everything needed can be rented affordably, or purchased outright.

For maximum efficiency and convenience, you should have access to a skid loader. This piece of equipment makes it much easier to handle the cement mixer, another mandatory piece of hardware for fence building. The hydraulic system on the skid loader can be used to power the mixer. The Monolithic Integrator SL 30 is a portable concrete mixer that is ideal for most fence jobs.

 

A shotcrete gun and attached hoses are also required for spraying the concrete in place. The hoses will attach to a concrete pump. Most fence builders will find the MudSlinger EHP 1500 to be a good choice when selecting a pump.

To Go Straight or Zigzag, That is the Question

Most fences are straight and they track property lines. Many people consider them to be more aesthetic and orderly looking than zigzag fencing.

 

But straight fences are not as strong as the zigzag variety. Prolonged exposure to high winds can eventually push a straight fence over. Failure occurs either by snapping the support posts at ground level or because the wind pressure causes the postholes to enlarge until the fence simply topples over. Because of how they are configured, it is almost unheard of for a zigzag fence to fall down as a result of wind pressure. 

A diagram showing the forces acting on a concrete fence A concrete fence that utilizes a zigzag layout

On a straight-line fence, the wind pressure and vibration can often enlarge the post holes and eventually push the fence over or snap the posts at ground level. However, on zigzag fences pressures act differently. A 2-inch thick fence with a 1′-foot offset acts as if it were a 2′-foot thick fence. Pressures applied act to turn over the fence rather than break it off at ground level, making it much more stable.

This is not to say that straight fences are not sturdy. Tremendous wind force would be required to snap concrete support posts. And the concrete fence itself is a formidable structure. The most critical variable is soil conditions. Straight-line fences have a greater chance of failure in regions where the subsoil undergoes a lot of expansion and contraction as the seasons change from hot to cold. The posts rock in this unstable soil, slowly expanding the size of the holes and resulting in the fence gradually tipping. If your subsoil conditions are similarly unstable, a zigzag fence is recommended.

 

Pulling Strings and Posting Up

With your plan in one hand and all applicable zoning regs in the other, it is now time to use a string line to lay out exactly where your fence will run. Measure carefully along the string line to determine the exact length of the fence. You also must measure out from the string line to determine the width of the fence throughout its entire length, whether you have decided on a straight or zigzag fence.

 

Next, select the post size your fence will need. In most soil conditions, an 8-inch diameter posthole is sufficient for straight fences. In areas with extremely unstable soil, up to a 12-inch diameter hole may be needed. Postholes for a straight-line fence should be spaced 8 feet apart. Generally, these postholes should extend to a depth of 3 feet.

For the stronger zigzag fence, spacing on the posts can vary from 12 to 16 feet apart. It should be noted that the amount of the zigzag has a great deal to do with how sturdy this type of fence can be, and it also affects the width and depth requirements of the postholes. For example, a fence with a 3-foot zigzag would require postholes of no more than 6 to 8 inches.

 

Now it is time to build your posts. After augering out the holes at the appropriate diameter and spacing intervals, they should be filledto ground level with concrete. It is then time to insert three pieces of Number 3 rebar arranged in the concrete in a flat triangle formation. The rebar should be uniformly sized to reach the top of the fence in each posthole.

A diagram showing the depth and width of the fence post hole Brace the form well enough to be able to withstand normal winds when installing forms.  

Posts should be poured 1/2 to 2 inches above ground level, then insert three vertical bars of No. 3 rebar. When installing forms, brace the form well enough to be able to withstand normal winds and some jobsite abuse.

Rebar posts set in concrete

 Rebar posts set in concrete.

 

Getting Into Fencing Form

With the posts in place, it is time to form your fence. Your forms can be made of just about anything, but a light framework of 2-inch by 4-inch, 7⁄16-inch thick waterboard makes for an inexpensive and reliable form. Your forms will have an “off” side, or bracing side, and a “near” side, which will be sprayed with the concrete. This “near” side must be coated with a concrete release agent such as diesel fuel.

 

The formwork should now be set so that it passes immediately adjacent to the post tops approximately 2 to 3 inches off the center of the post. The fence must be perfectly straight, or plumb, with 2-by-4-inch bracing and stakes sufficient to withstand wind gusts. The weight of the concrete will not appreciably add to the pressure on the forms.

This simple forming system can be repeated in sections or modules of 6 to 8 feet if you don’t want to build the entire fence in a single day.

 

Topping Off and Negotiating Corners

The top of your fence must be delineated. You may select something as simple as a flat runner board, or you may want to form up an interesting up-and-down or angle pattern top that adds aesthetics to the appearance of the fence. Regardless of the style chosen, this top form is necessary because when the concrete is sprayed in place later, this top form will provide the upper stopping place for the concrete.

 

Spray-in-place fences can also have a variety of corner types. Depending on the type of form you choose, you can make the corners square, round or angled.  

A fence that used shaped forms to create an unique look to the top of the wall 

You can use shaped forms to create a unique look to the topof the wall.

Behind (Re)Bars

When the forms, top and corners are in place, it is time to attach the vertical and horizontal rebar grid. The first step in the process is to drill a series of holes through the forms. Tie-wires will be inserted through these holes. The holes should mirror the support points of the rebar grid. Approximately four support points per 20 feet of rebarare required. 

A concrete fence form with rebar in place

Concrete fence form with rebar in place.

 

Generally, the horizontal bars are set first. The top rebar should be about 2 inches below the top of the fence. The bottom rebar should be 2 inches from the bottom of the fence. The rebar in between should be spaced evenly, but the rebar placement should never exceed 18 inches from the next nearest bar.

 

The next step is to set the vertical rebar with similar spacing. The rebar on each end of the section should be spaced 2 inches from the edge, with even spacing and no more than 18 inches between bars. This will ensure maximum possible strength throughout the entire surface of the fence. When the entire grid is wired together the rebar must be tight enough to prevent it from moving.

A diagram showing how to evenly space out the rebar for concrete fencing 

The rebar should be evenly spaced but not to exceed 18 inches apart, and come up to 2 inches from the top of the fence and 2 inches from the bottom.

Since the posts are the structural key to the strength of the fence, you should center the rebar extending from the post so that it fastens to the rebar of the fence. Make sure there is a reasonable diameter to the size of the post as it comes out of the ground. It can be thinned to the thickness of the fence after the first 2 to 3 feet from the ground. The rebar from the fence post should be spaced at about 3 inches.

 

Taking Dead Aim with Shotcrete

The use of Shotcrete requires a few prerequisites. You will need to have some familiarity with cement, shotcreting, aggregate and your available options.

A man is applying the shotcrete for the concrete fence 

Shotcrete application in progress. safety equipment is required, including facing shields or goggles, respiratory protection and waterproof gloves. Wet concrete can “burn” unprotected skin.

Shotcrete may be a new piece of terminology to some, but it is a process by which concrete is air placed, usually to a wall or overhead surface. Made of a mixture of cement, sand and water, Shotcrete is pumped to a special nozzle where air is injected to break it up and complete the application process. Shotcrete is also known as wet gunning or wet-placed concrete.

 

Note that in the best of projects, a certain amount of properly applied Shotcrete will reflect or bounce back. Generally speaking, the rebound should amount to about 10 percent of the total concrete applied, but this number can vary depending on the type of job and the experience of the operator.

 

Shotcrete tends to be much stronger than conventional concrete. One reason is that it contains more cement. The impact created by the spray application process tends to drive out the air spaces, making for a denser product. Shotcrete will usually have a compressive strength of 4,000 psi. The concrete strength will vary depending on the amount of aggregate, cement and the amount of water.

 

The best aggregate for Shotcrete is a very even gradation from 3/8 to nearly nothing. Aggregate that has been crushed is difficult to work with because that variety tends to jam in the pumping process. River or natural aggregates are preferable. Evenly crushed aggregate is hard to come by, but you will do yourself a favor by shopping for the finest gradation in your locality.

 

The water/cement ratio of your mix should be in the range of 0.4 to 0.45. This will create an extremely strong and workable concrete. Adding additional water may be necessary, usually because of off-sized chunks of aggregate. In theory, a slump test should give     an indication of the water/cement ratio, but slump tests with 3/8 minus aggregate are highly unreliable. Conversely, a slump test is an extremely valid measurement for a concrete with a 5 or 6 sack range and a 3/8 or larger aggregate.

Cement Types

Many fence builders will choose to use Type I Normal Portland Cement. It is available at almost any lumber yard and is a general-purpose cement suitable for uses such as pavement and sidewalk construction.

 

Type II Modified Portland cement is similar to Type I, with the exception of the fact that it has a lower heat of hydration rate. This means that it generates heat at a slower rate, but in many areas, Type I and Type II are combined.

 

A third option is Air Entraining Portland Cement. In this type, very small quantities of certain air entraining materials are inter-ground with the clinker during the manufacturing process. The cement was developed to    produce concrete that is resistant to severe frost action and to increase the durability of pavements subjected to the frequent application of salt for snow and ice removal. Concrete made with these cements contain minute, well-distributed air bubbles.

 

To Mix or Not to Mix

If you elect to use ready-mix concrete, remember a few things. You will have to have the fence completely formed. You will not be able to complete the project in sections or modules. Using ready mix will also require a larger pump. Obviously, if your job requires thousands of yards of concrete, ready mix is the only option.

 

Mixing on-site has advantages. Work scheduling is simpler and the pouring of things like fence posts is easier. Mixing on-site with small mixers does have to be well planned and organized. The mixer can be a Porta-mix mounted on a skid loader, a plaster mixer or a small-line concrete mixer.

 

Fence Thickness

The thickness of your fence is determined by the degree that the rebar is embedded in the concrete. You can pre-determine the desired thickness by calculating the height and thickness dimensions of the fence, and then figuring out how much concrete will be required to fill the space. In general, however, the overall exact thickness of the wall is less important than properly embedding all the rebar. If the rebar is embedded from 1⁄2 to 3⁄4 inch on both sides, the wall is thick enough to be extremely durable and strong.    

 

Spraying the Shotcrete

The strength of the posts is the critical factor in the sturdiness of the fence. The concrete of the upper part of the posts must rest securely on that poured in the hole. Blow and clean any material off the top of the lower portion before shotcreting the upper portion. Make sure to spray onto the top of the post first to prevent rebound or other materials from accumulating between the layers of concrete.

 

Apply the concrete as evenly as possible. This will ensure consistent thickness. The Shotcrete application begins at the bottom of the fence. The entire footing (top of the posts) should be covered first with a thick layer that extends about one foot up the wall. This first layer of concrete includes the nylon fibers to make the mix stick better. Make sure the form board is covered but don’t worry about embedding the whole rebar. If the first mix is sticking well, try to apply to a depth of 1⁄2 to 3⁄4 inch. On the other hand, if only 1⁄4 will stick, that is fine, too.

 

Allow this concrete to set overnight or until there is about 600 psi strength. In bad weather let it set for acouple of days. If it is to be subjected to heavy rain or frost, cover it. In cases of extreme cold, heat will have to be applied between the covering and the concrete.

 

On the second application, bring the thickness to about 11⁄2 inches. Two spray sessions may be required. Continue building the fence out, embedding the rebar thoroughly. The rebar should have 1⁄2 to 3⁄4 inch of concrete covering both sides. The second day’s application should not bring the fence to its final desired thickness because it is the final application of concrete that contains coloring.

A diagram of a man applying shotcrete in layers to the concrete fence

Shotcrete is placed in layers. The first layer should be about 1/2 to 3/4 inch. After it is sets overnight, apply the second layer to bury the rebar. Allow the second layer to set and then apply a third. By the time the second and third layers are in place, the fence will be very strong. 

Iced Tea Time: Your Fence is Finished

If you elect to build a spray-in-place concrete fence, finishing the job will probably lead to a revelation. You will realize that you have created something that just might outlive you, your children, and maybe even your grandchildren. In a small way, you will have made your mark, and the sense of satisfaction will be enormous. You will also have a great fence story to dine out on. So, fix a glass of iced tea, take a seat on the porch, and enjoy your new fence. It isn’t going anywhere.

A completed custom colorized concrete fence

With decorative forms, special colorants and a little imagination, your concrete fence can have a truly unique look of its own.

 

Editor’s Note: All information and images appear courtesy of Monolithic Dome Institute. To learn more about spray-in-place concrete fencing, visit www.monolithic.com or call (972) 483-7423.