How To Build A Spray-In-Place

Concrete Fence

Before

After

A spray-in-place concrete

fence adds a beautiful

touch to any yard. Not

only does it delineate, but

the fence also give a beau-

tiful backdrop to landscap-

ing. This fence by this

home in Italy, Texas varies

from six to four feet tall.

Disclaimer. All of the information given in this booklet is to the best of our knowledge factual and useful. The engineering data given is from practical experience. Any person building these fences is advised to have each particular fence engineered by a profes-sional engineer competent in this type of endeavor. The engineering will of necessity take into account the soil bearing, snow loads, wind loads, seismic values, etc.

How To Build A Spray-In-Place

Concrete Fence

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Step 1Design the Fence Layout

Where is the property line?Whenever a fence is built, it

should be ascertained where the prop-erty line is, or at least, exactly where the fence is to be constructed.

If the fence is between two prop-erty owners it is best to have an agree-ment on what the fence is to be like, how it should be laid out, and who is to pay for and install it.

The laws on fences vary from state to state, so if you don’t get an absolute agreement with your neigh-bor, it is probably best that you check with an attorney to see what your legal options are.

The property line must be determined, agree-ments between neighbors must be reached, and a plan decided on before construction can begin.

The zigzag fence is superior to the straight line fence. Notice the zigzag fence only requires a fence post every 16 feet, while the straight line fence requires a post hole every 8 feet.

After locating and marking the property lines decide with the neigh-bors on the best fence layout. This is doubly important if the fence is to be zigzag.

During planning remember to design the layout so it does not hinder future plans, remodeling, or expan-sions. For instance, if there is no exit to the back lot and some day you want to build a swimming pool there, you will find it very difficult to go through the concrete fence. Therefore, a gate would be in order.

Fences perform many functions, not the least is to “keep them out and us in.” “Them” can be critters, people, wind, snow drifts. “Us” usu-ally consists of the residents and the pets of the home.

An appropriate fence height

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Straight Fence vs. ZigzagFirst you will want to decide

whether or not to build a straight fence or one with a zigzag. Both types of fences, the straight line and the zigzag, have advantages. You must decide which of the two fences is most appro-priate for your application.

The Straight Fence — AdvantagesThe straight line fence is the tra-

ditional fence. It obviously can follow precisely along given property lines, and is utilized in most neighborhoods.

The straight fence has a series of posts, in line, all standing verti-cally. These posts acting together, in concert, keep the fence standing and prevent it from tipping over, even when loads are applied, such as: high winds, some one backing into it, or something piled against the fence.

A straight fence has the advantage of taking the least amount of land and being an exact delineator between two properties.The Straight Fence—Disadvantages

The straight fence’s main disad-vantage is strength. Expansion and

On a straight line fence (right) the wind pressure and vibration can often enlarge the post holes and eventually push the fence over or snap the posts at ground level.

Because of the zigzag, pressures act very differ-ently on this fence (below) from those of a straight fence. The 2” thick fence with a one foot offset zigzag acts as if it were a two foot thick fence. Pres-sures applied act to turn over the fence rather than to break it off at ground level. It is much more stable

needs to be established. Here again it is wise to check your local city ordinances. In many places a fence over three feet tall in the front of the house is not allowed. In other places a fence has a maximum height in the back of eight feet. You will find the fence ordinances vary everywhere in between. Normally a six foot fence is acceptable for everything but front yards, where they may be hazardous to motorists, especially on a corner lot. The best choice is get the rules.

The spray-in-place concrete fence is amazingly versatile. There are many creative options for layout and design.

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Well laid-out plan for a zigzag fence (above). It would be even better if the ends of the fence by the driveway had doubled fence posts, a box (see drawing page 5), or a turn to give greater stability and a more finished look.

contraction is more severe especially for long straight runs of fence. Twice as many posts need to be installed because they are all that hold it up. The posts need to be a little larger in diameter and be deeper in depth.

Consider wind pressure: A wind blowing 100 miles per hour against a vertical fence will push with 50 pounds per square foot against the fence. If the fence is six feet tall that means there is 300 pounds of push per foot down the length of the fence. The taller part of the fence has a lever action against the ground which increases its propensity to roll over. In other words, the taller the post the more leverage it exerts. This 300 pound push is considered in average three feet above the ground. If the posts are eight feet apart and we have an average of 300 pounds of push, that equals 2400 pounds of push on one fence post.

That fence post must be able to resist snapping off at ground level and the post hole must resist enlarging to prevent turn over at ground level. If the hole starts to enlarge even a little bit, the fence tends to pound the hole larger and larger as the wind hits it and backs off over and over again. A fence that can withstand being tipped over from a one-time push may fail to the buffeting of an intermittent wind. It is therefore very important that the post be larger in diameter and set deep enough to withstand this turn over at the ground line.

The Zigzag Fence — AdvantagesThe zigzag fence is much stron-

ger and more durable than a straight fence. It will stand up to expansive soils, wind loads and expansion and contraction much better than a straight fence. It proves interesting by being

Note the forms set up for shotcreting (left). This four-foot high fence is zigzagged one foot off center each way. Surprisingly, when completed one hardly notices the zigzags in this fence. The posts have already been poured. The bracing must resist wind until the fence is sprayed and solid.

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zigzagged, forming pockets for plant-ing trees and shrubs. It can be taller without significantly increasing post size.

A zigzag fence is a stronger fence because of the zigzag. It acts as if it were a wide fence. Note on the dia-gram (see page 3) the pressures act on a zigzag fence vastly differently from those of a straight fence.

When pressure is applied against the top of the fence it tries to turn the fence over. The posts certainly prevent some turn over, but mostly they pre-vent the fence from sliding sideways. It is far easier to prevent a fence from sliding sideways than it is to prevent it from turning over. In order to turn over the zigzag fence the wind must pick up one whole side of the zigzag.

The amount of zig (off set from center) determines how much pres-sure will have to be applied before the forward post can lift and bend over against the back post. In other words, the ability of the forces to hold against pressure is much greater as the “zigs” increase.

If the zigzagging is 2 feet every 16 feet the fence behaves as if it were a two-foot wide fence. Air pressure working against the fence is doing more to lift the high side of the fence than it is to turn over the fence. The zigzag fence is enormously strong.

The zigzag fence is a very pretty fence, besides being very structural. Small alcoves can have benches placed in them. Trees and plantings look great set in the enclosures of the insets.

The zigzag also acts as an expan-sion joint at each change of direc-tion, providing greater flexibility in expansion/contraction situations. The amount of the zig can be varied. A four-foot fence many only need twelve to eighteen inches of zigzag. A six to eight foot fence might be better with eighteen inches to two feet of zigzag.

The zigzag fence can easily be built eight, ten, twelve feet high. The straight fence is best kept at lower heights. The zigzag fence gets along very easily with posts 16 foot on

Above, an eight foot fence constructed for the Christmas Festival in Waxahachie, Texas has walls connected to the fence to delineate booths. More importantly, they significantly add to the strength of the fence.

Anytime an adornment or offset can be added strength will be added to the fence. This straight line fence has small offset every eight feet and finishes with a decora-tive and strengthening box at the end of the fence.

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center. The straight fence should have posts 8 foot on center.

The Zigzag Fence — Disadvantages

The zigzag fence takes up a little more space. If you want a zigzag fence the neighbors must absolutely determine where the fence posts are going to go. To be fair the posts should zigzag across the property line so that each of the parties have the same amount of property taken up by the fence itself. It takes a little more layout. It is somewhat nontraditional.

Summary of the Principle DifferenceThe major difference in the

strength dynamics of these two fences is — what it will take to knock the fence down.

The straight line fence is held in place by the soil. Force against the fence does two things. One, it tries to snap the post off. But, the con-crete post is very strong. Secondly, it tries to make the post hole bigger. Force acting against the top of the

This particular fence starts with twelve feet straight, then a two foot deep triangle on the right, then twelve straight feet of fence, then a two foot deep triangle to the left, and continues with alternating triangular niches. These niches create wonderful landscaping opportunities for benches, roses, anything. This fence acts as a 4 foot thick fence. It is extremely stable under all conditions.

There are few limitations to creative design. One could build a variation of the above using square offsets instead of triangles. The fence can be built serpentine. The spray-in-place concrete fence allows for the imagina-tion to run wild.

fence forces it to rock back and forth, enlarging the hole. The fence, to a certain extent, is only as strong as the soil.

For the zigzag fence the dynam-ics are totally different. It is not a soil issue, or snapping off of the posts. Here, force acting against the fence tries to tip the fence over. The posts serve mainly to keep the fence from sliding sideways in the soil -- the fence itself is the strength. A zigzag fence that zigs one foot off center one way and zags one foot off center the other acts as if it were a two foot thick fence.

Soil Considerations: Moving GroundMany parts of the United States

have expansive soil. This means that when the ground gets wet it expands and when it dries out it contracts.

Where we are here in Italy, Texas, it is extremely bad. The contraction will pull all of the dirt away from a post, or it may lift the post, or in the case of the concrete fence, it may try

to lift the fence.If you are building in that type of

soil condition, we suggest again that you very seriously look at the zigzag fence. If you don’t look at a zigzag fence, then look at a very sizable size post. The four foot high fence would have a three foot deep post. An eight foot high fence would have a four to five foot deep post. The straight line fence should have at least an eight inch post. The zigzag fence could get by with a six-inch post and in all cases it doesn’t need to be especially deep, three feet would be just right.

If the soils persist in moving the fence around, then you can dig the soil out from under the length of the fence and that should stop. I am hesi-tant to dig out from under the fence in most areas because that invites ani-mals to dig under the fence. In most places there is not going to be enough expansion or contraction to bother the fence itself. You should just monitor it and if it looks like it is going to be a problem then dig out half of the soil out from under it, or two thirds, or all of it if you need to.

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The Top of the FenceThe top of the fence in general

should be delineated. This can be as simple as a 2” x 2” runner along the top of the form or can have an inter-esting pattern fastened to the forms as illustrated.

As the fence is sprayed the con-crete is shot up underneath this top finish board so there is a place to stop. The imagination can be free on how to finish the top. It could be made to look like a picket fence, it could be made to have a flat top, or it could be let run wild and have an organic look.

Corners, Inclines, and more...This fence is marvelously ver-

satile. The corners can be square, angled or rounded. On an incline the fence can come down in steps or can follow the slope of the hill. The fence easily spans ditches because of its strength. It is simple to vary the height of the fence for privacy or a favorite view.

The top of the fence can have any desired finished shape. Likewise the fence itself is extremely versatile. On an incline it can come down in steps or follow the slope of the hill.

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The post should be poured up flush with the ground level or 1/2” to 2” above the ground level, then insert three vertical bars of #3 rebar. The fence post above is the thickness and depth for a zigzag fence. Fence posts poured in a zigzag pattern (below).

The next step in building either of the fences is installing the fence posts.

Measuring The LayoutPull a string along the property

line to delineate it. Measure carefully the distances along the string and from the string for the proper zigzag or other pattern.

Determine The Post Size And Distance

The posts are obviously key to the fence standing up. The size must be large enough to restrain movement in the soil. If the fence posts are in line they must be larger than for the zigzag fence.

The size of the post is best deter-mined by the type of the soil. If the soil is sand and not prone to loosing while wet, a 6 inch fence post may be all that is needed. More than likely, the fence post should be 8 inches in diameter. If the soil is subject to flooding and not much strength when wet, the post may need to be built 12 inches in diameter.

If the soil doesn’t have some good side to side holding characteristics then a straight fence is probably not the best to build. The zigzag fence may be used here, but may be even increased to 3 foot zigzag on 16 feet. The post hole is then of not much importance, a 6 inch or 8 inch post hole would be fine.

The post for the straight fence should be eight feet apart and for the zigzag fence they can be twelve to sixteen feet apart.

Installing The PostCare must be taken to fill the post

Step 2Install Fence Posts

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hole to the top with concrete. The concrete of the upper part of the post must rest securely on that poured in the hole.

Into the near center of the post should be placed three vertical bars of #3 rebar, extending up to the top of the fence. They should be arranged in a flattened triangle along the line of the fence. If the fence is higher than eight feet the rebar should probably be changed to #4 bar and the post holes may need to be increased to 18 inches in diameter for the straight fence; 8 inches will still be fine for the zigzag fence.

The post should be poured up flush with the ground level or slightly above ground level (above) with three vertical bars placed near center. Using a the side discharge door of a Monolithic Integrator SL30 (below) makes filling the post holes astoundingly quick and easy.

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Place the forms vertical and two to three inches off the center of the post. Note (above) the rebar coming out of the hardened concrete post are arranged in a flattened triangle. Waferboard, 2” x 4”s, and stakes for bracing (below) provide easy and inexpensive formwork for this four foot tall fence.

The forms can be made of virtu-ally anything that will hold still.

We have found a light framework of 2” x 4” faced with 7/16 inch thick waferboard makes inexpensive form-ing for this fence.

If the fence is only four feet tall, the waferboard can be laid down horizontally. If it is six to eight foot tall then the waferboard must be stood on end.

These form boards are called single side forms. One side of the forms we will call “off side” that is the side where we are going to do the least amount of spraying. It is the side for the bracing. The “near side” is the

Step 3Form The Fence

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Brace the form well enough to be able to withstand normal winds and some abuse (above). The formed offset (below) gives two benefits — a decorative finish to the end of the fence and extra strength to the straight wall.

side that we are going to do the spray-ing against. It needs to be the least obstructed side.

The next step is to set the form boards on the off side of the work area.

Coat the form on the near side with a concrete release agent. This release agent is often called form oil. Many times it is nothing more than diesel fuel. Form oil can be purchased at any concrete accessory supply house.

Putting Up the FormsAfter applying the form oil stand

up the single side forms. Place the formwork so that it is

vertical and passes immediately adja-cent to the post tops, approximately two to three inches off the center of the post.

The fence must be perfectly straight up and down (plumb) and it should be square at the corners with bracing of 2” x 4” and stakes to hold it still. Brace the form well enough to be able to withstand normal winds and some abuse, but the concrete is not going to add any significant pressure.

These photos are of the off side. You can see the temporary wafer board, braces, and stakes in place.

OptionsThis is an extremely simple

system. The pieces can be made in modules to be moved in four foot or eight foot sections.

If the fence length is short, you probably need to form the entire length of the fence.

If you are building a long fence, form and spray part of the fence. After the second spraying of concrete you can generally move the forms and start a new section while you are finishing up old section.

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Step 4Attach The Rebar

Now that the forms are in place, plumbed and staked, we can put the rebar in place. The rebar should be #3 (3/8 inch diameter) and run approxi-mately 18 inches each way.

In general the first layer of rebar is the horizontals, but it can be either the horizontal or verticals. Fasten the first layer of rebar by drilling a small hole on each side of the rebar through the form boards. Then push a tie wire through the holes and twist it tight in the back. It takes about 4 attaching points per 20 foot length of rebar. The rebar should be tightly attached to these forms so that it doesn’t wave and wiggle.

When it is time to remove the forms cut the wires on the off side which will free the form.

The top rebar should be about two inches below the top of the concrete. The bottom should be about two inches above the bottom of the con-crete and then spread out the rebar in-between, evenly, but not to exceed eighteen inches.

The vertical rebar can then be applied over the horizontal rebar and fastened with wire ties. These should also be spaced evenly but not to exceed eighteen inches.

The post is the structural key. Center the rebar so it fastens into 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 in the first 2 to 3 feet above the ground. The rebar from the fence posts is spaced about three inches apart and attached to the horizontal rebar.

(Above) Drill small holes above and below the rebar through the form boards. Fasten the first layer of rebar by wiring it to the off side. The rebar should be lightly attached to these forms so that it doesn’t wave and wiggle. Keep the rebar square and neat. It takes about 4 attaching points per 20 foot length of rebar.

Left is a 6’ tall zigzag fence formed with rebar attached, ready for shotcreting. Only a sec-tion of the fence is formed, the remainder will be four feet tall and formed later. At the fence posts there are three lengths of vertical out of the post rebar spaced 3” apart instead of one rebar every 18”.

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Place the rebar 15” to 18” on center. Deter-mine the distance by spacing the rebar evenly between the post. In general the hori-zontal rebar is placed first, but the first layer of rebar can be either the verticals or hori-zontals.

The forms are up with the vertical rebar in place. Note the single 2” x 4” brace on the near side of the form so there is a mini-mal amount of obstruction for the shotcreter. Once the horizontal rebar is in place the con-crete spraying can commence.

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

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Step 5Applying the Shotcrete

The next step is applying the shotcrete. This requires a working understanding of cement, shotcreting, aggregate and your options.

Ingredients for a fence: sand, cement, water, ad mixture, and colorant for the final coat.

The Monolithic Integrator SL30 attached to a skid loader is “self-bucketing” and easy to load up with sand.

Types of Portland CementType I. Normal Portland Cement.

Use this general all purpose portland cement. It is available at most lumber yards and cement supply stores. Following is the long explana-tion. It is here for those who wish to know more.

This is a general purpose cement suitable for all uses when the special properties of the other types are not required. It is used in pavement and sidewalk construction, reinforced concrete buildings and bridges, rail-way structures, tanks and reservoirs, sewers, culverts, water-pipe, masonry units, soil-cement mixtures, and for all uses of cement or concrete not subject to special sulfate hazard or where the heat generated by the hydration of the cement will not cause an objectionable rise in temperature.

Type II. Modified Portland CementThis cement has a lower heat of

hydration than Type I and generates heat at a slower rate. It is mixed with Type I in some locations. It will work well if available.

Air Entraining Portland Cement.There are three types of air-

entraining portland cement corre-sponding to Types I, II, and III. In these cements very small quantities of certain air-entraining materials are incorporated by intergrinding them with the clinker during the manufac-turing process. They have been devel-oped to produce concrete resistant to severe frost action and to effects of applications of salt to pavements for snow and ice removal.

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Two bags of cement are dumped into the mixer.

The hydraulic power from the skid loader is what powers the mixing (guard removed for picture).

Concrete made with these cements contains minute, well-distributed and completely separated air bubbles. The bubbles are so minute it is esti-mated there are many billions of them in a cubic foot of the concrete. The entrained air is reflected in reduced weight of the fresh concrete. Best results are obtained when the air con-tent is approximately 5% to 8%.

Shotcrete TermsShotcrete

Shotcrete is a process by which concrete is air placed usually on a vertical or an overhead surface. Shot-crete is made from a mixture of sand, cement and water which is pumped through a hose to the nozzle. At the nozzle air is injected to break up and actually apply the concrete. Shotcrete is also called wet gunning or wet placed concrete.

GuniteGunite is a term for a patented

process by which sand and cement are mixed together and transported down a hose with a large volume of air to the nozzle. At the nozzle water is injected to create the proper mois-ture to set the concrete. Gunite is also called dry gunning or dry gunned air placed concrete. Generally the sand and cement are mixed on the job site. Gunite takes approximately three times as much air as shotcrete. It will have about twice as much rebound.

ReboundWhenever air placed concrete,

either wet or dry process, is sprayed the surface will reflect or bounce back a certain amount. We have found it to be 10% with wet gunning and 20% with dry gunning. The rebound can vary substantially from these numbers depending on the situation and opera-

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tor’s experience.

Concrete StrengthsNormally shotcrete is much

stronger than conventional concrete because of several factors. First more cement is used in the mix. Also the impaction created by the air tends to drive out the larger air spaces thus providing a somewhat denser product. Usually shotcrete will have a compres-sive strength of more than 4,000 psi.

The concrete strength depends greatly on the aggregate, the amount of cement and the amount of water.

AggregateThe best aggregate for shotcrete

is a very even gradation from 3/8 to nearly nothing. If any of the sizes are left out that size must be replaced by the cement creating a much harder mixture to pump and work with. Rarely in the United States do we find nice, even graded aggregate. Aggre-gate that has been crushed is also much harder to work with than river or natural aggregates. The jagged edges of the crushed aggregate tend to hang up in the pumping process. Rarely do we find an even graded aggregate. So generally we have to make the best with what we can find in the locality.

Mix WaterWherever possible the water/

cement ratio needs to be held at .4 to .45. This creates an extremely strong, workable concrete. Sometimes addi-tional water must be used to create a pumpable mix. This is done most often when some of the aggregate is off-size or cracked. Theoretically the slump test will give you an indication of the water cement ratio. However, with 3/8 minus aggregate slump tests are very unreliable.

Slump TestThe slump test is performed by

filling an inverted cone with concrete, then removing the cone and measuring the distance the concrete slumps from the original height. A slump test is an

The concrete is being poured over the front of the Monolithic Integrator SL30 Concrete Mixer into the MudSlinger EHP 1500 con-crete pump (above). The mixer also has a side gate for letting smaller amounts out (below), such as would be needed for filling post holes. The side dump is demonstrated below.

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Safe Operations and PracticesThe use of appropriate safety equipment is required. This includes face shields, or safety goggles, respiratory protection (as needed) and waterproof gloves. Wet con-crete will “burn” unprotected skin. Shoes should be water proof and provide physical protection to the wearer. The first layer of shotcrete should be about 1/2 to 3/4 of one inch. After it is set, a second coat can be applied to bury the rebar.

The MudSlinger EHP1500 concrete pump is an ideal size for building fences. This is a very simple pump and can be driven by run-ning an air compressor.

extremely valid measurement for con-crete in the 5, 6 sack range with 3/4 or larger aggregate.

Compressive Strength TestsCompressive strengths in the

conventional concrete industries are performed by breaking cylinders that have been filled and set aside for that purpose. A good correlation between a series of cylinders is about 20%. Unfortunately shotcrete doesn’t test very well in cylinders unless they are special shotcrete cylinders which are hard to come by. The most valid system for testing shotcrete seems to be the Windsor probe or spraying a 2” thickness that is later cubed and tested.

Cubed tests and Windsor Probe tests are generally quite comparable. The Windsor Probe is much easier as it is simply a matter of firing a bolt with a predetermined charge into the concrete and measure the depth of penetration. It is somewhat destruc-tive in that there’s a small hole left with a bolt sticking out of it. Cubed tests are totally nondestructive unless they are taken as cores. The major problem with cubed tests is always the question -- was the concrete cured under exactly the same conditions that are in the facility itself.

Concrete ThicknessEvery endeavor is made to make

concrete the proper thickness. This thickness is gauged by the amount of embedment on the steel and by the experience of the nozzleman. It is further checked and corroborated by the calculations of the volumes needed for the project. In general, however, the thickness is less important than properly embedding all of the rein-forcing steel.

When 1/2 inch to 3/4 inch of con-crete covers the rebar on both sides, it is thick enough.

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The shotcrete is placed in layers. The first layer is left for several hours or overnight to gain strength. Then the second layer can be applied, then, of course, the third layer. By the time the second and third layers are in place the fence will be very strong.

Shown above the first layer of concrete is in place. There are places where some of the concrete sloughed off. These places will be easily filled in with the second layer of concrete.

Shotcrete Mix DesignMix Design for 1/3 cubic yard

Shotcrete mix design varies from job to job due to different types of materials and other conditions. How-ever, following is a mix design that works well in most areas.

Cement — 188 lb. Two sacks of standard Type I or II or I-II Portland cement. This is what the lumber yard will carry. It is not mortar mix.

Water — about 80 lbs. (10 gal-lons.) Water must be clean (potable).

To Mix or Not to MixDo you want to use ready-mix or

mix on-site? In order to use ready-mix you will need a larger pump and will want to have more of the fence formed. Mixing on-site has advan-tages, such as ease of pouring the fence posts and convenient schedul-ing.

Obviously, if the job uses thou-sands of yards of concrete, a ready mix plant is the only way to go for ordering shotcrete. But for the small jobs using the small pumps, we sug-gest you consider mixing on site.

Mixing on-site with small mixers must be well organized. The mixers can be a Porta-mix mounted on a skid loader, or a plaster mixer, or small line concrete mixer. The biggest advan-tage of on-site mixing is the ability to control the speed and delivery. There is nothing worse than waiting a few hours for the ready-mix truck and then immediately having a breakdown that prevents unloading it.

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The nozzleman applies the second layer of shotcrete. One can see that the rebar is being buried with this second layer. The fence is now approximately one and a half inches thick. The rebar needs to be scraped or ridges will be left. Do not apply all the shotcrete at one time. Cover the fence then go back and do it again. After a section sets for even a short time it can often be resprayed.

(Below) The nozzle should be held perpen-dicular to the work or slightly pointed up as shown on page 22. Never, never spray down with only one exception — spraying the base footing of the fence.

This will vary from job to job and must be adjusted. The water in the aggregate will cause a difference in the amount of water needed. Adjust to a proper slump. 2” to 6” as needed.

Concrete Sand — 690 lbs. (Ten, 5 gallon buckets is a good approximate measure).

Pea rock — 140 lbs. (Two, 5 gallon buckets.) The amount of pea rock is adjusted per gradation of the sand. If the sand is high in larger aggregate the pea rock may be left out. If it is low, then more may be needed. This is a place to start for maximum strength. The total of the sand and pea rock should equal the 830 pounds (twelve, 5 gallon buckets). Usually, the pea rock is left out of the final coats to provide a smoother finish.

Kel-Crete — use 2 oz. minimum per batch. Try up to four ounces per batch. Adjust between for the best results. We normally use 2.5 ounces. Add the Kel-Crete additive in with the water for easier dispersion. If the Kel-Crete is not added, another 1/3 of a bag of cement should be added.

Nylon fibers — use 1/3 lb. Per batch. These are the best fibers we have found for what we are doing.

Total weight of this batch will be about 1,100 pounds.

Spray the ShotcreteThe posts are obviously key to the

fence standing up. The size must be large enough to restrain movement in the soil. If the fence posts are in line they must be larger than for the zigzag fence.

The concrete of the upper part of the post 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.

And be sure and spray onto the top of the post immediately (first) to prevent rebound or other materials

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The fence is now structurally strong. It needs one more layer to even it up and then the final layer of colorant.

After the forms are removed spray another 3/4 inch to both sides of the fence. After this has cured a few hours finish the fence with a 1/4 inch thick coat of concrete with colorant added. One can choose to finish with the gray concrete and stain it or just leave it gray. This final coat can be etched to simulate rock, brick or block finish.

to accumulate between the layers of concrete.

Once all of the form work and rebar are in place a light coating of concrete is sprayed onto the form through the rebar.

The placing of the concrete is simple when done right. Apply the concrete as evenly as possible at all times. This helps in keeping the thick-ness correct.

The shotcrete is started at the bottom. The entire footing (top of posts) should be covered first with a thick layer that extends about 1 foot up the wall. This is to make sure that the concrete on the footing is good concrete and not just shotcrete rebound.

This first layer of concrete includes the nylon fibers to make the mix stick better. The idea is to cover the plywood completely and to build up some on the rebar.

It is not necessary that this first run of concrete be very thick. If it is working very well and sticking well, then go ahead and put a 1/2 to 3/4 inch on it. If all you can get to stick is a 1/4 of an inch, that is fine.

Let this concrete set over night, or until there is approximately 600 psi strength. This concrete will be tough, but it still can be scratched with a key. If the weather is inclement let the con-crete set for two days. If the concrete is going to be subjected to severe rain or frost it will need to be covered. If it is real cold then it will need to be heated between the covering and the concrete surface.

The next day spray the fence again with concrete, bringing the total thickness up to about an inch and a half. This may take two spray ses-sions. Often it can be sprayed twice the second day.

Build the fence out, embedding the rebar very thoroughly. Care should be taken to scrape the rebar after each spraying of the concrete so it does not have a large build up on it.

The rebar needs to be embedded in the concrete with about a 1/2 to 3/4 inch of cover. The spraying process the second day doesn’t need to get it

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Here can be seen the layering of the fence. The fence on the left has two layers of shot-crete and the forms have been stripped. The section on the right still has forms in place, some of which has been sprayed with the first layer of concrete.

With the forms removed from the back, and the third coat applied, the fence is now ready for the finish coat of colored concrete. This is the time to consider etching the fence to look like a rock wall.

out to full thickness because we are going to spray one more time with the colored concrete. The second day it is a good idea to have most of the thick-ness finished up. You don’t want to spend a lot of time with colored con-crete trying to cover up missed places.

Note: It is very difficult to judge the depth of sprayed concrete as it is being applied. A 1” layer can look very much like a 1/8” layer. To be sure of a uniform build up of thick-ness, a very uniform spraying pattern should be followed. This pattern can vary according to the nozzleman, but it should be consistent. To insure proper thickness, check the depth gauges.

It is important to use good shot-crete techniques when shooting around rebar, that is, shoot from close enough to the bar and with enough force that the concrete cannot build up on the face of the bar but closes around it from the back.

After the fence has set overnight, gently, and I use the word gently, remove the forms, clean them, and move them on to get ready for another section of fence.

The Last CoatThe concrete can now be applied

to both sides of the fence. This last coat of concrete to the front side and the complete coating on the back side should be done with a half to three quarter inch covering.

This spray of approximately 1/2 to 3/4 inch of concrete on the form side is to protect the rebar.

The fence will harden up consid-erably over the first thirty days. It is very helpful if it can be kept damp within that same thirty day period. This can be done by hanging cloth or burlap over the fence and spraying water on it from time to time during the day. It is extremely important that the fence be kept moist during periods of direct sun, hot sun, and dry winds. The fence can be protected by coating with a concrete sealer available from the concrete accessory supplier.

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It is a good idea to keep the fence moist, as with any concrete, as it goes through its curing phase. If possible the fence should be kept wet for up to 30 days or a concrete curing compound can be used. These com-pounds can be purchased from a concrete accesory store.

Final color coat sprayed is on the left. Note the position of the nozzleman for proper spraying. The nozzle should be held per-pendicular to the work or slightly pointed up as shown both above and below. Never, never spray down with only one exception — spraying the base footing of the fence.

ColorThe colorant will give a pleasing

color to the finished concrete fence. Spray a quarter inch of colored con-crete on both sides to produce the final finish.

If the final coat of shotcrete has colorant in it, the fence is now fin-ished other than the moisture cure.

There is nothing that says that the gray is not beautiful. Gray colored concrete will eventually get white and become very pretty.

The final spraying of concrete may be held up until the entire fence is finished with gray concrete so that the color can be the same from one end of the fence to the other.

The coloring in concrete is always a little bit tricky. It is difficult to get a true match from one batch to another. The batches tend to interfere with each other. They seem to get more water or something in one than they do in the other, but after a period of a few weeks it can be noticed that the fence tends to equal out to one color.

An option, a very valid option, is to spray out the full thickness of the concrete in the gray and then use a concrete stain. Most reputable paint/concrete shops have concrete stain. It is not a paint, it is a stain. It soaks into the concrete and will give you a permanent color finish.

If a really nice finish is desired, the stain is probably the safest way to go.

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This 8-foot tall, 800-foot long fence is for the annual Bethlehem Revisited program in Waxa-hachie, Texas. The fence encloses a half a block. It has concrete posts and wing walls to provide lateral stability. These fence/walls can be built very cost effectively. The walls are two-inches thick, except at the posts where they are three and a half inches thick. Without colorant it is a beautiful gray.

Costs The cost of shotcrete at $75 per

cubic yard translates to about 50 cents per square foot of fence at 2" of thickness. The 3/8" rebar at 30 cents per pound will cost about 15 cents per square foot. The additional amount for the post will add another ten cents per square foot. This totals about 55 cents per square foot for the materials used in the fence.

The cost of the forms must be amortized against the project. If we estimate another 10 cents for the forms we have about a 65 cents per square foot for the materials. Now we must add the labor and it will vary according to location and demand.

Each fence will need to be figured separately. Figured into the final cost of the fence will be travel expenses, time of year, accessibility, clean up, labor rates and other expenses.

Sale PriceMy suggestion is to price the com-

pleted fence at about $6 per square foot. This means a 6' tall fence will be about $36 per lineal foot. This is for a zigzag fence of at least a couple of hundred feet in length.

Note: The above sales prices was current in 1996. Adjust accordingly

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AppendixIn the past 20+ years, we have

gained significant know-how in building spray-in-place concrete fences. This Appendix adds to that knowledge.

Zigzags: Experience has taught us the importance of zigzags. They make a difference in strength, ver-satility, expansion and contraction.

Zigzags can be symmetrical, moving back and forth an extra foot on each side of the property line, every 16’. Half as many posts are needed with zigzag fence.

Or they can be even more ornamental. The fence can run straight down the property line for 12’, then have a short zigzag that creates a planter box or a place for a yard bench.

First do the one side and then do the other. If these are 4’ long, zig it 2’, first to one side and then to the other. You can imagine the stability this gives a fence and makes it very beautiful. Obviously neighbors need to agree on the fence, but it really creates a high-light for both properties.

Basalt Reinforcing: We have learned to use basalt reinforcing. Basalt is another name for lava rock. Reinforcing made of basalt is twice as strong as steel and -- most importantly -- it won’t rust. The number one cause of concrete failure is rusting rebar. Water soaks through the concrete’s pores and eventually rusts the rebar. As the rust grows, it explodes the con-crete.

Protecting the rebar is one of the concrete’s major functions. Using basalt reinforcing has made a dramatic difference in the abil-ity to build the fence. We suggest you use the basalt fabric. We call it a fabric. It has about a one-inch-square spacing each direction, and we can take this and put it directly into the concrete as we spray.

We first spray a 1/2” to 3/4” of concrete on the fence forming. After it sets up, we spray about another 1/2”. Then we just gently push the reinforcing into that con-crete. After it sets up to where it is relatively solid, we spray the next 1/2” or 3/4” of concrete to finish the fence.

At this point we have many options: We can peel the form work off and spray a light touch on the backside, or we can just leave it alone and do a hand trowel on either side, but in most cases we are done.

Now is the time to decide what to do about the fence color-wise. Concrete can have colorant mixed in with it. But if you have ever sprayed concrete, you know it’s super tough to make the same mix with each batch and get the same color.

We also can recommend coat-ing the fence with something like H&C Concrete Stain. This stain seems to last forever and is gor-geous and self-cleaning. The fence looks sharp and new for years.

The thickness described here

is 1 and ½” to 2” thick. Unless that fence will be subjected to heavy loads or heavy banging, that is all the fence you will ever need. In fact, it’s certainly stronger than any wood fence. And because we can keep it thin, we use less mate-rial and do the job quicker, simpler and less expensively.

As part of this Appendix, we can also tell you that you can use #2 steel rebar in the fence, as described in the book. I would move the bars closer to 15” on center -- or you could even go to 12”. Number 2 rebar is not readily available, but it can be used. But if you use steel rebar you have to keep the fence well coated to pre-vent rusting.

If you are using steel rebar, be sure you have about 2” of cover on each side of the rebar to pro-tect it from water. Or you have to paint the fence to protect it. To do a really good job of painting the fence, you need something beside the concrete stain. You need some-thing like silicone or first class acrylic that stops water migration.

We strongly recommend that you use basalt reinforcing. The rebar coming up out of the posts can still be steel, but it is even better if it’s basalt as well. Basalt rebar can be purchased at the same places as the webbing.