How To Build A Spray-In-Place Concrete Fence - Monolithic

How To Build A Spray-In-Place
Concrete Fence
With Appendix Added Spring 2012

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 beautiful
backdrop to landscaping.
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 professional
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.

Step 1
Design the Fence Layout
Where is the property line?
Whenever a fence is built, it
should be ascertained where the property
line is, or at least, exactly where
the fence is to be constructed.
If the fence is between two property
owners it is best to have an agreement
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 neighbor,
it is probably best that you check
with an attorney to see what your legal
options are.
After locating and marking the
property lines decide with the neighbors
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 expansions.
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” usually
consists of the residents and the
pets of the home.
An appropriate fence height
The property line must be determined, agreements
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.
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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.
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 differently
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. Pressures
applied act to turn
over the fence rather than
to break it off at ground
level. It is much more
stable
Straight Fence vs. Zigzag
First 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 appropriate
for your application.
The Straight Fence — Advantages
The straight line fence is the traditional
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 vertically.
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 disadvantage
is strength. Expansion and
4

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 — Advantages
The zigzag fence is much stronger
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.
5

zigzagged, forming pockets for planting
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 diagram
(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 prevent
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 pressure
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 expansion
joint at each change of direction,
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 decorative
and strengthening box at the end of the
fence.
6

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-inplace
concrete fence allows for the imagination
to run wild.
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 Difference
The 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 concrete
post is very strong. Secondly,
it tries to make the post hole bigger.
Force acting against the top of the
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 dynamics
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 Ground
Many 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 hesitant
to dig out from under the fence
in most areas because that invites animals
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.
7

The Top of the Fence
The 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 interesting
pattern fastened to the forms as
illustrated.
As the fence is sprayed the concrete
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 versatile.
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.
8

Step 2
Install Fence Posts
The next step in building either of
the fences is installing the fence posts.
Measuring The Layout
Pull 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.
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).
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 determined
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 Post
Care must be taken to fill the post
9

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.
10

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.
Step 3
Form The Fence
The forms can be made of virtually
anything that will hold still.
We have found a light framework
of 2” x 4” faced with 7/16 inch thick
waferboard makes inexpensive forming
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
11

side that we are going to do the spraying
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 Forms
After applying the form oil stand
up the single side forms.
Place the formwork so that it is
vertical and passes immediately adjacent
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.
Options
This 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.
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.
12

(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.
Step 4
Attach 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 approximately
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 concrete
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.
Left is a 6’ tall zigzag fence formed with rebar
attached, ready for shotcreting. Only a section
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”.
13

Place the rebar 15” to 18” on center. Determine
the distance by spacing the rebar
evenly between the post. In general the horizontal
rebar is placed first, but the first layer
of rebar can be either the verticals or horizontals.
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 minimal
amount of obstruction for the shotcreter.
Once the horizontal rebar is in place the concrete
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.
14

Step 5
Applying the Shotcrete
The next step is applying the
shotcrete. This requires a working
understanding of cement, shotcreting,
aggregate and your options.
Types of Portland Cement
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.
Type 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 explanation.
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, railway
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 Cement
This 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 airentraining
portland cement corresponding
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 manufacturing
process. They have been developed
to produce concrete resistant to
severe frost action and to effects of
applications of salt to pavements for
snow and ice removal.
15

Concrete made with these cements
contains minute, well-distributed and
completely separated air bubbles.
The bubbles are so minute it is estimated
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 content
is approximately 5% to 8%.
Shotcrete Terms
Shotcrete
Shotcrete is a process by which
concrete is air placed usually on a
vertical or an overhead surface. Shotcrete
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.
Gunite
Gunite 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 moisture
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.
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).
Rebound
Whenever 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-
16

tor’s experience.
Concrete Strengths
Normally 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 compressive
strength of more than 4,000 psi.
The concrete strength depends
greatly on the aggregate, the amount
of cement and the amount of water.
The concrete is being poured over the front
of the Monolithic Integrator SL30 Concrete
Mixer into the MudSlinger EHP 1500 concrete
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.
Aggregate
The 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. Aggregate
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 Water
Wherever possible the water/
cement ratio needs to be held at .4 to
.45. This creates an extremely strong,
workable concrete. Sometimes additional
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 Test
The 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
17

extremely valid measurement for concrete
in the 5, 6 sack range with 3/4 or
larger aggregate.
Compressive Strength Tests
Compressive 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 destructive
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 Thickness
Every 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 reinforcing
steel.
When 1/2 inch to 3/4 inch of concrete
covers the rebar on both sides, it
is thick enough.
The MudSlinger GHP1500 concrete pump is
an ideal size for building fences. This is a
very simple pump and can be driven by running
an air compressor.
Safe Operations and Practices
The use of appropriate safety equipment
is required. This includes face shields, or
safety goggles, respiratory protection (as
needed) and waterproof gloves. Wet concrete
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.
18

To Mix or Not to Mix
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.
Do you want to use ready-mix or
mix on-site? In order to use readymix
you will need a larger pump and
will want to have more of the fence
formed. Mixing on-site has advantages,
such as ease of pouring the
fence posts and convenient scheduling.
Obviously, if the job uses thousands
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 suggest
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 advantage
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.
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 Design
Mix Design for 1/3 cubic yard
Shotcrete mix design varies from
job to job due to different types of
materials and other conditions. However,
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 gallons.)
Water must be clean (potable).
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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.
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 perpendicular
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.
Spray the Shotcrete
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 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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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.
The fence is now structurally strong. It needs one more layer to even it up and then the
final layer of colorant.
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 thickness
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 concrete
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 sessions.
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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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 thickness
finished up. You don’t want to
spend a lot of time with colored concrete
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 thickness,
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 shotcrete
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 Coat
The 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 considerably
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.
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.
Here can be seen the layering of the fence.
The fence on the left has two layers of shotcrete
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.
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Final color coat sprayed is on the left. Note
the position of the nozzleman for proper
spraying. The nozzle should be held perpendicular
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.
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 compounds
can be purchased from a concrete
accesory store.
Color
The colorant will give a pleasing
color to the finished concrete fence.
Spray a quarter inch of colored concrete
on both sides to produce the final
finish.
If the final coat of shotcrete has
colorant in it, the fence is now finished
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 Waxahachie,
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.
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Appendix
In 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, versatility,
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 highlight
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
rusts grows, it explodes the concrete.
Protecting the rebar is one of
the concrete’s major functions.
Using basalt reinforcing has made
a dramatic difference in the ability
to build the fence. We suggest
you use the basalt fabric. We call
it a fabric. It has about a one-inchsquare
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.
See the basalt webbing being installed. Simple, fast and permanent for flower boxes or
fences.
Simple zig zag fence with colorant is 1 3/4 to 2 inches thick and six feet tall.
After it sets up, we spray about
another 1/2”. Then we just gently
push the reinforcing into that concrete.
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. And the color in the concrete
is less expensive and will stay that
color virtually forever.
We also can recommend coating
the fence with something like
H&C Concrete Stain. This stain
seems to last forever and is gor-
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The zig zag prevents trouble ith the black clay soil. Fence is permanent.
geous and self-cleaning. The fence
looks sharp and new for years.
The thickness described here is
1½” 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 material 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 prevent
rusting.
If you are using steel rebar, be
sure you have about 2” of cover
on each side of the rebar to protect
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 something
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.
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