Concrete Fence

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					How To Build A Spray-In-Place
Concrete Fence
                  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.
                                                After locating and marking the
             Step 1                        property lines decide with the neigh-
                                           bors on the best fence layout. This is
Design the Fence Layout                    doubly important if the fence is to be
                                           zigzag.
Where is the property line?                     During planning remember to
     Whenever a fence is built, it         design the layout so it does not hinder
should be ascertained where the prop-      future plans, remodeling, or expan-
erty line is, or at least, exactly where   sions. For instance, if there is no exit
the fence is to be constructed.            to the back lot and some day you want
     If the fence is between two prop-     to build a swimming pool there, you
erty owners it is best to have an agree-   will find it very difficult to go through
ment on what the fence is to be like,      the concrete fence. Therefore, a gate
how it should be laid out, and who is      would be in order.
to pay for and install it.                      Fences perform many functions,
     The laws on fences vary from          not the least is to “keep them out
state to state, so if you don’t get an     and us in.” “Them” can be critters,
absolute agreement with your neigh-        people, wind, snow drifts. “Us” usu-             The property line must be determined, agree-
bor, it is probably best that you check    ally consists of the residents and the           ments between neighbors must be reached,
with an attorney to see what your legal    pets of the home.                                and a plan decided on before construction
                                                                                            can begin.
options are.                                    An appropriate fence height




                                           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.



                                                                                                                                            3
needs to be established. Here again
it is wise to check your local city
ordinances. In many places a fence          On a straight line fence
over three feet tall in the front of the    (right) the wind pressure
                                            and vibration can often
house is not allowed. In other places       enlarge the post holes and
a fence has a maximum height in the         eventually push the fence
                                            over or snap the posts at
back of eight feet. You will find the       ground level.
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.   Because of the zigzag,
The best choice is get the rules.           pressures act very differ-
      The spray-in-place concrete fence     ently on this fence (below)
                                            from those of a straight
is amazingly versatile. There are many      fence. The 2” thick fence
creative options for layout and design.     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




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 appro-
priate for your application.

The Straight Fence — Advantages
     The 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



                                                                          4
                                                                                                  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.
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.
                                                                                                  The Zigzag Fence — Advantages
                                                                                                      The 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.

                                                                                                                                             5
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-           Above, an eight foot fence constructed for
sure will have to be applied before the     the Christmas Festival in Waxahachie, Texas
                                            has walls connected to the fence to delineate
forward post can lift and bend over         booths. More importantly, they significantly
against the back post. In other words,      add to the strength of the fence.

the ability of the forces to hold against   Anytime an adornment or offset can be
pressure is much greater as the “zigs”      added strength will be added to the fence.
                                            This straight line fence has small offset
increase.                                   every eight feet and finishes with a decora-
     If the zigzagging is 2 feet every      tive and strengthening box at the end of the
                                            fence.
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
                                                                                            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-in-
                                                                                         place concrete fence allows for the imagina-
                                                                                         tion to run wild.


                                                                                         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.
center. The straight fence should have     fence forces it to rock back and forth,            If the soils persist in moving the
posts 8 foot on center.                    enlarging the hole. The fence, to a           fence around, then you can dig the
                                           certain extent, is only as strong as the      soil out from under the length of the
The Zigzag Fence — Disadvantages           soil.                                         fence and that should stop. I am hesi-
    The zigzag fence takes up a little          For the zigzag fence the dynam-          tant to dig out from under the fence
more space. If you want a zigzag           ics are totally different. It is not a soil   in most areas because that invites ani-
fence the neighbors must absolutely        issue, or snapping off of the posts.          mals to dig under the fence. In most
determine where the fence posts are        Here, force acting against the fence          places there is not going to be enough
going to go. To be fair the posts          tries to tip the fence over. The posts        expansion or contraction to bother the
should zigzag across the property line     serve mainly to keep the fence from           fence itself. You should just monitor
so that each of the parties have the       sliding sideways in the soil -- the           it and if it looks like it is going to be
same amount of property taken up by        fence itself is the strength. A zigzag        a problem then dig out half of the soil
the fence itself. It takes a little more   fence that zigs one foot off center one       out from under it, or two thirds, or all
layout. It is somewhat nontraditional.     way and zags one foot off center the          of it if you need to.
                                           other acts as if it were a two foot thick
Summary of the Principle Difference        fence.
      The major difference in the
strength dynamics of these two fences      Soil Considerations: Moving Ground
is — what it will take to knock the              Many parts of the United States
fence down.                                have expansive soil. This means that
      The straight line fence is held in   when the ground gets wet it expands
place by the soil. Force against the       and when it dries out it contracts.
fence does two things. One, it tries             Where we are here in Italy, Texas,
to snap the post off. But, the con-        it is extremely bad. The contraction
crete post is very strong. Secondly,       will pull all of the dirt away from a
it tries to make the post hole bigger.     post, or it may lift the post, or in the
Force acting against the top of the        case of the concrete fence, it may try

                                                                                                                                      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 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,          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.
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.




                                                                                                                                            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.

                                                                                                   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
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
                                                                                                   side to side holding characteristics
a zigzag fence. Fence posts poured in a zigzag pattern (below).                                    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


                                                                                                          Step 3
for this four foot tall fence.



                                                                                                   Form The 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

                                                                                                                                   11
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 Forms
      After 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.       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.
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.




                                                                                                                                   12
                                                                                                                  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 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.
(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   Center the rebar so it fastens into the
so that it doesn’t wave and wiggle. Keep the rebar square and neat. It takes about 4 attaching     rebar of the fence. Make sure there is
points per 20 foot length of rebar.
                                                                                                   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 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”.
                                                                                                                                               13
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.




                                                                                              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
                                                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 explana-
                                                tion. It is here for those who wish to
                                                know more.
Ingredients for a fence: sand, cement, water,
                                                     This is a general purpose cement
ad mixture, and colorant for the final coat.    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
The Monolithic Integrator SL30 attached to      cement will not cause an objectionable
a skid loader is “self-bucketing” and easy to
load up with sand.                              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 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.

                                                                                     15
     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 Terms
Shotcrete
     Shotcrete is a process by which
concrete is air placed usually on a        Two bags of cement are dumped into the
vertical or an overhead surface. Shot-     mixer.
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.

Gunite                                     The hydraulic power from the skid loader is
     Gunite is a term for a patented       what powers the mixing (guard removed for
process by which sand and cement           picture).

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.

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 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.

                                               Aggregate
                                                    The best aggregate for shotcrete
                                               is a very even gradation from 3/8 to
                                               nearly nothing. If any of the sizes are
The concrete is being poured over the front    left out that size must be replaced by
of the Monolithic Integrator SL30 Concrete     the cement creating a much harder
Mixer into the MudSlinger EHP 1500 con-
crete pump (above). The mixer also has a
                                               mixture to pump and work with.
side gate for letting smaller amounts out      Rarely in the United States do we find
(below), such as would be needed for filling
post holes. The side dump is demonstrated
                                               nice, even graded aggregate. Aggre-
below.                                         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 Water
                                                    Wherever 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 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 con-
crete 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         The MudSlinger EHP1500 concrete pump is
it is simply a matter of firing a bolt      an ideal size for building fences. This is a
with a predetermined charge into the        very simple pump and can be driven by run-
                                            ning an air compressor.
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       Safe Operations and Practices
tests are totally nondestructive unless     The use of appropriate safety equipment
they are taken as cores. The major          is required. This includes face shields, or
                                            safety goggles, respiratory protection (as
problem with cubed tests is always          needed) and waterproof gloves. Wet con-
the question -- was the concrete cured      crete will “burn” unprotected skin. Shoes
                                            should be water proof and provide physical
under exactly the same conditions that      protection to the wearer. The first layer of
are in the facility itself.                 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.
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 rein-
forcing steel.
     When 1/2 inch to 3/4 inch of con-
crete covers the rebar on both sides, it
is thick enough.




                                                                                      18
                                                                                                       To Mix or Not to Mix
                                                                                                         Do 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-
The shotcrete is placed in layers. The first layer is left for several hours or overnight to gain   tage of on-site mixing is the ability to
                                                                                                    control the speed and delivery. There
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.
                                                                                                    is nothing worse than waiting a few
                                                                                                    hours for the ready-mix truck and then
                                                                                                    immediately having a breakdown that
                                                                                                    prevents unloading it.




                                                                                                      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. 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.
Shown above the first layer of concrete is in place. There are places where some of the                 Water — about 80 lbs. (10 gal-
concrete sloughed off. These places will be easily filled in with the second layer of concrete.
                                                                                                    lons.) Water must be clean (potable).
                                                                                                                                          19
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       The nozzleman applies the second layer of
                                           shotcrete. One can see that the rebar is
batch. Adjust between for the best         being buried with this second layer. The
results. We normally use 2.5 ounces.       fence is now approximately one and a half
                                           inches thick. The rebar needs to be scraped
Add the Kel-Crete additive in with         or ridges will be left. Do not apply all the
the water for easier dispersion. If the    shotcrete at one time. Cover the fence then
                                           go back and do it again. After a section
Kel-Crete is not added, another 1/3 of     sets for even a short time it can often be
a bag of cement should be added.           resprayed.
      Nylon fibers — use 1/3 lb. Per       (Below) The nozzle should be held perpen-
batch. These are the best fibers we        dicular to the work or slightly pointed up as
have found for what we are doing.          shown on page 22. Never, never spray down
                                           with only one exception — spraying the base
      Total weight of this batch will be   footing of the fence.
about 1,100 pounds.




   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
                                                                                    20
                                                                                                 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
After the forms are removed spray another 3/4 inch to both sides of the fence. After this has    strength. This concrete will be tough,
cured a few hours finish the fence with a 1/4 inch thick coat of concrete with colorant added.   but it still can be scratched with a key.
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.                                      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
The fence is now structurally strong. It needs one more layer to even it up and then the         inch of cover. The spraying process
final layer of colorant.
                                                                                                 the second day doesn’t need to get it
                                                                                                                                       21
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-     With the forms removed from the back, and
crete techniques when shooting around      the third coat applied, the fence is now ready
                                           for the finish coat of colored concrete. This
rebar, that is, shoot from close enough    is the time to consider etching the fence to
to the bar and with enough force that      look like a rock wall.
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.
                                           Here can be seen the layering of the fence.
The Last Coat                              The fence on the left has two layers of shot-
     The concrete can now be applied       crete and the forms have been stripped. The
                                           section on the right still has forms in place,
to both sides of the fence. This last      some of which has been sprayed with the
coat of concrete to the front side and     first layer of concrete.

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.


                                                                                     22
                                                Color
                                                     The 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.
Final color coat sprayed is on the left. Note
                                                     There is nothing that says that the
the position of the nozzleman for proper        gray is not beautiful. Gray colored
spraying. The nozzle should be held per-        concrete will eventually get white and
pendicular to the work or slightly pointed
up as shown both above and below. Never,        become very pretty.
never spray down with only one exception —           The final spraying of concrete
spraying the base footing of the fence.
                                                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
It is a good idea to keep the fence moist,      a little bit tricky. It is difficult to get a
as with any concrete, as it goes through its
curing phase. If possible the fence should
                                                true match from one batch to another.
be kept wet for up to 30 days or a concrete     The batches tend to interfere with each
curing compound can be used. These com-         other. They seem to get more water
pounds can be purchased from a concrete
accesory store.                                 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.




                                                                                         23
               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,      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
time of year, accessibility, clean up,     provide lateral stability. These fence/walls can be built very cost effectively. The walls are
labor rates and other expenses.            two-inches thick, except at the posts where they are three and a half inches thick. Without
                                           colorant it is a beautiful gray.
           Sale Price
    My 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




                                                                                                                                       24

				
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