Question: What is a prismoid?
Answer: “Prismoid” is the name of a three-dimensional shape. Some other threedimensional shapes are named cube, cylinder and sphere. Basically, a prismoid can be
described as a pyramid with the top point cut off. The remaining part must have the base
and top as parallel planes. A prismoid is also known as a frustum of a pyramid, but for
now we will use the word “prismoid” throughout this discussion. When the volume of a
prismoid must be calculated, several dimensions must be known.
Question: Why am I learning about a prismoid?
Answer: Because if you are using wooden forms and are thickening the edge of your
slabs around the perimeter of your block outs – this is the physical configuration of the
concrete that you are wasting! The sketch shown above may not look familiar, but if you
flip it upside-down it may look very familiar.
Question: So what if I am wasting a little concrete on these “prismoids” of
unnecessary concrete – how much could that possibly cost me?
Answer: A lot more than many contractors may realize! Every situation may vary, so
we have developed a method to determine the amount of wasted concrete on any
particular block out.
Question: Could this be why I always seem to need more concrete to cast a slab than
my calculations show?
Answer: Yes – this can cause you to need more concrete than you might expect to use.
Page 1 of 15
Question: How does POCKET FORM ISOLATORä prevent me from wasting this
“prismoid” of concrete volume?
Answer: PFIä does not require a thickened slab edge. As a matter of fact, the best slabs
do not have thickened slabs – but, instead have a uniform slab thickness throughout.
After the PFIä has been installed, backfill should be placed and compacted against the
exterior of the form up to the same elevation as the other parts of the slab. This is how
one can construct a slab with predictable shrinkage characteristics. Thickened slabs
actually prevent concrete from being able to shrink away from the block out, since the
thickened slab is actually bonded to the footing.
Question: What dimensions have to be known in order for me to calculate the
volume of a prismoid?
Answer: The area at the base, the area at mid-height, the area at the top and the height
of the prismoid.
Question: What is the formula for the volume of a prismoid?
Answer: (Ab+4Am+At)/6 x Hp…where Ab = Area at base…where Am = Area at midheight…where At = Area at top…and Hp = height of prismoid.
Question: Will you do an example calculation?
Answer: Yes! Let us use a small prismoid as a simple example, the formula is correct
for any size prismoid; but some of these numbers will be easily done without a calculator.
Assume for this example that we have a prismoid 10” in height, 12” square at the base
and 4” square at the top. The linear dimension at mid-height is an average between the
base plus the top. Now, let us plug these numbers into the formula shown above.
Ab = 12” x 12” = 144 square inches.
Am = 8” x 8” = 64 square inches, therefore;
4Am = 64” x 4 = 256 square inches.
At = 4” x 4” = 16 square inches.
Hp = 10”
(144”+256”+16”)/6 x 10” = 693.3 cubic inches.
Question: Is there an easier way to calculate the volume of a prismoid?
Answer: Yes! Use a math processing spreadsheet such as Microsoft Excel with the
formula plugged into the appropriate cells. At Forrester Manufacturing Company, we
have created a spreadsheet with this formula plugged into the appropriate cells. The
purpose of this spreadsheet is obvious when you see the name of the spreadsheet…
“CONCRETE WASTE CALCULATOR”. The “CWC” takes into affect that you are
attempting to learn how much concrete is wasted when a thickened slab is cast against
wooden forms for column isolation pockets.
Page 2 of 15
Question: What information must be given to determine how much concrete I am
wasting on any particular column isolation pocket?
Answer: There are several dimensions that must be furnished and also the cost of
concrete per cubic yard (be sure to add for pumping – if appropriate). Dimensions that are
necessary are as indicated in the sketch below. The numbers shown in parentheses
indicate position of within the spreadsheet format.
Question: I noticed that you used inches for your input. Should I use inches for my
input ?
Answer: Yes, the “CONCRETE WASTE CALCULATOR” is setup for input in
inches, because inches seemed to be the most straightforward method to use. A cubic
yard is one yard wide, one yard long and one yard tall. It is also 27 cubic feet (3 x 3 x 3)
and it is also 46,656 cubic inches (36 x 36 x 36). For ease of use, our spreadsheet is
designed to utilize the latter number [46,656] in the calculations.
Page 3 of 15
Question: How can I know you have the correct formulas in your spreadsheet?
Answer: There is no trickery here. This spreadsheet automatically subtracts the volume
of concrete that would be required to fill the block out (you are providing the necessary
dimensions), as this is a trade-off with either construction method. This spreadsheet also
calculates only the concrete within the prismoid below the slab (another tradeoff),
because the same slab is required in either scenario.
Question: Will the “CONCRETE WASTE CALCULATOR” tell me in dollars and
cents the dollar value of my wasted concrete?
Answer: Yes!
Question: What would happen if I didn’t thicken the slab edge around my block
outs?
Answer: First let me clarify something… the thickened slab would only be required if
your slab is cast prior to casting the block out void – but this describes about 90% of the
slabs we see. The other 10% of slabs that have the pockets cast prior to the slab casting
would not need to consider this calculation – because it would not affect them. If the
thickened slab were simply omitted – a cave-in of subbase material would most likely
occur.
Question: I am in the 10% crowd as you have defined in the question above. How
can PFIä help a contractor like me?
Answer: POCKET FORM ISOLATORä offers a time-saving method that is incredible
for projects that lend themselves to filling the block outs prior to slab casting! Obviously,
it is easier to take a form off of a casting than to pull a form out of a casting, but what if
you could set the form in-place and forget about ever removing it? We offer a way to do
just this! For a project to be ready to cast block out voids, forms must be set and any
penetrations into said forms must be made, therefore; PFIä could be set instead of a
removable form, backfilling can be completed for under-the-slab and the block outs and
the slab can be cast all at one time! How’s that for convenience?
Page 4 of 15
name
(name of customer)
date
(date) mm/dd/yy
job
(project name)
PROJECT-SPECIFIC TURNED-DOWN SLAB CALCULATOR
ITEM
NO.
DESCRIPTION of INPUT
INPUT
COLUMN
TYPE OF
UNITS
$0.00
DOLLARS
1
COST per CUBIC YARD of SLAB CONCRETE…
2
PLAN DIMENSION "x" of COLUMN BLOCKOUT…
0
INCHES
3
PLAN DIMENSION "y" of COLUMN BLOCKOUT…
0
INCHES
4
DIMENSION FROM T.O.F. UP TO F.F.E. …
0
INCHES
5
IDEAL UNIFORM SLAB THICKNESS…
0
INCHES
6
WIDTH OF BASE OF TURNED-DOWN SLAB…
0
INCHES
7
WIDTH OF TURNED-DOWN SLAB AT CONTACT POINT…
0
INCHES
FORMULA
COLUMN
TYPE OF
UNITS
DESCRIPTION of COMPUTER DERIVATIVE
ITEM
NO.
8
AREA at BASE of PRISMOID…
0
0
0
SQ. IN's.
9
AREA at TOP of PRISMOID…
0
0
0
SQ. IN's.
10
DEPTH of (wasted concrete) TURNED-DOWN…
0
INCHES
11
AVERAGE WIDTH of (wasted concrete) TURNED-DOWN…
0
INCHES
12
AREA at MID-HEIGHT of PRISMOID…
0
0
0
INCHES
13
VOL = AREA 8 + AREA 9 + (4 x AREA 12) / 6 =
0
14
0
CU. INS.
15
(pocket vol)
0
div. by 6 =
x depth of
0
equals
0
CU. INS.
CU. INS., or
0
CU. YARDS
$0.00
$0.00
THEREFORE, wasted concrete TURNED-DOWN SLAB COSTS YOU…
(top SECRET),
Page 5 of 15
0
name
almost
wasted
$0.00
$0.00
Question: How do I know which POCKET FORM ISOLATOR™ is correct for my
specific situation?
Answer: There are several factors that go into determining what PFI™ is best for each
situation, here are the factors:
1. The plan dimensions of the block out.
2. The shape of the block out.
3. The depth of the block out.
Question: How can I know what block out plan dimension is correct for my
situation?
Answer: There are several considerations:
1. Architectural or Engineering requirements.
2. Base Plate shape, size, rotation and location.
3. Allowance of minimum concrete cover from base plate to edge of block out.
4. Desired appearance of finished project.
Question: How can I know the best shape block out for odd situations?
Answer: Look at the various shapes of PFI™ that are available. Keep in mind, though;
we are always glad to manufacture other custom shapes as needs for these occur.
Question: How do I know the best depth of PFI™ for me to order?
Answer: The distance from the top of the footing up to the FFE is usually the depth that
is necessary, however; in situations where columns are bearing atop of piers – there is
often some design flexibility.
Question: What is the POCKET FORM ISOLATOR™ anchored to?
Answer: In most cases it is anchored to the footing, however; PFI™ can be staked in
place atop of compact fill material in some cases where piers have been utilized. In some
other cases, PFI™ is anchored to walls and/or footings or stakes. Wall anchorage is
required for perimeter block outs in many situations. In many other situations, PFI™ is
attached to temporary edge forms.
Question: How can I be certain that I am ordering the correct and best choice PFI™
for my project?
Answer: Call, fax or email one of our sales personnel. We will be happy to assist you in
making the right choice. There are over 400 shape and size combinations of PFI™ to
choose from. In some cases, more than one single option is available to meet your needs.
We want to assist you in determining what best fits your needs.
Page 6 of 15
Question: May I place my orders through your website?
Answer: No. Currently we are not set-up to accept orders via this website, but we plan
to make that service available in the future. For now, please use phone, fax or email.
Question: Will POCKET FORM ISOLATOR™ cause a problem with my laser
screeding process?
Answer: No. POCKET FORM ISOLATOR™ is intended to be shimmed and leveled to
have its top edge exactly at FFE. Certainly, you could knock PFI™ out of its intended
position if it is hit, but this is no more of a problem than avoiding wooden formwork.
Many contractors simply hand finish an approximately 6” wide perimeter around each
block out. When our orange safety lids are in place, a worker doesn’t scuff-up the floor as
the perimeter of each block out is hand troweled.
Question: Suppose I have several shape and size combinations on my project and
my workers have never used PFIä before…How can I be certain that they get each
PFI™ in the correct location?
Answer: If you can submit your drawings prior to ordering, we will provide you with an
easy-to-follow Placement Plan and Details of everything necessary. These drawings are
drawn on an AutoCAD® r2002 format and are suitable for submission to A/E Firms for
approval by the Design Professionals, if required.
Question: How long does it take to receive my order of POCKET FORM
ISOLATOR™ , once my order has been placed?
Answer: This varies with every order we accept. We utilize common carriers with
which we have a very good working relationship. Most orders are processed within one
week of placement, but this can vary due to our workload at the time your order is placed.
We try very hard to meet everyone’s deadlines. We can usually tell you the status of any
processed order by using computerized GPS Satellite Tracking available through our
carriers.
Question: Why do so many A/E Firms specify POCKET FORM ISOLATOR™ ?
Answer: Because it works so well. Architects and Engineers appreciate PFI™ because
it saves money, provides consistent workability and appearance, allows concrete slabs to
have predictable “behavior” due to the elimination of thickened slabs and has a fantastic
track record.
Page 7 of 15
Question: I see POCKET FORM ISOLATOR™ being used on projects all over the
country…What are some of the reasons that contractors choose to use PFI™ ?
Answer: There are many reasons that contractors choose to use our product!
Tilt-Up Contractors appreciate the simplicity, the Orange Safety Lids, the cost savings,
the nice finished appearance and the ability to reduce construction time.
Big Box Retail Center Contractors often choose to cast slabs and block outs at the
same time using our product and they appreciate not having utility penetrations into block
outs being wrecked when wooden forms are removed. Those electrical and plumbing
call-backs can cost a lot.
School Building Contractors benefit from the many shapes and sizes that we
manufacture. They also appreciate the Placement Plans and Detail Sheets.
Pre-Engineered Building Contractors often use PFI™ to form the piers between
footings and steel columns. Having permanent formwork saves a lot of time compared to
using temporary forms – it also allows reduced earth-working schedules and makes
placement of pier/slab reinforcing bars a very simple matter.
Question: I’ve heard that POCKET FORM ISOLATORä can save a lot of
headaches for the Electrical Sub-Contractor – how is this possible?
Answer: Very often conduit for electrical, security or fire detection equipment must be
run from under-the-slab to up-the-column. In these situations, the electrician cuts or drills
holes in the formwork for the column block out. The likelihood of the conduit being
damaged is quite high if the formwork it is passing through must be removed. Since the
PFIä is not removed, there can be no damage to the conduit from this means.
Question: I often build structures that have interior roof drains and it seems as
though my Plumbing Sub-Contractor is getting call-backs due to his rain leaders
being broken at the column block outs. Can your product help me?
Answer: Absolutely! Due to the greater footing depths required at this type of situation,
the formwork has to be deeper at these particular columns/footings. Due to the greater
depth needed, we simply provide you with a taller PFIä for these locations…and…Since
it is a permanent form – there is no form removal – there is no broken pipe! You will also
save money – because with wooden forms, there would have been a need for a thickened
slab edge around the block out.
Question: Do the lids come included free with the PFIä when I order for my job?
Answer: No, and the reason for this is simple – not everyone needs the lids. If lids are
not right for your project, we want to save you money.
Page 8 of 15
Question: What type of project has no need for the lids?
Answer: Any building that has the columns erected prior to placement of formwork for
the block outs. Obviously, many buildings have block out forms in-place prior to slab
casting – which is prior to column erection. Contractors using this construction sequence
can benefit from using the lids – in fact, it makes life easier for the concrete finishers (no
holes to avoid while troweling the concrete) and enables the project to comply with
OSHA’s “Open Pit Regulations”. Open column blockouts can be cause for some very
serious fines from OSHA – especially if columns are not yet erected!
Question: I am a Design Professional. Can you furnish me with drawings to
include in my contract documents?
Answer: Yes. We provide drawings on a daily basis for any of our products. We furnish
these in hard copy or electronic file formats. If you need a custom product, we will design
and detail the custom form and provide drawings in either format.
Question: My project requires 6” diameter pipe raceways (a.k.a. conduit) under
the slab and into the block outs – can these holes be cut in the field?
Answer: Yes, but why not let us do it for you? We have the specialized equipment to
cut and drill these holes exactly where you need them. We will be glad to quote a price
for this work to be included in your order!
Question: A friend of mine uses PFIä and tells me it is “just what the doctor
ordered” for use on his colored concrete projects. How can formwork help my
concrete coloring jobs go easier or be of better quality?
Answer: Integrally colored concrete has day-to-day variations in shade and density.
There is no good way to assure an integrally colored concrete slab will color match an
integrally colored concrete column block out that is cast days, weeks or months apart!
But, who said they had to be cast days, weeks or months apart? Normally, this would be
cause for a built-in problem, but with POCKET FORM ISOLATORä - all can be cast at
once in most case. Let’s face it – if an integrally colored concrete slab is cast prior to
steel or precast erection, what is going to keep the slab from getting damaged during
construction? After all, wouldn’t it stand to reason that colored concrete is going to be
exposed in the finished structure? That is why we recommend casting the slab and block
out voids simultaneously with concrete from the very same truck load!
Page 9 of 15
Question: One thing very frustrating to me with wooden block out forms is where
rains move-in and fill-up the voids with water, then the forms buckle and become a
nasty mess. What would happen if I used POCKET FORM ISOLATORä and the
same weather conditions moved-in? Would the forms buckle or delaminate?
Answer: PFIä is made of High Density PolyEthylene (HDPE). It will not take-on
water. It will never buckle or delaminate. For this reason, many contractors choose to
backfill around PFIä with gravel and then simply pump excess water from the open
voids. The gravel simply enables the region above the footing to become your “low spot”.
Question: Why does the size and rotation of the column base plate affect the size of
an isolation pocket? And…What if my base plate is not square?
Answer: At interior pocket cases where base plates are square and “concentric” around
the column’s centroid and not an edge situation, a simple formula applies. The formula
for sizing (plan dimensions) isolation pockets enclosing rectangular base plates is
2EC+BPD, where EC = Edge Clearance and where BPD = Base Plate Diagonal
(dimension). Since this formula works for all rectangular plate interior situations – it is
applicable to the majority of applications, however; for perimeter block outs much more
is sometimes necessary. There are several factors that govern the proper sizing of
perimeter block outs. Keep in mind that quite often perimeter base plates are eccentrically
positioned relative to the corners of a perimeter column isolation pocket. Since we
manufacture a wide variety of POCKET FORM ISOLATOR™ choices for consideration
– it is often best to email or fax a detail for us to consider – so we can recommend your
best option.
Question: At perimeter column isolation pockets, is there a means of connecting
the PFI™ to an adjacent wall, or edge form?
Answer: Yes! As a matter of fact, we manufacture numerous brackets to assist you in
making that connection very simple, yet solid. Many of our edge brackets are designed to
be easily attached to edge forms, also. This is another reason we really need to know your
particular situation – so we can send exactly what best fits your needs.
Page 10 of 15
Question: We try hard to get all of our footings cast to the exact elevation required,
but we always seem to have a couple that are off an inch or so. How can your
product deal with this concern?
Answer: This is one of our most commonly asked questions – and certainly a valid
point! We agree that getting all those footings cast to the exact elevations every time can
be extremely difficult. It is for this reason that we provide adjustable anchoring methods
for all of our POCKET FORM ISOLATOR™ units! For some configurations we can
even offer two methods of adjustable anchorage. We realized initially that we would
simply have to make hundreds of site visits and check with the field workers to learn
what methods work best. This has led us to adopt two quite successful solutions. By the
way – we are still interested in feedback in this area. We believe in learning as many
options as possible.
Question: I noticed that you only manufacture two sizes of Orange Safety Lids, a
24” square and a 28” square. What is your reasoning for this? And, how can I
provide lids for the other sizes of pockets that you manufacture?
Answer: With a field fabricated wooden cover made from either 2x lumber or layered
plywood. We provide an integral ledge on the interior face of every pocket we make.
This integral ledge is 1.5” down from the screed edge of all pockets. Obviously, for
several of our larger pocket sizes; a 1.5” thickness of wood would not carry heavy
enough loads and for this reason, we strongly recommend placing a temporary central
support under wooden lids.
Question: I noticed that some of your POCKET FORM ISOLATOR™ models have
only one piece [plastic form] per side, yet others may have as many as four pieces
per side. Why would your products be sold like this?
Answer: We initially began making pockets in two sizes – 24” square x 8” deep and
28” square x 12” deep. We then made a base extension part to increase the height of the
24” square pockets in 4” increments. We later added a base extension to increase the
height of our 28” square pockets in 12” increments. This 12” base extension can be
sawcut in our shop to enable the 28” square pockets to be 16” or 20” in height. We can
stack these base extensions under the basic units to build pockets in a wide variety of
height options.
Page 11 of 15
Question: I heard that your company is offering “segmental pockets”. What do you
mean by this?
Answer: The bulk of our sales are for 24” and 28” square pockets, but we are now
manufacturing segmental pockets in straight side lengths of up to 56.375”. What we call a
segmental pocket is where we have a single straight side of a pocket fabricated to exceed
28”. Our formwork has been engineered to have vertical as well as horizontal ribs. A
segment of our 24” x 8” or 28” x 12” basic part can be bolted through adjacent vertical
ribs to create a very sturdy greater length. These segmental sides can then be used with
segmental base extensions to manufacture pockets with individual side lengths up to
56.375” in triangular, square, pentagonal, hexagonal, heptagonal or octagonal shapes and
up to 48” tall. With the proper temporary bracing recommended, we can manufacture
segmental pockets of even greater lengths than shown above.
Question: Can your company manufacture POCKET FORM ISOLATOR™ in sizes
shorter than 24”.
Answer: Yes – we can do that in a manner similar to the answer above, except that
there would be only one short piece of form per side. We can make these in literally any
length, but sizes that align with the aforementioned vertical ribs are the most economical.
Question: What would you say if I told you I build and remove my wooden formed
24” square x 8” deep pockets for $15.00 each, and have never had any problems
with my way of doing these?
Answer: Well, if you are willing to do these for others, I’ll refer a lot of work to you.
Why not give us a call and we can discuss your method. I would be interested in hearing
from you.
Question: What would you say if I told you that I leave my wooden forms in place?
Answer: You are probably the best friend that a termite ever had.
Page 12 of 15
Question: My plans call for isolation pockets at the diagonal braces, how can I make
sure they will fit in the pockets?
Answer: Determine with slope and lengths shown on plan where the outermost point on
the bracing member lands at finished floor elevation – there should be a minimum of 3”
of concrete cover in our opinion. Please be mindful that many diagonal braces do not
originate from a footing centroid. If this cannot be easily verified, contact the project’s
structural engineer and have an assessment made. Often contract documents show one
single size of isolation pocket throughout an entire project, regardless of atypical cases.
Question: I need to provide future slab openings in a building. Can your product be
used for this?
Answer: Yes – in some cases POCKET FORM ISOLATOR™ can be your best choice
for this. This would have to be verified on a case-by-case basis.
Question: I need to install Refrigeration Line Pull Boxes on a project. The plans call
for sand fill up to 1.5” below FFE, then capped with a 1.5” slab flush with the floor.
Can your product be used for these installations?
Answer: Yes! We feel that an RLPB formed with POCKET FORM ISOLATOR™ is
second-to-none, we manufacture special bolt-on parts specifically for this application. We
are also glad to cut or drill the holes you need for pipeline penetrations. Just let us know
what you need and we’ll make it for you. Also, some sizes of our RLPB’s are available
with our Orange Safety Lids!
Question: Is it okay for my crews to use wooden wedges to elevate your plastic
forms?
Answer: If you only use the wooden wedges as a “tool” it is okay, but our
recommendation would be for you to use our shims & clips or our locking lift brackets –
either method will serve as a permanent part of your entire formwork. This is in strong
contrast to the wooden wedges – which often get overlooked and left in-place. The
problems with the wood are: invitation to termite intrusion, future deterioration and
reduction of concrete cover over base plates (since wood is more porous than concrete
and might occupy space adjacent to the base plate) due to water logging.
Page 13 of 15
Question: You said your products are made of plastic; I’m curious what kind of
plastic?
Answer: Our plastic products are made from High Density PolyEthylene, Structural
Foam. PFI™ is manufactured utilizing an injection molding process. Structural Foam is a
process of using less material than Dense Injection Molding, but getting an increase in
strength. Our particular specification for HDPE calls for 14.5% to 19.5% recycled
content – which not only benefits our customers with lower pricing, but is also friendly to
our environment. Instead of using trees (wood) to build column block outs, our products
enable you to place inert materials (non-biodegradable HDPE) into use – not using up our
valuable and limited land for landfills. We feel strongly that this is a manner in which our
company can take a stand for the cause of recycling. Think of plastic containers from
35mm film canisters, milk jugs and soda bottles all the way up to 55 gallon drums. These
products cannot ever be recycled back as food storage containers, etc. but they are perfect
in a concrete forming product.
Question: My construction company uses sawcut control joints in our slabs – placed
soon after the slab has been cast. If we use your forms, will it cause any problems
for the saw or the overall process?
Answer: No – it will not cause a problem. Cutting into the top corners of the PFI™ will
not cause any problems. This material is considerably softer than the concrete that you
are cutting. There have been no reports of saw blade gumming, either. I will recommend
that you temporarily remove the Orange Safety Lids immediately prior to making the
sawcut, so that over-cutting will not ruin your OSL’s. Replace the OSL’s immediately
after the saw is beyond the block out.
Question: It seems to me that your Orange Safety Lids will become lodged in the
PFI™ and be impossible or extremely difficult to remove. What is your
recommendation here?
Answer: Good question! We will agree with you that there appears to be a future
removal problem – but there is not. We have designed-in a special “mold-draft” and
ledge system, which simply never binds. OSL’s can easily be removed with our special
lifting tool – which is furnished in adequate quantities for any projects where OSL’s are
supplied. Even with built-up concrete paste, construction debris, etc. our OSL’s simply
never become lodged to the point of removal difficulty.
Page 14 of 15
Question: In an effort to ensure jobsite safety and to prevent premature removal of
the lids, is there any good way to [temporarily] “lock-in-place” your safety lids?
Answer: Simply place 2”-3” long wood screws in the joint between the grey and the
orange plastic parts. The wood screws will “self-drill” with minimal downward pressure,
the 2”-3” length will enable the wood screw to pass completely through the ledge, which
is an integral component of the grey PFI™. To increase the odds of preventing removal
that is unauthorized, consider using square drive or drive choices which discourage the
use of an ordinary screwdriver.
Question: My contract documents call for a “plinth” to be built atop of the
footings. I have never heard this term used. What is a “plinth”?
Answer: The word “plinth” is a term used to describe a column base that is atop of a
footing. We at Forrester Manufacturing Company typically refer to these as an integral
pier. Same meaning – just different words are chosen.
Page 15 of 15