Estimating Systems for Homes1

Post-Tension Slab
 A monolithic slab is a slab or a foundation that is supported all at once.
That is, to have the footing and the slab area of the foundation put together.
 That's why the forms need only be on the exterior. Note how clean the
footings are; how clean the slab portion of the foundation is
 In the preceding slide, note that all the pipes are wrapped with foam.
 This is a contractor that knows his business; he keeps a very tidy site; keeps all the
debris out of the footing. Why it is so important that you don't have debris
in the footing; what's the big deal?
 Well, if you have rocks, or boulders, or large pieces of dirt in the footing,
that takes the place of concrete that should be there, and you don't get the
foundation that you're supposed to have, according to your plans.
 Note that all the cables are up off the subgrade on chairs, and
all the pipes are wrapped with foam, you have your tub boxes back there,
the anchors are already there, there's no debris anywhere on the pre-slab,
there's no debris in the footings, and the pre-slab is ready to go, as a matter
of fact, an inspector was there, and passed it.
 What you're going to see, this is a foundation that they're getting started
on, just the form boards, but what you're going to see now, is, you can see
the anchors, you get the concrete poured, and those anchors are set in the
concrete.
 Cables are actually in a greased sleeve, so after the concrete sets, for several days,
they come in with the machine on the other side, they hook onto the cable on each side,
and they ratchet up to tens of thousands of pounds per square inch and it actually adds lift
and strength to the slab; and that's what the post tension is, then they clamp it off and
you're all done.

Estimating Post-Tension
There are four major categories of a post-tension slab-on-grade
estimate. Each of these categories requires a different type of
method of measurement. These categories are:
1. • Grade beam (aka“footing”) excavation
2. • Formwork
3. • Post Tension Reinforcement
4. • Concrete
The standard measurement for grade beam excavation
(trenching) is cubic yards (c/yd). The formula for calculating
c/yds of concrete is:
1. (L x W x D) / 27
2. L = Length of Beam
3. W = Width of Beam
4. D = Depth of Beam
The standard measurement of concrete formwork is square feet
of contact area (SFCA).

Estimating Post-Tension, cont.
! The standard measurement of concrete formwork is square feet of contact
area (SFCA).
! This is the way that labor is applied to formwork. If formwork is to be built
out of wood, then the quantity needs to be converted to board foot (BF) for
pricing.
! The formula for calculating SFCA is:
1. L x H
2. L = Length of Forms
3. H = Height of Forms
! To convert to BF take SFCA x 2.85
! The standard measurement for post-tension tendons is linear footage (lf),
and then converted to pounds.
! The formula for calculating weight of cables required is
(count x length x .62)
1. where count = count of tendons of a specified length, and
2. .62 is the weight of tendon assembly, including the
sheathing and anchors.

Estimating Post-Tension,
cont.
The standard measurement for concrete
is cubic yard (c/yd). The
formula for calculating c/yds is:
1. (l x w x t) / 27, where:
2. l = length in feet
3. w = nominal width feet
4. t = nominal thickness in inches

• In warmer climates, the use of admixtures can be used to slow
down the hydration process of the concrete.
• Water-reducing agents are helpful if they do not interfere with
the strength of the concrete.
• Several factors influence the rate of evaporation and thus the
strength of the concrete.
• These factors are concrete temperature, air temperature,
relative humidity and wind velocity.
• These conditions should be monitored and recorded during the
placement of concrete during hot weather.
• All of these factors should be considered in estimating post-
tension slab-on-grade foundations in adverse climate conditions.

I. OVERVIEW OF LABOR, MATERIAL, EQUIPMENT,
INDIRECT COST
There are four basic types of post-tensioned systems:
Type I – Un-reinforced
Type II – Lightly reinforced against shrinkage and temperature
cracking
Type III – Reinforced and stiffened.
Type IV – Structural (elevated).
This report will cover Type II post-tension slab-on-grade
systems with un-bonded tendons. The tendons discussed in
the report will be seven wire ½-inch tendons with a capacity of 270
kip per square inch (kips).

Soil Investigation Report:
Prior to preparation of an estimate on post-tension slab-on-grade, it is
important to get a clear understanding of the soil investigation report.
Most sites will have a minimum of one boring done for each building. All
boring will be a minimum of fifteen (15) feet unless un-weathered rock or shale
is encountered at a lesser depth. This report will give you the
following information:
1. Types of soil in the area – If clay materials are found in the area, they will be of
three types, Kaolinite, Illite or Montmorillonite in the order of their shrink-swell
potential from most to least.
2. The presence and type of rock found in the area - If rock is encountered in the
area, it will be one of three characteristics: soft, medium or hard.
Presence of high levels of water-soluble sulfate and chloride ion – If high levels of water-
soluble sulfate or chloride ion is found in the soil, the post-tension system will require
use of encapsulated tendons to reduce the risk of corrosion of the post-tension tendon
assembly.

Estimating Post-Tension
Formwork
The first item that is considered in estimating the cost of post-tension slabs is
the type of formwork that needs to be used. If the elevation of the slab is less
than 12 inches, then dimensional lumber may be used.
However, if it is over 12 inches, it will be more cost-effective to use
prefabricated steel or fiberglass forms. The use of plywood forms is not
recommended in the construction of post-tension slabs due to the flex of the
plywood material during the stressing operation of the tendon.
To calculate the square foot of contact area (SFCA) of forms, the length
of the perimeter of the foundation is multiplied by the height of the forms. If
dimensional lumber is to be used for formwork, the square foot of contact
area (SFCA) of the forms needs to be converted into board feet (BF). As a
rule of thumb in the industry, there is 2.85 BF of lumber in every SFCA of
forms.
Remember? – (LxH)(SFCA)(2.85) = BF

Example – Forming of Slab-on-Grade.
Slab is 50 ft x 100 ft in size.
Top of slab will need to be eight (8) inches above finish grade
of building.

GRADE BEAM EXCAVATION
 Grade beams (“footings”) are used to transfer the load of slab-on-
grade foundations to stable soil.
 Most Post-Tension slabs will have grade beams similar to the two
shown in figure 1.1 and 1.2.
 The exterior grade beams are typically 10 to 12 inches wide and
18 to 24 inches deep.
 The interior grade beams are typically 12 inches wide and 18 to
24 inches deep. Some post-tension slab-on-grade foundations do not
have interior beams (Those 6” or thicker).
 Interior beams, if required are usually located under load bearing
walls.
 Post-tension slab-on-grade that do not have any interior beams
are typically thicker than four inches and use a series of bounded
cables grouped together to form internal beams in the thickness of
the slab.
 It is more economical to pour a thicker slab than excavate and
pour the interior beams.

Grade Beam
Calculation
 To calculate the excavation of a grade beam, the length of each type
of grade beam is multiplied by the width and depth of the grade
beams.
 This will give the cubic feet (CFT) of material to be removed.
 To convert the cubic feet (CFT) quantity to cubic yard (CYDS)
divide the quantity by 27.
Example – Excavation of grade beams.
o Exterior grade beams – 10 inches wide x 18 inches deep.
o Interior grade beams – 12 inches wide x 18 inches deep
Slab thickness will be 4 inches.

VAPOR BARRIER
 The vapor barrier is placed between the gravel and the slab, and
is usually included in the concrete takeoff.
 The vapor barrier has two uses in a post-tension slab-on-grade.
 First, it serves to keep the moisture in the concrete after placement to
ensure proper curing; and
 Second, it keeps moisture out of the building after it has been
constructed.
 The vapor barrier material is typically polyethylene plastic,
usually 4 to 6 millimeter (mm).
 It usually is purchased in widths of four to 20 feet and
lengths of 100 feet.

Post-Tension
Tendons
POST TENSION TENDONS
 In post-tension systems the “tendon” is defined as a complete assembly consisting of the anchorages, the
prestressing strand, the sheathing and corrosion-inhibiting coating or grease that surround the prestressing steel.
 There are two types of Post-Tensioning:
• Bonded – Tendons that are bonded to concrete through use of grout, which is injected after the stressing operation
of the cable takes place. This type of system is very uncommon in
residential or multi-family construction due to the high cost of grouting the large amount of
smaller tendons.
• Unbonded – Tendons are not grouted or bonded to the concrete.
 Post Tension Tendons for slab-on-grade construction are typically seven wire, half-inch tendon, which means the
tendon is constructed on seven (7) wires of steel cable for a total of a half (1/2) inch diameter.
 The amount of prestressing force applied per tendon is a function of the size of the tendon.
 There are three typical sizes of strand tendons used in posttension slab-on-grade construction.
They are:
• Stressing End Anchor (SE) – This is the end by which the stressing operation will take place.
• Dead End Anchor (DE) – This is the anchor located at the opposite end of the stressing end.
• Intermediate Anchorage (IE) – The maximum length to stress a post-tension tendon from one
direction is 100 ft. If the cable is over 100 feet, then an intermediate anchor shall be placed between
the two stressing ends of the tendon.

Post-Tension Tendon Take-off
 Post-tension tendons are typically taken off by the
linear foot of cable and then converted to pounds of
cable by multiplying them by .62 which represents
the weight of the entire assembly of the tendon.
 When measuring the stressing end of the tendon,
two (2) feet must be added to the length to allow
for excess cable for the stressing operation.
 At the completion of the stressing operation, the
excess is cut off and grouted at the cut-off for the
protection of post-tension tendons.
 This assembly includes all the anchor devices that
are required to complete the system.

BACKUP STEEL
 With any Post-Tension System, there is a requirement for a minimum amount of
bonded steel to hold the anchor ends in place.
 Two #4 continuous rebar is required behind all posttension anchors to hold them in
place.
 There are also requirements of some tensile steel in the slab to strengthen the
tensile strength of the concrete prior to the stressing of the tendons.
 Stakes are required in the beams to hold the tendons in place prior to the
placement of the concrete. A post-tension supplier does not usually furnish backup
steel. However, the required tonnage of backup steel for a post-tension slab can be
calculated by multiplying the square footage (SF) of the slab by .20.
CONCRETE PLACEMENT, FINISHING AND CURING
 The minimum 28-day strength of concrete for post-tension slabon-grade
construction is typically 2,500 pounds per square inch (PSI) for single-family
residence and 3,000 pounds per square inch (PSI) for multi-family construction.
 It is important to have the mix design approved by the Engineer on record prior to
placement of the concrete to ensure it meets all the required specifications.
 Calcium chloride or admixtures that contain calcium chloride should never be used
for
post-tension construction due to the corrosion it causes on the steel tendons.
 Concrete volume calculations are based on cubic yards. When figuring the depth of
the beam, the thickness of the slab should be subtracted from the overall depth of

Estimating Post-Tension Concrete
(revisited)
The volume of the grade beam should
be divided by 27 to convert it to cubic
yards.
The volume of slabs is found by
multiplying the area of the slab (sf) by
the depth of the slab (inches).
The volume of the concrete in the slab
should be divided by 27, which
converts the measurement to cubic
yards (CYDS).

Division 3 - Concrete
Concrete Forms
Form in Place Plywood,CDX, 3/4", 4'x8' sheets,
SFCA X H X 2.85
BF 375 $0.95 $356.40
16d Duplex Framing Nails 50 lb. Box 5 $74.81 $374.05
Concrete Accessories
Steel Tension Cables Sq. Ft. 4600 $0.70 $3,220.00
Anchor Bolts, SSTB Concrete Anchors, 5/8" x 17" (SSTB16) Each 45 $4.99 $224.55
Sill Sealer Linear Ft. 260 $0.22 $57.20
3" Combo Rebar Chairs Each 450 $0.27 $121.50
Deck-O-Drain
Polyethylene plastic, 6 mm, 20'W, 100' rolls Each 4 $124.00 $496.00
Structural Concrete
House Slab and Footings, (Monolithic 3000 PSI), 6 Sack
Mix, pumped, trailer mounted, 30 CY/hour
CY 80 $116.00 $9,280.00
Exterior Concrete
Driveway Apron 6" Thick, City Walk 6" Thick across
Driveway
CY 9 $105.00 $945.00
Concrete Finishing
All Surfaces to be Broomed (Slabs/Driveway/Apron/City
Walk)
Concrete Curing
Curing, with sprayed membrane curing compound
Reinforcing Steel (Entire House)
Mechanical Compaction Inspection
Hold Down Inspection
Rebar Ties 6" precut ties (1000 per Box)
$15,074.70
03 38 00 - Post-Tension Concrete Forms and
Accessories
Ready-Mix Concrete
Concrete & Accessories, Total
This is simply the estimate
of materials, before labor
cost.

 Concrete curing is the process of maintaining proper concrete moisture content
and concrete temperature long enough to allow for hydration of the concrete.
 Concrete characteristics such as durability, strength and water tightness can be
obtained through the proper curing methods of a post-tension slab-on-grade.
 Prior to estimating the cost of a post-tension slab-on-grade, the means used to cure
the concrete will need to be decided. Most of the time, the specifications will
address the desired means of curing the concrete.
 There are several methods for curing concrete. These methods are:
• Curing with water
• Ponding – The use of water to cover the slab during the hydration process. The slab
is actually flooded with water using dams at the perimeter to keep the water in.
This is the most common curing process.
• Spraying – The use of steady fine spraying of water during the hydration process.
• Wet burlap – The use of wet burlap sheets to cover the slab during the hydration
process.
• Curing with barriers
• Liquid Membrane compounds – A membrane-forming compound that is sprayed
sprayed onto the concrete to form a chemical barrier to prevent loss of moisture from
the concrete
• Polyethylene film – The use of polyethylene film to cover the slab to keep the
moisture in the concrete during the hydration process.

Estimating HVAC (In a Nutshell)
1 Locate the scale of the mechanical engineering sheet of the building plans and the corresponding
scale factor on the architect scale; the scale of the drawing is most commonly located under the title of the
sheet.
2 Measure and mark the total length of mechanical duct lines (refer to the sheets key for line types)
and count the total number of turns the duct work makes, as turns in the duct work require the use of
either flex duct or angle duct material. Depending on the project, there may be more than one type of
duct (width or diameter) used on the project; in this case, make a list of the different duct sizes and mark
the total length and number of turns for each. The size of the duct will be noted on the sheet and will be
tagged to the specific duct area. Refer to the mechanical engineering sheet for specific product
information, if applicable.
3 Make a tallied list of all mechanical equipment; this category applies to fans, air handlers, heating
units and air-conditioning units. The specifications for this equipment will be noted clearly either next to
the individual units or on the key notes section of the sheet. The size (or capacity), model and manufacturer
of the units should be included on the list. Refer to the mechanical engineering sheet for specific product
information, if applicable.
4 Make a tallied list of all supplementary mechanical equipment. This category applies to
thermostats, vent grills and equipment mounting materials (may differ depending on local building code
and specifications). Refer to the mechanical engineering sheet for specific product information, if
applicable.
5 Call a local HVAC building supply center and quote the current unit cost of listed items found in
Steps 2 through 4.
6 Multiply the current unit cost of each item by the number of items needed to find the total cost of
materials for the project. For ventilation ducts, multiply the unit cost per foot by the total number of feet
measured in Step 2.

Checklist for Electrical Estimate
1. Service Line to House
2. Hook-up for temporary electrical service
3. Labor and material for rough-in (wiring, outlets, boxes
boxes and plates, boxes for fixtures, switches, connectors,
connectors, entrance panel & circuit breakers)
4. Light fixtures
5. Installation of light fixtures
6. Hook-up for appliances
7. Hook-up for HVAC
8. Hook-up & installation of special equipment - (This
would be fire suppression & detection, for example, &
& green alternatives)
9. Telephone boxes & service to house
10.Television boxes & service to house (if required)

Checklist for Plumbing Estimate
1. Cost of water supply line to the house, including the trench for the water pipe

References
 HOW TO ESTIMATE THE COST OF A POST-TENSION
SLAB-ON-GRADE
By: Frank Haas
DATE WRITTEN: MAY OF 2005
 Residential Zoned Ducted HVAC Systems
 2013 California Building Energy Efficiency
Standards
 California Utilities Statewide Codes and
Standards Team September 2011

Estimating Systems for Homes1

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