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Post-Tensioned
Prestressed Concrete
Post-Tensioned
Slab-on-Ground
Construction
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Reinforced vs Plain Concrete
It is important to recognize the difference between
reinforced and plain concrete when referring to post-
tensioned or conventionally reinforced concrete slabs-
on-ground. The American Concrete Institute (ACI)
defines reinforced concrete as concrete designed to
satisfy the minimum requirements of the code. The
category of plain concrete was developed by ACI to
refer to concrete used in ground supported construction
where loads are light, stresses are low, life safety
concerns are minimal or non-existent and the minimumrequirements for reinforced concrete are not necessary.
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Occupancy Loads
T e n s i o n T e n s i o n
f t = Mc /I or M/Sb
How Prestressing Works
The Basics
Edge Lift Edge Lift
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f t = M/Sb – (P/A)
CompressionForce Force
How Prestressing Works
For Slabs-on-Ground
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Advantages of Post-Tensioning
• Inherent compressive strength of concrete
• Less structural depth to achieve same design
strength
• Economical use of building materials
• Speed of construction
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• Single Family Residential
• Multi-Family Residential
• Commercial / Industrial
• Sports Courts
Applications of Post-TensioningSlab-on-Ground Foundations
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Post-Tension Steel
Tonnage Comparison
0
10000
20000
30000
4000050000
60000
70000
80000
1 9 9 4
1 9 9 5
1 9 9 6
1 9 9 7
1 9 9 8
1 9 9 9
2 0 0 0
2 0 0 1
2 0 0 2
2 0 0 3
Buildings SOG Bridges R&S Anchors
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Foundations TypesClassified by BRAB Report 33
In 1962, The Building Research Advisory
Board (BRAB) issued Report 33 which
classified residential foundations into four design categories based upon the degree
of severity of the supporting soil:
• Type I: Unreinforced
• Type II: Lightly Reinforced
• Type III: Reinforced and Stiffened
• Type IV: Structural
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The PTI design
procedure for expansive soil is
based on a ribbed
slab layout.
PTI Design Procedure
Slab-on-Ground Foundations
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The “key-word”
is Slab-on-GROUNDGROUND
• Slabs-on-Ground are designed to meet aspecific set of soil “parameters”.
• The performance of ANY foundation isdependent up on obtaining accurate soilinformation about the site.
• Expansive Soils
• Plasticity Index (PI) Greater than 15• Expansion Index (EI) Greater than 20
• Em, Ym, Bearing Capacity
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Site Inspection
Inspect site to look for unusual conditions
• Drainage ditches or low areas that hold water
• Trees that can influence soil moisture
• Anything that appears “out of the ordinary”
• Contact the geotechnical and structural engineer for recommendations
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Site Preparation
Strip Site of Organics & Trash
The site should be
initially stripped of all
surface vegetation and
other deleterious
material.
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Site Preparation
Recompact Scarified Surface Material
The exposed
subgrade shouldbe scarified and
recompacted.
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Site Preparation
Identify Voids & Recompact
Remove trees,
including theroot system
Proof roll the
site to identify
any loose soil
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Site Preparation
Drainage
Grade the lot for
positive drainageaway from the
foundation during
and after
construction.
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Site Preparation
Compaction tests should be performed on all fill material used
during the site development phase. The quality, as well as the
compaction, of all fill material should be documented.
Fill should exhibit low expansion properties, be free of organicsand other deleterious material, and be compatible with the
existing soil characteristics.
READ the General Notes sheet prepared by the structural
engineer. This sheet may contain special instructions about
specific site preparation requirements.
Contact the structural engineer should anything be unclear or in
question.
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Components of an
Unbonded PT System
• Unbonded Strand
• Anchorages
• Accessories
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Unbonded P-T Strand
Corrosion Protection "Grease"
High Density
PolyethyleneSheathing
P-T Coating
7 wire Steel
Strand
Unbonded strand consists of
three main components:
• Prestressing Steel
• PT Coating
• Sheathing
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Anchorage System
Anchor
2-pc Wedge
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Standard System
Fixed End Anchorage
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Standard System
Stress End Anchorage
Anchor Pocket Former
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Standard System Assembly
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Stressing Anchorage
Incorrect Correct
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Stressing Anchorage
• Replace Sheathing
• Tape
• 1 inch Rule
Tendon Sheathing Repair
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Installation
• Follow approved engineers drawings
• Anchors are placed 6 inches from edge
• Anchors are placed at T/2 or 3” below top of slab
• Anchors are securely attached to edge forms
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Installation
• Follow approved engineers drawings
• Anchors are placed 6 inches from edge
• Anchors are placed at T/2 or 3” below top of slab
• Anchors are securely attached to edge forms
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Installation
Horizontal transitions
are smooth, clear all
openings by 3 inches
and are straight past
the opening
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Chairs are placed
and tied at each
tendon intersection atless than 3’-6”
centers
Installation
Chair Placement
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Installation
Chair Placement
IncorrectCorrect
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Installation
Draped tendons are used in some designs to
provide compression in the bottom of beams
resisting tensile stresses caused from edge lift.
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Installation
Drops in Slabs
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Plumbing Blockouts
• Deflect tendons toavoid blockoutsthrough the slab.
• Maintain minimum of 3” concrete cover between tendon andblockout
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Plumbing Blockouts
Corner blockouts
can cause
concreting
difficulties. Rebar
solutions can
alleviate the
congestion and
prevent corner failures
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Anchorage Zone Penetrations
Penetrations through the anchorage zone
should be sleeved with schedule 40 pipe.
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Reentrant Corners
Initial curing tensile
stresses build-up at
reentrant (inside)corners causing
cracking to occur.
Rebar is typically
installed at these
locations.
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Stressing
• Initial Concrete Strength (2,000 psi MIN)
• Initial Marking (Elongations)
• Calibration of Jack/Gauge
• Stressing Force vs. Gauge Pressure
• Final Elongation Measurement
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Cutting Stressing Tails
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Finishing Stressing Recess
• Clean the pocket former recess of any
dirt, grit, oil or other material that will
prevent the grout from bonding to the
concrete
• Grout pocket former recess with non-
shrink cement, sand and water mix that
will reach the minimum compressive
strength of the concrete slab.
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Inspections
Inspections should be conducted to insure
the quality of the construction.
• Pre-pour: Installation of P-T and rebar
• Pour: Placement of Concrete
• Stressing: Tensioning of P-T Tendons
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Pre-Pour Inspection
• Check the tendon and bar count
• Check the placement of the fixed-end anchors for
the required cover over the end of the steel
• Check to be sure that the stressing ends are
securely nailed to the forms (2 nails) and that the
tendon is perpendicular to the anchor
• Check to be sure that you have adequate
concrete cover over all of the steel
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Pour Inspection
• Do not add excessive water at the site
• Place the concrete in one continuous
operation - NO COLD JOINTS
• Consolidate (vibrate) the concrete around the
P-T anchors, especially in the corners where
multiple anchors are located.
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Pour Inspection
DO NOT displace or walk on any of the reinforcement
- P-T or rebar
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Post-Concrete Placement
• Notify the P-T stressing company the day after
the concrete is placed to schedule stressing
• Remove the forms - ALL OF THEM. Do thiswithin 3 days after the concrete is placed
• Do NOT damage stressing tails in rough grade
• Do NOT cover tendon tails with dirt or lumber
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Trouble-Shooting
• Honeycombs
• Low strength and poor quality concrete
• Plumbing mistakes
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Trouble-Shooting
Correct Incorrect
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Foundation Maintenance
• Do not alter the drainage pattern of the site
• Provide a minimum of 3%-5% of slope away fromthe foundation with the first 5 feet
• Roof drains should not dischargewater at the perimeter of thefoundation
• Do not plant trees within the tree’s drip line
• Do not cut off natural moisture around thefoundation by constructing decks or pools
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The long-term performance of any
slab-on-ground foundation is dependent upon good drainage and a moisture
maintenance program by the property
owner.
Performance
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PTI Certification Program
• Consistency of material quality
• Quality of extrusion and fabrication process
• Traceability of components
• Stressing equipment calibration
• Company commitment to the long termperformance of the system
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Construction & Maintenance
More information concerning theconstruction & maintenance of post-tensioned slabs-on-ground, can be found ina manual published by the Post-Tensioning Institute.
The 3rd Editions is currently being reviewed by committee and will be available in thenext few months.
602-870-7540
www.post-tensioning.org
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Design Procedures & Code
602-870-7540
www.post-tensioning.org
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