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Ground slab design, presetressed, post tensioned design, crack free ground slabs
5/20/2018 Presentation- Ground Slabs-Post-Tensioned Prestressed Concrete
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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 reinforcedconcrete 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
Tension Tension
ft= Mc /I or M/Sb
How Prestressing Works
The Basics
Edge Lift Edge Lift
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ft= 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
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
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 fourdesign 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 forexpansive 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
Anythingthat appears out of the ordinary
Contact the geotechnical and structural engineerfor 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.
READthe 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 securelyattached 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 securelyattached 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 of3 concrete coverbetween tendon andblockout
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Plumbing Blockouts
Corner blockouts
can cause
concreting
difficulties. Rebar
solutions can
alleviate the
congestion and
prevent cornerfailures
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Anchorage Zone Penetrations
Penetrations through the anchorage zone
should be sleeved with schedule 40 pipe.
5/20/2018 Presentation- Ground Slabs-Post-Tension
