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SECTION 1
INTRODUCTION TO POST-TENSIONED
CONCRETE
DEVELOPED BY THE PTI EDC-130 EDUCATION COMMITTEE
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NOTE: MOMENT DIAGRAM CONVENTION
• In PT design, it is preferable to draw moment diagrams to the tensile face of the concrete section. The tensile face indicates what portion of the beam requires reinforcing for strength.
• When moment is drawn on the tension side, the diagram matches the general drape of the tendons. The tendons change their vertical location in the beam to follow the tensile moment diagram. Strands are at the top of the beam over the support and near the bottom at mid span.
• For convenience, the following slides contain moment diagrams drawn on both the tensile and compressive face, denoted by (T) and (C), in the lower left hand corner. Please delete the slides to suit the presenter's convention.
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REVIEW: FUNDAMENTALS OF PRESTRESSED CONCRETE
NEW: DIFFERENCES BETWEEN PRE-TENSIONING AND POST-TENSIONING
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REVIEW OF REINFORCED CONCRETE
Critical Point for Cracking
Stages of BehaviorUncracked Cracked (~Elastic) Ultimate
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REVIEW OF REINFORCED CONCRETE
Mcr
My
Mn
Mom
ent
Curvature
Large deflections due to cracking
Steel is not engaged until after cracking
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REVIEW OF REINFORCED CONCRETE
Reinforcement is PASSIVE
Steel crosses cracks, but does not prevent them
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QUESTION TO PONDERSuppose a R/C beam has too much cracking and too much deflection. How might you propose to fix it? (i.e. not replace it)
Tension (bending) + Compression (“squeezing”) =
Net Zero Stress“Sqeezed” Before Loading (Pre-compressed):
Pre-Compression (“prestressing”) + Tension (bending) =
Net Zero Stress
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HOW TO BUILD IT?
Prestressing: Concrete pre-compressed before loading in bending (flexural tension)
1. Pre-Tensioning: Steel tensioned beforeconcrete is placed
2. Post-Tensioning: Steel tensioned afterconcrete is hardened
Prestressing is ACTIVE – can prevent cracks from forming
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PRE-TENSIONING
1. Tension Strands
2. Cast Concrete – Bond strands to concrete
3. Cut Strands – Transfer force to concrete
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POST-TENSIONING
1. Cast Concrete with Duct
2. Feed Strands through Duct
3. Tension Strands
4. Grout Duct (or other corrosion protection)
Section
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POST-TENSIONING• Post-tensioning can take on any profile
• Draped configurations are much more common than straight tendons• Why?
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PRE-TENSIONING
Post-Tensioning
Force Transfer by Steel-Concrete bond
Force Transfer at end anchor
Strain Compatibility and Force Equilibrium:
Steel held at length longer than it “wants” to be: Tension
Concrete compressed shorter than it “wants” to be: Compression
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Pre-Tensioned elements are often precast in a factory and shipped to the site
Post-Tensioned elements can be cast and tensioned in the final location (cast-in-place). They can also be precast.
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PRE-TENSIONINGINSTALL PRESTRESSING STRANDS
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PRE-TENSIONINGTENSION STRANDS
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PRE-TENSIONINGSTRANDS AFTER TENSIONING
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PRE-TENSIONINGINSTALL MILD REINFORCEMENT
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PRE-TENSIONINGINSTALL INSERTS AND ASSEMBLIES
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PRE-TENSIONINGSET FORM SIDES
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PRE-TENSIONINGPLACE CONCRETE
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PRE-TENSIONINGCURE CONCRETE WITH ACCELERATED METHODS
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PRE-TENSIONINGREMOVE GIRDER FROM CASTING BED
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PRE-TENSIONINGMOVE GIRDER TO STORAGE
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PRE-TENSIONINGTRANSPORT TO JOBSITE
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PRE-TENSIONINGGIRDERS IN FINISHED STRUCTURE
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POST-TENSIONINGDucts for Post-Tensioning
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POST-TENSIONING
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POST-TENSIONING
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POST-TENSIONING
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POST-TENSIONING
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POST-TENSIONING
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POST-TENSIONING
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POST-TENSIONINGStressing Strands:
Multiple Strands: Multistrand
Single Strand: Monostrand
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HOW ARE STRANDS ANCHORED?
Cast against concrete at end of beam
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HOW ARE STRANDS ANCHORED?
Concrete
Duct
Strand
Anchor cast in concrete
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POST-TENSIONING:Bonded System(at high point)
Unbonded System
Grout“PT Coating” (grease)
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GROUTING POST-TENSIONED SYSTEMS
Grout In
Vent
Vent
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POST-TENSIONING
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STRUCTURAL EFFECT OF PRESTRESSING
True for Pre- and Post-Tensioning
Pre-Stressing
+
Applied Load
=
T C T C
Total Stress
T C
Stress Limits
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STRUCTURAL EFFECT OF PRESTRESSING
True for Pre- and Post-Tensioning
Pre-Stressing
+
Applied Load
=
T C T C
Total Stress
T C
+
T C
Service
Transfer
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ECCENTRIC PRESTRESSINGEccentricity in prestressing:
- Desirable at midspan- Not productive, even detrimental, at end of span
Strategies for pre-tensioned systems:- Draped / harped profiles
Temporarily held in place before concrete is hardened
- DebondingNot all strands are active at end of span
Strategies for post-tensioned systems:- Install ducts in desired profile
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COMMON CONFIGURATIONSPre-tensioning:
Draped
Debonded
Post-tensioning:
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PROBLEM FOR THOUGHT…Where should the prestressing be placed?
Tension
Tension
Mom
ent D
iagr
am
(T)
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PROBLEM FOR THOUGHT…Where should the prestressing be placed?
Tension
Tension
Mom
ent D
iagr
am
Option 1
Good:•Efficient at midspan•Easy to construct
Bad:•Counter-productive over support(T)
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PROBLEM FOR THOUGHT…Where should the prestressing be placed?
Tension
Tension
Mom
ent D
iagr
am
Option 2
Good:•Efficient over support•Easy to construct
Bad:•Counter-productive at midspan(T)
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PROBLEM FOR THOUGHT…Where should the prestressing be placed?
Tension
Tension
Mom
ent D
iagr
am
Option 3
Good:•Efficient over support•Efficient at midspan
Bad:•Difficult to construct
(T)
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PROBLEM FOR THOUGHT…Where should the prestressing be placed?
Tension
Tension
Mom
ent D
iagr
am
Option 4
Requires post-tensioning; very difficult to achieve by pre-tensioning
No net eccentricity
No net eccentricity
(T)
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SUMMARY: PRESTRESSED CONCRETE
Efficient use of materials – concrete maintained in compression, crack control
Smaller deflections/thinner members Longer spans Corrosion resistance Less material; reduced environmental impact