Behaviour of Anchors in PostTensioned High Strength Concrete Slabs

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Behaviour of Anchors in Post-Tensioned High
Strength Concrete Slabs
PhD Candidate Massoud Sofi
Supervisors Associate Professor Priyan
Mendis Dr Swee Mak (CSIRO)
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Outline

• Introduction
• Research Methodology
• Experimental and Analytical Work
• Work in Progress

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Behaviour of Anchors in P-T HSC Slabs
1. Introduction

• Post-tensioned concrete members are a form of
  prestressed concrete
• Steel tendons are stressed after concrete gains
  some strength
• Preferred construction method for long span
  slabs constructed in Australia and around the
  world

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Introduction (Continued)
Why Post-tension?

• Thinner slabs
• lighter structures
• Flexible column spacing
• Reduced cracking
• Low costs and availability

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Introduction (Continued)
Dead End Anchorage
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Introduction (Continued)
Live End Anchorage and surrounding reinforcement
Post-tensioning of a slab
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Introduction (Continued)
A typical post-tensioned transfer slab in
Melbourne Tendon Profiles
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Introduction (Continued)
The Problem?

• Failure of anchors (dead and live end)
• Results in repair costs
• Legal issues

Construction Company? Builder? Supplier? Is the
design adequate?
Canberra Failure shows grooves on the bottom of
the slab
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Introduction (Continued)
Criterion for application of Post-tensioning
fc 7 MPa, 25 P-T load at one day age fc
22 MPa, 75 P-T load at 3-7 days
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The Problem (continued)

• Concrete mix design
• Inadequate fc prediction of in-stiu concrete
• Ambient conditions (e.g., variable temperature)
• Anchor-concrete interaction
• Other critical early age properties

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2. Research Methodology

• Readymixed PT concrete mix - fixed

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Maturity
The STRENGTH of a given concrete mixture is a
function of its AGE and TEMPERATURE history. 
N.J. Carino, 1993
An example of concrete temperature over time
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Maturity (Continued)

• Correlation between strength and maturity values
• Less precise isothermal

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Methodology (continued)
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Methodology (continued)
- 10 slabs monitored in Melbourne - Field cured
cylinders
Compared with in-situ concrete slab strengths
using strength-maturity functions, only
approximately 62 of the field cured cylinder
strengths reached the target strength at 1 day
and 4 days -
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3. Experimental and Analytical Work

• - Simulate dead-end anchor failure mechanisms
  using beam-end specimen

- (ASTM A 944-99, 2000) describes the methods of
construction and testing for bond
Reaction 1
Reaction 2
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Experimental Work (continued)

• - Engineering property tests on the PT concrete
  mix, namely
• Compressive Strength
• Splitting Tensile
• Youngs Modulus
• Poissons Ratio
• Flexural Strength.

- Standard - Ambient
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Experimental Work (continued)

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Analytical Work
- Model beam-end specimen using FE program DIANA
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Analytical Work (continued)

• Make a geometry and mesh containing the concrete
  block and surrounding plywood
• The reinforcement is modelled using truss and
  interface elements (pullout)
• Potential flow boundaries convective type of
  heat transfer at the boundary (concrete-air)
• Ambient temperature is modelled as an external
  temperature of a boundary element

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Analytical Work (continued)
Phased Analysis

• Phase 1 early age concrete (potential flow) -
  hydration
• Phase 2 Linear static analysis Mechanical part
  of early age concrete (output crack and plastic
  strains)
• Phase 3 change the supports representing the
  plywood to those representing pullout supports
• Phase 4 Pullout tested using the same linear
  material properties used for mechanical part

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Analytical Work (continued)
Variable temperature shown across beam-end depth
Heat flow - stress staggered 3D
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4. Work in Progress and Concluding Remarks

• Completing and refining the model to use for
  different temperature scenarios, boundary
  conditions and geometries
• Further laboratory tests including TMC and
  pullout tests to compare with analytical
  prediction
• Use RAPT software to look at the macro-system
  behaviour
• Complex problem due to numerous variables