Validation and Usage of Digital Image Correlation for High Temperature Deformation Measurement in Modern Prestressing Steel

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1 Validation and Usage of Digital Image Correlation for High Temperature Deformation Measurement in Modern Prestressing Steel John Gales* PhD candidate Supervisor: Dr Luke Bisby 2nd Supervisor: Dr Martin Gillie Tech. Supervisor Dr Tim Stratford *Speaker

2 What is Prestressing Steel (PS)? Conventional steel rebar Unbonded PS steel Advantages of post-tensioning concrete with PS steel - Rapid construction - Shallow floors (high ceilings) Highly - Increased span lengths Optimized - Reduces building materials

3 UPT Building Optimization Current guidance is dated and has not kept up with modern optimization trends Today s flat-slab post-tensioned buildings, for example, with columns spaced (12 m) on center and span-depth ratios of 40 are more complex and require more engineering attention than typical flat-slab buildings of 40 years ago, with columns spaced at (6 m) on center and span-depth ratios of 20. -Randall Poston (chair ACI 318)

4 Real UPT Slab Behaviour in Fire is Unknown UPT optimization increases susceptibility to fire: - Prestressing steel more sensitive to high temperature than mild steel - Spalling of concrete cover (HS concrete, precompression can contribute to this) - Tendons run continuous, local damage effects the entire floor Only standard furnace tests of simple span slabs are available: - modern construction?, building materials?, real fires? 2008 standard fire test (Kelly and Purkiss)

5 Post and Korman, Engineering News Record June 12 th 2000 Can standard fire tests predict true fire behaviour of these UPT buildings? Key Biscayne, USA, 2000: UPT tendons continuous across 7 interior bays (> 50m) Localized fire on 2 nd floor spread vertically to 7 th floor over 2 bays triggered progressive failure of the UPT slab well beyond the zone of visible damage Columns UPT slabs Tendons released by fire Visible fire damage

6 Localized Fire Damage Localized fires may be due to spalling, travelling, ceiling jets Queens university tests in 2009 showed that tendon rupture is more probable under localized heating Complex stress relaxation / strength interaction - influenced by a permanent time dependent damage called, creep Localized UPT tendon tests (strong back tests) conducted at Queens university in Canada

7 Creep strain (ecr) Primary Creep Secondary Creep Tertiary Creep High Temperature Creep Creep is a time, temperature and load dependent deformation Possible to express creep through combined temperature time variable in a heated tensile test at constant load Dorn-Harmathy creep model Creep parameters for PS steel are from dated (1970) and different material compositions than modern counterparts- Errors in creep modelling? ecr,0 Δθ Δecr Secondary Creep rate=z= Δecr Δθ Temperature compensated time (θ) Temperature Compensated Time t 0 e H / RT dt Creep in Prestressing Steel e cr ecr cosh ln 2 (2 e Z 0 cr,0 )

8 Predicting Creep for Stress Relaxation? Typical Queens university high temperature stress relaxation test Regimes Transient heating Steady state hold Cooling Over estimation of (creep) May be due to parameter extrapolation, older steel derivations, inaccuracies in derivation

9 How Can We Derive Modern Creep Parameters?: Digital Image Correlation (DIC) in transient and steady state uniaxial tensile tests to measure deformation

10 What is Digital Image Correlation? Two photos showing deformation from a uniaxial ambient strength test Previously validated Image correlation algorithm (by Dr Andy Take, Queens) are used to measure virtual deformation (strain) through a series of sequenced test photos

11 Stress (MPa) Strain (mm/mm) Elastic range Plastic range DIC Validation (1 of 2) Uniaxial ambient strength test with comparison of DIC and bonded foil strain gauges MSG All strain gauges debonded MSG MSG PIV strain MSG1 MSG2 MSG3 PIV strain Strain Time (s)

12 Thermal strain (mm/mm) DIC Validation (2 of 2) Uniaxial transient thermal expansion test (unloaded) with comparison of DIC to theoretical calculation(ec2) EC2 mild steel DIC mild steel EC2 PS steel DIC PS steel Temperature ( C)

13 Creep strain (ecr) Primary Creep Secondary Creep Tertiary Creep Steady and Transient State Creep Tests Harmathy-Dorn Creep Model Steady and transient state testing are suppose to be equivalent in the Harmathy Dorn creep model based on activation energy ecr,0 Δθ Δecr Secondary Creep rate=z= Δecr Δθ Temperature compensated time (θ) Is this really valid and to what extent? Temperature Compensated Time t 0 e H / RT dt Creep in Prestressing Steel e cr ecr cosh ln 2 (2 e Z 0 cr,0 )

14 Total creep strain (ecr) ecr ecr Creep Test Equivalency Steady state and transient state test at 700 MPa Transient Steady State E E-20 θ(hr) θ (hr) Steady State Steady State Harmathy Prediction Transient Transient Harmathy Equation Prediction 0 5E-20 1E E-19 2E E-19 3E E-19 4E-19 Temperature compensated time (θ) (hr)

15 Total creep strain (ecr) Total creep strain (ecr) Varying Stress and Repeatability MPa 690 Mpa Steady State (427C) 690 Mpa 2C/min 800 Mpa 2c/min All tests 1000MPa 2C/min 2C/min MPA 2c/min (Test 1) 1000 Mpa 2c/min (Test 2) MPa 1000MPa 10c/min C/min MPA 2c/min (Test 1) MPa Mpa 2c/min (Test 2) MPa 10c/min E-20 1E E-19 2E E-19 3E E-19 4E-19 Temperature compensated time (θ) Varied stress levels E-21 1E E-20 2E E-20 Temperature Compensated Time (θ) Repeatability

16 Parameter Updates Secondary creep parameter updates (old vs new) New creep parameters do not match old, suggest less creep deformation in modern PS steel

17 Preliminary Modelling with New Parameters New creep parameters are conservative end

18 Key Insights: DIC deformation measurement is adequate when shown with traditional strain prediction and instrumentation Less limitations for usage (SGs fall off, extensometers can break in the tertiary phase of creep) New creep model params. more accurate for high temperature PS stress relaxation (creep) predictions More repeat testing needed at both transient and steady state in order to truly understand and quantify variability and validation using the Dorn-Harmathy creep model for prestressing steel

19 Impacts of Research Modern UPT concrete techniques continue to promote innovation in construction; however... - These innovations potentially sacrifice safety using out of date prescriptive testing and guidance - Research being conducted will allow us to rationally design for real fires in real buildings One Museum Park

20 Thank you The Ove Arup Foundation For additional information Web: