Evaluation of Corrosion Damage in Unbonded Post-Tensioned Concrete Structures:

Transcription

1 Evaluation of Corrosion Damage in Unbonded Post-Tensioned Concrete Structures: Misunderstandings, Misinformation and Moving Forward by Keith Kesner Randall W. Poston

2 Introduction 16 Years Ago Corrosion damage in unbonded post-tensioned concrete structures Today Myths, misconceptions and truths Corrosion damage in unbonded post-tensioned concrete structures Misinformation Misunderstandings Moving forward

3 Misinformation No guidelines for assessment not true State of practice guidelines ACI 423.4R Corrosion and Repair of Unbonded Single Strand Tendons ACI 222.2R Corrosion of Prestressing Steels ICRI Guide for the Evaluation of Posttensioned Concrete Structures ICRI Guide for the Repair of Unbonded Posttensioned Concrete Structures ASTM G Standard Guide for Examination and Evaluation of Pitting Corrosion

4 Misunderstandings All unbonded PT has problems not true Systems have improved Standards and awareness has improved

5 Misunderstandings Corrosion damage is a terminal flaw not true Evaluation of damage is possible Repaired structures can have indefinite service lives Address corrosion damage Stop water ingress Monitor and repair as needed

6 Moving Forward Methods to assess structures with damage Understand root causes Determine extent of damage Prevent future damage Corrosion prevention Repair details

7 Moving Forward Assessment of extent of damage Mild reinforcing steel damage Post-tensioning components Repair design Restore lost capacity Prevent future damage Long-term protection Limit moisture ingress Monitoring

8 Corrosion Assessment Mild Reinforcing Steel Half-cell corrosion potential (ASTM C 876) Corrosion rate Concrete resistivity Unbonded Strand Visual survey Measurement Physical strength tests

9 Corrosion Assessment Mild reinforcing steel damage Water ingress Concrete contamination Chlorides Carbonation

10 Visual Survey Difficult from slab edges Looks sorta bad Minor pitting

11 Visual Survey Difficult from slab edges Looks much worse No pitting

12 Corrosion Assessment Sampling Physically remove components Sample number -? Measurement and Testing Pit size Strength Metallurgical assessment

13 Corrosion Assessment Tendon sampling Balance problem Cost versus reward Number of samples Statistically significant versus damage Location of samples Random versus worst looking

14 Corrosion Assessment Testing Loss of strength Comparison with analysis results Comparison with code requirements Assembly Ultimate Load (kip) % GUTS Average

15 Corrosion Assessment Law School Library 56 Tendons extracted Category % Max. pit depth (in.) Ultimate Strength (kips) % GUTS Failure Severe in Intermediate in Mild / None in Total 100%

16 Corrosion Assessment Surface corrosion: No strength loss Intermediate pitting: ~10% reduction Mild pitting: No strength loss Severe pitting: ~25% reduction Adapted from ICRI Technical Guideline No , Guide for the Evaluation of Unbonded Post-Tensioned Concrete Structures

17 Corrosion Assessment Use of testing results Capacity predictions Repair design Informed decisions In-situ conditions Losses up to present Strategy for future

18 Repair Design Anchorage repair Replacement Relocation Strand repair Splices Full length

19 Repair Design Corrosion mitigation Stop water ingress Replace / seal grout pockets Sacrificial anodes Prevent future damage Inhibitors Protect embedded components

20 Long Term Monitoring Proven technology Locate / document future failures Failure Processor Collection SoundPrint Acoustical Monitoring System Pure Technologies, Inc. Calgary, Alberta Event Alert Drawing Provided by Pure Technologies, Inc.

21 Long - Term Monitoring system components

22 Long - Term Monitoring results Law Library Building # of Wire Breaks # of. Breaks Initial Construction Monitoring Starts Year

23 Summary Corrosion can occur in post-tensioned structures Not pervasive Not the end of the line Corrosion damage can be evaluated Diagnose the extent of damage from rational methods Use all available technology Proper repairs are effective Mitigate water intrusion Preventative maintenance