Evaluation of Corrosion Damage in Unbonded Post-Tensioned Concrete Structures: Misunderstandings, Misinformation and Moving Forward by Keith Kesner Randall W. Poston
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
Misinformation No guidelines for assessment not true State of practice guidelines ACI 423.4R - 1998 Corrosion and Repair of Unbonded Single Strand Tendons ACI 222.2R - 2001 Corrosion of Prestressing Steels ICRI 03736-2002 Guide for the Evaluation of Posttensioned Concrete Structures ICRI 03743-2006 Guide for the Repair of Unbonded Posttensioned Concrete Structures ASTM G 46-2005 Standard Guide for Examination and Evaluation of Pitting Corrosion
Misunderstandings All unbonded PT has problems not true Systems have improved Standards and awareness has improved
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
Moving Forward Methods to assess structures with damage Understand root causes Determine extent of damage Prevent future damage Corrosion prevention Repair details
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
Corrosion Assessment Mild Reinforcing Steel Half-cell corrosion potential (ASTM C 876) Corrosion rate Concrete resistivity Unbonded Strand Visual survey Measurement Physical strength tests
Corrosion Assessment Mild reinforcing steel damage Water ingress Concrete contamination Chlorides Carbonation
Visual Survey Difficult from slab edges Looks sorta bad Minor pitting
Visual Survey Difficult from slab edges Looks much worse No pitting
Corrosion Assessment Sampling Physically remove components Sample number -? Measurement and Testing Pit size Strength Metallurgical assessment
Corrosion Assessment Tendon sampling Balance problem Cost versus reward Number of samples Statistically significant versus damage Location of samples Random versus worst looking
Corrosion Assessment Testing Loss of strength Comparison with analysis results Comparison with code requirements Assembly Ultimate Load (kip) % GUTS 1 40.6 98.3 2 39.8 96.4 3 36.0 87.2 4 42.1 101.9 5 39.0 94.4 6 39.2 94.9 7 40.6 98.3 8 41.0 99.3 9 40.2 97.3 10 36.2 87.7 11 41.0 99.3 12 40.3 97.6 13 40.2 97.3 14 40.2 97.3 Average 39.7 96.2
Corrosion Assessment Law School Library 56 Tendons extracted Category % Max. pit depth (in.) Ultimate Strength (kips) % GUTS Failure 3 - - - Severe 52 0.04 in. 31.7 77 Intermediate 11 0.02 0.04 in 37.2 90 Mild / None 34 0.02 in. 40.3 97 Total 100%
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. 03736, Guide for the Evaluation of Unbonded Post-Tensioned Concrete Structures
Corrosion Assessment Use of testing results Capacity predictions Repair design Informed decisions In-situ conditions Losses up to present Strategy for future
Repair Design Anchorage repair Replacement Relocation Strand repair Splices Full length
Repair Design Corrosion mitigation Stop water ingress Replace / seal grout pockets Sacrificial anodes Prevent future damage Inhibitors Protect embedded components
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.
Long - Term Monitoring system components
Long - Term Monitoring results Law Library Building # of Wire Breaks 10 8 6 4 2 0 # of. Breaks Initial Construction Monitoring Starts 67 97 98 99 01 02 03 04 05 06 07 08 09 10 Year
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