Evaluation and Repair of Existing Post- Tensioned Buildings with Paper Wrapped Tendons Experiencing Corrosion Damage K. Dirk Bondy, S.E. President Seneca Structural Engineering, Inc. The Great American Cable Company II, Inc. CA C50 Contractor #812081 (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 1 of 40
Introduction The purpose of the talk is to discuss how to determine if a building is experiencing loss of post-tensioning due to corrosion, and what can be done to repair the structure if it has lost significant prestress. (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 2 of 40
What are the signs that the structure might have lost post- tensioning due to corrosion? (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 3 of 40
#1 Falling Concrete due to buildup of corrosion products on tendons (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 4 of 40
Evaluating Corrosion in P/T Systems (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 5 of 40
Evaluating Corrosion in P/T Systems (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 6 of 40
Evaluating Corrosion in Paper Wrapped P/T Systems (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 7 of 40
Evaluating Corrosion in Paper Wrapped P/T Systems (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 8 of 40
Corrosion in Other Types of P/T Systems (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 9 of 40
#2 Staining of Concrete Due to Corrosion Product Buildup (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 10 of 40
Evaluating Corrosion in P/T & Rebar Systems (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 11 of 40
Evaluating Corrosion in P/T & Rebar Systems (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 12 of 40
Evaluating Corrosion in P/T & Rebar Systems (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 13 of 40
Corrosion in Other Types of P/T Systems (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 14 of 40
#3 Evaluate the cracking patterns in the concrete Are the cracks typical shrinkage & restraint cracks or something else? (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 15 of 40
Evaluating Corrosion in Paper- Wrapped P/T Systems By definition, a through-crack cannot have any compression across the crack. Through cracks indicate that all of the post-tensioning in that region has been lost to either restraint or loss of prestress in the tendons. (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 16 of 40
Evaluating Corrosion in Paper- Wrapped P/T Systems If a through-crack exists at a location of high bending moment, and is not diagonal restraint-type cracking, there s a good chance that the loss of compression is due to loss of prestress in the tendons (corrosion). (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 17 of 40
Perform a Lift-off Test? Friction and bonding caused by deteriorated paper and buildup of corrosion product can cause erroneous conclusions from hydraulic jack lift-off tests (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 18 of 40
Evaluating Corrosion in Paper- Wrapped P/T Systems When evaluating corrosion, ALL of the forensic evidence available must be taken into account. Experience in distinguishing between typical restraint/shrinkage cracking and cracking due to corrosion or loss of P/T is critical. (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 19 of 40
What s s steps do you take when you have determined that the structure has lost significant prestess? (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 20 of 40
Repairing the System Remove Remove all loose concrete; Clean Clean (sand blast) exposed corroded reinforcement; Patch Patch large concrete spalls and epoxy significant cracks; Consider Consider sealing the top of the deck with an elastomeric coating. (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 21 of 40
Repaired Cracks (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 22 of 40
Repaired Cracks (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 23 of 40
Repaired Cracks (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 24 of 40
Repaired Cracks (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 25 of 40
The concrete has now been repaired, but what about the loss of reinforcement? (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 26 of 40
External Post-Tensioning External post-tensioning (EPT) can be very effective in the strengthening and repair of existing structures. An active force pushes on the existing structure without adding weight. By directly connecting to the columns or beams the slab is relieved by an amount equal to the vertical component of the EPT. (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 27 of 40
One-way External Post- tensioning (Shoring in Conjunction with EPT Layout) (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 28 of 40
One-way External Post- tensioning (X-Ray Sometimes Required) (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 29 of 40
One-way External Post- tensioning (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 30 of 40
Stressing Tendons (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 31 of 40
Galvanized Anchors (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 32 of 40
One-way External Post- tensioning (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 33 of 40
Sprinklers Used in Lieu of Applied Fireproofing (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 34 of 40
EPT with Applied Fire Proofing (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 35 of 40
EPT with Applied Fire Proofing (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 36 of 40
Two-way way External Post- tensioning (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 37 of 40
Two-way way External Post-tensioning tensioning (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 38 of 40
EPT w/o Moving Existing Equip. (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 39 of 40
The End (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHT RESERVED Page 40 of 40