Evaluation and Repair of Post-Tensioned Concrete

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1 Evaluation and Repair of Post-Tensioned Concrete Tank and Thin Shell Roof by Keith Kesner, Ph.D., PE P.E. Randall W. Poston, Ph.D., P.E. (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 1 of 24

2 Post-Tensioned Structures Corrosion Evaluation - Thin Shell Roof Bonded reinforcement electrochemical methods Unbonded reinforcement limited assessment methods Repair Design Concrete Tank Maintain integrity of existing structure Existing i PT needs to be considered d (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 2 of 24

4 Thin Shell Evaluation Concrete shell structure Hyperbolic paraboloid 5 inch thick shell Post-tensioned perimeter beams 30 Years Old Water leakage Staining Corrosion?? Post-tensioning system?? (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 4 of 24

6 Corrosion Assessment Electro-chemical methods Measure electrical activity associated with corrosion process Half-cell potential (ASTM C876) Concrete resistivity Corrosion rate (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 6 of 24

7 Corrosion Assessment Half-cell potential (ASTM C876) When bar is corroding, charge flow through concrete is associated with an electrical field Measure the electrical potential (voltage) of the field at the concrete surface Magnitude of the measured voltage, relative to a standard half-cell, is indicative of corrosion activity Provides only indication of likelihood of active corrosion (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 7 of 24

8 Corrosion Assessment Concrete resistivity Rate of corrosion is controlled by ease with which ions move from anodic to cathodic sites Concrete resistivity is, therefore, an important factor in rate of corrosion Resistivity, ρ Numerically equal to resistance, R, of a unit cube of material R = ρ L A (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 8 of 24

resistance of reinforcing bar Allows measurement of rate of

9 Corrosion Assessment Polarization resistance Measure of corrosion current Based upon evaluation of polarization resistance of reinforcing bar Allows measurement of rate of corrosion (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 9 of 24

10 Thin Shell Evaluation Corrosion testing on 3 bad and 2 good shells Assess reinforcement condition Materials testing Electro-chemical corrosion measurements Analysis Model effect of corrosion damage Evaluate reserve capacity (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 10 of 24

11 Thin Shell Evaluation Results Bad Shells Shell Corrosion Potential (mv) Resistivity (Ohm-cm) Corrosion Rate (μa/cm 2 ) 1 High Average 2 Low Average 3 High Average , ,000 1,000, , Average value from three tests at locations with highest corrosion potentials 2.Average of 84, 42 and 90 individual tests on Shells 1, 5 and 6, respectively 3.Average of 56, 42 and 90 individual tests on Shells 1, 5 and 6, respectively (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 11 of 24

13 Thin Shell Evaluation Electrochemical Isolated corrosion damage expected based upon corrosion rates in bad shells No chlorides or carbonation Good shells high concrete resistivity Post-tensioningtensioning Water intrusion apparent Strands intact (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 13 of 24

14 Thin Shell Summary Summary Isolated corrosion damage likely in 2 to 5 years Generally high resistivity (dry) concrete No accelerating factors Repair recommendations Address water leakage Isolated repairs to deck supports Corrosion inhibitor Grease reinjection (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 14 of 24

Wire Wrapped Tank Analysis Two Cases Failed dome

5 in Dome radius: 195 ft Dome thickness: 4 in

19 Wire Wrapped Tank Analysis Two Cases Failed dome ring Intact dome ring Loading and Geometry Inside diameter: 150 ft Wall height: 26.2 ft Wall thickness: 6.5 in Dome radius: 195 ft Dome thickness: 4 in Capacity: 3.3 MG (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 19 of 24

20 Wire Wrapped Tank Evaluation results failed dome ring Dead load only - unfilled Dome thrust ~ 4.7 kip / ft. circ. Prestress in top 4 ft. of tank resists dome thrust Prestress ~ 1.2 kip / ft. circ. Options need to replace dome ring External post-tensioning Waterproofing limit further corrosion (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 20 of 24

21 Wire Wrapped Tank Evaluation results intact dome ring Original design was adequate Stresses are within limits Repair options External post-tensioning tensioning Supplement existing dome ring Waterproofing limit further corrosion (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 21 of 24

23 Wire Wrapped Tank External post-tensioning Staged allow limited use during repair Retrofit angles increase dome ring support (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 23 of 24

24 Summary Evaluation Methods Bonded reinforcement Electro-chemical Unbonded reinforcement More limited, more conservative Repair Options Consider stresses Worker safety Prevent cracking Limit water ingress (C) COPYRIGHT POST-TENSIONING INSTITUTE, ALL RIGHTS RESERVED Page 24 of 24

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

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