EVALUATING PRESTRESSING STRANDS AND POST-TENSIONING CABLES IN CONCRETE STRUCTURES USING NONDESTRUCTIVE METHODS

EVALUATING PRESTRESSING STRANDS AND POST-TENSIONING
CABLES IN CONCRETE STRUCTURES USING
NONDESTRUCTIVE METHODS
By
Dr. Elin Jensen, Principal Investigator
Dr. Nabil Grace, Co-Principal Investigator
Mr. Samuel Kumi, Research Assistant
Department of Civil and Architectural Engineering
Lawrence Technological University
Southfield, MI 48075-0134
November 2015
TECHNICAL REPORT
DOCUMENTATION PAGE
1. Report No.
2. Government Accession No.
3. MDOT Project Manager
RC-1632
N/A
Beckie Curtis
4. Title and Subtitle
5. Report Date
Evaluating Prestressing Strands and Post-Tensioning Cable
in Concrete Structures using Nondestructive Evaluation
Methods
6. Performing Organization Code
7. Author(s)
8. Performing Org. Report No.
Dr. Elin Jensen, Dr. Nabil Grace, and Mr. Samuel Kumi
N/A
9. Performing Organization Name and Address
10. Work Unit No. (TRAIS)
Lawrence Technological University
21000 West Ten Mile Road
Southfield, Michigan 48075
N/A
N/A
11. Contract No.
2010-0293
11(a). Authorization No.
Z4
12. Sponsoring Agency Name and Address
Michigan Department of Transportation
Research Administration
8885 Ricks Rd.
P.O. Box 30049
Lansing MI 48909
13. Type of Report & Period
Covered
Final Report
10/01/12 to 6/30/2015
14. Sponsoring Agency Code
N/A
15. Supplementary Notes
16. Abstract
The objectives were to evaluate the ability of different NDE methods to detect and quantify
defects associated with corrosion of steel reinforcement and grout defects in post-tensioning
applications; and to evaluate the effectiveness of selected NDE methods to detect and quantify
the deterioration. The study included side-by-side box beam bridges for evaluation of the
condition of prestressing strands and web reinforcement. Decommissioned box beams were
tested in the laboratory to relate the quantified deterioration to the measured residual flexural
capacity. Also a segmental bridge was included for evaluation of the condition of the web
reinforcement and post-tensioning ducts. Laboratory specimens simulated debonding and voids
around steel reinforcement, loss of cross section area of steel reinforcement, and various
grouting challenges. The NDE methods used were 1) ultrasonic testing for delamination/void
detection; 2) electrochemical measurements for assessing the corrosive environment; and 3)
magnetic flux leakage (MFL) to estimate loss of cross sectional area of embedded steel.
Ultrasonic echo-pulse tomography was able to detect debonding of the steel reinforcement and
to detect internal voids and delamination. The method was able to quantify the depth of the
defect but not the defect length. Electrochemical testing isolated the areas with high chance of
corrosion. The MFL pilot demonstration showed that MFL can detect changes in the area of the
steel reinforcement in laboratory and field applications. Presently, the calibration overestimates
the steel loss. There was good agreement between estimated beam flexural capacity based on
NDE results and the measured capacity of full scale box beams.
17. Key Words
18. Distribution Statement
Corrosion, Concrete, Prestressed, Post-tensioning,
Nondestructive testing, ultrasonic testing, magnetic
flux leakage, laboratory and field investigation
No restrictions. This document is
available to the public through the
Michigan Department of Transportation.
19. Security Classification report
20. Security Classification - page
Unclassified
Unclassified
21. No. of Pages
22. Price
N/A
DISCLAIMER
The contents of this report reflect the views of the authors who are responsible for the facts and
accuracy of the data presented herein. The contents do not necessarily reflect the official views
or policies of the Michigan Department of Transportation, nor Lawrence Technological
University. This report does not constitute a standard, specification, or regulation. Trade or
manufacturer’s names, which may appear in this report, are cited only because they are
considered essential to the objectives of the report. The United States (U.S.) government and the
State of Michigan do not endorse products or manufacturers.
iii
TABLE OF CONTENT
TECHNICAL REPORT
i
DISCLAIMER
iii
EXECUTIVE SUMMARY
xxii
CHAPTER 1: INTRODUCTION
1
1.1 General
1
1.2 Statement of Problems
2
1.3 Motivation
3
1.4 Study Objectives
3
1.5 Scope of Study
4
CHAPTER 2: LITERATURE REVIEW
6
2.1 Application of Non-destructive Test Methods for Assessment
of Concrete Structures
6
2.2 Visual Methods
7
2.3 Mechanical Wave Methods
8
2.3.1 Ultrasonic Testing
10
2.3.2 Acoustic Emission
15
2.4 Magnetic Methods
16
2.4.1 Magnetic Flux Leakage (MFL)
16
2.4.2. Ground Penetrating Radar
19
2.5 Electrochemical Methods
21
2.5.1 Half-cell Potential
22
2.6 Summary
25
CHAPTER 3: EXPERIMENTAL METHODS
26
3.1 Introduction
26
3.2. Construction of Two Samples of Specimen 1 Box Beams, S1-1 and S1-2
28
3.2.1 Design and Construction of Formwork.
28
3.2.2 Reinforcement Cages
30
iv 3.2.3 Simulation of Defects
32
3.2.4 Prestressing Steel Strands
34
3.2.5 Concrete Placement and Preparation of Cylinders
35
3.2.6 Curing Box Beams and Releasing of Steel Strands
36
3.2.7 Compressive Strength of Box Beams
36
3.2.8 Ultrasonic Assessment of S1-1 and S1-2
38
3.3 Construction and Testing Phases of Three Specimen #2 Beams
40
3.3.1 Construction of Specimen #2 Beams
40
3.3.2 Ultrasonic Assessment of S2-1, S2-2 and S2-3
43
3.3.3 Calibration of Magnetic Flux Leakage (MFL) Mobile Scanner
55
3.3.4 Magnetic Flux Leakage Assessment of Specimen 2 Beams
(S2-1, S2-2 and S2-3)
59
3.4 Construction of Specimen 3 Beams for Simulation of Grouting Defects
3.4.1 Construction Details (S3-1, S3-2, S3-3 and S3-4)
3.5 Evaluation of Salvaged Box Beams (J11, H6 and A1)
60
60
72
3.5.1 Experimental Program for Residual Flexural Testing
72
3.5.2 Electro-chemical Testing of Salvaged Beams Using Canin
79
3.5.3 In-situ Hardness Testing of Salvaged Beams Using
Schmidt Hammer
81
3.5.4 Ultrasonic Assessment of Salvaged Beams J11, H6 and A1
82
3.5.5 Magnetic Flux Leakage Assessment of J11, H6 AND A1
84
3.6 NDT Field Testing
88
3.6.1 US-131 over Muskegon River, Southbound Lanes
88
3.6.2 Side-by-side Box Beam on I-96 over Canal Road, Lansing
100
CHAPTER 4: ANALYSIS AND DISCUSSION OF LABORATORY TEST RESULTS 104
4.1 Overview of Laboratory Specimen and Testing Set-ups
105
4.2 Analysis of Ultrasonic Assessment Data Using Statistical Matched Pairs
107
4.2.1 Laboratory Case study: S1-1 and S1-2 Box Beams
107
4.3 Analysis of Ultrasonic Assessment Data Using Comparison of
Reflected Intensities
114
v 4.3.1 Laboratory Case Study: S1-1 and S1-2 Box Beams
114
4.3.2 Laboratory Case Study: S2-1, S2-2 and S2-3 Beams
117
4.3.3 Laboratory Case Study for Grouting Defects of S3-1, S3-2, S3-3 and S3-4
120
4.4 Analysis of MFL Data
122
4.4.1 Background
122
4.4.2 Laboratory Case Study: S2-1, S2-2 AND S2-3 Beams
127
CHAPTER 5: ANALYSIS AND DISCUSSION OF FIELD TEST RESULTS
5.1 Salvaged Box Beams from Kent County
139
139
5.1.1 Ultrasonic Assessment of Salvaged Box Beams
139
5.1.2 Electrochemical Measurements on Box Beams
148
5.1.3 Impact Hammer Test on Box Beams
157
5.1.4 Magnetic Flux Leakage Assessment of Salvaged Box Beams
158
5.1.5 Analysis of Residual Flexural Testing of Salvaged Box Beams
162
5.2 I-96 Box Beams over Canal Road
165
5.2.1 Ultrasonic Assessment of I-96 Box Beams
166
5.2.2 Electrochemical Half-cell Assessment of I-96 Box Beams
168
5.3 Segmental Box Beam Bridge carrying US-131 over Muskegon River
170
5.3.1 Ultrasonic Assessment
171
5.3.2 Electrochemical Half-cell Assessment
182
CHAPTER 6: SUMMARY AND CONCLUSIONS
187
6.1 Research Objectives and Methods
187
6.2 Assessment of Defects Associated with Corrosion of Embedded Steel
192
6.2.1 Ultrasonic Testing
192
6.2.2 Half-Cell Potential Difference
194
6.2.3 Magnetic Flux Leakage Measurements
196
6.3 Assessment of Grouting Challenges in Post-tensioning Ducts
6.3.1 Laboratory Simulations
198
6.3.2 Field Explorations
199
vi 198
APPENDIX A:
Condition Assessment of Side-by-Side Box Beam Bridge Carrying US-24 over
Middle Rouge River
201
APPENDIX B:
Implementation Plan
225
APPENDIX C:
Design Calculations for Specimen1 Laboratory Box Beams
231
APPENDIX D:
Sample Calculations for Residual Strength of MDOT Salvaged Box Beam (J11)
265
APPENDIX E:
Scan Results of Ultrasonic Assessment of Grouting Defects of S3-1, S3-2, S3-3 & S3-4 273
APPENDIX F:
Magnetic Flux Leakage Assessment Graphs for Salvaged Box Beams J11, H6 & A1
281
REFERENCE
303
vii LIST OF FIGURES
Fig. 2.1: Longitudinal Wave Propagation (Top) and Transversal Wave Propagation
9
Fig. 2.2: Conversion of a P wave striking an interface between dissimilar materials,
following Snell’s law
10
Figure. 2.3: Configuration of a Transmitter and Receiver.
11
Figure 2.4: Schematic of pulse velocity device (Crawford, 1997)
12
Figure 2.5: Illustration of the Basis of Ultrasonic Pulse Echo Method
13
Figure 2.6: Illustration of the Basis of the IE method. (Pessiki et al, 2010)
15
Figure 2.7: Change in magnetic flux before and after detection of flaw (defect)
18
Figure 2.8: Propagation of EM energy through dielectric boundaries
20
Figure 2.9: Setup for electrochemical techniques
22
Figure 2.10: Schematic view of the electric field and current flow of an active/passive
macro-cell on steel in concrete
23
Figure 3.1: Specimen 1 Box Beams, S1-1 and S1-2
26
Figure 3.2: Specimen 2 Beams, S2-1, S2-2 and S2-3
27
Figure 3.3: Specimen Beams, S3-1, S3-2, S3-3 and S3-4
27
Figure 3.4: Construction of formwork.
29
Figure 3.5: Construction of interior cells
29
Figure 3.6: Formwork set-up
29
Figure 3.7: Cross-sectional details of box-beams
30
Figure 3.8: Longitudinal details of box-beams
31
Figure 3.9: Bending stirrups
31
Figure 3.10: Tiring and aligning reinforcement
31
Figure 3.11: Rebar cages for the box-beam
31
Figure 3.12: Installing Edge & Side Panels
31
Figure 3.13: Plastic defects around Top Rebar
32
Figure 3.14: Plastic defects around Bottom Strand
32
viii Figure 3.15: Honeycomb at the top of box beam
32
Figure 3.16: Plastic Tubes for Debounding
32
Figure 3.17: Top Plan of Box Beam 1 showing locations and defect Length around
Top Rebar
33
Figure 3.18: Bottom Plan of Box Beam 1 showing locations of defects around strands
33
Figure 3.19: South Elevation of Box Beam 1 showing locations of defects around strands 33
Figure 3.20: End Plates for prestressing bed
34
Figure 3.21: Prestressing steel strands
34
Figure 3.22: Casting of Box Beams, S1-1 and S1-2
35
Figure 3.23: Slump Test / Cylinders Preparation
35
Figure 3.24: Curing Beams
36
Figure 3.25: Releasing of steel strands
36
Figure 3.26: Four (4) Days at Transfer
37
Figure 3.27: 7 Days Compressive Strength
37
Figure 3.28: Scanning on the side of S1-1
39
Figure 3.29: Scanning on top of S1-2
39
Figure 3.30: Scanning on the bottom of S1-2
39
Figure 3.31: Constructing formwork
40
Figure 3.32: Cross-sectional area loss on rebar
40
Figure 3.33: Grinding configuration of 1inches-1inches-1inches for different percentage
losses for rebar
40
Figure 3.34: Grinding configuration of 5inches-2inches-5inches for different percentage
losses for rebar
41
Figure 3.35: 0.6” Strand length of wire cut
41
Figure 3.36: Wire cut covered with dual wall Polyolefin tube and duct tape
41
Figure 3.37: Cages of rebar and strands
42
Figure 3.38: Cross-sectional Area loss
42
ix Figure 3.39: Strands showing loss of wires covered with dual wall polyolefin tube
and duct tape
42
Figure 3.40: Placing of concrete
43
Figure 3.41: Leveling top of specimen 2 beams
43
Figure 3.42: Checking for slump
43
Figure 3.43: Preparation of concrete cylinders
43
Figure 3.44: Top Rebar Arrangement and Percentage Losses for S2-1 with
Total Length of 3 inches Cross-sectional Area Loss
45
Figure 3.45: Bottom Rebar Arrangement and Percentage Losses for S2-1 with
Total length of 3 inches Cross-sectional Area Loss
Figure 3.46: Cross-sectional Area of S2-1 showing Single Bottom Reinforcement
46
47
Figure 3.47: Top Rebar Arrangement and Percentage Losses for S2-2 with
Total Length of 12 inches Cross-sectional Area Loss
48
Figure 3.48: Bottom Rebar Arrangement and Percentage Losses for S2-2 with
Total Length of 12 inches Cross-sectional Area Loss
49
Figure 3.49: Cross-sectional Area of S2-2 showing Double Bottom Reinforcement
50
Figure 3.50: Top Strand Arrangement, Length and Number of Wire cuts for S2-3
51
Figure 3.51: Bottom Strand Arrangement, Length and Number of Wire cuts for S2-3
52
Figure 3.52: Cross-sectional Area of S2-3 showing Double Bottom Strands
53
Figure 3.53: Ultrasonic assessment set-up for S2-1
54
Figure 3.54: Scan on Rebar with defect at frequency of 70 kHz with dB of 33.8
54
Figure 3.55: Scan on rebar without defect at frequency of 70 kHz with dB of 33.8
55
Figure 3.56: MFL Mobile Scanner Calibration Setup
56
Figure 3.57: Magnetic flux leakage influence with depth
57
Figure 3.58: Graph of voltage change for each depth
58
Figure 3.59: Graph showing effect of 20% loss
58
Figure 3.60: MFL Mobile Scanner Assessing Cross-sectional Area loss or
Number and Length of Wire cuts for Specimen #2 Beams
x 59
Figure 3.61: Formwork Construction
60
Figure 3.62: Configuration of S3-1 showing draped post-tensioned duct with simulated
tendon paths in positive and negative moment regions and grout pumped in
to check complete grout coverage at angle breaks
61
Figure 3.63: Configuration of S3-2 showing draped post-tensioned duct to be partially
filled with water to be checked if the grout completely displaces the water. 61
Figure 3.64: Configuration of S3-3 showing straight steel duct with grout mixed with
expansion foam and plastic material
62
Figure 3.65: Configuration of S3-4 showing straight steel duct fully grouted in vertical
position to be used as a reference to assess S3-1, S3-2 and S3-3
62
Figure 3.66: Rebar preparation and installation of drapped post-tensioned duct
63
Figure 3.67: Setting Formwork
64
Figure 3.68: Beams Setup ready for casting
64
Figure 3.69: Placing Concrete into Formwork.
65
Figure 3.70: Checking Concrete Slump
65
Figure 3.71: Removal of Formwork
65
Figure 3.72: Curing of Beams
65
Figure 3.73: Grouting Equipment set-up
66
Figure 3.74: Compressive Strength Testing for Specimen #3 Concrete
68
Figure 3.75: Scanning Top of S3-1
70
Figure 3.76: Scanning Side of S3-1
70
Figure 3.77: Scanning Top of S3-2
70
Figure 3.78: Scanning Top of S3-3
70
Figure 3.79: Scanning Top of S3-4
70
Figure 3.80: Examples of Scan Reflections from Side of S3-1 at 50 kHz before and
after Grouting.
71
Figure 3.81: Examples of Scan Reflections from Side of S3-3 at 50 kHz before and
after Grouting.
71
xi Figure 3.82: Salvaged Box Beams from Kent County
72
Figure 3.83: Typical Cross-section of Salvaged Beams
73
Figure 3.84: Fabricating Spreader
74
Figure 3.85: Fabricating Spreader
74
Figure 3.86: Layout of Strain Gages Installed on top Surface of Beams
75
Figure 3.87: Layout of Strain Gages Installed on Fascia Side of Beams
75
Figure 3.88: Layout of Strain Gages Installed on Interior Side of Beams
76
Figure 3.89: Location of LVDT Installed on Fascia Side of Beams
76
Figure 3.90: Box Beam Residual Flexural Set-up at 4 kips Loading
77
Figure 3.91: Box Beam Residual Flexural Set-up at 8 kips Loading
77
Figure 3.92: Showing Set-up for Salvaged Box Beam J11 before and after failure
78
Figure 3.93: Showing Set-up for Salvaged Box Beam H6 before and after failure
78
Figure 3.94: Showing Set-up for Salvaged Box Beam A1 before and after failure
79
Figure 3.95: Electro-chemical Half-cell of salvaged beam
80
Figure 3.96: Schmidt Hammer Testing for MDOT Salvaged Beam #1
81
Figure 3.97: Ultrasonic Assessment of Salvaged Box Beams
83
Figure 3.98: Scanning Configuration at the Bottom of Box Beam J11
85
Figure 3.99: Scanning Configuration at the Bottom of Box Beam H6
85
Figure 3.100: Scanning Configuration at the Bottom of Box Beam A1
86
Figure 3.101: Typical Cross-section of Salvaged Beams
87
Figure 3.102: Scanning Strand #5 (S5) on North Half of Savage Beam with MFL
Mobile scanner
87
Figure 3.103: US-131 over Muskegon River (SB)
89
Figure 3.104: Plan and Elevation of US-131 over Muskegon River (SB)
89
Figure 3.105: Scanning on Transverse Post-tensioning duct within Segment 2
90
Figure 3.106: Location of Transverse Post-tensioning Duct has been highlighted
90
Figure 3.107: Typical Scan Result along Transverse Post-tensioning Duct,
showing location of duct and back reflection from thickness of Top Flange 91
xii Figure 3.108: Scanning on Longitudinal Post-tensioned Duct #24 and
Anchorages at the ends
92
Figure 3.109: Cross-section of SB, showing Duct #24, DWG No. BO1-4
92
Figure 3.110: Tendon anchorage details for bottom slab, DWG No. BO1-5
93
Figure 3.111: Typical Scan along Longitudinal Post-tensioning Duct 24,
showing location of duct and back reflection from thickness
of bottom flange
93
Figure 3.112: Typical Cross-section of Segment showing Bottom Flange and
Web Rebar Details, DWG No. BO1-7
94
Figure 3.113: Segments arrangement showing location of segment 2 between
abutment A and pier 1, DWG No. BO1-1
94
Figure 3.114: MFL Testing Grid Set-up within Bottom Flange of Segment #2
96
Figure 3.115: MFL Mobile Scanner with Temporary 1/8” Wood Strips
96
Figure 3.116: Locating Longitudinal and Transverse Rebar on web of Segment #2
Using Ground Penetrating Radar (GPR)
97
Figure 3.117: Electro-chemical half-cell Assessment of selected Section of
Web along Segment #2
98
Figure 3.118: Typical instrument configuration for half-cell measurements
98
Figure 3.119: Layout of Southbound Bridge indicating Test Areas #1 and #2
99
Figure 3.120: I-96 over Canal Road, Lansing
100
Figure 3.121: Scanning along web of Box beam 3-M-11
101
Figure 3.122: Typical scan along Web of box beam with back Reflection indicating
Web thickness
101
Figure 3.123: Locating Transverse Rebar for Electro-chemical Assessment using
Rebar Detector
103
Figure 3.124: Conducting Electro-chemical Analysis on the Web of External
Beam 3-M-11
103
xiii Figure 4.1: Over view of Specimen 1 Testing Set-up
105
Figure 4.2: Over view of Specimen 2 Testing Set-up
106
Figure 4.3: Over view of Specimen 3 Testing Set-up
106
Figure 4.4: Typical cross-section of box beams (S1-1 and S1-2)
109
Figure 4.5: Typical scan indicating diagnostic and actual thickness of top flange of S1-1
between diaphragms with y-axis showing scan depth and x-axis indicating
scan width at 50 kHz, 40 dB
109
Figure 4.6: Typical scan on stirrup without defect of box beam S1-1 between
diaphragms with y-axis showing scan depth and x-axis indicating scan
width at 80 kHz, 35 dB.
110
Figure 4.7: Typical scan on stirrup with defect (wrapped plastic tubes around rebar)
of S1-1 with y-axis showing scan depth and x-axis indicating scan width at
80 kHz, 35 dB.
110
Figure 4.8: Rebar cage set-up showing rebar with and without defects
115
Figure 4.9(a): scan on rebar with defect
116
Figure 4.9(b): scan on rebar with defect
116
Figure 4.9(c): scan on rebar with defect
116
Figure 4.10(a): scan on rebar without defect
116
Figure 4.10(b): scan on rebar without defect
116
Figure 4.10(c): scan on rebar without defect
116
Figure 4.11: Rebar cages set-up for S2-2 & S2-3
118
Fig. 4.12(a): scan on rebar with defect, 80 kHz
119
Fig. 4.12 (b): scan on rebar with defect, 90 kHz
119
Fig. 4.13(a): scan on rebar without defect 80 kHz
119
Fig. 4.13 (b): scan on rebar without defect 90 kHz
119
Figure 4.14: Configuration of S3-1 showing draped post-tensioned duct
121
Figure 4.15: Beam S3-1, showing draped post-tensioned duct
121
xiv Figure 4.16. Depiction of magnetic flux density lines around magnetic member
with or without cross sectional area reduction (ndt.net)
122
Figure 4.17. Variation in hall voltage output for area with or without cross sectional
area reduction.
123
Figure 4.18. Relationship between hall voltage and cross sectional area of rebar.
for concrete cover of 1.5 inches.
125
Figure 4.19. Predicted rebar cross sectional area loss versus actual rebar cross
sectional area loss over 12 inches in calibration beams.
126
Figure 4.20: Top rebar layout for S2-1 with rebar B highlighted
127
Figure 4.21: M3 Consolidated Graph for S2-1, Rebar B, Top
128
Figure 4.22: M3 Adjusted Graph for S2-1, Rebar B, Top
129
Figure 4.23: Bottom rebar layout for S2-2 with percentage cross-sectional losses
133
Figure 4.24: M3 Consolidated Graph for S2-2, Rebar H, Bottom
134
Figure 4.25: M3 Adjusted Graph for S2-2, Rebar H, Bottom
135
Figure 4.26: Top strand layout for S2-3 with number and length of wire cuts
137
Figure 5.1 Typical scans on corroded and less corroded strand locations
141
Figure 5.2 Typical scans at 100 kHz, 34.8 dB with (a) along web and
(b) at cell location.
142
Figure 5.3 Typical scans at 90 kHz, 34.8 dB with (a) along web and
(b) at cell location.
143
Figure 5.4 Typical scans at 80 kHz, 34.8 dB with (a) along web and
(b) at cell location.
144
Figure 5.5 Typical scans at 70 kHz, 34.8 dB with (a) along web and
(b) at cell location.
145
Figure 5.6: Typical scan on the side of Salvaged Box Beam J11
146
Figure 5.7 Typical scans from the side of box beam with scanned location
reference to end of beam
147
xv Figure 5.8: Electro-chemical Half-cell of salvaged beam
149
Figure 5.9: Combined Electro-chemical Potential Map for Salvaged Beam 1
(J11), Test 1-3
150
Figure 5.10: Electro-chemical Potential Map for Salvaged Beam 2 (H6), Test 1
150
Figure 5.11: Relative Frequency for Salvaged Beam 2 (H6), Test 1
151
Figure 5.12: Cumulative Frequency for Salvaged Beam 2 (H6), Test 1
151
Figure 5.13: Electro-chemical Potential Map for Salvaged Beam 2 (H6), Test 2
152
Figure 5.14: Relative Frequency for Salvaged Beam 2 (H6), Test 2
152
Figure 5.15: Cumulative Frequency for Salvaged Beam 2 (H6), Test 2
153
Figure 5.16: Electro-chemical Potential Map for Salvaged Beam 3 (A1), Test 1
153
Figure 5.17: Relative Frequency for Salvaged Beam 3 (A1), Test 1
154
Figure 5.18: Cumulative Frequency for Salvaged Beam 3 (A1), Test 1
154
Figure 5.19: Electro-chemical Potential Map for Salvaged Beam 3, Test 2
155
Figure 5.20: Relative Frequency for Salvaged Beam 3, Test 2
155
Figure 5.21: Cumulative Frequency for Salvaged Beam 3, Test 2
156
Figure 5.22: Schmidt Hammer Testing for MDOT Salvaged Beam 1
157
Figure 5.23: Typical Cross-section of Salvaged Beams
159
Figure 5.24: Graph of Load against Displacement for Salvaged Box Beam 1 (J11)
163
Figure 5.25: Graph of Load against Displacement for Salvaged Beam 2
164
Figure 5.26: Graph of Load against Displacement for Salvaged Beam 3
165
Figure 5.27: Typical scan on the side of Box Beam 3-M-11
166
Figure 5.28 Typical scans from the side of box beam with scanned location
reference to East end of beam
167
Figure 5.29: Mapping out of stirrup locations using profometer from abutment
end towards the central pier locations, evidence of corrosion stains.
168
Figure 5.30: Test locations and recorded potential values along Fascia of 3-M-11
169
Figure 5.31: US-131 over Muskegon River (SB)
170
xvi Figure 5.32: US-131 over Muskegon River (SB) showing side elevation of
segmental box beams
170
Figure 5.33: Scanning on Transverse Post-tensioning duct within Segment #2
172
Figure 5.34: Transverse Road Surface Crack
172
Figure 5.35: Typical scans from selected transverse post-tensioning duct within
segment 2 from north abutment
173
Figure 5.36: Scanning on Longitudinal Post-tensioned Duct #24 and
Anchorages at the ends.
174
Figure 5.37: Cross-section of SB, showing Duct #24, DWG No. BO1-4
175
Figure 5.38: Tendon anchorage details for bottom slab, DWG No. BO1-5
175
Figure 5.39: Typical scans along (a) anchorage location at 400 mm (1’-4”)
from anchorage end and (22’-8”) from anchorage end, assessment
in north-bound direction.
176
Figure 5.40: Typical Cross-section of Segment showing Bottom Flange and
Web Rebar Details, DWG No. BO1-7
177
Figure 5.41: Segments arrangement showing location of segment #2 between
abutment A and pier #1, DWG No. BO1-1
Figure 5.42 Typical scans along rebar showing reflections from rebar
177
178
Figure 5.43: Typical Cross-section showing Test location along the Web of
Segmental box beam, DWG No. BO1-7
Figure 5.44: Visible pockets of voids on inner surface of web.
179
180
Figure 5.45: Typical scans along rebar showing pockets of voids around rebar
from scans taken from side web within segment 2
181
Figure 5.46: Electro-chemical half-cell Assessment of selected Section of
Web along Segment #2
182
Figure 5.47: Layout of Southbound Bridge indicating Test Areas #1 and #2
xvii 183
LIST OF TABLES
Table 2.1 Interpretation of half-cell potential values as per ASTM C876
24
Table 2.2 Half-cell potential for different types of concrete
24
Table 3.1: Compressive Strength Development using 6 inch by 12 inch Cylinders
37
Table 3.2: Compressive Strength Development using 6 inch by 12 inch Cylinders
67
Table 3.3: Interpretation of half-cell potential values as per ASTM C876
80
Table 4.1: Checking concrete thickness of top & bottom flanges and side webs
112
Table 4.2: Localized scan on rebar and strands without defect.
113
Table 4.3: Localized scan on defects
114
Table 4.4: S2-1, Top Plan: Estimation of Cross-sectional Area Losses
132
Table 4.5: S2-2, Bottom Plan: Estimation of Cross-sectional Area Losses
136
Table 4.6: S2-3, Top Plan: Estimation of Cross-sectional Area Losses
138
Table 5.1: Interpretation of half-cell potential values as per ASTM C876
149
Table 5.2: Predicted Cross-sectional Area Loss
161
Table 5.3: Half-cell Potential difference along Fascia of 3-M-11
169
Table 5.4: Half-cell potential difference data of Test Area #1
183
Table 5.5: Average Half-cell potential difference data of Test Area #1
184
Table 5.6: Chance of rebar corrosion in Test Area 1
184
Table 5.7: Half-cell potential difference data of Test Area 2
185
Table 5.8: Average Half-cell potential difference data of Test Area 2
186
Table 5.9: Chance of rebar corrosion in Test Area 2
186
Table 6.1: Summary of Research Methods used to Detect and Quantify Defects
188
Table 6.2: Summary of Laboratory Specimens and Projects
189
xviii Table 6.3: Description of Laboratory Specimens
190
Table 6.4: Description of Field Projects
191
xix xx EXECUTIVE SUMMARY
Deterioration of prestressed and post-tensioned concrete members is highly related to the concrete
durability and corrosion of the steel reinforcement. In the case of prestressed structures the
corrosion causing loss of reinforcement material may also cause localized concrete damage such
as cracking, delamination and spalling. For post-tensioned structures the corrosion of the strands
is typically associated with voids in the otherwise grouted ducts. Common for both types of
reinforcement deterioration is that the onset of corrosion damage occurs without visual signs such
as cracks and stains on the concrete surface. Therefore, it is critical to be able to assess the
condition of the reinforcement to evaluate the safety of these concrete structures. Detailed
condition assessment data afford engineers the ability to make informed decisions to determine
short-term as well as long-term maintenance and repair activities. The general consensus among
researchers and practitioners is to use multiple nondestructive test methods along with visual
inspections and coring to effectively assess the structural performance.
The goal of this study was to assess the effectiveness of multiple nondestructive evaluation
(NDE) methods to evaluate the condition of prestressed concrete bridges exhibiting deterioration
associated with corrosion of the steel reinforcement. The research plan was designed around an
experimental approach and included both laboratory and field investigations. The objectives were
to (1) develop laboratory references, for selected NDE methods, for detecting and quantifying
defects associated with corrosion of steel reinforcement and grout defects in post-tensioning
applications; and (2) evaluate the effectiveness of the selected NDE methods to detect and quantify
deterioration in the field using both in service and decommissioned bridges.
Side-by-side box beam bridges were selected for evaluation of the condition of prestressing
strands and web reinforcement. Decommissioned box beams were tested in the laboratory to relate
the impact of the estimated material deterioration, by the NDE methods, and the measured residual
flexural capacity of the beams. Also a segmental bridge was included for evaluation of the
condition of the web reinforcement and post-tensioning ducts. Specialized laboratory specimens
were designed for this study to simulate debonding and voids around steel reinforcement, loss of
cross section area of steel reinforcement, and various grouting challenges. The principal NDE
methods used were ultrasonic, magnetic and electrochemical. The following primary methods
were used:
xxi 
Ultrasonic assessment for delamination, void detection and thickness measurements.

Electrochemical half-cell potential assessment for assessing the corrosive environment.

Magnetic flux leakage assessment to estimate loss of cross sectional area of steel
reinforcement.
The ultrasonic and the electrochemical assessments were conducted using commercially
available systems. The magnetic flux leakage assessment were performed using a strong earth
magnet mounted on a mobile unit. The change in magnetic field leakage as the magnet passes
over the steel reinforcement is correlated to steel loss. The pilot version of the MFL mobile system
was developed at the Center for Innovative Research at Lawrence Technological University.
Ultrasonic echo-pulse tomography was able to detect debonding of the steel reinforcement and
to detect internal voids and delamination. The method was able to quantify the depth of the defect
but not able to distinguish the debonding length around the reinforcement; unless cracking and
delamination had reached an advanced stage. Electrochemical testing isolated the areas with high
chance of corrosion. From the field evaluation of the box beams, the areas showing high chance
of corrosion coincided with areas experiencing increased ultrasonic reflections indicating various
stages of cracking and delamination around the steel reinforcement. The MFL pilot demonstration
showed that MFL can detect changes in the area of the steel reinforcement in laboratory and field
applications. Presently, the MFL calibration displays the strongest correlation between predicted
and actual loss when the loss is spread out over a longer area than when it is concentrated
(12 inches contrast to 3 inches).
There was good agreement between estimated beam flexural
capacity based on NDE assessment and the measured capacity of full scale box beams.
The study proposes an implementation strategy for integrating the use of ultrasonic echo-pulse
tomography to detect and quantify defects around the reinforcement. The method is able to
evaluate the extent of deterioration along the reinforcement beyond the immediate area suffering
from extensive visual deterioration such as rust stains, spalling and cracking. Furthermore,
ultrasonic assessment can detect early deterioration in the shear key region of the box beams. The
study also proposes the use of electrochemical assessment as part of monitoring the change in the
chance of corrosion in areas that exhibit surface cracking developed during construction, early
deployment, or other events. The MFL mobile scanner used in this study is not generally field
worthy until it is running on tracks mounted on the structure or a scaffold system.
xxii CHAPTER 1: INTRODUCTION
1.1 General
Detailed condition assessment data afford asset managers and structural engineers the ability to
make informed decisions to improve short-term as well as long-term maintenance and repair
activities of concrete structures. Recent advancements in nondestructive evaluation techniques
produce an excellent source of data for an informed decision.
The major cause of deterioration in prestressed strands and post-tension cables in bridges is
corrosion of prestressed strands which is usually associated with delamination and spalling as well
as grouting defects in post-tensioned ducts which also results in corrosion of post-tensioned cables.
It is against this background that a study was conducted to evaluate the condition of prestressing
strands and post-tensioning cables in concrete structures using nondestructive evaluation (NDE)
methods. The following primary nondestructive methods were deployed: ultrasonic assessment for
delamination and void detection; electro-chemical half-cell assessment for detecting corrosive
environment; and magnetic flux leakage to determine loss of cross sectional area of rebar and
strands.
Three groups of laboratory specimen with simulated defects of voids and cross-sectional area loss
were designed for testing using the above nondestructive methods. Three side-by-side field beams
from Kent County Road Commission decommissioned after 39 years of service were selected for
nondestructive evaluation and residual flexural testing. Field evaluations were also conducted on
portions of the segmental bridge carrying US-131 over Muskegon River, 6 miles south of Big
Rapids as well as segments of the side-by-side box beam bridge carrying I-96 over Canal Road in
Lansing. Results of nondestructive assessment conducted on selected box beams of northbound
US-24 over Middle Rouge River have been presented in the Appendix of this report as an
additional study.
1
1.2 Statement of Problems
Prestressed beams commonly used in highway bridge construction and segmental post-tensioned
bridges are widely used for medium and long span bridge construction. These types of structures
rely heavily on steel prestressing strands and post-tensioning cables for strength and durability.
Additionally, it is very important during the construction of these bridges to implement an effective
quality control plan to ensure proper placement of the strands and to ensure that the ducts are fully
grouted in accordance with the design. Methods to determine the overall conditions of these
strands are critical to verify the overall integrity and safety of these structures. Since the strands
are embedded in concrete, and often the area is complex and congested, nondestructive evaluation
(NDE) methods are needed to assess the condition of these structures. When the evaluation is
performed routinely the results can aid the management of the asset in terms of operation, repair
and replacement activities.
In particular, exposed structures like highway bridges are more vulnerable to environmental
deterioration. The deterioration of prestressed and post-tensioned concrete members is highly
related to the concrete durability and corrosion of the steel reinforcement. In the case of
prestressed structures the corrosion products may cause localized concrete damage leading to
delamination and spalling. In the case of post-tensioned structures the corrosion is typically
associated with regions of voids in the otherwise grouted ducts. Yet, common for both types of
structures is that the onset of damage occurs without visual clues such as cracks and stains on the
concrete surface. However, a significant indicator of the potential for ‘hidden’ deterioration is the
formation of voids. Voids may form either as a result of the corrosion product, delamination and
cracks in the prestressed concrete beam, or voids due to lack of proper grouting of the posttensioning ducts. A second indicator is the change in magnetic resistance of the strands due to the
loss of cross section area.
During the last decade, governments and industry have made significant investments into
developing new non-destructive testing (NDT) methods for assessing the conditions of prestressed
and post-tensioned concrete structures. The Federal Highway Administration's (FHWA) Office
of Infrastructure Research and Development has identified NDE as a primary activity in achieving
2
long term infrastructure performance as well as durable infrastructure systems (FHWA-HRT-08069).
At the present time, the general consensus among researchers and practitioners is to use multiple
NDT methods along with visual inspections and coring to effectively assess the structural
performance. There are different nondestructive evaluation (NDE) methods for monitoring and
instant assessing prestressed and post-tensioned concrete members. The methods range from semipermanently installed sensors such as Moisture Sensors, Acoustic Emission Sensors (AES) and
Micro Electro-Mechanical Sensors (MEMS) to manually operated mobile sensor systems such as
Magnetic Flux Leakage (MFL) and Ultrasonic Pulse-Echo Method.
1.3 Motivation
The research team at Lawrence Technological University (LTU) proposed a comprehensive study
to investigate and recommend NDE methods for assessing the conditions of prestressed and posttensioned concrete members. The study included a review of existing use of NDE methods to
evaluate the condition of prestressed and post-tensioned strands in concrete structures as well as
to verify the proper grouting of post-tensioned ducts in concrete structures. Experimental
laboratory and field investigations were performed to evaluate the NDE methods ability to detect
and quantify defects. MDOT engineers assisted in defining the selection criteria and the selection
of the candidate bridges.
The outcome of these investigations was to provide recommendations on suitable (rapid and
flexible) NDE methods to implement as an inspection technique in the bridge inspection protocol
for prestressed and post-tensioned concrete bridge systems.
1.4 Study Objectives
The main objectives of this project are as follows:

Review existing use of NDE methods to evaluate the condition of prestressed and posttensioned strands in concrete structures, and to verify the proper grouting of post-tensioned
ducts in concrete structures.
3

Evaluate the effectiveness of NDE methods to evaluate the condition of prestressed and
post-tensioned strands in concrete structures and verify proper grouting of post-tensioned ducts
in concrete. Detecting and quantifying defects are the primary measures used to evaluate the
methods.

Development of magnetic flux leakage device using the active MFL method.
1.5 Scope of Study
A detailed experimental investigation was designed to address the study objectives. The study
utilized laboratory prepared and field specimens tested in the laboratory as well as field inspection
on existing structures. Three sets of laboratory specimen and three salvaged box beams received
from Kent County Road Commission were evaluated in the laboratory. Two laboratory box beams
were constructed according to the geometry stipulated in MDOT Bridge Design Guides 6.65.10A
with pre-induced defects to simulate voids created around prestressed strands and post tensioned
cables as a result of corrosion by using plastic tubes as detailed in the construction phase. Three
laboratory beams were constructed with pre-induced grinding defects covered with plastic tubes
to simulate cross-sectional area loss for detection and quantification using ultrasonic assessment
and magnetic flux leakage system. Finally, four beams were constructed to simulate grouting
defects normally associated with post-tensioned cables.
Field evaluations were also carried out on sections of the following existing bridges in Michigan,
US-131 over Muskegon River, southbound 6 miles south of Big Rapids, MI, and segments of S0855602 of I-96 over Canal Road in Lansing, MI. The primary employed nondestructive evaluation
methods were ultrasonic, magnetic flux leakage and electro-chemical half-cell assessments. This
report consists of the following chapters:
Chapter (2): This chapter summaries existing use of NDE methods to evaluate the condition of
prestressed and post-tensioned strands in concrete structures as well as to verify
proper grouting of post-tensioned ducts in segmental concrete structures.
Chapter (3): This chapter presents details of construction of laboratory specimen as well as
methodology and testing on existing bridges in this study: US-131 over Muskegon
4
River, southbound 6 miles south of Big Rapids and segments of I-96 over Canal Road
in Lansing.
Chapter (4): This chapter presents the detailed analysis of the test results from the laboratory
investigations.
Chapter (5): This chapter presents the detailed analysis of the test results from the field
investigations.
Chapter (6): This chapter presents the summary and conclusions derived from this investigation.
Chapter (7): This chapter presents the recommendations derived from this investigation.
5
CHAPTER 2: LITERATURE REVIEW
2.1 Application of Non-destructive Test Methods for Assessment of Concrete Structures
Many structures are built of prestressed concrete in which prestressing steel wires are put into a
permanent state of tension to compensate for the low tensile strength of the concrete. Hence, tensile
cracking in the concrete is minimized by ensuring that the concrete is in compression under normal
working loads. However, some existing prestressed concrete bridge beams experience
deterioration that raises serious concerns about the long-term durability of structures. The major
cause of deterioration in prestressed strands in bridges is corrosion. In prestressed concrete
structures, the high stress level in the tendons strongly modifies the steel corrosion process. Stress
corrosion is characterized by the coupling between the conventional corrosion (pitting attacks in
chloride environment) and the steel micro cracking; the latter induced by the high stress level and
hydrogen embrittlement (Nürnberger, 2002). Steel micro cracking can lead to the brittle failure of
the prestressing steel even at very low corrosion level and under normal service loads (Spaehn,
1977). In such case, there may be no visible warning before tendon failure.
Deterioration of post-tensioned cables in concrete bridges is also a serious concern. The leading
cause of corrosion of post-tensioned cables is insufficient grouting of post-tensioned ducts
(Mutsuyoshi, 2001). Corrosion is a significant problem in numerous structures and the associated
cost is estimated at billions of dollars every year (Singh, 2000).
Unfortunately, corrosion of prestressing steel in both prestressed and post-tensioned concrete
members is not visible by indicators such as corrosion stain and concrete cracking during the early
stages. When visual signs of corrosion of prestressing steel occur on these members, the corrosion
activities would have been in progress for some time and extensive damage could have resulted
(Grace and Jensen, 2010).
Nondestructive evaluation techniques are commonly used in the civil engineering field. The recent
increased accuracy in detecting a specific type of parameter, such as material quality and defects,
is possible due in part to the development of new testing methods, improved data collection and
data analysis, as well as the development of user friendly systems. Contrary to destructive
6 methods, nondestructive methods provide information about material properties and structural
conditions without compromising the structure and its serviceability. At the same time, it is
paramount to select nondestructive test methods that are field worthy and effective, as well as
implement routine evaluation schedules of these structures for improved asset management
(Ghorbanpoor, 1999).
NDE technologies used for condition assessment of prestressed and post-tensioning systems can
be grouped according to their underlying methods (Azizinamini, 2012). These groups are:

Visual methods

Magnetic methods

Mechanical wave/vibration methods

Electrochemical methods
The following sections briefly presents the common methods used for condition assessment of
concrete structures with specific focus on detection and quantification of defects associated with
corrosion of reinforcement and concrete deterioration.
2.2 Visual Methods
Visual evaluation is an important assessment in non-destructive testing and it often provides
valuable information. Visual characteristics may be due to poor workmanship as well as structural
serviceability and material degradation. It aids the engineer to differentiate between the various
signs of distress types. Defects that are usually uncovered by visual evaluation are cracks, popouts, spalling, delamination, color stains, surface deterioration and lack of uniformity.
Considerable details can be collected from visual evaluation to ascertain the suitability of a
structures for its intended purpose and identifying the need for follow up in-depth inspection. An
engineer carrying out a visual evaluation of a structure should have a good knowledge of applicable
structural details, technical specification, past reports of evaluations done, construction records,
details of materials used, construction methods as well as dates of construction.
Furthermore, various technologies have extended the capability of visual inspection by using
devices like fiberscopes, bore scopes, magnifying glasses and mirrors, portable and permanent
7 video monitoring equipment, and robotic crawlers. Use of these technologies enable visual
inspection where direct perspective is impractical; for example, the bore scope provides enhanced
ability to inspect the interior portions of post-tensioned ducts for grout voiding and strand
corrosion (Azizinamini, 2012).
2.3 Mechanical Wave Methods
The most commonly used mechanical wave techniques are ultrasonic pulse velocity methods,
impact methods, and acoustic emission. These methods make use of (small amplitude) mechanical
motion that is imposed in a material. However, they differ in the way that the stress waves are
generated and on the signal processing techniques that are used (Azizinamini, 2012).
The two main modes of wave propagation in a solid are longitudinal (compression) waves and
transverse (shear) waves. In addition, the mechanical surface excitation will generate other tertiary
surface waves. The mechanical wave methods discussed in this section are based on the behavior
of the compression and the shear waves. The direction of the particle movement in these two
independent modes are shown in Figure 2.1. The compression waves moves faster (higher
velocity) than the shear waves and therefore, these wave forms are often referred to as the primary
and secondary waves, respectively. The typical primary wave velocity for good quality concrete
is about 4000 m/s (13,100 ft./sec) and the typically secondary wave velocity is about 2500 m/s
(8,200 ft./sec).
8 Fig. 2.1: Longitudinal Wave Propagation (Top) and Transversal Wave Propagation (Bottom)
(From Ultrasonic Technique for the Dynamic Mechanical Analysis of Polymers, BAM, 2007)
In a homogeneous, isotropic elastic material, the compression wave velocity, Vp, and the shear
wave velocity, Vs, can be expressed in terms of the elastic modulus, E, density, ρ and Poisson’s
ratio, .
(2-1)
and
(2-2) The mechanical wave technique for thickness and void detection is based on the fundamental
concept known as Snell’s Law. Assume that a compression (P) wave is imposed on a material
with velocity V1 and the P wave strike an interface with another material with different wave
velocity V2 (e.g. concrete and air). The incident P wave will be reflected and transmitted at the
interface as P and S waves, respectively (See Figure 2.2). Snell’s law governs the relationships
between the angles of the incident, reflected and transmitted waves.
9 Fig. 2.2: Conversion of a P wave striking an interface between dissimilar materials,
following Snell’s law (Mehta and Monteiro, 2014)
In addition to the reflection and transmission of waves, the waves in real materials are absorbed
and scattered. This phenomena is denoted attenuation. The scattering of waves result from the fact
that the propagating medium (like concrete) is not homogenous at the micro level. It contains
internal boundaries at which the waves changes abruptly because paste and aggregates have
different velocities (elastic properties) (KrautKramer, 1990). Scattering is the reflection of the
wave in directions other than its original direction of propagation. Absorption is the conversion
of the wave energy to other forms of energy. Therefore, in real materials, the amplitude of the
reflected wave is lower than the amplitude of the incident wave.
Generally, a reliable value of attenuation can only be obtained by determining the attenuation
experimentally for the particular material being used (NDT Resource Center, 2013). This can be
done by varying the frequency during testing, especially at locations where the internal component
of the structure is known to compare how best the internal structure matches with the scanned data.
2.3.1 Ultrasonic Testing
Ultrasonic inspections in concrete are used to detect flaws (such as voids) as well as to determine
dimensions and material properties (Green, 1987; Bray and McBride, 1992). Ultrasonic inspection
10 consists of sending and receiving waves in materials and measuring the associated travel time of
the waves. The wave frequency typically used for concrete structures is in the range of 20 to 150
kHz, however 50 to 60 kHz is suitable for most concrete applications. Low frequencies can be
used for very long concrete path lengths and high frequencies can be used for mortars or for short
path lengths. High frequency pulses have a well-defined onset but, as they pass through the
concrete, become attenuated more rapidly than pulses of lower frequency (International Atomic
Energy Agency, Vienna, 2002).
Condition and properties of the test material are determined by analyzing various properties of the
sent and received waves. Typical schematics of ultrasonic inspection systems are shown in Figure
2.3 and the configurations are classified as direct and indirect transmission between the transmitter
and receiver.
Fig. 2.3: Configuration of a Transmitter and Receiver for (a) Direct and (b) Indirect
Transmission (Mehta and Monteiro, 2014)
Several concrete variables can affect the wave velocity in concrete such as level of hydration
(porosity), moisture conditions, amount and type of aggregates, micro-cracking and presence of
reinforcement (Mehta and Monteiro, 2014). It is important to be aware of the influence of these
variables on the results.
Examples of nondestructive testing methods based on the ultrasonic testing principle are pulse
velocity, ultrasound pulse echo, impact echo, and acoustic emission. They are based on the
11 principle of sending and receiving waves in a material where condition and properties of the test
material are determined by analyzing various properties of the sent and received waves. The
methods differ in the way that the stress waves are generated and on the signal processing
techniques that are used.
2.3.1.1 Pulse Velocity Test
Pulse velocity testing is based on measuring the travel time of the compression wave between the
transmitter and receiver. The direct transmission method is preferred in pulse velocity testing with
sensors placed on either side of the concrete and immediately opposite each other (see Figure 2.3).
The pulse generated by the transmitter causes waves of several modes but only the travel time of
the fastest traveling wave form, the compression wave, is measured. The contact between the
concrete and transmitter/receiver is ensured using a liquid coupling material such as cellulose
paste. (International Atomic Energy Agency, 2002). A schematic of the pulse velocity system is
shown in Figure 2.4.
Fig 2.4: Schematic of pulse velocity device (Crawford, 1997)
The pulse velocity, V, is given as the simply ratio of path length, L, of the wave divided by the
time, T, it takes for the wave to traverse that path length as shown in equation 2-3
(2-3)
12 Pulse velocity testing is primarily used to determine:

the uniformity of the concrete expected to have the same properties

any concrete property changes over time

the concrete quality by correlating the pulse velocity and the concrete strength

the elastic properties (elastic modulus and Poisson’s ratio)
It is challenging to determine defects using the pulse velocity testing. When compression wave
strikes the interface between concrete and air, defects larger than the width of the
transmitter/receiver as well as longer than the wavelength of the compression wave may be
detected. In general, defects larger than about 4 inches (100 mm) can be detected (International
Atomic Energy Agency, 2002).
2.3.1.2 Ultrasound Pulse Echo
Ultrasound pulse echo testing can be used to detect objects, interfaces, and anomalies. A major
advantage is the application of the indirect setup requiring only a one sided access to the concrete
surface. If the wave velocity, V, is known, the wave travel time, t, between the transmitter, defect
and receiver is used to directly estimate the depth of the defect, d. The depth of the defect from
the surface of the concrete as shown in equation 2-4:
(2-4)
The illustration in Figure 2.5 shows the transmitted and reflected wave path. This reading is
denoted as A-scan. The method is also denoted pitch-catch method.
Fig 2.5: Illustration of the Basis of Ultrasonic Pulse Echo Method
(From NDT Systems catalog, Germann Instrument, 2010)
13 Ultrasound Pulse Echo testing is often conducted at an ultrasound frequency of 50 kHz because of
the scattering of the sound waves by the aggregates and air pores (Nead et al, 2013). Care should
be taken during testing and analysis with the adjustment of the measuring frequency to reduce the
amount of attenuation.
It is recommended to use an array of a minimum of 10 transducers (transmitter/receivers) when
using this method for thickness assessment. A mathematical method is used to determine the true
depth by use of a phase shift superposition technique to determine the maximum synthetic echo.
The ultrasound pulse echo is capable of assessing defects in concrete elements, debonding of
reinforcement bars, shallow cracking, and delamination. The method was also successfully used
in the detection of material interfaces, based on phase evaluations of the response. Examples
include the interfaces between concrete and steel (e.g., reinforcement) or concrete and air (e.g.,
grouting defects) (Taffe and Wiggenhauser 2006; Afshari et al. 1996; Krause et al. 2008; Hevin et
al. 1998). Very shallow flaws may remain undetected because the surface waves mask the needed
compressional or shear-wave signals. Also, as ultrasonic pulse echo works with lower frequencies,
some of the defects might remain undetected (Nead et al, 2013).
Investigating ducts is sometimes complicated, in particular for structures with thick concrete cover
greater than 4 inches (100 mm). Comprehensive analysis of many A-scan readings, using 3D
SAFT (Synthetic Aperture Focusing Technique), produces a 3D topography image and the position
of the duct and potential voids can be identified.
2.3.1.3 Impact-Echo
The impact echo (IE) method is a seismic or stress wave–based method used in the detection of
defects in concrete, primarily delamination (Sansalone and Carino 1989). The objective of the
method is to detect and characterize wave reflectors or “resonators” in a concrete. This is achieved
by striking the surface of the tested object and measuring the response at a nearby location (Nead
et al, 2013).
14 The concrete surface is struck (e.g. steel balls), and the response is measured by a nearby contact
or air-coupled sensor. The position of the defects or reflectors is obtained from the frequency
spectrum of concrete’s response to an impact (Nead et al, 2013).
This is an effective method of locating defects in piles, caissons, and plates. A mechanical impact
produces stress waves of 1 to 60 kHz. The wavelengths from 50 mm to 2000 mm propagate as if
in a homogeneous elastic medium. The mechanical impact on the surface generates compression,
shear and surface waves. Internal interfaces or external boundaries reflect the compression and
shear waves. When the waves return to the surface where the impact was generated, they can be
used to generate displacements in a transducer and subsequently, a display on a digital
oscilloscope.
The transducer acoustically mounted on the surface receives the reflections created from the
energy impact. The amplitude spectrum is then analyzed to determine if any discontinuities are
present (Pessiki et al, 2010). Figure 2.6 illustrates the basis of the IE method.
Figure 2.6: Illustration of the Basis of the IE method. (Pessiki et al, 2010)
2.3.2 Acoustic Emission
Acoustic emission is based on analyzing the response of the material during crack propagation.
The above discussed stress wave methods are different in that they are based on analyzing the
response of the material when subjected to a known signal.
15 Acoustic Emission (AE) is defined as the elastic energy released from materials which are
undergoing deformation and crack propagation. The rapid release of elastic energy, the AE event,
propagates through the structure to arrive at the structure surface where a transducer is mounted.
These transducers detect the displacement of the surface at different locations and convert it into
a usable electric signal. By analysis of the resultant waveform in terms of feature data such as
amplitude, energy and time of arrival, the severity and location of the AE source can be assessed
(Hellier, 2001).
The main problem in field application is to distinguish the emission caused by defects from
ambient emissions. Only breaks occurring after the installation of equipment can be detected. For
passive monitoring, vibration sensors are strategically mounted on structural members. These
vibration sensors are monitored to “listen” for fracture events due to corrosion of wire strands in
the tendons (Azizinamini et al, 2012). Furthermore, severe attenuation of elastic waves in strands
bonded with concrete substantially reduces the method's effectiveness (Ciolko et al, 1999).
2.4 Magnetic Methods
Magnetic methods make use of the interaction between magnetic (and associated electric) fields and
their interaction with matter. The magnetic based cover meters have been widely used in the concrete
industry for locating the steel reinforcement and to determine the concrete cover over the
reinforcement. In the last decades the application of the magnetic flux leakage (MFL) method has
been further developed to include the estimation of loss of steel reinforcing cross section area due to
corrosion (Azizinamini, 2012). While this is a very promising technique that would allow the engineer
to quantify reinforcement loss, standardized equipment is not readily available.
2.4.1 Magnetic Flux Leakage (MFL)
The use of MFL for inspecting steel in prestressed concrete members was studied by Kusenberger and
Barton (1981) and Sawade and Krause (2007) and was later extended to other on-site inspections
(Grosse 2007). Ghorbanpoor et al (2000) developed an MFL sensor using permanent magnets to
magnetize steel components in concrete and used the amplitude of detected MFL signals to determine
empirically the flaw volume. DaSilva et al. (2009) also reported the use of MFL to estimate the amount
of material loss of corroded steel strands in concrete. Studies performed have also shown that the
16 methodology for detecting corrosion in post-tensioned concrete bridges is promising
(Ghorbanpoor et al, 2000).
Instrumentation development has been subjected to evaluation and upgrade over the years. Various
data analysis techniques have been developed to aid the interpretation of relevant test results. Flaws
were recognizable when the flaw size was larger than approximately 10 percent of the crosssectional area of the specimen. With flaws smaller than 10 percent of the cross sectional area, the
correlation method, a signal analysis method based on the correlation concept was shown to be
effective (Ghorbanpoor et al, 2000). A study comparing residual magnetic field measurements to
magnetic flux leakage measurements, as a method to detect broken prestressing steel, was
conducted by Makar et al (2001). According to this investigation, analyses of two and three
dimensional magnetic field plots show that the residual magnetic field technique is capable of
detecting corrosion of a single wire in a seven-wire strand embedded in concrete with a maximum
concrete cover of 2.5 inches. MFL method has been used in Germany and reported that the
parameters associated with fractured wires are quantitatively identifiable in the laboratory (Scheel
et al, 2003). They also reported application of the method in the field.
Magnetic flux leakage for assessing condition of steel in concrete structures utilizes the
ferromagnetic property of the steel to detect disturbance of an externally applied magnetic field
due to the presence of flaws in the steel. If the direction of the applied magnetic field is set to be
collinear with the longitudinal prestressing steel, the flux lines will also be collinear with the steel
elements in the concrete. Any change in cross sectional area of the steel at any point will cause a
flux leakage, where the continuous flux lines within the steel will be forced into the medium
surrounding the flaw as shown in Figure 2.7. Since the effect of concrete on magnetic field is
negligible, the field leakage may be measured by Hall sensors in the air near the surface of the
concrete. If the magnetic source and sensors are moved along the length of the concrete member,
the changes in the field, due to the presence of flaws in steel, can be recorded as continuous in
terms of time and the field amplitude. This is then analyzed to obtain information relevant to the
location and extent of the flaw in the steel (Ghorbanpoor, 1999).
17 Fig 2.7: Change in magnetic flux before and after detection of flaw (ndt.net)
Magnetism is caused by the presence of molecular current loops, which is caused by two
phenomena: the motion of electrons within the atoms and the spin direction of the electrons within
the magnet material. In the presence of a magnetic field, a moving charge has a magnetic force
exerted upon it. The relationship that governs the force on moving charges through a magnetic
field is called Faraday’s Law and implies that as a charge q moves with a velocity V in a magnetic
field B, it experiences a magnetic force F.
(2-5)
where:
F
is magnetic force (newton)
q
is charge (coulomb)
V
is velocity (meter/second)
B
is applied magnetic field (weber/sq. meter)
When a magnetic field comes near ferromagnetic (steel) material, the magnetic flux lines will pass
through the steel bar because the steel offers a path of least resistance due to high magnetic
permeability compared to the surrounding air or concrete. When discontinuities of defects
(corrosion) are present, this low resistance path becomes blocked and the remaining steel may
become saturated, forcing some of the flux to flow through the air. Total saturation is not required
for detection as orientation of the magnetic field can be altered by even small levels of corrosion.
However, there is a saturation level at which all dipoles are aligned and no further alignment is
possible. At saturation level, the following relationship is valid:
18 (2-6)
where:
B
is Magnetic flux (weber/sq. meter)
µ
is Magnetic permeability of the material (weber/ampere meter)
H
is Magnetic field strength (ampere/meter)
To use the concept of MFL as a non-destructive evaluation (NDE) tool, the device must have the
ability to measure changes in the path of magnetic field force lines near a ferromagnetic material.
Such changes in the components of the flux can be detected by one or more sensors and can be
analyzed to determine the extent or severity of the flaw. Hall-effect sensors are often used to detect
and measure MFL. The sensors are made with semi-conductor crystals that when excited by a
passage of current perpendicular to the face of crystal, react by developing a voltage difference
across the two parallel faces. The possibility of fabrication of those sensors at any size allows the
detection of small flaws on the steel (DaSilva et al, 2009)
There are two primary methods for detecting these field anomalies: active and residual. In the
active method, the sensors are placed between the poles of the magnet and readings are obtained
as the device is passed over the specimen. In the residual system, the specimen is first magnetized;
it is important that the strands become magnetically saturated. Then the device is passed over to
read the residual magnetic field. Active MFL is appropriate when large areas of corroded regions
exist inside the ducts. However, when the corroded area is small, active MFL is no longer effective
(Marcel et al, 2009). An illustration of magnetic flux leakage method is shown in Figure 2.7.
2.4.2. Ground Penetrating Radar
Ground Penetrating Radar (GPR) is a pulse method for locating structural defects (voids and
delamination), location of steel reinforcement, pavement layer thickness and structural changes
(Loizes et al, 2006; Mehta and Moneteiro, 2014). GPR system is based on the concept of sending
an electromagnetic pulse through an antenna to the concrete surface of the testing specimen and
then recording the reflected pulses from the internal interfaces, where there is a variation in the
dielectric properties (AL-Qadi et al, 2005). It is this reflection phenomenon that allows radar
19 images to be created. A wide range of signal frequencies are used for various practical applications
ranging between 50 MHz and 2.5 GHz. Generally, for inspection of concrete structures, antennas
with frequencies of 1 GHz are used (Hugenschmidt, 2002; Mehta and Monteiro, 2014).
2.4.2.1 Physical Principle
The two most important factors affecting the propagation of radar pulses in any material are the
electrical conductivity and the dielectric constant (Sarrakento, 1997). The behavior of a beam of
electromagnetic (EM) energy as it hits an interface between two materials of dissimilar dielectric
constants, has been shown in Figure 2.8. Part of the energy would be reflected and the remaining
goes through the interface into the second material. The intensity of the reflected energy, AR, is
related to the intensity of the incident energy, AI, by the following relationship:
Fig 2.8: Propagation of EM energy through dielectric boundaries.
(International Atomic, Agency, 2002)
(2-7)
,
where
,
/
:
the reflection coefficient at the interface
:
the wave impedance of materials 1 and 2, respectively, in ohms
or written in terms of dielectric constants,
and
20 , of material 1 and 2 respectively:
,
√
√
√
√
(2-8)
This gives an indication that when a beam of electromagnetic energy hits the interface between
two media of different dielectric constants, the amount of reflection
,
is determined by the
values of their relative dielectric constants. The dielectric constant is strongly affected by the water
content and also several other factors such as mix proportions, porosity, pore fluid solution and
shapes of particles and pores.
The result of a GPR relief is a radar gram. The radar technique allows to establish the number,
position and diameter of all steel bars existing in a surveyed structure and it also allows to locate
possible cavities in both reinforced concrete structures and masonry structures. The
electromagnetic impulse technique offers the advantage of portable equipment and the capability
to detect rapidly large areas from one surface in a completely non-destructive and non-invasive
way. One of the main difficulties with the use of GPR is that the results are very difficult to
interpret and a skilled technician must be required to produce a reliable end result (Bungey et al,
2003).
2.4.2.2 Application
Locating of reinforcing bars in concrete is one of the most widespread applications of GPR in civil
engineering (Ulriksen, 1982). Recently, GPR is also used to locate ducts in post-tensioned bridges.
However steel ducts completely reflect the radar signal since they are conductive, so that the
tendon breaks or grout defects inside the duct cannot be detected (Pollock et al, 2008). GPR has
the potential to monitor grout conditions within non-conducting (plastic) ducts. In particular, GPR
is likely sensitive to the occurrence of soft, non-setting and chalky grout or cases of water intrusion
due to the changes in dielectric constants and hence reflection (Azizinamini et al, 2012).
2.5 Electrochemical Methods
These methods are used to monitor active corrosion in concrete structures by making use of the
electrochemical basis of the process. These approaches offer potential to measure meaningful data
related to active corrosion of strand. Most electrochemical techniques use the same measurement
21 set up. It consists of a reference electrode, a working electrode, a counter electrode, and a volt
meter. A closed electrical circuit is always required and direct electrical connection to the steel
reinforcement must be established as shown in Figure 2.9 (Azizinamini, 2012).
Fig 2.9: Setup for electrochemical techniques (Azizinamini and Gull 2012)
Two non-destructive techniques are commercially available based on the electrochemical process.
These are half-cell potential and polarization resistance. The former gives information on the probability of corrosion and the latter is related to the corrosion rate. The half-cell potential
measurement is practically and widely employed to identify the presence of corrosion. 2.5.1 Half-cell Potential
The half-cell potential (HCP) measurement is a well-established and widely used electrochemical
technique to evaluate active corrosion in reinforced steel and prestressed concrete structures. The
method can be used at any time during the life of a concrete structure and in any kind of climate,
provided the temperature is higher than 2°C (Elsener, 2003). Half-cell measurements should be
taken on a free concrete surface, because the presence of isolating layers (asphalt, coating, and
paint) may make measurements erroneous or impossible. Generally, the potential difference
between the reinforcement and a standard portable half-cell, using a Cu/CuSO4 standard reference
electrode, is measured when the electrode is placed on the surface of a reinforced concrete element.
When the reference electrode is shifted along a line or grid on the surface of a member, the spatial
distribution of corrosion potential can be mapped (Baumann 2008; Gu and Beaudoin 1998).
22 Using empirical comparisons, the measurement results can be linked to the probability of active
corrosion. Furthermore, it is meaningful to assess the change in potential over a larger surface area
to assess if there is an increased chance for an active corrosion cell.
2.5.1.1 Physical Principle
The procedure for measuring half-cell potentials is comparatively straight-forward. A good
electrical connection is made to the reinforcement, an external reference electrode is placed on the
concrete surface and potential readings are taken on a regular grid on the free concrete surface.
Different microprocessor-controlled single or multiple electrode devices have been developed and
are commercially available. The best way of representing the data has found to be a color map of
the potential field and contour plot, where every individual potential reading can be identified as
a small square. Alternatively, frequency plot can be used as well.
Electro-chemical half-cell potential mapping is used to identify localized corrosion of
reinforcement which greatly improve the quality of condition assessment of concrete structures
(ASTM C 876, Elsener 1992 & 2001, and Cairns and Melville, 2003, Pradhan and Bhattacherjee,
2009). Electrons flow through the steel to the cathode, where they form hydroxide (OH-) with the
available water and oxygen. This creates a potential difference that can be measured by the halfcell method. A schematic view of the electric field and the current flow on steel in concrete is
presented in Fig 2.10. Table 2.1 shows interpretation of half-cell potential values as per ASTM
C876.
Fig 2.10: Schematic view of the electric field and current flow of an active/passive macro-cell on
steel in concrete (Elsener 2001)
23 It is often necessary to use other data such as chloride contents, depth of carbonation, delamination
survey findings, rate of corrosion results and environmental exposure conditions, in addition to
corrosion potential measurement to formulate conclusions concerning corrosion activity of
embedded steel and its probable effect on service life of a structure. (ASTM Designation:
C876-09, Standard test method for corrosion potential of uncoated reinforcing steel in concrete)
Table 2.1 Interpretation of half-cell potential values as per ASTM C876
Half-cell potential (mV)
relative to Cu/CuSO4
Chance of rebar being corroded
< -500
Visible evidence of corrosion
-350 to -500
95%
-200 to -350
50%
> -200
5%
Typical order of magnitude for the half-cell potential of steel in concrete measured against a
Cu/CuSO4 reference electrode (RILEM TC 154-EMC) is presented in Table 2.2. It is to be noted
that Table 2.1 should not be used in isolation, but should be used considering the environmental
conditions of the concrete being tested, as this has an impact on the half-cell potential value as
shown in Table 2.2.
Table 2.2 Half-cell potential for different types of concrete
Concrete condition
Half-cell potential
Water saturated concrete without O2
-1000 to -900 mV
Moist, chloride contaminated concrete
-600 to -400 mV
Moist, chloride free concrete
-200 to +100 mV
Moist, carbonated concrete
-400 to +100 mV
Dry, carbonated concrete
0 to +200 mV
Dry, noncarbonated concrete
0 to +200 mV
24 The main disadvantage of this method is that the potentials measured are too sensitive to moisture
content, thickness of concrete cover, surface coating, resistivity of concrete, and the type of
electrode (Misra and Uomoto, 1990).
Theoretical considerations and practical experience on a large number of structures have
shown (Elsener et al, 1990, 1992 & 1979) that the results of potential mapping on existing
structures need careful interpretation. No absolute values of potential can be applied to indicate
corrosion hazard in a structure. Depending on concrete moisture content, chloride content,
temperature, carbonation of the concrete and cover thickness, surface coating, resistivity of
concrete, type of electrode, different potential values indicate corrosion of the rebar on different
structures. After repair work, interpretation of half-cell potentials can be even more difficult
because alkalinity, composition and resistivity of the pore solution in the repaired areas can be
greatly altered compared to the old existing concrete.
2.6 Summary
This chapter highlighted some of the most commonly used nondestructive testing method used for
condition assessment of steel reinforced concrete structures. The discussion was limited to topics
of visual inspection, mechanical wave methods, magnetic methods, and electrochemical methods.
The following basic principles governing the methods were briefly discussed as well as associated
applications.

Visual inspection is for surface evaluation of delamination, cracking, and stains.

Mechanical wave method is for detecting voids, defects as well as material interfaces.

Magnetic methods are for locating steel reinforcement and estimating the cross-sectional loss
of steel reinforcing material

Electrochemical methods are for assessing the chance for corrosive environment in
reinforced concrete.
25 CHAPTER 3: EXPERIMENTAL INVESTIGATION 3.1 Introduction
The experimental investigations were undertaken to evaluate the effectiveness of nondestructive
evaluation (NDE) methods to assess the condition of prestressed and post-tensioned strands in
concrete structures and verify proper grouting of post-tensioned ducts in segmental concrete
structures. The study utilized laboratory prepared and field specimens tested in the laboratory as
well as field inspection on existing structures. Three sets of laboratory specimen and three salvaged
beams received from Kent County Road Commission were evaluated in the laboratory.
Specimen 1 was made up of two (2) box beams constructed according to the geometry stiputed in
MDOT Bridge Guide section 6.65.10A. The box beams were simulated with voids created around
prestressing strands and transverse reinforcement using plastic tubes as detailed in section 3.2.3 of
this report. Specimen 2 was made up of two (2) beams constructed with reinforcement simulated
with grinding defects to depict cross- sectional area loss as detailed in section 3.3.1, for detection
and quantification using ultrasonic and magnetic flux leakage assessments. Specimen 3 was made
up of four (4) beams constructed with post-tensioned ducts simulated with grouting defects as
detailed in section 3.4.1 to be assessed using ultrasonic method. Laboratory prepared specimen
are shown in Figures 3.1 through 3.3.
Figure 3.1: Specimen 1 Box Beams, S1-1 and S1-2.
26 Figure 3.2: Specimen 2 Beams, S2-1, S2-2 and S2-3
Figure 3.3: Specimen Beams, S3-1, S3-2, S3-3 and S3-4
In addition, three (3) side-by-side salvaged box beams decommissioned by Kent County Road
Commission after they have been in service for 39 years were tested in the Structural Testing
Laboratory at Lawrence Technological University for both nondestructive evaluation as well as
residual flexural failure testing. Prior to flexural testing, the following nondestructive methods
were deployed: ultrasonic assessment for delamination and void detection; electro-chemical half27 cell assessment for detecting corrosive environment; impact hammer assessment of surfaces to
detect variations and potential delamination; and magnetic flux leakage to determine loss of cross
sectional area of reinforcement and strands. Details of all experimental procedures are presented
in subsequent sections.
Field inspections were also carried out on sections of the following existing bridges in Michigan,
US-131 over Muskegon River, southbound 6 miles south of Big Rapids and segments of I-96 over
Canal Road in Lansing. The employed nondestructive evaluation methods were ultrasonic,
magnetic flux leakage and electro-chemical half-cell assessments.
3.2. Construction of Two Samples of Specimen 1 Box Beams, S1-1 and S1-2
Two 15 feet long box beams were constructed with a cross section of 27 in x 36 in and in concept
designed according to MDOT Bridge Design Guides 6.65.10A with single layer of bottom
reinforcement. These laboratory beams represents box beam members in a typical reference bridge
with a span length of 60 feet and a deck width of 45 feet. The bridge would allow one lane of
traffic in each direction and two shoulders of 10 feet width. The laboratory beams were reduced
in length from 60 feet to 15 feet while still including a center transverse diaphragm and two cells.
This reduction enabled transporting the beams with the available lifts. Furthermore, due to the
limited scope of this project, no deck slab was incorporated during the construction of the beams,
and therefore, shear stirrups were not extended to integrate a slab system.
3.2.1 Design and Construction of Formwork.
The formwork was constructed mainly from plywood and consisted of base chairs, base-plates and
side plates. The base plate was supported on base chairs at a spacing of two feet along the length
of the base plate. Horizontal and vertical braces were also provided to support the side-plates and
to ensure straight alignment of the edges of the box-beams.
Interior cells were constructed using plywood boxes. The cells were fixed in place by suspending
them on threaded rods with end nuts outside the formwork. In the laboratory setting the final
placement of the cells were well controlled. For instance, one of the cells was intentionally shifted
horizontally towards one side of one box beam to create a thinner web. The intention was to
28 evaluate the effectiveness of the phased array 3D ultrasonic tomography method to detect and
quantify defects in the thinner web. Formwork was designed and constructed in the laboratory by
research assistants and laboratory technical staff as shown in Figures 3.4 through 3.6.
Figure 3.4: Construction of formwork.
Figure 3.5: Construction of interior cells
Figure 3.6: Formwork set-up
29 3.2.2 Reinforcement Cages
All reinforcements were cut, bent and installed in the laboratory. Each box-beam cage was made
up of mild steel #4 steel stirrups at different spacing and five #4 non-prestressing bars provided
as top and bottom reinforcement according to MDOT Bridge Design Guides 6.65.10A. The center
of top reinforcements were located at 2.5 inches from the top surface of the beam. The mild steel,
longitudinal bars and stirrups, for the box beams reinforcement were Grade 60.
In addition, there were a total of 12 prestressing steel strands located at the bottom of the beam.
The steel prestressing strands were Grade 270 relaxed strands with diameter of 0.6 inch. The center
of the prestressing strands were located at 2 inches from the bottom surface of the beam. A total
prestressing force of 280 kips was symmetrically distributed among the bottom steel strands along
the cross section of each beam as follows: 4 strands were stressed 10 kips each, another 4 were
stressed 23 kips each, 2 out of the remaining 4 were stressed 30 kips each and the remaining 2
were stressed 44 kips. Details of the cross-section and longitudinal sections of the beams have
been provided in Figures 3.7 and 3.8. Figures 3.9 through 3.12 depicts the preparation and
installation of rebar for S1-1 and S1-2.
Figure 3.7: Cross-sectional details of box-beams
30 Figure 3.8: Longitudinal details of box-beams
Figure 3.9: Bending stirrups
Figure 3.10: Assembling of reinforcement
Figure 3.11: Reinforcement cages for box-beam
31 Figure 3.12: Installing edge & side Panels
3.2.3 Simulation of Defects
Defects of voids were simulated along the length of reinforcement to depict voids created around
reinforcement as a result of the corrosion process, see Figures 3.13 through 3.16. Furthermore, two
additional types of defects were used; three (3) simulated honeycombs and heavy grease around
several of the prestressing strands. The goal of these activities were to determine if these defects
could be detected using the phased array ultrasonic 3D tomography. The defect types and their
locations were documented as shown in Figures 3.17 through 3.19.
Fig. 3.13: Plastic defects around reinforcement Fig. 3.14: Plastic defects around bottom strand
Fig. 3.15: Honeycomb at the top of box beam
Fig. 3.16: Plastic tubes for debonding
32 Figure 3.17: Top plan of box beam showing locations and defect length
around Top reinforcement
Figure 3.18: Bottom Plan of Box Beam 1 showing locations of defects around strands
Figure 3.19: South Elevation of Box Beam 1 showing locations of defects around strands
33 3.2.4 Prestressing Steel Strands
End plates for the prestressing bed were redesigned to suit the proposed beam size and to fit for
the strands spacing as shown in Figure 3.20
Figure 3.20: End plates for prestressing bed.
Figure 3.21: Prestressing steel strands
When construction of the steel cages and formwork were completed, the steel cages were placed
inside the formork. Plastic chairs with effective height of 1.5 in were attached to the underside of
the cages to provide the bottom cover. In addition, 1.25 in chairs were also attached to either side
of the cages to create the side concrete cover. Bulkheads were mounted at the two extreme ends of
the two beams. Conventional steel chucks were used as anchorage systems for the steel strands
and attached at both the live and dead-ends of the steel strands. Figure 3.21 depicts the application
of prestressing forces to the strands.
34 3.2.5
Concrete Placement and Preparation of Cylinders.
Six (6) cubic yards of concrete with compressive strength of 7000 psi ordered from MCCOIG
Materials was used for the casting of the two (2) box beams and preparation of 24 concrete
cylinders for compressive strength testing as shown in Figures 3.22 and 3.23. Two slump tests
were conducted prior to casting of the box-beams and immediately after in accordance with ASTM
C143/C143-05 to check the workability of the concrete. Two electrical vibrators were used for
consolidation as well as mechanical rods around the diaphragm locations and the end-block of the
beams where the steel reinforcements were closely spaced.
Figure 3.22: Casting of Box Beams, S1-1 and S1-2
Fig. 3.23: Slump Test / Cylinders Preparation
35 3.2.6
Curing Box Beams and Releasing of Steel Strands
The beams were cured under wet burlaps layered with plastic sheets to limit moisture loss and
surface cracking. Water was applied to the top beam surfaces for 7 days. Steel strands were
released after 4 days of curing by cutting them from the bulkheads in order to transfer the
compressive stresses to the concrete after it had gained adequate strength.
Figure 3.24: Curing Beams
Figure 3.25: Releasing of steel strands.
3.2.7 Compressive Strength of Box Beams
The average 28 day compressive strength of the cylinders was 7,694 psi. The development of
average compressive strength with age and the summative data are shown in Table 1. The
compressive strength was determined according to ASTM C39 - 05, Standard Test Method for
Compressive Strength of Cylindrical Concrete Specimens. Figures 3.26 and 3.27 shows
compressive strength testing for 4 days and 7 days respectively, the prestressing force was
transferred at the 4 days compressive strength.
36 Table 3.1: Compressive Strength Development using 6 inch by 12 inch Cylinders
Identification
Number
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
Max. Load
(Pound-force)
106,990
118,320
112,070
142,470
139,050
147,400
182,260
201,780
179,650
207,050
218,790
226,750
Compressive
Strength (psi)
3,784
4,185
3,964
5,039
4,918
5,213
6,446
7,137
6,354
7,323
7,738
8,020
Fig. 3.26: Four (4) Days at Transfer
Fig. 3.27: 7 Days Compressive Strength
37 Age of specimen
(days)
4
4
4
7
7
7
14
14
14
28
28
28
3.2.8
Ultrasonic Assessment of S1-1 and S1-2.
Testing was done by scanning the entire geometry of the box beams to ascertain how correctly the
ultrasonic assessment predicts the thicknesses of the various components of the box beams. This
was used as a reference during scanning of strands, for instance to check on the location of strand
from the concrete surface a reference was made to the thickness of the bottom flange.
The following testing protocol was used for S1-1 and S1-2.
(i)
Complete scan of all four longitudinal surfaces of each beam with varied frequencies of
30, 40, 50, 60, 70, 80, 90 and 100 kHz to examine how the different thicknesses of the
various components of the beam were predicted with the above frequencies.
(ii)
Localized scan on defects: plastic tubes (denoting voids and debonding) and grease with
varied frequencies of 30, 40, 50, 60, 70, 80, 90 & 100 kHz to simulate assessment of voids
which usually occur around corroded strands and reinforcement as a result of corrosion.
(iii)
Localized scan on defects: honey combs with frequencies of 30, 40, 50, 70, 80, 90 & 100
kHz to simulate deterioration in concrete structures such as cracks and delamination which
usually occurs as a result of corrosion of strands and reinforcement.
(iv)
Localized scan on reinforcement and strands without defect, with frequencies of 30, 40,
50, 60, 70, 80, 90 & 100kHz were compared with localized scan on plastic defects to check
on the differences of the various scans.
(v)
Localized scan on solid concrete sections without reinforcement, strand or any defect, with
frequencies of 30, 40, 50, 60, 70, 80, 90 & 100 kHz
Figures 3.28 through 3.30 show ultrasonic assessment from various locations along box beams
S1-1 and S1-2. Detailed analysis of the test results have been presented in sections 4.1 and 4.2.
38 Figure 3.28: Scanning on the side of S1-1
Figure 3.29: Scanning on top of S1-2
Figure 3.30: Scanning on the bottom of S1-2
39 3.3 Construction and Testing of Three Specimen 2 Beams
3.3.1
Construction of Specimen 2 Beams
3.3.1.1 Formwork and Preparation of Reinforcement/Strands
Formwork was designed and constructed in house in the laboratory by research assistants and
laboratory technical staff. Three sets of formwork were constructed to simulate three different
samples of Specimen 2 as shown in Figure 3.31. Reinforcement were cut and grinded to the
required percentage losses of cross-sectional areas as shown in Figure 3.32. The percentage losses
considered were 5, 10, 15, 20 and 30 with varying lengths of grinding along the reinforcement.
Two lengths of grinding configurations considered were 1inch-1inch-1inch and 5inches-2inches5inches as shown in Figures 3.33 and 3.34. Wires along 0.6 diameter strands were also cut varying
from one wire cut to four wires cut with varying cutting lengths of 0.25 inch to 1 inch and were
covered with dual-wall polyolefin plus duct tape as shown in Figure 3.36.
Fig. 3.31: Constructing formwork
Fig. 3.32: Cross-sectional area loss on reinforcement
Fig. 3.33: Grinding configuration of 1(0.5+0.5)inch-1inch-1(0.5+0.5)inch for different
percentage losses for reinforcementbar
40 Figure 3.34: Grinding configuration of 5(1+1+1+1+1) inches-2 inches-5(1+1+1+1+1) inches
for different percentage losses for reinforcement
Length of cut
in steel strand
Plastic tube &
duct tape
Figure 3.35: 0.6” Strand length of wire cut
Figure 3.36: Wire cut covered with dual wall
Polyolefin tube and duct tape
3.3.1.2 Preparing the cages
Reinforcemnet and strands were tied with documented defect locations as shown in
Figures 3.37 through 3.39. Three sets of cages were formed, one for single layer of top and bottom
reinforcement with defects in both top and bottom reinforcement, a second with double bottom
layer of reinforcement with defects in both top reinforcement as well as second layer of
reinforcemnt from the bottom. A third was formed for single layer of top strands with double layer
of bottom strands with defects in the top strands and the second layer of strands from the bottom.
The reinforcement grinded to simulate cross-sectional area losses were #5 steel of Grade 60 with
#3 stirrups of Grade 60. The longitudinal prestressing strands with wire cuts to simulate crosssectional area losses were 0.6 in, low-relaxation seven-wire steel strands of Grade 270.
41 Loss in cross sectional area Figure 3.37: Cages of reinforcement and strands
Figure 3.38: Cross-sectional Area loss
Wire cut defect Figure 3.39: Strands showing loss of wires covered with
Dual wall polyolefin tube and duct tape
3.3.1.3 Placing of Concrete and Preparation of Concrete Cylinders
Two cubic yards of concrete with compressive strength of 7000 psi ordered from MCCOIG
Materials was used for the casting of three (3) Specimen 2 beams and preparation of cylinders for
compressive strength testing as shown in Figures 3.40-43.
42 Fig. 3.40: Placing of concrete
Figure 3.42: Checking for slump
Fig. 3.41: Leveling top of specimen 2 beams
Figure 3.43: Preparation of concrete cylinders
3.3.2 Ultrasonic Assessment of S2-1, S2-2 and S2-3.
Ultrasonic testing of Specimen 2 beams was undertaken using the following testing protocol.
(i) Complete scan of top and bottom surfaces of each of S1-1, S2-2 and S2-3 with varied
frequencies of 30, 40, 50, 60, 70, 80, 90 and 100 kHz to examine how the different
cross-sectional area losses due to grinding of reinforcement and cutting of wires of strands
were detected with the above frequencies.
(ii) Localized scans on defect locations with varied frequencies of 30, 40, 50, 60, 70, 80, 90 & 100
kHz
43 (iii) Localized scan on reinforcement and strands without defect, with frequencies of 30, 40, 50,
60, 70, 80, 90 & 100 kHz.
Details of reinforcement configurations for S2-1, S2-2 and S2-3 with documented locations of
these simulated defects are shown in Figures 3.44 through 3.52. Figure 3.53 shows ultrasonic
assessment set-up for S2-1. Figures 3.54 and 3.55 show typical scan on reinforcement with defect
as well as scan on reinforcement without defect.
44 Figure 3.44: Top Reinforcement Arrangement and Percentage Losses for S2-1 with Total Length of 3 inches
Cross-sectional Area Loss
45 Figure 3.45: Bottom Reinforcement Arrangement and Percentage Losses for S2-1 with Total length of 3 inches
Cross-sectional Area Loss
46 Figure 3.46: Cross-sectional Area of S2-1 showing Single Bottom Reinforcement
47 Figure 3.47: Top Reinforcement Arrangement and Percentage Losses for S2-2 with Total Length of 12 inches
Cross-sectional Area Loss
48 Figure 3.48: Bottom Reinforcement Arrangement and Percentage Losses for S2-2 with Total Length of 12 inches
Cross-sectional Area Loss
49 Figure 3.49: Cross-sectional Area of S2-2 showing Double Bottom Reinforcement
50 Figure 3.50: Top Strand Arrangement, Length and Number of Wire cuts for S2-3
51 Figure 3.51: Bottom Strand Arrangement, Length and Number of Wire cuts for S2-3
52 Figure 3.52: Cross-sectional Area of S2-3 showing Double Bottom Strands
53 Figure 3.53: Ultrasonic assessment set-up for S2-1.
Reflection from
reinforcement with
cross-sectional area loss
Back wall reflection
from underside of beam
Figure 3.54: Scan on Reinforcement with defect at frequency of 70 kHz with dB of 33.8
54 Reflection from
reinforcement without
cross-sectional loss
Back wall reflection
from underside beam
Figure 3.55: Scan on reinforcement without defect at frequency of 70 kHz with dB of 33.8
3.3.3 Calibration of Magnetic Flux Leakage (MFL) Mobile Scanner
Calibration of the MFL Mobile scanner was done, using undamaged reinforcement with 10%, 20%
and 30% loss of cross-sectional area for reinforcement #4 - #9. Two timber formworks of size
48 inches x18 inches x 11inches were constructed with three (3) 2 inches holes punched on two
opposite sides of the shorter faces to calibrate the influence of the magnetic system on the
reinforcement at increasing depths away from the magnetic influence as shown in Figure 3.56. The
first formwork had holes at increments of 2.5 inches, while the second had holes at depths of
1.5 inches, 3 inches, and 6 inches. A baseline reading of 2.935 volts from the middle Hall-effect
sensor, M3, was taken by running the scanner over the formwork with no reinforcement present.
Undamaged reinforcement #4 through #9 each were scanned ten (10) times at each depth
increments. The voltage reading from the middle hall-effect sensor, M3, was averaged for each
run and then all ten runs were averaged together.
The process was repeated for reinforcement #4 - #9 with the center foot of the reinforcement
gradually milled to a 10%, 20% and 30% loss in cross-section area. The max voltage reading,
which indicates the peak voltage in the affected area, from the middle hall-effect sensor, M3, was
averaged for each run and then all ten runs were averaged together.
55 Figure 3.57 shows the trend line of the baseline adjusted average peak voltage for each depth. The
graph in Figure 3.57 shows the trend line for each depth up to three inches. Figure 3.58 shows a
scan of reinforcement #7 at the 1.5 inch depth. There is an increase in voltage as the MFL mobile
scanner passes over the affected area. This is due to more magnetic flux reaching the Hall-effect
sensor array than would be the case if the reinforcement did not have cross-sectional loss. Figure
3.59 shows magnetic flux voltage change of #7 reinforcement at 1.5 inches depth with crosssectional area loss of 20%.
Figure 3.56: MFL Mobile Scanner Calibration Setup
56 Effect of Reinforcement Depth Below Surface on Hall Voltage Change for Increasing Rebar Size
140
120
Depth
2.5
inches
y = 116.36x0.2332
R² = 0.91
y = 110.88x0.3616
R² = 0.96
Hall Voltage (‐mV)
100
y = 62.981x0.2571
R² = 0.69
80
60
y = 62.545x0.1826
R² = 0.45
40
y = 42.294x0.1111
R² = 0.50
20
0.20
0.40
0.60
0.80
1.00
Reinforcement Cross Section Area (in squared)
Figure 3.57: Magnetic flux leakage influence with depth.
57 Depth 5
inches
Depth 6
inches
Depth
7.5
inches
0
0.00
Depth 3
inches
1.20
#7 Reinforcement 1.5 inch Depth from Surface, 20 % Cross Section Loss
2.95
Change in Vertical Magnetic Flux Leakage
2.90
Hall Voltage (V)
2.85
2.80
2.75
M3 baseline reading: 2.935 Volts
2.70
2.65
2.60
2.55
Change in Horizontal Magnetic Flux Leakage
2.50
2.45
0
5
10
15
20
Distance along Surface with MFL Centered Directly Above Rebar (Inches)
Figure 3.58: Graph of voltage change for each depth
Magnetic Flux Voltage Change
#7 Reinforcement, 1.5 inch Depth, 20% Cross Section Loss
Hall Voltage Change (mV)
80
60
Change in mV proportional to change in cross section area
40
20
Vertical (M3)
0
‐20
0
5
10
15
20
Horizontal (F8)
‐40
‐60
Distance along Surface with MFL Centered Directly Above Rebar (Inches)
Figure 3.59: Graph showing effect of 20% loss
58