Carbon Fiber Reinforced Polymer Repairs of Impact-Damaged Prestressed I-Girders

ABSTRACT Three different Carbon Fiber Reinforced Polymer CFRP repair techniques are examined to determine how effectively each method can restore the ultimate flexural capacity of impact

Carbon Fiber Reinforced Polymer Repairs of Impact-Damaged Prestressed I-Girders

Ryan J Brinkman B.S University of Cincinnati Thesis submitted to:

School of Advanced Structures College of Engineering and Applied Science Division of Graduate Studies University of Cincinnati

Dr Richard A Miller, Ph.D., P.E., FPCI For partial fulfillment of the requirements for the degree of Master of Science

November 2012 Committee:

Dr Bahram Shahrooz, Ph.D, P.E., FACI

Dr Kent A Harries, Ph.D., P.Eng., FACI, University of Pittsburgh

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ABSTRACT

Three different Carbon Fiber Reinforced Polymer (CFRP) repair techniques are examined to determine how effectively each method can restore the ultimate flexural capacity of impact-damaged prestressed concrete girders The number of severed prestressing strands and the amount of CFRP repair material applied are varied to create a large repair matrix The ultimate moment capacity was calculated using the program XTRACT Capacities were also evaluated using AASHTO and ACI

specifications

The prototype girder’s geometry is based on an impact-damaged I-girder from a bridge in Eastland County, Texas, which was repaired using CFRP in 2006 using conventional externally bonded CFRP The CFRP repair methods examined in this study were near surface mounted (NSM) CFRP, externally bonded (EB) CFRP, and bonded post tensioned (bPT) CFRP The area of the girder available for repair was limited to the bottom soffit for near surface mounted CFRP and bonded post tensioned CFRP; however, the externally bonded CFRP was applied on the bulb as well as the soffit The range of the repairs examined were approximately 25%, 50%, and 75%, and 100% of the maximum practical amount of CFRP which could be applied based on the girder geometry, ACI guidelines, and

manufacturers’ recommendations

The geometry of the girder limited the amount of CFRP which could be applied, so the repairs could only completely restore the ultimate girder capacity when very few strands were damaged and the maximum amount of CFRP was used However, the CFRP techniques were shown effective at restoring some of the lost capacity and thus are an option if only a portion of the lost capacity must be restored The study also showed that, in some cases, repairs are completely ineffective and the girder capacity is not increased beyond the damaged state Using the results of this study, engineers can determine when repairing a girder will be effective and the extent to which the repair restores lost capacity

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ACKNOWLEDGEMENTS

First I would like to thank my advisor, Dr Richard Miller for his help and guidance throughout, as well as giving me the opportunity to assist in this research I would also like to thank Dr Bahram Shahrooz and

Dr Kent Harries for serving on my thesis committee

I would to thank the National Cooperative Highway Research Program (NCHRP), Program Director Dr Waseem Dekelbab, and the Project Panel for funding and providing comments on this research under

NCHRP 20-07/Task 307 I also wish to thank the University of Cincinnati for awarding me a Graduate

TABLE OF CONTENTS

ABSTRACT ii

ACKNOWLEDGEMENTS iv

TABLE OF CONTENTS v

LIST OF FIGURES vii

LIST OF TABLES viii

CHAPTER 1: INTRODUCTION 1

1.1– INTRODUCTION 1

1.2 – OBJECTIVES 2

CHAPTER 2: LITERATURE REVIEW 3

2.1 – GUIDELINES FOR REPAIR OF PRESTRESSED CONCRETE BRIDGE ELEMENTS 3

2.1.1 - Shanafelt and Horn (1980) 3

2.1.2 - Shanafelt and Horn (1985) 4

2.1.3 - Post NCHRP 280 - Harries, Kasan and Aktas (2009) 7

2.2 - TESTING OF CFRP REPAIR TECHNIQUES ON EXPERIMENTAL GIRDERS 7

2.2.1 - Quattlebaum, Harries, and Petrou (2005) 7

2.2.2 - Nordin and Täljsten (2006) 9

2.2.3 - Casadei, Galati, Boschetto, Tan, Nanni, and Galeki (2006) 10

2.2.4 - Aram, Czaderski, and Motavalli (2008) 11

2.3 - TESTING OF CFRP REPAIR TECHNIQUES ON EXTRACTED GIRDERS 12

2.3.1 - Aidoo, Harries, and Petrou (2006) 12

2.3.2 - Miller, Rosenboom, and Rizkalla (2006) 13

2.3.3 - Reed, Peterman, Rasheed, and Meggers (2007) 14

2.4 – FIELD REPAIRS OF IMPACT-DAMAGED GIRDERS 15

2.4.1 - Schiebel, Parretti, and Nanni (2001) 15

2.4.2 - Tumialan, Huang, Nanni, and Jones (2001) 16

2.4.3 - Klaiber and Wipf (2003) 17

2.4.3 - Kim, Green, and Fallis (2008) 18

2.4.4 - Yang, Merrill, and Bradberry (2011) 19

2.5 - FRP REPAIR GUIDELINES 20

2.5.1 - ACI Committee 440 (2008) 20

2.5.2 - Zureick, Nowak, Mertz, and Triantafillou (2010) 21

2.7 SUMMARY 22

CHAPTER 3: PROTOTYPE BRIDGE 24

3.1 – BRIDGE AND GIRDER GEOMETRY 24

CHAPTER 4: CFRP REPAIR METHODS 26

4.1 – GENERAL REPAIR METHOD INFORMATION 26

4.2 – NEAR SURFACE MOUNTED (NSM) REPAIR METHOD 26

4.3 – EXTERNALLY BONDED (EB) REPAIR METHOD 30

4.4 – BONDED POST TENSIONED CFRP (bPT) REPAIR METHOD 31

CHAPTER 5: MODELING 34

5.1 - XTRACT PROGRAM 34

5.2 – MODELING THE GIRDER 34

5.3 – MODELING STRAND DAMAGE 38

CHAPTER 6: CALCULATING THE EFFECTIVENESS OF THE REPAIR 39

6.1 - NORMALIZED RATING FACTOR 39

6.2 ANALYSIS OF PROTOYPE GIRDERS 40

CHAPTER 7: DISCUSSION 44

7.1 – GENERAL ANALYSIS 44

7.2 – NEAR SURFACE MOUNTED METHOD 46

7.3 – EXTERNALLY BONDED METHOD 47

7.4 – BONDED POST TENSIONED METHOD 48

7.5 – METHOD COMPARISON 49

7.6 – UPDATED REPAIR SELECTION CRITERIA TABLES 54

CHAPTER 8: SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS 59

8.1 - SUMMARY 59

8.2 - CONCLUSIONS 60

8.3- ADDITIONAL RESEARCH RECOMMENDATIONS 63

LITERATURE REFERENCES 64

APPENDIX A SAMPLE CALCULATIONS 69

APPENDIX B BRIDGE RATING EXAMPLE 70

LIST OF FIGURES

Figure 3.1.1 Prototype bridge and girder (Yang et al 2011) 25

Figure 4.2.1 NSM slot spacing requirements based on ACI 440.2R-08 27

Figure 4.2.2 NSM spacing for prototype girder 29

Figure 4.3.1 EB spacing for prototype girder 31

Figure 4.4.1 bPT spacing for prototype girder 33

Figure 4.4.2 Schematic representations of CFRP applications (Harries et al 2012) 33

Figure 5.2.1 XTRACT model of EB method with 17 CFRP strips and 8 prestressed steel strands severed from the bottom row 37

Figure 5.2.2 XTRACT model of the girder 37

Figure 7.2.1 Normalized rating factors for NSM method 47

Figure 7.3.1 Normalized rating factor for EB method 48

Figure 7.4.1 Normalized Rating factor for bPT method 48

LIST OF TABLES

Table 1.1.1 Repair selection criteria (Shanafelt and Horn 1985) 5

Table 4.2.1 Optimization of NSM strip dimensions 29

Table 5.2.1 XTRACT concrete properties 35

Table 5.2.2 XTRACT reinforcing element properties 37

Table 5.3.1 Repair scenarios considered in this study 38

Table 6.2.1 Girder capacity and normalized rating factor 42

Table 6.2.2 Change in girder capacity and normalized rating factor 43

Table 7.5.1 Approximate values of α and β 52

Table 7.5.2 Maximum number of severed prestressing strands, nmax, that can be replaced by CFRP (Harries et al 2012) 53

Table 7.6.1 Repair Selection Criteria – EB-CFRP Techniques (Harries et al 2012) 55

Table 7.6.2 Repair Selection Criteria – PT-CFRP Techniques (Harries et al.2012) 56

Table 7.6.3 Repair Selection Criteria – Steel-based Techniques (Harries et al 2012) 57

CHAPTER 1: INTRODUCTION 1.1– INTRODUCTION

There are many prestressed bridges currently in service in the United States which have been subjected to various levels of impact damage In 1980 it was reported there was an average of 201 damage incidents per year to the nation’s 23,344 prestressed concrete bridges, resulting in an incident rate of 0.86% (Shanafelt 1980) The actual incident rate is likely to be higher, as not every incident of damage is reported Combined with the continuously increasing load demands bridges are required to support, the damage can cause the bridges to be rated below the American Association of State

Highway and Transportation Officials (AASHTO) inventory rating of 1.0 In these cases, transportation officials must make the decision to replace the bridge, replace the damaged elements only, repair damaged elements, post the structure, or otherwise accept the substandard rating

The benefits of repairing a larger bridge element compared to replacing one are plentiful; it is quicker, less expensive, less inconvenient for users of the bridge, and usually less complex However, if information on bridge repair methods is inadequate, bridges elements may be needlessly replaced without examining all possible repair techniques In 1985, a National Cooperative Highway Research Program Report estimated the cost of repairing a girder with “severe” damage was 15-50% of the cost of replacing the girder (Shanafelt 1985) The range of this figure demonstrates the importance having sufficient and updated data on all repair techniques and examining every possible repair technique

Various repair techniques developed by commercial and academic institutions have been

proven effective Although these repair methods have been proven as suitable alternatives to replacing

a structure, there has not been extensive research comparing the different methods and their ranges of applicability There exists no guideline to inform decision makers whether to replace, repair, or do nothing to damaged bridge girders, or what techniques are applicable and the most efficient for

restoring damaged capacity to a bridge There is no “one size fits all” repair technique because

variables, such as the extent of damage and girder geometry, will affect the effectiveness of each repair

method Almost every repair method can be utilized to restore some lost capacity, thus, a table

comparing the selection criterion and comparing the different repair options would be beneficial in selecting a repair technique

1.2 – OBJECTIVES

The need for an updated repair selection guidance that compares different repair methods is great, as it will provide bridge engineers and evaluators with better resources from which to make well-educated decision A crucial component of such a repair matrix would be the inclusion of data on recently developed repair techniques utilizing carbon fiber reinforced polymers (CFRP) There has been successful implementation of these techniques academically and in the field, but the entire range of applicability for which they can be utilized is still uncertain The objective of this study is to populate a matrix involving different types of CFRP repair techniques over various damage ranges to demonstrate what methods are applicable for restoring the ultimate flexural capacity of damaged I-girders to provide

a reference for engineers considering CFRP repairs A full load rating example and CFRP repair design calculations will also be provided as additional reference

CHAPTER 2: LITERATURE REVIEW 2.1 – GUIDELINES FOR REPAIR OF PRESTRESSED CONCRETE BRIDGE ELEMENTS

2.1.1 - Shanafelt and Horn (1980)

The National Cooperative Highway Research Program (NCHRP) Report 226 - Damage Evaluation and Repair Methods for Prestressed Concrete Bridge Members presents a guide on how to assess

prestressed concrete bridge damage and how to select an appropriate repair The aim of the report was

to assist the people responsible for the bridge repair in their decision making process by using value engineering and taking into consideration service life, safety performance, maintenance, cost,

aesthetics, user convenience, speed of repair, and inconvenience to drivers

To help guide its users, the report defines three different damage classifications: minor damage, moderate damage, and severe damage These damage classifications were used in a survey given to the state departments of transportation to try to establish common definitions and determine each

department’s course of action regarding each classification Minor damage is described as damage only

to the concrete portion of the girders, with no exposed reinforcing bars or prestressing strands

Moderate damage is described as damage only to the concrete portion of the girders or extensive spalling, which results in exposed reinforcing bars, and, or prestressing strands Severe damage is described as damage to concrete and reinforcing elements of the girder

The report goes into detail describing four different repair options: external post tensioning, metal sleeve splicing, strand splicing, and a combination of external post tensioning and metal sleeve splicing External post tensioning is the application of high strength steel rods or strands to the girder by means of a bolsters (also called corbels) which are cast against the girder The steel rods are tensioned

by jacking against the bolster drawing the girder into compression to help restore lost prestressing force Metal sleeve splicing, also referred to as steel jacketing, is the application of steel plates to enclose the girders Shear studs may be required and grouting is usually required to account for

dimensional inconsistencies of the girder The metal sleeves can help strengthen the girder by providing

sufficient additional reinforcement and some degree of confinement Internal splicing is the process of

reconnecting severed strands with a mechanical splice Once, spliced prestress can be restored by

heating of the strand, preloading the bridge, or tightening the splice with a torque wrench These

methods can be used in combination to improve the effectiveness of the repair The report also

provides a hybrid example of a metal sleeve splice and then installing external post tensioning

Eleven cases of these repair methods are described in detail and eight of the methods include

some sample calculations However, though sample calculations are presented, the specific repair

methods were not tested, and there is no evidence of the success or failure of these detailed repairs

2.1.2 - Shanafelt and Horn (1985)

NCHRP Report 280 - Damage Evaluation and Repair Methods for Prestressed Concrete Bridge

Members, presents Phase II of the project that was published five years earlier The stated purpose was

to “further evaluate promising methods of repair and to prepare guideline for damage evaluations and

repair techniques.” The report suggests that even though some of the practices were years old upon

the writing of NCHRP 226, not all of the methods were widely accepted It was believed the lack of

guidance regarding the assessment of damages and the process of repairs, as well as the lack of repair

method testing, were all causes for the repair methods not being implemented It was also believed

that since replacing a girder is the most conservative approach, some engineers simply preferred this

option

The report tried to address the lack of guidance by presenting a repair selection criteria table,

Table 2.1.1, to help assist bridge engineers and evaluators This table provides qualitative assessment

values of four different repair methods: post-tensioning, internal strand splicing, metal splice splicing or

steel jacketing, and replacement Although somewhat limited in scope, this table does proved a quick

and easy to use reference that is useful when comparing different repair methods

Damage Assessment Factor

Repair Method to Consider Post-

tensioning

Internal Splicing

Metal Sleeve

Adding Strength to

Non-Damaged Girders

Splicing Tendons or

Bundled Strands

N/A means not applicable

* Can be improved to excellent by extending corbels on fascia girder

Table 1.1.1 Repair selection criteria (Shanafelt and Horn 1985)

This report also tried to address the lack of repair processes and lack of experimental testing to back up the design recommendations The methods examined in this were the same repair methods

examined in NCHRP 226: external post tensioning, internal splicing of severed strands, a metal sleeve

splice, and a combination, hybrid example of internal strand splicing and external post tensioning

For the experimental program, ten different tests were performed on one girder During the first nine tests (described below), at least two full loading cycles were applied during the trials, but the maximum load was limited to 75% of the calculated ultimate moment capacity of the girder For test 10, 100% of the ultimate load was applied in one cycle The test load was a single point load located at midspan of a simply supported girder Prior to some tests, spalled concrete was patched, however none

of the cracks were repaired by epoxy injection A record of loads, strains, and deflections were made The tests performed were:

Test 1 – Instruments were set up and installed on the undamaged girder and it was loaded until it

reached 75% of its calculated ultimate moment capacity

Test 2 – Concrete corbels and post-tensioned high strength bars were installed and the girder was loaded until 75% of its calculated ultimate moment capacity

Test 3 – The high strength bars were removed and the girder was loaded until 75% of its calculated ultimate moment capacity

Test 4 – Concrete was broken away in order to sever four of the sixteen prestressed strands and the girder was loaded until 75% of its calculated ultimate moment capacity

Test 5 – The four severed strands were each spliced with a single-strand internal splice and a preload was applied The girder was patched and the preload was released after the patch regained adequate strength The girder was loaded until 75% of its calculated ultimate moment capacity

Test 6 – The post-tension high-strength bars were reconnected to the girder and the girder was loaded until 75% of its calculated ultimate moment capacity

Test 7 – The post-tension bars were disconnected, concrete was broken away, the four strand splices were severed, and the girder was loaded until 75% of its calculated ultimate moment capacity

Test 8 – The girder was patched and the external high strength bars were post tensioned It was loaded until 75% of its calculated ultimate moment capacity

Test 9 – The post-tension bars were disconnected, concrete was broken away, two more strands were severed, for a total of six out of sixteen strands severed The girder was patched and a metal sleeve splice was installed The girder was loaded until 75% of its calculated ultimate moment capacity

Test 10 – The girder was loaded until 100% of its calculated moment capacity

Although this procedure addressed the need of experimental testing, a problem exists in that each test was performed one right after the other The strand splice testing was completed after the external post tensioning had already been installed, and the metal sleeve was installed after the

completion of eight other tests The various tests affect the concrete in different ways; even though the

tensions bars were removed or the girder was patched, it does not mean the girder did not still exhibit affects from the previous tests Such a test protocol generally yields inconclusive results

2.1.3 - Post NCHRP 280 - Harries, Kasan and Aktas (2009)

This report serves as post NCHRP 280 guidelines for the repair of prestressed concrete girders Its contents include updated information on the available repair techniques, survey results on current practices, and repair examples that now cover FRP The authors suggest updating the damage

classification and breaking down the “Severe Damage” classification from NCHRP 280 into three

different categories The Severe I category is described as damage which requires a structural repair but does not necessitate a prestressed or posttensioned method The Severe II category is described as damage which requires prestress or posttensioning with the repair method The Severe III category is described as damage too great to be practically repaired and the member should be replaced

2.2 - TESTING OF CFRP REPAIR TECHNIQUES ON EXPERIMENTAL GIRDERS

2.2.1 - Quattlebaum, Harries, and Petrou (2005)

In this experimental program twelve girders were cast; the girders were 15.6 ft (4750 mm) long, 10.0 in (254 mm) deep, and 6.0 in (152 mm) wide with a clear span of 15.0 ft (4572 mm) The beams had one layer with three #4 (13 mm) reinforcing bars The girders were retrofitted with three different CFRP systems: conventional adhesive application (CAA), near-surface mounted (NSM), and powder actuated fastener (PAF) CAA is now more commonly referred to as externally bonded (EB) A single 2.0

in (51 mm) wide CFRP strip was used for the CAA method Two - 0.98 in (25 mm) strips adhered together and inserted into two - 0.25 in (6.4 mm) wide by 1.3 in (32 mm) deep slots were used for the NSM method A 3.3 in (84 mm) wide strip was used for the PAF method and the nails were embedded 1.6 in (41 mm) into the soffit, staggered with a longitudinal spacing of 2.5 in (64 mm), and had a transverse spacing of 1.0 in (25 mm) A total of six girders were tested under cyclic loading; one of each method was tested under high stress fatigue loading, and one of each method was tested under low

stress fatigue loading A total of four girders were tested monotonically until failure, one control beam and one for each retrofit method Two of the girders were inadvertently damaged and thus there were

no control beams for cyclic loading

The specimens were loaded using a single point load at midspan To determine the strain levels

of the steel, three linear displacement transducers were installed on either side of the girder at the level

of the steel and, three strain gauges centered on the midspan were attached to steel CFRP strains were also determined using strain gauges, and the deflection was measured using draw wire transducer The

conclusions drawn were as follows:

 All three methods resulted in strength increases over the control specimen The yield load increased 21-26% and the ultimate load increased28-33%

 Under monotonic testing, concrete crushing was the dominate failure mode except for the CAA specimen, which was controlled by midspan debonding

 The NSM specimen had the best bond after fatigue testing and was the sole specimen which appeared to have a complete bond after monotonic loading testing

 The PAF specimens had lower strength increases compared to NSM and CAA because of less effective stress transfer between CFRP and concrete

 The PAF and NSM methods noticeably weakened the concrete

 Care must be taken when using powder actuated faster because the reinforcing steel can be damaged by nails, as was the case with the low stress fatigue specimen

 PAF outperformed others in fatigue loading, but its long-term performance is still very

questionable because shearing of the fasteners occurred shortly after the initial failure and there were concerns of corrosion with the metal fasteners

 With cyclic loading, all methods exhibited increases of FRP and steel strain indicating degrading bond characteristics

 PAF is the quickest method since it requires less labor and equipment; hence it may be

applicable for emergency situations CAA requires more time since the soffit must be prepared, the epoxy precisely mixed, and the epoxy applied before it cures NSM requires the most time since the soffit preparation takes longer than the CAA method because slots for the CFRP must

be cut and epoxy is used as well

2.2.2 - Nordin and Täljsten (2006)

Fifteen girders were cast for this experimental program The beams were 13.12 ft (4 m) long, 11.81 in (300 mm) deep, and 7.87 in (200 mm) wide All the retrofitted girders utilized the near surface mounted method (NSM) One beam was not strengthened in order to be used as a reference, four beams were retrofitted with non-prestressed NSM bars, and ten beams were retrofitted with

prestressed NSM bars Two different types of CFRP material with different moduli of elasticity were

used; six beams were retrofitted with a 36,000 ksi (250 GPa) CFRP and eight with 23,000 ksi (160 GPa) CFRP In addition, the length of the CFRP was varied; eight beams had a CFRP length of 13.1 ft (4.0 m) that continued under the supports and the six other beams had a CFRP length of 10.5 ft (3.2 m) The beams were subject to two point loads placed equidistant from midspan and the beam was loaded to failure The beams were fitted with two strain gauges for the concrete at the top of the beam, three strain gauges for the reinforcing steel, and four strain gauges for the CFRP The midpoint deflection was also measured The conclusions drawn were as follows:

 All the methods resulted in flexural strength increases over control specimen

 All the methods resulted in smaller cracks, which will increase durability and service life

 The failure mode of every retrofitted specimen was fiber failure of the NSM rod, thus indicating efficient stress transfer and good bond

 The non-prestressed beams were more ductile and had a larger deflection at every load than prestressed beams

 The stiffer rods resulted in stiffer behavior from the beam and a higher steel yielding load

2.2.3 - Casadei, Galati, Boschetto, Tan, Nanni, and Galeki (2006)

For this study three I-girders were cast The girders were 36 ft (11 m) long, had a free span of

34 ft (10.3 m), were 32 in (810 mm) deep, had a 13 in (330 mm) wide top flange, and a 17 in (430 mm) wide bottom flange These dimensions fall between standard AASHTO Type I and Type II I-girders The concrete deck was 32 in (810 mm) wide and 6 in (152 mm) deep There were two layers of six

prestressing strands For the retrofitted girders, concrete was chiseled out to expose the strands and two strands were cut to simulate impact damage Prior to CFRP application, the loose concrete was removed and patched One specimen served as the control and was undamaged, one specimen was retrofitted using the externally bonding (EB) method, and the third was retrofitted using the prestressed NSM method A 0.0065 in (0.165 mm) thick CFRP sheet was used in the EB method, which was then covered with U-wraps to prevent delamination Three - 1.125 in (2.8 cm) deep and 0.75 in (1.9 cm) wide slots were cut and a prestressed 0.375 in (9.5 mm) diameter, 12 ft (3.66 m) long CFRP bar was inserted into each slot for the NSM method The NSM bars were prestressed using a steel wedge anchorage system to achieve the same level of prestress the beam had before the strands were cut The beams were subject to two point loads placed equidistant from midspan until failure Three string transducers, placed at mid-span and under loading points, measured the vertical displacements The conclusions drawn were as follows:

 Both systems were capable of restoring the ultimate capacity of the two cut prestressing wires

 The EB CFRP repair failed by debonding of the CFRP and was then immediately followed by the U-wraps rupturing

 NSM performed in a more ductile behavior and failed when the concrete cover on the sides of the severed tendons, close to the NSM bars, split open

2.2.4 - Aram, Czaderski, and Motavalli (2008)

In this study four girders were cast; the girders were 7.9 ft (2.4 m) long, 9.8 in (250 mm) deep, and 5.9 in (150 mm) wide The CFRP repair method was the externally bonded method One specimen served as the control and was undamaged, one specimen was retrofitted using unstressed externally bonded CFRP, and two specimens were retrofitted using prestressed externally bonded CFRP with the CFRP prestressed at various levels: one at 36% of maximum value of tensile strength and one at 18% of maximum value of tensile strength The strips were 2.0 in (50 mm) wide and 0.047 in (1.2 mm) thick The gradient method was used to prestress the strips to avoid the use of mechanical anchorage For the gradient method, the prestressing force is gradually reduced from the middle toward both ends of the CFRP strip via heating which cures the adhesive used to bond the CFRP to the beam The beam was tested using two point loads placed equidistant from midspan until failure or CFRP debonding Strain gauges were applied on the concrete and CFRP The conclusions drawn were as follows:

 Under greater loads, the retrofitted specimens had a smaller deflection and strain at midspan

 CFRP prestressing caused unloading of the steel reinforcement, which in turn increased the load

at which the steel began to yield

 The CFRP failure modes were debonding failure, delamination, and sudden separation of the CFRP strip

 The load capacity increase of prestressed FRP was less than the unprestressed CFRP because of the premature debonding of the strips

 Prestressing the strips resulted in lower shear stresses because cracks occurred at higher loads

 The higher prestressed CFRP had higher stresses at the CFRP strip ends

 The gradient method was not effective because the gradient anchorage was in the region of shear stresses due to loading; however, this can be avoided with a longer span

2.3 - TESTING OF CFRP REPAIR TECHNIQUES ON EXTRACTED GIRDERS

2.3.1 - Aidoo, Harries, and Petrou (2006)

This experimental program examines the CFRP repair and testing of eight reinforced concrete girders removed from a forty-two year old decommissioned Interstate bridge The bridge was cast-in-place reinforced concrete, designed in 1957, erected in 1961, and replaced in 2001 because it was not wide enough to meet traffic demands The bridge had five simple spans of 30.0 ft (9.140 m) each, with girders spaced at 6.50 ft (1980 mm) The test specimens had a total depth of 32.5 in (825 mm), a width

of 13.5 in (343 mm); the test span was reduced to 26.33 ft (8025 mm) The deck was 6.50 in (165 mm) thick and 36.5 in (927 mm) wide There were three layers of reinforcing bars; the bottom layer

contained three -#11 (36 mm) bars, the middle layer contained three -#10 (32 mm) bars, and the top layer contained two -#8 (25 mm) bars The girders were retrofitted with three different CFRP systems: conventional adhesive application (CAA), near surface mounted (NSM), and powder actuated fastener (PAF) Two - 4.02 in (102 mm) strips placed adjacent to each other on the girder soffit were used in the CAA method Four 1.3 in (32 mm) deep slots were cut and two - 0.98 in (25 mm) strips adhered

together and inserted into each slot were used for the NSM method Two - 4.09 in (104 mm) hybrid strips were placed on top of each other and fastened to the bottom girder soffit were for the PAF method All strip lengths were 25.0 ft (7620 mm) to avoid going over the beam supports and thus replicate real world application

The girders were loaded using a single point load at midspan; four were subjected to monotonic loading until failure and four were subjected to fatigue loading To determine the strain levels of the steel, the girders were fitted with seven horizontally mounted linear variable resistance displacement transducers centered at midspan on the soffit and five supplementary LVR transducers were fitted on the both sides of the beam at the level of the steel The vertical deflections were measured with a draw wire transducer Strain gauges were also applied to the CFRP coinciding with the seven horizontal LVRs

to determine the CFRP strain and investigate debonding The conclusions drawn were as follows:

 All retrofitted specimens had a higher flexural strength than the control

 All retrofitted specimens showed debonding characteristics in fatigue loading despite the

relatively low fatigue stress levels

 CAA and NSM were much better in monotonic loading than PAF

 Powder actuated fasteners were not a good choice to use on older concrete because significant cracks and spalling can occur

 NSM showed the best bond characteristics and the best ductility, however took the longest to install

2.3.2 - Miller, Rosenboom, and Rizkalla (2006)

In this study, the CFRP repair and testing of an impact-damaged girder extracted from the North Carolina Bridge 169 in Robeson County is analyzed The span was 54.8 ft (16.7 m), but in the test setup, the clear span between supports was reduced to 53.8 ft (16.4 m) The girder was an AASHTO Type II I-girder with a total depth of 36.9 in (913 mm), top flange width of 12.0 in (305 mm), and bottom flange width of 17.9 in (457 mm) The deck was 5.98 in (152 mm) thick and 14.8 in (377 mm) wide The girder had been impacted close to midspan and one of the sixteen prestressed strands was severed while others were exposed due to spalling

The CFRP repair method was EB Prior to the CFRP application, the loose concrete was removed and patched Three layers of CFRP, applied one on top of each other, were adhered to the bottom soffit and one layer of CFRP was adhered to the rest of bulb A 5.9 ft (1.8 m) long CFRP U-wrap was applied to the damaged area and then five U-wraps spaced at 1.3 ft (0.4 m) were applied on either side of the damaged girder The U-wraps were needed to prevent crack growth at the damaged area and to

mitigate the possibility of CFRP delamination The beam was instrumented with potentiometers and strain gauges The girder was tested in three point bending and was first subjected to fatigue loading and then loaded until failure The conclusions are as follows

 Girders subjected to impact damage with one of sixteen strand missing can be repaired using CFRP

 The beam failed as a result of cracks propagating outside of the CFRP repaired areas; thus the CFRP repaired portion behaved better than undamaged portion even with severed strands

 The CFRP did not delaminate before the concrete began crushing; therefore, it is assumed the CFRP did not debond prematurely

2.3.3 - Reed, Peterman, Rasheed, and Meggers (2007)

The CFRP repair and testing of damaged girders extracted from Bridge #56 located in Graham County, Kansas near Penokee is examined in this report The girder was constructed in 1969 designed for H-15 loading, and thus had been subjected to many cases of overloading, which led to significant cracking and spalling Damaged girders were replaced and three of the extracted girders were obtained for testing The bridge was four spans and the girders were double-T shaped The girders were then saw cut to create six single-T shapes which were 40 ft (12.2 m) long The total depth of the beam was

23 in (585 mm) and the top flange was 5 in (125 mm) deep and 3 ft (915 mm) wide Each stem

contained four prestressing strands placed in a single vertical layer The CFRP repair methods were EB and a combination of EB and NSM Five specimens were tested; specimens 1-3 were tested until failure, and Specimens 4-5 were tested under fatigue loading

Prior to CFRP application, the loose concrete was removed and patched Specimen 1 was left unrepaired and served as a reference, and Specimens 2-5 had two plies of CFRP externally bonded to the soffit and continuing 3.75 in (95 mm) up the stem Specimen 2 had only one 12 in (305 mm) wide ply of CFRP U-wrap covering the CFRP on both ends Specimen 3 and 4 had two plies of CFRP U-wraps spaced at 18 in (455 mm) as well as CFRP U-wraps on the ends In addition to the CFRP externally bonded to the surface, Specimen 5 had two 60 in (1500 mm) long #6 (19 mm) NSM CFRP bars inserted

at midspan on both sides of the web In addition, Specimen 5 had three plies of 20 in (510 mm) long

CFRP wrap applied at midspan which were covered by three plies of 40 in (1020mm) long CFRP wrap to help confine the NSM bars Specimens 1-4 were subjected to loading a single point load at midspan A hydraulic actuator applied the load at midspan and a transverse spreader beam distributed the load across the entire top flange width at midspan The midspan deflection was measured with one linearly variable displacement transducer applied on each side of flange In Specimen 5 a longitudinal spreader beam applied the load from the hydraulic actuator to the beam symmetrically about midspan

U-at two separU-ate points The conclusions are as follows:

 All repaired specimens had higher flexural strength than the control

 Specimen 2 failed when a horizontal crack formed near the loading point Adding more CFRP wraps, as exhibited in Specimen 3 strengthened the beam, which ultimately failed due to CFRP reaching its ultimate capacity and rupturing

 Specimens 4 and 5 failed due to fatigue of the prestressing strands and the CFRP behaved as predicted

 The bond between the CFRP and the girder was sufficient as no cases failed from debonding

2.4 – FIELD REPAIRS OF IMPACT-DAMAGED GIRDERS

2.4.1 - Schiebel, Parretti, and Nanni (2001)

The repair of impact-damaged girders of Bridge A-4845 on Route 24 over Route 291 in

Independence, Missouri is detailed in this report The girders are Standard MoDOT Type 6 I-girders; the dimensions are similar to an AASHTO Type IV I-girder The girders have a total depth of 4.5 ft (1.3 m), top and bottom flange widths of 2.0 ft (610 mm), and a deck thickness of 7.5 in (19 mm) Each girder has twenty-four prestressed strands The impact damage caused spalling, which exposed several strands It was assumed that two strands were damaged enough to reduce them to 50% of their ultimate tensile capacity, hence essentially one strand was fractured The CFRP repair method was EB Prior to CFRP application, the loose concrete was removed and patched For the repair, two plies 22 in

(560 mm) wide were externally bonded to the soffit; first, a 14.0 ft (4.27 m) long strip was applied to the soffit and then a 12.0 ft (3.66 m) long strip was adhered on the other strip Twelve - 6 in (150 mm) wide, 60 in (1500 mm) long CFRP U-wraps spaced at 16 in (400 mm) center to center were adhered to the soffit and continued up to cover the bulb The conclusions drawn were as follows:

 The calculated capacity of the girder is nearly equal to that of the undamaged capacity

 The bond durability tests indicated a good bond between the CFRP and the girder

 This method was efficient and was carried out without major difficulty

2.4.2 - Tumialan, Huang, Nanni, and Jones (2001)

This paper describes the repair of an impact-damaged girder at the Interstate 44 and 270 interchange in St Louis County, Missouri The girders are MoDOT Type 2 I-girders with the bottom and top widths increased by 2 in (51mm); the dimensions are similar to an AASHTO Type II I-girder The girders have a total depth of 32 in (810 mm), a 15 in (380 mm) top width, a 19 in (480 mm) soffit width, and a deck thickness of 7.5 in (190 mm) Each girder has twenty prestressed strands, but the impact damage severed two of the strands

The CFRP repair method was EB Prior to CFRP application, the loose concrete was removed and patched For the repair, two - 18 in (460 mm) wide plies were externally bonded to the soffit; first, a 10’-8” (3250 mm) long strip was applied to the bottom soffit and then a 9’-4” (2850 mm) long strip was adhered on the other strip Sixteen - 4 in (100 mm) wide CFRP U-wraps spaced at 8 in (200 mm) on center were adhered to the soffit and continued upwards to cover the bulb A small problem arose when a blister formed in the repaired area The blister was 33 in (840 mm) long, 9.5 in (240 mm) wide, and 0.75 (19 mm) deep, caused by excess of saturant The blister was repaired by epoxy injection The conclusions drawn were as follows:

 Based on ACI and AASHTO calculations the original flexural capacity of the girder was restored

 This method was efficient and the one problem that did arise, the blister, was handled

efficiently; the CFRP application was completed in only two hours

 This method was minimally disruptive because only one lane was shut down during CFRP application and repairing of the blister

2.4.3 - Klaiber and Wipf (2003)

This case study examines the repair of impact-damaged girders of the Highway 65 Bridge in Altoona, Iowa The bridge consists of four spans with two - 96.5 ft (29.4 m) middle spans The girders have a total depth 4.5 ft (1.4 m) with a soffit width of 1’-10” (559 mm); the dimensions are similar to an AASHTO type IV I-girder All six girders of the bridge were impacted in the second span about 30 ft (9.1 m) from the center pier The first impacted beam had some spalling and one strand severed; however, the second beam had considerable spalling which exposed a total of five strands, two of which were severed To help quantify the effectiveness of the repair, measurements were taken before and after the CFRP were installed Twenty-four strain gauges were set up, three on each of the spans adjacent to the impacted span and eighteen on the impacted span Deflections of all six girders in the impacted span under truck load were measured as well

The CFRP repair method was EB Prior to CFRP application, the loose concrete was removed and patched For the repair, four - 4 in (100 mm) wide by 75 ft (22.9 m) long CFRP strips were externally bonded to the soffit An 80 ft (24.8 m) long CFRP U-wrap was adhered to the girder and covered the soffit and all but the top inch of the web on either side of the girder The conclusions are as follows

 Girders subjected to impact damage with a small percentage of strands severed can be repaired using CFRP

 The CFRP U-wraps helped confine the patch material, as well as prevent CFRP debonding

 Girders repaired with CFRP have reduced deflections

2.4.3 - Kim, Green, and Fallis (2008)

This document covers the repair of an impact-damaged girder of the Main Street Bridge located

in Winnipeg, Canada It was built in 1963 and repaired in 2003 The bridge has four spans, each 46.10

ft (14.05 m) in length The girders are C-shaped with a total depth of 26 in (660 mm), top width of 48.0

in (1220 mm), and each stem is 7.01 in (178 mm) wide Each stem contains ten prestressing strands and the impact damage severed several of the strands

The CFRP repair method was prestressed EB Laboratory tests were conducted and an FEA model was made to confirm prestressed EB was an appropriate repair method Prior to CFRP

application, the loose concrete was removed and patched The girder was repaired in the following sequence: three layers of CFRP sheets were bonded onto jacking plates and allowed to cure for one week; the bottom soffit of the damaged stem was prepared; shear plates were installed on the damaged stem with high strength bolts; the CFRP was adhered to the bottom soffit; a hydraulic jack applied prestress; the prestress was locked off by tightening nuts on the rods securing the jacking plate to the girder; the jack was removed; and finally a temporary support system involving plastic covered plywood attached to the bottom and drawn into contact with wire clamps to ensure no gaps existed between the CFRP and the concrete was put in place The conclusions drawn were as follows:

 The effectiveness of the repair at fully restoring the flexural load carrying capacity was

confirmed by an FEA model and AASHTO load rating calculations The AASHTO load ratings calculations were up to 25% more conservative than what the FEA model predicted

 Serviceability and crack control were improved

 An anchor system was needed in order to apply a high level of prestress

 This method was effective however it was very time involved and cumbersome

 This method was non-disruptive as traffic was not prohibited over the bridge

2.4.4 - Yang, Merrill, and Bradberry (2011)

In this case study, Texas DOT’s use of CFRP is described as it is used to repair bridges damaged

by impact, fire, corrosion, and alkali-silica reaction Four different cases studies involving CFRP repair of impact-damaged girders in four different bridges are examined

Case 1 is Texas’s first field application of CFRP to repair an impact damaged member An

overheight vehicle impacted the exterior girder of a bridge The impact fractured the entire web

between two diaphragms; however only one of the forty-four prestressing strands was severed The girder was repaired using EB CFRP using the following sequence: all loose concrete was removed; an H20 truck was used to preload the beam and remained in place during the patching and epoxy injection stages; the truck was removed; and finally one ply of continuous CFRP U-wrap was externally bonded to girder covering the entire member length between the diaphragms and all of the web and bottom soffit This process was completed in only five days

Case 2 is the prototype structure used in this thesis and is discussed in Section 3.1

Case 3 examines the case of two impact-damaged girders in which the bottom flange and nearly all the web of both girders were fractured Four strands were severed in one beam and three were damaged in the other, however between 85% and 90% of the strands maintained their original strength The beam was repaired with strand splicing and EB CFRP The strands were spliced and tensioned, the concrete was patched, and then CFRP was wrapped around the soffit and webs One year later the girder was impacted again, however, the CFRP appeared to strengthen the beam and only one

additional strand was severed The damaged CFRP was saw cut and removed, the severed strand was spliced, and new CFRP was spliced over the existing CFRP layers Both of these repairs were completed

in one week

Case 4 examines a repeatedly impacted bridge in which CFRP was used as sacrificial

reinforcement Sacrificial reinforcement is used to protect the primary reinforcement from damage and thus increase the likelihood of the structure surviving The bridge sustained major damage in 2006 and

was repaired with EB CFRP It was impacted and repaired with EB CFRP again in 2007, and then it was impacted and repaired with EB CFRP a third time in 2008 Due to budget constraints, the bridge is not able to be raised and thus sacrificial CFRP is proving to be a viable option until enough funds are

available The conclusions drawn from these cases are as follows:

 External CFRP is an excellent option for impact-damage repairs

 External CFRP is simple to install and may not require road closures

 CFRP can be used as a sacrificial reinforcement

 It is possible to effectively combine CFRP repair and strand splicing

2.5 - FRP REPAIR GUIDELINES

2.5.1 - ACI Committee 440 (2008)

Many previous studies demonstrated the effectiveness of fiber reinforced polymer (FRP) and

their applicability to real world scenarios This led to the development of FRP standards found in ACI 440.2R-02 (2002) which were revised in ACI 440.2R-08 - Guide for the Design and Construction of

Externally Bonded FRP Systems for Strengthening Concrete Structures (2008) ACI 440.2R-08 covers a

wide range of topics including general information about FRP as well as proposed guidelines based on experimental research and field applications of FRP It also gives nine examples calculating various properties of FRP including the use of EB and NSM methods to increase the flexural strength of concrete

beams The revisions from ACI 440.2R-02 to ACI 440.2R-08 include specifications on development

length, updated axial compression specifications, recommendations on the NSM groove dimensions, separate sections for flexural strengthening of prestressed and reinforced members, and updated design examples Neither report mentions the PAF method, indicating that there remains too little data on the PAF method to warrant inclusion

2.5.2 - Zureick, Nowak, Mertz, and Triantafillou (2010)

NCHRP also recognized the importance of the fiber reinforced polymers (FRP) repairs and

developed guidelines which were specific to bridge elements NCHRP Report 655 - Recommended Guide Specification for the Design of Externally Bonded FRP Systems for Repair and Strengthening of Concrete Bridge Elements (Zureick 2010) is the third revision of the NCHRP recommended specifications for FRP

repairs The goal was to create an FRP guide specification to ensure the expected performance of CFRP systems was achieved, and thus, increase the likelihood CFRP would be used to repair or retrofit

concrete in the future The first report, NCHRP Report 514 - Bonded Repair and Retrofit of Concrete Structures Using FRP Composites—Recommended Construction Specifications and Process Control Manual (Mirmiran 2004), used completed surveys from the state departments of transportation, state

representatives of the Transportation Research Board, members of the ACI committee on FRP, members

of AASHTO, and the FRP industry to determine the current state of practice and write the first

recommended specifications The report includes background on FRP, issues related to FRP, a

recommended FRP construction specification manual, and a process control manual The specification manual gives recommended practices for handling, storing, preparing, and installing near surface

mounted and externally bonded CFRP systems; and the process control manual gives details on a quality assurance program to guarantee the repair is completed in a correct manner The manual also provides

a fundamental introduction into FRP repair, but does not provide many threshold values; most of the specifications are standard practices or simply recommend that the manufacturer’s instructions be followed

The second report, NCHRP Report 609 - Recommended Construction Specifications and Process Control Manual for Repair and Retrofit of Concrete Structures Using Bonded FRP Composites (Mirmiran 2008), improved upon the Report 514 by providing more experimental data regarding the threshold

values of NSM groove sizes, temperature, defects in the FRP repair, and surface cracks, voids,

roughness, and out-of-flatness These findings were used to update the recommended specification; however the bulk of the specifications and control manual remained the same

NCHRP Report 655 (Zureick 2010) revised the recommended specifications once again The

report includes more experimental data that influenced the new specifications, such as debonding strain and peeling stress of FRP However, the format of the specifications was changed so it resembled the 4th

edition of the AASHTO LRFD Bridge Design Specifications (AASHTO 2007) This was done to help users

who were most likely familiar with the AASHTO specifications, as well as help the specifications become accepted by AASHTO The guide has information on LRFD limit states and load cases, contains the CFRP material requirements, and has sections dedicated to members subject to shear and torsion, flexure, and combined axial force and flexure The final part of the report includes six step-by-step illustrative example calculations of using CFRP to strengthen members flexurally and axially This report provides

an excellent guide on how to implement FRP repairs on concrete bridge elements; however, does not

define many FRP limiting values For this reason, ACI 440.2R-08 is used to calculate the FRP properties and NCHRP Report 655 is used to relate FRP repair specifically to concrete bridge members

2.7 SUMMARY

The studies described have demonstrated the effectiveness and applicability of CFRP repair It has been demonstrated through many examples that CFRP can help restores the flexural capacity, increase serviceability, and protect the girder if it is impacted again CFRP repairs are simple, quick, relatively inexpensive, and may be completed without restricting bridge traffic ACI recognized these benefits and established guidelines for repairs using EB and NSM CFRP

A valuable feature of CFRP is its ability to be prestressed which improves its in situ performance This has been accomplished in the field using steel plates but it would be beneficial to avoid steel

anchors since they are vulnerable to corrosion Techniques such as the gradient method, if applied

correctly, may be able to reduce the susceptibility to corrosion Debonding is one of the main concerns

of using CFRP, however using U-wraps can greatly improve the bond strength

Since CFRP will continue to be used more commonly, more examples on repairing prestressed

damaged girders with CFRP are needed The Repair Method for Prestressed Girder Bridge (Harries 2009) and NCHRP Report 655 (Zureick 2010) are quality references, but a wider range of CFRP examples and

comparisons of the different methods are needed to help engineers become more familiar with the range over which CFRP techniques are applicable Data on these repair methods can then be used to

update the repair method guidelines table from the NCHRP report 280 (1985), Table 2.1.1 This table

provides qualitative assessment values of four different repair methods: post-tensioning, internal strand splicing, metal splice splicing or steel jacketing, and replacement It is a good template but more

categories such as what type of girders the repair may be applied to, the environmental impact of the repair process, and if the girder will retain its capacity if it is impacted again This updated table would provide a valuable tool to engineers when deciding on a repair option

CHAPTER 3: PROTOTYPE BRIDGE 3.1 – BRIDGE AND GIRDER GEOMETRY

For the purposes of this research, a prototype girder has been chosen for analysis The

prototype girder is based on the girders described in case study #2 reported by Yang, Merrill, and Bradberry (2011) The case study examines the CFRP repair of a bridge located in Eastland County, Texas The bridge was designed in accordance with the 1965 AASHO (now AASHTO) design

specifications for H20 loading It was impacted in March of 2004 The repairs took ten days to

complete An overall view of the bridge is shown in Figure 3.1.1a and the extent of damage in Figure 3.1.1b

The bridge has four spans, the end spans are 54’-8” (16.7 m) and the interior spans are 84’-8” (25.8 m) Five girders spaced at 7’-3” (2.21 m) make up the cross section, as shown in Figure 3.1.1c The girders, shown in Figure 3.1.1d, are Texas Type C with a total depth of 40 in (1010 mm), a 14 in (356 mm) top flange width, 22 in (559 mm) soffit, and a deck thickness of 7 in (180 mm) Each girder has a total of thirty-two - 0.5 in (12.7 mm) diameter, Grade 270 low relaxation strands, eight of which are harped The bridge was impacted near midspan (Figure 3.1.1b), where the harped strands are in the bottom soffit Eight of the strands were severed in the exterior corner, and based on pictures of the damage, it is assumed three strands in the bottom row were severed, three in the second row, and the final two in the third row It was determined that a CFRP repair would be sufficient and thus the girder would not have to be replaced

The actual repair method used was a combination of strand splicing and externally bonded (EB) CFRP Prior to CFRP application, the loose concrete was removed and patched To repair the girder, five

of the eight severed strands were internally spliced, the girder was patched, and three layers of CFRP were externally bonded to the soffit Geometric constraints prevented all the strands from being internally spliced The FRP was then surrounded by U-wraps to improve the bond behavior and confine the patch

The bridge was closed to one lane during the repair process Although this closure adversely affected traffic flow, this was more desirable than having to reroute traffic if the entire bridge had been closed The splices were completed in three days Five days later, after the patch material had cured, the two days of EB repair began This case is a successful real world application that can be used as a reference for future CFRP repairs and thus a good prototype to use In this study, multiple CFRP-based repair techniques for the same bridge will be considered; these are described in the following chapter

c) Section of prototype bridge

d) Section of prototype girder

Figure 3.1.1 Prototype bridge and girder (Yang et al 2011)

CHAPTER 4: CFRP REPAIR METHODS 4.1 – GENERAL REPAIR METHOD INFORMATION

Three different repair methods will be analytically examined to determine their effectiveness at restoring the lost ultimate capacity of the prototype girder The repair methods that will be examined are near surface mounted (NSM), externally bonded (EB), and bonded post tensioned (bPT) All three repair methods have advantages and disadvantages, thus the best repair method for this case is not immediately obvious The CFRP material that will be used for the EB and NSM methods and the

prestressing system for the bPT method are commercially available The manufacturers’ specifications

and specifications of ACI 440.2R-08 will be used determine the CFRP material properties

4.2 – NEAR SURFACE MOUNTED (NSM) REPAIR METHOD

The near-surface mounted (NSM) method involves cutting slots into the girder soffit with a concrete saw, and fully enclosing the CFRP material into the slots with structural adhesive The CFRP strip properties are based on a commercially available product The strips are 0.047 in (1.2 mm) thick, 2.0 in (50 mm) wide, have a modulus of elasticity of 23.2 x 103 ksi (160,000 MPa), a ultimate tensile strength of 406 ksi (2,800 MPa), and ultimate tensile strain of 0.017 The optimal area of the strips to be

inserted into the slots will be based on ACI 440.2R-08 specifications

The number of NSM CFRP strips that can be applied will be limited by the geometry of the bottom soffit Although it is less efficient, NSM strips can be applied to the sides of the bulb However, due to the labor involved with cutting slots, the small amount of CFRP that can be installed, and the possibility of overstressing the concrete with extra cuts, the option of adding NSM to the sides of the

bulb is not considered ACI 440-08 gives recommendations on the minimum spacing of the slots to

prevent the possibility of overstraining the concrete The clear spacing of the slots should be at least 2.0 times the depth of the slot cut, and the clear distance from the edge of the beam should be 4.0 times the depth of the slot The width of the slot should be at least 3.0 times the thickness of the CFRP strip

and the depth of the slot should be at least 1.5 times the width of the CFRP strip The dimensions requirements for the grooves are displayed in Figure 4.2.1

Figure 4.2.1 NSM slot spacing requirements based on ACI 440.2R-08

For this study, the greater of 1.5 times the width of the CFRP strip or 0.125 in (3.18 mm) plus the width of the CFRP strip was used to determine the slot height To prevent the saw from impacting the reinforcing steel, the top of the slot must maintain a clear distance of 0.25 in (6.4 mm) from the reinforcing steel In addition, a tolerance of 0.125 in (3.18 mm) was used to account for any mistakes made when cutting the slot depth and for any possible dimensional discrepancies when the beam was constructed Thus, a total of 0.375 in (9.53 mm) was subtracted from the clear dimension of bottom of the soffit to the reinforcing steel on the bridge drawings The depth from the bottom of the beam to the middle of the prestressing strands is 2.0 in (50.8 mm), the strands are 0.50 in (12.7 mm) diameter, and the stirrup size is #4 (13 mm) With the tolerance and variance accounted for, the maximum depth of a cut is 0.875 in (22.2 mm)

The maximum width of the slot was limited to 0.25 in (6.35 mm), the maximum width a saw can cut in one pass The thickness of the CFRP strips is 0.047 in (1.2 mm); however the thickness can be increased by adhering two strips together to create a width of 0.094 in (2.39 mm) The slot width of

0.25 in (6.35 mm) is slightly less than 0.282 in (7.2 mm), which is the recommended 3.0 times the thickness of the strip, but the difference is sufficiently nominal that the double strips can be used and the CFRP should still have adequate bond with the girder

To determine the maximum amount of CFRP that can be applied an optimization process is carried out The number of slots that can be cut is reduced when the height of the slots is increased therefore the maximum area of CFRP that can be applied is not immediately obvious To determine the optimal area, the thickness of the strips was held constant and the width of the CFRP strips was varied in 1/8” (3.18 mm) increments The total depth of the slots varied from 0.25 in (6.35 mm) to 0.75 in (19.1 mm) and the number of slots across the soffit varied from twenty-seven to ten Although the maximum possible depth of a cut is 0.875 in (22.2 mm), increasing the width of the CFRP in 1/8” (3.18 mm)

increments, limits the depth of slot to 0.75 in (19.1 mm) because of the depth constraints Table 4.2.1 displays the optimization process

It can be seen that even though a smaller height will allow for more cuts, the maximum area of CFRP that can be applied occurs when the maximum height is used Using fewer slots is beneficial since

it reduces the amount of labor involved and reduces the amount of stress applied to the concrete from the saw cut Based on these calculations, the optimal number of cuts is ten and the optimal groove dimensions are 0.25 in (6.35 mm) wide, by 0.75 in (19.1 mm) deep and the optimal dimensions of the NSM strips 0.094 in (2.39 mm) thick, by 0.50 in (12.7 mm) wide To provide a range of repairs, the four cases considered are: three NSM strips on the soffit (30% of the maximum repair), five NSM strips (50%), seven strips (70%), and the maximum ten NSM strips on the soffit The ten-NSM strip repair is shown in Figure 4.2.2

Width of

strip (in.) Depth of cut (in.)

Depth of cut rounded

Dim check - cuts &

clear spacing ≤ soffit (in.)

Area of CFRP provided (in 2 ) Using one strip – total thickness of strip = 0.047 in

Light shaded entry is the optimal height

Darker shaded entries exceed maximum slot height (≥ 0.875 in.)

Table 4.2.1 Optimization of NSM strip dimensions