Guide for Design and Construction of

NSM Titanium Alloy Bars for

Strengthening Concrete Structures

Christopher Higgins, Ph.D., P.E.

Deanna Kuhlman

Laura Baughman

Mackenzie Lostra

Jonathan Knutdsen

Sharoo Shresta, Ph.D.

Eric Vavra

Overview

• Background

• ASTM Specification

• AASHTO-LRFD Compatible Design Guide Overview

• 63 Questions and Comments -> Responses

1

Strengthening Existing BridgesFlexural girder strengthening with CFRP laminate

http://aslanfrp.com/Aslan400/Resources/Aslan400.pdf

• Post-tensioning• Wrapping/confining

• Carbon fiber reinforced polymer (CFRP) laminate

• Near-surface mounted (NSM)• Carbon fiber reinforced polymer rod/strip• Glass fiber reinforced polymer (GFRP) rod• Stainless steel bars

FRP rods and laminates fail due to bond and anchorage and materials are nonductile

Concerns with corrosion at surface for most metals, relatively low strength (stainless reinforcing bars)

Strengthening with NSM CFRP strips

http://aslanfrp.com/Aslan500/aslan500-pg2.html

2

Ductile FRP?

Environmentally insensitive material with high strength, well defined properties, good surface

bonding characteristics along length, and efficient mechanical

anchorages

3

Titanium Alloy Material Properties (Ti-6Al-4V)

4

1210

690

860

1030

520

340

170

1380

Str

ess

(M

Pa)

Extensometer Strain (in/in)

Titanium Alloy Material Properties (Ti-6Al-4V)

• Aircraft fastener quality (6% Aluminum 4% Vanadium)

• Well-defined, high strength, and ductile (limited hardening-

>protects bond, structural fuse)

• High fatigue resistance (CAFL~ 75 ksi), low notch sensitivity

• Impervious to chlorides due to stable oxide layer

• Coeff. of thermal expansion (8.6me/ oC) (8-12 Con. and 12 St.)

• Conventional fabrication (shear, cut, and bend)

• Relatively lightweight of 281 lb/ ft3 (steel 1.7x)

• Bends facilitate anchorage

5

Strengthening – Flexure and Diagonal Tension (Shear)

6

26 full-scale specimens

Fabrication and Installation

7

ACI 440.2R

• Groove Spacing

• Groove dimensions

Durability High Cycle Fatigue and Freeze-Thaw Combined

8

• Largest combined structural-environmental testing chamber

• Thermocouples at 0.5, 1.5, and 3 in. ensure temperature targets

• 1.6 million cycles @ steel stress range >50 years of life.

Time

Tem

pera

ture

(C

)

6/1/2016 4:34:00 PM 6/2/2016 12:34:00 AM 6/2/2016 7:25:00 AM-10

-5

0

5

10

15

0.5 in. embedment1.5 in. embedment

3 in. embedmentAmbient

T Beam Experimental Results – Durability (s=10 in.)

9

TiAB Env. and Fatigue

TiAB

Base

Field Demonstration: Mosier Bridge Over I84

10

DL produces M-

LL produces M+

Results

11

• Design strength of Ti girder exceeds factored demands even with conservative assumptions

• Reserve strength of Ti girder substantially exceeds factored demands• Strengthening of failed girder better response than unfailed girder

Desi

gn

Reserve Capacity

Predicted strength w Ti

12

30% less expensive than CFRP

Main Committee: Committee B10 –

Reactive and Refractory Metals and Alloys

Sub-Committee: Committee B10.01 on Titanium

ASTM Specification for NSM Titanium

Approved Nov. 2018

ASTM B1009-18 Requirements:

• Tensile properties (Class 120 and 130)

• Chemical requirements

• Bond strength

• Cross-Sectional area calculation

• Bending requirements

Design Guide Ballot Item

• “Guide for Design and Construction of Near-Surface Mounted

Titanium Alloy Bars for Strengthening Concrete Structures”

• AASHTO-LRFD Format

• General Conditions

• Materials

• Construction

• Installation

• Design

• Flexure and Shear (MCFT)16

Design Guide

• Conventional analysis methods

• Design TiABs at yield if conditions are met

• Includes environmental durability factor (epoxy)

• 3 Limit states for flexure and 1 for shear

• Strength

• Service (check bond stress at cutoffs and where retrofitted strength above base capacity)

• Fatigue (not of TiAB but of reinforcing steel)

• Comprehensive design example (shear and flexure)

17

Review Comments

• 63 Comments

• TX, MN, IN, PA, NY, NC, AK

• Responses provided by Dr. Tanarat Potisuk, ODOT and Chris Higgins, OSU

• Document revised per comments and updated to portal

• Example revised based on updated ASTM grade classes.

18

Article/ Section Comment Resolution

3

Chapter 3 does the low Coefficient of

thermal expansion relative to steel and

concrete need to be taken into account?

(Durability and fatigue?)

The coefficient of thermal expansion for the Ti alloy (8.6me/ oC) is closely correlated to

that of concrete (8 to 12 me/ oC) and thermal stresses are no higher than that of

conventional reinforcing steel (CoTE=12me/ oC). A number of specimens with NSM Ti

were subjected to about 120 freeze-thaw cycles in a climate-controlled chamber. Tests

after the freeze-thaw cycles showed that capacity reduction of the specimens was

insignificant compared to the control specimens.

5

Chapter 5 No mention of required cover,

hole diameters, bar clearance in the

tolerance section.

The groove size equals 1.5 x db, which results in 5/ 32 in. of clearance around the bar. This

value is very close to 1/ 8 in., the smallest/ practical tolerance dimension. Titanium bars

are specified in the center of the groove surrounded by epoxy resin. Verification of

position tolerances during epoxy curing time or after fully cured epoxy is not

recommended. Hole diameters will be controlled by the drill bit size. Therefore,

tolerances for only hole depth and hole locations are needed

5 & 10

Sections 5.1, 5.2 & 10.need to be reconciled.

Tolerances are custom and do not agree

with Standard hook fabrication. Cursory

review shows that use of re-bar standard as

a whole may be acceptable.

The titanium alloy material properties are quite distinct from rebar. The

material is more closely associated with that used for aircraft fasteners.

Thus, the details are not tied to reinforcing bars. The bond and

development is achieved using structural epoxy which is different than

reinforcing bars in concrete. The hook tail lengths followed the

requirements for conventional reinforcing steel stirrup hooks and the bend

diameters followed standard hooks according to CRSI. Material and

specimen tests showed that the combination of the proposed bend radius

and tail dimensions performed satisfactory such that the Ti alloy material

yield stress can be achieved for design (as demonstrated in test results).

5

Section 5.3 states that TiAB must not be

placed in contact with steel. This is

contradiction with the findings related to

galvanic corrosion. If they are non reactive,

non magnetic why is this disallowed?

The other purpose of this section is to avoid damage to existing rebar. A hammer drill

with carbide tip bit is required for drilling holes for the hooked ends, so existing rebar

should not be adversely impacted. If a hole is too close to existing bars, the drill bit can

damage existing bars. It is true that titanium is non reactive and non magnetic, but when

it touches existing rebar, there is a possibility of galvanic corrosion. The epoxy should

isolate the Ti alloy bars from the steel, and recent research (Platt, S., & Harries, K. (2018).

Study of Galvanic Corrosion Potential of NSM Titanium Reinforcing Bars. Case Studies in

Construction Materials, 9, [e00175]. https:/ / doi.org/ 10.1016/ j.cscm.2018.e00175) shows

that this is not of significant concern, but it is something we try to avoid.

7

Section 7.2No upper limit to strength

added? or do we adhere to AASHTO in all

cases not stated.

No. Yes, we adhere to AASHTO for all cases.March 12, 2014 19

7

Section 7.3 Material resistance

to fire as well as upper thermal

load limits should be included

in section 3.

The maximum undesirable temperature to open flame is

mentioned in Section 5.2. In general, the design is no more

sensitive to high thermal loads than conventional CFRP NSM.

The anchorage of hooks into the core concrete provides

additional protection from high temperatures (fire) compared to

conventional CFRP surface bonded and NSM applications.

7Section 7.4 Is this section

necessary?

Titanium is a relatively expensive material and typically applied

only to those regions requiring strengthening. This section is a

reminder for designers who will normally design a

strengthening system for targeted areas.

7

Section 7.5 Are these factors in

l ieu of, or addition to vendor

environmental exposure

adjustments

This factor is equivalent to those shown in Table 9.1 of ACI

440.2R, but is more specific to titanium embedded in epoxy

resin.

8

Section 8.2 Is this material valid

for application in non beam

theory sections. Beam theory is

the only condition addressed.

This material w ith NSM application is well suited for anchorage

strengthening in D regions as well as other locations requiring

tension ties. Determination of demands for supplemental Ti

alloy bars in D regions can be performed using strut-and-tie

methods similar to conventional reinforcing steel.

8

Section 8.5 should be revised to

clarify that 36 ksi is no longer

valid on the low end. Some

older structures this limit

exceeds yield.

While the purpose of the stress limit is to limit crack width for

new designs (which would not use grade 40 steel), for a retrofit

design of an existing bridge, we would permit a higher stress in

the reinforcing bars for serviceability considerations, which

would still be elastic for Grade 40. If the rebar stress is elastic

and below 36 ksi one could expect similar crack widths for

Grade 40 and 60 conditions. The addition of Ti alloy bars will

help reduce the rebar stress. When combined with the fatigue

provisions we are ensuring long-term performance. To account

for even lower grade steels, we added that it should not exceed

the yield stress of the existing reinforcing steel.

Pages 4-6,

Notations and

Symbols

Use consistent units as AASHTO

(e.g., kips rather than pounds, etc.)

We will re-format this design guide to make it consistent with AASHTO

LRFD format. The units will be revised.

Page 7, Section

1.2

Mentions that at ultimate strength,

debonding of the TiABs is

anticipated. Does it become a more

brittle failure then?

According to specimen tests, after debonding of the TiABs, the end hooks

embedded in holes filled with epoxy resin were able to provide anchorage

for the titanium bars to fully develop yield and associated ductility. The

maximum load is achieved when loss of Ti alloy bar bond is observed.

However, the steel achieves yield and the Ti bars do as well. The beam

specimen behavior was ductile. The ideal retrofit shifts a nonductile failure

mode to a ductile one.

Page 9,

Chapter 2

Tests mention an increase in

capacity over standard. Was a

"base" test done to determine actual

capacity without TiAB added?

Yes. OSU has tested large numbers of full-size RC bridge girder specimens

including control specimens and specimens with many strengthening

methods since 2001. References were included in the guide and research

reports are available at ODOT Research website.

Page 11,

Section 4.1

The description of the bonding

material is pretty general overall. It

could be more specific/ helpful in

showing how to determine if a

material is adequate or not and what

tests might be required in the field to

verify it.

This is what NSM-CFRP industry found as well. Most high-strength resins

will be adequate to develop the NSM system. Currently there is not an

ASTM standard testing procedure to test NSM bond. The research

developed its own bond specimen tests. A standard test for NSM bond is

being developed.

Page 12,

Section 5.2

Cracking at the bend is mentioned.

Is any testing needed to check for

cracks?

Tests were conducted to investigate the possibility of crack initiation in the

bend. The test results on even tighter bend radius than proposed were

included in the referenced research reports. Warm-working the bars during

bending prevented cracking of the bars (even for tighter bend radii than

proposed). The temperatures reported in the design guide are

recommended for fabrication and are easy to achieve and verify visually

from color change on the Ti alloy bars.

Page 12,

Section 5.2

Surface grinding is mentioned as an

option on the inside bend radius.

Does a reduction in capacity need to

be taken?

No additional reduction is required. As described in Section 3.2, 96% of

nominal area is recommended for use in the design. Yes, a reduction of

cross-sectional area is taken into account.

Page 12,

Section 5.3

What if standard hooks can't be

used (i.e., if concrete isn't thick

enough to allow them)?

Detailing adjustment will be required, such as shifting the hook

end locations to beam or intermediate diaphragm location for

strengthening a bridge deck or use a smaller bar size to reduce a

required tail length.

Page 13,

Section 5.4

It notes that concrete cover

shall be sound. What if it isn't?

Need to know what to do if you

get in the field and concrete is

in worse condition than

anticipated.

Any strengthing approach that makes use of the existing

concrete to transfer stress will require sound concrete. For very

damaged sections of concrete, other strengthening methods

could be more economical, such as section enlargement w ith

new rebar. Before making design decisions on any strengthening

approach, the condition of concrete substrate should be

evaluated as best as possible.

Page 13,

Section 5.4

What if there is not a uniform

groove? Many applications

remove additional concrete

due to condition. What about if

shotcrete was previously

applied?

This is detailing issue. Grinding can be specified before cutting

grooves given that there is adequate remaining clear cover. There

may also be other construction techniques that can be used to

produce grooves in concrete substrate.

Page 13,

Chapter 6

More figures would be helpful

at this location to better show

what is being discussed.

Figures will be added when the guide is re-formatted to

AASHTO LRFD format.

Page 14,

Section 7.2

Why is such a low value used

for the load factor on DC load

(1.05)?

For this check, it does not require full-design dead load at

strength level. See comment below.

Page 14, Section

7.2

What is the basis of equation 7-1? Any

statistics used in determining load

factors? Probabilities?

This is similar to the new AASHTO Redundancy I load combination for fracture

critical bridges. The factor proposed for DC is the same and the factor proposed

for DW is slightly higher (1.1 instead of 1.05). The live load factor proposed is also

slightly higher (because it includes 15% impact-> effective factor of 0.863

compared to 0.85 in Redundancy I). The Redundancy I load combination was

calibrated and intended to show the bridge member has sufficient capacity to

survive in a faulted state. It is possible to establish a specific reliability level for

this condition. While we do not believe designs should necessarily be limited by

this provision, it provides guidance on other considerations like inspection

intervals to ensure long-term performance.

Page 14, Section

7.2

In lines following equation 7-1, should

read "…from weight of components,

weight of wearing surface, and fatigue

l ive load…."

"fatigue" will be added in front of "live load".

Page 14, Section

7.2

The second to last paragraph in the

section seems a bit too liberal. Need a

bit more guidance if the equation is not

being met.

It is desirable for the structure to be able to support it's own weight and some live

load by calculation. However, design calculations can show that a bridge cannot

support any live load (or even it's own weight), yet carries traffic and even

without signs of distress. Research with Ti alloy bars has shown that a completely

failed bridge girder can be restored to full design strength. The use of hooks

embedded in the core is an added benefit for the method as this protects the

anchorage and the structural adhesive. Hence, this section provides for

engineering judgement while expressing caution.

Page 14, Section

7.2

Suggest adding a comma in last

sentence "…exercised by the designer,

and inspection…"

Will modify as recommended.

Page 15, Section

7.6

Why are strains noted for bonding

materials w ith varying sensitivity and

different bar grades and what is meant

by them? Was this done to bound the

values or..?

The paragraph demonstrates how strains are calculated using two different

combinations of the bar grade and environmental exposure factor. Will add "For

example," in front of the sentence. Some structural adhesives show reduced

performance under environmental exposure. The exposure factor is based on

research done on full-scale girders with Ti alloy bars and also based on the factors

used in ACI 440.

Page 16, Section

8.2

Mentions there is no relative slip

between the concrete and steel or

TiABs. Has the research (tests) shown

this to be true?

Yes, strain compatibility works for sections with Ti alloy bars in NSM application.

Page 16,

Figure 8.1

Should note that the DL strain

is only original dead load and

not added dead load.

This is described on the following page (Pg. 17), 1st paragraph.

Page 17,

Equations 8-

4a and 8-4b

Values are only for

reinforcement Grade 60 or less.

Should note this and refer to

AASHTO for other grades of

reinforcement higher than 60.

Most bridge structures that would require NSM -TiABs

strengthening are more likely reinforced with Grade 40 or Grade

60 bars.

Page 17,

Equations 8-

4a and 8-4b

Shouldn't these equations also

check for the TiABs and use the

most conservative value? If so,

what other values should be

used since the grade falls

outside of what is specified in

AASHTO.

Strain is calculated at the existing rebar elevation as an indicator.

TiABs are located near the extreme fiber, therefore TiABs will see

larger strain. This is predictable. Both existing rebar and TiABs

need to reach yield stress to ensure ductile behavior. If TiABs

strain is used, we may not know if the existing rebar yields or

not.

Page 18,

Section 8.5

Mention the upper bound of 36

ksi for grades of 60 or less.

What if it is higher? What

about for the TiABs?

In the service range, the existing rebar w ill control crack width

due to the higher elastic modulus, therefore the stress in existing

rebar is used for crack control. Note that TiABs will lower

stresses in existing rebar in a strengthened section. It is also

noted that strengthening would be more likely on bridges with

lower yield stress reinforcing bars.

Page 18,

Section 8.6

Mention for steel w ith grade of

60 or less. What if it's higher?

Most bridge structures that would require NSM -TiABs

strengthening are more likely reinforced with Grade 40 or Grade

60 bars.

Page 19,

Figure 9.1

How are the transverse bars

installed? Field bending?

#2 TiABs were used in the tests and recommended for shear

strengthening. Given the low elastic modulus and small cross-

sectional area, the bar can be field-bent easily. The TiAB stirrup

can be opened up around the beam section and it w ill return to

its original shape due to its high strength. The bends can also be

done by titanium manufacturer before shipment of material to a

construction site.

Page 20,

Equation 9-2

Should be greater than or equal

to, not just equal to.Agreed. Will revise.

Page 20,

Equation 9-4

Equation doesn't match

AASHTO since you are not

using the same units. Should

use the same units as

AASHTO.

The units and equations will be revised in the re-formatted

version.

Chapter 9

Why is prestressing component

taken out of equations? Should

include since this is a

possibility that it would need

to be included for those

situations.

Will modify as recommended.

Page 21,

Section 9.4

Recommend stating what aE is

in last sentence again.Will modify as recommended.

Page 21,

Equations 9-

10a and 9-

10b

How are these determined?

Seems there should be some

correlation with how much

steel shear reinforcement is

present.

The typo is fixed. The minimum spacing comes from those

provided in ACI 318 and 440 whereby as the demand in the

section increases, the minimum stirrup spacing decreases to

ensure a sufficient number of stirrups cross the potential

diagonal crack. This is more strict than that from AASHTO

5.7.2.6 and further provides improved crack control.

Page 21,

Equations 9-

10a and 9-

10b

Should use same units as

AASHTOWill modify as recommended.

Page 23,

Table 10.1

Formatting of numbers is

inconsistentWill modify as recommended.

Page 23,

Section 10.1

What about if strengthening

both flexure and shear? Need

to show spacing requirements

for that condition.

This w ill be shown in the example later on. Will consider adding

language in this section as well.

Page 25,

Section 10.5

What about hook termination

locations relative to regular bar

terminations?

At design, a designer is more likely find that TiABs are required

beyond the existing rebar termination. Therefore, only TiABs

hook termination needs to be determined. The design guide

further requires check of Ti alloy bar bond stresses at locations

where the strengthened element capacity exceeds demands.

Page 26,

Figure 11.1

Note in figure that girder is

symmetrical about CL?Will modify as recommended.

Page 27,

Figure 11.2

Should use arrows on

dimension lines.Will modify as recommended.

Page 30,

Section

11.1.1

Have it written as "…with 2-

5/ 8 in. diameter…" but

recommend "…with two 5/ 8

in. diameter…" for clarity.

Will modify as recommended.

Page 33,

Section

11.1.2

Recommend showing calc for

change in stress of 2.5 ksi.

The method for computing stresses in the Ti alloy bars was

previously demonstrated in Section 11.1.1 and would be

redundant here.

Page 36,

Section

11.1.4

Recommend showing calcs for

stresses of 13.7 ksi, 2.7 ksi, and

1.8 ksi.

The method for computing stresses in the Ti alloy bars was

previously demonstrated in Section 11.1.1 and would be

redundant here.

Page 38,

Section

11.2.1

For VC equation, should show

the equation using units

consistent w ith AASHTO.

Will modify as recommended.

Page 39,

Section

11.2.1

For equation 6 sqrt(f'c) bw dv,

should use units consistent

w ith AASHTO.

Will modify as recommended.

Page 44,

Figure

11.15a

There may not be room for

both flexural and shear TiAB

depending on requirements for

spacing between layers.

For most situations of conventional cover, there is sufficient

room to deploy Ti alloy bars (smaller diameter for shear and

larger for flexure) to be able to do both flexural and shear

strengthening simultaneously. To avoid the chance of

accidentally cutting existing steel stirrups, the bar placement as

shown in the example is preferred.

3.2

Geometric

Properties

Unclear whether the intent is

to use smooth or deformed

rebar. Please clarify what types

of bars are acceptable.

The bars are required to have surface deformations to enhance

bond.

10.4 Standard

Hook Details

There are details on minimum size of

grooves and spacing. However, the

same details were not provided for

the shear/ hook bars. Please provide

details on size of hole required for

drilling and minimum spacing

between stirrup bars. Or should

same logic of 1.5Dti and >=3Dti be

applied for the stirrups? If so,

specify.

In this section, there is a sentence indicating that the hook details are the

same for shear and flexure. Yes, the same requirements apply to both

stirrups and flexural Ti alloy bars.

Section 5.4

Can NSM TiAB be used on surfaces

that have spalled and been patched?

Many older structures needing

strengthening will have some girders

with areas of delamination/ spalling.

All delaminations and spalling should be located and repaired prior to

construction of NSM-TiABs, as would similarly be required for other

materials.

New

Document

Why is the section limited to

titanium only? It would seem that

these procedures could/ should be

used for any appropriate metal

reinforcing bar.

Other metallics could similarly be applied in NSM applications. The

materials need to be highly corrosion resistant. Some stainless steels meet

this requirement and have been used in the NSM method. However, they

have limited strength compared to Ti alloy bars and thus require more bars.

To achieve the same strength levels as Ti alloy bar requires more stainless

steel (more bars and bigger diameter bars). This adds construction costs

and increases time (to add more cutting, drilling, cleaning, structural

adhesive, etc.). The need for larger diameter bars also becomes problematic

for saw cutting deeper and wider grooves. In addition, there is not

sufficient data at the present time to develop design standards for such

materials. The design guide, as proposed, forms a framework design with

other metallics in NSM applications and could be modified as new

materials are tested and demonstrated.

Can this guide also include

other materials that can be used

for strengthening (ie MMFX,

Stainless, CFRP)?

AASHTO T6 is updating the AASHTO guide for bonded FRP

systems to include NSM-CFRP. Other metallics could similarly

be applied in NSM applications. The materials need to be highly

corrosion resistant. Some stainless steels meet this requirement

and have been used in this method. However, some have limited

strength compared to Ti alloy bars and thus require more bars

(or larger bars). This can add cost and increase time for

construction (to add more cutting, drilling, cleaning, structural

adhesive, etc.). The need for larger diameter bars also becomes

problematic for saw cutting deeper and wider grooves. In

addition, there is not sufficient data at the present time to

develop design standards for other metallics. The design guide,

as proposed, provides a framework design with other metallics

in NSM applications and could be modified and adapted as new

materials are tested and demonstrated.

Not in AASHTO double

column formatThe double column format can be added if required.

29

Acknowledgements

• Oregon Department of Transportation

• Bruce Johnson, TACs, and Research group

• Perryman Company, Houston, PA

• Undergraduate Research Assistants: Kyle Logan, Jonathon Roy, Aléxia Ribeiro, Lance Parson, Hunter Anderson, Kyle Sonnevile, John Huntoon, Glen Galant, Corey Groshong, James Kemp, and Spencer Maunu

30

The findings and conclusions are those of the author and do not necessarily reflect those of the project sponsors or the individuals or companies acknowledged.