Institute of Bridge Engineering Civil, Structural and

Dr. Pinar Okumus, Assistant Professor Joshua Rodems, EIT, Graduate Student

Garrett Miller, Graduate Student

Institute of Bridge Engineering Civil, Structural and Environmental Engineering

University at Buffalo

April 19-24, Saratoga Springs, NY

Education

Professional Development

Research

www.buffalo.edu/bridge

Motivation

• Cover Externally Bonded (EB) systems

• Do not explicitly address Near-Surface Mounted (NSM) systems

Basis for AASHTO Guide Specs, 2012

Objectives

• Review literature on NSM FRP Concrete

Strengthening

• Evaluate AASHTO Guide Specifications,

2012 for NSM FRP

• Prepare provisions for NSM FRP systems

to implement into the AASHTO Guide

Specifications, 2012

Groundwork

Includes both

• EB Laminates

• NSM Bars

Outline

Section 1-General Requirements

Section 2-Material Requirements

Section 3-Members under Flexure

Section 4-Members under Shear and Torsion

Section 5-Members under Combined Axial Force and Flexure

AASHTO Guide Specifications, 2012

Members under Flexure

1) The strain in the FRP system at FRP debonding / rupture:

𝑇𝑓𝑟𝑝 = tensile force in the FRP reinforcement

corresponding to an FRP strain of 0.005 AASHTO Guide


ACI 440.2R 𝜀𝑓𝑑 = 0.083𝑓`𝑐

𝑛𝐸𝑓𝑡𝑓 ≤ 0.9𝜀𝑓𝑢

𝜀𝑓𝑑 = 0.7𝜀𝑓𝑢

for FRP laminates

for NSM FRP

Members under Flexure

2) The development length, ld:

for FRP laminates

for FRP circular bars

for FRP rectangular bars

𝐿𝑑 ≥𝑇𝑓𝑟𝑝

𝜏𝑖𝑛𝑡𝑏𝑓𝑟𝑝

𝑙𝑑𝑏 =𝑑𝑏

4 𝜏𝑏𝑓𝑓𝑑

𝑙𝑑𝑏 =𝑎𝑏𝑏𝑏

2(𝑎𝑏 + 𝑏𝑏) 𝜏𝑏𝑓𝑓𝑑

𝑙𝑑𝑓 = 0.057𝑛𝐸𝑓𝑡𝑓

𝑓`𝑐

AASHTO Guide


ACI 440.2R

Outline

Section 1-General Requirements

Section 2-Material Requirements

Section 3-Members under Flexure

Section 4-Members under Shear and Torsion

Section 5-Members under Combined Axial Force and Flexure

AASHTO Guide Specifications, 2012

FRP Shear Strength

𝑽𝒓 = ∅ 𝑽𝒄 + 𝑽𝒔 + 𝑽𝒑 + ∅𝒇𝒓𝒑𝑽𝒇𝒓𝒑

∅𝒇𝒓𝒑 = 0.85 (additional resistance factor for FRP only)

𝑽𝒇𝒓𝒑 = Nominal shear strength of FRP system

- Externally Bonded (EB): outlined explicitly

- Near-Surface Mounted (NSM): described but

no confirmed design methodology

Current design spec – AASHTO 2012:

• Strength of NSM FRP reinforced concrete beam:

- Mofidi et al (2012) – 6 beams

- Wiwatrojanagul et. al (2012) – 6 beams

- Cisneros et. al (2010) – 8 beams

- Dias and Barros (2004 & 2006) – 8 beams

• Average bond stress and groove size:

- DeLorenzis and Nanni (2002) – 22 tests

- DeLorenzis et. al (2002) – 36 tests

- Mirmiran et. al (NCHRP 609, 2008) – 12 tests

• Total: 28 beams, 70 bond/pullout tests

Experimental Studies

Cisneros et. al (2010)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

B90-3 B90-6 B45-3 B45-6 S90-3 S90-6 S45-3 S45-6

Str

eng

th I

ncr

ease

Beam Code

Shear Strength Increase due to NSM FRP

Key: B45-3

FRP incline angle

Number of FRP bars in given length


FRP Bar or Strip 𝑆𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝐼𝑛𝑐𝑟𝑒𝑎𝑠𝑒 =

𝐹𝑎𝑖𝑙𝑢𝑟𝑒 𝑆ℎ𝑒𝑎𝑟 (𝑤𝑖𝑡ℎ 𝑁𝑆𝑀 𝐹𝑅𝑃)

𝐹𝑎𝑖𝑙𝑢𝑟𝑒 𝑆ℎ𝑒𝑎𝑟 (𝑤𝑖𝑡ℎ𝑜𝑢𝑡 𝐹𝑅𝑃)


Beam S45-3: Cisneros et al (2010) Typical Test Setup

Two-Opposite-Side


• Experimental Failure Shear

• 2012 AASHTO Guide Spec: Bonded FRP Systems

- Rupture failure

- Nonrupture failure

• FRP Draft Spec: EB Systems (ACI 440.2R-08)

• Near-Surface Mounted: De Lorenzis & Nanni (2001)

• Others investigated by not practical for code use

Graphical comparison of shear strength:

∅𝒇𝒓𝒑 = 0.85


0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

B90-3 B90-6 B45-3 B45-6 S90-3 S90-6 S45-3 S45-6

Sh

ea

r F

or

ce

(k

ips

)

Beam Code

Comparison of Calculated and Actual Shear Strength (Vn)

Failure Load

Calculated Strength - 2012 AASHTO, Rupture Failure

Calculated Strength - 2012 AASHTO, Nonrupture Failure

Calculated Strength - ACI 440.2R-08

Calculated Strength - De Lorenzis & Nanni NSM (2001) Tb = 1.00 ksi


0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

B90-3 B90-6 B45-3 B45-6 S90-3 S90-6 S45-3 S45-6

Sh

ea

r F

or

ce

(k

ips

)

Beam Code

Comparison of FRP Shear Strength (Vfrp)

2012 AASHTO, Rupture Failure 2012 AASHTO, Nonrupture Failure ACI 440.2R-08 De Lorenzis & Nanni NSM (2001)

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70Ca

lcu

late

d S

hea

r S

tren

gth

(k

)

Experimentally Measured Shear Force at Failure (k)

Calculated Strength vs Actual Strength: 2012 AASHTO, Rupture Failure

Cisneros et al (2010) Wiwatrojanagul et al (2012)

Mofidi et al (2012) Dias and Barros (2004)

Calculated Strength = Failure Load

Over-estimated

Under-estimated

Shear strength over-estimated

Shear strength under-estimated

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70Ca

lcu

late

d S

hea

r S

tren

gth

(k

)


Calculated Strength vs Actual Strength: 2012 AASHTO, Nonupture Failure




Over-estimated

Under-estimated

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70Ca

lcu

late

d S

hea

r S

tren

gth

(k

)


Calculated Strength vs Actual Strength: Draft Spec (ACI 440.2R-08)




Over-estimated

Under-estimated

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70Ca

lcu

late

d S

hea

r S

tren

gth

(k

)


Calculated Strength vs Actual Strength: NSM – De Lorenzis & Nanni (2001)




Over-estimated

Under-estimated

0

2

4

6

8

10

12

14

16

1.00 1.20 1.40 1.60 1.80 2.00 2.20 2.40 2.60 2.80

Ex

per

imen

tal

Pu

llo

ut

Lo

ad

(k

)

Groove Ratio, k = Groove Depth/Bar Diameter

Groove Ratio vs Pullout Strength

DeLorenzis & Nanni (2002) DeLorenzis et al - Epoxy (2002) Optimum Groove Size (min) Optimum Groove Size (max)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

1.00 1.20 1.40 1.60 1.80 2.00 2.20 2.40 2.60 2.80 3.00

Av

era

ge

Bo

nd

Str

ess

(ksi

)

Groove Ratio, k = Groove Depth/Bar Diameter

Groove Ratio vs Average Bond Stress

DeLorenzis & Nanni (2002) DeLorenzis et al - Epoxy-Conc Interface (2002)

DeLorenzis et al - Epoxy-Bar Interface (2002) Optimum Groove Size (min)

Optimum Groove Size (max) Average Bond Stress, Tb = 1.0 ksi

Shear Strength Equation: De Lorenzis & Nanni (2001)

The term Vf shall be determined as:

For round bars: 𝑽𝒇 = 𝟐𝝅𝒅𝒃𝝉𝒃𝑳𝒕𝒐𝒕

where:

db = diameter of NSM FRP bar (in)

τb = average bond stress of NSM FRP reinforcing (ksi).

(May be taken as 1.0 ksi).

Ltot = total length of NSM FRP bars crossed by a 45°shear crack (in)

The term Vf shall be determined as:

For rectangular bars: 𝑽𝒇 = 𝟒 𝒂𝒃 + 𝒃𝒃 𝝉𝒃𝑳𝒕𝒐𝒕

where:

ab = smallest cross-sectional dimension of NSM FRP bar (in)

bb = largest cross-sectional dimension of NSM FRP bar (in)

τb = average bond stress of NSM FRP reinforcing (ksi).

(May be taken as 1.0 ksi).

Ltot = total length of NSM FRP bars crossed by a 45°shear crack (in)


The term τb (average bond stress) specified in the

nominal shear strength equations:

• Direct relationship to shear strength

• Experimental data shows can be taken as 1.0 ksi

• Further testing can produce results > 1.0 ksi

(permissible to use in design).


FRP spacing and incline


l0.004 = length of NSM FRP bar based on limiting

strain in the FRP to 0.004.

lnet = Net length of NSM FRP bar used for reinf. (in).

n = the number of FRP bars crossed by a single

45°shear crack (integer).

- calculated in manner that prevents a

longer bonded length from over-

estimating shear capacity (Vn).


L4

L3 L

2

L1

Vn based on 45-degree shear cracks:

- Mild steel only – accurate

- Prestressing steel present– conservative (α < 45°)

n = 4


Groove Size • Groove size has impact on average bond stress

• Constructability vs. optimum bond

• Round bars:

Optimum groove size: 1.5 to 2.0 db

Maximum groove size: 2.0 db

• Rectangular bars:

Optimum groove size: 1.5 to 2.0 ab and bb

Maximum groove size: 3.0 ab and 2.0 bb

- ACI 440.2R-08 (13.3) and Nanni et. al (2004)

• Tolerance: +/- 1/8” for 9/16” groove is OK

- Mirmiran et. al, NCHRP Report 609 (2008)

Groove Size

High aspect

rectangular bar

Standard

circular bar

Steel Stirrup

(typ.)

ACI 440.2R-08 (13.3) and Nanni et. al (2004)

[+/- 1/8” tolerance]

Conclusions

• NSM FRP can increase Vn significantly depending on

beam characteristics & FRP properties and spacing.

• Current AASHTO Guide Spec can estimate Vf for

NSM FRP but De Lorenzis & Nanni (2001) method

has explicitly outlined a reliable design procedure.

• Draft code language was prepared on:

o Debonding/rupture strain

o Development length

o Shear strength contribution of NSM FRP

o Groove size

Conclusions • Optimum groove size = 1.5 to 2.0x bar width unless

constructability is a factor (Max = 3.0x).

• 1/8” +/- tolerance for 9/16” groove size OK

• The FRP failure mode changes from epoxy splitting

to concrete splitting as groove size is increased for

both FRP bars and strips.

• Best bond - epoxy adhesive (vs alternatives)

• Experimental testing can be used to obtain

increased values for avg. bond stress (τb) higher

calculated shear strength.

Recommended Actions

• Extend the literature review, cover chapters of

the AASHTO Guide Specifications, 2012 other

than flexure and shear

• Conduct research to fill knowledge gaps such as

the ones on prestressed concrete and torsion,

generate more test data

• Draft additional code language that will be added

to the AASHTO Guide Specifications, 2012

• Develop design examples for NSM FRP

Strengthening

References

AASHTO, 2012. Guide Specifications for Design of Bonded FRP Systems for Repair and Strengthening of Concrete Bridge Elements, 1st ed. American Association of State Highway and Transportation Officials, Washington D.C. 2012.

ACI Committee 440. 2008. Guide to the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures, ACI 440.2R-08, American Concrete Institute, Farmington Hills, MI.

Barros, J. A. O., Dias, S. J. E., “Shear Strengthening of RC Beams with Near-Surface-Mounted CFRP Laminates.” 7th International Symposium on Fiber-Reinforced Polymer (FRP) Reinforcement for Concrete Structures (FRP7RCS), Kansas, USA, Vol. 1, pp. 807-823. 2005.

Cisneros, D., Artega, A., De Diego, A., Alzate, A., Ricardo, P., 2012, “Experimental Study on NSM FRP Shear Retrofitting of RC Beams.” 6th International Conference on FRP Composites in Civil Engineering (CICE2012), Rome, Italy, 13-15 June.

De Lorenzis, L., Nanni, A., 2001. “Shear Strengthening of Reinforced Concrete Beams with Near-Surface Mounted Fiber-Reinforced Polymer Rods,” ACI Structural Journal, Vol. 98, No. 1, pp. 60-68. American Concrete Institute, Farmington Hills, MI.

De Lorenzis, L., Nanni, A. 2002. “Bond Between Near-Surface Mounted FRP Rods and Concrete in Structural Strengthening.” ACI Structures Journal, Vol. 99, No. 2, March-April 2002, pp. 123-133.

De Lorenzis, L., Rizzo, A., and La Tegola, A., (2002). “A Modified Pull-out Test for Bond of Near-Surface Mounted FRP Rods in Concrete.” Composites, Part B, 33(8), 589-603.

Mofidi, A., Chaallal, O., Benmokrane, B., Neale, K., “Experimental Tests and Design Model for RC Beams Strengthened in Shear Using the Embedded Through-Section FRP Method.” Journal of Composites for Construction, Vol. 16, No. 5, October 2012, American Society of Civil Engineers.

Nanni, A., Parretti, R., “Strengthening of RC Members Using Near-Surface Mounted FRP Composites: Design Overview.” Advances in Structural Engineering, Vol. 7 No. 5, 2004.

NCHRP. 2008. Recommended Construction Specifications and Process Control Manual for Repair and Retrofit of Concrete Structures Using Bonded FRP Composites, NCHRP Report 609. Transportation Research Board, National Research Council, Washington, DC.

NCHRP. 2011. Design of FRP Systems for Strengthening Concrete Girders in Shear, NCHRP Report 678. Transportation Research Board, National Research Council, Washington, DC.

Wiwatrojanagul, P., Ayudhya, B. I.N., Sahamitmongkol, R., “NSM FRP Shear Strengthening of RC Beams with Internal Stirrups.” Thammasat International Journal of Science and Technology, Vol. 17, No. 1, January-March 2012.

Thank You

Pinar Okumus, PhD

[email protected]

Joshua Rodems, EIT

[email protected]

www.buffalo.edu/bridge

mailto:[email protected]

mailto:[email protected]

http://www.buffalo.edu/bridge

𝒍𝒆𝒇𝒇 = 𝒍𝒃 𝒔𝒊𝒏 𝜶𝒇 − 𝟐𝒄

𝒍𝒃 = 𝒍𝒆𝒏𝒈𝒕𝒉 𝒐𝒇 𝑵𝑺𝑴 𝑭𝑹𝑷 𝒃𝒂𝒓

𝒏 =

𝒍𝒆𝒇𝒇 𝟏 +𝟏

𝐭𝐚𝐧 𝜶𝒇

𝒔𝒇

𝒍𝟎.𝟎𝟎𝟒 = 𝟎. 𝟎𝟎𝟏𝒅𝒃𝑬𝒇

𝝉𝒃

Proposed Design Equations (circular bars):

Proposed Shear Strength Equation: De Lorenzis & Nanni NSM (2001)

Proposed Design Equations (circular bars):

𝑳𝒊 =

𝒔𝒇

𝒄𝒐𝒔 𝜶𝒇 + 𝒔𝒊𝒏 𝜶𝒇

𝒊 ≤ 𝒍𝟎.𝟎𝟎𝟒 𝒇𝒐𝒓 𝟏 ≤ 𝒊 ≤𝒏

𝟐

𝒍𝒏𝒆𝒕 −𝒔𝒇

𝒄𝒐𝒔 𝜶𝒇 + 𝒔𝒊𝒏 𝜶𝒇

𝒊 ≤ 𝒍𝟎.𝟎𝟎𝟒 𝒇𝒐𝒓 𝒏

𝟐 < 𝒊 ≤ 𝒏

𝑳𝒕𝒐𝒕 = 𝜮𝑳𝒊

𝑽𝒇 = 𝟐𝝅𝒅𝒃𝝉𝒃𝑳𝒕𝒐𝒕

𝒍𝒏𝒆𝒕 = 𝒍𝒃 −𝟐𝒄

𝐬𝐢𝐧 𝜶𝒇

Proposed Shear Strength Equation: De Lorenzis & Nanni NSM (2001)

Documents

Institute of Bridge Engineering Civil, Structural and