Composites: Part B 58 (2014) 510–517

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Composites: Part B

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Effect of temperature on strength and stiffness properties of near-surfacemounted FRP reinforcement

1359-8368/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesb.2013.10.055

⇑ Corresponding author. Tel./fax: +1 (517) 353 9813.E-mail addresses: yubaolin@msu.edu (B. Yu), kodur@egr.msu.edu (V. Kodur).

Baolin Yu a, Venkatesh Kodur b,⇑a Dept. of Civil and Environmental Engineering, 3503 Engineering Building, Michigan State University, East Lansing, MI 48824, USAb Dept. of Civil and Environmental Engineering, 3580 Engineering Building, Michigan State University, East Lansing, MI 48824, USA

a r t i c l e i n f o

Article history:Received 30 April 2013Received in revised form 7 September 2013Accepted 25 October 2013Available online 11 November 2013

Keywords:A. Carbon–carbon composites (CCCS)B. High-temperature propertiesB. Mechanical propertiesD. Mechanical testingE. Near-surface mounted (NSM)reinforcement

a b s t r a c t

This paper presents results from an experimental study on the effect of temperature on tensile strengthand elastic modulus of two types of near-surface mounted (NSM) reinforcement, CFRP strip and CFRP rod,in 20–600 �C temperature range. Results from these tests indicate that both CFRP strip and rod retainmost of their initial tensile strength and elastic modulus properties up to 200 �C. However, these proper-ties degrade significantly beyond 300 �C due to decomposition of FRP resin. Further study shows NSMstrip and rod possess relatively higher strength than those used for internal reinforcement and externallaminates. Data from tests is utilized to propose empirical relations for variation of strength and modulusof NSM CFRP reinforcement with temperature.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, these is an increasing use of fiber-reinforcedpolymer (FRP) reinforcement in strengthening and retrofitting ofreinforced concrete (RC) structures. Currently, the most popularstrengthening technique is to apply FRP laminates to the surfaceof concrete element, which is designated as externally bondedreinforcing (EBR) technique. In this technique, FRP reinforcementusually fails due to premature debonding, and thus the full poten-tial of FRP is not efficiently utilized. This drawback of prematuredebonding can be overcome through near-surface mounted(NSM) technique, wherein slots are cut into the concrete cover ofan RC member, and then FRP rebars or strips are inserted intothe slot, and filled with epoxy adhesive or cementitious grout. ThusFRP reinforcement is bonded to concrete on three surfaces in thecase of strip or on all surfaces in the case of rebar, and this ensureshigher stress transfers between concrete and FRP. Combininginherent advantages of FRP reinforcement, such as light weight,non-corrosive, NSM FRP strengthening is becoming an attractivealternative to other strengthening forms in retrofitting ofstructures.

The other important advantage of NSM technique, over that ofexternal strengthening technique, is the protection to FRP rein-forcement from environmental, vandalism and fire damage [1].

Since FRP is embedded well inside concrete, the concrete coverprovides good insulating effect in the event of fire. However, thereare limited studies to quantify the level of fire resistance that canbe achieved in NSM FRP strengthened RC members. For evaluatingfire resistance of an NSM FRP strengthened structural member,temperature dependant strength and modulus properties of FRPreinforcement are required. The properties of NSM FRP reinforce-ment are different from that of conventional FRP used as internaland external reinforcement, since NSM FRP is usually of differentcomposition and of small cross-sectional area so as to facilitatesmaller groove size and easy installation. Further, in most practicalapplications, carbon FRP (CFRP) reinforcement is used in NSMstrengthening, due to relatively high strength and elastic modulusof CFRP as compared to that of GFRP and AFRP [2,3]. So far there islack of reliable data on high temperature properties of NSM FRP.

2. Previous studies

There is good amount of published information on high temper-ature performance of composites used in aeroplane and automo-bile applications. However, only limited data is available onspecific FRP types used in civil construction applications. Previousstrength tests on FRP reinforcement were mainly conducted in20–300 �C temperature range [4–6], but this is not sufficient toevaluate the performance of FRP under extreme temperaturesencountered in fire. Other notable studies focused on strengthand modulus degradation of specific internal FRP rebars in

B. Yu, V. Kodur / Composites: Part B 58 (2014) 510–517 511

20–600 �C temperature range [7,8]. These studies indicated thatFRP follows a linear stress–strain response at both room and hightemperature. However, the mechanical properties of FRP reinforce-ment degrade rapidly at higher temperatures, and carbon and glassFRPs retain about 50% of their original strength at 325 �C and250 �C respectively. Further, the authors inferred that the variationof strength and modulus is highly dependant on composition ofFRP.

Besides strength tests, few researchers [9,10] proposed empiri-cal relations for strength and stiffness degradation of FRP withtemperature, utilizing limited test data available in literature.However, these relations are mainly for FRP rebars and laminatesover a narrow temperature range, and may not be applicable toNSM FRP reinforcement, since not specific test data on NSM FRPwas utilized in developing these relations.

To develop data on strength and modulus degradation of NSMFRP at elevated temperatures, a series of high temperature tensilestrength tests were carried out at Michigan State University. Datafrom the tests is utilized to develop empirical relations for strengthand modulus degradation of NSM FRP reinforcement over a widetemperature range.

3. Preparation of test specimens

The experimental program consisted of tension tests on 25 CFRPstrips and CFRP rods at various temperatures. 13 of these test spec-imens were CFRP strips, while remaining 12 were CFRP rods. CFRPstrips were of 4.5 mm thickness and 13.5 mm width, and CFRP rodswere of 6.4 mm diameter. The nominal tensile strength and mod-ulus of CFRP strip, as specified by the manufacturer, is 2790 MPaand 155 GPa respectively, and the ultimate strain is 0.018. For CFRProd, the corresponding nominal tensile strength, elastic modulusand ultimate strain are 2070 MPa, 124 GPa and 0.017 respectively.CFRP specimens for tests were provided by FYFE Co. LLC. Otherproperties of FRP reinforcement used in the test program are givenin Table 1.

It is well established that CFRP reinforcement possesses hightensile strength at ambient conditions. However, in a tension test,two ends of CFRP are susceptible to crushing under the pressure ofgripping. Thus strong anchors have to be provided at the two ends,to facilitate gripping of CFRP specimen. The provision of properanchors ensures failure to occur in the central region of the speci-men, rather than at ends (in the anchorage zone). A specializedanchorage system was implemented while preparing CFRP strip/rod specimens for tension tests. The anchor system was developedfollowing ACI 440.3 specifications [11] and those recommended byWang et al. [8]. This is achieved through filling high strength adhe-sive into a circular steel tube (confinement), as shown in Fig. 1a.

In this experimental program, both high strength epoxy (Tyfo Sepoxy) and expansive cement (RockFrac NEDA) were applied asfilling materials to evaluate their relative bond performance. TyfoS epoxy is a two-component matrix material used in bondingapplications and is marketed by FYFE Co. LLC. This epoxy was pre-pared by adding component A (modified epoxy resin) to compo-nent B (hardener) in a volumetric ratio of 100:42 (or a weightratio of 100:34.5). The added ingredients were mixed for 5 minusing a mixer at a speed of 400–600 RPM until two components

Table 1Properties of NSM CFRP reinforcement as specified by manufacturer.

NSM reinforcement Tensile properties

Strength (MPa) Modulus (GPa) Ultimate s

Strip 2790 155 0.018Rod 2070 124 0.017

are uniformly blended. Another filling material used in the fabrica-tion of anchorage system is RockFrac NEDA expansive cement,which is used as non-explosive demolition agent and is marketedby RockFrac Company. The cement mortar was prepared by addingRockFrac cement into cold water (30% of the overall weight), andthen thoroughly mixing cement and water to get a uniform mortar.

Commercially available steel pipes were selected as confine-ment for filling materials, to ensure sufficient bond is generatedbetween filling material and CFRP specimen. The nominal dimen-sions of steel pipes are 42 mm in outer diameter and 1.6 mm inthickness, and the pipes were cut into tubes of 356 mm length.These dimensions are as per recommendations of ACI 440.3standard [11] and previous researchers [8]. To increase friction be-tween filling material and tube, 102 mm long thread was fabri-cated inside the surface of the tube. To prevent sliding betweenCFRP and filling material, some small dents were created on CFRPstrip or rod, and steel wires were bound to these dents, as shownin Fig. 1b. Through this procedure a higher interaction (friction)was generated between CFRP and anchor system.

When epoxy (or cement) is filled into the tube, CFRP strip or rodhad to be aligned vertically and centrally in the steel tube, to avoidany eccentric forces generated during tension test. For this a steelframe was fabricated to align CFRP and tube in the vertical direc-tion, as shown in Fig. 1c. The steel tubes sit on a wooden boardand they were clipped by two aluminum plates. A wooden plug,with a hole in the center, was installed at the bottom of the tubeso that CFRP specimen can be placed centrally. CFRP specimenwas also fixed at the top of steel frame to ensure it was aligned ver-tically. Once the epoxy gets hardened in the steel tube, CFRP spec-imen is turned around for casting anchor system at the other end.

4. Test set-up

Room temperature tensile strength tests on NSM CFRP speci-mens were carried out using Hydraulic Materials Test System(MTS). This is mainly attributed to the fact that MTS machine iscapable of providing high compression pressure to grip the twoends of test specimens, as well as applying higher tension load torecord high strength and stiffness of CFRP specimens at room tem-perature. CFRP strip and rod specimens for room temperature testswere specially prepared to fit MTS machine set-up. The test appa-ratus and specimens for room temperature tests are shown inFig. 2.

For high temperature tests, a different test set-up was devel-oped, and an illustration of this set-up is depicted in Fig. 3. In thisset-up, two ends of CFRP specimen (with anchor system), areclipped to two pairs of clamping brackets respectively, which areconnected to top and bottom beams. The CFRP specimen is loadedin tension by adjusting the distance between these two beams.Two hydraulic jacks, sitting on the bottom steel beam, can directlyapply specified loading to the top beam through an extension rod.When hydraulic jacks apply an increasing load, the top beammoves upward and thus CFRP specimen gets stretched longitudi-nally. The top beam is always maintained perfectly horizontal tominimize eccentric loading occurring during the test. The heatingdevice comprised of a small scale furnace which is placed betweentwo pairs of clamping brackets. Through this set-up, tensile

Dimension (mm) Density (g/cm3) Fiber content (%)

train

13.5 � 4.5 1.81 62dia. 6.4 N/A 60

(a) Epoxy filling (b) Wires on FRPs (c) Steel frame (d) Test specimen

Fig. 1. Fabrication of anchor system for FRP specimens.

MTS

CFRPstrip

CFRProd

Fig. 2. Test apparatus and specimens for room temperature test.

Steel bracket

Inside furnace

Furnace

LVDT

Hydraulic lack

Fig. 3. Test setup for FRP tension test at elevated temperatures.

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0 20 40 60 80 100 120

Tem

pera

ture

(°C

)

Time (min)

600°C

500°C

400°C

300°C

200°C

100°C

Fig. 4. Temperature progression with time to attain a target temperature in CFRPspecimen.

512 B. Yu, V. Kodur / Composites: Part B 58 (2014) 510–517

strength test can be conducted by heating the CFRP specimen to adesired temperature and then subjecting it to tensile loading.

During the test, CFRP specimen is heated to a target tempera-ture, and then the heating is continued for additional 20–30 minto ensure the entire specimen attains target temperature. Toaccurately monitor the temperature of CFRP specimen, two ther-mocouples are installed on the surface of CFRP specimen at twodifferent locations (mid-height and quarter height), and theaverage of these two thermocouple readings is taken as the actualtemperature of the specimen. The heating rate of furnace is set tobe at 5–10 �C/min, depending on the target temperature: a fasterrate is used for higher target temperatures. The progression ofmeasured CFRP temperature with time is shown in Fig. 4. It canbe seen in the figure that in each case, temperature gradually in-creases to a target temperature, and then the specimen is main-tained at this target temperature for about 20 min. This ensuresthat the specimen and furnace reach thermal equilibriumconditions and that the internal and surface temperatures of thespecimen were sufficiently close to the target temperature.

Following the specimen attaining a target temperature, tensiontest is carried out using hydraulic jacks. To measure elongation of

CFRP in tension tests, a linear variable differential transformer(LVDT) is placed between the upper and the lower clamping brack-ets. The variation of distance between these two brackets is takenas elongation of CFRP specimen placed between two anchors, since

B. Yu, V. Kodur / Composites: Part B 58 (2014) 510–517 513

the elongation of CFRP in anchor parts is negligible. The elongationmeasurements start as soon as loading is applied, and the displace-ment of the upper pair of brackets is recorded until CFRP specimenfails. The reliability of loading equipment and elongation measure-ments are verified through two preliminary tests, one using steelstrand and the other using CFRP strip. In these two tests, straingauges were placed along the longitudinal direction of the speci-men, and the measurement of strain gauges was compared withthe readings from loading cell. As shown in Fig. 5, in steel strandtest, tensile stress in specimen kept increasing until steel enteredyielding phase. While in CFRP strip test, tensile stress in specimenincreased linearly. It can be seen that the stress values based onload reading match well with those obtained from strain gauges(product of strain and modulus), and thus the measurement frominstrumentation is considered to be reliable.

5. Results and discussion

Data recorded in tension tests is utilized to evaluate tensilestrength and elastic modulus of NSM CFRP at various tempera-tures. The tensile strength was calculated by dividing the maxi-mum load at failure by the actual cross-sectional area of testspecimen, while elastic modulus was evaluated as the slope oflinear part of stress–strain curve. At each target temperature, twotension tests were conducted, and the average of two values wastaken as tensile strength and elastic modulus of CFRP. Results fromthese tests at various temperatures are tabulated in Tables 2 and 3for CFRP strips and rods respectively.

The tensile strength and elastic modulus of NSM CFRP strip,based on room temperature tests, were found to be 1641 MPaand 150.8 GPa respectively, and the corresponding values for CFRProd are 1577 MPa and 130.9 GPa respectively. The measured roomtemperature elastic modulus of CFRP strips and rods are very closeto those specified in manufacturer data (2.7% error for strip and5.6% error for rod). However, room temperature tensile strengthobtained from tension tests is relatively lower than manufacturerspecified nominal strength. This is mainly attributed to the factthat CFRP resin fractures at a relatively low load. In the room tem-perature tests, failure of CFRP specimen gets initiated throughcracking of resin. With increase in load, CFRP specimen graduallysplit into bunch of fibers, and some of these fibers were fracturedor pulled out from anchors at the end. This resulted in drop intension load due to reduction in the amount of fibers in a CFRPspecimen. Although CFRP specimen does not break totally, thepeak tension load is attained when majority of resin cracks. In fact,the strength specified in the manufacturer data is essentially thestrength of carbon fibers, but in tension test CFRP specimens hardlyreach this strength due to fracture of resin. Thus, the tensilestrength obtained in the test is taken as the actual room tempera-ture strength of CFRP.

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0 10 20 30 40 50

Stre

ss (M

Pa)

Time (min)

Load cell - steel strand testStrain gauge 1 - steel strand testStrain gauge 2 - steel strand testLoad cell - FRP strip testStrain gauge - FRP strip test

Fig. 5. Comparison of measured stresses using loading cell with strain gauges.

Results and observations from strength tests are tabulated inTables 2 and 3. It can be seen in these two tables that strengthand elastic modulus of NSM CFRP strip and rod decrease with in-crease in temperatures. The variation of tensile strength of CFRPstrip and rod with temperature is plotted in Figs. 6a and 7a. Thetrends in both figures indicate that the degradation of tensilestrength in CFRP can be grouped into three stages. In 20–200 �Ctemperature range, tensile strength of CFRP decreases graduallyat a slow pace, and CFRP strip and rod retain about 80% of originalstrength at 200 �C. In current practice, CFRP is assumed to lose sig-nificant strength past its glass transition temperature (around80 �C). However, data from these strength tests clearly indicatethat CFRP resin remains intact till about 200 �C, and thus CFRP re-tains much of its initial strength. In 200–400 �C temperature range,CFRP strip and rod experience faster degradation of their strength,and this is mainly due to decomposition of polymer resin ataround 300 �C. As noted from observations (see Tables 2 and 3), re-sin starts melting at 300 �C, but does not get totally decomposed,hence CFRP splits into bunches of fibers and these fibers primarilyresist tension load. Based on linear interpolation, the tensilestrength of CFRP strips and rods drop to 50% of their originalstrength at about 305 �C and 330 �C respectively. This temperaturecan be treated as critical temperature for CFRP strip or rod. Thecritical temperature analogy used for conventional steel reinforc-ing bars is defined as the temperature at which steel loses 50%of its room temperature strength. In the third stage (400–600 �C), majority of polymer resin gets decomposed, and only indi-vidual fibers contribute to load resistance. The strength of CFRProd and strip degrades at a very high rate at this stage and reachesabout 10% of their original strength. The amount of strength reten-tion is highly dependent on the extent of oxidation of carbonfibers.

It can be seen in Figs. 6 and 7 that the measured strength andmodulus data at elevated temperatures is relatively scattered ascompared to those obtained at room temperature. This is mainlyattributed to two factors, variation of heat flux in a specimen andsliding (slip) occurring between CFRP and anchors. Since heatingrate of furnace is controlled manually, the heat flux introducedby furnace is different from one test to another, and this resultsin variation in specimen temperature at the time of test. Also, insome high temperature tests, there was slight sliding that occurredbetween CFRP and epoxy at the anchors, which also lead to varia-tions in the measured strength. CFRP strips have relatively largertensile strength in tests as compared to those of CFRP rods, andthus resulted in slightly larger variation in test data. The use ofexpansive cement in anchors generates higher bond performanceas compared to that of epoxy, and only negligible slip occurred inspecimens with expansive cement anchors.

The stress–strain relationships for CFRP strips and rods at vari-ous temperatures are shown in Figs. 8 and 9 respectively. It can beseen that CFRP strip and rod exhibit almost linear stress–strain re-sponse at both ambient and high temperatures. Also, the ultimatestrain of CFRP decreases with increase in temperature. Thus theductility of CFRP reinforcement decreases at higher temperatures,which is contrary to that occurring in conventional steel reinforc-ing bars. The slope of stress–strain curves at different temperaturesis taken as the elastic modulus of CFRP specimens, and they areplotted in Figs. 6b and 7b. It can be seen in Figs. 6b and 7b thatthe decrease in elastic modulus follows similar trend as that of ten-sile strength. However, at most target temperatures, relativelyhigher percentage of elastic modulus is retained as compared tothat of tensile strength. Based on the observations in tests,degradation of elastic modulus is more dependant on the state ofpolymer resin. Prior to decomposition of polymer resin (300 �C),the integrity of CFRP specimen is well maintained, and thus higherlevel of elastic modulus is retained. Once polymer resin melts and

Table 2Tensile strength and elastic modulus of CFRP strips at various temperatures.

Temp.(�C)

Strength(MPa)

Averagestrength(MPa)

% Of initialstrength

Elasticmodulus(GPa)

Averagemodulus(GPa)

% Of initialmodulus

Failure mode

20 1724 1641 100 141.2 150.8 100 CFRP split into a bunch of fibers, and then fibers got fracturedor were pulled out1559 160.5

100 1431 1461 89.0 160.1 135.9 90.1 CFRP split into a bunch of fibers, and then fibers got fracturedaround the mid-height1491 111.7

200 1532 1122 76.2 137.5 123.5 81.9 CFRP split into a bunch of fibers, and fibers fractured aroundthe mid-height968 109.5

300 705 768 50.5 89.0 79.9 52.9 Resin melted and fibers fractured around the mid-height831 70.7

400 585 717 43.7 51.5 70.0 46.4 Majority of resin got decomposed, fibers were stretched apart850 88.5

500 657 512 31.2 8.1 33.1 22.0 Little resin left and fibers were stretched apart367 58.1

600 175 175 10.6 – – – No resin left, fibers got separated and stretched apart

Table 3Tensile strength and elastic modulus of CFRP rods at various temperatures.

Temp.(�C)

Strength(MPa)

Averagestrength (MPa)

% Of initialstrength

Elasticmodulus (MPa)

Averagemodulus (MPa)

% Of initialmodulus

Failure mode

20 1536 1577 100 127.2 130.9 100 CFRP split into a bunch of fibers, and then fibers got fracturedor were pulled out1618 134.6

100 1536 1399 88.7 991.0 115.0 87.8 CFRP split into a bunch of fibers, and then fibers got fracturedaround the mid-height1261 130.9

200 1199 1274 80.8 916.2 96.9 74.0 CFRP split into a bunch of fibers, and then fibers got fracturedaround the mid-height1349 102.1

300 1114 927 58.8 105.3 93.9 71.7 Resin melted and fibers got fractured around the mid-height741 82.4

400 484 484 30.7 69.4 69.4 53.0 Majority of resin got decomposed, fibers were stretched apart

500 223 312 19.8 65.9 52.9 40.4 Little resin left and fibers were stretched apart401 39.9

600 126 126 8.0 26.6 26.6 2.0 No resin left, fibers got separated and stretched apart

(a) Tensile strength (b) Elastic modulus

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Tens

ile s

treng

th (M

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Test data

Average values

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Elas

tic m

odul

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Pa)

Temperature (°C)

Test data

Average values

resin intact resin decomposed

resin evaporated

resin intact resin decomposed

resin evaporated

Fig. 6. Variation of tensile strength and elastic modulus of CFRP strips with temperature.

514 B. Yu, V. Kodur / Composites: Part B 58 (2014) 510–517

evaporates, CFRP specimens turn into a bunch of separate fibers,and thus elastic modulus gets significantly reduced.

The failure modes of CFRP strips and rods at various tempera-tures are illustrated in Figs. 10 and 11. The failure pattern of CFRPspecimens in 20–300 �C range are quite similar, wherein CFRPsplits into bunches of thin fibers due to cracking of polymer resin.These fibers then gradually are stretched or pulled out, and

eventually CFRP specimen loses its integrity as well as strength.Beyond 300 �C, polymer resin starts to decompose, and carbonfibers also oxidize at temperatures above 400 �C. It can be seenin Figs. 10 and 11 that the fibers get more softened and separateout in 400–600 �C temperature range. In these tests, thespecimens eventually failed due to stretching of fibers at themid-height.

(a) Tensile strength (b) Elastic modulus

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Test dataAverage values

resin intact resin decomposed

resin evaporated

resin intact resin decomposed

resin evaporated

Fig. 7. Variation of tensile strength and elastic modulus of CFRP rods with temperature.

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0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016

Stre

ss (M

Pa)

Strain

20°C

100°C

200°C

300°C

400°C

500°C

Fig. 8. Stress–strain response of CFRP strips at various temperatures.

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0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014

Stre

ss (M

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Strain

20°C100°C200°C300°C400°C500°C

Fig. 9. Stress–strain curves of CFRP rods at various temperatures.

20°C

100°C

200°C

300°C

400°C

500°C

600°C

Fig. 10. Failure modes of CFRP strips at different temperatures.

20°C

100°C

200°C

300°C

400°C

500°C

600°C

Fig. 11. Failure modes of CFRP rods at various temperatures.

B. Yu, V. Kodur / Composites: Part B 58 (2014) 510–517 515

6. Comparison of results with published data

Figs. 12 and 13 present a comparison of current test results withpublished data on the variation of tensile strength and elasticmodulus of CFRP with temperature. It should be noted that thepublished data is for CFRP rebars and laminates, while the currenttest data is specifically for CFRP strips and rods. It can be seen inboth cases, tensile strength and elastic modulus of CFRP decreasewith temperature. However, there is an obvious variation in theextent of strength and modulus retention at different tempera-tures, and this variation is mainly due to different varieties (com-position) of CFRP reinforcement used in NSM strips/rods, EBRlaminates and internal rebars. As can be seen in Fig. 12, tensile

strength of NSM CFRP strip and rod are close to upper bound ofthese data, which indicates NSM CFRP possess relatively betterresistance to high temperature. However, in Fig. 13, the elasticmodulus of CFRP strips and rods show a relatively larger fluctua-tion as compared to previously published data. In the temperature

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ngth

rete

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n (%

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Temperature (°C)

Test data (strip) - Yu and KodurTest data (rod) - Yu and KodurTest data - Wang et al.(2007)Empirical curve - Bisby (2003)Empirical curve - Saafi (2002)

Fig. 12. Comparison of measured tensile strength with published results.

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ulus

rete

ntio

n (%

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Temperature (°C)

Test data (strip) - Yu and Kodur

Test data (rod) - Yu and Kodur

Test data - Wang et al.(2007)

Empirical curve - Bisby (2003)

Empirical curve - Saafi (2002)

Fig. 13. Comparison of measured elastic modulus with published results.

516 B. Yu, V. Kodur / Composites: Part B 58 (2014) 510–517

range of 20–200 �C, elastic modulus of NSM CFRP strip and rod islower bound of the plotted data, while in 400–600 �C range, theyalmost become the upper bound. This variation is probably dueto rough estimation of elastic modulus of NSM CFRP, which is eval-uated as the slope of linear portion of stress–strain curve at a giventemperature.

7. Relations for strength and modulus of NSM CFRP withtemperature

Data generated from the above tests is utilized to developempirical relations for strength and modulus of NSM CFRPreinforcement as a function of temperature. These relations areexpressed in terms of temperature dependant reduction factors,which are normalized to room temperature values. A review of lit-erature shows that there is very little information on degradationof mechanical properties of CFRP after resin decomposition. Mou-ritz and Gibson [12] proposed the following general relation forthe variation of mechanical properties of FRP with temperature.

PðTÞ ¼ PU þ PR

2� PU � PR

2tan h k T � T 0g

� �� �� �Rn ð1Þ

P(T) represents a particular property, either tensile or compressivestrength, or elastic modulus; Rn is a power law factor to accountthe residual resin content. For tensile strength and elastic modulus,n can be considered to be zero, since tensile strength is mainlydependant on the strength of fibers after the decomposition of poly-mer resin, and thus Rn = 1. PU and PR are unrelaxed (low tempera-ture) and relaxed (high temperature) values of that property,respectively. T 0g is the critical temperature of FRP, correspondingto a 50% reduction in the property value. k is a constant describingthe extent of relaxation. This relation takes into account the effect of

decomposition of FRP occurring at high temperatures on mechani-cal properties, and thus can be used over a wide range of tempera-tures. By dividing Eq. (1) by PU, the retention factor for tensilestrength and elastic modulus at a given temperatures can beobtained

FðTÞ ¼ 1þ PR=PU

2� 1� PR=PU

2tan h k T � T 0g

� �� �ð2Þ

F(T) is the retention (%) factor of mechanical properties at temper-ature T (�C).

The above equation is taken as the basis for developing anexpression for strength and modulus retention factors for NSMCFRP. As discussed above, the resin of CFRP strips and rods getscompletely evaporated at 600 �C. Therefore, the strength and mod-ulus at 600 �C were used as PR, and the strength and modulus atroom temperature (20 �C) were used as PU. The critical tempera-ture (T 0g) corresponding to 50% reduction in tensile strength andmodulus of NSM CFRP strip is 305 �C and 340 �C respectively,and the corresponding values for NSM CFRP rod are 330 �C and320 �C respectively. Then k is the only coefficient to be determinedin Eq. (2).

A regression analysis was carried out using ‘‘Solver’’ function inExcel [13] to determine k. The ‘‘Solver’’ is an advanced program inExcel which is able to obtain an optimum function to match a spec-ified dataset. The prerequisite for using this Solver function is toprovide a basic format of a function and coefficients to be deter-mined. In the current analysis, Eq. (2) is the basic format of thefunction and k is the coefficient to be determined. Then a regres-sion analysis was carried out so as to achieve a minimum errorvalue between predictions from empirical formula (Eq. (2)) andthe above measured test data. Based on the regression analysisresults, the following relations were arrived for strength andmodulus retention factors in CFRP strip and rod as a function oftemperature.

CFRP strip:

Strength : f ðTÞ ¼ 0:56� 0:44 tan hð0:0052ðT � 305ÞÞ ð3Þ

Modulus:

EðTÞ ¼ 0:51� 0:49 tan hð0:0035ðT � 340ÞÞ ð4Þ

CFRP rod:

Strength : f ðTÞ ¼ 0:54� 0:46 tan hð0:0064ðT � 330ÞÞ ð5Þ

Modulus:

EðTÞ ¼ 0:51� 0:49 tan hð0:0033ðT � 320ÞÞ ð6Þ

The comparison of predicted strength and elastic modulus fromproposed empirical relations with measured values in above dis-cussed tests is plotted in Figs. 14 and 15. It can be seen in Fig. 14that the proposed empirical relations closely match with measureddata for tensile strength of CFRP strip and rod, and the average er-ror between predicted strength and test data is 7% and 6.3% respec-tively. The values of standard derivation (also called mean squareerror) are 10% and 7.9% for CFRP strips and rods. The errors be-tween test data and empirical equations range from 0% to 19%,and the maximum error occurred in 100–200 �C range. This ismainly due to the fact that CFRP strength is still high at these tem-perature levels and there is slight slip that occurred between strip/rod and anchor system. The elastic modulus predictions, as shownin Fig. 15, also show reasonable agreement with test data, and theaverage error is 10% and 11.2% for CFRP strips and rods respec-tively. The values of standard derivation (also called mean squareerror) of elastic modulus are 12% and 13% for CFRP strips and rods,and the error ranges from 0% to 21%. This slight larger error in elas-tic modulus of CFRP strip and rod is mainly due to relatively scat-tered data obtained in tests.

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600 700

Stre

ngth

rete

ntio

n

Temperature (°C)

Test data- strip

Empirial formula- strip

Test data- rod

Empirical formula- rod

Fig. 14. Comparison of tensile strength predicted by empirical formula with testdata.

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600 700

Mod

ulus

rete

ntio

n

Temperature (°C)

Test data - stripEmpirical formula - stripTest data - rodEmpirical formula - rod

Fig. 15. Comparison of elastic modulus predicted by empirical formula with testdata.

B. Yu, V. Kodur / Composites: Part B 58 (2014) 510–517 517

8. Conclusions

Based on the results presented in the paper, the following con-clusions can be drawn:

1. At ambient conditions, NSM CFRP strips and rods generallyattain failure through fracture of resin polymer. Withincrease in loading, CFRP splits into a bunch of fibers andgradually loses its tensile strength.

2. NSM CFRP strips and rods retain much of their tensilestrength and modulus till about 200 �C. This is mainly dueto the fact that polymer resin of CFRP remains intact up to200 �C.

3. Beyond 300 �C, tensile strength and elastic modulus of NSMCFRP decrease at a faster pace due to decomposition of poly-mer resin.

4. CFRP strips and rods lose 50% of their initial strength at305 �C and 330 �C respectively. Hence the critical tempera-ture of NSM CFRP strips and rods is 305 �C and 330 �Crespectively.

5. CFRP strips and rods exhibit linear stress–strain response atboth ambient and high temperatures. However, ultimate(failure) strain of CFRP decreases with increase in tempera-ture, which is contrary to that occurring in reinforcing steel.

6. NSM CFRP strips and rods exhibit slightly better resistance tohigh temperature as compared to conventional CFRP rebarsand laminates.

7. The proposed relations for strength and elastic modulus ofCFRP strip and rod can be used in evaluating fire responseof concrete structures strengthened with NSM FRP reinforce-ment.

Acknowledgements

This material is based upon work supported by the National Sci-ence Foundation under Grant No. CMMI-0855820. The authors alsothank FYFE Co. LLC and RockFrac Company for providing test mate-rials. Any opinions, findings, and conclusions or recommendationsexpressed in this paper are those of the authors and do not neces-sarily reflect the views of the sponsors.

References

[1] Sena-Cruz JM, Barros J. Bond between near-surface mounted carbon-fiber-reinforced polymer laminate strips and concrete. J Compos Constr2004;8(6):519–27.

[2] De Lorenzis L, Teng JG. Near-surface mounted FRP reinforcement – anemerging technique for strengthening structures. Compos B2007;38(2):119–43.

[3] Bisby LA, Kodur VKR. Evaluating the fire endurance of concrete slabs reinforcedwith FRP bars: considerations for a holistic approach. Compos B2007;38(5):547–58.

[4] Dimitrienkon Y. Thermomechanics of composites under hightemperatures. London: Kluwer; 1999. p. 347.

[5] Fujisaki T, Nakatsuji T, Sugita M. Research and development of grid shaped FRPreinforcement. In: Proceedings of international symposium on fiber reinforcedpolymer reinforcement for reinforced concrete structures. Detroit, Michigan,March, 1993. p. 177–92.

[6] Kumahara S, Masuda Y, Tanano H, Shimizu A. Tensile strength of continuousfiber bar under high temperature. In: Proceedings of international symposiumon fiber reinforced polymer reinforcement for reinforced concrete structures.Detroit, Michigan, March, 1993. p. 731–42.

[7] Wang YC, Kodur VKR. Variation of strength and stiffness of fiber reinforcedpolymer reinforcing bars with temperature. Cem Concr Compos2005;27(9):864–74.

[8] Wang YC, Wong PMH, Kodur VKR. An experimental study of the mechanicalproperties of fibre reinforced polymer (FRP) and steel reinforcing bars atelevated temperatures. Compos Struct 2007;80(1):131–40.

[9] Bisby LA, Green MF, Kodur VKR. Response to fire of concrete structures thatincorporate FRP. Prog Struct Eng Mater 2005;7(3):136–49.

[10] Saafi M. Effect of fire on FRP reinforced concrete members. Compos Struct2002;58(1):11–20.

[11] American Concrete Institute Technical Committee 440. In: Guide test methodsfor fiber-reinforced polymers (FRPs) for reinforcing or strengthening concretestructures. ACI 440.3R-06, Farmington Hills, MI, 2006.

[12] Mouritz AP, Gibson AG. Fire properties of polymer compositematerials. Dordrecht: Springer; 2006. p. 179–80.

[13] Microsoft Inc. Office 2007 Help Document. Redmond: Microsoft, 2007.