GCA-400-1

DURABILITY OF GLASS FRP COMPOSITE BARS FOR CONCRETESTRUCTURE REINFORCEMENT UNDER TENSILE SUSTAINED LOADIN WET AND ALKALINE ENVIRONMENTS

A Masmoudi, R., B Nkurunziza, G., C Benmokrane, B., and D Cousin, P.A Associate Professor ,Ph.D. Candidate B , C NSERC Chair Professor , D Research AssociateNSERC Chair, ISIS Canada, Department of Civil Engineering,Université de Sherbrooke, Sherbrooke, Québec, J1K 2R1, Canada

Abstract: This paper describes a new experimental set-up specially designed for accelerated ageingtests for Fibre-Reinforced Polymer (FRP) reinforcements. The set-up uses a combination of temperature,environmental exposure (alkaline, saline, water, and concrete), and tensile sustained loads. Two series-tests I and II of Glass Fibre-Reinforced Polymer (GFRP) bars 12.7 mm-diameter were exposed to twodifferent environments, namely the alkaline solution and de-ionized water, respectively. Temperaturevarying between 45 to 63°C is used to accelerate the degradation mechanism. Also, the bars weresubmitted to sustained tensile loads varying from 20 to 29 % of the short-term ultimate tensile strength.Both series specimens were exposed for a 104 days (15 weeks) duration. Following the acceleratedageing tests, the GFRP specimens were removed and tested for residual tensile strength. Results ofthese residual tensile tests are reported in this paper and compared to the guaranteed tensile and designstrengths of the GFRP bar used in this experimental investigation.

1. INTRODUCTION

During the last decade, there has been an important increase in the use of Fibre Reinforced Polymer(FRP) Composites bars as concrete reinforcement in the construction industry because of their inherentadvantages in terms of light weight, high specific strength and stiffness ratios and their non-corrosiveproperties. However, the existing design guidelines and codes (ACI 440.1R-01, CAN/CSA-S06-00,CAN/CSA- S806-02, JSCE, ISIS-M03-01) indicate that the prescribed safety factors and environmentalreduction coefficients are very conservative and therefore, limiting the large scale use of FRPreinforcements for concrete structures. This conservative design philosophy is based on the lack ofvalidated experimental data related to long term durability of FRP bars.

In recent years, a number of studies have been undertaken to enhance the state of the art relatedto the long term durability of FRP materials exposed to civil engineering infrastructure environments(Tannous 1997; Devalapura et al. 1998; Benmokrane and El-Salakawy 2002, Benmokrane and Rahman,1998; Benmokrane et al. 1998, Malvar, 1998; Mutsuyoshi et al. 2001, Dejke 2001; Karbhari and Li 2002;Karbhari and Lee 2002; Nkurunziza et al. 2002).

The current design guidelines and codes (ACI 440.1R-01; CAN/CSA-S6-00; CAN/CSA- S806-02;JSCE; ISIS-M03-01) specify strength reduction factors in order to take into account the effects ofsustained stress, fatigue, and environmental conditions. Table 1 presents the reduction factors as

Congrès annuel de la Société canadienne de génie civil

Annual Conference of the Canadian Society for Civil Engineering

Moncton, Nouveau-Brunswick, Canada4-7 juin 2003 / June 4-7, 2003

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suggested by ACI 440.1R-01, CAN/CSA-S06-00, and JSCE. For the Japanese society of civilengineering (JSCE) code, the specified material factor takes into account the standard deviation of testdata, damage, differences in test strength in real structures, effects of material characteristics on the limitstate, service temperatures and environmental conditions.

Table 1 Reduction factors for GFRP offered by Design Guides and Codes

Factor ACI 440.1R-01 CAN/CSA-S06-00 JSCEReduction due toenvironment causeddeterioration

CE ″environmentalreduction factor″

0.70-0.80

φFRP ″resistancefactor″

0.75

1/γfm″material factor″

0.77Reduction due to sustainedstress NA 0.8-1.0 NATotal strength reduction dueto environment and stress 0.70-0.80 0.60-0.75 0.77Specified upper tensilestress limits in reinforcementdue to permanent load

0.14-0.16 0.60-0.75 ≤ 0.7

2. OBJECTIVES

The objectives of the presented study are:− To develop accelerated ageing laboratory tests to evaluate the long term durability of FRP reinforcing

rods;− To define acceptable stress limit levels for FRP reinforcing bars for concrete structures. These limits

will take into account the environmental conditions as well as the sustained stress level.

3. EXPERIMENTAL PROGRAM

3.1 Test Specimens

A large number of FRP reinforcing bars are being tested as a part of a large scale experimental programfocused on the durability of Glass FRP reinforcements for concrete structures. The FRP bars (12.7 mmdiameter) used in this study are the second generation of ISOROD glass FRP (manufactured by Pultrall,Thetford Mines, Québec) and are composed of 73 % E-glass fibres and a vinylester resin. Table 2presents the experimental parameters in terms of the level of the sustained tensile load, elevatedtemperature and solution exposure for each individual test specimen. The elevated temperature is usedas accelerating agent. In this testing program (Table 2), the elevated temperature varied from 45 to 63 oC,while the tensile sustained load varied from 18 to 29 % of the short-term tensile strength.

Table 3 presents the tensile properties in terms of average ultimate tensile strength, guaranteedand design tensile strengths (according to ACI 440.1R.01), modulus of elasticity and average ultimatetensile strain.

The GFRP specimens used in this study were cut to a length of 1300 mm and grouted with a resin-mortar matrix inside 410-mm long steel tubes at both ends, as shown in Figure 1. Each specimen wasinstrumented with electrical strain gages (ESG) to monitor the longitudinal strains.

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Table 2. The experimental parameters

Seriesno. Specimen no.

Level of the sustained tensileload in % of the average

short-term tensile strength

Elevated temperature(oC)

DW-12-1 29 63DW-12-2 21 62DW-12-3 20 61DW-12-4 20 60DW-12-5 20 60DW-12-6 27 60DW-12-7 23 57

I(De-ionized water)

DW-12-8 20 56AS-12-1 21 62AS-12-2 18 61AS-12-3 20 61AS-12-4 24 51AS-12-5 22 49AS-12-6 20 46

II(Alkaline solution)

AS-12-7 24 45

Table 3. Tensile properties of the GFRP bar used in this study

Average ultimatetensile strength

(MPa)

Guaranteed tensilestrength,

(MPa)

Design tensile strength

(MPa)

Modulus ofelasticity

(GPa)

Ultimate strain

(%)

fu,ave,

f*fu

(f*fu= fu,ave-3σ) (ffu=CEf*fu), where CE=0.7( ACI 440) fE uε

639±27 560 392 42±2 1.5±0.12

410 410 480

Threaded steel tube filled with resin-mortar matrix

FRP rebar

Electrical strain gage

Figure 1. Details and dimensions of GFRP accelerated ageing test specimen

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A 63-mm diameter and 250-mm long plastic cylinder is installed around the middle part of the barspecimen. One end of the cylinder is sealed with an adhesive resin, and then filled with the appropriatesolution to simulate the selected type of exposure. Figure 2 shows a close up view of typical specimensready for installation. Two different aqueous solutions were used in this study: The first one is de-ionizedwater simulating 100 % relative humidity and having a pH value of approximately 7. The second is asimulated pore water solution representing an alkaline concrete environment and is composed of 1185 gof Ca(OH)2, 9 g of NaOH and 42 g of KOH in 10 litres of deionized water according ACI 440-Krecommendations. The pH of this solution is 12.9. This value of pH was periodically checked and keptconstant during all the time of exposure.

Figure 2. View of the GFRP test specimens ready for installation

3.2 Experimental Set-up and Test Procedure

A new experimental set-up for accelerated ageing tests has been developed and built at the Departmentof Civil Engineering, Université de Sherbrooke. The test set-up consists of several steel frames and aheating tunnel system. The test specimens are installed into the steel frames where the sustained loadsare applied. These sustained loads are kept constant by the counterweight of a gravimetric load, asshown in Figure 3. The steel frames are positioned in a way that the bar specimens are placed into theheating tunnel (Figure 4 and 5). The temperature is automatically controlled and monitored, and canreach up to 80°C. Figure 4 shows different views with specimen inside the heating tunnel system.Electrical strain gage and thermocouple wires were inserted inside the solutions for strain andtemperature monitoring. After the time of exposure, the specimens were removed and tested for residualtensile strength evaluation. Glass transition temperature, Tg, was also evaluated for reference andconditioned specimens using the DSC (Differential Scanning Calorimeter) technique.

4. TEST RESULTS

4.1 Residual tensile strengthTable 3 presents the experimental residual tensile strength results of the 12.7 mm-GFRP bars after 104days (15 weeks) of exposure in de-ionized water and in alkaline solution, under a stress level of 20 to 29% of the short term ultimate tensile strength with temperature varying from 45 to 63°C. The ratios betweenthe individual residual tensile strength and the short-term ultimate, guaranteed and design tensilestrengths are also presented. A 13 to 15 % reduction for tensile strength in average is observed for bothseries tests I and II after 104 days of exposure. It can be noted however that the residual tensile strengthsfor conditioned specimens is still in the magnitude range of the specified guaranteed ultimate tensile (94 to96 %) and is higher (135 to 138 %) than the specified design strength as recommended by ACI 440design guidelines. Further statistical analysis taking into account the standard deviation of theexperimental test results are in progress. It can be concluded that the tested GFRP bars perform verywell, taking into account the aggressive environmental conditions used for testing.

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Fulcrum Weight

Frame base

1440mm

1900

mm

Swivel head Lever arm

Pipe grip

FRP rebar

Outline of heating chamber Spherical nut

Plastic tube for environmental solution or concrete cylinder

Load calibrating system

Figure 3. Sustained load system – creep frame

(a) General view (b) Back view of the experimental set-up

Figure 4. The new experimental set-up for accelerated ageing tests

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Figure 5. The specimen inside the heating tunnel

Table 3 Ratio between residual tensile strength to guaranteed and design tensile strengths

Series no.Specimen no.

Residual tensilestrength after

104 days

(MPa)

Ratio: residualtensile strength to

the short-termultimate tensile

strength

Ratio: residualtensile strength toguaranteed tensile

strength

Ratio : residualtensile

strength to thedesign tensile

strength

DW-12-1 467 0.73 0.83 1.19DW-12-2 568 0.89 1.01 1.45DW-12-3 488 0.76 0.87 1.24DW-12-4 527 0.82 0.94 1.34DW-12-5 545 0.85 0.97 1.39DW-12-6 504 0.79 0.90 1.28DW-12-7 597 0.93 1.07 1.52

I(Deionized

water)

DW-12-8 624 0.98 1.11 1.59Average 540 0.85 0.96 1.38

AS-12-1 479 0.75 0.85 1.22AS-12-2 520 0.81 0.93 1.32AS-12-3 545 0.85 0.97 1.39AS-12-4 534 0.84 0.95 1.36AS-12-5 548 0.86 0.98 1.40AS-12-6 555 0.87 1.0 1.42

II(Alkalinesolution)

AS-12-7 524 0.82 0.94 1.34Average 529 0.83 0.94 1.35

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The residual modulus of elasticity (RMOE) was also measured. Experimental results indicate that theoverall RMOE (series tests I and II) is equal to 39 ± 1 GPa in average. This value is within the range ofthe standard deviation of the reference modulus of elasticity (42 ± 2 GPa), indicating that the usedaccelerated environmental conditions have no significant effects on the modulus of elasticity of GFRPbars.

Figures 6 and 7 present the typical strain variations for the considered duration. The plots indicatea small increase in tensile strains due to the creep behaviour of GFRP bars.

2000

2500

3000

3500

4000

4500

0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 105

Time (days)

App

lied

stra

in (µ

e)

DW-12-02 DW-12-03

DW-12-06 DW-12-07

Figure 6. Typical strain versus time for specimens exposed to deionized water during 104 days.

2000

2200

2400

2600

2800

3000

3200

3400

3600

0 7 14 21 28 35 42 49 56 63 70 77 84 91 98 105

Time (days)

App

lied

stra

in (µ

e)

AS-12-01 AS-12-02 AS-12-05

Figure 7. Typical strain versus time for specimens exposed to alkaline solution (pH=12.9) during104 days.

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4.2 Differential Scanning Calorimeter (DSC) analysis results

The glass transition temperatures (Tg) for reference and conditioned samples were determined by DSCmeasurements using a Hewlett Packard DSC 7 apparatus, under nitrogen up to 200°C at a heating rate of10°C/min. The glass transition temperature has been determined for the first and second heating scan.The measurement of the Tg is an important aspect in evaluating the durability of FRP composites. Anydegradation of FRP materials will affect the structure and visco-elastic properties of the resin matrix.Degradation may involve chemical mechanisms such as plasticization or polymer chain breaking. Thesedegradation phenomena generally lead to a Tg decrease.

Table 4 presents Tg values for the reference and conditioned samples. For reference specimens, itis observed that the Tg corresponding to the second scan is slightly higher than for the first scan. Thisshift shows that the samples were not fully cured and that a post-curing phenomenon occurred during thefirst heating run at 200°C. However, the difference between the two Tg is small.

For specimens conditioned in de-ionized water at temperature of 60oC during 104 days (DW), itmay be noted that, for the first scan, the glass transition occurs at a lower temperature than with thereference samples, whereas they are similar in the second scan. This may be explained by the presenceof water absorbed during the conditioning. Water acts as a plasticizer and decreases the glass transitiontemperature. However, this water evaporates during the first heating run and its plasticizing effectdisappears, leading the reference Tg.

The same phenomenon occurs with the specimens conditioned in the alkaline solution attemperature of 60oC during 104 days (AS). The Tg of the second scan is not affected by the conditioningwhich shows that no irreversible degradation mechanism yielded glass transition temperature decrease.The only Tg decrease is reversible and is due to the presence of water in the matrix.

Table 4. Differential Scanning Calorimeter (DSC) analysis results

Specimen Tg (scan 1)(°C)

Tg (scan 2)(°C)

Reference 114 120Conditioned (De-ionized Water) 105 120Conditioned (Alkaline Solution) 106 120

5. CONCLUSIONS

A new experimental set-up for accelerated ageing tests was designed and fabricated to evaluate the long-term durability of FRP reinforcement for concrete structures. The temperature, sustained stress level, andenvironmental exposure (alkaline/saline solutions, deionized water, concrete) can be controlled with thisnew test equipment. Two series-tests of 12.7 mm diameter GFRP bars, were investigated under twodifferent environmental conditions and sustained loads. A 13 to 15 % reduction for tensile strength inaverage is observed for both series tests I and II after 104 days of exposure. It can be noted however thatthe residual tensile strengths for conditioned specimens is still in the magnitude range of the specifiedguaranteed ultimate tensile (94 to 96 %) and is higher (135 to 138 %) than the specified design strengthas recommended by ACI 440 design guidelines. Further statistical analysis taking into account thestandard deviation of the experimental test results are in progress. It can be concluded that the durabilityperformance of the 12.7 mm GFRP bars is very good, taking into account the relatively very aggressiveenvironmental conditions being used. Preliminary experimental observations indicate that the usedenvironmental conditions have no significant effect on the modulus of elasticity.

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6. REFERENCES

ACI 440.1R-01, (2001) Guide for the Design and Construction of Concrete Reinforced with FRP Bars,American Concrete Institute, Farmington Hills, Michigan, 41p.

Benmokrane, B. and El-Salakawy, E., (2002) Second International Conference on Durability of FibreReinforced Polymer (FRP) Composites for Construction (CDCC 02), Montréal (Québec), Canada,May 29-31.

Benmokrane, B. et Rahman, A.H.(1998) Avant propos, Comptes rendus de la 1ière conférenceinternationale sur la durabilité des composites en polymères renforcés de fibres (PRF) pour laconstruction (CDCC'98), Sherbrooke, Canada.

Benmokrane, B., Rahman, A.H., Ton-That, M.-T and Robert, J.-F(1998) Improvement of the DurabilityPerformance of FRP Reinforcement for Concrete Structures, Proceedings of the First InternationalConference (CDCC 98), Sherbrooke (Québec) Canada, July 18, p. 571-598.

CAN/CSA-S6-00, (2000). "Canadian Highway Bridge Design Code," CSA International, Rexdale, Toronto,Ontario, Canada, 734 p.

CSA S806–02, (2002) Design and Construction of Building Components with Fiber Reinforced Polymers,Canadian Standards Association, Rexdale, Ontario, 177p.

Devalapura,R.K, Greenwood, M.E., Gauchel, J.V., And Humphrey (1998) Evaluation of GFRPPerformance using Accelerated Test Methods, in Proceedings of the Durability of Fiber ReinforcedPolymer (FRP) Composites for Construction, Benmokrane, B. and Rahman, H. Eds., Université deSherbrooke, Canada, August, pp.107-116.

Dejke, V. (2001), Durability of FRP Reinforcement in Concrete-Literature Review and Experiments, Thesisfor the Degree of Licentiate of Engineering, Departement of Builiding Materials, Chalmers Universityof Technology, Göteborg, Sweden.

ISIS-M03-01, (2001) Reinforcing Concrete Structures with Fibre Reinforced Polymers, The CanadianNetwork of Centres of Excellence on Intelligent Sensing for Innovative Structures, ISIS Canada,University of Winnipeg, Manitoba, 81 p.

Karbhari, V. and Lee, L.(2002) Design of FRP Composites in Rehabilitation for Durability and DamageTolerance, Proceedings of the Second International Conference (CDCC 02), Montréal (Québec)Canada, May 29-31, p. 429-440.

Karbhari, V. and Li, Y.(2002) Design Allowables and Materials Resistance Factors- E Review,Proceedings of the Second International Conference (CDCC 02), Montréal (Québec) Canada, May29-31, p. 429-440.

Malvar, L.J.(august 1998) Durability of Composites in Reinforced Concrete, 1st International Conferenceon Durability of Fiber Reinforced Polymer for Construction (CDCC'98), Sherbrooke, Canada, p.361-372.

Masin, M. and Chvoj, Z. (2000), Analysis of the Arrhenius Shape of the Diffusion Coefficient on AN fcc(111) Surface, Surface Review and Letters, Vol.7, No.3, pp.219-225

Mutsuyoshi, H., Sumida, A. And Uomoto, T. (2001), Alkali Resistance of Fibers, FRP Rods and EpoxyResins, in Proceedings of the Fifth International Conference on Fibre-Reinforced Plastics forreinforced Concrete Structures, Burgoyne, C.J., Ed., Cambridge, U.K, Vol.1, July, pp.479-513.

Nkurunziza, G., Masmoudi, R. and Benmokrane, B. (2002) Effect of Sustained Tensile Stress andTemperature on Residual Strength of GFRP Composite Bars, Proceedings of the SecondInternational Conference, Canada, May 29-31, pp.347-358.

Tannous, F. E. (1997) Durability of AR Non-Metallic Reinforcement Bars and Prestressing Tendons,Dissertation, Department of Civil Engineering and Engineering Mechanics, University of Arizona,287 p.