Investigation of GFRP-concrete bond: experimental and numerical studies.

Abdelmonem Masmoudi1, Atef Daoud1, Mongi Ben Ouezdou1,3, Radhouane Masmoudi2

1 Civil Engineering Laboratory, National Engineering School of Tunis (ENIT) BP 37,1002 Tunis Belvédère, Tunisia. abdelmonem.masmoudi@gmail.com

mongi.benouezdou@enit.rnu.tn Daoudatef@yahoo.fr 2 Department of civil engineering, Faculty of engineering - Sherbrooke University

2500, boul. of Sherbrooke University (Québec), J1K 2R1 Canada. radhouane.masmoudi@usherbrooke.ca

3 Department of Civil and Environmental Engineering, Engineering and Architecture College, University of Nizwa, POBox 33, 616 Birkat Al Mouz, Nizwa, Sultanate of Oman

Abstract. This paper constitutes a contribution to investigate the interface between GFRP bars and concrete at high temperature and saturated water. A total of 280 specimens were under different temperatures in dry and saturated environments. The specimens were remained for 24h, 120 and 240 days to high temperatures and saturated water up to 80 °C. Thereafter, they were subjected to pullout-tests. Experimental results and Microscopic analysis showed a damage and a reduction of bond strength due to temperatures up to 16,8 % after 240 days of immersion in water up to 80 °C. A contribution for numerical analysis of GFRP/Concrete interface was developed using finite elements code CASTEM2000. The numerical distribution of strength and deformations are in a good accuracy with experimental finding.

Keywords: Glass Fiber Reinforced Polymer (GFRP), Bond, Pullout Testing, Temperature, Microscopic analysis, saturated water, deformations.

1 Introduction Glass Fiber Reinforced Polymer (GFRP) object of this research are considered as a new technology for reinforced concrete structures and are gaining popularity worldwide. The Transverse Coefficients of Thermal expansion (CTE) of GFRP are different from the concrete CTE. For this reason, the temperature and moisture may significantly affect the shear strength behavior, as well as, the bond performance when embedded in concrete. A literature review suggested that the degradation of FRP should be tested with coupling effects between moisture and elevated temperatures. Due to this coupling effect, the degradation could be accelerated by high diffusion rates. Comparing the effect of environmental temperature with the effect of mechanical loading (Xi and Asiz 2004) indicated that the effect of temperature is dominant.. A recently study showed that the thermal axial elongation of the GFRP Rebar did not affected slip values but it affected the

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confinement action by the concrete surrounding the GFRP rebar at the embedment length 5 Db , witch affected indirectly slip values (Masmoudi et al 2008a), (Masmoudi et al 2008b). The coupling effect between moisture uptake and temperature is quite complex because a change of temperature can result in a change of diffusion coefficient of FRPs. As a result, the rate of moisture uptake may increase or decrease depending on the orientation of temperature changes. (Verghese et al 1998) In the literature, the results of degradation of mechanical properties of FRPs obtained by different research groups showed a wide range of deviation. The long term bond performance between FRP and concrete, has been evaluated using concentric pullout test method witch consist in the immersion of the specimens in saturated calcium hydroxide solution (Nanni and Bakis 1998). They founded that until 80°C a degradation over time in terms of residual pullout strength. The hygro thermal treatment, pull-out tests were subjected during 4 and 8 months. The hygrothermal effect on the average long-term bond strengths and concrete were then compared to untreated specimens (20°C).

2 Experimental programs The principal idea of the test program is to evaluate the long term performance (under high temperatures in dry and wet environments) of the bond strength of FRP bars embedded in concrete. Pullout bond testing will be performed on specimens submitted to high temperatures of 40, 60 and 80 C for a period of 4 and 8 months each, in air and in water

2.1 Material properties The material under investigation is a unidirectional Glass FRP bars "ComBAR®" composite .Mechanical properties of the GFRP bars such as nominal strength and elastic modulus are listed in Table 1. These properties are based on the experimental tests conducted at laboratories of Schock Bauteile GmbH, Munich Technical University (Schock Co. 2006), and Syracuse University (Aboutaha, R. 2004). The measured tensile strength for all diameters is greater than 1000 MPa . Two different bar sizes chosen with nominal diameters of Db= 8 and 16 mm. Table 1. Properties of the GFRP and Steel bars used in this study (Schock Combar 2006)

Nominal Diameter (mm)

Tensile Modulus of Elasticity (GPa)

Ultimate Tensile Strength (MPa)

CTE x 10-5

(mm/mm.°C) Density

16 60 ±1.9 738 ± 22 0.6 (axial) 2.2 8 2.2 (radial)

2.2 Concrete design Normal weight concrete was prepared in the following composition (Kg/m3) : two coarse aggregate 4/12 and 12/20 mm respectively 296 kg and 691 kg , sand 857kg ,ordinary Portland cement type CEM I 42.5, 300 kg and water 204 kg (water-to-cement ratio w/c=0.68) . The 28 day uniaxial cylinder compressive strength was fc= 30 MPa. These are average values obtained by testing five 160 x 320 mm cylinder specimens. Concrete cylinders were cast and cured for 4 and 8 months at room temperature (20°C). The pullout specimens and the standard concrete cylinders were cast in two layers and compacted until using a vibrator. The control cylinders and the pullout specimens were stored at the room temperature until the date of testing. The control cylinders were tested for compression strength just after the completion of the pullout test

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2.3 Specimens Description The proposed pullout specimens to examine the local bond strength consist of 500 mm long GFRP bar embedded vertically in 150 x 150 mm concrete cube for bar diameter of 8 mm. For 16 mm diameter, a 180 mm x 180 mm concrete cube was used .This difference in concrete cube dimensions is intended to avoid the failure mode of concrete splitting. All specimens were prepared following the specifications of Guide Test Methods (ACI 440.3R 2004). The embedment length for all specimens is 5Db, where Db is the GFRP-bar diameter. 2.4 Test procedure The pullout tests were conducted in a Lloyd 50 KN machine. The pullout tests were monotonic by increasing slip at 1.2 mm/min rate following the recommendation of the Guide Test Methods (ACI 440.3R 2004).All measurements, including pullout load and displacements (slips) were recorded by a computer controlled data acquisition system at a rate of 5 data/sec. The data acquisition system was started a few seconds before starting the loading. Four LVDTs with accuracy equal to 0.001mm were used for the GFRP bar to monitor the displacements. Three LVDTs were placed at 120 degree segment orientation at the loaded end, and one LVDT at the free end. A particular care has been taken to ensure that the longitudinal axis of the specimen coincides with the line joining the two anchorages fitted to the testing machine.

2.5 Testing Plan Accelerated ageing on GFRP Rebars embedded in concrete used in this study as showed in Figure 1, were designed to simulate the effect of moisture and temperature on the interface GFRP Rebar /concrete. The embedded GFRP Rebars were immersed in water under different temperatures (20°C,40°C,60°C and 80°C). The pH of solution is controlled by a pH meter (pH=7). Five specimens were removed from the conditioning basin and tested under pullout test to compare their bond strength retention to the reference specimens. The temperature of immersion was chosen to accelerate the degradation effect of ageing. The level of water is controlled by Solenoid valve.

Figure.1 Accelerated ageing on GFRP Rebars embedded in concrete

3 Results and discussion

3.1Temperature effect in dry environment The average max bond stress τ was calculated following expression:

beb LD

F

.

max

..πτ = (1)

GFRP Rebars

Water Specimen

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Where Fmax is the Peak recorded bar load (N) during the pullout test, Db is the nominal bar diameter (mm), and Leb is the embedment length of GFRP bar (mm). For temperature up to 60°C, the average bond strengths do not show any significant reduction. For the 80°C temperature, the maximum reductions after 8 months of ageing in dry environment are 9.64 % and 14.14 %, respectively for the 8 mm 16 mm GFRP bars when compared to the reference results at 20°C

3.2 Temperature effect in saturated water After 240 days of ageing, the average bond strengths do not show any significant reduction for temperature up to 60°C. For the 80°C temperature, the maximum reductions are 10.8 % and 16.8 % respectively for the 8 mm and 16 mm when compared to the reference results at 20°C. The coupling effect between saturated water and temperature show some reduction in bond strength.

3.3 Micro structural effects Scanning Electronic Microscopy (SEM) observations and image analysis were performed to observe the microstructure of specimens before and after aging. All specimens observed in the SEM were first cut, polished and coated with a thin layer of gold. After the coating of surfaces, micro structural observations were performed on a PHILIPS XL 30 SEM (Fig. 2). The visual and micro structural observations showed no significant damage on the interface GFRP Rebars/concrete after 240 days of immersion in Dry environment. Therefore, degradation was observed for specimens in saturated water up to 80 °C.

Figure.2 Micrographs of GFRP bar coating interface (a) Reference, (b) Saturated in water up to 80°C for 8 months.

Figure.2 Micrographs of GFRP bar coating interface (a) Reference, (b) Saturated in water up to 80°C for 8 months.

3 mm 3 mm

2 mm2 mm

(a) (b)

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4. Finite element modelling of a deformation distribution The deformations on the embedded length were determined using finite elements code CASTEM2000. The numerical results were compared to experimental finding. Non-linear material behavior was defined for concrete and GFRP. Most of the material properties values were taken from concrete design books. A smeared cracking analogy was defined for concrete elements. This analogy uses the concept of oriented damaged elasticity and isotropic compressive plasticity to represent the inelastic behaviour of concrete and anisotropic traction plasticity to represent the inelastic behaviour of GFRP The crack material caused by loading is represented by a variable parameter D 0<D<1. This parameter will ponderate the elasticity modulus of material The law comportment is us following:

klijklij CD εσ ).1( −= (1)

Where σij et εij respectively the strength and deformations components tensor damage form, Cijkl are the non damage material elasticity tensor components. The variable damage D increases from 0 to 1, witch corresponding to failure of material. Axi-symmetry conception was used, and load is applied as a concentrated force in the loaded end GFRP bar. To allow a comparison between finite element results and the experiment tests, we will simulate the same load and boundary conditions. The load is applied by a traction machine that applies load by displacement-rate control. A 2-D triangular nodes element was used to mesh both the concrete and GFRP bar. A comparison of the finite element déformations was established with the experimental finding. (Fig. 3).

GFRP 16 mm F= 2.5 KN

0,0E+00

2,0E-05

4,0E-05

6,0E-05

8,0E-05

1,0E-04

1,2E-04

0 50 100 150 200 250 300

Distance from loaded end (mm)

Def

orm

atio

ns Data measured Finite element

GFRP 16 mm F= 24 KN

0,0E+00

2,0E-04

4,0E-04

6,0E-04

8,0E-04

1,0E-03

1,2E-03

1,4E-03

0 50 100 150 200 250 300

Distance from loaded end (mm)

Def

orm

atio

ns

Data measured

Finite element

Figure.3 Comparison of the longitudinal deformation distribution (F= 2.5 KN and 24 KN).

5. Conclusions The bond mechanics of glass FRP bars in normal strength concrete was investigated through experimental testing coupling effect, between moisture and temperature, to see how the combinations of the influential parameters affect the degradation of the GFRP specimens. The primary conclusions from the experimental research are as follows:

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1- No significant reduction on the bond strength after 8 months of ageing in temperature and saturated ranging from 40 to 60°C. 2- Coupling effect, between moisture and temperature up to 80°C show a reduction in the average bond strengths. For the 80°C temperature, the maximum reductions are 10.8 % and 16.8 % respectively for the 8 mm and 16 mm GFRP bars. 3- Microscopy observations shows that aging the embedded GFRP Rebars in water up to 80°C, will affect the micro structural properties of the Rebars 4- The model bases on the concrete damage, allows a distribution and the deformations evolution in the interface GFRP/concrete. 5-The numerical distribution of strength and deformations are in a good accuracy with experimental finding

Acknowledgments

The authors wish to acknowledge the " Civil Engineering Laoratory LGC of ENIT " ,"ISET de Sfax" , "Schock Bauteil GmbH Combar®" and "Sika Tunisienne" for their help and financially supporting this research.

References

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ACI 440.3R, (2004); Guide Test Methods for Fiber-Reinforced Polymers (FRPs) for Reinforcing or Strengthening Concrete Structures

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Nanni A, Bakis CE (1998) Acceleration of FRP bond degradation, Proceeding of the 1st Int. Conf. on the Durability of Composites for Construction.”, CDCC’98, Sherbrooke, Canada, eds: Benmokrane, B., Rahman, H., p 45-55.

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