Durability study and long-term predictions of new GFRP dowels for 1

concrete pavement 2

3 Mathieu Montaigu1, Mathieu Robert2 and Brahim Benmokrane3 4

5 6 Abstract 7

New research using fibre-reinforced polymers (FRP) as a load transfer element in concrete 8

pavement structures has been gaining attention. Dowel bar commonly used for load transfer in 9

concrete pavement slab is made from steel. However, once the steel dowel bar corrodes, it may 10

cause faults, such as binding due to lockout of the dowel bar in concrete pavement, level 11

differences resulting from spalling or decreased efficiency of load transfer. The corrosion of steel 12

dowels has been characterized as the primary source of premature concrete pavement failure by 13

the American Concrete Paving Association (ACPA). To solve this problem, many studies on the 14

performance and feasibility of FRP dowel bars are being undertaken. In particular, glass fibre-15

reinforced polymer (GFRP) dowel bar is now considered as a potential solution. However, GFRP 16

dowels could be damaged by the capillary water of concrete, which typically has high alkalinity. 17

This study investigates the deterioration characteristics of GFRP dowel bars manufactured using 18

two types of resin (vinylester and polyester), in simulated environmental conditions experienced 19

by concrete pavement. An accelerated test method was applied to examine the long-term 20

deterioration of the GFRP dowel bar within a limited period. Different test parameters were 21

studied: i) the type of resin; ii) the diameter of the dowel; and (iii) the temperature and time of 22

conditioning. The test results will contribute to introduce new GFRP dowel bars with enhanced 23

durability characteristics for field applications. 24

25

Keywords: Concrete pavement, GFRP dowel, durability, alkaline resistance, mechanical 26 properties, SEM, FTIR. 27

1 Master Student, Department of Civil Engineering., University of Sherbrooke, Sherbrooke, 1

Quebec, Canada, J1K 2R1, Phone: (819) 821-8000 ext. 65269, Fax: (819) 821-7974, E-mail: 2

Mathieu.Montaigu@USherbrooke.ca 3

2 Post-Doctoral Fellow, Department of Civil Engineering., University of Sherbrooke, Sherbrooke, 4

Quebec, Canada, J1K 2R1, Phone: (819) 821-8000 ext. 62967, Fax: (819) 821-7974, E-mail: 5

Mathieu.Robert2@USherbrooke.ca 6

3 NSERC Research Chair Professor in Innovative FRP Composite Materials for Infrastructures, 7

Department of Civil Engineering, University of Sherbrooke, Sherbrooke, Quebec, Canada, J1K 8

2R1, Phone: (819) 821-7758, Fax: (819) 821-7974, E-mail: 9

Brahim.Benmokrane@USherbrooke.ca 10

11

12

Introduction 1

FRP composites, mainly based on thermoset polymers and glass or carbon fibres, are being used 2

in infrastructures exposed to harsh conditions involving de-icing salts or marine environments. A 3

typical dowel bar is more vulnerable to corrosion when it is used to transfer loads in concrete 4

pavement due to the penetration of water by the joint opening [Mauricio et al., 2005]. When 5

corrosion occurs in a dowel bar, freezing could be caused by the expansion, followed by fractures 6

due to the curling of the concrete pavement slab. To solve these problems, GFRP dowel bars 7

could be considered now as a feasible solution [Eddie et al., 2001; Porter et al., 2002] with high 8

industrial productivity [HITEC, 1998; Saad et al., 2000]. Unfortunately, in some special 9

conditions, such as in high alkalinity environment, the long-term performance of the GFRP is still 10

unresolved question and remains an important point for designers and owners. 11

Won et al. [2006] have estimated the long-term durability of 25 and 38 mm GFRP dowels 12

manufactured with a vinylester resin by Aslan Pacific. Specimens were conditioned at 70°C for 13

60 days in an alkali environment consisting in 0.16% Ca(OH)2, 1% NaOH, 1.4% KOH with a pH 14

around 12.6 close to that of concrete and simulate the pore water. The Litherland’s method 15

[1981] was used to establish a Time Shift Factor (TSF) and to evaluate the durability of the 16

dowels for the Korean environment. The short beam shear test was used to characterize the 17

degradation of the matrix/fibre interface. No significant degradation of the mechanical properties 18

were observed (strength retention over than 95%) and the short-term acceleration tests have 19

assumed this integrity over 60 years of service life in the Korean road conditions. Thus, longer 20

ageing tests and more representative conditioning have to be achieved. 21

The deterioration of GFRP dowel bars occurs when free hydroxyl ions and H2O molecules 22

diffuse through the matrix of the GFRP dowel [Robert et al., 2009]. The relative weak adhesion 23

of the polyester or the vinylester resin could result in serious deterioration, when the water 24

penetrates with ions into the structure as shown on the Figure 1. The failure of the matrix is 1

mainly caused by expansion and plasticization mechanisms, due to the diffusion of the 2

surrounding solution. In water or alkaline solution, the deterioration mechanisms of the glass 3

fibres essentially depend on leaching and etching. External alkaline ions leaching into the glass 4

fibres lead to the most important chemical reaction that dissolves the glass fibres into water. The 5

other critical reaction is etching, in which hydroxyl ions break Si-O-Si bonding. The 6

concentration and growth of hydration products between individual filaments can also damage 7

the fibres [Murphy et al., 1999]. The deterioration at the interfaces between the fibres and the 8

matrix involves a much more complex mechanism [Chen et al., 2007]. The interface is a 9

nonhomogeneous region with a thickness of about 1 µm. This layer is weakly bonded and is most 10

vulnerable to deterioration. Therefore, the degradation of GFRP is not only due to high pH level, 11

but also due to the combination of alkalis, high pH, and moisture. The matrix formed by 12

vinylester, which contents less ester units as compared to polyester, is less vulnerable to hard 13

deterioration by hydroxyl ions. However, the use of cheaper resin, like polyester, could lead to 14

cost-effective structures. 15

In order to evaluate long-term durability performance of FRP in alkaline environment, extensive 16

studies have been conducted to develop accelerated aging procedures and predictive models for 17

long-term strength estimates, especially for GFRP bars [Porter et al., 1997; Dejke, 2001; Bank et 18

al., 2003; Chen and Davalos, 2006]. These models are based on Arrhenius type model proposed 19

by Litherland and his colleagues [Litherland et al. 1981]. Research studies on the effects of 20

temperature on the durability of FRP bars in concrete alkaline environment indicates that an 21

accelerated factor for each temperature difference can be defined by using Arrhenius laws. These 22

factors differ for each product, depending on the types of fibre and resin and bar size. In addition, 23

the factors are affected by the environmental conditions, such as surrounding solution media, 24

temperature, pH, moisture, and freeze-thaw conditions. Predictive models based on Arrhenius 1

laws make the implicit assumption that the elevated temperature will only increase the rate of 2

degradation without affecting the degradation mechanism or introducing other degradation 3

mechanisms of FRP bars. Gerritse [1998] indicated that at least 3 elevated temperatures are 4

necessary to perform an accurate predication based on Arrhenius law. Moreover, the measured 5

data should be in continuous time intervals [Robert et al., 2009]. 6

This study investigates the long-term behaviour of GFRP dowels after accelerated ageing tests at 7

different temperatures and period of time. To see the effects of the type of resin on long-term 8

durability of GFRP dowels, vinylester- and polyester-based dowels were compared in this study. 9

To characterize the deterioration mechanisms and propose a long-term prediction of the dowels 10

durability, specimens were immersed at different temperatures in alkaline cement Portland 11

solution until 180 days, then tested under direct shear and 4 points flexure assuming that these 12

stresses are the dominant forces acting on dowels during their entire service life. Master curves 13

based on Arrhenius relation give us extrapolation of the very long-term durability of the GFRP 14

dowels. Microstrural analysis using scanning electron microscopy (SEM), differential scanning 15

calorimetry (DSC) and Fourier Transform Infrared spectroscopy (FTIR) techniques were also 16

used to evaluate the characteristics of the dowel specimens. 17

18

Experimental Program 19

Material 20

Glass FRP dowels manufactured by a Canadian company (Pultrall inc., 2010) were used in this 21

study. GFRP dowels were made of continuous E glass fibres impregnated in polyester (PE) or 22

vinylester (VE) resin (Figure 2) using the pultrusion process. Durability tests were performed on 23

several diameters from 25.4 to 44.5 mm for the vinylester-based dowels and from 25.4 to 34.9 24

mm for the polyester-based dowels and the reference properties determined by preliminary tests 1

are shown in Table 1. As there was no effect of the diameter on the degradation of dowels (<2%), 2

this durability study only presents results for 34.9 and 38.1 mm diameters dowel, which are the 3

most employed diameters in roads infrastructure. For comparison, epoxy coated steel dowels of 4

28.6 and 38.1 mm were tested under direct shear and present ultimate direct shear strength of 240 5

MPa with a standard deviation less to 1%. All dowels were cut into 300 and 1067 mm lengths so 6

that the direct shear test and flexural test can be performed using ACI 440.3R-04 and ASTM D 7

4476 standard, respectively. The bars were divided into two series; 1) the unconditioned 8

reference samples; 2) conditioned samples immersed in alkaline solution. 9

10

Conditioning of the GFRP Dowels 11

Accelerated ageing tests were conducted according to the method DBT-2 “Recommended FRP 12

Dowel Bar Test Protocol”, proposed by Market Development Alliance of the Composite Institute 13

[MDA, 1998], in accordance with the ASTM C 581 standard. This empirical method develops 14

data in 180 days to prove that a GFRP dowel bar can be used to provide satisfactory service over 15

the pavement life with minimal maintenance (design of 30 to 40 years). Conditioning of samples 16

includes the combination of environmental exposures and application of an elevated temperature. 17

Temperature and aqueous media will influence the time to failure as temperature is the 18

accelerator factor. Cement extract solution, which is a real representative solution of concrete 19

pores water, was prepared by mixing commercial Portland cement type 30 (highest rate of CaO) 20

with tap water to get a pH of 12.60, measured at the beginning of the test and maintained constant 21

throughout the conditioning period with Ca(OH)2 and cement. Specimens were immersed at three 22

different temperatures (23, 50 and 60°C). Once the solution reaches the prescribed temperature, 23

the conditioning time was started. Robert et al., [2009] have shown that the increasing of reaction 24

rate is almost linear between room temperature and 50°C, whereas at higher temperatures (more 1

than 60°C), the rate leads to an exponential increase of the reaction/degradation. Therefore to 2

avoid any thermal degradation, the maximum conditioning temperature used in this study was 3

60°C. 4

Specimens were place in wood containers waterproofed with a high density polyethylene film 5

and hermetically closed to avoid excessive evaporation and changes of the pH solution. The 6

specimens were maintained spaced by PVC sticks to allow the solution covering the entire dowel 7

surface [ASTM C 581; ACI 440.3R-04]. The containers were placed in environmental rooms at 8

isothermal conditions. Specimens were weighted and their diameters were measured throughout 9

the conditioning period to follow the water absorption and eventually characterize a mass change 10

or change in diameter. For each diameter and resin type of dowel, six specimens were removed 11

from solution and tested under short beam shear and flexural tests after 30, 60 and 180 days at 23, 12

50 and 60°C of conditioning to measure the mechanical properties of aged dowels. The obtained 13

data were used to estimate the long-term behaviour in shear and flexure performances and 14

characterize the degradation mechanisms in the dowel structure. 15

16

Moisture Uptake 17

The moisture uptake at saturation of reference samples was determined before and after 18

conditioning according to ASTM D 570 standard. Three specimens of 75 mm long of each type 19

of dowel were cut, dried and weighted. They were then immersed in water at 50°C during 3 20

weeks. The samples were periodically removed from water, surface dried and weighted. The 21

water content in weight percent was calculated with the equation 1. 22

23 W = 100 · (Ps – Pd)/Pd (1) 24

1 Where Ps and Pd are the weights of the bar in saturated and dry state, respectively. The percentage 2

moisture uptake was calculated and the gain in mass was corrected to take into account possible 3

mass loss of the specimens due to various dissolution phenomena during the aging procedure, 4

such as hydrolysis, by drying later completely the immersed specimens by placing them in an 5

oven at 100°C for 24 h and comparing their masses to their initial masses. 6

7

Direct Shear Test 8

The major structural solicitation of the dowels in service conditions is the direct shear. Direct 9

shear tests were conducted to characterize the GFRP dowels according to the method B4 of ACI 10

440.3R-04 code. The setup consists in a 230 x 100 x 110 mm steel base equipped with lower 11

blades spaced of 50 mm face to face, allowing the double direct shear of the specimen by an 12

upper blade as shown on the Figure 3. 13

Six unconditioned specimens of 300 mm long were tested as reference for each type of dowel at 14

the laboratory conditions (23 ± 2°C, 50 ± 10% relative humidity) using MTS 810 testing machine 15

equipped with a load cell of 500 kN. The displacement speed was 1.5 mm/min giving between 30 16

and 60 MPa/min, until the failure of the specimen. The direct shear strength is given by the 17

equation 2. 18

19

APs

u 2=τ (2) 20

21 Where �u is the direct shear strength (MPa); Ps is the failure load (N); and A is the specimen 22

section (mm²). Due to the perpendicular action of the load to the dowel, this test especially 23

solicits the fibres resistance [Porter et al., 1997]. 24

25

Four-Points Flexural Test 1

Due to the length of the dowels in the concrete pavement, the second solicitation in service 2

conditions is the flexure. To characterize the GFRP dowels, four-points flexural tests were 3

conducted according to the method developed by Zhang et al. [2007]. This new method 4

developed in accordance with ASTM D 4476 standard, is more appropriate for pultruded bars 5

allowing the failure to occur in the weakest zone of the material as compared with the three-6

points test which leads to failure under the loading nose. The tests were conducted on 1067 mm 7

long specimens with spans equal to 16 times the diameter as shown on the Figure 4 where 8

L/3=600 mm for 25.4 mm diameter. Six unconditioned specimens were tested as reference for 9

each type of diameter at the laboratory conditions (23 ± 2°C, 50 ± 10% relative humidity) with a 10

Baldwin hydraulic press of a capacity of 1,800 kN. The chosen displacement speed was 60 11

mm/min giving between 250 and 350 MPa/min, until the failure of the specimen by tension, 12

compression or for a maximum fiber strain of 5%, whichever occurs first. The flexural stress is 13

given by the equation 3. 14

15

332

RPL

u πσ = (3) 16

17 Where �u is the flexural stress in the outer fibers at midspan (MPa); P is the failure load (N); L is 18

the support span (mm); and R is the dowel radius (mm). The flexural modulus is calculated with 19

equations 4 and 5, for a chosen P and a corresponding deflexion Y measured with a Linear 20

Variable Displacement Transducer (LVDT). 21

22

YP

IL

E ×=129623 3

(4) 23

With 4

4RI

π= (5) 1

2 The maximum outer fiber strain may be calculated with the equation 6. 3

4

Euσε =max (6) 5

6 GFRP materials have better tensile strength than steel reinforcements, but have relatively poor 7

resistance against lateral loads called shear forces (direct or interlaminar) [Park et al., 2008]. This 8

is due to the fact that GFRP dowel bar is manufactured by a pultrusion process in which the 9

fibres are arranged unidirectionally and bonded using polymer matrix. Flexural test solicits fibres 10

by tension and the composite bar by flexure; it develops horizontal strength which could illustrate 11

interface degradation better than the direct shear test. 12

13

Differencial Scanning Calorimetry (DSC) 14

Twelve-milligram to 15-milligram specimens from both unconditioned and aged samples were 15

sealed in aluminum pans and analyzed in a TA Instruments DSC Q10 calorimeter equipped with a 16

refrigerated cooling system. Analysis was conducted in modulated DSC mode. Specimens were 17

heated from 25°C to 195°C at a rate of 5oC/min. Glass transition temperature was determined by 18

DSC for each specimen in accordance with ASTM D 1356 standard. Two scans were performed 19

for each specimen. The first scan is useful to determine the difference of Tg between reference 20

and conditioned specimens. If a decrease of Tg is observed for conditioned samples, this is an 21

indication of plasticizing effect or chemical degradation. The second scan gives information 22

about the mechanism of degradation and if it is irreversible. 23

24

Fourier Transformed Infrared Spectroscopy (FTIR) 1

Fourier transform infrared spectroscopy (FTIR) spectra were recorded using a Nicolet Magna-550 2

spectrometer equipped with an attenuated total reflectance device. Fifty scans were routinely 3

acquired with an optical retardation of 0.25 cm to yield a resolution of 4 cm−1. 25.4 mm diameter 4

dowels were investigated after 180 days conditioning at 60°C. 5

6

Scanning Electron Microscopy (SEM) 7

Scanning electron microscopy (SEM) observations and image analysis were performed to 8

observe the microstructure of both vinylester- and polyester-based dowels specimens before and 9

after aging in alkaline solution during 180 days at 60oC. All specimens observed in the SEM were 10

first cut, polished, and coated with a thin layer of gold-palladium by a vapor-deposit process. 11

After coating the surfaces, microstructural observations were performed on a JEOL JSM-840A 12

SEM. These observations were conducted to see the potential degradation of polymer matrix, 13

glass fibers, or interfaces, if any. 14

15

Arrhenius Relation 16

Arrhenius concept was used for the prediction of long-term behaviour of GFRP dowels. The 17

equation 7 expresses the Arrhenius relation, in terms of the degradation rate [Nelson, 1990]. 18

19

RTEa

Aek−

= (7) 20

21 Where k is the degradation rate; A is a constant relative to the material and degradation process; 22

Ea is the energy of activation of the reaction; R is the universal gas constant; and T is the 23

temperature in °C. The primary assumption of this model is that only one dominant degradation 24

mechanism of the material operates during the reaction and that this mechanism will not change 1

with time and temperature during the exposure [Chen et al., 2007]. Only the rate of degradation 2

will be accelerated with the temperature increase. Mechanical properties of the dowels at 3

different ageing temperature and periods can lead to a long-term extrapolation of the material 4

properties in pavement structure. 5

Another approach was used by Dejke (2001) to generate a relative time shift factor (TSF). Dejke 6

(2001) proposed to use the TSF to transform the time in the accelerated test to actual service lives 7

for GFRP reinforcement. Because the time for a certain reaction to take place must be 8

proportional to the inverse of the rate of reaction, Dejke (2001) proposed determining the TSF in 9

accordance with the equation 8. 10

���

����

�

+−

+== 15,2731

15,2731

2

1 21 TTB

ett

TSF (8) 11

12 Where, T1 and T2 are the exposure temperatures (°C); and t1 and t2 are the times required to 13

obtain a certain level of decrease in mechanical property at temperatures T1 and T2, respectively. 14

The TSF is sensitive to the activation energy, and a good estimate of Ea is needed to generate a 15

reasonable TSF. 16

17

Experimental Results and Discussion 18

Direct Shear Strength 19

Table 2 shows the direct shear strength of both vinylester- and polyester-based dowels after 20

different conditionings in alkaline solution. Figure 5 shows the retention of the direct shear 21

strength after 180 days of ageing of the GFRP dowels at various temperatures. As shown in the 22

Figure 5, vinylester-based dowels were slightly affected by ageing in alkaline solution with a 23

retention of more than 90% of the initial shear performance after 180 days at 60°C in alkaline 1

solution (pH=12.60). Polyester-based dowels present shear strength retention above 75%. All 2

specimens have kept elastic behaviour until the shear failure of the fibres. It can be seen that the 3

degradation rate between 23°C and 50°C is nearly the same that between 50°C and 60°C, 4

characterizing the exponential effect of the temperature on mechanisms of degradation. 5

6

Flexural Strength 7

Table 3 shows the flexural properties of both vinylester- and polyester-based dowels after the 8

conditioning period of 180 days. Figure 6 shows the retention of the direct shear strength after 9

180 days of ageing of the GFRP dowels at various temperatures. As shown on the Figure 6, 10

vinylester-based dowels present are very durable. Even after a 180 days conditioning in alkaline 11

solution at 60oC, the retained flexural strength is equal to 95% of the initial flexural. Polyester-12

based dowels show strength retention above 65%. All specimens have kept elastic behaviour until 13

the flexural failure of the dowels. Vinylester-based dowels have failed under different modes of 14

failure (compression, balanced or tension) and polyester-based dowels have presented tensile 15

rupture. This observation confirms the higher bond of vinylester resin as compared to polyester 16

resin which has led to failure at the fibre/matrix interface. 17

18

Flexural Modulus of Elasticity 19

Figure 7 shows the change in the flexural elastic modulus of aged dowels after immersion of 180 20

days in alkaline solution at various temperatures for vinylester- and polyester-based dowels. 21

Indeed, it can be seen from the experimental results that even after 180 days of immersion, the 22

loss of flexural elastic modulus is negligible for both vinylester- (2%) and polyester-based (5%) 23

dowels and that all aged vinylester-based dowels are not significantly affected by the higher 24

temperature or the exposure to alkaline solution. On the other hand, it can be seen that a slight 1

tendency exists between the temperature of immersion and the loss of flexural modulus of 2

elasticity for polyester-based dowels. These results show that flexural elastic modulus of GFRP 3

dowels is not significantly affected after 180 days by aging in alkaline solution. 4

5

Effects on Polymer Matrix 6

Table 4 gives the values of Tg before and after aging in alkaline solution during 180 days at 60oC. 7

No significant effect was observed on vinylester-based dowels. Polyester-based dowels were 8

affected by the ageing, showing a loss of Tg of 6% after conditioning, with no effect of the 9

diameter. The shift of Tg to lower temperature obtained during the second run can be explained 10

by non-reversible chemical reaction, like hydrolysis. It corroborates the observed mechanical 11

losses and the local degradations of the matrix structure shown by SEM (Figures 9 and 11). 12

A FTIR analysis of unconditioned dowel and specimens aged in alkaline solution during 180 13

days at 60oC was conducted (Figure 8). The most interesting region of the FTIR spectra is 14

located between 3300 cm-1 and 3600 cm-1, which corresponds to the stretching mode of the 15

hydroxyl groups of the vinylester and polyester resin. When hydrolysis reaction occurs, new 16

hydroxyl groups are formed and the corresponding infrared band increases. Changes in the peak 17

intensity are quantified by determining the ratio of the OH- peak to the carbon-hydrogen 18

stretching peak of the resin, which is not affected by the conditioning. The experimental ratios of 19

the OH peak to the carbon-hydrogen stretching peak of the core and the surface of vinylester-20

based dowel immersed in alkaline solution for 180 days at 60oC were 0.44 and 0.54, respectively, 21

compared to 0.49 for unconditioned samples. The hydroxyl peak did not show any significant 22

changes. This observation lead to the conclusion that no chemical degradation of the vinylester 23

resin occurred during the immersion of the dowels in alkaline solution at 60°C for 180 days. On 24

the other hand, the experimental ratios for the core and the surface of polyester-based dowel 1

immersed in alkaline solution for 180 days at 60oC were 0.50 and 0.87, respectively, compared to 2

0.48 for unconditioned samples. These results lead to the conclusion that chemical degradation of 3

the polymer occurred at the surface of the dowel, which is in direct contact with the solution 4

during the immersion of the dowels. This observation explains the losses in mechanical 5

properties of polyester-based dowels and it can be supposed that the thickness of degraded 6

polymer matrix is related to the moisture absorption and that this degradation is a time-dependent 7

mechanism. 8

9

Moisture Absorption 10

Table 5 presents the moisture uptake at saturation for reference and aged specimens. As shown 11

on the Table 5, no change of moisture uptake was observed for both vinylester- and polyester-12

based dowels. Reference values remain strictly equals. In spite of the slight polyester resin 13

degradation characterized by DSC analysis and SEM observations (Figure 9), aging had no effect 14

on moisture uptake of the polyester-based GFRP dowels. 15

16

Microstructural observations (SEM) 17

External surface 18

SEM observations of external surface of dowels were performed to investigate the surface 19

deterioration of the polymer matrix after conditioning in alkaline solution. Figure 9 presents 20

micrographs of the surface of reference and vinylester- and polyester-based aged dowels. It can 21

be observed that no degradation of the vinylester matrix has occurred after 180 days at 60°C 22

aging in alkaline solution. No increase of the number or dimensions of the pores was observed 23

and the surface remains intact without any cracking or microcracking. On the other hand, same 24

conditioning on polyester-based dowels has led to local degradations on the surface. The matrix 1

presents a more porous aspect and some cracks have appeared which could lead to an increase of 2

the penetration of the surrounding solution and a higher diffusion of the hydroxyl ions in the 3

polymer matrix. No dimension or weight changes of the dowels have been noticed after the 4

accelerated ageing in alkaline solution. 5

6

Microstructural Effects 7

SEM observations were performed on reference and aged dowels after flexural tests to investigate 8

the mechanisms of failure at the fibre/matrix interface. Figure 10 presents micrographs of the 9

fibre/matrix interface after failure of reference specimen and specimens aged in alkaline solution 10

during 180 days at 60°C. It can be observed that the vinylester-based dowel presents more 11

residual matrix on the fibres after failure compared to polyester-based dowel. This observation 12

corroborates the different modes of failure observed on vinylester-based dowels (compression, 13

balanced or tension) and characterizes the higher bond of vinylester resin as compared to 14

polyester resin which has led to failure at the fibre/matrix interface. The micrograph of polyester-15

based dowel shows smooth fibres after debonding and failure, which indicates the weaker 16

adhesion of the polyester resin and the easier delamination of fibres at the interface. This 17

observation is in accordance with the tension mode of failure observed for polyester-based 18

dowels. 19

Figure 11 presents micrographs for both reference specimens and specimens aged in alkaline 20

solution during 180 days at 60oC for both vinylester- and polyester-based dowels. The visual and 21

microstructural observations showed no significant damage on vinylester-based dowel after 180 22

days of immersion in the alkaline solution at the highest temperature (60oC). Observations of the 23

fibre/matrix interface and of the microstructure, in general, demonstrate that the conditionings of 24

vinylester-based dowel in alkaline solution do not affect the microstructural properties of the 1

GFRP dowel (Figure 11c). For the polyester-based dowel, it can be seen on Figure 11b), that a 2

certain number of pores are present at the initial state which can explained the higher moisture 3

uptake measured at saturation as compared to vinylester-based dowels. Micrograph of Figure 4

11d), taken near the surface, shows the degradation at the fibre/polymer matrix interface 5

explaining the lower flexural strength and modulus of elasticity for polyester-based dowels. 6

However these observations should be considered carefully due to the local effect of the 7

degradations near the surface. 8

9

Long-term predictions 10

Long-term predictions of mechanical properties can be made according to the method DBT-2, 11

based on shear and flexural properties obtained after 30, 60 and 180 days at three temperatures of 12

conditioning. Simulations are proposed for the vinylester- and polyester-based dowels of 34.9 13

mm diameter. Following the procedure proposed by Bank et al. (2003), the natural logarithm of 14

time to reach a set of levels of normalized performances versus 1/T, expressed as the inverse of 15

absolute temperature (1000/K), was used to predict the service life at the Mean Annual 16

Temperature (6.2oC) in Montréal, Québec, Canada. A coefficient of determination (R2) value 17

close to 1 is desired. However, the ASTM procedures recommend a minimum value of 0.80 for 18

acceptability and the obtained R2 values are between 0.96 and 0.99. From the Arrhenius plot, the 19

service life time necessary to reach the established direct shear and flexural strengths retention 20

levels (PR) can be extrapolated for any temperature. Consequently, predictions are made for both 21

direct shear and flexural strengths retention as a function of time for an immersion at 6.2°C and 22

the general relation between the PR and the predicted service life at the average temperature of 23

6.2oC are drawn (Figure 12). It can be seen from Figure 12 that vinylester-based dowels present 24

a very high durability in concrete pavement environment. In fact, the predicted service life of 1

vinylester-based dowels immersed in alkaline solution at an isotherm temperature of 6.2oC to 2

reach a PR less than 90% can be estimated to be infinite. On the other hand, the prediction curves 3

for polyester-based dowels predict direct shear and flexural strengths retention of 75 and 55%, 4

respectively, after a service life of 40 years, corresponding to the design period of concrete 5

pavement structure. It can be considered, regarding to these results, that flexural forces solicit 6

more the fibre/matrix interface and is more representative to the occurred degradations. 7

An approach proposed by Dejke (2001) was also used to generate a relative time shift factor 8

(TSF) for the aging in alkaline solution. According to the equation 8, TSF for polyester dowels 9

were calculated. t1 and t2 parameters were determined for a strength retention of 90% at 50 and 10

60°C, respectively. TSF curves for polyester-based dowels are shown in the Figure 13 where 11

y=N/C with N=equivalent service days at 6.2°C; and C=number of conditioning days. According 12

to these predictions, one day of conditioning at 60°C corresponds to 17 service days under 13

flexural load. So, the 180 days of conditioning used in the present study leads to flexural strength 14

retention of 65% corresponding to a service life of more than 8 years. As vinylester-based dowels 15

have been very slightly damaged by the accelerated ageing tests, it was not possible to generate a 16

reasonable TSF. On the other hand, The ACPA estimates that steel dowel bars can fail in as little 17

as 7 to 15 years depending on design and location, whereas the concrete highway slab itself can 18

easily perform for 35 to 40 years [Porter, 2002]. It was confirmed by field inspections by the 19

Ohio Department of Transportation (ODOT) on many road structures aged over than 10 years 20

[MDA, 1998]. 21

22

23

SUMMARY AND CONCLUSIONS 1

Based on the results of this study the following conclusions may be drawn only for the product 2

tested and considering the conditionings as worst conditions of service life and environmental 3

conditions: 4

- Vinylester-based dowels present very high durability after 180 days of accelerated ageing 5

in alkaline solution at 60°C. Strength retentions of 90 and 95% were measured in shear 6

and flexure, respectively, after 180 days of immersion in alkaline solution at 60oC. 7

Mechanical losses could be explained by a plasticization phenomenon of the polymer 8

matrix confirmed by the fact that 50% of the measured degradation was reached after 60 9

days of conditioning. 10

- The microstructural observations have shown that no significant damage has occurred to 11

the internal microstructure or at the fibre/matrix interface for vinylester-based GFRP 12

dowel. No physical parameter was affected by the ageing and no significant chemical 13

degradation were observed even near the dowel bar surface. 14

- According the to long-term predictions, the shear and flexural strengths retention of 15

vinylester-based GFRP dowel aged in alkaline solution will decreased by less than 10% 16

after 200 years at 6.2oC, showing the high stability of vinylester-based GFRP dowels in 17

concrete environment. 18

- Polyester-based dowels were significantly affected by the ageing in concrete environment. 19

Strength retentions of 75 and 65% were measured in shear and flexure, respectively, after 20

180 days of immersion in alkaline solution at 60oC. 21

- The degradation of polyester-based dowels was confirmed by a loss of Tg of 6% (5°C), 22

showing a slight irreversible degradation of the matrix. The observation of the 23

microstructure of the polyester-based dowel corroborates these results, showing local 1

delaminations and a degraded surface aspect after ageing. Some delaminations and 2

debonding of fibres have also occurred near the surface of the dowel. An increased of 3

unconnected pores was also observed. The significant losses of properties could be also 4

explained by a plasticization phenomenon of the matrix. 5

- According to the long-term predictions, the shear and flexural strengths retention of 6

polyester-based GFRP dowel aged in alkaline solution will decreased by 25 and 45% in 7

shear and flexure, respectively, after 200 years at 6.2oC. For steel reinforced concrete 8

pavement, the ACPA estimates that steel dowel bars can fail in as little as 7 to 15 years 9

depending on design and location, whereas the concrete highway slab itself can easily 10

perform for 35 to 40 years. 11

12

Finally, this study clearly shows that the durability of tested vinylester-based GFRP dowel is not 13

significantly affected by immersion in alkaline solution. It is reasonable to assume that this kind 14

of aging is harsher in comparison to the real concrete environment. Moreover, the long-term 15

predictions made in this study are pertinent to compare durability of GFRP dowels to other 16

products using equivalent method, but should be put into perspective because a mean annual 17

temperature is used without any effect of the temperature fluctuation. In future works, polyester-18

based products could be investigated during longer conditioning period (1 or 2 years) to assume 19

their very-long term behaviour. 20

21

22

23

Acknowledgements 1

The authors thank the Natural Sciences and Engineering Research Council of Canada (NSERC), 2

the Center for Applied Research on Polymers (CREPEC), the Fonds québécois de la recherche 3

sur la nature et les technologies (FQRNT), and Pultrall Inc. for support. 4

5

6

References 1

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Reinforcing or Strengthening Concrete Structures. American Concrete Institute, Farmington 3

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Conshohocken, Etats-Unis, 4 p. 6

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Resins Used in Glass-Fiber-Reinforced Structures Intended for Liquid Service. American 8

Society for Testing and Material, Conshohocken, Etats-Unis, 5 p. 9

ASTM D 792 (2000). “Standard Test Methods for Density and Specific Gravity (Relative 10

Density) of Plastics by Displacement.” American Society for Testing and Materials. 11

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Analysis (TMA). American Society for Testing and Material, Conshohocken, Etats-Unis, 5 p. 13

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Material, Conshohocken, Etats-Unis, 10 p. 15

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Unis, 4 p. 18

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Reinforced Polymers annexe N. CSA-S806-02, Canadian Standards Association, Toronto, 22

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Association, Toronto, Canada, 44 p. 2

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Göteborg, Sweden. 7

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Eddie, D., Shalabi, A., and Rizkalla, S. (2001). Glass-Fiber-Reinforced Polymer Dowels for 15

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Sherbrooke, Québec, Canada, 285-297. 19

Highway Innovative Technology Evaluation Center (HITEC). 1998. HITEC evaluation plan for 20

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of Transportation: Ohio. 22

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Process Engineering, Long Beach, Calif., 2222–2230. 11

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Park, C. Jang, C. Lee, S. et Won, J. (février 2008) Microstructural investigation of long-term 14

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2, 6 p. 11

Table 1 Reference properties of the GFRP dowels 1

Phys

ical

Property VE-based GFRP dowels (mm) PE-based GFRP dowel (mm) 25.4 28.6 31.8 34.9 38.1 41.3 44.5 25.4 28.6 31.8 34.9

Fibre content (%) 81.5 81.3 80.5 80.7 80.6 80.9 80.3 80.6 80.6 79.6 79.8 Cure ratio (%) 100 100 100 100 100 100 100 99.84 99.91 99.95 99.98

Tg (°C) 118 119 124 124 123 121 125 87 83 84 90 Moisture uptake (%) 0.07 0.08 0.05 0.06 0.07 0.07 0.07 1.46 1.59 1.50 1.53

Relative density 2.28 2.22 2.14 2.12 2.14 2.12 2.08 2.18 2.20 2.13 2.11 LTE long. (x10-6°C-1) 7.5 7.6 7.7 7 7.6 7.6 7.8 7.8 6.9 7.6 6.8

LTE transv. (x10-6°C-1) 25.4 25.9 27.1 23.5 24 27.6 26.6 23.2 23.9 26.5 21

Mec

hani

cal Direct shear strength (MPa) 168 194 160 184 173 197 182 154 166 151 164

Short beam shear strength (MPa) 53.2 59..9 60.1 61.1 53.9 58.3 60.2 29.1 34.8 40.1 36.7 4 points flexural strength (MPa) 1324 1213 1165 1210 1077 1015 1040 1081 788 839 759

Flexural modulus of elasticity (GPa) 54.1 53.4 52.6 50.3 51.6 50.2 49.3 51.1 50.2 51.6 49.5

Table 2 Direct shear properties of aged GFRP dowels 1

Time of immersion (days)

Temperature (oC)

Mean Shear Strength (MPa)

Standard deviation (MPa)

VE35 dowel

VE38 dowel

PE35 dowel

VE35 dowel

VE38 dowel

PE35 dowel

0 23 184.3 173.4 163.7 1.81 3.19 4.44

30 23 182.1 177.7 152.5 2.67 1.24 1.87 50 180.0 175.7 149.9 2.43 2.54 2.65 60 178.1 171.0 148.4 3.65 1.87 3.67

60

23 180.3 181.9 142.9 1.97 1.93 3.62 50 175.7 177.6 136.2 4.17 1.84 3.97 60 172.4 168.6 133.1 1.85 0.86 4.16

180

23 178.9 176.5 138.4 1.87 2.14 2.79 50 172.7 169.8 131.4 3.11 0.73 1.47 60 167.5 157.7 125.4 3.65 3.27 1.55

2

3

Table 3 Flexural properties of aged GFRP dowels 1

Time of immersion (days)

Temperature (oC)

Mean Flexural Strength (MPa) Standard deviation (MPa)

VE35 dowel

VE38 dowel

PE35 dowel

VE35 dowel

VE38 dowel

PE35 dowel

0 23 1,210.2 1,077.0 758.7 50.37 61.01 36.41

30 23 1,202.4 1,080.1 713.7 30.64 25.68 25.96 50 1,200.0 1,075.4 690.4 38.14 36.24 16.47 60 1,190.5 1,065.4 647.9 75.96 55.98 12.87

60

23 1,195.2 1,084.6 668.7 26.57 20.15 20.88 50 1,190.1 1,076.5 621.5 37.18 35.86 17.55 60 1,183.0 1,058.3 605.5 73.62 53.88 6.81

180

23 1,189.6 1,087.4 611.8 17.52 70.25 22.80 50 1,179.3 1,058.4 555.1 19.45 36.91 24.71 60 1,154.3 1,048.5 497.5 53.37 12.47 13.12

2

3

Table 4 Tg of reference and aged dowels 1

Conditioning Temperature (oC)

Duration (days)

Tg run 1 (oC) Tg run 2 (oC)

VE35 dowel

VE38 dowel

PE35 dowel

VE35 dowel

VE38 dowel

PE35 dowel

Unconditioned 123 124 90 124 123 90

Alkaline solution

60 180 122 123 84 123 122 85

2

3

Table 5 Moisture uptake at saturation 1

Conditioning Temperature (oC)

Duration (days)

W (%)

VE35 dowel

VE38 dowel

PE35 dowel

Unconditioned 0.06 0.07 1.53

Alkaline solution 60 180 0.06 0.07 1.53 2

3

4

1

Figure 1 Penetration of ions with water in the joint [Park et al., 2008] 2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

1

Figure 2 Typical epoxy-coated steel, Polyester- and vinylester-based GFRP dowels 2

3

4

1

Figure 3 Direct shear test setup 2

3

4

1

Figure 4 Four-points flexural test setup 2

3

4

1

Figure 5 Direct shear strength retention of GFRP dowels after conditioning in alkaline 2 solution at 60oC 3

4

5

6

7

1

Figure 6 Flexural strength retention of GFRP dowels after conditioning in alkaline solution 2 at 60oC 3

4

5

6

7

8

9

10

11

12

13

1

Figure 7. Elastic modulus retention of conditioned GFRP dowel aged 180 days in alkaline 2 solution at 23o, 50o, and 60°C 3

4 5

6

7

8

9

10

11

12

13

14

15

1

a) b) 2

Figure 8 FTIR spectra for unconditioned and aged samples for: a) vinylester-based dowel, 3 and b) polyester-based dowel 4

5

6

1

a) b) 2

3

c) d) 4

Figure 9 Micrographs of the dowels surface for: a) 34.9 mm VE reference dowel; b) 34.9 5 mm PE reference dowel; c) 34.9 mm VE dowel aged in alkaline solution during 6 180 days at 60oC; d) 34.9 mm PE dowel aged in alkaline solution during 180 days 7 at 60oC 8

9

10

1

a) b) 2

3

c) d) 4

Figure 10 Micrographs at the fibre/matrix interface after failure for: a)VE reference dowel, b) 5 PE reference dowel, c) 34.9 mm VE dowel aged in alkaline solution during 180 6 days at 60oC; d) 34.9 mm PE dowel aged in alkaline solution during 180 days at 7 60oC 8

9

10

1

a) b) 2

3

c) d) 4

Figure 11 Micrographs at the fibre/matrix interface before mechanical tests for: a)VE 5 reference dowel, b) PE reference dowel, c) 34.9 mm VE dowel aged in alkaline 6 solution during 180 days at 60oC; d) 34.9 mm PE dowel aged in alkaline solution 7 during 180 days at 60oC 8

9

10

1

Figure 12 Long-term predictions for 34.9 mm dowels 2

3

4

5

1

Figure 13 TSF for polyester-based GFRP dowels for equivalent service time at 6.2°C 2

3

4

5