355
Field study of glass-fibre-reinforced polymer durability in concrete1 A.A. Mufti, M. Onofrei, B. Benmokrane, N. Banthia, M. Boulfiza, J.P. Newhook, B. Bakht, G.S. Tadros, and P. Brett
Abstract: The Canadian Highway Bridge Design Code (CHBDC) does not permit the use of glass-fibre-reinforced polymer (GFRP) for primary reinforcement or prestressing tendons in concrete components. The restriction on the use of GFRP in concrete was based on published laboratory studies indicating that GFRP is not stable in the alkaline environment of concrete. In 2004, ISIS Canada sponsored an extensive study of the durability of GFRP in concrete by removing cores from GFRP-reinforced concrete components of five 5- to 8-year-old structures from across Canada. Three teams working independently at several Canadian universities used a variety of analytical methods to (i) investigate whether the GFRP in concrete field structures had been attacked by alkalis and (ii) compare the composition of GFRP removed from in-service structures with the composition of control specimens that were saved from the projects and not exposed to the concrete environment. The analytical results have confirmed that the GFRP in concrete did not suffer any damage during the 5–8 years of exposure. As a result of this study, the CHBDC in its forthcoming (second) edition has permitted the use of GFRP for both primary reinforcement and prestressing tendons in concrete components, provided the maximum stress level in GFRP at the serviceability limit state is kept at or below 25% of its ultimate strength. It was also found that, contrary to some claims, concrete over GFRP bars does not crack even if the depth of cover is as thin as 28 mm. Key words: alkali attack, barrier wall, crack, 366 deck slab, depth of cover, fibre-reinforced polymer (FRP), glass-fibrereinforced polymer (GFRP). Résumé : Le Code canadien sur le calcul des ponts routiers (2000) ne permet pas l’utilisation de polymères renforcés de fibres de verre (« GFRP ») comme armature principale de renforcement ou de précontrainte dans les composantes en béton. La restriction de l’utilisation des « GFRP » dans le béton était basée sur des études en laboratoire publiées qui indiquaient que les « GFRP » n’étaient pas stables dans l’environnement alcalin du béton. En 2004, ISIS Canada a parrainé une étude détaillée sur la durabilité des « GFRP » dans le béton en prélevant des carottes de composantes en béton armé de « GFRP » de structures âgées de 5 à 8 ans de divers endroits au Canada. Trois équipes indépendantes dans plusieurs universités canadiennes ont utilisé diverses méthodes pour (i) examiner si les alcalis avaient attaqué ou non les « GFRP » dans les structures de béton sur le terrain et (ii) comparer la composition des « GFRP » prélevés des structures en utilisation par rapport aux échantillons de référence qui avaient été mis de côté lors des projets et non exposés au béton. Les résultats analytiques ont confirmé que les « GFRP » dans le béton n’ont subi aucun dommage durant les cinq à huit années d’exposition. Grâce aux résultats de la présente étude, le Code canadien sur le calcul des ponts routiers permettra, dans sa seconde édition à paraître bientôt, l’utilisation des « GFRP » comme armature de renforcement principale et de précontrainte dans les composantes en béton, à la condition que le niveau maximum de
Received 25 November 2005. Revision accepted 20 September 2006. Published on the NRC Research Press Web site at cjce.nrc.ca on 17 May 2007. A.A. Mufti2 and M. Onofrei. ISIS Canada Research Network, Agricultural and Civil Engineering Bldg., University of Manitoba, A250 – 96 Dafoe Road, Winnipeg, MB R3T 2N2, Canada. B. Benmokrane. Department of Civil Engineering, Université de Sherbrooke, 2500 University Blvd., Sherbrooke, QC J1K 2R1, Canada. N. Banthia. Department of Civil Engineering, CEME Bldg., The University of British Columbia, 2024–2324 Main Mall, Vancouver, BC V6T 1Z4, Canada. M. Boulfiza. Department of Civil Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N 5A9, Canada. J.P. Newhook. Department of Civil Engineering, D Bldg., Dalhousie University, 1360 Barrington Street, Halifax, NS B3J 1Z1, Canada. B. Bakht. JMBT Structures Research Inc., 21 Whiteleaf Crescent, Scarborough, ON M1V 3G1, Canada. G.S. Tadros. SPECO Engineering Ltd., 43 Schiller Crescent NW, Calgary, AB T3L 1W7, Canada. P. Brett. KRM Consulting Ltd., 4509 Seawood Terrace, Victoria, BC V8N 3W1, Canada. Written discussion of this article is welcomed and will be received by the Editor until 31 July 2007. 1 2
This article is one of a selection of papers in this Special Issue on Intelligent Sensing for Innovative Structures (ISIS Canada). Corresponding author (e-mail:
[email protected]).
Can. J. Civ. Eng. 34: 355–366 (2007)
doi:10.1139/L06-138
© 2007 NRC Canada
356
Can. J. Civ. Eng. Vol. 34, 2007 contrainte dans les « GFRP » à l’état-limite d’utilisation est gardé à ou sous 25 % de la résistance à la rupture. Contrairement à certaines revendications, le béton recouvrant les tiges de « GFRP » ne se fissure pas, même si le recouvrement est aussi mince que 28 mm. Mots-clés : attaque alcaline, mur coupe-feu, fissure, tablier de pont, profondeur du couvert, polymères renforcés de fibres (« FRP »), polymères renforcés de fibres de verre (« GFRP »). [Traduit par la Rédaction]
Mufti et al.
Introduction In locations where deicing salts are used to keep the roads free of ice, bridge deck slabs and barrier walls are the components most prone to corrosion. The use of fibre-reinforcedpolymers (FRPs) in place of steel reinforcement can make these concrete components virtually corrosion-free. However, two of the FRPs — carbon-fibre-reinforced polymer (CFRP) and aramid-fibre-reinforced polymer (AFRP) — are too expensive to be considered natural alternatives to steel reinforcement. The cheapest of the FRPs is glass-fibre-reinforced polymer (GFRP), which on the basis of laboratory studies was declared unstable in the alkaline environment of concrete by several researchers. Accordingly, the Canadian Highway Bridge Design Code (CHBDC) (CSA 2000) did not permit the use of GFRP for either primary reinforcement or prestressing tendons in concrete components. Reviewing the activities of ISIS Canada, Meier and Brönnimann (2003) recommended that Canada, having invested significantly in innovative concrete structures with GFRP, should undertake a field study on the durability of GFRP in concrete. Following this advice, in 2004 ISIS Canada initiated a project in which concrete cores containing GFRP were removed from several 5- to 8-year-old exposed structures, and the composition of the GFRP was analyzed at a microlevel. The purpose of this paper is to present the findings from the field study.
Previous studies on glass-fibre-reinforced polymer durability Malvar (1998) summarized the results of several published studies on the alkali resistance of GFRPs. All these studies were conducted either by immersing the GFRPreinforced concrete specimens in water (e.g., Sen et al. 1993) or by using alkaline solutions to simulate the concrete alkali environment. The conclusion of these studies, reported by Malvar (1998), was that “GFRP should not be used in direct contact with concrete”. Uomoto (2000) also used his extensive laboratory experience to state categorically that “GFRP is not recommended for internal reinforcements” (in concrete). Several years after Malvar’s state-of-the-art report, Sen et al. (2002) conducted a study of E-glass– vinylester GFRP in a strong alkaline solution and reported that GFRP bars in a strong alkaline solution and stressed to 25% of their failure loads failed 15–25 d after stressing; they concluded that the GFRP bars they had tested were “not durable and should not be used to reinforce concrete.” As a part of a research project on the feasibility of restraining bridge deck slabs with pretensioned concrete straps, several straps, with the GFRP tendons stressed to 55% of the 5th percentile tensile strength of their individual ultimate strength, were built at the University of Manitoba
(Banthia 2003). Three of these specimens were already 365 d old when the tendons were checked to see whether they were holding their strains. Vogel (2005) pretensioned concrete beams with GFRP stressed to 30% of its ultimate strength. One of his beams, reserved for future investigation, is already more than 2 years old and shows no sign of distress. Similarly, a number of structural concrete components containing GFRP and exposed to the elements for many years have not shown any apparent signs of GFRP degradation. In a discussion paper, Mufti et al. (2003) wondered why the pretensioned specimens had not failed and why the GFRP-reinforced concrete components were not showing any sign of distress. Could it be that the fluids necessary to transport alkalis to glass fibres are more freely available in laboratory studies than in hardened concrete not immersed in water? To obtain field data on the durability of GFRP in concrete exposed to the natural environment, an extensive study was undertaken recently.
Field studies Structures Five field structures, exposed to a wide range of environmental conditions and reported in the technical literature, were chosen for the study under consideration. • The Hall’s Harbour wharf was the first marine structure in Canada to be built using GFRP-reinforced concrete (Newhook et al. 2000). The wharf, located on the shore of the Bay of Fundy in Nova Scotia, comprises externally restrained precast concrete deck slab panels with GFRP bars and concrete pile cap beams reinforced with a hybrid GFRP – steel bar system. Concrete with a compressive strength of 45 MPa was used in the panels and beams. At the time of investigation, the structure was 5 years old and had been exposed to a thermal range of +35 °C to –35 °C, frequent wet–dry and freeze–thaw cycles, splash, and tidal saltwater. • The Joffre Bridge, located in Sherbrooke, Quebec, over the Saint-François River, contains GFRP bars as reinforcement in the 45 MPa concrete in the sidewalks and traffic barriers (Benmokrane et al. 2000). At the time of the investigation, the structure was 7 years old. The temperature in the region ranges between +35 °C and –35 °C, and the bridge is subjected to frequent wet–dry and freeze–thaw cycles. Deicing salts are used on the bridge during the winter. • The four-span Chatham Bridge, located in Chatham, Ontario, contains in the two outer spans steel-free deck slabs to which the barrier walls are attached by means of doubleheaded stainless steel bars (Aly et al. 1997). Ordinary concrete, with a nominal strength of 35 MPa, reinforced © 2007 NRC Canada
Mufti et al.
with NEFMAC™ glass fibre G-13 grid at a pitch of 100 mm × 100 mm is used in the barrier walls. At the time of the investigation, the structure was 8 years old. The temperature range in Chatham is between +30 °C and –24 °C. The bridge deck experiences frequent wet–dry and freeze–thaw cycles and is sprayed with deicing salt in the winter months. • The Crowchild Trail Bridge, located in Calgary, Alberta, utilizes ribbed-deformed GFRP C-bar as reinforcement in its barrier walls and deck slab (Tadros et al. 1998). Concrete with a compressive strength of 35 MPa was used in these components. At the time of the investigation, the structure was 8 years old; it experiences a thermal range of +23 °C to –15 °C and frequent freeze–thaw cycles, and it is sprayed with deicing salts in the winter months. • The Waterloo Creek Bridge, located on Vancouver Island, British Columbia, has barrier walls reinforced with GFRP and connected to the steel-free deck slab with doubleheaded steel bars. Concrete with a compressive strength of 35 MPa was reinforced with NEFMAC GFRP in the barrier walls (Tsai and Ventura 1999). At the time of the investigation, the structure was 6 years old. The thermal range in the region is +23 °C to 0 °C. Deicing salts are used frequently on the bridge deck. Like the GFRP bars studied by Sen et al. (2002), the GFRP reinforcement in the selected structures was made of E-glass and a vinylester matrix. Experienced contractors were employed to extract at least ten 75 mm diameter cores from each of the five structures. Figure 1 shows the barrier wall of the Chatham Bridge during the coring process. Sample preparation Three concrete cores from each of five structures, containing GFRP, were sent for analysis to three teams of material scientists working independently at various Canadian universities. The removal of GFRP samples along with surrounding concrete and the polishing of the specimens required special care, mainly because GFRP and concrete have different hardnesses. The procedure for sample preparation used by the three investigation teams was as follows. (1) Cut the specimen with a diamond saw. (2) Pregrind the specimen on a 125 grid diamond plate. (3) Dry the specimen in a special holder. (4) Allow the specimen to harden at room temperature on a holder with epoxy and dry for about 12 h. (5) Keep the specimen in an oven at 45 °C for about 2 h. (6) Grind the surface of the specimen successively with 125, 45, and 15 µm diamond plates. (7) Polish on a pellon polishing cloth with a mixture of glycol ethanol and 1–2 µm diamond powder at 150 rpm for 2–4 h. (8) Place the specimen in a carbon coater at 10–4 Torr (0.013 Pa) to make the prepared surface conductive for examination with electron microprobe. In the absence of a 3-D analytical technique, both longitudinal and transverse cross sections of the specimens were examined.
357 Fig. 1. Barrier wall of Chatham Bridge.
Methods of analysis After being prepared for microanalysis, the GFRP reinforcement and surrounding concrete were analyzed by several methods: optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, differential scanning calorimetry (DSC), and Fouriertransformed infrared (FTIR) spectroscopy. The OM was used to examine the interface between the GFRP and concrete. The entire surface of each sample from each core was examined, and photos were taken at random locations. The SEM was used for detailed examination of the glass fibre – matrix interface and individual glass fibres. The specimens used in SEM analyses were also analyzed by EDX, which detects potential chemical changes in the matrix and glass fibres due to the ingress of hydroxyl and chloride ions from the concrete pore solution. Chemical changes of the matrix were characterized by FTIR spectroscopy. Small sections of the GFRP extracted from the cores were crushed and ground into a powder. The pellet method with spectroscopic-grade potassium bromide was used to obtain the infrared spectra. Changes in the glass transition temperature (Tg) of the matrix due to exposure to concrete environmental conditions were determined with DSC. The Tg measurements were carried out on small pieces cut from the GFRP extracted from the cores. The measurements were taken in air between 40 °C and 200 °C at a heating rate of 10 °C/min.
Results and discussions The results obtained by the three research teams were similar. A complete documentation of the analyses by the three teams is provided by Benmokrane and Cousin (2005), Boulfiza and Banthia (2005), and Onofrei (2005). Optical microscopy Optical microscopy (OM) examination was used to visually assess the interfacial transition zone (ITZ) and the bond between the GFRP and concrete. This bond is likely to deteriorate first, because of alkalis from concrete: the presence © 2007 NRC Canada
358 Fig. 2. Optical microscopy images of concrete–GFRP interface: (a) Crowchild Trail Bridge; (b) Chatham Bridge.
Can. J. Civ. Eng. Vol. 34, 2007 Fig. 3. Micrograph of cross section of GFRP from Crowchild Trail Bridge.
Fig. 4. Micrograph of longitudinal section of GFRP from Chatham Bridge.
of delaminated zones at the GFRP–concrete interface can facilitate the accumulation of water at the interface and consequent formation of localized areas with different moisture contents and alkalinity, thus accelerating the degradation of the GFRP. Deterioration of the matrix as a result of different combinations of exposure conditions, such as alkalinity, temperature, moisture, freeze–thaw cycles, and cyclic loading, can also lead to the deterioration of the GFRP–matrix bond. The OM examination of the interface between the GFRP and concrete of the cores from the five field structures exposed to natural environmental conditions showed no deterioration of ITZ or damage to the bond between the GFRP and concrete. The OM results for cores from the Crowchild Trail and Chatham bridges, shown in Figs. 2a and 2b, respectively, are typical of all OM observations. It can be seen
in these figures that at this magnification there are no gaps between the GFRP and concrete, thus confirming a good bond between the two materials. The OM analyses confirmed that the 5–8 years’ exposure to alkalinity, freeze–thaw and wet-dry cycles, deicing salts, saltwater, and thermal loading had no effect on the integrity of the GFRP–concrete interface. Scanning electron microscopy Scanning electron microscopy (SEM) was used to examine in detail the effect of exposure on the constituent materials of the GFRP. The SEM examination of cores from all five structures confirmed that there was no sign of any damage to the FRP. A micrograph of the cross section of a GFRP bar from the Crowchild Trail Bridge is presented in Fig. 3, in which it can be seen that on the plane under examination, the fibres have not lost any cross-sectional area. Similarly, a micrograph of a longitudinal section of a GFRP sample from the Chatham Bridge, presented in Fig. 4, shows no degradation of the fibres in the direction of the fibre axes. Although the entire surface of each specimen was examined, © 2007 NRC Canada
Mufti et al.
359
Fig. 5. Energy-dispersive X-ray spectrum for in-service specimen from Joffre Bridge.
Fig. 6. Energy-dispersive X-ray spectrum for control specimen from Joffre Bridge.
attention was focused primarily on areas close to the GFRP– concrete interface. Results from SEM analyses presented in Figs. 3 and 4 are typical of all other SEM analyses, which have shown that the individual fibres are intact, with no gaps between the fibres and the matrix. There is no evidence of deterioration of the glass fibre – matrix interface. A good contact between individual glass fibres and the matrix, between sand grains and the matrix, and between concrete and the matrix was ob-
served for all specimens. The SEM analyses confirmed that the GFRP in the five structures did not show any evidence of attack from the alkali in concrete. Energy-dispersive X-ray analysis It is well known that silica glass dissolves in strong alkaline solutions, such as concrete pore solutions. To attack glass fibres, alkalis from the concrete pore solution must first enter the polymer matrix. When the glass fibres degrade as the re© 2007 NRC Canada
360
Can. J. Civ. Eng. Vol. 34, 2007
Fig. 7. Energy-dispersive X-ray spectra for the matrix of GFRP from Hall’s Harbour wharf: (a) close to GFRP–concrete interface; (b) some distance from GFRP–concrete interface.
sult of various processes, such as dissolution, leaching, and ion exchange, the chemical compositions of the glass and matrix change. The energy-dispersive X-ray (EDX) analysis was used to assess such potential chemical changes in the matrix and glass fibres. Examples of EDX spectra are discussed in the following. The chemical composition of fibres in all samples showed the absence of Zr, thus confirming that the investigated GFRP contained E-glass and not alkali-resistant glass.
Samples of GFRP used in the Hall’s Harbour wharf and Joffre Bridge were saved and kept in storage for possible future investigation. These samples (referred to here as the control specimens) provide an opportunity to compare the chemical composition of the GFRP extracted from in-service structures (referred to as the in-service specimens). The EDX spectra of the in-service and control GFRP specimens from the Joffre Bridge are presented in Figs. 5 and 6, respectively. It can be seen that the matrix in both specimens con© 2007 NRC Canada
Mufti et al.
361
Fig. 8. Fourier-transformed infrared spectra for GFRP in Joffre Bridge: (a) control specimen; (b) specimen from structure.
tains mainly carbon, as it should. However, some additional elements, such as Si, Al, and Ca, were also detected. Since the EDX spectrum of the in-service specimen is almost identical to that of the control specimen, the presence of elements other than C in the matrix could be attributed to contamination introduced during preparation of the specimens for microscopic examination. The concrete pore-water solution consists mainly of Na+ and K+ with OH– as the counter ion. Other elements present
in the solution either are very insoluble (Ca2+) or have low solubility (Mg, Al, Si, Fe, and SO42–). Since EDX cannot detect elements or ions lighter than sodium, the OH– ion cannot be detected. However, the OH– ions and the cations Na+ and K+ will diffuse together to maintain charge neutrality. Therefore, a strong indication of alkali migration from a concrete pore solution toward the glass fibres would be the presence of Na or K in the matrix (Figs. 5 and 6). Observations on several specimens indicated that neither Na nor K is © 2007 NRC Canada
362
Can. J. Civ. Eng. Vol. 34, 2007
Fig. 9. Fourier-transformed infrared spectra for GFRP in Hall’s Harbour wharf: (a) control specimen; (b) specimen from structure.
present in the matrix. As can be seen in Figs. 7a and 7b, the EDX spectrum close to the GFRP–concrete interface was identical to that in the middle of the GFRP bar, thus confirming that no change in the chemical composition had occurred within the cross section of the GFRP. It is also important to note that none of the basic elements of glass fibres — Si, Al, Ca, and Mg — were found in the matrix adjacent to the glass fibres, thus confirming again the absence of alkali attack on glass fibres.
Infrared spectroscopy The durability of the polymeric matrix in FRP is governed to a large extent by the chemical nature of the structure of the polymeric chain. In their structure, all resins have ester bonds, which are the weakest links of the polymer. A possible matrix degradation mechanism is alkali hydrolysis of the ester linkages. Because of the alkaline environment in concrete, alkali hydrolysis is expected to some extent. During the hydrolysis reaction, the OH– induces ester linkage attack, © 2007 NRC Canada
Mufti et al.
363
Table 1. Ratio of OH/CH for specimens removed from structures and control specimens. OH/CH ratio Structure
GFRP
Joffre Bridge, barrier wall Joffre Bridge, barrier wall Hall’s Harbour, wharf
GFRP C-bar (Φ = 9 mm) GFRP C-bar (Φ = 16 mm) GFRP IsoRod™ (Φ = 16 mm)
Control specimen
Exposed specimen
0.64
0.60
0.50
0.46
0.97
0.91
Note: Φ is diameter.
and the resin chain is broken. Consequently, the structure of the resin is disrupted, and the material properties are changed. If the resin degrades it is not able to protect the glass fibres against alkaline attack. Changes in the amount of hydroxyl groups present in the composite material provide insight into the extent of polymer hydrolysis. The relative amounts of hydroxyl groups in the specimens were measured by determining the ratio of the maximum of the band corresponding to the hydroxyl groups at 3430 cm–1 and the band corresponding to the carbon–hydrogen groups at 2900 cm–1 in the FTIR spectra. The CH content is assumed to be constant. Since the vinylester resins naturally contain hydroxyl groups, all the spectra present a strong absorption band in this region. Typical results of the FTIR analysis of control and in-service GFRP samples from the Joffre Bridge and Hall’s Harbour wharf are presented in Figs. 8 and 9, respectively. It can be seen in these figures that there is no significant change between the spectra of the specimens removed from in-service structures and those from the specimens not exposed to the concrete environment. Table 1 lists the OH/CH content ratios for the control and in-service specimens from the Joffre Bridge and the Hall’s Harbour wharf. The results show very little change in the hydroxyl content of the in-service specimens. In fact, the decreased OH/CH ratio for the in-service sample indicates that the hydrolysis reaction was extremely unlikely in these specimens. Glass transition temperature The properties of GFRP are governed by the properties of both its constituents, the glass and the polymer matrix. The glass transition temperature (Tg), an important physical property of the matrix, is not only an indicator of the thermal stability of the material but also an important indicator of the structure of the polymer and its mechanical properties. For example, as a result of breakage of the van der Waals bond between the polymer chains, moisture in the matrix reduces the Tg of the resin, because of its plastification. The swelling stresses associated with moisture uptake or the presence of alkalis can cause permanent damage in the resin, such as matrix cracking, hydrolysis, and fibre–matrix debonding. The CHBDC requires that matrices and (or) the adhesives of FRP systems should have a wet glass transition temperature (Tgw) of more than 20 °C plus the maximum daily mean temperature as specified elsewhere in the code. The glass
transition temperature associated with moisture uptake, denoted by Tgw, is lower than Tg. Although Tg is often taken to represent the effective operational limit for the polymer and its composite in terms of temperature, it is advisable to use the wet glass transition temperature (Tgw) as the effective limit in cases where water uptake is likely to be a result of normal exposure. The glass transition temperatures for the GFRP specimens removed from the in-service structures are presented in Table 2. Where possible the results are benchmarked to control specimens. The results presented in the table indicate that the structure of the matrix was not disrupted by exposure to the environment at the Hall’s Harbour wharf. No significant differences in the Tg values for the control and those for the exposed specimens from the Joffre Bridge were observed. However, it should be noted that the Tg value for the 16 mm diameter GFRP C-bar was 108 °C, which, even for full curing, is low compared with the Tg for other GFRPs used in other structures. The low values of Tg are attributed to the manufacturing process. Higher Tg values, ranging between 123 °C and 128 °C, were measured for the 9 mm diameter GFRP C-bar. The Tg values for both types of GFRP bar suggest no structural disruption of the matrix due to natural exposure at the Joffre Bridge (Fig. 10). For the Chatham, Crowchild Trail, and Waterloo Creek bridges, control specimens were not available, and the effect of service conditions on these GFRP composites could not be evaluated comparatively. However, the Tg results for the GFRP C-bar material used in the Crowchild Bridge suggest that the material was fully cured. The same is not the case for the GFRP grid sample extracted from the Chatham Bridge, since the Tg measured in the first and second runs was 98 °C and 116 °C, respectively. A similar behavior was observed for the GFRP grid extracted from the Waterloo Creek Bridge. The Tg values obtained during the first scan were 40 °C lower than measured in the second run (78 °C versus 117 °C), possibly indicating that the matrix in GFRP in both the Chatham and Waterloo Creek bridges was not fully cured. Carbonation and pH values Phenolphthalein tests were carried out to determine the depth of carbonation on freshly cut surfaces. No evidence of carbonation was found in concrete cover over the GFRP bars. The leach test method was used to determine values of pH. Crushed concrete, taken separately from the near the surface cores and near the GFRP bars, was mixed with double deionized water, and the pH of the solution lying above material deposited after precipitation was measured. As can be seen in Table 3, the measured pH values range between 11.95 and 12.13, suggesting a drop from the pH of the original concrete, which is usually higher than 12.5. High dissolution rates are expected if the glass fibres are in direct contact with strong alkali solutions, such as the original concrete pore solution. The dissolution rates become low when pH drops below a certain level, such as in carbonated concrete. The long-term performance of the GFRP is controlled by two factors: (i) the rate of penetration of alkalis through the polymeric matrix and (ii) the rate of drop of pH values. It is expected that the future reduction in the values of pH will render the concrete pore solution less harm© 2007 NRC Canada
364
Can. J. Civ. Eng. Vol. 34, 2007 Table 2. Glass transition temperature (Tg). Tg (°C) Structure
GFRP
Specimen
1st run
2nd run
Hall’s Harbour, wharf
GFRP IsoRod™ (Φ = 16 mm)
Control
105
125
Joffre Bridge, barrier wall
GFRP C-bar (Φ = 16 mm)
From structure Control
123 107
125 108
From structure Control
107 123
108 126
From From From From
127 126 98 78
128 129 116 117
GFRP C-bar (Φ = 9 mm) Crowchild Trail Bridge, barrier wall Chatham Bridge, barrier wall Waterloo Creek Bridge, barrier wall
GFRP C-bar NEFMAC™ grid NEFMAC™ grid
Fig. 10. Thermographs for GFRP from Joffre Bridge: (a) inservice specimen; (b) control specimen.
structure structure structure structure
Table 3. Values of pH from leach tests. Structure
Location
Hall’s Harbour, wharf
Near Near Near Near Near Near Near Near Near Near
Joffre Bridge, barrier wall Chatham Bridge, barrier wall Crowchild Bridge, barrier wall Waterloo Creek Bridge, barrier wall
ful, even if it manages to penetrate the polymeric matrix, which has low permeation and transportation properties. Concrete cover over fibre-reinforced polymers The first edition of the CHBDC (CSA 2000) required the clear cover over FRP bars to be a minimum 35 mm with a construction tolerance of ±10 mm. In light of some published (e.g., Rahman et al. 1995) and unpublished analytical research, some engineers believed that the depth of cover over FRPs should be greater than that required by the CHBDC. The durability study presented above provided a
pH
surface of GFRP bar surface of GFRP bar surface of GFRP bar surface of GFRP bar surface of GFRP bar
core core core core core
11.97 12.00 12.06 12.08 12.06 12.08 12.07 12.07 11.95 12.13
unique opportunity to study the effect of cover depth on concrete cracking. None of the nearly 50 cores removed from GFRP-reinforced concrete had cracks over the GFRP bars or grids. Unfortunately, a record of the depth of cover in all the cores was not kept. However, 15 cores with GFRP are still stored in the Structures Laboratory of the University of Manitoba. The cover in these samples was measured and was found to vary from 28 to 111 mm. The surfaces of concrete that were exposed in the parent structures were examined carefully with a handheld microscope. No radial crack was observed in any of the samples. The concrete core in which the cover was only 28 mm can be seen in Fig. 11. Vogel (2005) examined a number of concrete beams prestressed with GFRP and CFRP tendons with minimum cover and subjected to the thermal gradients expected in Canada; he noted that “the flexural specimens regularly monitored during the experimental program with a handheld microscope never revealed the presence of cracks within the cover”. Aguiniga (2003) also reported that even shallow covers over FRP bars do not show cracks in structures exposed to the environment. As noted by Boulfiza and Banthia (2005), there might be several reasons for the lack of agreement between analytical predictions and field observations. It is possible that the relatively soft lateral modulus of elasticity of the FRP and the plastic nature of concrete in its early life were not modeled correctly. Also, as discussed by Bakht et al. (2004), the reason for the absence of cracks above FRP bars in concrete structures might be that during the setting of concrete, the © 2007 NRC Canada
Mufti et al.
365
Fig. 11. A core with a cover of 28 mm, showing no crack in the concrete.
FRP bars are “locked” into the concrete at a higher temperature than they are likely to experience later. In light of the above discussion, the requirements for cover over FRP reinforcement in the forthcoming second edition of the CHBDC have remained unchanged.
Conclusions On the basis of the results of analyses described above, it is confirmed that there was no degradation of the GFRP in the concrete environment of real-life engineering structures exposed to natural environmental conditions for durations of 5–8 years. From the OM analyses, it is clear that there is no evidence of debonding between GFRP and concrete in all the examined structures, which have been subjected to wet–dry cycles, marine environment, freeze–thaw cycles, and deicing salts. The matrix in all GFRPs was intact and unaltered from its original state. It is encouraging to note that the results from the FTIR spectroscopy and differential scanning calorimetry analyses support the results from the OM examinations. The latter results indicate that neither hydrolysis nor significant changes in the glass transition temperature of the matrix took place after exposure to the combined effects of the concrete alkaline environment and the external natural environmental exposure for 5–8 years.
The results from SEM and EDX analyses confirm that there is no degradation of the GFRP in the real-life concrete structures. The EDX analyses also indicate no alkali ingress in the GFRP from the concrete pore solution. The results of the study on cores removed from actual engineering structures are not in agreement with the results obtained in simulated and accelerated laboratory studies. As expected, the pH value of concrete drops with time because of carbonation; this observation, coupled with the permeability of the polymeric matrix, suggests that glass in the GFRP will not be attacked by alkalis in future. No cracking of concrete was observed above GFRP bars and grids, even when the depth of cover was as little as 28 mm. The overall conclusion of the research project is that the GFRP is durable in concrete. The first edition of the CHBDC (CSA 2000) was conservative in restricting the use of GFRP to only secondary reinforcement. As a result of the study reported in this paper, the second edition of the CHBDC, expected to be published in 2006, allows the use of GFRP as primary reinforcement and prestressing tendons provided the maximum stress levels are kept below 25% of the ultimate strength of the GFRP.
Acknowledgments The financial support of the ISIS Canada Research Network for the work reported in this paper is gratefully ac© 2007 NRC Canada
366
knowledged. The assistance of technical staff at the participating universities is also gratefully acknowledged.
References Aguiniga, G. 2003. Characterization of design parameters for fibre reinforced polymer composite reinforced concrete systems. Ph.D. thesis, Texas A&M University, College Station, Tex. Aly, A., Bakht, B., and Schaeffer, J. 1997. Design and construction of steel-free deck slab in Ontario. In Annual Conference of the Canadian Society for Civil Engineering, Sherbrooke, Que., 27–30 May 1997. Canadian Society for Civil Engineering, Montréal, Que. Vol. 6, pp. 81–90. Banthia, V. 2003. Transverse confinement of steel free deck slabs by prestressed concrete straps. M.Sc. thesis, Department of Civil Engineering, University of Manitoba, Winnipeg, Man. Bakht, B., Mufti, A., and Tadros, G. 2004. Discussion of “Fibrereinforced polymer composite bars for the concrete deck slab of Wotton Bridge.” Canadian Journal of Civil Engineering, 31(3): 530–531. Benmokrane, B., and Cousin, P. 2005. University of Sherbrooke GFRP durability study report, project 5.17. ISIS Canada, University of Manitoba, Winnipeg, Man. Benmokrane, B., Rahman, H., Mukhopadhyaya, P., Masmoudi, R., Chekired, M., Nicole, J.-F., and El-Safty, A. 2000. Use of fibre reinforced polymer reinforcement integrated with fibre optic sensors for concrete bridge deck slab construction. Canadian Journal of Civil Engineering, 27(5): 928–940. Boulfiza, M., and Banthia, N. 2005. University of Saskatchewan and University of British Columbia durability study, project 5.17. ISIS Canada, University of Manitoba, Winnipeg, Man. CSA. 2000. Canadian highway bridge design code. Standard CAN/ CSA-S6-00. Canadian Standards Association, Toronto, Ont. Malvar, L.J. 1998. Durability of composites in reinforced concrete. In Durability of Fibre Reinforced Polymer (FRP) Composites for Construction, Proceedings of the 1st International Conference (CDCC ’98), Sherbrooke, Que., 5–7 August 1998. Edited by B. Benmokrane and H. Rahman. Department of Civil Engineering, University of Sherbrooke, Sherbrooke, Que. pp. 361– 372. Meier, U., and Brönnimann, R. 2003. ISIS Canada research network, intelligent sensing for innovative structures. External Research Assessment Report, Swiss Federal Laboratories for Materials Testing and Research (EMPA), Zurich, Switzerland.
Can. J. Civ. Eng. Vol. 34, 2007 Mufti, A.A., Onofrei, M., Bakht, B., and Banthia, V. 2003. Discussion paper on durability of e-glass/vinylester reinforcement in alkaline solution. ACI Structural Journal, 99(2): 265. Newhook, J.P., Bakht, B., and Mufti, A.A. 2000. Design and construction of a concrete marine structure using innovative technology. In Proceedings of the 3rd International Conference on Advanced Composite Materials in Bridges and Structures, Ottawa, Ont., 15–18 August 2000. Edited by J.L. Humar and A.G. Razaqpur. Canadian Society for Civil Engineering, Montréal, Que. pp. 777–784. Onofrei, M. 2005. Durability of GFRP reinforced concrete from field demonstration structures. ISIS Canada, University of Manitoba, Winnipeg, Man. Rahman, A.H., Kingsley, C.Y., and Taylor, D.A. 1995. Thermal stress in FRP-reinforced concrete. In Proceedings of the CSCE Annual Conference, Ottawa, Ont., 1–3 June 1995. Vol. II. Canadian Society for Civil Engineering, Montréal, Que. pp. 605–614. Sen, R., Marsical, D., Issa, M., and Shahawy, M. 1993. Durability and ductility of advanced composites. In Structural Engineering in Natural Hazards Mitigation, Proceedings of the 1993 Structures Congress, Irvine, Calif., 19–21 April 1993. Edited by A.HS. Ang and R. Villaverde. American Society of Civil Engineers, New York. Vol. 2, pp. 1373–1378. Sen, R., Mullins, G., and Salem, T. 2002. Durability of E-glass/ vinylester reinforcement in alkaline solution. ACI Structural Journal, 99(3): 369–375. Tadros, G., Tromposch, E., and Mufti, A.A. 1998. Superstructure replacement of Crowchild Trail Bridge. In Proceedings of the 5th International Conference on Short and Medium Span Bridges, Calgary, Alta., 13–16 July 1998. Edited by L. Dunaszegi. Canadian Society for Civil Engineering, Montréal, Que. pp. 499–506. Tsai, P., and Ventura, C.E. 1999. Waterloo Creek Bridge project, field assessment. The University of British Columbia, Vancouver, B.C. Report No. 2. Uomoto, T. 2000. Durability of FRP as reinforcement for concrete structures. In Proceedings of the 3rd International Conference on Advanced Composite Materials in Bridges and Structures, Ottawa, Ont., 15–18 August 2000. Edited by J. Humar and A.G. Razaqpur. Canadian Society for Civil Engineering, Montréal, Que. pp. 3–17. Vogel, H.M. 2005. Thermal compatibility of bond strength of FRP reinforcement in prestressed concrete applications. M.Sc. thesis, Department of Civil Engineering, University of Manitoba, Winnipeg, Man.
© 2007 NRC Canada
