DURABILITY OF GLASS FIBER REINFORCED PLASTIC BARS

Transcription

1 DURABILITY OF GLASS FIBER REINFORCED PLASTIC BARS Project 5/417 Submitted to The Research Center College of Engineering King Saud University Riyadh, Saudi Arabia By Abdulrahman M. Alhozaimy Saleh H. Alsayed Department of Civil Engineering College of Engineering King Saud University Riyadh, Saudi Arabia

2 ABSTRACT A total of 112 specimens of E-glass fiber reinforced plastic (GFRP) rebars with 12.7 mm in diameter were used to investigate the effect of alkalinity concentration and moist curing temperature on their tensile strength and weight. The main variables in the study include alkalinity (NaOH) concentration (0, 5 and 20 g/l), moist curing temperature (21±1 and 40 ±1 C), GFRP rebar conditions (coated or uncoated with cement paste) and immersion period (1, 4, 8 and 12 months). The test results indicate that alkaline solution and temperature may cause great loss to the tensile strength of the GFRP rebars and, in general, that loss is in direct proportion with the moist curing temperature, alkalinity concentration and the immersion period. These same factors also affect, although to a much less extent, the weight reduction of the GFRP rebars. The results also reveal that under normal temperature and drinking water temperature conditions, the presence of cement paste around the GFRP rebars plays a paramount factor in deteriorating the rebars. However, under high moist temperature, the temperature becomes the controlling factor in deterioration process of the rebars.

3 1: INTRODUCTION Fiber reinforced plastics (FRP) are advanced man-made composite materials manufactured by a synthetic assembly of high strength fibers with a light matrix and some selected fillers. FRP have been widely used in aeronautical and chemical engineering for more than 20 years. However, their applications in civil engineering works are rather recent. Currently, there are many types of FRP commercially produced but the most common type used for structural applications, for economic reasons, is the glass fiber reinforced plastics (GFRP). GFRP are non corrosive, lighter, and stronger (in tension) than steel. Unfortunately, in some special conditions, such as in high alkalinity environment, the long term performance of the GFRP is still unresolved question. This is so because of the fact that glass fibers (GF), one of the main constituents of the GFRP materials, loose great portion of their strength when subjected to wet alkaline environment. Tests carried out on glass fiber reinforced concrete (GFRC) [1, 2] indicated that even if alkali-resistance glass fibers are used, composites fabricated with glass fibers show a high reduction in flexural strength, toughness and modulus of rupture when subjected to accelerated aging tests. Sen et al [3] also investigated the durability of

4 S-2 glass/epoxy pretentioned beams exposed to wet/dry cycles in a 15% salt solution and found that GFRP rebars lost their effectiveness within 3-9 months of exposure. Porter et. al. [6] exposed three different types of number 3 E-glass FRP rebars (manufactured using an isophthalic polyester resin) to high alkaline solution and a maximum temperature of 60 C for a period of 2 to 3 months. Their test results indicated that the accelerated aging severely reduced the ultimate tensile strength and the maximum strain capacity of the GFRP rebars. Tannous and Saadatmanesh [7] investigated the influence of eight different environments on the tensile strength, elastic modulus and ultimate strain at failure for number 3 and number 5 E-glass FRP manufactured through pultrusion process using polyester or vinyl ester resin matrix for a period of 6 months. The environments considered in the study included water at 25 C, saturate Ca (OH) 2 solution with ph of 12 at 25 C, saturated Ca (OH) 2 solution with ph of 12 at 60 C, HCl solution with ph of 3 at 25 C, NaCl 3.5% by weight solution at 25 C, NaCl + CaCl 2 (2:1) 7% by weight solution at 25 C and NaCl + MgCl 2 (2:1) 7% by weight solution at 25 C. The investigators reported that, under the environments considered in the study, the highest losses in strength after 6 months (in both

5 vinyl ester and polyester fibers) were in deicing salt and alkaline solution. However, the losses in the vinyl ester rebars were lower. They also reported that although significant losses in strength were observed, limited changes in the elastic modulus and ultimate strain at failure were recorded. Such degradation in the durability of GFRP brought into view the urgent need to evaluate the long term performance of GFRP rebars and, in turn, the concrete structures reinforced by them before they can be prescribed for reinforcing concrete structures. It is of particular importance to point out here that GFRP materials are relatively new and there is only little experience with their long term performance in natural environment, as a reinforcing material for civil applications. One of the well known technique for predicting the service life at some specified conditions is via the application of accelerated test methods. The most common one is immersion in hot water or hot liquid, identical to that the structure will be subjected to for various period of time and determining the change in the engineering properties induced by such exposure [6,7] 2: MAIN OBJECTIVE OF THE PROJECT

6 The main objective of this project was to investigate the longterm durability of GFRP bars through monitoring the weight reduction and tensile strength deterioration (tensile strength is the main characteristic used by structural engineers to design reinforced concrete structures by steel or GFRP bars). The effect of impure water, alkalinity, and temperature (accelerated aging process through immersion the GFRP bars in hot water and/or alkaline solution) on the durability of the GFRP bars were assessed. 3: MATERIALS AND EQUIPMENT The following describes the material and equipment incorporated in the experimental program. 1. GFRP bars 12mm diameter. This type of GFRP is manufacturing through pultrusion process. Chemical composition of GFRP is given in Table 1 2. Steel tanks for immersion the specimens. 3. Plastic molds to cast specimen with cement paste. (0.50 w/c ratio). 4. Ordinary Portland Low alkali cement (Type 1) for the cement paste, used for coating the specimens. Chemical composition of cement is given in Table 1

7 5. Anhydrous Sodium Hydroxide (NaOH), to obtain the specified alkaline solution. 6. ph meter to monitor the ph of water/solution. 7. Electronic balance, with an accuracy of 0.1g, for weighing specimen. 8. Glass thermometer for monitoring the temperature of water/solution. 9. Epoxy for sealing ends of bar pieces and performing tension test. 10. Tension test machine. 11. Specially made (fabricated) grips to perform tension test. 12. LVDT s and strain gages to monitor elongation and strain in tension specimen. 13. Load cells to monitor the loading through data acquisition system. 14. Personal computer and data acquisition system to record the test data.

8 Table 1: Chemical composition of GFRP bar and Ordinary Portland Low alkali cement (Type 1) by weight. GFRP bar Material Composition E-Glass Urethane Modified Vinyl Easter Unsaturated Polyester Ceramic Reinforcement Corrosion Inhibitor 1.5 Ordinary Portland Low alkali cement (Type 1) SiO 2 CaO Al 2 O 3 Fe 2 O 3 MgO Na 2 O K 2 O LOI SO

9 4: DESIGN OF EXPERIMENT To achieve the stated objective, an extensive experimental program has been devised which takes into consideration the following variables: (1) water conditions (drinking water and alkaline solution), (2) GFRP bar conditions (coated with cement paste and uncoated), and (3) moist curing temperature (21±1 and 40±1 C). The test ages were selected as ending of 1,4,8, and 12 months. The experimental program is detailed in Table 2. Table 2 : Factorial Design of Experiments Water Conditions Moist Drinking water Alkaline solution (Sodium Hydroxide) curing Control 5 gm/l 20 gm/l temp. FRP bars conditions FRP bars conditions FRP bars conditions (deg. C) Uncoated Coated Uncoated Coated Uncoated Coated 21±1 # * * * * * * 40±1 # * * * * * * * 2 samples will be tested at different ages: 1,4,8,12 months # 2 Control samples will be tested as received from manufacture for all ages as specified above. 4.1 Specimen Preparation Initially twenty GFRP bars φ12 mm, were procured. Each bars was cut into seven pieces of length 700 mm each. The ends of all pieces were sealed with epoxy so that uniform resin coating can be

10 achieved at the cross sections also. Each bar piece was cleaned labeled and weighed to an accuracy of 0.1 g. Out of these seven pieces four pieces were kept as uncoated, two pieces were coated with cement paste (0.50 w/c ratio) and the seventh piece, termed as the control specimen, was tested for the tensile strength in the condition as received from manufacturer. Two replications were considered for each condition, as mentioned in Table 1, thus requiring, including the control specimens, twenty eight specimens for each age (12 specimens for 21±1 C,12 specimens for 40±1 C and 4 specimens for control). Therefore, a total of 112 specimens of GFRP were prepared for all ages of testing. To have cement paste coating on them the specimens, the specimens were placed in plastic molds (50 x 50 mm in cross section). The cement paste of 0.50 water/cement ratio poured into the mold and the mold slightly shaken to remove entrapped air. The specimens were demolded after approximately twenty four hours and put in water tanks for curing. After two weeks of moist curing under standard laboratory conditions, the specimens were taken out and placed on wooden racks for air curing for another week before immersion them in tanks.

11 4.2 Immersion Tanks Eight steel tanks, with dimensions of 400 mm x 900 mm in plan and 600 mm deep, were fabricated. They were provided with cover at the top, an opening, with valve at the bottom and painted with an enamel paint. Four of the tanks were covered from outside with an insulation material and fitted with electrical heaters, thermostat controlled, so as to maintain the temperature of water/solution to 40 ± 1 C. The tanks were designated as T1 through T4 (without heaters) and T5 through T8 (with heaters). Perforated racks, made up of Galvanized steel, were fabricated to facilitate placing of bar pieces inside the tanks. The placement of prepared specimens in immersion tanks is summarized in Table 3. The uncoated and coated specimens in drinking water were placed in separate tanks, this has been done to minimize the leaching effect of coated specimen in drinking water. It should be noted that ph values in the drinking water of the tanks T2 and T6 were raised from ph of 7.10 to 9.96 and respectively, due to placing the coated specimens.

12 Table 3: Detail of the immersion tanks Tank Designation Temperature ( C) Contents of the Tank T 1 21 ± 1 Uncoated specimen in drinking water T 2 21 ± 1 Coated specimen in drinking water T 3 21 ± 1 Coated and uncoated specimens in NaOH solution concentration 5 g/ l T 4 21 ± 1 Coated and uncoated specimens in NaOH solution concentration 20 g/ l T 5 40 ± 1 Uncoated specimen in drinking water T 6 40 ± 1 Coated specimen in drinking water T 7 40 ± 1 Coated and uncoated specimens in NaOH solution concentration 5 g/ l T 8 40 ±1 Coated and uncoated specimens in NaOH solution concentration 20 g/ l 5: TEST PROCEDURE At the specified age of testing, the specimens were taken out from tanks and cement paste coating around coated specimens were removed. All the specimens were cleaned with tap water, thoroughly and were dried and weighted to an accuracy of 0.1 g. Then the specimens were tested in direct tension. To assure proper bonding and to protect the rebars against the machine jaws, specially made grips were used in the test.

13 6: TEST RESULTS AND DISCUSSION The effect of different environmental conditions on the tensile strength of GFRP bars for 12 months are presents in Table 4 and schematically presented in Figs. 1 through 4. The corresponding results that represent the influence of different environmental conditions on the weight of the GFRP bars are reported in Table 5 and Figs.5 to 8. As can be seen in Figs. 1 and 2 that, except for the uncoated specimens immersed in drinking water at 21 C, for coated and uncoated specimens, the reduction in the tensile strength is in direct proportion with the immersion period. However, for the uncoated specimens immersed in drinking water at 21 C, the reduction in the tensile strength is almost constant throughout the immersion period, implying that under normal temperature and in the absence of the alkaline solution the moisture has little effect on the tensile strength of GFRP rebars. The results shown in Fig. 1 also indicate that when the water is at 21 C and contains 5 or 20 gm/ l of NaOH, the reduction in the tensile strength is almost the same whether the specimens were coated with cement paste or not, i.e. in the presence of alkaline solution and under normal temperature, cement paste has no influence on the durability of the GFRP rebars. However, when no sodium hydroxide was added to water, the reduction in tensile strength under the normal temperature (21 o C) is about 2% in one year for uncoated specimens and 19% for coated specimens. The difference in the reduction of the tensile strength can be attributed to the alkalinity

14 leached from the cement paste used for coated specimens. On the contrary to this, the results shown in Fig. 2, reveal that at high moist curing temperature (40 o C), the reduction in the tensile strength is almost the same for both coated and uncoated specimens irrespective of alkalinity concentration. It seems increasing the moist curing temperature from 21 C to 40 C greatly accelerated the rate of the tensile strength deterioration. For instance, at one year of immersion period, the reduction in tensile strength of uncoated specimens in drinking water under the (40 o C) temperature is about 33%, compared to 2% under the temperature (21 o C). This can be attributed to the effect of heat (water temperature) on the resin used in manufacturing this type of GFRP rebars. It is interesting to observe in Figs. 1 and 2 that the tensile strength reduction in the uncoated specimens immersed in alkalinity solution (NaOH concentration = 5 g / l ) is similar to the tensile strength reduction of the coated specimens immersed in drinking water. It means that the alkalinity of cement paste used in this study (low alkali cement, Na 2 O equivalent = 0.2) or alkaline solution (NaOH = 5 g / l ) produced almost the same rate of the GFRP rebars deterioration. The influence of moist curing temperature is further highlighted in Figs. 3 and 4. The results reported in these two figures clearly show that under both low (21 o C) and high (40 o C) temperature conditions, the reduction in the tensile strength is in direct proportion with the increase of the immersion period and the alkalinity concentration. Comparison between the results presented in Figs.3 and 4 shows that the percentage of the reduction in the tensile strength is further aggravated by the increase in moist curing temperature. This is

15 especially the case for uncoated specimens immersed in drinking water. The tensile strength reduction increased generally twofold or more with increasing moist curing temperature from 21 o C to 40 o C. Furthermore, Figs. 3 and 4 show that at one year immersion period, the maximum reduction in the tensile strength occurs when the conditions of high temperature and high alkalinity prevail simultaneously. In such conditions, for coated and uncoated specimens, the reduction in the tensile strength is more than 50% of the original strength of the bar. Although this percentage reduction occurred in the laboratory after one year, which may represent several years in the field, it is still an indication. Thus this type of GFRP rebars is not suitable for civil work applications if the concrete structure will be used under humidity and relatively hot environmental conditions. The results presented in Figs. 5 to 8 show that the reduction in the weight of the rebars has the same trend as the reduction in the tensile strength. Exception from that is for the uncoated specimens immersed in drinking where the weight of specimens stayed almost constant up to the end of the 12 months immersion periods. This implies that for drinking water, temperature has no influence on the weight reduction of GFRP bars. However, the percentage in the reduction of the weight under any of the condition consider in this study is much lower than the corresponding tensile strength reduction

16 under the same curing conditions. The maximum recorded reduction in the weight after one year of immersion, when high alkalinity and temperature conditions prevail is 5% corresponding to 50% reduction in the tensile strength. Thus, alkaline solution has about 10 fold deterioration effect on GFRP bars tensile strength than the of the weight. This may lead to the conclusion that alkalinity even at relatively high temperature is not an aggressive solution to the weight of the GFRP bars but it is very much-deteriorated chemical element to their tensile strength. Thus it may be stated that alkalinity is causing unacceptable deterioration to the tensile strength of the currently investigated GFRP rebars and the deterioration effect is very much aggravated by the temperature such that it may impair the safety of the structure that is reinforced with them. It may also be true to argue that this finding is based on accelerating tests which may provide indications rather than absolute figures to the long-term behavior of the materials under actual service conditions.

17 CONCLUSIONS Based on the results of the study carried out as part of this investigation the following conclusions are drawn: 1. Deterioration of the tensile strength and reduction of the weight of GFRP rebars due to alkalinity and temperature are in direct proportion with alkalinity concentration, moist curing temperature and immersion period. 2. Deterioration of the tensile strength and weight of GFRP rebars due to alkalinity and temperature are highly dependent on the chemical composition and manufacturing quality of the outermost layer of the rebars. 3. Under normal temperature (21 o C) and drinking water condition, the presence of cement paste around the GFRP rebars greatly increases the deterioration rate of the rebar. However, such influence disappears when the water contains 5 g/ l or more of NaOH. 4. Increasing the moist curing temperature from 21 to 40 o C highly aggravates the rate of deterioration of the rebars. Furthermore, under hot temperature (40 o C), the deterioration of the GFRP rebars is almost the same for both coated and uncoated specimens irrespective of the alkalinity concentration (NaOH= 0, 5 or 20 g/ l ). 5. The alkalinity of cement paste used in this study (low alkaline cement, Na 2 O equivalent = 0.2) or alkaline solution (NaOH) with a concentration of 5g/ l produced almost the same rate of deterioration for Type A GFRP rebars considered in this investigation.

18 RECOMMENDATION As the long term behavior of GFRP rebars is very much dependent on the chemical composition and manufacturing techniques of the rebars, it is advisable not to use any type of GFRP rebars as a major reinforcing material for concrete structures that are anticipated to be used under hot moist environmental conditions unless the actual long term behavior of that particular type under such field conditions is well established. REFERENCES 1. Shah, S.P., Ludirdsa, D., Daniel, J.1. and Mobasher, B., "Toughness- Durability of Glass Fiber Reinforced Concrete Systems, " ACI Material Journal, Sep.-Oct. 1988, pp Soroushian, P., Tlili, A, Yohena, M., and Tilsen, B. "Durability Characteristics of Polymer-Modified Glass Fiber Reinforced Concrete, " ACI Material Journal, Jan.-Feb. 1993, pp Sen, R-, Mariscal D., and Shahawy, "Investigation of S-2 Glass/Epoxy Strands in Concrete, Fiber-Reinforced-Plastic Reinforcement for Concrete Structures, International Symposium held at Vancouver on March 28-31, 1993, SP-138, pp Porter, M.L., Mehus, J., Young, K.A., O Neil, E.F., and Barnes, B.A., Aging for Fiber Reinforcement in Concrete, Proceedings of the Third International Symposium on Non-Metallic (FRP)

19 Reinforcement for Concrete Structures, Sapporo, Japan, Oct. 97, Vol. 2, pp Tannous, F. E., and Saadatmanesh, H., Environmental effects on the Mechanical Properties of E-Glass FRP Rerebars, ACI Material Journal, Vol. 95, No. 2, March-April 1998, pp Litherland, K.L., Oakley, D.R, and Proctor, B.A., "The Use of Accelerated Aging Procedures to Predict The Long Term Strength of GRC Composites, " Cement and Concrete Research, Vol. 11, No. 3, 198 1, pp Aindo, A.J., bbb Oakley, DR., and Proctor, B.A., "Comparison of the Weathering Behavior of GRC with Predictions made from Accelerated Aging Tests, " Cement and Concrete Research, V. 14, No. 2, 1983, pp Gangarao, H.V.S., and Vijay, P.V., Aging of Structural Composites Under Varying Environmental Conditions, Proceedings of the Third International Symposium on Non-Metallic (FRP) Reinforcement for Concrete Structures, Sapporo, Japan, Oct. 97, Vol. 2, pp