TENSILE LOAD RELAXATION OF FRP CABLE SYSTEM DURING LONG-TERM EXPOSURE TESTS

Transcription

1 TENSILE LOAD RELAXATION OF FRP CABLE SYSTEM DURING LONG-TERM EXPOSURE TESTS Iwao SASAKI Senior Researcher, Advanced Materials Research Team Public Works Research Institute 1-6 Minamihara, Tsukuba, Ibaraki, , Japan * Itaru NISHIZAKI Head, Advanced Materials Research Team Public Works Research Institute 1-6 Minamihara, Tsukuba, Ibaraki, , Japan Abstract FRP cables have been used as tendons for pre-stressed concrete or ground anchors. The tensile load relaxation of FRP cables is usually verified by stress relaxation tests for 1,000hours in a laboratory. However, demonstrative data for long term behaviour of FRP cables have not been sufficiently provided, and more empirical studies are required, particularly for stress relaxation. The authors have carried out outdoor exposure tests for 17 years to verify the long-term durability. FRP cable specimens using carbon, aramid, glass, and vinylon fibers, exposed various conditions such as several initial prestressing tensile load, with/without direct sunlight radiation and salt splash conditions. The specimens were retrieved and investigated with several properties including the residual prestressing tensile load. The results suggest that practical durability of carbon and aramid FRP cables seems to be still good, but initial loading level should be carefully considered for glass and vinylon FRP in the case tensile load permanently applies. Keywords: Exposure test, FRP cable, long-term durability, relaxation, tensile load. 1. Introduction Application of FRP cables to tendons of a pre-stressed concrete member has been studied from early 1980s. Design methods and codes have also been already established for the application as a tendon. In comparison with steel tendons, FRP cables are expected to greatly improve durability particularly for high corrosive environment. However, its application examples are still limited in trial stage in Japan. Present main application of FRP cables in construction is for PC tendons and ground anchors, but other application, such as for external reinforcing cables or for suspension cables for a bridge, seems to be promise. The application also usually treated as a trial construction. There are some main reasons for the situation, one of the reasons seems to be in the lack of the evaluation of cost-benefit effect and in the lack of the evaluation of the durability. When we consider the application of FRP cables to the external reinforcing cables or for the suspension cables for a bridge, the main deterioration factor becomes outdoor environment including maritime condition. Page 1 of 8

2 In order to evaluate the durability of FRP cables under these conditions, outdoor exposure test seems to be indispensable. Some studies have been reported in this area [1] (Uomoto et al. 1996), [2] (Tomosawa et al. 1997), however the data is based on laboratory experiments and the long term durability of this material has not be verified enough. The authors have been carrying out several series of outdoor exposure tests of FRP cables mainly in maritime conditions for 20years. In this report, some of the results will be reported based on the tensile stress relaxation behavior of FRP cables after 17 years exposure with the obtained durability data at 3.5 years of these materials that the authors have reported [3] (Katawaki et al. 1992), [4] (Sasaki et al. 1997) as interim reports. 2. Test Programs 2.1 Materials Six types of FRP cables; two types of CFRPs, two types of AFRPs, one GFRP and one Vinylon FRP (VFRP); were selected. GFRP and VFRP were not considered to be applied to a tendon at the time this study started, however added to the exposure tests to know the durability behavior of these new materials. Table 1 shows the detail of each cable tested in this study. The values of tensile strength and modulus in the table were measured values at the beginning of the exposure test using cables manufactured in the same lot and the anchor systems that were suggested by the manufacturers of the FRP cables. Table 1. Characteristics of the FRP cables used in the test 2.2 Sample name CFRP1 CFRP2 AFRP1 AFRP2 GFRP VFRP Shape Fiber type Matrix resin Vf (%) Diameter (mm) Ultimate load (kn) Modulus (GPa) Anchor system Strand Carbon Epoxy Rod Carbon Epoxy Wedge Rod Aramid Vinyl ester Braided Aramid Epoxy Rod E-glass Vinyl ester Rod Vinylon Epoxy Tensile Load Level In order to evaluate tensile stress relaxation in the long term maritime exposure, two levels of pre-stressing load were applied for each cable material. Tensile load levels were adjusted to 0.8Pu and 0.6Pu for CFRP and VFRP, 0.75Pu and 0.55Pu for AFRP, 0.4Pu and 0.25Pu for GFRP, considering the findings at the beginning of the exposure test. 2.3 Exposure Tests Exposure location In the practical use of external reinforcing cables, FRP cables are suffered from various modes of damages such as temperature fatigue, humidity permeation, salinity attack, and ultra-violet ray radiation. Effect on each single deterioration factor can be evaluated by laboratory testing, but comprehensive testing must be required for durability validation. In Page 2 of 8

3 order to apply these deterioration factors in parallel, outside maritime exposure tests site were carried out. Without With direct direct Sunlight Sunlight Fig. 1. Exposure test site in Suruga bay Fig. 2. FRP cables pre-stressed in SUS flames A platform steel deck facility located in Suruga-bay, Shizuoka prefecture of Japan facing to the Pacific Ocean, was used as the exposure site. The distance of the platform from the coast is about 250 meters. Figure 1 shows the view of the platform. The platform has three decks and has square dimension in 15 meters. The exposure test reported here was carried out at the second deck of the platform where the height is 8.9 meters from the tidal level. Two locations on the second deck were used to expose with /without direct sunlight as shown in Figure Exposure conditions In order to evaluate the effect of the direct sunlight to the deterioration of the FRP cables, two different places on the second deck of the platform (Fig.1) were selected. One was a place at the open area of the second deck where well exposed to sunlight, and the other was a place under the top deck where sunlight less reaches than the place at open area. In 1993, all specimens in this report were pre-stressed and installed on the exposure test site. Figure 2 shows the specimens of FRP cables set into SUS flames for exposure. Among four specimens of same test condition, each of two specimens was retrieved in November 1996, and the results of the FRP cables exposed for 3.5 years were already reported by the authors [4] (Sasaki et al. 1997). All of the remaining SUS steel frames with FRP cables were finally retrieved in July Unloading measurement for residual tensile load was conducted in May 2010, therefore, the apparent exposure time for these FRP cables was 17 years in terms of tensile load application. This report illustrates the results on both durations of exposure. As for six types of FRP cables (Table 1), two tensile levels were applied stated in the previous section, two conditions on direct sunlight and two durations to retrieval were prepared for this exposure testing. For these test conditions, two specimens were prepared for the each same condition as the repetition. Thus, 96 cable specimens (flames) were basically installed at the beginning of the exposure test. Page 3 of 8

4 2.4 Retrieval and Evaluation Evaluations for the retrieved FRP cables were carried out with the appearance check at first. Subsequently, unloading procedures were conducted with stress-strain measurements to evaluate residual tensile load. After the unloading procedures, retrieved cables will be used for rupture tests and chemical investigations in future studies. In unloading procedures, an exposed cable was pulled by a jack with the measurements of load and the displacement of anchor and load flame for exposure as shown in Figure 3. Figure 4 shows the example of load-displacement measurement. An inflection point was observed in each case that the point where a SUS flame does not bear jack load anymore. In this study, the load of this point was determined as the residual tensile load Load kn Displacement gauge Jack Load cell 20 Displacement of anchor mm Fig. 3. Residual tensile load measurement Fig. 4. Residual tensile load determination 3. Results This section states the results focusing on residual tensile load. Figure 5 shows residual tensile load after 3.5 years exposure with/without direct sunlight which previously reported [4] (Sasaki, et al., 1997) by the authors. In the same manner, Figure 6 shows residual tensile load after 17 years exposure with/without direct sunlight that obtained in this study. Residual tensile load of CFRP and AFRP cables were 70-80% after 17 years exposure with direct sunlight. Those of without direct sunlight were about 90% for CFRP1 and 80% other than CFRP1. The progress of load reduction during 3.5 and 17 years is very small for CFRP and AFRP cables. GFRP cables represent significant effect on initial tensile level. All GFRP cable specimens in 0.4Pu tensile load ruptured by creep during exposure. In contrast, in 0.25Pu, all specimens preserve good condition and residual tensile load were over 90% without sunlight and about 80% with sunlight. As the authors have reported [4] (Sasaki, et al., 1997), slight tensile strength reduction (10% without direct sunlight, 20% with direct sunlight) was observed after Page 4 of 8

5 3.5 years exposure. Although GFRP has fairly good relaxation characteristic in lower stress level, GFRP showed sensitive creep properties against the initial stress level and creep rupture. Residual tensile load of VFRP cables indicates less than 50%, and the higher initial loading gave basically higher relaxation ratio. However, the progress of load reduction after 3.5 years may be insignificant. (a) With direct sunlight (b) Without direct sunlight (The values show the ratio of residual tensile load against each initial load.) Figure 5. Residual tensile load after 3.5 years exposure. (a) With direct sunlight (b) Without direct sunlight (The values show the ratio of residual tensile load against each initial load.) Figure 6. Residual tensile load after 17 years exposure. Page 5 of 8

6 4. Discussions 4.1 Relaxation Ratio Figure 7 shows semi-logarithmic plot of time-relaxation relation for CFRP and AFRP cables of 3.5 and 17 years (about 150,000 hours) exposure applied direct sunlight. The stress at one hour after the initial loading was assumed as the initial value. In order to evaluate the results in this test by comparing with the standards for relaxation (i.e. at 0.7Pu tensile loading) [5] (JSCE), each apparent relaxation was calculated as interpolated value at 0.7Pu by different loading levels (e.g. 0.8Pu and 0.6Pu for CFRP). Apparent relaxation ratios are derived from the complement ratio of the residual tensile load to initial applied load; thus, it includes relaxation of FRP cable itself and other loss factors such as loss in anchors and load frame creep. Although the procedure for introducing tensile load (in 1993) may not be completely equal to the procedure at releasing load after exposure (1997, 2010), the influence may not so significant. However, the temperature difference between initial and release can be one of the factors, particularly for some cables of which fibers have zero or negative coefficient of linear expansion. Focusing on the apparent relaxation after 1,000,000hours, all CFRP and AFRP cables indicate around 30-35% in rough estimation. In a semi-logarithmic scale, duration between 3.5 and 17 years is only one step (order) difference on the horizontal axis; therefore, the results of 17 years exposure do not indicate large difference from those after 3.5 years. However, the order must be demonstrative and persuasive compare to 1,000hours which conventional quality standard. The figure implies that we can evaluate extremely long term relaxation by semi-logarithmic prediction base on the empirical findings in this study. 4.2 Direct Sunlight Effect Figure 8 shows apparent relaxation behavior without direct sunlight also for CFRP and AFRP cables of 3.5 and 17 years exposure. The apparent relaxation after 1,000,000hours can be estimated 10%, 20%, 30%, respectively CFRP1, CFRP2, AFRP1&2. This means that, in terms of the influence of direct sunlight application, direct sunlight may affect negative implication to CFRP, though AFRP cables show slight difference due to sunlight effect. One of the reasons for the difference of apparent relaxation with/without sunlight is material degradation due to ultra-violet lays. However, chemical deterioration may not be influential because no deterioration has been found in same types of cables without pre-stressing in the previous study [6] (Nishizaki & Sasaki). Therefore, thermal fatigue resulting from temperature stress induced by sunlight can also be influential phenomenon. Evaluation in mechanical properties such as ultimate strength and chemical properties are also planned to carry out in the future, and the results may suggest further implications. 4.3 Relaxation Value of Standard Relaxation performance of PC tendons is usually determined at 1,000hours in 0.7Pu tensile loading. Figure 7 and its interpolated estimation indicates that apparent relaxation ratio at this condition are 16-19% with direct sunlight. In the case without direct sunlight, apparent relaxation ratio at 1,000hours in 0.7Pu tensile loading indicates 5% for CFRP1 and 11-14% for the other cables. These results are almost consistent with the values that previous papers and guidelines have stated. Page 6 of 8

7 Figure 7. Apparent relaxation of FRP cables at 0.7Pu in exposure with direct sunlight. Figure 8. Apparent relaxation of FRP cables at 0.7Pu in exposure without direct sunlight. 5. Conclusions (1) Apparent relaxation levels of FRP cables after 17 years exposure were as follows: CFRP: 10-20% without direct sunlight or 20-30% with direct sunlight. AFRP: 20-30% irrespective of direct sunlight. GFRP: around 10% under 0.25Pu tensile stress, however, all cables loaded 0.4Pu ruptured by creep behavior. VFRP: more than 50%. (2) CFRP cables showed a negative implication of direct sunlight for apparent relaxation. (3) Estimated apparent relaxation of CFRP and AFRP cables after 1,000 hours under 0.7Pu stress was within ranges broadly consistent with the previous laboratory findings. (4) The results prove that long term relaxation can be estimated by semi-logarithmic plot. Page 7 of 8

8 Acknowledgement The authors acknowledge the supports of KAKENHI for retrieval and investigations of specimens after 17 years exposure. REFERENCES [1] Uomoto, T., Ohga, H., Performance of Fiber Rein-forced Plastics for Concrete Reinforcement, Proceedings of the Second Advanced Composite Materials in Bridges and Structures (ACMBS2), 1996, pp [2] Takewaka, K., Khin, M., Deterioration and Stress-Rupture of FRP Rods in Alkaline Solution Simulating as Concrete Environment, Proceedings of the Second Advanced Composite Materials in Bridges and Structures(ACMBS-2), 1996, pp [3] Katawaki, K. et al., Evaluation of the Durability of Advanced Composites for Applications to Prestressed Concrete Bridges, Proceedings of the First Advanced Composite Materials in Bridges and Structures (ACMBS-1), 1992, pp [4] Sasaki, I. Nishizaki, I., Sakamoto, H., Katawaki, K., Kawamoto, Y., Durability Evaluation of FRP Cables by Exposure Tests, Proceedings of the third International Symposium on Non Metallic Reinforcement for Concrete Structures (FRPRCS-3), 1997, pp [5] Japan Society of Civil Engineers, Recommendation for Dsign and Construction of Reinforced Concrete Structures using Continuous Fiber Reinforcing Materials (draft), Concrete Library, No.88, [6] Nishizaki I., Sasaki, I., Long-term durability of FRP cables under maritime conditions, Proceedings of the 5th International Conference on FRP Composites in Civil Engineering (CICE-5), Page 8 of 8