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TENSILE LOAD RELAXATION OF FRP CABLE SYSTEM DURINGLONG-TERM EXPOSURE TESTS

Iwao SASAKISenior Researcher, Advanced Materials Research TeamPublic Works Research Institute1-6 Minamihara, Tsukuba, Ibaraki, 305-8516, Japanisasaki@pwri.go.jp *

Itaru NISHIZAKIHead, Advanced Materials Research TeamPublic Works Research Institute1-6 Minamihara, Tsukuba, Ibaraki, 305-8516, Japannisizaki@pwri.go.jp

AbstractFRP cables have been used as tendons for pre-stressed concrete or ground anchors. Thetensile load relaxation of FRP cables is usually verified by stress relaxation tests for1,000hours in a laboratory. However, demonstrative data for long term behaviour of FRPcables have not been sufficiently provided, and more empirical studies are required,particularly for stress relaxation. The authors have carried out outdoor exposure tests for 17years 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 wereretrieved and investigated with several properties including the residual prestressing tensileload. The results suggest that practical durability of carbon and aramid FRP cables seems tobe still good, but initial loading level should be carefully considered for glass and vinylonFRP in the case tensile load permanently applies.

Keywords: Exposure test, FRP cable, long-term durability, relaxation, tensile load.

1. IntroductionApplication of FRP cables to tendons of a pre-stressed concrete member has been studiedfrom early 1980s. Design methods and codes have also been already established for theapplication as a tendon. In comparison with steel tendons, FRP cables are expected to greatlyimprove durability particularly for high corrosive environment. However, its applicationexamples are still limited in trial stage in Japan. Present main application of FRP cables inconstruction is for PC tendons and ground anchors, but other application, such as for externalreinforcing cables or for suspension cables for a bridge, seems to be promise. The applicationalso usually treated as a trial construction.

There are some main reasons for the situation, one of the reasons seems to be in the lack ofthe evaluation of cost-benefit effect and in the lack of the evaluation of the durability. Whenwe consider the application of FRP cables to the external reinforcing cables or for thesuspension cables for a bridge, the main deterioration factor becomes outdoor environmentincluding maritime condition.

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In order to evaluate the durability of FRP cables under these conditions, outdoor exposure testseems 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 andthe 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 cablesmainly in maritime conditions for 20years. In this report, some of the results will be reportedbased on the tensile stress relaxation behavior of FRP cables after 17 years exposure with theobtained 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 oneVinylon FRP (VFRP); were selected. GFRP and VFRP were not considered to be applied to atendon at the time this study started, however added to the exposure tests to know thedurability behavior of these new materials. Table 1 shows the detail of each cable tested inthis study. The values of tensile strength and modulus in the table were measured values atthe beginning of the exposure test using cables manufactured in the same lot and the anchorsystems that were suggested by the manufacturers of the FRP cables.

2.2 Tensile Load Level

In order to evaluate tensile stress relaxation in the long term maritime exposure, two levels ofpre-stressing load were applied for each cable material. Tensile load levels were adjusted to0.8Pu and 0.6Pu for CFRP and VFRP, 0.75Pu and 0.55Pu for AFRP, 0.4Pu and 0.25Pu forGFRP, considering the findings at the beginning of the exposure test.

2.3 Exposure Tests

2.3.1 Exposure location

In the practical use of external reinforcing cables, FRP cables are suffered from variousmodes of damages such as temperature fatigue, humidity permeation, salinity attack, andultra-violet ray radiation. Effect on each single deterioration factor can be evaluated bylaboratory testing, but comprehensive testing must be required for durability validation. In

Table 1. Characteristics of the FRP cables used in the test

Sample name CFRP1 CFRP2 AFRP1 AFRP2 GFRP VFRP

Shape Strand Rod Rod Braided Rod Rod

Fiber type Carbon Carbon Aramid Aramid E-glass Vinylon

Matrix resin Epoxy Epoxy Vinyl ester Epoxy Vinyl ester Epoxy

Vf (%) 64 65 66 65 65 72

Diameter (mm) 12.5 8.0 6.0 8.0 6.0 6.0

Ultimate load (kN) 141 70.6 52.4 65.7 36.3 19.6

Modulus (GPa) 145 168 55.6 62.1 52.9 28.6

Anchor system Adhesive Wedge Adhesive Adhesive Adhesive Adhesive

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order to apply these deterioration factors in parallel, outside maritime exposure tests site werecarried out.

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 tothe Pacific Ocean, was used as the exposure site. The distance of the platform from the coastis about 250 meters. Figure 1 shows the view of the platform. The platform has three decksand has square dimension in 15 meters.

The exposure test reported here was carried out at the second deck of the platform where theheight is 8.9 meters from the tidal level. Two locations on the second deck were used toexpose with /without direct sunlight as shown in Figure 1.

2.3.2 Exposure conditions

In order to evaluate the effect of the direct sunlight to the deterioration of the FRP cables, twodifferent places on the second deck of the platform (Fig.1) were selected. One was a place atthe open area of the second deck where well exposed to sunlight, and the other was a placeunder 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 fourspecimens 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 finallyretrieved in July 2009. Unloading measurement for residual tensile load was conducted inMay 2010, therefore, the apparent exposure time for these FRP cables was 17 years in termsof 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 previoussection, two conditions on direct sunlight and two durations to retrieval were prepared for thisexposure testing. For these test conditions, two specimens were prepared for the each samecondition as the repetition. Thus, 96 cable specimens (flames) were basically installed at thebeginning of the exposure test.

Without

direct

Sunlight

With direct

Sunlight

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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 toevaluate residual tensile load. After the unloading procedures, retrieved cables will be usedfor rupture tests and chemical investigations in future studies.

In unloading procedures, an exposed cable was pulled by a jack with the measurements ofload 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 wasobserved in each case that the point where a SUS flame does not bear jack load anymore. Inthis study, the load of this point was determined as the residual tensile load.

Fig. 3. Residual tensile load measurementFig. 4. Residual tensile load determination

3. ResultsThis section states the results focusing on residual tensile load. Figure 5 shows residualtensile 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 tensileload 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 withdirect sunlight. Those of without direct sunlight were about 90% for CFRP1 and 80% otherthan CFRP1. The progress of load reduction during 3.5 and 17 years is very small for CFRPand AFRP cables.

GFRP cables represent significant effect on initial tensile level. All GFRP cable specimens in0.4Pu tensile load ruptured by creep during exposure. In contrast, in 0.25Pu, all specimenspreserve good condition and residual tensile load were over 90% without sunlight and about80% with sunlight. As the authors have reported [4] (Sasaki, et al., 1997), slight tensilestrength reduction (10% without direct sunlight, 20% with direct sunlight) was observed after

Load cell

60

50

40

30

20

Load

kN

Displacement of anchor mm

Jack

Displacementgauge

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3.5 years exposure. Although GFRP has fairly good relaxation characteristic in lower stresslevel, 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 loadinggave basically higher relaxation ratio. However, the progress of load reduction after 3.5 yearsmay 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.

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4. Discussions

4.1 Relaxation Ratio

Figure 7 shows semi-logarithmic plot of time-relaxation relation for CFRP and AFRP cablesof 3.5 and 17 years (about 150,000 hours) exposure applied direct sunlight. The stress at onehour after the initial loading was assumed as the initial value. In order to evaluate the resultsin 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 differentloading levels (e.g. 0.8Pu and 0.6Pu for CFRP).

Apparent relaxation ratios are derived from the complement ratio of the residual tensile loadto initial applied load; thus, it includes relaxation of FRP cable itself and other loss factorssuch as loss in anchors and load frame creep. Although the procedure for introducing tensileload (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 differencebetween initial and release can be one of the factors, particularly for some cables of whichfibers have zero or negative coefficient of linear expansion.

Focusing on the apparent relaxation after 1,000,000hours, all CFRP and AFRP cables indicatearound 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 indicatelarge difference from those after 3.5 years. However, the order must be demonstrative andpersuasive compare to 1,000hours which conventional quality standard. The figure impliesthat we can evaluate extremely long term relaxation by semi-logarithmic prediction base onthe empirical findings in this study.

4.2 Direct Sunlight Effect

Figure 8 shows apparent relaxation behavior without direct sunlight also for CFRP and AFRPcables of 3.5 and 17 years exposure. The apparent relaxation after 1,000,000hours can beestimated 10%, 20%, 30%, respectively CFRP1, CFRP2, AFRP1&2. This means that, interms of the influence of direct sunlight application, direct sunlight may affect negativeimplication 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 materialdegradation due to ultra-violet lays. However, chemical deterioration may not be influentialbecause no deterioration has been found in same types of cables without pre-stressing in theprevious study [6] (Nishizaki & Sasaki). Therefore, thermal fatigue resulting fromtemperature stress induced by sunlight can also be influential phenomenon. Evaluation inmechanical properties such as ultimate strength and chemical properties are also planned tocarry 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 tensileloading. Figure 7 and its interpolated estimation indicates that apparent relaxation ratio at thiscondition are 16-19% with direct sunlight.

In the case without direct sunlight, apparent relaxation ratio at 1,000hours in 0.7Pu tensileloading indicates 5% for CFRP1 and 11-14% for the other cables. These results are almostconsistent with the values that previous papers and guidelines have stated.

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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 0.4Puruptured 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 0.7Pustress 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.

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AcknowledgementThe authors acknowledge the supports of KAKENHI for retrieval and investigations ofspecimens 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 (ACMBS-2), 1996, pp. 125-132.

[2] Takewaka, K., Khin, M., “Deterioration and Stress-Rupture of FRP Rods in Alkaline SolutionSimulating as Concrete Environment”, Proceedings of the Second Advanced CompositeMaterials in Bridges and Structures(ACMBS-2), 1996, pp. 649-656.

[3] Katawaki, K. et al., “Evaluation of the Durability of Advanced Composites for Applications toPrestressed Concrete Bridges”, Proceedings of the First Advanced Composite Materials inBridges and Structures (ACMBS-1), 1992, pp. 119-127.

[4] Sasaki, I. Nishizaki, I., Sakamoto, H., Katawaki, K., Kawamoto, Y., “Durability Evaluation ofFRP Cables by Exposure Tests”, Proceedings of the third International Symposium on NonMetallic Reinforcement for Concrete Structures (FRPRCS-3), 1997, pp. 131-137.

[5] Japan Society of Civil Engineers, “Recommendation for Dsign and Construction of ReinforcedConcrete Structures using Continuous Fiber Reinforcing Materials (draft)”, Concrete Library,No.88, 1996.

[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), 2010.