# Long-Term Bending Creep Behavior of Thin-Walled CFRP Tendon Pretensioned Spun Concrete Poles

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## Abstract

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## 1. Introduction

## 2. Materials

_{g}, of 110 °C. The tendons’ average experimental tensile strength was 3375 MPa with a longitudinal elastic modulus E

_{11}of 180.7 GPa and an average ultimate strain ε

_{11u}of 1.87%. One should note that the carbon fiber T700S has a guaranteed ultimate tensile strain of 2.1%, which could not be reached in tensile testing, due to premature anchorage failure. In order to increase the bond to the concrete, the tendons’ surface was roughened by coating it with a layer of epoxy (type Scotch Weld 2216 by 3M (St. Paul, MN, USA); average layer thickness: 0.25 mm), including hard aluminum oxide (Al

_{2}O

_{3}) sand granules of 0.5 mm in diameter. Additionally, a novel kind of shear reinforcement was used for the poles consisting of a CFRP tape spiral with polyamide matrix of pitch of 40 mm and a cross-section of 7 mm × 0.3 mm [25] (Figure 1). The low density, excellent stress-corrosion resistance and low creep and relaxation of CFRP are well known [28]. The above properties make unidirectional CFRP tendons particularly suitable as prestressing reinforcements for concrete elements [29].

**Figure 1.**Cross-section of carbon fiber-reinforced polymer (CFRP) prestressed pole specimens and the bending load arrangement.

^{−3}(of type CEM I 52.5 R). Silica fume (54 kg·m

^{−3}were mixed into the concrete) and high performance superplasticizer (type Sika

^{®}ViscoCrete-20 HE

^{®}(Baar, Switzerland) at a content of 30 kg·m

^{−3}) were important to achieve a high strength and a good flowability for the spinning process. This particular HPSC mix design allows for optimum spinning of hollow cylinders at water/(cement + silica fume) ratios in the range of 0.31–0.32. 20 mm long polypropylene (PP) fibers were included in the concrete mix at 1 kg·m

^{−3}with the objective of preventing shrinkage cracking. Concrete compaction was carried out by centrifugal casting for 15 min with a maximum revolution speed of 800 rpm in a pretensioning-spinning mold [25]. Prestress was released after 2 days, and then, the elements were demolded. The pole specimens were then kept in a 20 °C/90% R.H. (relative humidity) chamber for 7 days and, thereafter, left to cure under indoor ambient conditions.

## 3. Experimental Specimens and Bending Test Setup

_{R,exp}) determined in the flexural tests of poles Nos. 7 and 8 (5.645 kNm, Table 1).

**Table 1.**Experimental program and main test results (n.a. means “not applicable”, the maximum deflection and maximum strain values for pole No. 14 were last taken 1.4 years before failure *). HPSC, high-performance spun concrete.

Pole No. | Initial CFRP prestress (MPa) | Bending test | Cracking moment (kNm) | Creep moment (kNm) | Failure moment (kNm) | Time to failure | Midspan deflection at failure (mm) | Δε_{c,max} (‰) | Δε_{CFRP, max} (‰) | Failure mode |
---|---|---|---|---|---|---|---|---|---|---|

7 | 1600 | quasi-static | 2.58 | n.a. | 5.76 | 1 h | 66.4 | −5.37 | 10.55 | HPSC crushing |

8 | 1600 | quasi-static | 2.41 | n.a. | 5.53 | 1 h | 70.6 | −5.90 | 11.33 | HPSC crushing |

12 | 1600 | creep | 2.07 | 2.07 | none | running | n.a. | −2.37 | 1.55 | none |

13 | 1600 | creep | n.a . | 2.80 | none | running | n.a. | −4.14 | 3.92 | none |

14 | 1600 | creep | n.a. | 4.07 | 4.07 | 16.54 y | 68.6 * | −6.84 * | 9.27 * | bond |

## 4. Results and Discussion

_{CFRP,max}of the outermost CFRP tendon (near the tensile edge of the pole) is not measured directly, but can be calculated from the measured values of the maximum compression strain at the top edge of the pole Δε

_{c,max}and the maximum tensile strain at the lower edge Δε

_{t,max}(which were both determined by measuring strains over DEMEC gauges with a base length of 200 mm). In this case, the assumption is that “plane sections remain plane” during loading, which was experimentally proven in [25] to be a valid assumption for the short-term bending behavior. One should note that for the long-term bending behavior, this assumption is still valid [21] and that the tensile creep of the unidirectional CFRP tendons is very limited, even at high tensile stress levels [32,34], while the creep of the tendon surface bond layer is also rather limited in the central (cracked) span of the poles, due to the limited bond stresses present between the cracks [18,25]. Table 1 shows the high values Δε

_{CFRP,max}of the outermost (lowest) CFRP tendons in the quasi-static bending tests to failure (around 11‰), which demonstrate that the outermost tendon reaches considerable tensile stresses if one takes into account that the tendon strain resulting from prestress after 28 days is 7.3‰ (considering prestress losses after [25,32]).

**Figure 3.**Quasi-static bending behavior of CFRP prestressed HPSC pole No. 7: Comparison of measured with calculated moment vs. curvature relationships.

#### 4.1. Short-Term Bending Behavior

_{c,max}in Table 1, on top of which, the initial concrete strain due to prestress of 0.36‰ has to be added). Figure 3 shows the experimental moment vs. curvature diagram for pole No. 7, with the curvatures in the zone of pure bending derived from the strain measurements (DEMEC gauges) on the top and bottom edges during flexure. In the lower part of the diagram, the pole remained uncracked until the bending stresses overcame the centric prestress and the tensile strength of the HPSC (which was determined to be 10.9 ± 1.3 MPa in standard three-point bending tests of unreinforced spun cylinders of an age of 28 days). The kink in the moment-curvature curve was caused by the formation of the first bending cracks in the tensile zone of the central pole span. With the load increasing, the number of cracks in the central span increased, and the cracked zone developed from the central span to the shear spans. Therefore, the curvature (and deflection) increased considerably. Figure 4 shows a state of increased deflection for the test of pole No. 8 at 98% of the failure load.

_{11}= 180.7 GPa). The determination of the position of the neutral axis c is carried out by iteration of the equilibrium condition of the internal forces. The position of the neutral axis allows now for the calculation of the curvature dφ/dx based on the initially assumed compressive strain on the top edge. The internal moment can thus be calculated from the known compressive stress distribution in the HPSC and the tensile forces in the CFRP tendons. This procedure is repeated by adjusting the compressive strain on the top flange until the internal moment corresponds to the external bending moment, for which one aims to calculate the curvature. Figure 3 shows the comparison of the calculated curvatures following this method with the measured curvatures from the DEMEC readings in the central span of pole No. 7. A good agreement is observed between the calculated and measured curvatures. The pole’s failure moment for concrete crushing is then calculated (to M

_{R,calc}= 5.71 kNm) following the above procedure and considering a HPSC failure strain on the top flange of 6‰, a value that was experimentally determined in a series of additional flexural tests [25] and which is considerably higher than the usual crushing strains of high strength concretes with other mix designs and compacted by other means [36]. Note that the thus calculated crushing moment of the poles is only 1% lower than the experimentally determined value of M

_{R,exp}= 5.76 kNm for pole No. 7 (Figure 3) and 3% higher than the experimentally determined value for pole No. 8.

#### 4.2. Long-Term Bending Behavior Outdoors

_{2}O

_{3}sand coating from the CFRP tendon surface could be clearly observed at the northern tendon end and by carefully removing the HPSC cover at the tensile edge of the failed cross-section of pole No. 14 (Figure 7, left part of the tendon). Note that this highest loaded CFRP tendon of pole No. 14 (near the tensile edge) was strained to 13.6‰ (2457 MPa) at the beginning of the creep test and that 1.4 years before failure, its strain increased to 16.28‰ (2942 MPa). This increased tendon stress was calculated from the measured values of Δε

_{c,max}at the top edge and Δε

_{t,max}at the lower edge of the pole and shows the stress redistribution over the section due to bending creep and crack growth. It is hypothesized that this increased bottom tendon strain and the coating’s bond creep led to the CFRP anchorage failure by debonding of the Al

_{2}O

_{3}sand coating from the CFRP tendon surface after 16.5 years.

**Figure 6.**Failure of pole No. 14 due to slippage of the lowest CFRP tendon in the tensile zone at the north end of the pole, which led to local crushing of the high strength spun concrete.

**Figure 7.**Detail of the tensile edge of the failed cross-section of pole No.14 after removing the HPSC cover.

_{c}(t) is then calculated as a function of the time t under sustained load:

_{c}(ε

_{c}), measured for short-term loading [35]:

_{c}(ε

_{c}) is derived from a bilinear compression stress-strain relationship determined in [25].

_{c}(t, r

_{a}) is assumed. The elastic modulus of the HPSC after t is then estimated with Equation (3) under consideration of the creep coefficient given by Equation (1). These parameters allow the calculation of the strain-plane of the prestressed cylindrical cross-section under consideration of the force equilibrium condition, of the kinematic conditions of the slender beam and of the stress-strain relationships of CFRP and HPSC (3) under the assumption of the absence of bond creep. In particular, the cross-section analysis considered that the prestressed tendons in the compressive zone have a tensile pre-strain that is reduced by the flexural bending strain. All tendon forces (i.e., also for the tendons in the compressive zone) are taken into account in the section’s equilibrium of internal compression (in the concrete) and tension force. The iteration of ε

_{c}(t, r

_{a}) till equilibrium of the internal moment with the (external) bending creep moment is reached, provides the desired strain-plane of the cross-section after loading time t. During iteration, the failure strains of CFRP (21‰) and HPSC compressive edge (−6‰) are checked for. In accordance with [37], the assumption is made that the failure strains of CFRP and HPSC are independent of the creep time. The bending creep curvatures of poles Nos. 13 and 14 estimated following this procedure are compared with the experimentally determined curvatures in Figure 8. The calculated curvatures show a reasonably good agreement with the experimental ones. The high curvature after one year of sustained creep is shown in Figure 2 for the highest loaded pole, No. 14. Note that the rate of creep is initially lower (for the first six months) and subsequently higher than observed in practice. This is explained with the influence of the climate: the outdoor creep tests were started at the beginning of a hard winter, during which, temperatures stayed between −10 °C and +15 °C. This fact led to a low creep rate during the first six months of testing. With the higher summer temperatures after that (20 °C to 35 °C), the creep rate increased in the next six months. From the second testing year on, the creep deflection under sustained bending loads increased always during the warmer summer months.

_{2}O

_{3}sand coating of the CFRP surface: bond creep at the HPSC/CFRP interface is thus very limited over 16.5 years. Note that the bending creep moment applied to pole No. 12 represents a maximum long-term design bending load state [33]. This result confirms the above hypothesis that creep and shrinkage of the HPSC are controlling the long-term deformations of the pole specimens.

^{−3}), while a slight increase in average and maximum crack widths occurs with increasing time, reaching end values of 0.4 mm and 0.6 mm, respectively. Pole No. 12, which was loaded with over 80% of the short-term cracking moment, shows 10 very thin bending cracks (width 0.02 mm) after six months of testing. Six months later, the amount of thin cracks has increased to 17, a number that has been constant for the following 15.5 years. Note that the tensile edge of pole No. 12 is subjected to an initial bending stress of 8.8 MPa, which is very near to the average tensile strength of 10.9 MPa for the HPSC determined in three-point bending tests at 28 days.

## 5. Conclusions

- The short-term bending behavior of CFRP prestressed HPSC poles is bilinear with considerable rotation capacity at failure. The moment vs. curvature behavior and the failure moment for HPSC crushing can be modeled by a simple cross-sectional analysis following the beam theory of hybrid prestressed cross-sections.
- The long-term bending serviceability of CFRP prestressed HPSC poles is satisfactory for realistic service moments, represented by the lowest loaded pole, No. 12, in the test series presented. Long-term curvatures and deflections stabilize after six months of sustained loading. Furthermore, pole specimens under realistic long-term service moments showed crack patterns that were stable over time and minimal slippage of the tendons with respect to the pole’s end faces. The latter proves the successful and durable anchorage of the Al
_{2}O_{3}sand-coated CFRP prestressing tendons of this study in thin-walled precast concrete members under realistic long-term service loads. - Pole No. 14, which was loaded with twice the maximum long-term service moment, failed after 16.5 years, due to bond failure of the highest loaded CFRP tendon. The debonding of the Al
_{2}O_{3}sand coating from the CFRP tendon surface could be clearly observed. - The long-term evolution of curvatures due to bending creep of the poles could be modeled analytically with reasonable accuracy using a simple, direct analysis based on the assumption that HPSC creep governs the strain and stress redistribution over the cross-section with time.

## Acknowledgments

## Author Contributions

## Conflicts of Interest

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**MDPI and ACS Style**

Terrasi, G.P.; Meier, U.; Affolter, C. Long-Term Bending Creep Behavior of Thin-Walled CFRP Tendon Pretensioned Spun Concrete Poles. *Polymers* **2014**, *6*, 2065-2081.
https://doi.org/10.3390/polym6072065

**AMA Style**

Terrasi GP, Meier U, Affolter C. Long-Term Bending Creep Behavior of Thin-Walled CFRP Tendon Pretensioned Spun Concrete Poles. *Polymers*. 2014; 6(7):2065-2081.
https://doi.org/10.3390/polym6072065

**Chicago/Turabian Style**

Terrasi, Giovanni P., Urs Meier, and Christian Affolter. 2014. "Long-Term Bending Creep Behavior of Thin-Walled CFRP Tendon Pretensioned Spun Concrete Poles" *Polymers* 6, no. 7: 2065-2081.
https://doi.org/10.3390/polym6072065