Experimental Evaluation of Ductile Fiber Reinforced Cement-based Composite Beams Incorporating Shape Memory Alloy Bars Academic research paper on "Materials engineering"

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Procedía Engineering 79 (2014) 506 - 512

Procedía Engineering

www.elsevier.com/locate/procedia

37th National Conference on Theoretical and Applied Mechanics (37th NCTAM 2013) & The 1st International Conference on Mechanics (1st ICM)

Experimental evaluation of ductile fiber reinforced cement-based composite beams incorporating shape memory alloy bars

Chung-Chan Hunga, Wei-Ming Yena*

aDepartment of Civil Engineering, National Central University, No.300, Jhongda Rd, Taoyuan 32001,Taiwan

Abstract

The objective of the study is to explore the use of two promising materials, i.e., ductile fiber reinforced cement-based composites (DFRCCs) and shape memory alloys (SMA), in structural beams for enhanced seismic performance. DFRCCs are distinguished from regular concrete materials by their tensile strain hardening behavior accompanying by multiple hairline cracks. The unique tensile behavior in the material scale transforms into the enhanced ductility, shear and moment resistance, and damage tolerance in the structural scale. SMAs, a type of smart materials, are featured by their shape memory effect and superelasticity. The properties allow deformed SMAs with a strain as large as 8% to recover their original shape. The cyclic behavior of DFRCC beams reinforced by SMA bars is examined in the study. Four different beam specimens with experimental parameters, including DFRCCs, SMA bars, and bond strength between rebar and DFRCCs, are designed and tested. The results show that the use of DFRCCs to replace conventional concrete materials is able to enhance the energy dissipation capacity and failure pattern of the beam specimen. The reduced bond strength between rebar and the surrounding cement composites in the critical region of the beam is found to improve the ductility of the beam under displacement reversals. The beam specimen that employs SMA bars to replace regular steel bars in the longitudinal reinforcement exhibits appealing self-centering properties with very minor residual deformation after undergoing a drift response of 5%.

© 2014 ElsevierLtd. Thisisanopenaccess article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of the National Tsing Hua University, Department of Power Mechanical Engineering

Keywords: Ductile fiber reinforced cement-based composites; shape momory alloy bars; cyclic behavior; cantilever beams

1. Introduction

Ductile fiber reinforced cement-based composites (DFRCCs) and shape memory alloys (SMAs) are two materials that offer promising advantages for development of next-generation earthquake-resistant structures. DFRCCs are

* Corresponding author. Tel.: +3-422-7151 ext.34189; fax: +3-422-5960. E-mail address: bf9525@yahoo.com.tw

1877-7058 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of the National Tsing Hua University, Department of Power Mechanical Engineering doi:10.1016/j.proeng.2014.06.373

distinguished from conventional concrete materials from their strain-hardening behavior under tension accompanying by multiple narrow cracks. The unique tensile strain-hardening behavior in the material scale transforms into enhanced shear and moment resistance, damage tolerance, and energy dissipation capacity in the structural scale, compared to conventional concrete materials [1-5]. Depending on the mix design and types of fibers, the major crack of DFRCCs occurs at a tensile strain between 0.2% and 8%, which can be up to 100 times larger than that of conventional concrete materials. After formation of major cracks, DFRCCs exhibit strain-softening behavior. When under compression, DFRCCs also outperform conventional concrete materials in terms of strength and ductility. SMAs possess desirable characteristics for applications in areas of earthquake-resistant structures [6]. Nickel-titanium (NiTi) alloys, the main type of SMAs, have two unique properties: shape memory and superelasticity, resulting from transformation between austenite and martensite phases of the alloy triggered by temperature or stress variations [7]. Shape memory refers to the ability of NiTi SMAs to undergo deformation at one temperature, then recover its original shape upon heating above their transformation temperature. Superelasticity refers to the fully recovery of the deformed NiTi SMAs with a strain as large as 8% to its undeformed shape when the applied stress is released. The superelasticity of NiTi SMAs occurs at a narrow temperature range above the transformation temperature of the alloy. In addition, NiTi SMAs provide high strength, large energy dissipation capacity through hysteretic behavior, and high resistance to corrosion and fatigue.

The objective of the study is to investigate the seismic performance of DFRCC beams reinforced by SMA bars. Four slender cantilever beam specimens are designed with experimental parameters including SMA bars, DFRCCs, and bonding strength between steel bars and DFRCCs. The seismic performance of the beam specimens is assessed using cyclic loading tests. The performance of the newly developed beams is discussed in terms of hysteretic responses, failure patterns, and base rotations.

2. Test set-up

Four slender cantilever beams with dimensions of 25cm*35cm*280cm (width*height*length) and reinforcing details shown in Figures 1-3 are made. While one specimen is made of conventional concrete materials (termed RC), the other three are made of DFRCCs (termed ES, ESMA, and EST). The RC specimen, employed as the control specimen, is a conventional RC beam. The ES specimen is made of DFRCCs and reinforced using steel bars. The ESMA specimen is made of DFRCCs and uses SMA bars to replace the longitudinal steel bars in the plastic hinge region of the beam. In the EST, as shown in Figure 2, the longitudinal steel bars in the plastic hinge region of the beam are wrapped using rubber mastic tapes to reduce the bond strength between steel bars and DFRCCs, with an attempt to extend the plastic hinge region. For the reinforcement details of RC, ES, and EST, four #8 longitudinal steel bars are used. Due to the manufacturing limitation on the size of SMA bars, eight #5 SMA bars connected with steel bars via couplers are used as the longitudinal reinforcement in the ESMA. In particular, the SMA bars which are 82.5cm long are placed in the critical regions of the beam as shown in Figure 3. The mechanical properties of the SMA bars used are summarized in Table 1. All four specimens have the same design of lateral steel reinforcements.

12@7.5

6@15.42

Figure 1. Dimensions and reinforcing details of RC and ES specimens (unit: cm)

60. (Rubber Mastic Tape)

3@7.5 10 5

12@7.5 6@15.42

Figure 2. Dimensions and reinforcing details of EST specimen (unit:cm) _280_

82,5 (SMA)

5 10 3@15

12@7.5 6@15.42 3@7.5 10 5

Figure 3. Dimensions and reinforcing details of ESMA specimen (unit:cm)

■75—a

u_a_a_a

Table 1. Mechanical properties of NiTi SMAs

Superelastic Ni-Ti

Mechanical Properties SMA

Specimen 1

Tensile Strength (MPa) 925

Yield Strength(0.2% offset) (MPa 465

Elongation in 4D (%) 21.5

Reduction of Area (%) 30.5

The seismic performance of the beam specimens are investigated using cyclic-loading tests. Experimental setup is shown in Figure 4. One end of the specimen is clamped by two precast RC blocks connected using 10 prestressed bars. Vertical displacement reversals are applied to the other end of the specimen via a hydraulic actuator. The length of the tested region of the beam is 1850 cm. Figure 5 shows the locations of the installed linear variable displacement transducers (LVDT) on each beam specimen. The locations of the attached strain gauges on the reinforcing bars are shown in Figure 6.

Figure 4. Test set-up (unit: mm)

Figure 5. Locations of LVDTs

FT-V FT-3r FT-4r FT-5r FT-6r FT-7

FB-1 v

FB-2V FB-3 FB-4 FB-5 FB-fi FB-7

, 25 ,15 15 .15, 15 ,15.

li ill ill ii ,M il .I

Figure 6. Locations and notations of strain gauges (unit:cm)

3. Test results

Figure 7 shows the load-drift responses of the four specimens. The RC specimen, as shown in Figure 7(a), exhibits stable hysteretic responses before the drift reaches 8%. After the drift response is larger than 8%, the strength drops significantly. When the drift reaches 9%, the strength is less than 80% of the maximum strength. The benefits of using DFRCCs to replace conventional concrete in the beam specimen can be observed from the test results of ES plotted in Figure 7(b). It can be seen that the hysteretic loops of the ES specimen are obviously fatter than that of the RC specimen. In addition, after the maximum strength is reached at 1% drift , the strength stably maintains at the peak value as the drift response increases to 8%. The comparison of the load-drift responses of the ES (Figure 7(b)) and EST (Figure 7(c)) indicates that the reduced bond strength between the DFRCC and longitudinal steel bars at the plastic hinge region does not affect the strength capacity of the beam specimen. Nevertheless, after the maximum strength is reached, unlike the stable strength observed in the ES specimen, the strength of EST slightly decreases as drift increases. It is worth noting that after the EST reaches 10% drift, the pinching behaviour becomes obvious. The reduction of the bond strength in the critical region is found to considerably increase the ductility of the specimen. While both the RC and ES specimens fails at a drift of less than 9%, the EST specimen reaches a drift of more than 12% without obvious degradation in strength. The test on the EST specimen is terminated at a drift response of 12% due to the limitation of stoke of the wire gauge and hydraulic actuator. Figure 7(d) shows the influence of using SMA bars to replace the longitudinal steel reinforcement in the DFRCC beam specimen. It can be seen that after the applied load is fully unloaded in each loading cycle, the residual displacement is very minor. In particular, when the ESMA is fully unloaded from a drift of 5%, the residual drift is less than 0.05%.

10 8 6 4 2

3 0 ® i ta -2

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14

Drift (%)

(a) RC

10 8 6

§2 g 0 ¿5-2

-4 -6 -8 -10

-14-12-10 -8 -6 -4 -2 0 2 4 6 Drift (%)

8 10 12 14

10 8 6

) 4 ( 2 c 0 ' -2 -4 -6 -8 -10

IE S w

JKnT //

im h 7

/rmi wim m

m mi mm Va

10 8 6

(c) EST

Figure 7. Load-displacement responses of the specimens

Drift (%)

(d) ESMA

-14-12-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 Drift (%)

(b) ES

|esma|

Figure 8 shows the failure patterns of the specimens. It can be seen in Figure 8(a) that significant concrete spalling accompanying by widening cracks occurs at the plastic hinge region in the RC specimen. For the other three specimens that are made of DFRCCs, multiple hairline cracks are observed before major cracks widen. In additional, no obvious concrete spalling is observed for the three DFRCC beams. The ES specimen fails due to the facture of a longitudinal steel bar. In the EST specimen, the wrapped longitudinal bars at the base of the beam facilitate the development of two major plastic hinges, which are located at the two ends of the wrapped region. The additional plastic hinge leads to an improved ductility of the beam under displacement reversals, compared to other beams. It is observed in the test that before major cracks take place, a dense array of hairline cracks develops at the two plastic hinge zones whereas only a few cracks develop in the wrapped region. The ESMA specimen fails due to the fracture of the thread portion of the SMA bar connected with the rebar coupler, as shown in Figure 9. The fracture of the tread SMA bar is likely caused by the heat treatment in the rebar coupling process that changes the lattice of the SMA bar and thus reducing the ductility and strength.

(c) EST (12%) (d) ESMA (5%)

Figure 8. Failure patterns of the specimens

Figure 9. Fracture of the thread NiTi SMA bar

Figure 10 shows the cyclic base rotation - drift responses of the beam specimens. It can be seen that when the drift response researches 4%, the RC specimen has the smallest base rotation. The use of DFRCCs to replace the conventional concrete materials in the beam considerably increases the base rotation response to 0.023, as can be seen in Figure 10(b). For the EST and ESMA specimens, the rotation demand is further increased to be more than 0.03.

0.03 0.02 0.01 0

0.04 0.03 œ 0.02 g 0.01

g -0.01 « -0.02 -0.03 -0.04

Drift (%)

(a) RC (9%)

Drift (%)

(c) EST (12%)

~ 0.02

T 0.01

-2 0 Drift (%)

(b) ES (8%)

_ 0.03

w 0.02 s

-0.01 -0.02 -0.03

Figure 10. Base rotation - drift responses

-2 0 Drift (%)

(d) ESMA (5%)

4. Summaries and Conclusions

The study presented herein investigated the potential of using DFRCCs and SMA bars to replace conventional concrete materials and steel bars, respectively, in structural beams, with the aim of enhancing the seismic performance of beams. Cyclic loading tests on four different beam specimens were carried out. The results showed that using DFRCCs to replace conventional concrete in the beam leaded to enhanced energy dissipation capacity,

ductility, and failure pattern. In particular, while the conventional RC specimen failed due to excessive crack widening and concrete spalling, the DFRCC specimens showed a dense array of hairline cracks before crack localization. In addition, no obvious concrete spalling was observed when the DFRCC specimens failed. The reduced bond strength between the longitudinal steel bars and the surrounding DFRCCs facilitated development of an additional plastic hinge, leading to enhanced beam ductility under displacement reversals. Although the energy dissipation capacity of the DFRCC beam reinforced by SMA bars was substantially less than that of the other beams, the use of SMA bars significantly reduced the residual deformation of the beam specimen after experiencing displacement reversals.

Acknowledgements

The research described herein was sponsored in part by the National Science Council under Grant No. 102-2221-E-008-061. The opinions, findings, and conclusions expressed in this paper are those of the authors and do not necessarily reflect the views of the sponsor.

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