ACTIVE CONFINEMENT

Active confinement of concrete columns through SMAs is a particular case of the prestressing technique, but with a conceptual difference of the function performed by the strengthening material (the SMA) with respect to the above-mentioned cases (figure 2.24).

Active confinement would make sense in the rehabilitation of existing structures, when it is necessary to gain ductility, but the cost of SMAs would hardly justify this application in planning new structures (Janke et al. 2005). The first tests on active confinement with SMAs date back to the year 2000 (Krstulovic-Opara & Thiedeman 2000), and it is in recent years that most experimental results have been published (Choi et al. 2008), (Andrawes et al.

2010), (Shin & Andrawes 2010), (Park et al. 2011), (Dommer & Andrawes 2012), (Shin &

Andrawes 2012), (Shin & Andrawes 2013), (Shin & Andrawes 2014), (El-hacha &

Abdelrahman 2015), (Tran et al. 2015) (Tavousi Tehrani et al. 2017), (Tazarv & Saiidi 2017), (Chen & Andrawes 2017a).

In 2008, (Choi et al. 2008) studied the behavior of concrete test specimens confined with 1 mm diameter wires. They used two Ni-Ti alloys: one in martensite phase with a prestrain of 3%, which actively compressed the test specimens upon heating through an electrical circuit;

and a second Ni-Ti alloy in austenite phase. The active confinement of the first case slightly increased the concrete compressive strength, with a significant rise in ductility. The confinement effect produced with the austenite alloy was similar. It is worth noting that the geometrical imperfections of the wire produced significant losses after cooling of the martensite, which undoubtedly decreased the confining stress.

(Shin & Andrawes 2010) published the results of an experimental study on the feasibility of the use of Ni-Ti-Nb spirals to apply active confinement pressure. The used alloy enabled recovery stresses of 565 MPa to be mobilized at 108 ºC with a prestrain of 6.4 %, maintaining a residual recovery stress of 460 MPa at ambient temperature. Uniaxial compression tests were carried out in actively (SMA) and passively (GFRP) confined concrete test specimens.

Through active confinement, the ultimate strength increased 21% compared to identical plain concrete test specimens; with the ultimate strain rising 24 times (figure 2.25). In the experimental campaign, they also studied the use of GFRP fabrics with SMAs, which enabled delaying the rupture of the GFRP.

State of the art

Figure 2.24. Schematic drawings of a confined concrete column uniaxial compressive test: a) Concrete element confined with SMA spirals, b) Cross-section before heating, c) Cross-section after heating, and d)

Uniaxial compressive test stress–strain path comparison between confined and unconfined test. From (Cladera, Oller, et al. 2014), adapted from (Shin & Andrawes 2010)

Figure 2.25. Stress-strain relationship for test specimens without confinement, confined with SMAs, GFRP, or SMA-GFRP hybrid fabrics (Cladera, Oller, et al. 2014)

A subsequent study (Park et al. 2011), also using a Ni-Ti-Nb alloy, revealed that the load and unload cycles that produced strains above the initial prestrain, led to the loss of residual stress of the prestressing upon unloading, therefore it was necessary to study this phenomenon in more depth. In 2012, (Choi, Nam, et al. 2012) reported a maximum recovery stress of 325 MPa for Ni-Ti-Nb wires (1 mm diameter and composition Ni - 47.75 %, Ti – 37.86 %, Nb – 14.69 %) with a hysteretic gap (As - Ms) of 122.5 ºC (Ms =-17.59 ºC) suitable for confining civil structures but concluded that not all Ni-Ti-Nb alloys are appropriate for post-tensioning applications, which is also likely to occur with different alloys, as when faced with load and unload cycles after initial prestressing, active confinement may be lost,

Chapter 2

leaving only passive confinement, which would also give the concrete greater resistance and ductility, although to a lesser extent than active confinement.

In 2012, (Dommer & Andrawes 2012) reported a maximum recovery stress of 574 MPa for Ni-Ti-Nb wires with an acceptable level of stability within a range of -10 ºC to 50 ºC. Other SMAs could be also suitable for this purpose as (Leinenbach et al. 2012) reported recovery stresses of about 500 MPa for Fe-based SMAs.

Ni-Ti-Nb wires have been successfully used to actively confine circular and non-circular columns to improve their behavior under axial compression forces and bending. (Shin &

Andrawes 2010; Dommer & Andrawes 2012) reported good performance on active confinement of RC columns (compression) by means of Ni-Ti-Nb spirals in retrofitting, i.e.

seismic retrofit of highway bridge columns (figure 2.26a).

(Choi, Choi, et al. 2012) ascertained good performance in a study of application of Ni-Ti-Nb martensitic SMA wires to retrofit for reinforced concrete columns with lap splices using wire jackets (figure 2.26b). Also, they compared the results with those of Ni-Ti wires with the same application. Furthermore, the study investigated the recovery and the residual stresses of Ni-Ti-Nb wires and its behavior. The study showed that the Ni-Ti-Nb wire jackets were more adaptive for retrofit of RC columns than the Ni-Ti wire jackets since the Ni-Ti-Nb disposed of more appropriate temperature windows for the applications of SMAs in civil structures.

Figure 2.26. a) Example of application of SMA spirals for active confinement of bridge piers (Dommer &

Andrawes 2012) and b) general view and cross section of lab specimen of RC columns retrofit by SMA wire jackets (Choi, Choi, et al. 2012)

State of the art

Recently, (Chen & Andrawes 2017a) investigated the cyclic behavior of Ni-Ti-Nb SMA confined concrete performing a series of uniaxial cyclic tests on confined concrete cylinders.

The test results showed that the effectiveness of Ni-Ti-Nb SMA confinement on strength and ductility enhancement increases as active confining pressure increases. Using the test results the authors developed an empirical stress-strain model for Ni-Ti-Nb SMA confined concrete, and proposed (Chen & Andrawes 2017b) a plasticity-based confined concrete constitutive model. This model is capable of predicting and simulating three-dimensional stress-strain behavior of Ni-Ti-Nb SMA confined concrete under both monotonic and cyclic loading. New hardening/softening function, dilation rate function, and damage parameters are derived and validated based on experimental results. The proposed model (figure 2.27) is able to closely simulate the axial and lateral stress-strain behaviors of the retrofitted cylinders. Therefore, novel confinement techniques using SMA spirals are proved to be promising for seismic retrofit of concrete structures.

(Chen et al. 2014) studied active confinement of concrete prisms using SMA wires. Their study focuses on developing a novel scheme for applying external active confinement to non-circular concrete elements that lack ductility using SMAs (figure 2.28). Concrete elements (prisms) were tested under monotonic and cyclic uniaxial compression loads. The compressive stress–strain relationships of the SMA confined concrete elements were examined and compared with the behaviors of elements confined with conventional GFRP jackets.

Figure 2.27. Comparison of Stress-strain relationship for proposed plasticity model and experimental results (Chen & Andrawes 2017b).

Chapter 2

Figure 2.28. Scheme of active confinement of non-circular prisms in (Chen et al. 2014)

The results clearly showed a significant improvement in the ultimate strain and residual (post-peak) strength of concrete elements actively confined with SMA compared to that of GFRP confined elements. The new technique clearly showed effectiveness in delaying the damage of concrete, which helped significantly in improving the ductility of non-circular concrete members.