Behavior of Concrete Driven by Uniaxially Embedded Shape Memory Alloy Actuators Zongcai Deng 1 ; Qingbin Li 2 ; Anquan Jiu 3 ; and Lei Li 4 Abstract: The behavior of a concrete specimen uniaxially embedded with a shape memory alloy SMA wire actuated by electrical current is studied in this paper. The main factors influencing the axial strain of the concrete specimen were examined through a sophisticated experimental program. The experimental results indicate that the SMA actuator can be employed as a way of prestressing for axial concrete specimens and can be used for controlling axial strain of concrete specimens. Also the influences of initial prestrain of SMA wires, modes activating electrical current, the actuation times, etc., on the axial strain behavior of concrete specimen are significant. DOI: 10.1061/ASCE0733-93992003129:6697 CE Database subject headings: Concrete; Compression; Prestressing; Temperature. Introduction Improvements in materials science, engineering technology, data acquisition and processing, and computer technology have made it possible to apply sophisticated active control to structures. Among the possible actuators for smart structures, shape memory alloy SMA is widely used for its shape memory effect and pseu- doelasticity Chandhay and Rogers 1989, 1991. SMA wire has many advantages over other actuators, such as a larger load ca- pacity, a higher recovery strain in the order 8 –10%, excellent fatigue performance, convenient accommodation with concrete or steel, a variable modulus with phase transformation state, etc. Much effort has been made to apply the shape recovery force of SMA actuators to modify structural response, such as vibration control Baz et al. 1990; Rogers 1990; Chen and Levy 1996; Bin et al. 2000, acoustic control, bucking, and postbucking control Sup et al. 2000, shape control of composite structures Liang and Rogers 1990; Brett et al. 1998, reinforcement of composite materials and structures using fine NiTi wires Liang et al. 1997; Kawai et al. 1999, seismic isolation elements for civil engineer- ing Graesser and Cozzarell 1991; Liu and Singh 1995; Thomson et al. 1995; Shiba et al. 1998; Krzysztof et al. 2000, and dynamic behavior control of composites materials and structures Baz and Ro 1992; Santis et al. 1997. Only limited work on the deforma- tion control of concrete specimens with SMA wire actuators can be found in the current available literature Maji and Negret 1998. Maji Maji and Negret 1998 performed experimental re- search on active deflection control using mortar beams with ec- centrically embedded SMA wire actuators, the results of which showed that the recovery strain of SMA could be used to generate an active prestress force in concrete or adjust the deformation for concrete specimens on an as-needed basis. The behavior of a uniaxial concrete specimen with symmetrically embedded SMA wires has not been reported to date. We consider it useful to determine the interaction mechanism between SMA and the host material because the stress state of a uniaxial specimen is simple. The objective of this study is to investigate the prestressing char- acteristics of a uniaxial concrete specimen with symmetrically embedded NiTi actuators. Many factors affecting the behavior of concrete specimens, such as mode of actuating electrical current, initial prestrain of SMA, actuation times, and initial ambient tem- perature of air, were examined through experiments. The experi- mental results indicate that the axial strain or the prestressing state of a concrete specimen can be adjusted easily by changing the value of electrical current intensity or the actuating time of the SMA actuator. Experimental Program Materials The materials for the host concrete used in this paper were type I Portland cement, crushed limestone aggregate with a maximum size of 15 mm, and fresh river sand. The mix proportion by weight was cement: aggregate: sand: water=1.0:1.88:1.14:0.36. In this mix, a high-rang water reducer was used to achieve good workability. The compressive strength of the concrete cubic specimen for 28 days was 21.5 MPa, the modulus of elasticity was 18.7 GPa, and the linear coefficient of expansion was 0.71 10 -5 /°C. The SMA used in these tests was Ti-50 wt % Ni wire, its maximum recovery strain is  L =8.1%. The material properties for SMA wires which are not prestrained are the same as those published in Brinson 1993 and listed in Table 1. NiTi wires were all heat-treated to 500°C and had shape memory effect ac- 1 Postdoctoral Research Fellow, Dept. of Hydraulic Engineering, Tsin- ghua Univ., Beijing 100084, P.R. China; currently, Associate Professor, School of Civil Engineering, Beijing Polytechnic Univ., Beijing 100022, P.R. China. 2 Professor, Dept. of Hydraulic Engineering, Tsinghua Univ., Beijing 100084, P.R. China. 3 Postgraduate Research Assistant, Dept. of Hydraulic Engineering, Tsinghua Univ., Beijing 100084, P.R. China. 4 Doctoral Research Assistant, Dept. of Hydraulic Engineering, Tsing- hua Univ., Beijing 100084, P.R. China. Note. Associate Editor: Franz-Josef Ulm. Discussion open until No- vember 1, 2003. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this techni- cal note was submitted for review and possible publication on November 6, 2001; approved on September 5, 2002. This technical note is part of the Journal of Engineering Mechanics, Vol. 129, No. 6, June 1, 2003. ©ASCE, ISSN 0733-9399/2003/6-697–703/$18.00. JOURNAL OF ENGINEERING MECHANICS © ASCE / JUNE 2003 / 697