Abstract Shape memory alloys (SMAs) are a class of smart materials that recover apparent plastic deformation (~6-8% strain) after heating, thus remembering the original shape. This shape memory effect (SME) can be exploited for self post-tensioning applications. NiTi-based SMAs are promising due to their corrosion resistance and resistance against low frequency/cycle fatigue failure. This study investigates self-post-tensioned (SPT) bridge girders by activating the SME of NiTiNb, a class of wide-hysteresis SMAs, using the heat of hydration of grout. Both NiTiNb and activation via hydration heat have yet to be explored for prestressing of concrete using SMAs.
First, the localized strain fields during the tensile stress-induced martensitic transformation in NiTiNb wide-hysteresis shape memory alloys are studied, and the material design and characterization of the SMA tendons are discussed. Then, the temperature increase due to the heat of hydration of four commercially available grouts is investigated. Pull-out tests are conducted to investigate the bond between the grout and SMA bar. The use of self post-tensioned SMA tendons in concrete girders will increase overall sustainability of bridge structures by (i) minimizing the susceptibility of post-tensioning tendons to corrosion; (ii) enabling the adjustment of prestressing force during service life; and (iii) simplifying the tendon installation.
Prestressed concrete is a construction method where permanent compressive stresses are created in a concrete structure to counteract tensile stresses induced by externally applied loads. By prestressing the concrete, which is weak in tension, it is ensured that the structure remains within its tensile and compressive capacity. Two common techniques of prestressing are pretensioning and posttensioning. In pretensioning, prestressing tendons are tensioned prior to casting concrete and the tendons are released upon hardening of concrete. When the tendons are put in tension after concrete placement, the process is called post-tensioning. In post-tensioning, the tendons are placed in pre-positioned ducts, stressed through jacking and anchored at the ends of the concrete member once the concrete has hardened. The duct is then grouted to ensure bonding of the tendon to the surrounding concrete, to protect the tendons from corrosion and to improve the resistance of the member to cracking .
Post-tensioned (PT) structural elements are used quite often in bridges due to their ability to span long widths economically while providing an aesthetically pleasing structure. PT systems are also preferred in bridge construction because they greatly increase structural capacities and are fairly easy to implement effectively. Although PT systems provide many advantages for designers and constructors, these systems have raised concerns regarding corrosion of the PT tendons. The degree of corrosion of PT tendons is critical to the structural performance of PT systems and the cost to replace tendons can exceed several hundred thousand dollars per tendon.
Over the past decade, there has been an increasing interest in the use of shape memory alloys (SMAs) for various civil engineering applications . SMAs are a class of metallic alloys that can remember their original shape upon being deformed. This shape recovery ability is due to reversible phase transformations between different solid phases of the material. The phase transformation can be mechanically induced (superelastic effect) or thermally induced (shape memory effect). Besides their ability to recover large strains with minimal residual deformations, SMAs possess excellent corrosion resistance, good energy dissipation capacity, and high fatigue properties. Superelastic SMAs can undergo large strains, in the order of 7 to 8%, and recover these deformations upon removal of stress. Due to their excellent re-centering and good energy absorbing capabilities in passive nature, the superelastic SMAs have been considered in a number of seismic applications [3-6].
SMAs that exhibit shape memory effect (SME) generate large residual deformations when the material is mechanically loaded over a certain stress level and unloaded. However, SME SMAs recover those residual strains upon being heated. Several researchers have explored the use of SME SMAs as actuators for active vibration control [7-8]. Since relatively large amount of material needs to be activated in very short time to generate an active control force, the application of SME SMAs for seismic control of civil structures has been mostly limited to theoretical studies. Several attempts have been made to use SME SMAs for active confinement of reinforced concrete (RC) columns. Andrews et al.  investigated the feasibility of using SME SMA
*Research supported by Mid-Atlantic University Transportation Center. O. E. Ozbulut is with the Department of Civil and Environmental Engineering, University of Virginia, Charlottesville, VA 22901 USA (phone: 434-924-
7230; fax: 434-982-2951; e-mail: firstname.lastname@example.org). M. Sherif is with the Department of Civil and Environmental Engineering, University of Virginia, VA 22901 USA (e-mail: email@example.com). R. F. Hamilton is with the Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA 16802 USA (e-
mail: firstname.lastname@example.org). A. Lamba is with the Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA 16802 (e-mail:
Feasibility of Self-Post-Tensioned Concrete Bridge Girders Using Shape Memory Alloys*
Osman E. Ozbulut, Muhammad Sherif, Reginald H. Hamilton and Asheesh Lanba
spirals to retrofit RC bridge columns through experimental and analytical studies. They found that high recovery stress associated with shape recovery of SME SMAs can be utilized to apply an active confinement pressure on concrete columns to significantly improve the strength and ductility of the columns. In another experimental study, Choi et al.  studied the bond behavior of concrete actively confined by SME SMAs by conducting monotonic and cyclic tests. The potential use of thermally induced SMAs to prestress concrete has been another research topic.
Maji and Negret  were the first to utilize the SME in NiTi SMAs to induce prestressing in concrete beams. SMA strands were pretensioned into the strain-hardening regime and then embedded in small-scale concrete beams. Once the beams were cured, the SMA strands were activated by the applied heat. El-Tawil and Ortega-Rosales  tested mortar beam specimens prestressed with SMA tendons. They considered two types of SMA tendons: 2.5 mm and 6.3 mm diameter wires. Test results showed that significant prestressing could be achieved once the SMA tendons were heat-triggered. Sawaguchi et al.  investigated the mechanical properties of mini-size concrete prizm specimens prestressed by Fe-based SMAs. Li et al.  examined the performance of concrete beams with embedded SMA bundles. Through an extensive experimental program, they studied the development of smart bridge girders that can increase their prestressing force to resist the excessive load as needed. In all of these studies, SMA tendons are triggered by an electrical source.
This paper investigates the development of self-post-tensioned (SPT) bridge girders by activating SME of SMAs using the heat released during grout hydration. First, self-post-tensioning with SMA tendons and the required conditions on the transformation temperatures of the SMAs are discussed. Then, the design of NiTiNb wide-hysteresis SMAs for self-post-tensioning application and material characterization are described. Heat of hydration of different commercially available grout products is studied to measure the temperature increase during grouting and find the optimum grout composition. The bond strength between the SMAs and the grout is investigated through pull-out tests. Using NiTiNb SMAs as post-tensioning tendon instead of conventional steel tendons will not only address the critical problem of corrosion-induced deterioration, but also will greatly simplify the construction and enable adjusting the pre-stress level as needed during the service life of concrete bridge structures.
II. SELF-POST-TENSIONING WITH SMAS
SMAs have two main microstructural phases, which have different atomic crystal structures. One is called martensite that is stable at low temperatures and high stresses and the other is called austenite that is stable at high temperatures and low stresses. The key characteristic of SMAs is a solid-solid, reversible phase transformation between martensite and austenite phases. SMAs have four characteristic temperatures at which phase transformations occur: (1) the austenite start temperature As, where the material starts to transform from twinned martensite to austenite, (2) austenite finish temperature Af, where the material is completely transformed to austenite, (3) martensite start temperature Ms, where austenite begins to transform into twinned martensite, (4) martensite finish temperature Mf, where the transformation to martensite is completed. If the temperature is below Ms, the SMA is in its twinned martensite phase. When a stress above a critical level is applied, the material transforms into detwinned martensite phase and retains this phase upon the removal of the load. It can regain its initial shape when the SMA material is heated to a temperature above Af. Heating the material above Af results in the formation of the austenite phase and a complete shape recovery. By a subsequent cooling, the SMA transforms to initial twinned martensite phase without any residual deformation. Figure 1 illustrates the shape memory effect on a