1

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.

I. INTRODUCTION

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 [1].

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 [2]. 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. [9] 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: ozbulut@virginia.edu). M. Sherif is with the Department of Civil and Environmental Engineering, University of Virginia, VA 22901 USA (e-mail: mms4bx@virginia.edu). R. F. Hamilton is with the Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA 16802 USA (e-

mail: rfhamilton@psu.edu). A. Lamba is with the Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA 16802 (e-mail:

aul179@psu.edu).

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. [10] 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 [11] 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 [12] 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. [13] investigated the mechanical properties of mini-size concrete prizm specimens prestressed by Fe-based SMAs. Li et al. [14] 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 stress-strain curve and a temperature diagram.

STR

ESS Detwinned

Martensite

Twinned Martensite

HEATING

STRAIN

TEM

PER

ATU

RE

Twinned Martensite

DetwinnedMartensite

Austenite

Ms

Mf

As

Af

LOADING

HEA

TIN

GCOOLING

DetwinnedMartensite

UNLOADING

DetwinnedMartensiteAustenite

COOLING

LOADING

UNLOADING

Figure 1. Shape memory effect

Significant heat is generated during the hydration of cement products. Numerous factors such as the type and composition of cement, the proportion of the mix, and the ambient temperature affect the heat evolution during the hydration process. In concrete structures, internal temperatures of 70°C are not uncommon [14]. In grouting applications, higher temperatures can be developed since grout is generally composed of very high portion of cement and heat dissipation is mostly restricted [15].

Therefore, heat of hydration of grout can be used to trigger the SME of SMAs to obtain SPT concrete members. Figure 2 shows the process for development of the SPT concrete girder using SMAs. First, the SMA tendons, in the martensitic state, are pre-strained. Then, concrete is poured and after hardening, the SMA tendons are installed in post-tensioning ducts. The void between the duct and the SMA tendons is then filled with grout. Due to the heat of hydration of grout, the temperature of the SMA tendons increases which induces the transformation to austenite when the temperature is over the As. A complete transformation to austenite phase occurs when the temperature reaches the Af. As the SMA tendons attempt to return back to their original shorter length, while being constrained at both ends, a tensile stress is produced in the tendons, causing pre-stress in the concrete girder.

Austenite

DetwinnedMartensite

TwinnedMartensite

STEP 1

STEP 2

Pre-stretch original SMA while in

martensite phase

Cast concrete, and install the tendon in post-tensioning duct

Fill the duct with grout, and trigger the tendons using the

heat of hydrationSTEP 3

Figure 2. Self-post-tensioning Process

The conditions on the phase transformation temperatures and the required temperature window (service temperature) for self-stressing application are shown in Figure 3. First, the As should be larger than the highest possible ambient temperature as the pre-strained SMA tendons must stay in the martensite state at ambient temperature. This will prevent pre-stretched SMA tendons from recovering their deformations at the storage temperature or during the installation of tendons to the concrete member. Second, the Ms should be below the lowest possible ambient temperature. This will ensure that the heated SMA tendons maintain their recovery stress after cooling to the ambient temperature. If the temperature of the SMA tendons becomes lower than the Ms, the SMA tendons will lose their recovery stress due to a phase transformation to martensite. This requirement for Ms coupled with the aforementioned requirement for As necessitate the use of the current NiTiNb class of wide-hysteresis (i.e. TH = As - Ms) SMAs. Furthermore, the Af should be as close as possible to the As, which requires minimizing the differential TM-A = Af - As, to complete the phase transformation using the hydration heat. When the temperature rises over the As, the SMA tendons start to transform to austenite, and thus recovery stresses are induced. However, the maximum recovery stress will not be obtained until the microstructure is completely austenitic, at a temperature over the Af.

Af

As

Ms

Mf

Temperature

Martensite fraction

100%

0%

AusteniteTwinned

Martensite

Should be over maximum ambient temperature

Should be below minimum ambient temperature

Service Temperature Should be close

to the As

Figure 3. Phase transformation temperatures of SMAs

III. MATERIAL DESIGN

In general, the microstructure of cast NiTiNb alloys consists of a matrix and the constituent phases β Nb- particles in a eutectic mixture with the matrix and Nb-rich nano-precipitates in the matrix [17]. Only the matrix undergoes the MT. The matrix contains mostly Ni and Ti with a smaller amount of Nb having a B2 type structure [18]. A eutectic structure is present and authors indicate that β-Nb rich phase (aka Nb-rich particles) are “in” the eutectic. This eutectic mixture forms a net-like configuration in the matrix phase. These particles have a BCC structure [18]. Siegert et al. found that these β-Nb rich particles form at the grain and sub-grain boundaries [19]. Nb-rich nano-precipitates have been differentiated from the β-Nb rich particles. In Figure 4 (a) two regions are seen, a light region labelled ‘X’ and a darker region labelled ‘Y’. The region Y is the α NiTi phase. The lighter region X has features that are indicative of a eutectic structure. Observe Nb-rich particles that exhibit a granular structure from those that exhibit a lamellar structure.

A"

(a)$

Y$

X$

Zooming$in$on$this$area$

(b)$Y"

X"

Granular$Structure$

Figure 4. Scanning electron micrographs of NiTiNb in the as-cast ingot form (a and b).

During thermo-mechanical processing (e.g. hot or cold working, rolling, and extrusion), the net-like eutectic structure breaks down and the β Nb-particles align in the primary processing direction. Severe deformation takes place in the primary NiTi(Nb) matrix phase and the eutectic structure consisting of Ni-Ti(Nb) and β-Nb particles during processing, resulting in a more uniform microstructure than as-cast samples. Figure 5(a) shows the as-cast microstructure for Ni47.3Ti44.1Nb8.6 (at%). The microstructure consists of the eutectic mixture (lighter net-like area) in the matrix phase (large darker areas). Figure 5(b) shows a zoomed in view of the eutectic and matrix of the dashed-red box in Figure 5(a). This image shows that the β Nb-rich particles in the eutectic exhibit a lamellar and globular structure. Figure 5(c) and (d) shows the microstructure of rolled Ni47.7Ti43.5Nb8.8 (at%) material. The images show that rolling breaks down the eutectic structure and the β Nb-rich particles have been elongated and oriented in the rolling direction. The particles are now mostly ellipsoidal or globular in shape. Owing to the vastly different microstructures in as-cast and processed NiTiNb materials, the deformation of martensite, as well as the martensitic transformation, can proceed very differently in the as cast versus as processed microstructures.

Figure 5. The microstructure of microstructure of as-cast Ni47.3Ti44.1Nb8.6 (at%) at (a) 9000X and (b) 100000X of the region bounded by the dashed red box. The microstructure of rolled Ni47.7Ti43.5Nb8.8 (at%) is shown at (c) 10000X and (d) 100000X of region bounded by the dashed red box.

The material response was characterized using constrained recovery experiments conducted on Ni47.7Ti43.5Nb8.8 (at%) rolled strip material. The material was first pre-strained to a set percentage of strain and then unloaded. The displacement of the actuator on the load frame wass then fixed, and the material was heated. As the material wants to recover the residual deformation via the SME but was constrained, recovery stresses were generated in the material. In an attempt to lock martensite in the material at room temperature, the specimens were dipped in liquid nitrogen at around -194 °C prior to pre-straining at room temperature. The geometries of the specimens are different: the one strained to 14% strain is a dog-bone specimen (Type A) whose gage section is 57.2 mm long and 4.2 mm wide, and the second one strained to 15% is a flat specimen (Type B) which is 241 mm long. The strain in Figure 6(a) was found using the displacement of the actuator. The pre-straining results in Figure 6(a) show that the deformation during loading takes place in three stages: a linear elastic response, followed by a stress drop onto a stress plateau, and finally a strain hardening type response. After unloading, both specimens have residual strains: 10 % for the type A dog-bone specimen and 11% for the type B specimen.

The constrained recovery stress versus temperature result for these specimens is shown in Figure 6(b) during heating. The results show that recovery stress begins to be generated instantly upon heating, and saturates at around 102.8 °C for the type A specimen and around 138.4 °C for the type B specimen. As the specimens were cooled back down to RT, a small amount of pre-stress loss takes place. From the constrained recovery stress versus time plots in Figure 6(c), the maximum recovery stress generated for the type A specimen is 534.4 MPa with a pre-stress loss of 22.1 MPa for a final recovery stress of 512.3 MPa. The maximum recovery stress generated for the type B specimen is 505.2 MPa with a pre-stress loss during cooling down to RT of 68.1 MPa for a final recovery stress of 437.1 MPa. These results indicate that NiTiNb SMAs can produce a prestress more than 500 MPa for self-stressing applications.

Figure 6. The constrained recovery results for rolled strip Ni47.7Ti43.5Nb8.8(at%) material. (a) Stress-Strain curves during pre-straining at RT. (b) The recovery stress vs temperature during heating. (c) The recovery stress vs time during heating and subsequent cooling to RT.

IV. HEAT OF HYDRATION OF GROUT Portland cement, potable water and along with any admixtures to obtain required properties are the basic grout materials.

The chemical reaction between Portland cement and water is exothermic, i.e. producing heat. This heat is called the heat of hydration. In order to determine the temperature increase during grouting post-tensioning ducts, four commercially available tendon grouts were tested: (1) Euco Cable Grout PTX (Euco), produced by Euclid Chemical; (2) SikaGrout 300 PT (Sika), produced by Sika Corp.; and (3) Five Star Special Grout 400 (Five Star), produced by Five Star Products, Inc.; (4) MasterFlow 1205 Grout (MasterFlow), produced by BASF Chemical. All grouts were prepackaged and approved by Virginia Department of Transportation for post-tensioning applications. The water-to-grout ratios for each commercial grout were set per manufacturer’s direction and given in Table 1. To prepare test specimens with each grout, a mixing cylinder was cleaned and a bag of selected grout and the required water were placed in the cylinder. The contents were mixed in the cylinder for 3 minutes with a variable speed high shear mixer and the resulting grout mixture was poured into a 102 x 203 mm (4 x 8 inch) cylinder with a thermocouple attached to a single tendon placed in the center. The thermocouple was connected to a data logger that monitored the temperature of the curing grout every minute for 48 hours.

TABLE I. SUMMARY OF GROUT TEMPERATURE TEST RESULTS

Specimen Grout Water-to-

Grout Ratio

Initial Temperature (°C)

Maximum Temperature (°C)

Temperature Increase (°C)

S1 Euclid 0.25 21 41 20 S2 Euclid 0.25 22 41 19 S3 Euclid 0.25 21 41 20 S4 Sika 0.24 21 48 27 S5 Sika 0.24 22 53 31 S6 Sika 0.24 21 48 27 S7 Five Star 0.25 22 41 19 S8 Five Star 0.25 22 41 19 S9 MasterFlow 0.27 22 41 19

S10 MasterFlow 0.27 22 41 19 S11 MasterFlow 0.32 22 40 18 S12 MasterFlow 0.32 22 40 18

The time versus grout temperature plots for each specimen as well as ambient temperature are given in Figure 7. Three specimens were prepared and tested for Euco and Sika grouts on three different days. The results for Euco grout are consistent for each specimen. The highest temperature recorded during curing is 41°C, which indicates a temperature increase of 19°C to 20°C from initial temperature of 21°C to 22°C depending on specimen due to the heat produced by the cement hydration. The temperature of Sika grout reaches 48°C for two specimens and 51°C for one specimen. At three different tests of Sika grout, the average temperature increase is 28°C. The peak temperature and average increase in temperature for Five Star and MasterFlow grouts are similar to the results obtained from Euco grout. For MasterFlow grout, two samples at two different water-to-grout ratios were tested. It is observed that the peak temperature is slightly higher and occurs a few hours earlier when a lower water-to-grout ratio is used (Figure 7d). The grout temperature reaches its peak value at 10 to 18 hour after casting for Euco, Sika and MasterFlow grouts whereas the peak temperature occurs at 2.5 hour after casting for Five Star grout. For all specimens, the grout temperature reduces to values between 22°C and 24°C near 30 hour after casting and remained almost constant thereafter. The results of experimental tests conducted to characterize the grout temperature during curing are summarized in Table 1. These results suggest that a commercially available tendon grout (Sika grout) can provide an average of 28°C increase in temperature during the hydration, process, which can be used to activate SMA tendons.

0 5 10 15 20 25 30 3515

20

25

30

35

40

45

Time (hour)

Tem

pera

ture

(°C

)

EucoS1S2S3S1−AmbientS2−AmbientS3−Ambient

0 5 10 15 20 25 30 35

15

20

25

30

35

40

45

50

55

Time (hour)

Tem

pera

ture

(°C

)

SikaS4S5S6S4−AmbientS5−AmbientS6−Ambient

(a) (b)

0 5 10 15 20 25 30 35 40 4515

20

25

30

35

40

45

Time (hour)

Tem

pera

ture

(°C

)

Five Star

S7S8S7/8−Ambient

0 5 10 15 20 25 30 35 40 45

15

20

25

30

35

40

45

Time (hour)

Tem

pera

ture

(°C

)

MasterFlow S9

S10S11S12S9/10/11/12−Ambient

(c) (d) Figure 7. Temperature measured in different commercially available grouts during curing

V. PULL-OUT TESTS To investigate the bond behavior of SMA bars with grout, pull-out tests were conducted. SMA bars with a diameter of 3.5

mm were cut into 220 mm segments using a cutoff wheel. Two 102 x 102 mm (4 x 4 inch) cylindrical molds were used to manufacture pull-out specimens. Holes with a diameter of 3.5 mm were drilled at the center of the top and bottom of the molds to allow SMA to pass through. The SMA bar was secured at the bottom hole of each mold and Sika grout was poured inside the mold. The specimens ere left to cure for 3 days. Figure 8(a) shows a schematic diagram of the specimen used for the pull-out test.

Since it is difficult for standard grips to fully hold on to SMA bars because of their small diameter size, special aluminum sleeves were fabricated. Two 50 mm-long sections were cut from a 10-mm aluminum rod. These sections were then placed on a lathe, and a 3.5-mm hole was drilled all the way through. The sleeve was then attached to the specimen by means of twisting since the aluminum sleeve hole was a little bit smaller than the diameter of the SMA bar. The tight fit was useful to establish a mechanical interlock to help the SMA resist slippage out of the sleeve during testing.

The pull-out tests of the SMA bar conducted using an MTS servo-hydraulic load frame. The specimen was held in place by a testing cage attached to the top head of the load frame by a large bolt. The load was applied to the SMA bar at a rate of 0.075 mm/s and measured by a built-in load cell of the load frame. The slip of the SMA bar relative to grout were measured using Digital Image Correlation (DIC) method at the loaded and free ends of the specimen. Two cards with a speckle pattern were attached to the loaded and free ends of the SMA bar. The bottom card was attached directly at the end of the grout cylinder to reduce any errors in the calculation of slippage due to strains in the SMA. The optical system capture the movement of speckle patterns on the cards and provide an output of the average vertical movements at each time step. Figure 8(b) shows pull out test set-up.

Figure 8. (a) Pull-out test specimen and (b) test-set-up

Two specimens were tested and the applied tensile force and the slip of the bar were recorded. Bond strength is defined as the shear force per unit surface area of the bar and calculated by the following equation:

τ =Tπdbl b

(1)

where T is the tensile load on the SMA bar, db is the nominal bar diameter, and lb is the embedment length of the bar. Figure 9 illustrates bond stress-slip curves both at the loaded and free ends of the SMA bar. The bond behavior is characterized by an initial increase in the bond stress up to 1.2 MPa for the first specimen and up to 1.4 MPa for the second specimen, and with insignificant slippage and a softening thereafter. Since SMA bars had a very smooth surface, mechanical bearing forces were very low and the load transfer was primarily provided by friction. The bond stress is important to ensure that the strains are uniformly distributed along the length of the SMA bar. Yet, for post-tensioning applications, the recovery stresses in the SMA bar are transferred through the anchorage system at the end of the beam even if the bond strength is low.

0 10 20 30 400

0.2

0.4

0.6

0.8

1

1.2

1.4

Bond

Stre

ss (M

Pa)

Slip (mm)

Loaded End

0 10 20 300

0.2

0.4

0.6

0.8

1

1.2

1.4

Bond

Stre

ss (M

Pa)

Slip (mm)

Free End

(a)

0 10 20 30 400

0.2

0.4

0.6

0.8

1

1.2

1.4

Bond

Stre

ss (M

Pa)

Slip (mm)

Loaded End

0 10 20 300

0.2

0.4

0.6

0.8

1

1.2

1.4

Bond

Stre

ss (M

Pa)

Slip (mm)

Free End

(b)

Figure 9. Bond stress-slip relationship for SMA bars at free end and loaded end for (a) Specimen 1 and (b) Specimen 2

VI. CONCLUSION This paper investigates the feasibility of activating SMA tendons using heat of hydration of grout in order to develop self-

post-tensioned concrete bridge girders. Material characterization tests were conducted on two types of NiTiNb SMAs. It was shown that a recovery stress more than 500 MPa could be achieved after cooling to ambient temperature. Further studies are currently being conducted to optimize the phase transformation temperatures for self-post-tensioning application. The increase in temperature during the hydration of four commercially available grouts was evaluated. A typical 102 x 203 mm (4 x 8 inch) cylinder specimen was filled with the grout. A thermocouple and a data acquisition system were employed to measure the temperature during 48 hours. The time versus temperature plots were created for each tests. It was observed that the highest temperature increase was for the Sika grout. At three different tests of Sika grout, an average of 28°C temperature increase was observed. The temperature increases for other grouts were between 18°C and 20°C. The results indicate that a commercially available grout can provide considerable temperature increase during the heat of hydration of grout, which can be used to activate SMA tendons. In addition, two pullout tests were conducted on cylindrical specimens to investigate the bond between the grout and SMA bar. The tests concluded that the maximum pullout load is between 1.4 to 1.6 kN and a bond stress between 1.2 and 1.4 MPa for 3.5-mm plain SMA bars.

The use SMA tendons, which possess high fatigue and corrosion resistance, as post-tensioning elements in concrete girders will provide and increase the service life and life-cycle cost savings for concrete bridges. The replacement of steel tendons with SMA prestressing tendons will prevent corrosion-induced deterioration of tendons in concrete structures. The use of heat of hydration of grout to activate the shape memory effect of SMA tendons will provide self-stressing capability. This will greatly simplify the tendon installation. The need for jacking equipment or electrical source will be eliminated. This will also enable to form the reinforcement in any shape in two or three-dimensional space without special devices. The use of SMA tendons will lead to better control of the prestress and higher levels of effective prestress. During the service life of a bridge, heat-triggering the SMA tendons can repair possible damage observed in girders and reduce excessive deflections.

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