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1
Feasibility of self-prestressing concrete members 1
using shape memory alloys 2
3 4
5
6 Osman E. Ozbulut 7
Department of Civil and Environmental Engineering 8 University of Virginia 9
Charlottesville, VA 22901 USA 10 11
Reginald F. Hamilton 12 Department of Engineering Science and Mechanics 13
The Pennsylvania State University 14 University Park, PA 16802-6812 15
16 Muhammad Sherif 17
Department of Civil and Environmental Engineering 18 University of Virginia 19
Charlottesville, VA 22901 USA 20 21
Asheesh Lanba 22 Department of Engineering Science and Mechanics 23
The Pennsylvania State University 24 University Park, PA 16802-6812 25
26 27 28 29
30 Corresponding Author: O. E. Ozbulut, Department of Civil and Environmental 31
Engineering, University of Virginia, Charlottesville, VA 22901 USA (phone: 434-924-32 7230; fax: 434-982-2951; e-mail: [email protected]). 33 34 35
2
Feasibility of self-prestressing concrete members 36
using shape memory alloys 37 38
39
Abstract 40
Shape memory alloys (SMAs) are a class of smart materials that recover apparent plastic 41
deformation (~6-8% strain) after heating, thus “remembering” the original shape. This 42
shape memory effect (SME) can be exploited for self post-tensioning applications and 43
NiTi-based SMAs are promising as SME is possible at elevated temperatures amenable 44
to practical application compared to conventional NiTi. This study investigates the 45
feasibility of self-post-tensioned (SPT) concrete elements by activating the SME of 46
NiTiNb, a class of wide-hysteresis SMAs, using the heat of hydration of grout. First, the 47
microstructure characterization of the NiTiNb wide-hysteresis shape memory alloys is 48
discussed. Then, the tensile stress-induced martensitic transformations in NiTiNb SMA 49
tendons are studied. Next, the temperature increase due to the heat of hydration of 50
four commercially available grouts is investigated. Pull-out tests are also conducted to 51
investigate the bond between the grout and SMA bar. Results show that the increase in 52
temperature due to hydration heat can provide significant strain recovery during a free 53
3
recovery experiment, while the same temperature increase only partially activates the 54
SMAs during a constrained recovery. 55
Keywords: self-stressing, shape memory alloy, concrete structures, post-tensioning, 56 heat of hydration 57
Introduction 58
Prestressed concrete is a construction method where permanent compressive stresses 59
are created in a concrete structure to counteract tensile stresses induced by externally 60
applied loads. By prestressing the concrete, which is weak in tension, it is ensured that the 61
structure remains within its tensile and compressive capacity. Two common techniques of 62
prestressing are pre-tensioning and post-tensioning. In pre-tensioning, prestressing tendons 63
are tensioned prior to casting concrete and the tendons are released upon hardening of 64
concrete. If the tendons are put in tension after concrete placement, the process is called 65
post-tensioning. In post-tensioning, the tendons are placed in pre-positioned ducts, 66
stressed through jacking and anchored at the ends of the concrete member once the 67
concrete has hardened. The duct is then grouted to ensure bonding of the tendon to the 68
surrounding concrete, to protect the tendons from corrosion and to improve the resistance 69
of the member to cracking (Naaman, 2004). The well-bonded pre-stressing tendons enable 70
the transfer of pre-stressing force to the structural concrete and therefore provide integral 71
4
behavior. In particular, any strain experienced by the concrete is also experienced by the 72
post-tensioning tendon after good bonding. 73
Over the past decade, there has been an increasing interest in the use of shape memory 74
alloys (SMAs) for various civil engineering applications (Ozbulut et al., 2011). SMAs are 75
a class of metallic alloys that can remember their original shape upon being deformed. 76
This shape recovery ability is due to reversible phase transformations between different 77
solid phases of the material. The phase transformation can be mechanically induced 78
(superelastic effect) or thermally induced (shape memory effect). Superelastic SMAs can 79
undergo large strains, in the order of 7 to 8%, and recover these deformations upon 80
removal of stress. SMAs that exhibit shape memory effect (SME) generate large residual 81
deformations when the material is mechanically loaded over a certain stress level and 82
unloaded. However, the SME SMAs recover those residual strains upon being heated. 83
Several attempts have been made to use the SME SMAs for civil engineering applications 84
(Andrews et al., 2010; Choi et al., 2011; Aguilar et al., 2013). In particular, the potential 85
use of thermally-induced SMAs to prestress concrete has been investigated by several 86
researchers, as discussed next. The use of SMA tendons in prestressed concrete elements 87
can increase overall sustainability of structures by minimizing the susceptibility of 88
5
prestressing tendons to corrosion and by enabling the adjustment of prestressing force 89
during their service life. 90
Maji and Negret (1998) were the first to utilize the SME in NiTi SMAs to induce 91
prestressing in concrete beams. SMA strands were pretensioned into the strain-hardening 92
regime and then embedded into small-scale concrete beams. Once the beams were cured, 93
the SMA strands were activated by the applied heat. El-Tawil and Ortega-Rosales (2004) 94
tested mortar beam specimens prestressed with SMA tendons. They considered two types 95
of SMA tendons: 2.5 mm and 6.3 mm diameter wires. Test results showed that significant 96
prestressing could be achieved once the SMA tendons were heat-triggered. Sawaguchi et 97
al. (2006) investigated the mechanical properties of mini-size concrete prizm specimens 98
prestressed by Fe-based SMAs. Li et al. (2007) examined the performance of concrete 99
beams with embedded SMA bundles. Through an extensive experimental program, they 100
studied the development of smart bridge girders that can increase their prestressing force 101
to resist the excessive load as needed. In all of these studies, SMA tendons are triggered 102
by an electrical source. 103
In this paper, the development of self-post-tensioned (SPT) concrete elements by 104
activating SMAs using the heat released during grout hydration is investigated. This 105
current work takes advantage of the unique capability of stress-induced martensite in 106
6
NiTiNb to remain stable and not recover via the superelastic effect (Cai et al., 1994; Zhao, 107
2000; Kusugawa et al., 2001). Pre-straining at room temperature may be more practical 108
for the pre-stressing application and sets this work on NiTiNb apart from commercially 109
available NiTi alloys which are known to exhibit superelasticity at temperatures above Af 110
(Liu and Galvin, 1997). First, the process of self-post-tensioning with SMA tendons and 111
the required conditions on the transformation temperatures of the SMAs are discussed. 112
The alloys are typically cast and further cold- or hot-worked into rods, sheets, tubes, wires, 113
etc. for practical application (Wang et al., 2014; Siegert et al., 2002; Yan et al., 2012). 114
Thus in this work commercially available rod and sheet NiTiNb materials are contrast 115
with respect to a cast NiTiNb alloy in order to benchmark the influence of differential 116
deformation-processes. 117
For materials characterization, the microstructure is reported along with the 118
characteristic thermally-induced martensitic transformation temperatures and the 119
mechanical properties. Then, the NiTiNb SMAs are pre-strained and subsequently heated 120
in order to assess them for the self-post-tensioning application. The SME recovery during 121
heating without constraint is described with respect to the influence of pre-strain/pre-stress 122
on the reverse transformation temperatures ( As* and Af
* ). The recovery is constrained 123
during heating in order to assess the influence of pre-strain/pre-stress on stress generation 124
7
as well as the reverse transformation temperatures ( As** and Af
** ). Next, heat of hydration 125
of different commercially available grout products is studied to measure the temperature 126
increase during grouting and to identify the optimum grout composition with respect to the 127
capability to match the temperature differential between the reverse transformation 128
temperatures. Finally, the bond strength between the SMAs and the grout is investigated 129
through pull-out tests and the results of experiments are discussed. 130
Self-post-tensioning with SMAs 131
132 The key characteristic of SMAs is a solid-solid, reversible phase transformation 133
between its two main microstructural phases, namely martensite and austenite. SMAs have 134
four characteristic temperatures at which phase transformations occur: (i) the austenite 135
start temperature As, where the material starts to transform from twinned martensite to 136
austenite, (ii) austenite finish temperature Af, where the material is completely transformed 137
to austenite, (iii) martensite start temperature Ms, where austenite begins to transform into 138
twinned martensite, (iv) martensite finish temperature Mf, where the transformation to 139
martensite is completed. If the temperature is below Mf, the SMA is in its twinned 140
martensite phase. When a stress above a critical level is applied at a temperature below Mf, 141
the twinned martensitic material converts into detwinned martensite phase and retains this 142
8
phase upon the removal of the load. It can regain its initial shape when the SMA material 143
is heated to a temperature above Af. Heating the material above Af results in the formation 144
of the austenite phase and, in the ideal case, a complete shape recovery. By a subsequent 145
cooling, the SMA transforms to initial twinned martensite phase without any residual 146
deformation. 147
Significant heat is generated during the hydration of cement products. Numerous 148
factors such as the type and composition of cement, the proportion of the mix, and the 149
ambient temperature affect the heat evolution during the hydration process. In concrete 150
structures, internal temperatures of 70°C are not uncommon (Dwairi et al., 2010). Since 151
grout is generally composed of very high portion of cement, high temperature increases 152
can be also observed during grouting applications. Therefore, hydration heat of grout can 153
be used to trigger the SME of SMAs to obtain SPT concrete members. Figure 1 shows the 154
process for development of the SPT concrete beams using SMAs. First, the SMA tendons, 155
in the martensitic state, are pre-strained. Then, concrete is poured and the SMA tendons 156
are installed in post-tensioning ducts after concrete hardening. The void between the duct 157
and the SMA tendons is filled with grout. Due to the heat of hydration of grout, the 158
temperature of the SMA tendons increases, which induces the transformation of the 159
material to austenite when the temperature is over the As. A complete transformation to 160
9
austenite phase occurs when the temperature reaches the Af. As the SMA tendons attempt 161
to return back to their original shorter length, while being constrained at both ends, a 162
tensile stress is produced in the tendons, causing pre-stress in the concrete beam. 163
Austenite
Detwinned Martensite
Twinned Martensite
STEP 1
STEP 2
At T<As, Pre-stretch original SMA while in
martensite phase
Cast concrete, and install the tendon in post-tensioning duct
Fill the ducts with grout, and trigger the tendons using the
heat of hydrationSTEP 3
Figure 1. Self-post-tensioning process.
The conditions on the phase transformation temperatures and the required temperature 164
window (service temperature) for self-stressing application are shown in Figure 2. First, 165
the As should be larger than the highest possible ambient temperature as the pre-strained 166
SMA tendons must stay in the martensite state at ambient temperature. This will prevent 167
10
pre-stretched SMA tendons from recovering their deformations at the storage temperature 168
or during the installation of tendons to the concrete member. Second, the Ms should be 169
below the lowest possible ambient temperature. This will ensure that the heated SMA 170
tendons maintain their recovery stress after cooling to the ambient temperature. If the 171
temperature of the SMA tendons becomes lower than the Ms, the SMA tendons will lose 172
their recovery stress due to a phase transformation to martensite. This requirement for Ms 173
coupled with the aforementioned requirement for As necessitate the use of the current 174
NiTiNb class of wide-hysteresis (i.e. ΔTH = As −Ms ) SMAs. The ternary alloying with 175
Nb facilitates hysteresis 130 – 150 °C compared to 30 °C in binary NiTi alloys (Zhang et. 176
al., 1990; Zhao, 2000). Furthermore, the Af should be as close as possible to the As, which 177
requires minimizing the differential ∆TR=Af - As, to complete the phase transformation 178
using the hydration heat. When the temperature rises over the As, the SMA tendons start to 179
transform to austenite, and thus recovery stresses are induced. However, the maximum 180
recovery stress will not be obtained until the microstructure is completely austenitic, at a 181
temperature over the Af. 182
11
Af
As
Ms
Mf
Temperature
Martensite fraction
100%
0%
AusteniteMartensite
Should be over maximum ambient temperature
Should be below minimum ambient temperature
Service Temperature Should be close
to the As
Figure 2. Phase transformation temperatures of SMAs.
Materials Characterization 183
The phase transformation temperatures As and Af and their differential ∆TR=Af - As 184
depend on the microstructure of the NiTiNb alloy, i.e. the composition and the micro-185
constituent morphology. The influence of Nb addition has been systematically 186
investigated with respect to the microstructure and transformation temperatures (Piao et 187
al., 1992; Siegert et al., 2002). A common ternary alloy composition for widening the 188
thermal hysteresis (Ms - As) while providing useful shape memory effect recovery 189
behavior is Ni47Ti44Nb9 (at%) (Otsuka and Wayman, 1998). Tailoring the micro-190
constituent morphology via deformation processing is the fundamental means to control 191
12
the phase transformation temperatures (Siegert et al., 2002). The alloys are typically cast 192
and further cold- or hot-worked into final forms for practical application. 193
In this work, the microstructure of a cast and deformation-processed (sheet) alloy with 194
similar compositions are reported. Atlantic Metals and Alloys LLC supplied a cast alloy 195
with the composition Ni47.3Ti44.1Nb8.6 at.%. Medical Metals LLC supplied a deformation-196
processed sheet that was 6 mm wide and 0.25 mm thick with the composition 197
Ni47.7Ti43.5Nb8.8 at.%. The compositions of both alloys are nearly equal to Ni47Ti44Nb9 198
at.%, which is the recommended ternary composition for wide hysteresis applications 199
above. The grain sizes for the cast and sheet material were determined as 300 µm and 300 200
nm respectively using the Intercept Procedure from ASTM E112-12. Texture is rarely 201
reported (Yan et. al., 2012) and it is likewise beyond the scope of this work. The impact 202
of differential thermo-mechanical processing was contrast by studying an extruded rod 203
material (3.45 mm diameter) provided by. Memry Corporation with the composition 204
Ni44.6Ti42.8Nb12.6 (at%). Specimens for mechanical testing and microstructure analysis 205
were wire electro-discharge machined from the rod material with an 8 mm gage length 206
and 1.1 x 0.5 mm2 cross-section and from the sheet material with a 10 mm gage length and 207
3 x 0.25 mm2 cross-section. 208
13
The microstructure is characterized along with the characteristic thermally-induced 209
martensitic transformation temperatures and mechanical properties. The cast 210
microstructure consists of a net-like arrangement of a characteristic eutectic micro-211
constituent encompassing a NiTiNb matrix and a scanning electron microscopy (SEM) 212
image is shown in Figure 3(a). The martensitic transformation occurs in the matrix 213
regions. The as-cast microstructure is representative of the microstructure prior to thermo-214
mechanical processing. The SEM image in Figure 3(b) shows the typical eutectic micro-215
constituent, which is made up of β Nb-rich particles and α-NiTiNb matrix (Zhao 2000; 216
Siegert et al., 2002; Wang et al., 2014). Deformation processing breaks up the net-like 217
structure (Siegert et al., 2002; Wang et al., 2014; Yan et al., 2012). The micro-constituent 218
morphology for the sheet material is shown in Figures 3(c). The image reveals a 219
composite-like microstructure with β Nb-rich particles (appearing as the lighter streaks) 220
that are elongated and discontinuous fiber-like reinforcements aligned in the primary 221
processing direction within the NiTiNb matrix. 222
It is well known that the unique stabilization of martensite, which is the cornerstone of 223
NiTiNb shape memory behavior, is attributed to the microconstituent morphologies 224
(Melton et. al., 1988; Duerig et. al., 1989; Cai et. al, 1994; Zhao, 2000; Kusagawa et. al., 225
2001; Seigert et. al., 2002; Yan et. al., 2012; Wang et. al., 2014). The microstructure 226
14
images in Figure 3(c) and 3(d) underscore differential microconstituent morphologies 227
resulting from different thermomechanical processing done by the different companies, 228
which show the sheet and rod materials respectively. The images reveal a composite-like 229
microstructure with β Nb-rich particles (appearing as the lighter streaks) that are elongated 230
and discontinuous fiber-like reinforcements aligned in the primary processing direction 231
within the NiTiNb matrix. The area fractions of the second particles were estimated using 232
a digital image pixel thresholding technique (Sahoo et. al., 1988; Pal and Pal, 1993), 233
which takes advantage of the dark and light contrasts of the matrix and particles 234
respectively, and uses the average of several SEM images. The particle average area 235
fraction for the rod microstructure was 18%. Consistent with the similar compositions, the 236
fractions for the cast and sheet materials were 10%. The average inter-particle spacings 237
were determined from cross-sectional SEM images. For the sheet material, the inter-238
particle spacing was about 100 nm and it was 500 nm for the rod material. These findings 239
convey a refined microconstituent morphology for the sheet compared to the rod. 240
Moreover, the microstructure characterization illustrates that deformation-processing after 241
castings affords the ability to tailor the microstructure, via orienting the microconstituents 242
and inter-particle spacing. 243
15
Figure 3. SEM images showing (a) the microstructure and (b) the micro-constituents in a cast Ni47.3Ti44.1Nb8.6 (at%) alloy and the microstructures of (c) the Ni47.7Ti43.5Nb8.8 (at%) alloy sheet (d) theNi44.6Ti42.8Nb12.6 (at%) alloy rod materials.
244 Differential scanning calorimetry (DSC) analysis was carried out to determine 245
transformation temperatures. DSC was carried out using a power compensated Perkin-246
Elmer DSC8500. The temperature scan rate was 40 °C/min. The methodology was as 247
follows: (i) heat from 50 °C to 100 °C, (ii) hold for 1 minute at 100°C, (iii) cool to -120 248
°C, (iv) hold at -120 °C for 1 minute, (v) reheat to 200°C, (vi) hold at 200°C for 1 minute, 249
(vii) cool to 50 °C. The transformation temperatures for the cast alloy were Ms = -64 °C, 250
Mf = -106 °C, As = -81 °C and Af = 11 °C. For deformation-processed NiTiNb alloys 251
16
endothermic and exothermic events did not arise in DSC measurements down to liquid 252
nitrogen temperature. 253
Thermal cycling under constant bias load is typically employed to determine 254
characteristic transformation temperatures for processed NiTiNb materials (Melton et al., 255
1988; Kim et al., 2011). Experimental details are reported elsewhere (Hamilton et al., 256
2015). Constant bias loads were increased from 10 MPa up to a load that facilitated 257
measurable transformation strain. Biasing with a constant stress of 150 MPa did reveal the 258
thermally-induced transformation. The transformation temperatures for the sheet were Ms 259
= -64 °C, Mf = -75 °C, As = -29 °C and Af = -6 °C, and those for the rod were Ms = -52 °C, 260
Mf = -71 °C, As = -29 °C and Af = -1 °C. 261
The strength properties were determined from uniaxial tension loading until failure and 262
the stress-strain responses are shown in Figure 4. The uniaxial tension test for each 263
material was determined at room temperature (~23 °C) with the material in the austenite 264
state and thus the martensitic transformation is stress-induced. The tests were conducted in 265
displacement control using an equivalent strain rate of 2.0 x10-4 /s. Strain was measured 266
via a miniature extensometer within the gauge length and computed via digital image 267
correlation (DIC), using the ‘Inspect Extensometer (IE)’ tool from the DIC software Vic-268
2D®. The IE gage length of the matches the gage lengths of the specimens. The material 269
17
properties are summarized in Table 1. The moduli for the deformation-processed materials 270
are greater than that for the cast material. Deformation processing improves ductility and 271
the fracture strain increases compared to the cast material. For the cast and rolled sheet 272
materials with similar composition, deformation processing improves the mechanical 273
strength. 274
Stress-strain curves for processed composite microstructures evolve as follows: 275
austenite linear-elastic response, stress drop/softening and plateau (indicative of the phase 276
transformation), linear-elastic response of martensite, non-linear strain-hardening 277
response, and fracture. Upper critical stress (referred to as “Austenite Critical Stress” in 278
Table 1) levels for the rod and sheet materials reach about 500 and 640 MPa respectively 279
and lower plateau stress levels are 460 and 590 MPa. The strain throughout the plateau 280
response is slightly larger for the rod (7.5 %) compared to the sheet (7.0 %). The cast net-281
like microstructure facilitates an initial linear-elastic slope followed by a deviation from 282
linearity, and then a critical stress that precedes the transformation. Moreover, a plateau is 283
apparent for the processed materials with strain accruing throughout the plateau. Rather 284
than a plateau, the transformation for the cast material exhibits a hardening-like response. 285
The martensitic sheet exhibits higher strength properties and failure strain. 286
18
Figure 4. The tensile stress-strain response of the NiTiNb Alloys.
Table 1. Mechanical properties for the NiTiNb alloys. Note that the critical and yield stresses 287 are based on a 0.2% offset. 288
Composition (at%)
Modulus (GPa)
Austenite Critical Stress (MPa)
Martensite Yield Stress (MPa)
Ultimate Tensile Strength (MPa)
Failure Strain (%)
Cast Ni
47.3Ti
44.1Nb
8.6 63 330 - - 9.4
Sheet Ni47.7Ti43.5Nb8.8
70 640 740 980 42.2
Rod Ni
44.6Ti
42.8Nb
12.6 54 500 600 710 29.1
289
Stre
ss (M
Pa)
19
Pre-straining and Shape Memory Effect Recovery Experiments 290
The pre-strain experiments were conducted on an MTS 810 servo hydraulic load frame 291
and at room temperature, which averaged about 23°C and was well above the Af 292
temperatures so the starting microstructure was austenitic. Pre-strain for binary NiTi 293
SMAs is typically carried out in the martensitic state and martensite reorientation takes 294
place rather than the stress-induced austenite-to-martensite transformation (Duering and 295
Melton, 1989; Cai et al., 1994; Zhao, 2000; Wang et al., 2014). In this work, the 296
martensite was stress-induced, which is made possible by the Nb addition in the ternary 297
NiTiNb SMAs. The specimens were loaded in displacement control at an average strain 298
rate of about 2.0 x10-4 /s and they were unloaded upon reaching the desired pre-strain 299
level. Residual strain remained after unloading. To assess the recovery ratio of residual 300
strain, specimens were heated at zero load (referred to as free recovery). Thermo-301
mechanical experiments were conducted for the current work on an MTS 810 servo 302
hydraulic load frame equipped with a custom thermal-cycling set-up. The specimens were 303
heated via induction heating. The temperature was measured via a thermocouple affixed to 304
the specimen. The induction coil design (Semiatin and Zinn, 1988) minimized thermal 305
gradients in the specimen. Heating and cooling rates were controlled so that they were 306
maintained around 10-15 °C/min. Specimens were allowed to cool in the ambient back to 307
20
room temperature. Preliminary free-recovery experiments were conducted on both 308
deformation-processed material in order to assess the material response of the differential 309
composite-like microstructures. 310
For the sheet and rod materials, free recovery is contrast for the pre-straining stress-311
strain response shown in Figure 5(a). After unloading, there is residual strain . The 312
strain recovery begins at the *sA temperature in Figure 5(b). The strain saturates at a 313
temperature *fA when the reverse transformation is complete. Since saturation is achieved, 314
the reverse transformation temperature interval * * *R f sT A AD = - fully activates the shape 315
memory effect. As shown in Figure 5(b), not all the strain is recovered during heating and 316
there is permanent irrecoverable strain . The strain that is recovered during heating 317
is shown as . The percentage of residual strain that is recovered via free SME recovery 318
is defined as the recovery ratio ( ). The recovery ratio (58%) for the 319
sheet material is higher than that of the rod (49%). The recovery will be incomplete if the 320
temperature is raised by a fraction of *RTD ; therefore, the material will be partially 321
activated. In order to achieve full activation, as well as maximize the recovery ratio, the 322
rese
perme
εrecfull
*100%fullrec
res
ee
æ öç ÷è ø
21
material microstructure must be designed such that *RTD matches the heat of hydration of 323
the grout. 324
(a) (b)
Figure 5. (a) The stress-strain curves for pre-straining of sheet and rod; (b) the subsequent strain-temperature (𝜀 − 𝑇) response during shape memory recovery for sheet and rod materials. 325
The recovery behavior can be further characterized based on the heating strain-326
temperature (ε-T) curves. A dotted line is drawn tangent to the curves in Figure 5b and 327
demonstrates the extent of strain recovery within a select temperature range. The strains 328
for the sheet and rod respectively recover with temperature at 0.12 %/°C and 0.09 %/°C. 329
Contrasting the initial slopes, the sheet exhibits a higher recovery ratio over a smaller 330
22
temperature interval, which may better match the possible heat of hydration. After the 331
initial slope the strain recovers gradually and the sheet alloy exhibits a *RTD = 49 °C and 332
*RTD = 57 °C for the rod. However, it can be seen that when the temperature is increased 333
to 50°C, which corresponds to a 26°C increase from the pre-straining temperature, 91% of 334
for the sheet and 77% of for the rod is achieved. The lower temperature interval 335
and higher recovery ratio of the sheet as compared to the rod confirms that extent of 336
deformation experienced by sheet material resulted in a more promising microstructure for 337
activation via heat of hydration of the grout. Thus, understanding the microstructure 338
property relationships can enable tailoring the eutectic microconstituent orientation 339
(giving rise to an apparent elongation in Figures 3(d) compared to 3(c)) and inter-particle 340
spacing in order to tune the activation strain and * * *R f sT A AD = - . The following section 341
focuses on stress generation during heating with a fixed displacement constraint for the 342
sheet material, as the martensite exhibited higher strength and fracture strain and the 343
recovery ratio is substantial over a lower temperature interval. 344
Pre-straining and Stress Generation Experiments 345
The stress-strain responses for the pre-straining of the sheet material are shown in 346
Figure 6. Pre-strain levels of more than 12% are commonly suggested (Otsuka and 347
εrecfull εrec
full
23
Wayman, 1998) and in this work pre-strain levels approach that value and exceed it. The 348
pre-strain levels were 5.4, 8.8, 10.1, 12.2 and 16.1% in order to deform martensite to 349
different extents of the stress-strain response. During loading, the curve exhibits an initial 350
linear-elastic response up to a stress peak, followed by a stress drop. Then a stress plateau 351
indicative of the phase transformation is observed. The evolution of the morphology 352
depends on whether the pre-strain in Figure 6 is within the plateau region, at the 353
completion of the plateau, within the linear elastic response of martensite after the plateau, 354
or within the strain-hardening type behavior of martensite after the elastic response. For 355
the 5.4% pre-strain level, the microstructure is a mixture of martensite and austenite. The 356
martensite volume fraction increases throughout the plateau and the material should be 357
completely martensitic at the 8.8% pre-strain. Beyond the plateau, the martensite deforms 358
for the 10.1% pre-strain and the stress-strain response exhibits a linear-elastic type 359
response. The highest pre-strain levels (12.2 and 16.1%) are within the non-linear 360
response and the martensite likely yields. 361
24
Figure 6. The tensile stress-strain curves for the sheet pulled to increasing pre-strain levels at room temperature.
362
Residual strain remains after unloading in Figure 6 and the corresponding displacement 363
is fixed during heating which constrains shape memory effect recovery and generates 364
recovery stresses. The stress generation experiments utilize the heating set-up described 365
in the previous section. The displacement sensor of the MTS machine measures the 366
change in actuator position and thus the change in length of the specimen and the grips. 367
For the experiments in this work, an extensometer would not easily mount to the thin sheet 368
specimens. Hence, the displacement was used for stable feedback that maintained the 369
constant constraint and avoided damaging the extensometer. Note that ASTM standard 370
25
E328 outlines Standard Test Methods for Stress Relaxation for Materials and Structures at 371
constant temperature and recommends mounting an extensometer within the gage section 372
to fix/constrain the strain. Using the extensometer measurement to maintain a fixed strain 373
constraint can fix the sample length, as the grips would be excluded from the constraint, 374
and the impact on the results will be considered in future work. The current experiments 375
best represent the constraining conditions for pre-stressing via heat of hydration of the 376
grout. During heating, the displacement was programmed so that is was fixed at the 377
residual strain after pre-straining and the stress generation results are expected to be 378
reliable for the context of the discussion in this work. 379
In Figure 7, the recovery stress is plotted as it evolves throughout heating. The stress 380
generation begins at the onset of heating and thus close to the test temperature. For the 381
lowest pre-strain in Figure 7(a), the recovery stress reaches a maximum (or peak) and 382
drops. For the higher pre-strain levels in Figures 7(b) and 7(c), the recovery stress 383
increases to a maximum and decreases slightly up to the maximum temperature. The 384
temperature at the maximum stresses generated during heating should correspond to the 385
temperature at which stress-induced martensite, which was stabilized during pre-straining 386
deformation, recovers deformation. Note that the deformation recovery mechanism may 387
be attributed to detwinned SIM martensite reverting to twinned martensite (Zhao, 388
26
2000; Zheng et al., 2000; Liu et al., 2013) to recovery of reoriented SIM or deformation 389
induced martensite (Liu et al., 2013; Zhao, 2000; Zheng et al., 2000) to the conventional 390
reverse martensite to austenite transformation; or to multiple mechanisms occurring in 391
different volume fractions of martensitic material. Hence, in Figure 7, that temperature is 392
designated as **fA (associated with constrained residual strain recovery). Recovery stresses 393
reaching maximums during heating have been observed for NiTiNb (Wang et al., 2014) 394
and a Fe-based SMA (Dong et al., 2005) For each pre-strain level, recovery stress accrues 395
after heating is complete as the temperature decreases to room temperature. Recovery 396
stresses increased during cooling for the Fe-based SMA (Dong et al., 2005). The 397
observations that the generated stress reaches a maximum during heating and that it 398
increases during cooling merit further study beyond the scope of the current work. The 399
findings pertinent to the pre-stressing application, which are discussed later in the 400
discussion, are that the maximum recovery stresses are generated under constraint after 401
straining to 12.2% and the resulting transformation temperature interval ** ** **R f sT A AD = - . 402
403
27
(a) (b) (c)
Figure 7. The stress-temperature response during constrained heating and cooling of specimens that have been pre-strained to (a) 5.4%, (b) 10.1% and (c) 12.2%. The double arrows indicate heating, and the single arrows cooling.
Heat of hydration of grout 404
Portland cement, potable water along with any admixtures to obtain required properties 405
are the basic grout materials. The chemical reaction between Portland cement and water is 406
exothermic, i.e. producing heat. This heat is called the heat of hydration. In order to 407
determine the temperature increase during grouting post-tensioning ducts, four 408
commercially available tendon grouts were tested. All grouts were prepackaged and 409
approved by Virginia Department of Transportation for post-tensioning applications. The 410
water-to-grout ratios for each commercial grout were set per manufacturer’s direction and 411
given in Table 2. To prepare test specimens with each grout, a mixing cylinder was 412
28
cleaned and a bag of selected grout and the required water were placed in the cylinder. 413
The contents were mixed in the cylinder for 3 minutes with a variable speed high shear 414
mixer and the resulting grout mixture was poured into a 102×203 mm ( 4×8 inch) 415
cylinder with a thermocouple attached to a single tendon placed in the center. The 416
thermocouple was connected to a data logger that monitored the temperature of the curing 417
grout every minute for 48 hours. 418
419
420
421
Table 2. Summary of grout temperature test results 422
Specimen Grout
Water-to-
Grout Ratio
Initial Temperature
(°C)
Maximum Temperature
(°C)
Temperature Increase
(°C) S1 Grout I 0.25 21 41 20 S2 Grout I 0.25 22 41 19 S3 Grout I 0.25 21 41 20 S4 Grout II 0.24 21 48 27 S5 Grout II 0.24 22 53 31 S6 Grout II 0.24 21 48 27 S7 Grout III 0.25 22 41 19 S8 Grout III 0.25 22 41 19 S9 Grout IV 0.27 22 41 19 S10 Grout IV 0.27 22 41 19 S11 Grout IV 0.32 22 40 18 S12 Grout IV 0.32 22 40 18
423
29
The time versus grout temperature plots for each specimen as well as ambient 424
temperature are given in Figure 8. Three specimens were prepared and tested for Grout I 425
and Grout II on three different days. The results for Grout I are consistent for each 426
specimen. The highest temperature recorded during curing is 41°C, which indicates a 427
temperature increase of 19°C to 20°C from initial temperature of 21°C to 22°C due to the 428
heat produced by the cement hydration. The temperature of Grout II reaches 48°C for two 429
specimens and 51°C for one specimen. At three different tests of Grout II, the average 430
temperature increase is 28°C. The peak temperature and average increase in temperature 431
for Grout III and Grout IV are similar to the results obtained from Grout I. For Grout IV, 432
two samples at two different water-to-grout ratios were tested. It is observed that the peak 433
temperature is slightly higher and occurs a few hours earlier when a lower water-to-grout 434
ratio is used (Figure 8d). The grout temperature reaches its peak value at 10 to 18 hour 435
after casting for Grout I, Grout II and Grout IV whereas the peak temperature occurs at 2.5 436
hour after casting for Grout III. For all specimens, the grout temperature reduces to values 437
between 22°C and 24°C near 30 hour after casting and remained almost constant 438
thereafter. The results of experimental tests conducted to characterize the grout 439
temperature during curing are summarized in Table 2. These results suggest that a 440
30
commercially available tendon grout (Grout II) can provide an average of 28°C increase in 441
temperature during the hydration, process, which can be used to activate SMA tendons. 442
(a) (b)
(c) (d) Figure 8. Temperature measured in different commercially available grouts during curing.
0 10 20 30 4015
20
25
30
35
40
45
Time (hour)
Tem
pera
ture
(°C
)
Grout I S1S2S3S1−AmbientS2−AmbientS3−Ambient
0 10 20 30 4015
20
25
30
35
40
45
50
55
Time (hour)
Tem
pera
ture
(°C
)
Grout II S4S5S6S4−AmbientS5−AmbientS6−Ambient
0 10 20 30 4015
20
25
30
35
40
45
Time (hour)
Tem
pera
ture
(°C
)
Grout IIIS7S8S7/8−Ambient
0 10 20 30 4015
20
25
30
35
40
45
Time (hour)
Tem
pera
ture
(°C
)
Grout IVS9S10S11S12S9/10/11/12−Ambient
31
Pull-out tests 443
When the post-tensioning ducts are filled with grout after the tendon has been anchored 444
at both ends, it is possible to obtain some degree of bond between the pre-stressing tendon 445
and the concrete. The bond of pre-stressing tendon is important with regard to failure 446
behavior, cracking, and the factor of safety. For prestressed concrete beams with well-447
bonded pre-stressing tendons, the ultimate tensile stress of tendon is an important factor 448
that affects the strength of the member. However, if the bonding of the grout to the post-449
tensioning tendon is not satisfactory or the tendon is unbonded, the tendon rarely reaches 450
its ultimate resistance before the failure of concrete in compression. The insufficient bond 451
will also result in a uniform distribution of tensile strains along the length of the tendon, 452
which leads to the development of fewer but wider cracks in concrete (Abeles, 1981). In 453
addition, in case of a ruptured tendon, good bonding between the grout and pre-tensioning 454
tendon enables re-anchoring of ruptured tendon and contributes to the residual structural 455
capacity (Abdelatif et al., 2012). 456
To investigate the bond behavior of SMA bars with grout, pull-out tests were 457
conducted. SMA bars with a diameter of 3.5 mm were cut into 220 mm segments using a 458
cutoff wheel. Two 102×102 mm (4×4 inch) cylindrical molds were used to manufacture 459
pull-out specimens. Holes with a diameter of 3.5 mm were drilled at the center of the top 460
32
and bottom of the molds to allow SMA to pass through. The SMA bar was secured at the 461
bottom hole of each mold and Grout II was poured inside the mold. The specimens were 462
left to cure for 3 days. Figure 9(a) shows a schematic diagram of the specimen used for the 463
pull-out test. 464
Since it is difficult for standard grips to fully hold on to SMA bars because of their 465
small diameter size, special aluminum sleeves were fabricated. Two 50 mm-long sections 466
were cut from a 10-mm aluminum rod. These sections were then placed on a lathe, and a 467
3.5-mm hole was drilled all the way through. The sleeve was then attached to the 468
specimen by means of twisting since the aluminum sleeve hole was a little bit smaller than 469
the diameter of the SMA bar. The tight fit was useful to establish a mechanical interlock to 470
help the SMA resist slippage out of the sleeve during testing. 471
The pull-out tests of the SMA bar conducted using an MTS servo-hydraulic load frame. 472
The specimen was held in place by a testing cage attached to the top head of the load 473
frame by a large bolt. The load was applied to the SMA bar at a rate of 0.075 mm/s and 474
measured by a built-in load cell of the load frame. The slip of the SMA bar relative to 475
grout were measured using Digital Image Correlation (DIC) method at the loaded and free 476
ends of the specimen. Two cards with a speckle pattern were attached to the loaded and 477
free ends of the SMA bar. The bottom card was attached directly at the end of the grout 478
33
cylinder to reduce any errors in the calculation of slippage due to strains in the SMA. The 479
optical system capture the movement of speckle patterns on the cards and provide an 480
output of the average vertical movements at each time step. Figure 9(b) shows pull out test 481
set-up. 482
(a)
(b) Figure 9. (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 483
recorded. Bond strength is defined as the shear force per unit surface area of the bar and 484
calculated by the following equation: 485
τ =Tπdbl b
(1)
where T is the tensile load on the SMA bar, db is the nominal bar diameter, and lb is the 486
embedment length of the bar. Figure 10 illustrates bond stress-slip curves both at the 487
loaded and free ends of the SMA bar. The bond behavior is characterized by an initial 488
34
increase in the bond stress up to 1.2 MPa for the first specimen and up to 1.4 MPa for the 489
second specimen, and with insignificant slippage and a softening thereafter. Since SMA 490
bars had a very smooth surface, mechanical bearing forces were very low and the load 491
transfer was primarily provided by friction. Maximum pullout load is found to be 1.4 kN 492
and 1.6 kN for the two specimens. 493
(a)
(b) Figure 10. Bond stress-slip relationship for SMA bars at free end and loaded end for (a) Specimen 1 and (b) Specimen 2.
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
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
35
Discussion of results 494
The comparison of the rod and sheet material demonstrates the importance of 495
microstructure to tailor ∆TR to the thermal inputs that the heat of hydration can provide. It 496
has been postulated that the increase in reverse transformation temperatures after pre-497
straining in these alloys is related to the interaction of the martensitic transformation with 498
the β Nb-rich particles (Duerig and Melton, 1989; Piao et al., 1993; Shi et al., 2012). The 499
deformation of these particles could lead to the stabilization of transformed martensite in 500
the surrounding matrix. Stabilization refers to the reverse martensitic transformation 501
requiring a higher thermal driving force, which facilitates an increase of the reverse 502
transformation temperatures (Liu and Favier, 2000). Pre-strain for NiTi-based SMAs is 503
typically carried out at temperatures between Ms and As; the stress-induced austenite-to-504
martensite transformation is expected to remain after unloading and thus heating facilitates 505
recovery via SME (Duerig and Melton, 1989; Cai et. al., 1994; Zhao, 2001; Kusagawa et. 506
al., 2001; Wang et. al., 2014). This work demonstrates stress-induced martensite that 507
remains after pre-strain deformation at a constant room temperature, which is well above 508
Af determined via the thermal cycling, can be partially recovered. The sheet 509
microconstituent morphology appears refined in SEM images compared to the rod and the 510
results reflect that the sheet exhibits the highest activation strain and improved martensite 511
36
strength properties. Hence, the findings confirm that the material response can be tuned 512
via tailoring Nb concentration and deformation-processing after casting in order refine the 513
inter-particle spacing and volume fraction of the microconstituent morphology. 514
The grout temperature characterization tests reveal that it is possible to increase the 515
temperature of a post-tensioning tendon up to about 50°C when a post-tensioning duct is 516
filled with a commercially available grout. When the SMA sheet material is pre-strained at 517
different levels, the strain remained after unloading, the strain recovered upon a 518
temperature increase to 50°C, which can be reached through hydration heat, and the strain 519
recovered after fully activating the material (heating above *fA are provided in Table 3. 520
Note that *fA is stress dependent and it increase under applied stress as shown in Table 3. 521
The material should be heated above the *fA under generated stress to achieve maximum 522
strain recovery. It can be seen that a great percentage of the maximum recoverable strain 523
can be activated when the temperature reaches 50°C. For instance, for an SMA tendon 524
with a 10.1% pre-strain, a temperature increase to 50°C increase will achieve 91% of 525
recoverable strain during free recovery experiments. 526
527
528
529
37
530
531
532
Table 3. Characteristic metrics for pre-strain deformation of NiTiNb alloy sheet material. 533
534
Figure 11 shows the dependence of the reverse transformation temperature interval 535
* * *R f sT A AD = - and recovery ratio on the pre-strain for the free recovery experiments. The 536
∆TR values continually increase while the recovery ratio reaches maximum for the 10.1% 537
pre-strain and decreases thereafter. The increasing *RTD and the pre-strain reaching a 538
maximum reflect the role of the Nb-rich particles in the composite-like microstructure. 539
Particles can have a stabilizing effect and prohibit the reverse martensitic transformation 540
and thus the thermal energy and Af must increase to complete (or fully activate) the 541
recovery (Duerig and Melton, 1989). 542
Pre-strain
(%)
Applied Stress (MPa)
Strain remaining
after unloading (%)
Strain recovered up to 50 °C
(%)
Free Recover
y *fA
(°C)
Strain recovered
after heating to
*fA (%)
5.4 590 2.3 1.0 63 1.1 8.8 630 5.2 2.7 75 3.0 10.1 720 6.8 4.1 80 4.5 12.2 800 9.0 1.3 111 5.3 16.1 860 12.7 0.5 123 4.8
38
543
(a) (b)
Figure 11. The variation of (a) *RTD and (b) recovery ratio with pre-strain level for free
recovery experiments 544
Figure 12 shows the variation of the **RTD and recovery stress with the pre-strain for 545
the constrained recovery experiments. For the pre-strain level of 10.1% and 12.2%, the 546
stresses generated during constrained recovery are 530 MPa and 550 MPa, respectively. 547
The temperature intervals under constrained recovery are in general larger than the *RTD 548
during free recovery, especially when the pre-strain level is greater than 8.8%. 549
Presumably, the constraining stress augments the stabilization effect. This disparity needs 550
to be considered when designing these alloys. 551
552
5.4 8.8 10.1 12.2 16.10
10
20
30
40
50
60
70
80
90
Pre−strain (%)
∆T* R
(°C
)
5.4 8.8 10.1 12.2 16.10
10
20
30
40
50
60
70
Pre−strain (%)
Rec
over
y R
atio
(%)
39
(a) (b)
Figure 12. The variation of (a) **RTD and (b) recovery stress with pre-strain level for
constrained recovery experiments 553
The results obtained from free and constrained recovery tests show that pre-stressing 554
significantly affects the transformation temperatures and shape recovery. Figure 13(a) 555
shows the amount of strain recovered per unit temperature increase for free recovery 556
experiments at different pre-strain levels. It can be seen that the optimum pre-strain level 557
that result in maximum strain recovery for a unit temperature increase is 10.1%. That pre-558
strain level also provides the maximum recovery ratio as can be seen from Figure 11(b). 559
On the other hand, for constrained recovery, the recovery stress per unit temperature 560
increase is decreasing with increasing pre-strain level as shown in Figure 13(b). However, 561
the maximum recovery stress increase with the pre-strain level up to 12.2% pre-strain, and 562
decrease thereafter as shown in Figure 12(b). Therefore, if the SMA tendons will be 563
5.4 8.8 10.1 12.2 16.10
20
40
60
80
100
120
Pre−strain (%)
∆T** R
(°C
)
5.4 8.8 10.1 12.2 16.10
100
200
300
400
500
600
Pre−strain (%)
Rec
over
y St
ress
(MPa
)
40
activated partially with limited temperature increase, a relatively low level of pre-strain (6-564
9%) can be more favorable. If the SMA tendons will be fully activated with sufficient 565
heat, then a pre-strain level that is just beyond the plateau region (10-12%) is more 566
preferable. 567
(a) (b) Figure 13. (a) Strain recovered per unit temperature increase as a function of pre-strain level; (b) recovery stress per unit temperature increase as a function of pre-strain level
568
It should also be noted that only four types of grouts approved by the state 569
transportation agency are considered here. The hydration heat of grout is a result of a 570
number of exothermic chemical reactions that can be influenced by several factors such as 571
water/cement ratio, air entrainment, chemical admixtures, cement type, cement fineness, 572
and ambient temperature (Ball and Camp, 2014). Therefore, further studies can be 573
conducted to obtain larger hydration heat (i.e. higher increases in temperature) by altering 574
5.4 8.8 10.1 12.2 16.10
0.02
0.04
0.06
0.08
0.1
Pre−strain (%)
Rec
over
ed S
train
/∆T* R
(%/°C
)
5.4 8.8 10.1 12.2 16.10
2
4
6
8
10
12
Pre−strain (%)
Rec
over
y St
ress
/∆T** R
(MPa
/°C)
41
some of these factors. However, this needs to be done carefully since excessive hydration 575
heat can cause other problems during grouting and adversely affect the performance of the 576
grout. 577
Pull-out tests showed the bond between the SMA bars and the grout is low. However, it 578
is comparable to the bond strength (less than 1 MPa) between a single steel bar and grout 579
(Watanabe et al., 2012) and the bond strength (less than 2 MPa) between smooth 580
prestressing bars and normal strength concrete (CEB-FIP, 2010). In self-post-tensioning 581
with SMAs, the prestress is transferred to the concrete element by the end anchorages. 582
Therefore, the recovery stresses in the SMA bar will be transferred through the anchorage 583
system at the end of the beam despite the low bond strength. Nonetheless, as discusses 584
earlier, good bonding is still favorable for better cracking behavior and ultimate strength 585
response. The use of SMA strands instead of a single SMA bar can provide better bond 586
performance as higher bond strength was reported for steel strands compared to steel bars 587
in the literature. Superelastic SMAs in the strand or cable form have recently been 588
developed and studied by several researchers (Reedlunn et al., 2013; Daghash et al., 589
2014). When SMA strands with shape memory effect properties are also developed, they 590
can be used in prestressing applications with better bond characteristics. 591
42
Conclusions 592
This paper explores the feasibility of activating SMA tendons using heat of hydration 593
of grout in order to develop self-post-tensioned concrete elements. Material 594
characterization tests were conducted on NiTiNb SMAs. In particular, the influence of 595
differential thermo-mechanical processing on shape memory behavior is assessed for a 596
rolled sheet and extruded rod. The recovery behavior of the material was studied during 597
free and constrained recovery experiments. The increase in temperature during the 598
hydration of four commercially available grouts was evaluated. A typical cylinder 599
specimen was filled with the grout and a thermocouple and a data acquisition system were 600
employed to measure the temperature during 48 hours. In addition, two pullout tests were 601
conducted on cylindrical specimens to investigate the bond between the grout and SMA 602
bar. The major findings of this study can be summarized as follows: 603
1. Tailoring deformation-processing after casting, such that the eutectic 604
microconstituent is oriented and closely spaced, can facilitate tuning the 605
activation strain and reverse transformation interval. 606
2. Pre-strain level considerably influences the reverse transformation interval and 607
recovery ratio or stress. 608
43
3. For free recovery experiments, reverse transformation interval increases with 609
the increasing pre-strain level while recovery ratio reaches a maximum value 610
and then decreases. 611
4. For constrained recovery experiments, both reverse transformation interval and 612
recovery stress increase up to 12.2% strain and then slightly decrease with the 613
increasing pre-strain. It is shown that a recovery stress more than 550 MPa 614
could be achieved after cooling to ambient temperature. 615
5. An average of 28°C temperature increase was observed during hydration of a 616
pre-packaged grout material. 617
6. The temperature increase due to heat of hydration of the grout can activate most 618
substantial percentage of recoverable strain during a free recovery experiment. 619
However, for constrained recovery, higher temperature increases are needed to 620
fully activate the SMA material. 621
7. Bond strength between plain SMA bars and grout material is found to be about 622
1.3 MPa. To achieve higher bond strength, the surface of SMA bars can be 623
sand-blasted. 624
Although this feasibility study indicates that the concrete elements can be prestressed 625
by partially activating NiTiNb shape memory alloy bars using hydration heat of the grout, 626
44
pre-straining in the austenitic state via the stress-induced martensitic transformation has 627
limited potential. Decreasing pre-strain deformation temperatures below room temperature 628
down to Ms and below Mf can facilitate lower critical stress levels and differential reverse 629
transformation intervals for SME recovery as well as higher recoverable strains and 630
recovery stresses (Cai et. al., 1994; Zhao, 2000; Kusagawa et. al., 2001). Hence, a similar 631
systematic study for pre-straining NiTiNb in the martensitic state is warranted. Further 632
research is needed to investigate other SMA materials that possess more favorable phase 633
transformation ranges for self-stressing and higher recovery stresses. Potential of 634
obtaining higher temperature increases during the hydration of grout can also be explored. 635
Furthermore, long-term prestress losses, the use of larger size SMA tendons, and the 636
effects of field conditions need to be examined. 637
Funding 638
The authors would like to acknowledge the support of the Mid-Atlantic University 639
Transportation Center to conduct this research. 640
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