7 Pavement damage occurs in cold-region concrete exposed to high concentrations of 8 calcium chloride (CaCl2). The damage is caused by a combination of conventional freeze-thaw 9 damage and the formation of a deleterious phase known as calcium oxychloride from a reaction 10 between CaCl2 and calcium hydroxide in the concrete. Much research has focused on calcium 11 oxychloride mitigation in cement pastes, but not on concrete damage due to calcium oxychloride. 12 In this study, the synergistic roles of air and supplementary cementitious materials (SCMs) in 13 reducing damage in concrete exposed to high concentrations of CaCl2 and freeze-thaw conditions 14 is studied. Concrete mixtures were made with different SCM replacement levels and air contents 15 ranging from 1.8 to 8.0% and immersed in 25% CaCl2 solutions and subject to freeze-thaw cycles 16 (-8 °C to 25 °C) for 600 days. Bulk resistivity and visual assessment of damage were found to be 17 excellent descriptors of the damage progression. Damage was reduced as the SCM content and air 18 content was increased. Mixtures which had 20% SCM and 8% air and mixtures which had 35% 19 SCM and more than 4% air showed strong durability against damage due to calcium oxychloride 20 formation. 21 22 Keywords: Deicing salt; calcium oxychloride; freeze-thaw; non-destructive testing; 23 supplementary cementitious materials. 24 1. Introduction 25 Premature deterioration along pavement joints has been a major issue limiting service life 26 in many cold regions in the U.S. [1-7]. While concrete pavements generally demonstrate adequate 27 long-term performance, joint deterioration can severely affect the service life, in addition to being 28 laborious and expensive to repair [1-3]. The deterioration is caused in part by freeze-thaw and in 29 part due to the formation of a deleterious phase known as calcium oxychloride. This phase forms 30 from a reaction between calcium chloride (CaCl2) or magnesium chloride (MgCl2) deicers and 31 calcium hydroxide in the concrete [1-3,8,9]. As the intensity of deicing and the use of stronger 32 deicers such as CaCl2 and MgCl2 increases, it stands to reason that the incidence of such 33 deterioration will increase [10]. Several authors have investigated freeze-thaw damage in concrete, 34 and while the mechanisms are not completely understood, freeze-thaw damage can generally be 35 mitigated by having adequate amounts of well-distributed and appropriately sized air voids in the 36 concrete [2,11]. The effects of CaCl2 on concrete are more complex due to interactions with 37 various hydrate phases [12]. Calcium oxychloride forms when higher concentrations of CaCl2 are 38 used [8,13]. The most commonly suggested form of calcium oxychloride in concrete has the 39 chemical formula 3Ca(OH)2•CaCl2•12H2O [14] and the chemical reaction showing its formation 40 is shown in Eq. 1 [8]. 41 CaCl2 + 3Ca(OH)2 + 12H2O → 3Ca(OH)2 •CaCl2 •12H2O (1) 42 Supplementary cementitious materials (SCMs) are natural or industrial by-product 43 materials that improve the performance, durability, and sustainability of concrete mixtures [15,16]. 44 The use of SCMs in cement pastes causes a reduction in the amount of calcium hydroxide and 45 calcium oxychloride due to dilution and pozzolanic reactions [3,17-19]. The reduced calcium 46 hydroxide and calcium oxychloride contents are reflected in reduced damage in cement pastes 47 exposed to high concentration CaCl2 solutions [19-22]. Calcium hydroxide and calcium 48 oxychloride contents are reduced as SCM reactivity and replacement are increased [18]. Based on 49 the studies listed above and certain other studies [18,19,22-25], specifications to limit damage 50 induced by calcium oxychloride formation in concrete pavements were introduced in the United 51 States (performance based specification AASHTO PP 84). Two suggestions are used in this 52 specification for pavement concretes, which are 30% SCM replacement, or limiting measured 53 calcium oxychloride amounts in cement pastes to below 0.15 g/g cement paste. The AASHTO PP 54 84 specifications are mainly based on the work done on cement pastes [18,19,22-25], and they 55 have not been validated for a range of concrete mixtures. Studies of concrete damage due to 56 calcium oxychloride are somewhat limited, but these studies suggest that the damage reduces as 57 the air content and SCM replacement increases [26-29]. While little research has been performed 58 on the damage to concrete exposed to both freeze-thaw and concentrated CaCl2 solutions, two 59 important studies merit mention. Ghazy et al. showed that fly ash and nanosilica could mitigate 60 calcium and magnesium oxychloride damage under such conditions [26]. Julio-Betancourt showed 61 that pessimum concentrations of CaCl2 for damage were dependent on exposure temperature, and 62 increased as the temperature increased [28]. However, neither of these studies dealt with mixtures 63 in which both air and SCM contents were significantly varied. 64 Realistic concrete pavement mixtures for cold regions are air entrained and have SCMs at 65 different replacement levels. The synergistic effects of these two parameters in reducing damage 66 due to calcium oxychloride in freezing conditions is not clearly understood. The objective of this 67 study is to critically evaluate the synergies between air and SCM contents in reducing damage due 68 to calcium oxychloride formation in concrete. Through this study, we seek to further examine the 69 SCM replacement criteria in AASHTO PP 84 that deal with concrete pavement joint damage in 70 the presence of CaCl2. This study is the first part of a larger project where the performance of 71 concretes immersed in CaCl2 solutions and subjected to a variety of temperature conditions with 72 and without freezing are studied. 73 Concrete mixtures with fly ash and slag replacement levels of 0, 20, and 35% by mass with 74 air contents ranging from 1.8 to 8.0% were made. The concretes were characterized using 75 extensive fresh and hardened concrete testing. At 28 days, the cylinders were immersed in 25% 76 CaCl2 solution and were subjected to freeze-thaw cycles (-8 °C to 25 °C), and monitored using 77 mass loss, bulk resistivity, surface resistivity, ultrasonic pulse velocity, and visual assessment up 78 to 600 days. 79 80 2. Materials 81 Type I/II OPC was used in all mixtures in this study. Class F fly ash and ground granulated 82 blast furnace slag (also known as slag here, but simply termed slag throughout the text here) were 83 used as SCMs. The chemical compositions of the cementitious materials obtained from the 84 manufacturer or through X-ray fluorescence testing performed on beads using a calibrated device 85 are provided in Table 1. 86 87 Table 1. Chemical compositions of the cement, fly ash, and slag (weight %). 88 Oxide Cement* Fly ash Slag SiO2 19.90 56.01 32.09 Al2O3 4.55 21.71 12.88 Fe2O3 3.77 12.98 0.75 CaO 63.96 4.81 42.44 MgO 1.17 1.03 6.53 SO3 2.35 0.37 3.03 K2O 0.37 2.57 0.27 Na2O 0.18 0.30 0.13 * The cement has a limestone content of 3.8%, an LOI of 2.5%, and a Blaine fineness of 386 m2/kg. 89 The chemical admixtures used for testing include a commercial superplasticizer and an air 90 entraining agent. The aggregates used for testing include coarse aggregates (oolitic limestone, 91 nominal maximum size 25.4 mm, 3.75% absorption capacity) and fine aggregates (siliceous sand, 92 nominal maximum size 4.75 mm, 2.84% absorption capacity). Aggregates were corrected for 93 water content before mixing. Commercial deicing CaCl2 with >95% purity was used to prepare 94 solutions with 25% concentration of CaCl2 by mass by mixing the salt with tap water. 95 96 3. Experimental methods 97 3.1. Mixing and curing procedures 98 Concrete mixture designs were made with water-to-cementitious materials ratio (w/cm) of 0.40 99 and with fly ash and slag replacement levels of 0, 20, and 35% (by mass) and are shown in Table 100 2. Mixtures were designed to have a slump between 20 mm to 120 mm and targeted air contents 101 of 2 ± 1%, 5 ± 1%, and 8 ± 1% which were achieved using the air entraining agent. The tolerances 102 in the air content values are somewhat smaller than the 1.5% which is common in field conditions. 103 These mixtures are modified variants of a realistic base mixture design, which was obtained in 104 consultation with a ready mix concrete producer. 105 Mixing was conducted using a rotating drum mixer. First, aggregates were added, and mixed 106 for one minute. Then, the cementitious materials were added and mixed for two minutes. Finally, 107 water and admixtures were added. Small amounts of superplasticizer were added in four cases for 108 stiff mixtures (as shown in Table 2). The total mixing time was about 15 minutes and fresh concrete 109 properties (slump and air content) were tested for each mixture. Actual, measured air contents 110 ranged from 1.8% to 8.0%. The design slump was not achieved for two mixtures and the target air 111 content of 5 ± 1% was not obtained for one mixture. After mixing, six 100 × 200 mm concrete 112 cylinders were cast for each mixture and moist cured for 28 days. Three of the concrete cylinders 113 were tested destructively at 28 days for evaluating their compressive strength, whereas the rest 114 were tested non-destructively at 28 days and during the freeze-thaw cycles up to failure or for 600 115 days. 116 117 Table 2. Concrete mixtures tested for this study. OPC is ordinary portland cement, FA is fly ash, 118 SL is slag, and (SP) is superplasticizer. 119 Mixture TAC* (%) w/cm Cement (kg/m3) Fly ash (kg/m3) Slag (kg/m3) Sand (kg/m3) Coarse aggregate (kg/m3) Air entrainer (%) SP (%) OPC-2.6 2 0.40 372 – – 757 1051 – 0.30 OPC-6.6 5 0.40 372 – – 757 1051 0.10 – OPC-7.9 8 0.40 372 – – 757 1051 0.38 – FA20-2.1 2 0.40 298 74 – 757 1051 – 0.11 FA20-5.6 5 0.40 298 74 – 757 1051 0.24 0.09 FA20-7.9 8 0.40 298 74 – 757 1051 0.66 – FA35-1.8 2 0.40 242 130 – 757 1051 – – FA35-4.5 5 0.40 242 130 – 757 1051 0.30 – FA35-8.0 8 0.40 242 130 – 757 1051 0.75 – SL20-2.8 2 0.40 298 – 74 757 1051 – 0.26 SL20-4.9 5 0.40 298 – 74 757 1051 0.08 – SL20-8.0 8 0.40 298 – 74 757 1051 0.20 – SL35-1.9 2 0.40 242 – 130 757 1051 – – SL35-4.0 5 0.40 242 – 130 757 1051 0.10 – SL35-7.5 8 0.40 242 – 130 757 1051 0.30 – *TAC: Target Air Content. 120 121 3.2. Test methods 122 3.2.1. Slump and air content of concrete 123 After five to ten minutes of mixing, the concrete was tested for slump according to ASTM 124 C143 and air content in accordance with ASTM C231. 125 3.2.2. Compressive strength 126 At the age of 28 days, concrete cylinders were tested for compressive strength in 127 accordance with ASTM C39. All cylinders were capped with sulfur prior to testing. The coefficient 128 of variation (COV) values of strength ranged from 0.26 to 7.9% and averaged 4.5%. At 600 days, 129 after the end of the exposure duration, the remaining cylinders which did not fail were tested for 130 compressive strength and compared with their initial values at 28 days. 131 3.2.3. Freeze-thaw cycles 132 At 28 days, three concrete cylinders from each mixture design were submerged in 25% 133 CaCl2 solutions in a sealed container (liquid-solid ratio of 0.7 by mass) and were then moved into 134 a freezer for 24 hours. After 24 hours in the freezer, the container was taken out of the freezer and 135 placed under ambient conditions for 24 hours after which the procedure was repeated. Using this 136 method, each cycle took 48 hours. Residue and spalled material in the containers were removing 137 on average every 30 cycles. The CaCl2 solution was replaced with fresh solution every 100 cycles. 138 Solution replenishment guidelines for such exposures are unavailable, and the frequency was 139 limited by the amount of effort and solution volumes needed. Temperature monitoring was 140 performed for ambient conditions, the CaCl2 solution, and the core of the concrete cylinders during 141 the cycles and the average values are shown in Figure 1. The temperature in the freezer varied 142 somewhat depending on the location, and the average ambient temperature is shown in Figure 1. 143 The average ambient temperature was -8 ± 2 °C. The lowest average temperature for the cylinders 144 and the solution were measured to be -4 ± 2 °C in the freezer. In each cycle, the cylinders spend 145 about 10 hours at freezing temperatures. On the thawing side, the ambient temperature reached 25 146 ± 2 °C while the sample and solution reached 24 ± 2 °C before they were moved back inside the 147 freezer. In order to account for the varying temperatures in the different locations in the freezer, 148 the sealed containers were placed in a randomized manner inside the freezer. The test conditions 149 are somewhat realistic in terms of temperature exposure in cold regions, however, the test is an 150 accelerated test, as immersion in 25% CaCl2 solutions is a harsh exposure condition compared to 151 field conditions. 152 153 Figure 1. Measured temperature for ambient conditions, CaCl2 solution, and concrete cylinders 154 during a full freeze-thaw cycle. 155 156 3.2.4. Mass change 157 The mass of the concrete cylinders was measured at 28 days and after approximately every 158 ten cycles during the first 150 cycles of experiment and approximately every 30 cycles on average 159 for the rest of experiment up to 300 cycles. The slowdown in the measurements was because many 160 mixtures showed damage which did not change substantially with time in the duration of 150 to 161 300 cycles. The surface of the concrete was air dried for 2 hours in the laboratory at 25 ± 2 °C 162 before mass was measured. The mass and dimensions were used to calculate density at 28 days. 163 The COV values of the density measurements at 28 days ranged from 0.9 to 2.3% and averaged 164 1.4%. 165 3.2.5. Bulk resistivity 166 The bulk resistivity of the concrete cylinders was measured at 28 days and after 167 approximately every 10 cycles for the first 150 cycles and every 30 cycles for the rest of the 168 experiment based on ASTM C1760 at a frequency of 1 kHz. The saturation state of the concrete 169 is unknown and variable, therefore interpretation of resistivity numbers is somewhat complexity, 170 however, as discussed later in the text, the resistivity proved to be an accurate indicator of the 171 concrete quality during exposure and damage. The COV values of bulk resistivity measurements 172 at 28 days ranged from 1.7 to 9.6%, and averaged 4.9%. 173 3.2.6. Surface resistivity 174 The surface resistivity of the concrete cylinders was measured at 28 days and after 175 approximately every 10 cycles for a limited number of cycles based on ASTM WK37880. Due to 176 the excellent correlations that were observed between surface and bulk resistivity, the surface 177 resistivity measurements were discontinued. Measurements were made using the four-point 178 Wenner probe method at a frequency of 40 Hz. The COV values of surface resistivity 179 measurements at 28 days ranged from 4.1 to 11.8% and averaged 6.5%. 180 3.2.7. Ultrasonic pulse velocity (UPV) 181 The UPV of the concrete cylinders was measured at 28 days and after approximately every 182 10 cycles for the first 150 cycles of the experiment and on average every 30 cycles for the rest of 183 the experiment in accordance with ASTM C597 using a pulse voltage equal to 250 V. A couplant 184 was used to ensure adequate contact between the transducers and the concrete. The COV values 185 of UPV measurements at 28 days ranged from 0.3 to 2.4% and averaged 1.3%. 186 3.2.8. Visual damage monitoring 187 Based on the damage observed in the concrete during the first 30 cycles, a visual damage 188 classification scheme was developed and is shown in Table 3. Each cylinder was carefully 189 observed and assigned one classification for each measurement. As the visual damage is only 190 assessed periodically, the condition of the concrete in between cycles is unknown. Here, an 191 averaging assumption is made – in the first half of the period between cycles, the condition of the 192 concrete is the same as the prior visual assessment, and in the second half of the period, the 193 condition of the concrete is the same as the post visual assessment. 194 195 Table 3. Visual damage classification. 196 Damage classification Description Representative image None No signs of cracks on the ends or the sides of the cylinders. Minor Short and thin cracks on the ends but no damage on the sides of the cylinders. Moderate Long and deep cracks on the ends; some cracking on the sides of the cylinders. Severe Large amount of spalling at the ends of the cylinder; some cracking on the sides and middle but middle is still intact. Failure More than half of the cylinder has spalled; cracks throughout the volume 197 3.2.9. Post-cycle measurements 198 For each cylinder that failed prior to the end of the 600 days test duration, cylinders were 199 crushed and small, representative, pieces from the core were extracted and oven dried at 100 °C 200 for 72 hours to evaluate the solution absorption. At the end of experiment, mixtures which did not 201 fail were tested for their compressive strength and then solution absorption was evaluated in a 202 manner similar to the failed mixtures. 203 204 205 4. Results and discussion 206 4.1. Fresh and hardened initial properties of concrete 207 Table 4 shows the fresh and hardened properties of the concrete tested at the time of mixing 208 and at 28 days. Measured slump ranged from 0 to 165 mm and air contents ranged from 1.8 to 209 8.0%. Loss of air content over time and the quality of the air void system is not considered in this 210 study but these important parameters and how they impact damage due to calcium oxychloride 211 formation should be studied in the future. 212 Table 4. Fresh and hardened properties for the concrete mixtures. 213 Mixture Slump (mm) Air content (%) Density (kg/m3) Strength (MPa) UPV (m/s) Bulk resistivity (Ohm-m) Surface resistivity (Ohm-m) OPC-2.6 0 2.6 2339.5 41.2 4538.5 38.1 71.8 OPC-6.6 75 6.6 2163.6 29.1 4231.7 43.8 72.3 OPC-7.9 50 7.9 2165.2 25.6 4314.5 52.0 93.8 FA20-2.1 65 2.1 2310.9 37.2 4439.2 46.6 75.7 FA20-5.6 50 5.6 2209.3 31.1 4294.5 51.5 83.8 FA20-7.9 75 7.9 2127.1 18.8 4186.3 50.8 86.2 FA35-1.8 65 1.8 2285.6 28.8 4423.3 53.4 97.1 FA35-4.5 115 4.5 2228.4 20.0 4414.2 63.2 106.2 FA35-8.0 165 8.0 2022.1 8.38 3796.7 63.4 124.4 SL20-2.8 75 2.8 2275.5 37.9 4565.7 57.3 101.9 SL20-4.9 35 4.9 2220.7 36.0 4377.8 56.7 96.9 SL20-8.0 90 8.0 2162.5 21.1 4284.2 66.4 114.4 SL35-1.9 25 1.9 2311.5 37.2 4672.3 92.9 153.6 SL35-4.0 40 4.0 2261.4 34.8 4545.7 109.5 197.7 SL35-7.5 110 7.5 2114.8 16.9 4286 106.7 188.4 214 Figure 2 illustrates the compressive strength for different concrete mixtures as a function 215 of the corresponding air content at 28 days. The compressive strength values range from 8.4 MPa 216 (FA35-8.0) to 41.2 MPa (OPC-2.6). The strength decreases as the air content and SCM amounts 217 increase [30-32]. Fly ash causes a greater decrease in the strength than slag at the same replacement 218 levels. The mixture with 35% fly ash shows poor strength performance at high air contents, 219 consistent with literature [32]. However, the fly ash is a Class F fly ash, which reacts slowly, 220 especially at early ages, so the strength of several mixtures is expected to improve over time. 221 Exposure at later-ages is being evaluated in a companion study. 222 The compressive strength roughly decreases linearly as the air content increases, though in 223 certain mixtures with SCM, the behavior may be considered to be bilinear, with greater slope at 224 higher air contents. The slope (the dependence of strength on air content) does not depend strongly 225 on SCM amount or type and is 3 ± 1 MPa for each percentage of air for the mixtures tested. These 226 findings, while in line with literature, highlight a potential issue with increased amounts of SCMs 227 that are being suggested in AASHTO PP84 – significant decreases in strength may result, with 228 some fly ashes. These decreases, which may be prominent at early ages, will then have to be 229 compensated through the use of lower w/cm and chemical admixtures, which could potentially 230 increase the concrete cost. 231 232 Figure 2. Compressive strength at 28 days plotted against air content. 233 234 Bulk resistivity values measured at 28 days as a function of air content are plotted in Figure 235 3. The bulk resistivity increases as the SCM replacement increases, which is well known from 236 literature [33]. The highest bulk resistivity is measured for the mixtures with 35% slag. A longer 237 curing duration is anticipated to further increase the bulk resistivity of the fly ash mixtures. The 238 general effect of increasing air content is to increase the bulk resistivity. However, different SCM 239 types and amounts produce different results, and at higher air contents, the resistivity is not 240 strongly affected by air content. Comparing Figure 2 and Figure 3 suggests that no obvious 241 correlation can exist between the two measurements; in fact, many of the strongest mixtures (OPC) 242 have low bulk resistivity values. 243 While not discussed in detail, analysis of the other initial hardened properties showed a 244 strong linear relationship between surface resistivity and bulk resistivity (discussed later in the 245 text), and a moderate linear relationship between compressive strength and UPV (consistent with 246 literature [34,35]). 247 248 Figure 3. Bulk resistivity values at 28-days for all concrete mixtures. 249 4.2. Visual monitoring during exposure 250 Figure 4 shows the number of cycles for each mixture to reach the designated damage 251 classification. Other studies monitoring concrete cylinders exposed to deicing salt and freeze-thaw 252 cycles have reported similar schemes of damage classification [36-38]. 253 Increasing air contents and increasing SCM amounts significantly reduce the damage at 254 any given cycle and the increase the time to failure. The improvement of calcium oxychloride 255 durability with air is due to two reasons. The first reason is a reduction in the degree of saturation 256 that is provided by the presence of air voids at any given exposure duration compared to mixtures 257 without air [3,29]. The second reason is the air voids provide space which can accommodate the 258 crystal growth or expansion of the calcium oxychloride [3]. Both these mechanisms are similar to 259 the role that air plays in reducing freeze-thaw damage. SCMs improve calcium oxychloride 260 durability because they reduce the amount of calcium oxychloride that forms (Eq. 1) through 261 dilution and pozzolanic reaction [19,23,39]. Greater SCM replacement levels and use of more 262 pozzolanic SCMs will further reduce the amount of calcium oxychloride that forms. SCMs are 263 also likely to modify the transport of CaCl2 into concrete, but a detailed discussion of transport 264 phenomena is out of the scope of this work. 265 Mixtures which have high amounts of air but no SCM, such as OPC-6.6 or OPC-7.9 fail 266 rapidly. While the air does provide space for expansion, the large amount of calcium hydroxide in 267 these concretes enables continuous formation of calcium oxychloride, which ultimately leads to 268 failure. Therefore, the usage of SCM to reduce damage due to calcium oxychloride formation in 269 concrete is essential. However, using only SCM but low amounts of air does not confer adequate 270 durability either, as seen in mixtures FA20-2.1 and SL20-2.8. While 20% SCM reduces the amount 271 of calcium oxychloride that forms, from paste studies, it is known that substantial amounts of 272 calcium oxychloride can still form at such replacement levels [9,19]. The lack of air to reduce 273 expansion/crystallization stresses leads to rapid damage. Therefore, visual observations confirm 274 that the use of air to reduce damage due to calcium oxychloride formation, in addition to the use 275 of SCMs which reduces the amount of calcium oxychloride formed, is essential. 276 As the air content and SCM replacement increase, the time duration spent in each damage 277 classification stage increases, meaning the increase in the time to failure with use of air and SCMs 278 is substantial. OPC-2.6 fails less than 40 cycles, FA20-5.6 fails at 160 cycles, whereas FA20-7.9 279 does not show any damage after 300 cycles. When 20% SCM is used, the highest air contents (8%) 280 show limited or no damage. When 35% SCM is used, even mixtures with 4% air do not fail during 281 exposure. Six of the mixtures with the qualities specified above did not fail during the duration of 282 the exposure. Based on these results, we can state that there is an interplay between air content and 283 SCM replacement. Specifications should take such an interplay into account, as suggesting 30% 284 SCM replacement or more may limit freedom for mixture designs. Considering that typical 285 mixture designs in cold regions call for 6.5% air content, mixtures with 35% SCM are not likely 286 to suffer damage due to calcium oxychloride formation under such conditions. However, it is 287 unclear whether mixtures with 20% SCM will show adequate performance, especially when 288 tolerances and variability in air content measurements are taken into account. In addition, 289 considering the considerable differences between FA20-5.6 and FA20-7.9, mixtures with 20% 290 SCM and 5% air content may show very different performance from mixtures with 20% SCM and 291 7% air content. The exact type of SCM used and the nature of the air void system are further 292 parameters which will influence behavior. Many of these parameters and their effects on calcium 293 oxychloride resistance have never been studied in literature; our findings suggest that such studies 294 are warranted. 295 OPC and fly ash mixtures became soft and mushy as damage increased and the mixtures 296 approached failure. However, in concretes containing slag, a similar softening phenomenon was 297 not observed. Instead, cylinders appeared to maintain their hardness, but showed spalling with 298 pieces fracturing with sharp faces. OPC and fly ash mixtures showed spalling with rounded edges. 299 300 Figure 4. Visual damage classification plotted against number of cycles. 301 302 4.2. Surface resistivity and UPV changes during exposure 303 Figure 5 shows the relationship between surface resistivity and bulk resistivity. For all 304 concretes tested, initially, and after exposure to several freeze-thaw cycles, there is a strong linear 305 relationship between surface resistivity and bulk resistivity. Similar relationships for concrete have 306 been reported by other studies [40,41]. Therefore, surface resistivity is not further explored in this 307 study. The conclusions that have been drawn for bulk resistivity are also valid for surface 308 resistivity, considering the linear nature of their relationship. 309 310 Figure 5. Relationship between surface resistivity and bulk resistivity for hardened concrete and 311 hardened concrete immersed in 25% CaCl2 solution and subject to freeze-thaw cycles. 312 313 Figure 6 shows the variation of the UPV during the exposure for concrete cylinders. The 314 UPV values reduce as the mixtures undergo damage. The OPC cylinders (Figure 6a) manifest 315 damage early, which leads to sharp reductions in the UPV values. As the air content in the OPC 316 mixtures increases, damage is manifested later (time to failure is increased). When the mixture 317 fails (based on Table 3 and Figure 4), UPV values reach zero. The reduction in the UPV values is 318 somewhat sudden and not continuous. As the SCM content increases, the time to failure increases 319 (Figure 6b and Figure 6c). Similarly, as the air content increases, the time to failure increases, 320 consistent with visual observations. Mixtures with high SCM and air contents, for example FA35-321 8.0 appear to not manifest any damage over the 300 cycles, as the UPV does not change over time. 322 The effect of air and SCMs is considerable – the OPC-2.6 mixture fails at less than 50 cycles, 323 whereas the FA35-8.0 mixture appears to not be damaged over 300 cycles. Therefore, air and 324 SCMs are essential in mixtures exposed to conditions in which calcium oxychloride formation can 325 occur. The UPV observations are in line with the visual observations. These findings suggest that 326 strength does not in any way control damage due to calcium oxychloride formation, as OPC-2.6 is 327 the strongest mixture based on compressive strength. Therefore, increasing strength is not a valid 328 strategy to improve durability against calcium oxychloride; whereas increasing air content and 329 SCM replacement are valid strategies. The decrease in UPV of concretes as they are damaged has 330 been studied for exposure to freeze-thaw cycles and salt scaling conditions [42,43], but studies 331 appear to be limited for conditions in which calcium oxychloride can form. Curve fitting to explore 332 the UPV behavior was explored but simple curves which explain the UPV changes were not 333 discovered. 334 Based on Table 3 and Figure 4, the number of cycles when the cylinders reach a visual 335 classification of severe is known. An attempt was made to define a UPV level or a reduction in 336 UPV values which correspond to a severe damage classification. For the OPC mixtures, a 4% 337 decrease in UPV values corresponded to the severe damage classification. However, this 338 “threshold” UPV reduction ranged from 2.5% to 8.1% for different mixtures, as it showed a strong 339 dependence on the air content and SCM replacement. While in certain mixtures a 5% UPV 340 reduction was reflected as severe damage (for example the OPC mixtures), other mixtures (SL35-341 7.5) did not show any damage even when the mass change exceeded 5%. Therefore, defining a 342 general threshold for damage based on UPV level is not trivial. UPV values drastically drop when 343 damage exceeds the severe classification, but the exact behavior again depends on air and SCM. 344 345 346 347 Figure 6. UPV plotted against number of cycles for mixtures a) OPC, b) fly ash, c) slag. 348 349 4.3. Mass changes during exposure 350 The mass change values for the control, fly ash and slag mixtures are plotted in Figure 7. 351 During the freeze-thaw cycles, all concrete mixtures demonstrate a generally increasing trend in 352 their mass as they absorbed CaCl2 solution. Increases in mass during exposure to freeze-thaw and 353 salt scaling conditions are known [42]. The OPC mixtures, which showed early failure, show rapid 354 increase in mass. This increase was especially seen in OPC-2.6, which failed after very few 355 measurements. For OPC-6.6 and OPC-7.9, the mass increases rapidly to values greater than 5%, 356 before failure occurs. Mass change measurements are complex because mixtures leach calcium 357 hydroxide and show spalling, in addition to solution sorption. Other studies have observed similar 358 complexities and have suggested measurement of the spalled material separately in addition to the 359 specimen mass to better evaluate mass changes [44]. When large-sized chunks of the concrete are 360 spalled, then the mass is reduced, which is reflected as a sharp drop in the curves in Figure 7. 361 “Final” values of mass change are not shown, as often mixtures are broken into pieces by this time 362 and the mass change measurement does not have any meaning when mixtures are in this condition. 363 Similar to the UPV, the use of air entrainment and SCM, causes slower increases in mass. The 364 slope of the curves in the initial portion appears to be reduced as SCM replacement is increased. 365 However, the curves are not particularly linear, which makes a quantification of the slope 366 somewhat complex. Mixtures with both air and SCM commonly show a two-stage behavior where 367 the mass loss increases only slightly after a certain point, but this is not the case for all mixtures. 368 It appears that mixtures with greater amounts of air and SCM show a lower mass change (increase) 369 at a given numbers of cycles. The mass change needed to induce failure increases with air content: 370 OPC-6.6 failed at 4.6% mass change whereas OPC-7.9 failed at 6.2% mass change. Corresponding 371 values for FA20-2.1 and FA20-5.6 are 2.8% and 6.1%, respectively. 372 A mass change “threshold” for severe damage/failure cannot be defined as it depends on 373 air content and SCM replacement. Some mixtures (FA20-2.1) show failure at mass change values 374 below 3%, but other mixtures (SL35-7.5) show no damage even at mass change values of 4% or 375 greater. 376 377 378 379 380 Figure 7. Mass change vs. number of cycles a) OPC, b) fly ash, c) slag. 381 382 4.4. Bulk resistivity changes during exposure 383 Bulk resistivity values for the control, fly ash and slag mixtures are shown in Figure 8. In 384 all mixtures, the bulk resistivity reduces as the number of cycles increases and the CaCl2 ingresses 385 into the concretes. The bulk resistivity of the concretes increases with air content and SCM 386 replacement, which is consistent with visually better-quality concretes (after a fixed number of 387 cycles) and the results obtained from UPV and mass change. It is evident from the different tests 388 that air and SCM improve the performance of concrete undergoing damage due to calcium 389 oxychloride formation for reasons discussed in Section 4.2. 390 As the mixtures are exposed to CaCl2 solution and freeze-thaw cycles, the bulk resistivity 391 values decrease. The initial decrease is rapid, likely because the solution ingress changes the pore 392 solution conductivity and the degree of saturation [3,6]. For concretes which show poor 393 performance (for example OPC-2.6), cracking and spalling initiate early, and the concrete sorbs 394 large amounts of the ionic solution, leading to very low values of bulk resistivity before failure. 395 As the amount of air increases, for example, in the mixtures OPC-6.6 and OPC-7.9, the bulk 396 resistivity decreases rapidly, but then the rate of decrease reduces. However, the bulk resistivity in 397 these mixtures continues to reduce, as they suffer damage and ultimately fail. As the amount of air 398 and SCM replacement increases, the initial bulk resistivity at 28-days increases. The bulk 399 resistivity reduces during exposure, but at a given number of cycles, the bulk resistivity is higher 400 in mixtures with greater amounts of air and SCM replacement. In addition, many mixtures with 401 greater amounts of air and SCM (for example FA35-8.0) have bulk resistivity values that are 402 unchanged after an initial reduction. The lack of a further reduction in the bulk resistivity is 403 consistent with a lack of damage in these mixtures from visual observations (Figure 4). 404 The initial bulk resistivity should in principle be a strong predictor of calcium oxychloride 405 durability, as bulk resistivity increases as SCM replacement increases. However, the bulk 406 resistivity does not depend strongly on air content, so a function which combines the bulk 407 resistivity and air content is likely to be a reliable indicator of mixture resistance to damage due to 408 calcium oxychloride formation. This is a powerful finding, because air content and bulk resistivity 409 are relatively simple measurements to perform on any concrete. However, in our study, the initial 410 bulk resistivity is not always an accurate indicator of calcium oxychloride durability. For example, 411 the initial bulk resistivity values of OPC-7.9 and FA20-7.9 are similar, although the former fails 412 at 140 cycles whereas the latter shows no damage even at 300 cycles. This discrepancy is because 413 the fly ash reaction extent is low at 28-days [45,46]. The bulk resistivity of FA20-7.9 would have 414 continued to increase as the fly ash reaction increases, although bulk resistivity values of OPC 415 mixtures typically plateau at 28-days. Therefore, mature measurements of bulk resistivity (56- or 416 91-days) would be better predictors of calcium oxychloride durability. Using bulk resistivity would 417 be an attractive and practical option as the measurements are relatively simple to perform and 418 could be performed in a routine manner by concrete producers. The use of surface resistivity 419 (which correlates with bulk resistivity) would be especially attractive as it could be used to rapidly 420 assess the threat of calcium oxychloride damage in existing pavements. 421 While UPV and mass change did not appear to show the same type of behavior for all 422 mixtures, bulk resistivity appears to be more consistent in this regard. For the bulk resistivity, most 423 mixtures appear to show a power law type decay in their response when exposed to the 25% CaCl2 424 solution. Saturation functions that describe the behavior of partially saturated concrete have been 425 suggested to follow a power law [47]. The reduction in bulk resistivity in mixtures exposed to 25% 426 CaCl2 solution is driven by both increase in the degree of saturation and a decrease in pore solution 427 resistivity. As the behavior observed here is similar to the power law saturation function, one 428 interpretation is that the changes in degree of saturation may dominate the changes in the pore 429 solution resistivity. 430 While threshold values of UPV and mass change were not found, for the bulk resistivity, 431 by comparing to the visual damage (Figure 4), it appears that when the bulk resistivity is reduced 432 to a value below 4 Ohm.m, the cylinders reach a severe/failure damage classification. Therefore, 433 one can tentatively consider a value of 4 Ohm.m of bulk resistivity as a failure threshold when 434 concrete is damaged by calcium oxychloride formation. A corresponding threshold for 435 minor/moderate damage is not evident, and such a threshold may depend on mixture design. A 436 correlation between the degree of saturation and damage due to calcium oxychloride formation 437 has not been established, but it is reasonable that mixtures which reach high degree of saturation 438 values quicker will also fail sooner. This thinking is consistent with freeze-thaw damage, where 439 concrete fails rapidly after exceeding a critical degree of saturation [6]. For freeze-thaw, the air 440 content strongly affects the time needed to reach a critical degree of saturation. For calcium 441 oxychloride, if a critical degree of saturation exists, in addition to the air content, it would also 442 depend on the SCM replacement. 443 Since the bulk resistivity appears to show a consistent response, and a threshold value for 444 failure could be defined, curve fitting for the bulk resistivity was performed using power law fits, 445 as shown in Figure 8. All mixtures, except two mixtures which showed rapid failure (FA20-2.1 446 and SL20-2.8) were fit using power laws. The fits appear to largely have good R2 values. As a 447 result, bulk resistivity may be used to predict failure in concrete in such conditions. While 448 saturation driven reductions in the bulk resistivity are feasible [47], the exact physical significance 449 of such fitting is not clear. Nevertheless, the fits are important as they can in principle be used to 450 predict the time to failure for mixtures which have not failed. 451 452 453 454 455 456 Figure 8. Bulk resistivity values and fitted lines for a) OPC, b) 20% fly ash, c) 35% fly ash, d) 457 20% slag, e) 35% slag. 458 459 The predicted time to failure for all mixtures is shown in Table 5. Assuming the failure 460 threshold of 4 Ohm.m for samples as point of failure, the expected number of cycles to failure was 461 evaluated for all mixtures where fitting was performed. The prediction error is also shown in Table 462 5. The predicted error ranges from -12% to +30%, depending on the mixture design, with average 463 of 10%. Considering the wide variety of mixtures tested, these numbers appear to be rather 464 reasonable. The table demonstrates quantitatively that the time to failure is increased very 465 significantly as the air and SCM contents are increased, consistent with results obtained other 466 measurements. 467 4.5. Post-exposure measurements 468 After 300 cycles, the cylinders from the six remaining mixture designs which failed were 469 tested in compression. Cylinders were left to air dry for 24 hours before the test. The measured 470 strength values and corresponding 28-day values are shown in Table 6. The table shows that all 471 mixtures experienced some level of damage during exposure, except FA35-8.0. Mixtures with 472 higher air contents and SCM replacements show lesser damage, as evidenced by smaller strength 473 reductions. Fly ash in some mixtures may have continued to react during exposure as the 28-day 474 curing was not adequate for full fly ash reaction [45,46,48]. 475 The measured solution absorption is shown for all mixtures in Figure 9. There appears to 476 be a relationship between the durability of the concrete and the absorption values. Specifically, 477 mixtures which fail early appear to have absorbed a greater amount of solution. This is unsurprising 478 as the solution sorption is what ultimately leads to failure. The cylinders which did not fail all 479 show absorption below 8%, whereas almost all mixtures which failed show sorption greater than 480 8%. Absorption values decrease as SCM replacement and air content increases. 481 482 Table 5. Measured and predicted number of cycles to failure using power law fits for the bulk 483 resistivity (N/A: For FA20-2.1 and SL20-2.8, fitting was not possible due to early failure). 484 Mixture name Measured cycles to failure Damage status at 300 cycles Predicted cycles to failure Prediction error (%) OPC-2.6 33 Failure 43 30% OPC-6.6 88 Failure 71 -19% OPC-7.9 141 Failure 170 21% FA20-2.1 29 Failure N/A - FA20-5.6 152 Failure 138 -9% FA20-7.9 - None 1287 - FA35-1.8 85 Failure 109 28% FA35-4.5 - Moderate 2871 - FA35-8.0 - Minor 12382 - SL20-2.8 67 Failure N/A - SL20-4.9 150 Failure 197 31% SL20-8.0 - Moderate 1406 - SL35-1.9 117 Failure 103 -12% SL35-4.0 - Severe 473 - SL35-7.5 - None 1080 - 485 Table 6. Changes in compressive strength of mixtures after 300 cycles (SD is the standard 486 deviation of the compressive strength measurements). 487 Mixture name 0 Cycles 300 Cycles Change (%) Damage Status Compressive Strength (MPa) SD (MPa) Compressive Strength (MPa) SD (MPa) FA20-7.9 18.85 1.28 15.69 0.33 -17% None FA35-4.5 20.04 0.05 17.13 1.50 -15% Moderate FA35-8.0 8.38 0.13 13.06 0.13 56% Minor SL20-8.0 21.08 0.58 13.72 2.15 -35% Moderate SL35-4.0 34.85 2.58 4.94 0.36 -86% Severe SL35-7.5 16.95 0.68 14.78 0.31 -13% None 488 489 Figure 9. Solution absorption of the various mixtures. 490 491 As discussed earlier, the bulk resistivity and air content should in principle be able to 492 predict concrete durability when exposed to calcium oxychloride formation. As exposure was 493 performed at early ages, here, SCM replacement level is used as a proxy for the bulk resistivity. 494 Figure 10 shows a plot of cycles to failure (from Table 5, measured values used where available, 495 predicted values used otherwise) plotted against the multiplied SCM replacement level in (wt. %) 496 and air content (volume %) with the number cycles to failure. The cycles to failure increase as the 497 SCM replacement level multiplied by the air content increases, especially beyond a threshold level. 498 Depending on the cycles to failure, each mixture is classified into one of two performance groups 499 as defined below: 500 • Poor durability: Concrete is expected to fail in less than 300 cycles. 501 • High durability: Concrete is expected to fail after more than 300 cycles. 502 According to this classification, if concrete mixture SCM replacement multiplied by air 503 content is more than 125, mixtures would be highly durable when exposed to high concentrations 504 of CaCl2. Some error in such assessments is likely, but this finding appears to be a good starting 505 point for further evaluation of realistic field mixtures. Concrete mixtures with 20% SCM and more 506 than 7.9% air or 35% SCM and more than 4.0% air fall into the highly durable group. Using SCM 507 replacement multiplied by air content, we are able to theoretically predict the time to failure for 508 other mixtures. Minimum levels of air needed for adequate durability at different SCM levels are: 509 15% SCM (8.3% air), 20% SCM (6.25% air), 25% SCM (5% air), and 30% SCM (4.2% air). 510 Recommended air contents in ACI documents vary depending on aggregate size, but for severe 511 exposure, they are usually around 6.5%. However, field tolerance on air content is typically 1.5%. 512 For an 8% air mixture, 15% SCM would be adequate, but for a 5% air mixture, 25% SCM would 513 be adequate. 514 When one considers the AASHTO PP84 guidelines, the 30% SCM replacement appears to 515 be on the higher side. However, our calculations do not have an inbuilt factor of safety and are 516 based on limited data in the air content range of 5% to 8%. Therefore, at this time, we suggest that 517 the 30% SCM replacement in AASHTO PP84 is reasonable. Further data on impacts of the curing 518 duration, effects of SCM chemistry, later-age bulk resistivity evolution, air void system, and 519 performance of realistic mixtures (5% to 8% air and SCM replacements between 15% and 30%) 520 are needed before implementation of changes in the AASHTO PP84 replacement level. 521 Developing suitability criteria for concrete that will be exposed to deicing salts based on a 522 combination of air contents, SCM replacements, and bulk resistivity measurements will an 523 important and implementable advantage. If such relationships can be developed, the other 524 AASHTO PP84 provision, which calls for the quantification of calcium oxychloride in 525 cementitious pastes, a rather complex measurement, could potentially be simplified. 526 527 Figure 10. Performance classification of mixture designs based on air content and SCM 528 replacement. 529 4. Conclusions 530 This study investigated the synergistic roles of air and supplementary cementitious 531 materials (SCMs) in reducing concrete damage when exposed to concentrated CaCl2 solutions and 532 freeze-thaw cycles. The following conclusions can be drawn based on the monitoring of damage 533 over 600 days and post-exposure measurements: 534 • The use of air entrainment and SCMs reduces the damage at any given cycle. Damage is 535 reduced as the SCM replacement increases. 536 • Bulk resistivity, surface resistivity, and visual assessment appear to be clear indicators of 537 damage progression over time. UPV and mass change are also damage indicators, however, 538 they are somewhat more complex to interpret. 539 • Mixtures which had 20% SCM and 8% air and mixtures which had 35% SCM and more 540 than 4% air showed strong durability against damage caused by calcium oxychloride 541 formation. 542 • Based on power law fits for bulk resistivity, time to failure can be estimated for mixtures 543 which have not failed. 544 • SCM content multiplied by air content can differentiate mixtures which show poor and 545 high durability under such exposure conditions, with a threshold value of 125 between the 546 two behavior types. 547 • Further testing of realistic field mixtures and establishment of thresholds based on air 548 content, SCM replacement, and bulk resistivity is an important step in better designing 549 mixtures for cold regions. 550 551 5. Acknowledgements 552 RMC Research & Education Foundation is thanked for providing the funding needed to 553 carry out this study. Titan America, specifically Dr. Eric Koehler, are thanked for support and for 554 providing materials used in this study. The support of Ozinga Inc., specifically Dr. Amir 555 Hajibabaee, is also acknowledged. Dr. Micah Hale (University of Arkansas), Casey Jones 556 (University of Arkansas), Dr. Colin Lobo (NRMCA), Luca Montanari (SES Group & Associates, 557 LLC), Dr. Karthik Obla (NRMCA), and Dr. Robert Spragg (Federal Highway Administration 558 Turner-Fairbank Highway Research Center) are thanked for their insightful thoughts on calcium 559 oxychloride which have helped improve the quality of this manuscript. 560 561 562 6.