8 Construction using concrete through additive manufacturing is gaining attention. This approach 9 involves a layer-wise deposition of concrete. A layer of printed concrete needs to be \strong" enough 10 to sustain the weight of layers to be printed above. These layers may not bond well with each other, 11 however. Self-compacting concrete (SCC) may lead to a better bonding, but it may not have sucient 12 strength. This paper presents a method to print SCC through controlled heating. Properties of printed 13 concrete in fresh and hardened states are studied. Heating leads to a sharp rise in the strength of 14 freshly-printed concrete layers, but a long duration of heating may lead to a reduction in strength of 15 printed concrete after hardening particularly if water-to-cement ratio is small or if loads are applied 16 parallel to the printed layers. 17 Keywords: self-compacting concrete; additive manufacturing; 3D printing; heating; infrared 18 re ectance; surface texture; buildability 19 1. Introduction 20 Conventional construction using concrete includes preparation of concrete mix, placement of the mix 21 into the formwork, compaction, removal of the formwork after the concrete has gained sucient strength, 22 and curing. This process is labour-intensive, time-consuming, and the shape of a structural member 23 is often limited by the available formwork. Additive manufacturing (also known as three-dimensional 24 (3D) printing) of concrete can help overcome these challenges [1{7]. 25 Fresh-state properties of cast-in-situ concrete are characterized in terms of setting time, workability 26 etc. Similarly, strength and sti ness parameters are used to characterize the hardened concrete cast- 27 in-situ. Parameters such as buildability, shape stability, extrudability, open time and surface moisture 28 have been reported for freshly-printed concrete in the past [8, 9]. Similarly, shrinkage and strength of 29 printed concrete after hardening also have been reported [10]. A brief discussion on the properties of 30 printed concrete is presented in the paragraphs below. 31 Buildability is an indicator of the strength of freshly-printed concrete. There is no widely accepted 32 1 de nition of buildability. Maximum number of free-standing layers (e.g., [8]), compressive strength of 33 a freshly-printed specimen (e.g., [9, 11]), ultrasonic pulse velocity through a freshly-printed specimen 34 (e.g., [12]), and depth of penetration of Vicat plunger into freshly-printed concrete layers [13] have 35 been considered as measures of the buildability in the past. A parameter related to buildability is 36 shape stability, which refers to the similarity between the achieved and intended dimensions of a printed 37 specimen. Ali Kazemian et al. [9] printed a lament of concrete through a 38.1 mm 25.4 mm nozzle, 38 and the width of the printed lament ranged between 38.1 mm and 48 mm. They considered a printed 39 specimen to have \dimension consistency" if the dimensions of the printed line were within 10% of the 40 dimensions of the nozzle. Rahul et al. [14] printed a lament of concrete through a nozzle of size 30 mm 41 20 mm, and considered a mix to be \extrudable" if dimensions of the cross-section of the printed 42 lament was within 0.5 mm of the dimensions of the nozzle (approximately 2% of the dimension of the 43 nozzle). Panda et al. [15] de ned a \shape retention factor" as the ratio of cross-sectional area of the 44 nozzle to that of the printed line, and observed that the factor for the printed specimens was between 45 0.7 and 0.9. Open time for printing a concrete mix is de ned as the duration for which the mix can be 46 extruded with a consistent rate without chocking the pump. Le et al. [8] have de ned the open time as 47 the duration in which the yield strength of the mix increases by 0.3 kPa from its initial value. Surface 48 moisture can be characterized as the mass of water extracted from the surface of printed layer through 49 a paper cloth (e.g., [16]). 50 Strength of the printed specimens vary depending on whether the direction of loading is parallel or 51 perpendicular to the printed layers [10, 17, 18, 18]. A standard method to perform the tests (e.g., size 52 of specimens) is yet to evolve. Drying shrinkage in printed concrete has been reported to be a ected by 53 curing conditions (e.g., [10]). Pore sizes ranging between 0.2 mm and 4 mm have been reported (e.g., 54 [10]); also see Figure 1(b). Porosity of the printed concrete after hardening has also been reported [18]. 55 A wide spectrum of concrete mixes have been printed in the past: high strength concrete (e.g., 56 [10]), ultra-high strength concrete (e.g., [19]), bre-reinforced concrete (e.g., [20, 21]), and geo-polymer 57 concrete (e.g., [22, 23]). Reported initial yield strength of the concrete printed till date has been in the 58 range of 300 { 4,000 Pa [8, 11, 22, 24{27]. Rahul et al. [14] suggest that initial yield strength of the 59 concrete mix for acceptable extrudability and buildability should be in the range of 1,500 { 2,500 Pa. 60 Filaments of the printed concrete are \strong" as a consequence, and do not bond well with each other 61 leaving considerable amount of voids after hardening [16, 18, 28, 29]. Panel (a) of Figure 1 presents the 62 schematic of a section through \strong" laments, and panel (b) shows a surface of the printed concrete 63 with strong laments cut after hardening [10]. A better bonding between laments can be achieved using 64 a self-compacting concrete (SCC); initial yield strength of SCC is less than 100 Pa (e.g., [30, 31]). A 65 challenge in printing the SCC, however, is to ensure that a printed layer achieves the capacity to support 66 2 the layers above in a reasonable time frame. The initial strength gain of the SCC can be accelerated 67 using chemical admixtures (e.g., calcium sulpho-aluminate, calcium chloride, calcium aluminate and 68 sodium sulphate [9, 13, 21, 32{39]). However, the admixtures may cause choking in the printing set-up 69 due to accelerated setting, lead to expansive hydration reaction [40], produce high heat of hydration [41], 70 and/or have chemicals that can accelerate corrosion in reinforcement. These accelerators often need to 71 be added to the mix near the nozzle (e.g., in shotcrete applications [32, 42, 43]), which complicates the 72 design of the printing set-up. 73 (a) Schematic of voids between laments (b) Voids in printed concrete (adapted from [10]) Figure 1: Voids between laments of printed concrete This paper presents a method to print self-compacting concrete mixes through controlled heating. 74 An in-house concrete printing set-up has been developed at Indian Institute of Technology (IIT) Gandhi- 75 nagar, India. The set-up comprises of motion assembly, extrusion assembly, heating system, and infrared 76 re ectance feedback system [44]. The motion assembly enables the horizontal and vertical motion of 77 the printing platform. Self-compacting concrete is deposited on the printing platform through the ex- 78 trusion assembly. Heating system removes a portion of the moisture from printed layer, which increases 79 the buildability of the printed layers. Infrared (IR) re ectance feedback system measures the surface 80 re ectance of the printed layers. The concrete printing setup is operated using an ATmega-2560 micro- 81 controller on Arduino integrated development environment (IDE) [45]. Three self-compacting concrete 82 mixes are considered, which have identical workability but di erent water-to-cement ratios. Fresh-state 83 properties of printed concrete, namely, shape stability, buildability, layer moisture, surface moisture, 84 infrared surface re ectance, and early-age shrinkage are studied. Properties of printed concrete after 85 hardening, namely, pore size, shear strength, and compressive strength, are also studied. 86 Section 2 of the paper presents the details of the concrete printing set-up. Details of self-compacting 87 concrete mixes considered for printing are presented in Section 3. Test methods used to characterize the 88 properties of freshly-printed concrete are described in Section 4. Properties of freshly-printed concrete 89 are presented in Sections 5. E ect of heating on the evolution of buildability is presented in Section 6. 90 Properties of hardened concrete are presented in Section 7. 91 3 2. Concrete printing set-up 92 The process of printing concrete involves deposition of a layer of concrete on a printing platform, 93 and heating the printed layer until \buildability" is achieved. The next layer is then printed and the 94 cycle is continued. Figure 2 shows the concrete printing set-up developed at IIT Gandhinagar. Details 95 on the components of the printer are presented in sections below. 96 Figure 2: Concrete printing set-up at IIT Gandhinagar (adapted from [46]) 2.1. Motion assembly 97 Motion assembly enables the translation of the printing platform independently in the three orthog- 98 onal directions (two horizontal, one vertical). The assembly comprises of stepper motors [47], threaded 99 rods, pulley-belt system, guiding rails, and the top platform that is connected to the printing platform 100 with a workspace of 300 mm 300 mm 300 mm. Figure 3(a) shows the schematic of the vertical 101 motion assembly, wherein four stepper motors are placed with their shafts along the threaded rods. 102 This set-up facilitates the vertical movement of the horizontal motion assembly. The horizontal motion 103 assembly enables the horizontal movement of the top platform along the two orthogonal horizontal 104 directions (see Figure 3(b)). Figure 3(c) shows the schematic of the complete motion assembly. 105 2.2. Extrusion assembly 106 Extrusion assembly is used to extrude the fresh concrete through a nozzle onto the printing platform. 107 The assembly comprises of a 250 watt direct current (DC) motor (torque capacity of 200 kg-cm, and 108 maximum revolution per minute (rpm) of 200), 25 mm-diameter steel screw, rigid coupling, conical 109 hopper, nozzle with an inside diameter of 20 mm, and a load cell with capacity of 40 kg mounted 110 between the printing platform and the top platform. Figure 4 shows the schematic of the extruder 111 that remains static during the printing (also see Figure 2). The rate of printing is controlled using a 112 proportional-integral-derivative (PID) control algorithm (e.g., [48]). 113 4 (a) Vertical motion assembly (b) Horizontal motion assembly (c) Vertical and horizontal motion assembly Figure 3: Motion assembly of the concrete printer (adapted from [46]) 2.3. Heating system 114 Heating system comprises of two commercially available 1,000 Watt quartz radiation heaters and six 115 12 V DC fans (not shown in Figure 2). The system is mounted between the extrusion assembly and the 116 printing platform. This system removes a portion of moisture from the printed layer, which changes 117 the texture of the printed surface. Figure 5 shows the surface of freshly-printed concrete before ( glossy 118 texture) and after ( matt texture) heating. 119 2.4. Infrared re ectance feedback system 120 The infrared (IR) re ectance feedback system comprises of an array of IR re ectance sensors (see 121 Figure 6) mounted between the extruder and the top of motion assembly. An IR sensor consists of an 122 infrared wave emitter and a photo-transistor (e.g., [49]) receiver. The IR waves generated by the emitter 123 are partially re ected from the printed surface, and the intensity of the re ected wave is detected using 124 the photo-transistor receiver. A 10 bit ATmega2560 microcontroller converts the analog signal received 125 from the IR sensor to a digital signal valued between 0 and 1023, where 0 corresponds to a perfectly 126 5 Figure 4: Schematic of the static extruder and the printing platform (drawing not to scale) Figure 5: Freshly-printed concrete surface before and after heating (adapted from [46]) re ecting surface and 1023 corresponds to a perfectly non-re ecting surface. 127 3. Self-compacting concrete mixes 128 Method proposed by Aitcin [50] was followed to design a self-compacting concrete mix (referred to 129 as \HSC1") with a target mean strength of 80 MPa after 28 days. Ordinary Portland cement (OPC) 130 of 53 grade per IS 12269 [51] was used (make: Ultratech). Water-to-cement and cement-to-sand ratios 131 were kept at 0.32 and 1.65 (by mass), respectively. Ten percent silica fume (% of cement by mass) was 132 added to increase the packing density and viscosity [50]. Coarse aggregates were not used. Workability 133 of the mix was increased by adding 1.1% (% of cement by mass) ASTM Type-F polycarboxylate ether- 134 6 Figure 6: Infrared re ectance sensor array (drawing not to scale) based superplasticizer [52] (make: Agrosyn Impex). The concrete mix had 744 kg cement, 1,227 kg 135 sand, 75 kg silica fume, 238 kg water, and 8 kg superplasticizer per cubic meter. The mix had a ow 136 table spread [53] of 270 mm. Two more concrete mixes HSC2 and HSC3 were prepared, which had 137 water-to-cement ratios 0.36 and 0.40, respectively. Suitable amounts of superplasticizers were added 138 to the two mixes so that their ow table spreads were same as HSC1 mix. Other parameters, namely, 139 cement-to-sand and cement-to-silica fume were identical for the three mixes. Table 1 summarizes the 140 details of the three mixes. 141 Table 1: Proportion (by mass) of ingredients in concrete mixes Mix ID OPC Silica fume Sand Water Superplasticizer HSC1 1 0.1 1.65 0.32 0.011 HSC2 1 0.1 1.65 0.36 0.005 HSC3 1 0.1 1.65 0.40 0.002 Roussel [54] had proposed an expression to calculate yield strength of fresh concrete ( 0) based on 142 mini-cone spread: 143 0= 1:747V2R5R2=V (1) where,is density of concrete, Vis volume of mini-cone, Ris mini-cone spread, and is a function of 144 surface tension and contact angle of the SCC mix. Mini-cone spread for the three mixes of Table 1 were 145 168 mm, 170 mm and 162 mm, respectively. Since the mini-cone spread was lower than 350 mm, the 146 surface tension e ects were neglected in the yield strength calculations [54]. Accordingly, initial yield 147 strength of the three mixes were 27 Pa, 25 Pa and 33 Pa, respectively. 148 4. Tests to characterize fresh-state properties of printed concrete 149 Tests were carried out to characterize the following properties of freshly-printed concrete: 1) shape 150 stability, 2) buildability, 3) layer moisture, 4) surface moisture, 5) infrared surface re ectance, and 6) early-age 151 7 shrinkage. Following sections present the details of the tests. 152 4.1. Shape stability 153 Shape stability test was carried out to determine the thickness of a freshly-printed layer that remains 154 \stable" under gravity. The test set-up comprises of hollow cylinders with an inner diameter of 20 mm, 155 and heights of 30 mm, 20 mm, 15 mm, 10 mm, 8 mm, 6 mm and 4 mm (see Figure 7). A cylinder is 156 rst lled with an SCC mix. The cylinder is then pulled upwards (e.g., in slump test). No visible change 157 in height and diameter of the cylinder-shaped SCC specimen indicates shape stability. 158 Figure 7: Shape stability test set-up 4.2. Buildability 159 Two measures to characterize buildability were considered: 1) compressive strength of printed spec- 160 imens, and 2) yield strength of printed layers determined through a Vicat penetration test. Details of 161 the two approaches are presented in the sections below. 162 4.2.1. Compressive strength test 163 A compression testing set-up with a capacity of 300 N was developed. The set-up also comprised 164 of two heat guns of 1,000 watt each and an IR re ectance feedback array with ve IR sensors (see 165 Section 2.4). Figure 8 shows the schematic of the compression testing set-up. Figure 9 shows the 166 compression testing set-up developed at IIT Gandhinagar. Variation in the re ectance of the printed 167 surface could be recorded, while the surface was being heated. A caulking gun shown in Figure 10 (e.g., 168 [21]) was used for printing specimens for this test. A fresh concrete layer was printed on a metal plate 169 rst, and was then placed in the test set-up. The layer was heated in the set-up using the two heat 170 guns placed at an angle of 10with respect to the horizontal and pointing towards the surface of the 171 freshly-printed layer. The approximate horizontal distance between the nozzle of the heat guns and 172 the printed layer was 150 mm. The printed layer was taken out after a speci ed duration of heating 173 and next layer was printed. The procedure was followed to prepare a specimen with desired number of 174 8 layers. A piece of chosen size was cut from the printed specimen, which would then be subjected to the 175 compression test. 176 Figure 8: Schematic of compression testing set-up (drawing not to scale) Figure 9: Compression testing set-up 4.2.2. Vicat penetration test 177 Penetration of Plunger G used for the Vicat standard consistency test [55] in a printed specimen 178 was used to determine yield strength of the printed specimen [56]. The yield strength is considered the 179 second measure of buildability. The caulking gun of Figure 10 was used to print a layer on a metal plate. 180 The layer was heated using two 1,000 Watt heat guns arranged with respect to the printed specimen in 181 a manner similar to the compression testing set-up. The penetration test was conducted after printing 182 a sucient number of layers, and the yield strength ( 0) was determined using the following expression 183 [56]: 184 0=F=(2Rh) (2) 9 where,his the penetration depth of Plunger G, and RandFare radius (= 5 mm) and weight of Plunger 185 (= 300 grams), respectively. 186 4.3. Layer moisture 187 Specimens to determine the moisture in a layer during heating were printed using the caulking gun of 188 Figure 10. A printed layer was placed on a weighing balance with 0.01 gram resolution, and was heated 189 in a manner similar to the Vicat test. Change in weight of the printed layer was manually recorded 190 every 10 seconds. The change indicates the loss of moisture in the layer due to heating. 191 Figure 10: Caulking gun 4.4. Surface moisture 192 Surface moisture in a printed layer was measured using the method proposed by Sanjayan et al. [16]. 193 The layer was prepared using the caulking gun of Figure 10, and it was heated in the manner similar to 194 the Vicat test. Subsequently, a paper towel with plan dimensions same as the printed layer was placed 195 on the printed layer for a duration of 20 seconds. Increase in the weight of the paper towel was taken 196 as surface moisture. 197 4.5. Infrared surface re ectance 198 Specimens to determine IR surface re ectance of a printed layer were prepared using the caulking 199 gun. The printed specimen was placed in the compression testing set-up. The specimen was heated 200 and the surface re ectance was measured in parallel using the array of ve IR sensors placed in the 201 compression testing set-up (see Section 4.2). 202 4.6. Early-age shrinkage 203 A concrete layer with plan dimensions of 300 mm 100 mm was printed using the caulking gun on 204 a lubricated granite slab. Two light-weight re ectors were placed centrally and 250 mm apart on the 205 printed layer. The layer was heated in a manner similar to the Vicat test. Change in distance between the 206 re ecting surfaces was measured using two SHARP GP2Y0A51SK0F analog IR laser distance sensors, 207 and an Arduino Due-R3 ARM-Cortex-M3 control board was used to record the data. Figure 11 shows 208 the schematic of the early-age shrinkage test set-up and Figure 12 shows the actual set-up. 209 10 Figure 11: Schematic of the shrinkage test set-up (drawing not to scale) Figure 12: Shrinkage test set-up 5. Properties of freshly-printed concrete 210 Properties of freshly-printed concrete identi ed in Section 4 were determined for the three mixes 211 presented in Table 1. The results are presented in the sections below. 212 5.1. Shape stability 213 The hollow cylinders shown in Figure 7 were lled with fresh HSC1 mix, and the cylinders were 214 pulled out immediately. The nal shapes of the specimens with initial heights 8 mm or greater were 215 substantially di erent compared to the respective initial shapes, as seen in Figure 13. The nal shapes for 216 specimens with 6 mm and 4 mm heights were comparable to their respective initial shapes. Observations 217 for HSC2 and HSC3 mixes were similar to HSC1. These results can be a basis for determining the layer 218 thickness (controlled through ow rate) during printing. 219 Figure 13: Shape stability test of HSC1 Mix 11 5.2. Buildability 220 5.2.1. Compressive strength test 221 A layer of HSC1 mix (see Section 3) was printed on a metal plate using the caulking gun. The plate 222 was placed in the compression testing set-up (see Section 4.2). The layer was heated for 60 seconds. A 223 second layer could not be printed if the printed layer was heated for a duration less than 60 seconds. The 224 metal plate was taken out of the test set-up, and another layer was printed and heated for 60 seconds. 225 This process was carried out a total of ve times. Figure 14 shows the plate with ve printed layers 226 placed in the test set-up. Total thickness of the ve layers was 30 mm. A cube of size 30 mm 30 mm 227 30 mm was prepared from the specimen of Figure 14. The cube was placed back in the compression 228 test set-up, as shown in Figure 15. The compressive strength test started a total of 20 minutes after 229 water was added to the HSC1 mix. Compression head of the test set-up was lowered 2 mm every minute 230 (e.g., [57]). 231 Figure 14: Sample preparation for compression testing of freshly-printed concrete Figure 16(a) plots recorded compressive force against axial strain in the cube1;2. The cube could be 232 loaded without experiencing any visible cracks till an axial strain of approximately 6%. Results for mixes 233 HSC2 and HSC3 are presented in panels (b) and (c) of Figure 16, respectively. Strains corresponding 234 to rst visible cracks for the two specimens was 8% and 9%, respectively. Figure 16 also presents the 235 results for specimens with layers heated for 120 and 180 seconds. Strains corresponding to the rst 236 visible cracks for HSC1 (HSC2, HSC3) mix was 6% (7%, 5%) and 4% (7%, 7%) corresponding to the 237 two durations of heating, respectively. It is proposed to de ne buildability as the axial stress in the 238 1The test set-up was loaded without any specimen and deformation per unit load (also known as compliance) was determined to be 0.015 mm/N. 2Total deformation recorded by the compression test set-up is the sum of actual deformation in the specimen and deformation in the test set-up itself. Axial strain in the specimen was calculated as the ratio of actual deformation in the specimen to original height (30 mm in the present case). 12 Figure 15: Cube cut from the printed specimen placed in compression test set-up cube corresponding to an axial strain of 5%. Buildability for the three mixes and the three durations 239 of heating are presented in Table 2. Buildability was higher for a smaller water-to-cement ratio and/or 240 a longer duration of heating. 241 (a) HSC1 (b) HSC2 (c) HSC3 Figure 16: Axial force-strain response of cubes Table 2: Buildability of freshly-printed concrete obtained through compressive strength tests Heating duration (s)Compressive strength at 5% axial strain (kPa) HSC1 HSC2 HSC3 60 20.3 8.9 4.5 120 23.9 23.2 22.8 180 45.5 30.7 24.1 5.2.2. Vicat penetration test 242 A 6 mm-thick layer of HSC1 mix was printed using the caulking gun on a metal plate, and was 243 heated using two 1,000 Watt heat guns for 60 seconds. Subsequently, two more layers were printed 244 13 through the same process (total thickness of printed specimen was 18 mm). The Vicat test plunger for 245 standard consistency test (Vicat Plunger G according to IS 5513 [55]) penetrated the printed specimen 246 6 mm deep (yield strength 15.9 kPa per Eq. 2). This test was conducted approximately six minutes 247 after water was added to the dry mix. Subsequent to the penetration test, the printed surface was 248 heated again for 60 seconds. The penetration test was carried out again. The plunger could penetrate 249 the specimen 1 mm deep (yield strength of 95.5 kPa). The surface was heated again for 60 seconds, and 250 the penetration test was conducted. The plunger could not penetrate the specimen. These results are 251 summarized in Table 3. Results for HSC2 and HSC3 mixes are also presented in Table 3. Buildability 252 decreased with an increase in water-to-cement ratio, and increased with an increase in duration of 253 heating. 254 Table 3: Buildability of freshly-printed concrete through Vicat penetration test Heating duration (s) Penetration (mm) Yield strength (kPa) Layer 1 Layer 2 Layer 3 HSC1 HSC2 HSC3 HSC1 HSC2 HSC3 60 60 60 6 7 8 15.9 13.6 11.9 60 60 120 1 2 4 95.5 47.7 23.8 60 60 180 0 0 1 - - 95.5 5.3. Layer moisture 255 A layer of HSC1 mix was printed using the caulking gun. Plan dimension of the printed layer was 256 80 mm 80 mm, and the layer was 6 mm-thick. Total mass of the printed layer was 98.4 grams and 257 the amount of water in the printed layer was 10.2 grams. The printed layer was placed on the weighing 258 machine and heating was initiated. Details of the set-up are presented in Section 4.3. Figure 17(a) 259 presents the total mass of the printed layer with time. Figure 17(b) presents the moisture content 260 (ratio of the mass of available water in the printed layer to that of dry mix) in the layer with time. 261 Approximately 1% moisture was lost after 60 seconds of heating. 262 5.4. Surface moisture 263 A layer of HSC1 mix with plan dimensions of 250 mm 25 mm was printed using the caulking gun. 264 Surface moisture was measured using the method proposed by Sanjayan et al. [16] (see Section 4). Gain 265 in the weight of the paper towel was 0.7 grams, which represents the surface moisture of freshly-printed 266 layer [16]. A similar layer of HSC1 mix was printed, and heated for a duration of 60 seconds. The 267 layer was immediately covered with a paper towel for 20 seconds to measure the surface moisture of 268 HSC1 mix after 60 seconds of heating. This process was repeated to measure the surface moisture after 269 120 and 180 seconds of heating. Similar exercise was conducted for HSC2 and HSC3 mixes. Figure 18 270 presents the variation of the surface moisture with heating durations mentioned above. Surface moisture 271 decreased with increasing duration of heating for the mixes considered. 272 14 (a) Total mass of printed layer (b) Moisture content Figure 17: Moisture in a printed layer Figure 18: Surface moisture in a printed layer 5.5. Infrared surface re ectance 273 A layer of HSC1 mix (see Section 3) was printed on a metal plate using the caulking gun. The plate 274 was placed in the compression test set-up (see Section 4.5). Heating was initiated along with the IR 275 re ectance measurement of the printed surface. This process lasted for about 60 seconds, at the end of 276 which the surface of the printed layer acquired a matt texture (as observed visually). The metal plate 277 was taken out of the test set-up, and another layer was printed and heated for 60 seconds. This process 278 was carried out a total of ve times. Figure 14 shows the plate with ve printed layers placed in the 279 test set-up. 280 Figure 19(a) presents the average of the surface re ectance measured using the ve sensors of the 281 re ectance array plotted against time for the rst layer printed. The average value increased from 671 282 to 722 in the 60 seconds of heating, during which the surface texture changed from glossy tomatt. 283 Figure 19(b) presents the results for the second layer. The initial average value of re ectance was 593. 284 The di erence compared to the rst layer can be attributed to the manual placement of the metal plate 285 15 and the change in the level of the surface whose re ectance is being measured. The average re ectance 286 increased to 642 after 60 seconds of heating while the surface texture changed from glossy tomatt. 287 Figures 19(c) through 19(e) present the results for third through fth layers, respectively. Texture of 288 each of the three layers turned from glossy tomatt after 60 seconds of heating. Average re ectance for 289 the ve layers increased by 51, 49, 59, 57, and 56 during the heating, respectively. This change can be 290 a basis to determine whether the next layer can be printed. 291 (a) Layer 1 (b) Layer 2 (c) Layer 3 (d) Layer 4 (e) Layer 5 Figure 19: Average re ectance at the surface of printed layer 5.6. Early-age shrinkage 292 A layer of HSC1 mix was printed on the lubricated granite base of the shrinkage test set-up (see 293 Section 4.6). The specimen was kept undisturbed for 500 minutes, during which the re ectors came 294 closer by 19 microns (a shrinkage strain of 75 106). Figure 20(a) presents the history of shrinkage for 295 the printed layer. Similarly, the re ectors came closer by 24 microns and 45 microns for HSC2 and HSC3 296 mixes, and corresponding shrinkage strains were 96 106and 180 106, respectively. Corresponding 297 histories of shrinkage are presented in panels (b) and (c) of Figure 20, respectively. 298 Another layer of HSC1 mix was printed on the lubricated granite slab. The layer was heated 299 for 60 seconds. The re ectors rst moved away from each other by 24 microns (expansive strain of 300 96106), and then came closer to each other by 43 microns (shrinkage strain of 172 106) at the end 301 of 500 minutes. Corresponding shrinkage history is presented in Figure 20(a). These results for HSC2 302 and HSC3 mixes are presented in panels (b) and (c) of Figure 20, respectively. It is clear from the gure 303 16 that 1) maximum shrinkage was achieved much faster when the specimens were heated for 60 seconds 304 compared to when they were not, 2) shrinkage was generally greater when the specimen was heated 305 compared to when it was not, and 3) e ect of heating on maximum shrinkage decreased with increase 306 in water-to-cement ratio. 307 (a) HSC1 mix (b) HSC2 mix (c) HSC3 mix Figure 20: Early-age shrinkage of printed layers 6. Evolution of buildability 308 As noted in Section 1, a concrete mix with a moderate initial yield strength (e.g., 1,500 Pa) is often 309 used to print concrete. Admixtures are added to the mix to accelerate the setting process, thereby 310 increasing the capacity of the printed layer to bear the weight of the layers above (or to increase 311 buildability). Khalil et al. [13] studied the e ect of adding an admixture in concrete mix on the 312 evolution of buildability of printed concrete. Two binders were considered: 1) 100% OPC, and 2) 313 93% OPC mixed with 7% calcium sulpho-aluminate by weight. Yield strength of the two mixes were 314 2,618 Pa (15 minutes after water was added to dry mix) and 2,730 Pa (10 minutes after water was added 315 to dry mix), respectively, as determined through the Vicat penetration test described in Section 4.2.2. 316 The yield strength increased to 2,730 Pa (38,217 Pa) and 95,541 Pa (95,541 Pa) for the two mixes, 317 respectively, 20 minutes (45 minutes) after water was added to the two mixes. Figure 21 presents the 318 evolution of buildability in the two mixes. 319 Initial yield strength of HSC1 mix was 27 Pa per Eq. 1 (see Section 3). Yield strength of the mix 320 was considered to increase linearly with time per the relationship below [58]: 321 o(t) = o(0) +Athixt (3) where, o(0) is the initial yield strength, o(t) is the yield strength after time  t, andAthixis the 322 occulation or structuration rate. Lecompte et al. [31] noted that the above relationship is applicable 323 17 Figure 21: Evolution of buildability of printed concrete only before setting of concrete has begun. Roussel [58] suggest that the occulation rate for a thixotropic 324 material can be considered to be 0.3 Pa/s. Yield strength of the mix was 117 Pa ve minutes after water 325 was added to the dry mix, assuming mix HSC1 is a thixotropic material. Subsequent values of yield 326 strength of HSC1 mix are based on the Vicat test results reported in Section 5.2.2. Accordingly, the yield 327 strength of the mix at the end of six minutes (60 seconds of heating) and seven minutes (120 seconds 328 of heating) were 15,923 Pa and 95,541 Pa, respectively. These results and those for HSC2 and HSC3 329 mixes are shown in Figure 21. It is clear that heating can lead to a much sharper rise in buildability 330 compared to that obtained through the usage of chemical admixtures. 331 7. Properties of printed concrete after hardening 332 A layer of HSC1 mix was printed using the set-up described in Section 2. The plan area of the layer 333 was 250 mm 250 mm. Thickness of the layer was 6 mm. The layer was heated for 60 seconds. A total 334 of seven layers were printed using this approach. The printed specimen was covered with a moist cloth 335 for 24 hours, and was kept in water for next 27 days. A total of 16 cubes were cut from the printed 336 specimen, each with edges 40 mm long. A total of 48 such cubes were prepared for the combination 337 of HSC1 mix and 60 seconds of heating for a layer. A total of nine combinations of mixes (HSC1, 338 HSC2 and HSC3) and duration of heating (60 seconds, 120 seconds and 180 seconds) were considered. 339 Forty eight cubes were prepared for each combination. Following properties of the printed concrete after 340 hardening were studied through tests on the cubes: 1) pore size, 2) shear strength, and 3) compressive 341 strength. Results are presented in the sections below. 342 7.1. Pore size 343 Figure 22 shows an enlarged view of the cut surface of a concrete cube (see Section 7). Pore size 344 on the surface was measured using the \CTL Crack Comparator." The maximum pore size was 1 mm 345 18 with most pores smaller than 0.4 mm. 346 Figure 22: Printed concrete cross-section 7.2. Shear strength 347 Figure 23 shows the 300-ton Instron Compression Testing Machine (CTM) with a cube supported 348 at the bottom from two sides and loaded at the top using a 10 mm thick plate. The nearest distance 349 between the two supports is 12 mm. This set-up enables a double shear strength test of a cube (e.g., 350 [59]). Five cubes for each combination of mix and duration of heating were tested for shear strength 351 with layers aligned parallel to the direction of loading. Figure 24 identi es the directions parallel and 352 perpendicular to the printed layers in a 50 mm-cube. Rate of loading the specimen was 1 mm per minute. 353 Shear strength was calculated as the maximum load divided by the area under shear, i.e., 2 40 mm 354 40 mm. Figures 25(a) presents the average shear strength of ve cubes for each combination. Also 355 presented in the gure are the average strength of three cubes (e.g., [60]) cast in moulds. Figure 25(b) 356 presents the results for cubes tested with the printed layers perpendicular to the direction of loading. 357 Figure 23: Shear strength test set-up Average shear strength of mould-cast cubes of HSC1 mix was 4.2 MPa. The strength corresponding 358 to a duration of heating of 60 seconds was 3.8 MPa (4.0 MPa) for cubes tested parallel (perpendicular) to 359 the layers. The strength was 3.3 MPa (3.8 MPa) and 3.0 MPa (3.9 MPa) corresponding to 120 seconds 360 and 180 seconds of heating, respectively. The e ect of duration of heating was smaller for greater 361 water-to-cement ratios, in general. 362 19 Figure 24: Printed concrete after hardening (adapted from [46]) (a) Testing parallel to the layers (b) Testing perpendicular to the layers Figure 25: Average shear strength of concrete cubes 7.3. Compressive strength 363 Five cubes for each of the nine combinations of mix and duration of heating were tested for compres- 364 sive strength in directions parallel and perpendicular to the layers. Compression testing was performed 365 in accordance with IS 516 [61]. Figures 26(a) and 26(b) present the average compressive strength of 366 printed cubes in directions parallel and perpendicular to the printed layers, respectively. Average com- 367 pressive strength of three mould-cast specimens are also presented in the gure. Average compressive 368 strength of mould-cast cubes of HSC1 mix was 71.6 MPa. The strength corresponding to a duration of 369 heating of 60 seconds was 61.5 MPa (65.7 MPa) for cubes tested parallel (perpendicular) to the layers. 370 The strength was 54.2 MPa (62.2 MPa) and 49.1 MPa (56.4 MPa) corresponding to 120 seconds and 371 180 seconds of heating, respectively. The e ect of duration of heating on the compressive strength was 372 smaller for greater values of water-to-cement ratios. 373 20 (a) Testing parallel to the layers (b) Testing perpendicular to the layers Figure 26: Average compressive strength of concrete cubes 8. Conclusions 374 A new method to print concrete through controlled heating of printed layers is developed. The 375 method allows for printing of a self-compacting concrete mix with initial yield strength as low as 30 Pa. 376 The method o ers following potential advantages over existing practices: 1) a better bonding between 377 adjacent layers, 2) a sharper rise in buildability of the printed layers, and 3) a smaller possibility of 378 choking in the printing set-up. 379 Properties of printed concrete in the fresh state were evaluated. It was concluded that a \stable" 380 layer thickness for the considered SCC mixes can be 6 mm. Buildability of the printed layers, i.e., the 381 capacity to hold the layers above were characterized through two approaches, namely, direct compression 382 test, and Vicat penetration test. The buildability was greater for a smaller water-to-cement ratio and/or 383 a longer duration of heating. A longer duration of heating would be associated with a greater loss of 384 moisture from the printed layer, and it may adversely a ect the bonding between adjacent layers. A 385 sucient level of buildability was considered achieved when the texture of the printed surface had 386 turned into matt. The same could be characterized through the measurement of surface re ectance of 387 the printed surface. Early-age shrinkage in a printed layer was greater if the layer was subjected to 388 heating for 60 seconds compared to when it was not. The di erence was smaller for a greater water-to- 389 cement ratio. The \ultimate" early-age shrinkage was achieved faster when the printer layer was heated. 390 For the mixes considered in the present study, the shrinkage strain was smaller than 200 106. 391 A visual inspection of the printed specimens after hardening indicated that the size of the largest pore 392 was 1 mm, and that most pores were smaller than 0.4 mm. Shear and compressive strength of printed 393 cubes were found smaller when the direction of loading was parallel to the printed layers compared to 394 when it was perpendicular. The e ect of duration of heating was greater in the former case. As an 395 21 example, the average compressive strength of a set of mould cast cubes was 71.6 MPa. The strength 396 for the corresponding printed specimen was 61.5 MPa when the direction of loading was parallel to 397 the layers and each layer was heated for 60 seconds. The strength was 49.1 MPa corresponding to 398 180 seconds of heating. For loading perpendicular to printed layers, the average compressive strength 399 for the two durations of heating was 65.7 MPa and 56.4 MPa, respectively. The average shear strength 400 of the printed cubes were 3.8 MPa (4 MPa) and 3 MPa (3.9 MPa) for the two durations of heating, 401 respectively, and when the direction of loading was parallel (perpendicular) to the layers. The average 402 shear strength of the mould-cast cubes was 4.2 MPa. 403 Acknowledgement 404 Authors are grateful to IIT Gandhinagar for the nancial support. 405