Stiffness behavior of soil stabilized with alkali-activated fly ash from small to large strains

Rios, S., Cristelo, C., Viana da Fonseca, A., Ferreira, C. (2016). Stiffness behavior of a soil stabilized with 1 alkali activated fly ash from small to large strains. International Journal of Geomechanics, 2 DOI: 10.1061/(ASCE)GM.1943-5622.0000783. 3 (http://ascelibrary.org/doi/abs/10.1061/(ASCE)GM.1943-5622.0000783) 4 © 2016 American Society of Civil Engineers 5 Stiffness behavior of a soil stabilized with alkali activated fly ash 6 from small to large strains 7 Sara Rios a; Nuno Cristelo b; António Viana da Fonseca c; Cristiana Ferreira d 8 a Post-Doc Research Fellow (corresponding author), CONSTRUCT-GEO, Faculty of Engineering of the 9 University of Porto 10 E-mail: [email protected] 11 12 b Assistant Professor, CQVR, School of Science and Technology, University of Trás-os-Montes e Alto Douro 13 14 c Associate Professor, CONSTRUCT-GEO, Faculty of Engineering of the University of Porto, 15 16 d Assistant Professor, CONSTRUCT-GEO, Faculty of Engineering of the University of Porto 17 18 19 20 Abstract 21 Alkaline activation of fly ash creates a geopolymeric cement that can replace the ordinary 22 Portland cement in several applications such as soil improvement, with the advantage of much 23 lower CO2 emissions and reusing an industrial by-product otherwise landfilled, thus averting 24 several environmental problems. In this paper, the behavior of a silty sand improved by the 25 alkaline activation of fly ash is analyzed from small to large strains by presenting uniaxial and 26 drained triaxial compression tests’ results, as well as seismic wave velocities measured 27 throughout the curing period. The dynamic, cyclic and static tests show a significant increase 28 in stiffness with curing time, even beyond the 28 days of curing period. Based on the non29 destructive wave propagation technique, the increase of the shear and compression wave 30 velocities with time were drawn giving the evolution of the elastic shear modulus as well as the 31 Poisson ratio values. The dynamic Young modulus was compared to the correspondent secant 32 Young modulus obtained from the mechanical tests. Additionally, the evolution of the 33 properties of this stabilized soil with curing time was compared and confronted to that of soil34 cement, based on the elastic stiffness of both materials, showing that the most significant 35 difference lies on the curing rate. 36 37 Keywords: Fly-ash, Alkaline activation, Soil improvement, Triaxial tests, Seismic Wave 38 Measurements 39 40 Introduction 41 42 Soil stabilization with cement and/or lime-based binders has been the subject of many research 43 programs over the last few decades (e.g., Dupas and Pecker, 1979; Little, 1995; Camusso and 44 Barla, 2009; Consoli et al., 2011; Rios et al., 2012; Houssain and Yin, 2014; Rahimi et al., 45 2016). Recently, other materials have been tested successfully for artificially cementation of 46 soils, like biopolymers (Chen et al., 2014; Khatami and O’Kelly, 2013), polymer-infused roots 47 (Sauceda et al., 2014), carbonating reactive magnesia (Yi et al., 2013) or microbial-induced 48 calcite precipitation (e.g., Cheng et al., 2013). The interest in soil improvement is based on the 49 environmental, economic, social and technical advantages of improving the geotechnical 50 properties of the original soil, instead of, for instance, replacing it by a soil with better 51 mechanical properties. However, environmental issues related to cement production and 52 durability concerns regarding its application to a soil layers constitute a significant motivation 53 to develop new binders. In particular, the amount of carbon dioxide released to the atmosphere 54 by the cement industry is estimated to represent 5% to 8% of the global carbon dioxide 55 emissions (Scrivener and Kirkpatrick, 2008). In that sense, the use of increasing volumes of 56 waste materials, such as fly ash (Kang et al., 2016) in the construction industry is becoming a 57 more and more significant contribution for the reduction in cement consumption. 58 59 Several studies have recently been made aiming the characterization of alkali activated fly ash 60 as a possible substitute for traditional Portland cement from the mechanical and environmental 61 point of view (e.g., Palomo et al., 1999; Turner and Collins, 2013). However, most of them are 62 focused on structural applications, as a substitute for concrete (e.g., Bernal et al., 2011). The 63 few studies for soil improvement applications were only based on a high consumption of 64 alkaline activator (Cristelo et al., 2011, 2013, Sukmak et al., 2013), and the final product was a 65 viscous grout, with an almost liquid consistency, very different from that of a typical soil66 cement mixture. The mechanical behavior of these mixtures is therefore far from that of a lightly 67 cemented soil. The high levels of activator have also a significant impact on the cost of the 68 technique, producing strength levels which can be much higher than needed. 69 70 Therefore, this research project intended to characterize the geotechnical behavior of a well 71 graded silty-sand resulted from remolded residual soil from granite masses, abundant in Porto 72 region, stabilized with fly ash (FA) activated with low rates of a sodium-based grout. The low 73 rates of activator are expected to have three major consequences: 74 75 − The generation of lower strength levels than those reported in the scarce available 76 literature regarding soil stabilization with alkali activated fly ash, but still high enough 77 for most geotechnical applications. 78 − The lower percentages of activator will reduce the total cost of the technique, to a level 79 for what it becomes competitive with cement from a financial point of view. 80 − It will also produce a final mixture with a soil-like structure, which will enable the use of 81 common geotechnical laboratory tests and procedures, namely in the triaxial apparatus. 82 83 In the present paper, the deformation behavior of this stabilized soil is assessed based on 84 uniaxial and triaxial tests – using local strain instrumentation; and seismic wave analysis - using 85 ultrasonic transducers, throughout the loading process, from very small to very large shear 86 strains. This large range characterization is essential to accurately predict the stress-strain 87 behavior, enabling the design of geotechnical structures with this material. Considering the 88 extensive worldwide experience of soil-cement behavior (Rios et al., 2014), a comparison 89 between both materials is presented. 90 91 92 Materials and Methods 93 94 This study presents the characterization of mixtures composed by silty sand (characterized in 95 Viana da Fonseca et al., 2013), fly ash and an alkaline activator. Low calcium content fly ash 96 (Class F according to ASTM C618, 2003), produced by a Portuguese coal-fired thermo-electric 97 power plant, was used. The activator was prepared using a sodium silicate (SS) to sodium 98 hydroxide (SH) ratio of 1:2. The SS was originally in solution form, with a bulk density of 99 1.464 g/cm3 at 20ºC, a SiO2/Na2O weight ratio of 2.0 (molar oxide ratio of 2.063) and a Na2O 100 concentration in the solution of 13.0%. The SH was originally supplied in pellets with a specific 101 gravity of 2.13 at 20ºC (99 wt%), and was dissolved in water to form a 7.5 molal solution. 102 103 Three types of mixtures were studied, with different FA percentages (relatively to the total 104 solids weight), activator contents (liquid to solids ratio) and dry unit weights. Furthermore, 105 specimens with the same ash contents and a liquid phase constituted solely by water, that is, 106 without activator, were molded and tested for comparison purposes. Characterization of all the 107 fabricated mixtures is shown in Table 1. More details may be found in Rios et al. (2016). 108 109 To fabricate the specimens, the dry soil was first mixed with fly ash until a homogeneous 110 mixture was obtained. Then, the activator solution (produced 6 h before use to allow 111 temperature stabilization) was added, followed by further mixing. The resulting paste was 112 compacted in three layers inside a cylindrical stainless steel mold with 70 mm of diameter and 113 140 mm height in order to obtain the desired unit weight. After 48 h the specimen was removed 114 from the mold and wrapped in cling film, to avoid moisture loss, before being stored again in a 115 controlled temperature room (20ºC). Curing periods of 28 and 90 days were considered. 116 117 Uniaxial compression strength (UCS) and drained triaxial compression (CD) tests were 118 performed according to ASTM 1633 (1996) and ASTM D7181 (2011), respectively, on 119 specimens cured for 28 (UCS and CD) and 90 days (UCS). A 100-kN automatic hydraulic 120 testing machine was used for the uniaxial compression strength (UCS) tests, fitted with a 50121 kN capacity and 0.006-kN resolution load cell (Figure 1a). For reproducibility reasons, each 122 UCS result is the average of three tested specimens. The tests were carried out under monotonic 123 displacement control, at a rate of 0.1 mm/min. This speed is slower than the value recommended 124 by ASTM 1633 (1996), so that it could be possible to perform small unload-reload cycles. Local 125 deformation transducers (LDTs) were used with the UCS tests for increased strain measurement 126 accuracy (Goto et al., 1991; Hayano et al., 1997) and, consequently, more reliable unload-reload 127 stiffness moduli (Figure1b). These small unload-reload cycles were included in some UCS 128 tests, at 15%, 30% and 60% of the expected uniaxial compression strength. The cycle amplitude 129 (qcycmax-qcycmin) was established at 20% of the maximum deviator stress of each cycle (qcycmax). 130 131 Triaxial tests were performed using Hall-effect Transducers (Clayton et al., 1989) glued directly 132 onto the specimen membrane (Figure 2). The specimens were saturated applying a back133 pressure of 500 kPa, anisotropically consolidated considering a coefficient of earth pressure at 134 rest (K0) of 0.5, and sheared under displacement control at a rate of 0.01 mm/min. During the 135 triaxial tests, unload-reload cycles were performed at 5%, 15%, 30% and 60% of the 136 corresponding unconfined compressive strength, assuming that this value is a lower bound 137 estimate of the peak deviator stress. A large amplitude (qcycmax-qcycmin) of 90% of the qcycmax was 138 used, allowing a clear definition of the cycle. 139 140 Ultrasonic compression (P) and shear (S) wave velocities were measured by ultrasonic non141 destructive transducers (Figure 3) in all the UCS test specimens, at the curing periods of 3, 7, 142 14, 21 and 28 days, and at 90 days for the long-term curing specimens. For wave generation 143 and acquisition, commercially available equipment was used (Figure 3a), comprising a pair of 144 piezoelectric ultrasonic compression transducers, for measuring P-wave velocities, with a 145 nominal frequency of 82 kHz and 30 mm in diameter; a pair of piezoelectric ultrasonic shear 146 transducers, for measuring S-wave velocities, with a nominal frequency of 100 kHz and 35 mm 147 in diameter; and a pulse waveform generator and data acquisition unit, equipped with an 148 amplifier, directly logged to a PC, using specific software to operate as an oscilloscope. 149 150 The input signal was configured for an excitation voltage of 500 V and a pulse signal frequency 151 of 82 kHz, both for P and S-wave transducers. The same frequency was used for both 152 transducers since this is the closest frequency available in the function generator. Calibration 153 of each pair of transducers was achieved by measuring the wave velocity through a calibration 154 rod, with known density and wave velocity. The measurements were taken along the 155 longitudinal axis of the specimens, with the specimen vertically aligned and the transducers 156 installed on opposite faces. Therefore, the path length corresponded to the height of the 157 specimens of approximately 140 mm. The exact travel length and the weight of each specimen 158 were measured before each reading, with a precision of ±1%. In terms of wave propagation, the 159 transmitter was located at the bottom of the specimen, while the receiver was at the top end. 160 The acoustic coupling between the transducers and the specimen during the measurement was 161 ensured by a layer of ultrasound conductive gel. Furthermore, the transducers were firmly and 162 uniformly pressed against the top surface of the specimen, by the use of a 1 kg disk (Figure 3b) 163 assuring a similar pressure on the transducers throughout the entire experimental program. The 164 readings were taken at generic curing periods of 3, 7, 14, 21, 28 and 90 days. Each presented 165 result corresponds to the average of at least ten consecutive pulse velocity readings. 166 167 168 169 Results 170 171 Assessment of stiffness by compression tests 172 173 The unload-reloading cycles performed on the unconfined compression tests allowed the 174 evaluation of the unload-reload modulus (Eur) at three different strain levels, as expressed in 175 Figure 4 for mixture M2 after 90 days curing. From the stress-strain curves the secant stiffness 176 modulus was determined, using the values plotted in Figure 5 against the deviator stress q 177 normalized by its peak value (qpeak). The secant moduli are significantly higher in the alkali 178 activated mixtures than in non-activated soil-ash specimens. A clear difference was also 179 observed between the two curing times (28 and 90 days) at all stages of these UCS tests, 180 including at peak (where bonding has been partially destroyed) meaning that a strong type of 181 bonding is present (Cuccovillo and Coop, 1999). On the other hand, the stiffness degradation 182 pattern appears to be steeper at 90 days than at 28 days, as typically happens when cementation 183 increases (Leroueil and Vaughan, 1990, Viana da Fonseca et al., 2011). 184 185 Triaxial test results showed very stiff stress-strain curves, as illustrated in Figure 6. Although 186 large cycles were performed, resulting in considerable yielding, an attempt was made to recover 187 the elastic modulus considering the initial part of the unloading branch, as reported in Gomes 188 Correia et al. (2004). Figure 7 illustrates this analysis for one of the tests, namely the test of M2 189 specimen at σV0 = 50 kPa and σH0 = 25 kPa. 190 191 As presented for the UCS, the secant modulus evolution during the triaxial compression tests 192 was also plotted against q/qpeak (Figure 8). The data is very clear indicating that M1 mixture is 193 definitely the stiffer, and that the confining stress contributed to an increase in stiffness. This 194 shows that strong cemented bonds (as it is the case in M1) do not break when the confining 195 stress is applied. In the other mixtures the results are not so evident and it is possible that a 196 weaker type of cementation is present resulting in some damage of cemented bonds due to 197 confining stress, especially at M3. However, more results were needed to confirm this. 198 199 Assessment of stiffness by compression and shear wave measurements 200 201 Compression and shear wave velocities (P and S waves, respectively) were used to evaluate the 202 development and evolution of the elastic stiffness of the cemented specimens to be tested in 203 unconfined compression, throughout curing time. This was possible by the non-destructive 204 nature of these ultrasonic wave measurements. Figure 9 shows the obtained output signal for P 205 and S waves, indicating the propagation time registered in each measurement, using a classical 206 time-domain approach. The determination of P-wave travel time is straightforward, 207 corresponding to the first break of the received wave signal, as clearly indicated in Figure 9a). 208 On the other hand, the selection of the shear wave arrival is slightly more complex, due to the 209 interference of compressional waves and near-field effects in the received signal, as previously 210 recognized by other authors (Arroyo et al., 2003; Viana da Fonseca et al., 2009). As a result, S211 wave arrival was defined as the first major downward break (the polarity of the signals was 212 determined during calibration), corresponding to the beginning of a low frequency wave, 213 typical of shear waves, as evident in Figure 9b. 214 215 From the theory of elasticity, is it well known that compression and shear wave velocities are 216 related to the confined (M0) and shear (G0) moduli, respectively, according to Equations (1) and 217 (2). 218 219 𝑴𝟎= 𝝆 𝑽𝑷 𝟐 (1) 𝑮𝟎= 𝝆 𝑽𝑺 𝟐 (2) 220 where  is the bulk density of the material. Equation (3) provides the Poisson’s ratio value (), 221 from which the dynamic Young’s modulus (E0) can be derived, using Equation (4). 222 223  =(𝑽𝑷 𝑽𝑺)𝟐−𝟐 𝟐(𝑽𝑷 𝑽𝑺)𝟐−𝟐 (3) 𝑬𝟎= 𝟐𝑮𝟎 (𝟏 +  ) (4) 224 Figure 10 and 11 illustrate the evolution of these elastic parameters with curing time for the 225 three different mixtures up to 90 days. Three specimens were molded for each mixture as 226 expressed by the symbols and the average line is plotted for a clear comparison. A significant 227 evolution of these moduli with curing time has been found. M1 and M3 mixtures have a parallel 228 linear trend, although M1 presents higher stiffness evolution. M2 mixture consistently shows a 229 different behavior, with a trend close to M1 at earlier curing periods but with lower stiffness 230 values at 90 days of curing time. This may indicate that M2 mixture tends to cure at a faster 231 rate, stabilizing at an earlier age than the other two mixtures; however, further investigation is 232 needed to confirm this statement. 233 234 Poisson’s ratio also shows an interesting trend slightly reducing at shorter curing times and then 235 increasing up to 0.25 for M1 and M2 mixtures and 0.3 for M3. 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Dry unit weight (kN/m3) b SiO2 / Na2O (wt.) c M01 0.15 - - 11.7 - - 18.22 - M02 0.20 - - 15.6 - - 17.08 - M03 0.25 - - 19.5 - - 16.04 - M1 0.15 0.125 7.5 8.8 11.7 0.781 18.22 0.552 M2 0.20 0.125 7.5 11.7 15.6 0.781 17.08 0.552 M3 0.25 0.125 7.5 14.7 19.5 0.781 16.04 0.552 a For a SS/SH mass ratio of 0.5; b For a unit weight of 20 kN/m3; c Quantities from the activator 493 494 Table 2: Stiffness modulus of the analyzed mixtures from dynamic, cyclic and static tests 495 Type of tests Parameter 28 days 90 days M1 M2 M3 M1 M2 M3 Dynamic tests E0 (MPa) 3239 2831 2597 7123 5852 5924 UCS tests Eur (MPa) - - - - [3954-5027] [2000-3972] Et0 (MPa) 1452 1274 1010 3740 3016 2696 Triaxial Tests Eur (MPa) [2220-3165] [1118-2030] [500-2560] - Et0 (MPa) [1950-4050] [982-1347] [587-1865] 496 Table 3: Stiffness modulus of the analyzed mixtures 497 Parameter 28 days 90 days M1 M2 M3 M1 M2 M3 E0 (MPa) 3239.48 2831.27 2596.89 7123.15 5852.23 5924.41 Esec10% (MPa) 1378.58 1344.85 876.46 3629.85 2825.00 2556.17 Esec10% /E0 0.43 0.47 0.34 0.51 0.48 0.43 498 499 Table 4: Power fit constants 500 Source data (Fig. 12) Constant A Exponent n R2 28 d 0.0117 -0.40 0.14 90 d 0.0002 -0.88 0.93 M1 0.001 -0.69 0.74 M2 0.0003 -0.82 0.81 M3 0.003 -0.80 0.74 501 Figures 502 503 504 Figure 1: (a) Load frame for uniaxial compression tests; (b) strain measurement setup 505 506 507 508 Figure 2: Triaxial compression strain measurement setup 509 510 511 Figure 3: Seismic Pand S-wave velocity measurement equipment (a) and setup (b) 512 513 514 Figure 4: Stiffness modulus obtained from cycles performed during the unconfined compressive strength test of 515 one of the M2 specimens after 90 days curing 516 517 Figure 5: Evolution of the secant stiffness modulus throughout the unconfined compression test 518 519 Figure 6: Stress-strain-volume curves obtained in drained triaxial compression tests of stabilized soil for the 520 three mixtures (M1 M2 and M3). 521 522