Abstract
The durability of stabilized subgrades is the backbone for ensuring pavement’s longevity. Its realistic evaluation is the key to accurate prediction of a pavement’s performance, aiding in the planning of maintenance and rehabilitation. Current ASTM standards for durability assessment of cement-treated soils discuss wetting–drying (WD) and freezing–thawing (FT) separately (in ASTM D559 and ASTM D560, respectively) and do not account for sequential interactions between moisture and temperature fluctuations. With weather patterns becoming more complex, most regions have already started experiencing all four seasons with varying intensities and frequencies. This makes the need for coupled durability assessments highly relevant to mimic the sequential interactions between environmental stresses induced by all four seasons. This paper thus presents two novel coupled durability methods, namely, wetting-drying-freezing-thawing (WDFT) and freezing-thawing-wetting-drying (FTWD), developed by a sequential combination of the existing ASTM standards for WD and FT. Low-plasticity clay specimens stabilized with two stabilizers, ordinary Portland cement (OPC) and Portland limestone cement (PLC), were considered for the coupled durability studies. The influence of the WDFT and FTWD cycles (0, 3, 7, and 10) on the performance of stabilized soils is determined through conventional volumetric and mass measurements. Additionally, rigorous engineering strength evaluations, including the unconfined compressive strength and resilient modulus, were employed, with pavement applications as the primary focus. Findings revealed that the sequence of environmental stressors greatly influences the performance, with WDFT being more detrimental by causing rapid volumetric changes and faster stiffness degradation in both OPC and PLC-stabilized soils, the effect being more pronounced in the latter case.
Introduction
Pavement performance relies heavily on the stability of underlying subgrade soils, which must endure seasonal environmental stresses in addition to cyclic loads imposed by traffic. Distresses in pavements such as rutting, heaving, and cracking are directly linked to the presence of weak subgrades ( 1 , 2 ). A well-practiced approach that has established records of improving subgrade performance is through chemical stabilization, usually by calcium-based stabilizers such as cement. Cement is often regarded as the most promising all-purpose stabilizer; it enhances the unconfined compressive strength ( 3 ), lowers the hydraulic conductivity ( 4 ), and improves the stiffness ( 5 ), which are all crucial for long-term pavement integrity. These improvements stem from cement hydration and pozzolanic reactions that alter the soil’s microstructure and bonding characteristics ( 6 , 7 ).
Despite their benefits, cement-stabilized soils remain vulnerable to degradation under environmental loading. Moisture variations and temperature fluctuations (by freezing and warming) promote the development of cracks in soil because of volume changes arising from swelling/shrinking behavior of clay minerals, compounded with the formation and melting of ice lenses within the soil matrix, aggravating the situation further. Durability assessment of geomaterials under environmental stressors is not new, as several laboratory testing methods were already being used by researchers to simulate the long-term strength deterioration over time. Methods such as capillary suction ( 8 ) or soaking for 3 days and drying in 20°C with 65% relative humidity conditions ( 9 ) are practiced for simulating moisture variations and evaluating moisture-resistant behavior of various types of stabilized geomaterials ( 10 ). Tebaldi et al. ( 11 ) had alternating freezing and thawing temperatures of 20°C and −17°C conditions over a 24-h period for simulating temperature fluctuations. The extent of moisture and temperature fluctuations varies from region to region, depending on the local climatic conditions, with researchers using custom ranges for their durability studies, making direct comparison of the performance of stabilizers across different studies challenging. The ASTM standardized the durability evaluation process of stabilized soils to provide a framework that makes comparisons across studies more meaningful if followed.
For wetting–drying (WD), ASTM D559 recommends complete submersion of samples in potable water for 5 h, followed by oven-drying at 71 ± 3°C for 42 h. For freezing–thawing (FT), ASTM D560 suggests freezing at a temperature of −23°C for 24 h and thawing in an environmental chamber maintained at 23 ± 2°C for 23 h. Both standards require measurements of volume, water content changes, and soil-cement mass loss by wire brushing as indicators for durability assessment. While there is a debate on these extreme conditions imposed by the ASTM standards that are of low natural prevalence in the real world, ASTM D559 and ASTM D560 are still the widely followed guidelines for evaluating WD and FT durability of cement-stabilized soils, respectively. Methodology from these standards is widely used by researchers ( 12 – 14 ) across the world and is referred to by federal agencies in developing guidelines such as the Unified Facilities Criteria (UFC) 3-250-11 from the United States Department of Defense.
With ever-changing weather patterns, it is now common to see the dynamic interaction between all four seasons in most parts of the world, rather than a conventional tropical or temperate climate. Extreme events such as intense precipitation, severe cold, and heat waves are becoming more erratic and recurring in nature. These coupled sequential interactions between moisture and temperature changes, such as wetting-drying-freezing-thawing (WDFT) and freezing-thawing-wetting-drying (FTWD), have compounding effects on the durability of soils and could lead to faster strength deterioration when compared with the influence of WD or FT alone ( 15 , 16 ). While few studies have looked at the combined sequential effects of environmental stressors on untreated pavement materials ( 15 – 17 ), plenty of research into stabilized soils with WD and FT methods has given the basis for promising investigations into pavement material behavior when subjected to combined environmental stressors ( 18 – 21 ). The WDFT and FTWD coupled methods are mainly applicable to cold semi-arid regions, such as the Upper Plains and Northern Midwest regions of the United States of America. The season at the time of construction will determine which coupled durability stressors will be most applicable. For instance, heavy rainfall following construction in the early summer would be suitable for the WDFT method. Alternatively, the FTWD method corresponds to a hard freeze following pavement construction. Evaluating the durability of geomaterials under these coupled events is crucial to mimic realistic environmental stressors and to accurately predict the pavement response ( 22 ). This is particularly relevant to resilient transportation infrastructure systems because pavement construction and performance are highly sensitive to the season in which they are built ( 23 , 24 ).
The current durability procedures in the literature lack a unified established protocol to simulate multiple environmental stressors in sequence as, to date, there is no standard for coupled durability evaluations. Existing standards are limited to either WD or FT and do not facilitate simulating the full spectrum of seasonal variations experienced in regions with four-season climates. Regions that experience local four-season variability, especially during spring or autumn, are poorly represented in the current durability testing protocols. Also, no strength assessments, such as the test to determine the unconfined compressive strength (UCS), were included in the existing WD and FT durability standards, even though such testing is practiced in general by several researchers, as it offers great insights into the material’s structural conditions post-durability. Including this strength evaluation in the standards would significantly raise the reliability of the durability protocols.
Overall, a significant research gap exists within the ability of current durability standards to capture strength and stiffness degradation in cement-stabilized soils under realistic, multi-seasonal environmental conditions. Addressing this gap is essential for pavement engineers seeking reliable performance predictions and design criteria when durability is of utmost consideration. With this as the objective, this paper introduces two new coupled durability testing protocols that simulate all four seasonal stressors in two distinct sequences: WDFT and FTWD. These sequences were designed to reflect field conditions and construction times, and to examine their impact on the durability performance of stabilized soils.
The methodology integrates and extends existing ASTM procedures for individual WD and FT cycles to create a combined protocol for evaluating cumulative effects. A low-plasticity clay stabilized with OPC and PLC was subjected to 0, 3, 7, and 10 cycles of the developed WDFT and FTWD sequences. Durability performance was assessed through measurements of void ratio, volumetric strain, resilient modulus, and UCS, offering new perspectives on the long-term behavior of stabilized soils under four-season environmental loading and the importance of the environmental loading sequence.
The rest of the paper is structured as follows. Materials and methods will be presented next to describe the soil, stabilizers, and methodologies for coupled durability assessments that were developed in this study. After this, the results and discussion will be presented to interpret the impact of the developed coupled durability methodologies. Toward the end, conclusions and remarks will be made, summarizing the major findings of this research.
Materials and Methods
The flow of the research methodology and testing procedures employed in the present study is schematically shown in Figure 1 for a quick overview; the details of which are provided in the following subsections.
Research methodology for novel four-season durability methods.
Soil and Stabilizers
Low-plasticity clay (CL) of gray-black color was collected locally from Bryan, Texas. Grain size distribution, Atterberg limits, specific gravity, and moisture-density relations at modified compaction efforts were all determined in accordance with ASTM standards for soil characterization. The evaluated properties are listed in Table 1. Initial assessments on the swelling potential of the soil revealed that it is expansive in nature, therefore needing stabilization strategies.
Characterization of Soil
Note: MDUW = maximum dry unit weight; OMC = optimum moisture content.
Two types of cement, ordinary Portland type 1 cement (OPC) and Portland limestone cement (PLC), were used as stabilizers. While OPC is a traditional all-purpose stabilizer, PLC is relatively new in applications such as soil stabilization. PLC has more sustainable benefits as a result of the partial replacement of clinker with limestone, which reduces the energy intensiveness of its manufacturing process. Choice of stabilizers was made based on the rationale to include the most widely used OPC, and highly potential and relatively sustainable PLC to enable direct comparisons between them, from both strength and durability perspectives. OPC and PLC were supplied by local vendors from Houston, Texas. UFC 3-250-11 provides guidelines for selecting the optimal dosages for soil stabilization. For CL soils, it recommends starting with 9% as the initial trial with a limiting dosage as 11%. Following this, pilot durability tests (WDFT) were performed on soil (CL) stabilized with 9% OPC and PLC. Both stabilized soils could not pass the durability conditioning, although they meet the prior-durability UCS criteria of 250 psi set by UFC 3-250-11. Therefore, 11% dosage was considered for carrying out further studies on understanding the impact of the developed novel coupled durability methods on the performance of OPC- and PLC-stabilized soils. Hereafter, the CL stabilized with 11% OPC and 11% PLC will be simply referred to as OPC- and PLC-treated soils and be identified just as OPC and PLC in the figures for convenience. The moisture-density relations for untreated and cement-treated clay (with 11% dosage) obtained through modified Proctor compaction tests are presented in Figure 2. The optimum moisture content (OMC) for the OPC- and PLC-treated soil is 13.7% and 14.0%, respectively. The maximum dry unit weight (MDUW) was found to be 17.77 kN/m3 and 18.21 kN/m3, respectively, for the OPC- and PLC-treated soil.
Compaction curves for untreated and cement-treated CL.
Specimen Preparation and Coupled Durability Conditioning (WDFT and FTWD)
Cement-treated specimens for the coupled durability assessments were prepared through the following steps: a) Initially, oven-dried soil was crushed to pass the #4 sieve (mesh opening of 4.75 mm) to ensure that any lumps within the soil mass were broken down to ease the mixing process with the stabilizer. b) Dry weights of required soil and cement dosages corresponding to MDUW were measured and mixed thoroughly to achieve a homogenous soil-cement mixture, after which the required amount of water to match the OMC was added and mixed again, ensuring uniform moisture distribution throughout the sample before compaction. c) The mixture was then transferred into a mold to be compacted for a total of three layers. Cylindrical specimens were statically compacted by an Instron Universal Testing System at a loading rate of 7.5mm/min to 144 mm in height and 72 mm in diameter for durability and strength tests. Specimens were prepared using a displacement-controlled method with static compaction to mimic modified Proctor compaction conditions (2,700 kN-m/m3). d) After compaction and demolding, the density of the specimens was verified by recording mass and three measurements of height and diameter. Height measurements were conducted at three equally spaced points along the cylinder’s circumference, while measurements for the diameter were performed at the mid-height of every layer. e) The treated specimens were then cured for 7 days in a humidity room set to 100% relative humidity and a temperature of 23°C to enhance cement hydration and strengthen the compacted geomaterials, in accordance with UFC 3-250-11, after which they are ready for durability conditioning. Triplicates of each stabilized soil type were prepared for each duration of durability cycles, per method. In addition, six treated specimens were also prepared in a similar fashion for resilient modulus and UCS testing that had undergone no durability testing to act as a 0-cycle benchmark. Therefore, a total of 42 treated specimens were prepared for experimentation.
Two different durability methods, WDFT and FTWD, were proposed in this study and imposed on cement-treated specimens to simulate the interplay between environmental stressors arising from all four seasons. The conditions for each environmental phase followed the conditions from the uncoupled ASTM standards (ASTM D559 for WD and ASTM D560 for FT), with a slight deviation in the wetting and thawing phase durations. Wetting included 6 h of complete submersion of specimens in water. The wetting phase was extended to an hour beyond the recommendations from the ASTM D559 (i.e., 5 h of wetting) to make the timing of measurements consistent across the entire duration of durability cycles. One hour of extended wetting may also leave specimens prone to more degradation. In the drying phase, specimens were placed in an environmental chamber for 42 h at a temperature of 70°C. Freezing took place in a cold chamber at −23°C for 24 h. For this, specimens were placed on a metal tray in a plastic tub before being sealed and placed into the cold chamber. Also, between the metal tray and the specimen, a saturated felt pad was placed to simulate capillary suction during freezing, as recommended by ASTM D560. Then, during the thawing phase, specimens were placed in a humidity room with a relative humidity of 100% and a temperature of 23°C for 24 h on the same tray from the freezing phase. Note that an additional hour of thawing was implemented as a deviation from ASTM D560, which recommends 23 h of thawing. Analogously with the extended wetting period, this additional hour of thawing also ensures consistent times for specimen measurements. The extended thawing period will also allow for specimens to become more saturated. A noteworthy point about the durability methods is that the drying phase occurs before the freezing phase in both WDFT and FTWD methods. After the first drying cycle of each sequence, the specimens are relatively dry when they are transferred to the freezing chamber. However, moisture ingression into the specimens is simulated throughout the freezing phase of the durability cycles by placing the specimens over saturated felt pads. The number of durability cycles included three, seven, and 10 cycles for each stabilized soil, with a total duration of 12 days, 28 days, and 40 days, respectively. Although ASTM standards for uncoupled durability assessments suggest continuing for 12 cycles, the upper limit for coupled durability cycles was set to 10 cycles in the present study, considering the aggressive nature and prolonged test durations of the WDFT and FTWD conditions. After each environmental phase in all cycles, mass and volume measurements were recorded as described above to track the mass and volume changes at every stage of the durability cycles. In addition, stiffness and strength measurements at the end of 0, 3, 7, and 10 cycles were taken with the aim of correlating them to the structural changes occurring in the soil by the environmental conditioning.
Stiffness and Strength Evaluation
Resilient Modulus
Although the stiffness of a material can be evaluated using various moduli such as Young’s modulus, bulk modulus, and so forth, resilient modulus (M R ) is often the most widely used one for studies focusing on pavement applications. It is a direct measure of dynamic stiffness and is a critical parameter in determining the elastic response of subgrade and pavement materials under repeated traffic loading. In view of its significance in the Mechanistic-Empirical Pavement Design, M R is considered in the present study as the parameter of interest for understanding the stiffness of cement-stabilized soils and its reduction with coupled environmental stress cycles.
M R is defined as the ratio of applied deviatoric stress to the resilient strain. The GCTS Universal Soil Dynamics System was used in the present study for M R determination via repeated load triaxial (RLT) test stages programmed in accordance with the American Association of State Highway and Transportation Officials (AASHTO) T307 standard. The testing included 15 loading stages performed in a sequence on the specimen with different (confining and deviatoric) stress combinations. Applied confining stresses included 41.4 kPa, 27.6 kPa, and 13.8 kPa. Deviatoric stresses applied during the test included 13.8 kPa, 27.6 kPa, 41.4 kPa, 55.2 kPa, and 68.9 kPa. The loading sequence was structured to first perform all the deviatoric stress combinations in the increasing order of magnitude (given above) for the highest confining pressure (41.4 kPa) and then repeat the similar procedure of sweeping the deviatoric stresses in an incremental fashion for the medium (27.6 kPa) and then for the low (13.8 kPa) confining stress. The triaxial apparatus applies deviatoric stress on the specimens similar to the Haversine function, having 0.1 s of loading and unloading phase and a 0.9-s rest period until the next load application. The loading, unloading, and resting phases altogether are counted as one cycle. For each cycle 100 load cycles were applied (in total 15), and the corresponding M R is reported by averaging the values obtained from the last five cycles. Also, the testing protocol by AASHTO T307 requires “pre-conditioning” of the specimen with 500 cycles (performed at 41.4 kPa confining stress and 27.6 kPa deviatoric stress) before running the actual sequential loading to remove non-uniformities across the contact area of the specimens and the loading top cap. Therefore, the same has been followed as well. M R evaluation is nondestructive. Thus, the prepared specimens (post 0, 3, 7, and 10 coupled durability cycles) were first tested for this, followed by the destructive strength evaluation (UCS) on the same specimen.
Unconfined Compressive Strength
The strength of cement-stabilized soils and their deterioration with the impact of coupled environmental stress (WDFT and FTWD) cycles is quantified according to the most widely used parameter in the literature, that is, UCS. ASTM D1633 provides guidelines specifically for evaluating the compressive strength of molded soil-cement cylinders. It recommends that the specimens be submerged in water for 4 h before the UCS test. Although it does not talk about how to evaluate the strength deterioration post-durability cycles, as there is no existing standard dealing with this aspect, it would be fair to have the post-durability specimens also be soaked, similar to the pre-durability/unconditioned cement-stabilized specimens (as specified by the ASTM D1633), to ensure a consistent testing protocol when comparing the pre- and post-durability UCS values. Therefore, all the UCS tests in the present study were conducted following the ASTM D1633 with a 4-h pre-soaking just before the test. After the soaking and before the unconfined compression strength test, mass and volume measurements were also taken on the specimens in the same manner as the measurements for durability, to demarcate the influence of this soaking on the soil structure. After all these measurements, the specimens were uniaxially compressed along the longitudinal axis at a rate of 1.3 mm per minute until failure. After the completion of the UCS test, soil samples from the specimens were dried in an oven at 105°C for moisture content measurements.
Results and Discussion
The impact of coupled environmental stresses (both WDFT and FTWD) imposed by all four-season weather patterns on the stiffness and strength of cement-treated soil is discussed based on the observations from the test results in the following subsections.
Impact of WDFT and FTWD on Resilient Modulus
The average M R and standard deviations of the tested specimens obtained from the RLT tests for the OPC-treated clays after FTWD and WDFT conditioning are shown in Figures 3 and 4, respectively. A noteworthy assumption to point out is that specimens were assumed to have failed if two of the three triplicate specimens could not withstand durability cycles. Here, the term specimens being “failed” or “could not withstand/survive” refers to severe damage to the specimens by the durability cycles, making them too fragile to handle for the stiffness and strength evaluations.
Resilient modulus curves for OPC-treated clays after the FTWD conditioning for: (a) 0 cycles; (b) three cycles; (c) seven cycles; and (d) 10 cycles.
Resilient modulus curves for OPC-treated clays after the WDFT conditioning for: (a) 0 cycles; and (b) three cycles.
In Figure 3, a clear reduction of M R was observed as the FTWD cycles progressed, indicating gradual deterioration of OPC-stabilized soil. For example, under a confinement pressure (σ c ) of 41.4 kPa and a deviatoric pressure of 13.8 kPa, the M R of OPC-treated CL was 458.61 MPa before durability tests; however, the values dropped to 270.49 MPa, 247.71 MPa, and 150.22 MPa after three, seven, and 10 FTWD cycles, respectively, showing a continuous reduction in stiffness. Such trends were also observed under all the other stress conditions for OPC-treated soil undergoing FTWD durability test. Despite the deterioration, all the triplicate specimens (treated) survived the 10 cycles of FTWD.
In contrast, for WDFT method, more severe damage to the OPC-treated specimens was observed. Only one specimen out of three replicates showed structural integrity for M R test after seven cycles of WDFT, while the other two deteriorated significantly, and their M R could not be determined. In addition, all triplicate specimens treated with OPC could not survive 10 cycles of WDFT. Therefore, the resilient moduli of these specimens could not be measured. As a result, Figure 4 only represents the average M R values of specimens after 0 and 3 WDFT cycles. A reduction of 65% to 77% in M R after three cycles of WDFT was observed, depending on the confinement and deviatoric stress levels. It can be seen collectively from both Figures 3 and 4 that the M R values drop significantly as the coupled durability cycles progress, the effect being more rapid in the case of WDFT method.
For the soil specimens treated with PLC, similar trends in M R over different durability cycles were observed. Specifically, all the triplicate specimens survived 10 cycles of FTWD when treated with PLC, and the average resilient moduli of these samples are presented in Figure 5. For the WDFT method, however, the PLC-treated clay specimens could not survive through seven and 10 WDFT cycles; therefore, only resilient moduli of PLC-treated CL after 0 and three cycles are collected in Figure 6. Comparing FTWD and WDFT methods, the WDFT sequence resulted in more damage to PLC-treated specimens as well, analogous to the OPC-treated specimens. This further confirms that the WDFT coupled curability method is more critical and often causes more material deterioration.
Resilient modulus curves for PLC-treated clays after the FTWD conditioning for: (a) 0 cycles; (b) three cycles; (c) seven cycles; and (d) 10 cycles.
Resilient modulus curves for PLC-treated clays after the WDFT conditioning for: (a) 0 cycles; and (b) three cycles.
A more detailed analysis on M R values was conducted based on the 13th loading stage among the 15 total testing sequences, which corresponds to 13.8 kPa of confining pressure and 41.4 kPa of deviatoric stress. Many practitioners and engineers use the M R values corresponding to this stress combination in their designs, as it represents the most frequently experienced stresses by a pavement ( 25 ). Therefore, the same has been considered for detailed analysis, while similar relations can be discussed for other stress combinations too.
For 0-durability cycles, at the 13th stage, PLC specimen has an M R value of 303.25 MPa (Figure 5a) while OPC specimen exhibited an M R of 241.43 MPa (Figure 3a). This is a clear indication of PLC specimens being relatively stiffer compared with OPC specimens when subjected to no environmental stress. This higher initial stiffness of the PLC specimens is attributed to the lower initial void ratio, indicating a more compact specimen. This is clearly evident from its MDUW of 18.21 kN/m3, which is higher than that of the OPC specimens (17.77 kN/m3). However, if we look at the 13th stage M R data of both OPC and PLC specimens subjected to either seven cycles or more of FTWD (Figure 3c versus Figure 5c, or Figure 3d versus Figure 5d), it can be clearly seen that OPC specimens retained their stiffness relatively longer. The formation of cementitious gels is greater in the case of OPC specimens because of the higher chemical reactive nature of OPC when compared with PLC, thereby making them more durable.
After three FTWD durability cycles (Figure 3b), the 13th stage M R value for OPC specimen is around 118.63 MPa, while it is 103.71 MPa for PLC specimens (Figure 5b). However, after three WDFT durability cycles, the 13th stage M R values are interestingly similar, with 55.99 MPa for the OPC-treated specimens (Figure 4b) and 56.47 MPa for the PLC-treated specimens (Figure 6b). Considering that PLC-stabilized clay has high initial stiffness, these comparisons imply that they could potentially be more vulnerable to degradation when subjected to environmental stressors (both WDFT and FTWD), but further supporting tests are required to verify this.
To quantify the stiffness reduction or retention of the treated clay specimens after the coupled FTWD and WDFT durability tests, the stiffness retention parameter, defined as the ratio of the residual M R and the original M R , was calculated. Table 2 summarizes the average M R , standard deviation, and stiffness retention for all OPC- and PLC-treated specimens under coupled durability cycles. At three durability cycles of the WDFT and FTWD methods for the OPC-treated specimens, the stiffness retention was 23.19% and 49.14%, respectively. Analogously, the trend is the same for the PLC-treated specimens, with the 18.62% and 34.20% stiffness retention for the WDFT and FTWD methods at three cycles, respectively. Interpretations from the stiffness retention values further support the WDFT method being more damaging across both OPC- and PLC-treated soils.
Average Resilient Modulus and Stiffness Retention for OPC- and PLC-Treated Specimens Subjected to WDFT and FTWD conditioning
Note: WDFT = wetting-drying-freezing-thawing; FTWD = freezing-thawing-wetting-drying; OPC = ordinary Portland cement; PLC = Portland limestone cement; na = not applicable.
Impact of WDFT and FTWD on Unconfined Compressive Strength
The UCS results of both OPC- and PLC-treated clays for 0, 3, 7, and 10 WDFT and FTWD cycles are presented in Figure 7. The UCS values and their standard deviation throughout all UCS tests are also presented in Table 2. One key observation that can be made by a quick comparison of the UCS and M R values for 0-durability cycles in OPC (Figure 3a) and PLC (Figure 5a) cases is that, while PLC specimens have exhibited relatively high stiffness, they have low UCS when compared with OPC specimens. This indicates that unconditioned PLC specimens are relatively stiffer at small strains, while OPC specimens are stronger at large strains. Based on the observed trends in Figure 7, the subplots have been divided into multiple zones (Zones I, II, and III, respectively, for regions between 0 and 3, 3 and 7, and 7 and 10 cycles) to make explanations clearer.
Evolution of UCS with coupled durability methods of WDFT and FTWD.
From Figure 7, the general trend of the UCS with WDFT durability conditioning for both OPC- and PLC-treated specimens can be observed to be continually decreasing in all Zone I as the durability cycles progress. The UCS value drops from 3.51 MPa to 2.98 MPa after three cycles of WDFT, for the OPC case. For PLC, the initial strength of the specimens was 2.62 MPa, and it dropped to 2.15 MPa after three WDFT cycles. WDFT has a similar influence (of continuous decrease as the cycles progress) on both the stiffness (Figures 4 and 6) and strength (Figure 7) of stabilized soils. On the other hand, while FTWD has shown a similar response of continuous decrease in the stiffness as the cycles progress (Figures 3 and 5) to WDFT, it has a slightly different influence on the UCS response of the specimens, especially showing a rise in strength during 0 to three cycles (Zone I), followed by reduction during three to seven cycles (Zone II), and ultimately approaching a residual value during seven to 10 cycles (Zone III), for both OPC and PLC cases, as can be seen in Figure 7. It can be observed that at the end of 10 cycles in the FTWD method, samples did survive the environmental conditions, showing almost similar strengths as those at the end of cycle 7. Therefore, Zone III for the FTWD could be treated as a residual strength zone. Based on the overall impacts of WDFT (specimens’ failure after seven cycles) and FTWD (still leaving residual strength), it is clear that the WDFT sequence is more critical compared with FTWD from the UCS point of view as well.
To allow for a quick comparison of the relative performance of OPC and PLC and to bring out the impacts of WDFT and FTWD coupled durability cycles on stabilized clays, similar to the stiffness retention, strength retention, defined as the ratio of residual and original UCS, expressed in percentage, from both cases for different cycles, were computed and tabulated in Table 3. It can be observed for both WDFT and FTWD cycles that OPC outperformed PLC by retaining its strength for more cycles, which is as expected. However, considering the associated sustainable benefits, PLC can still be effectively used for sites that expect more FTWD patterns, as it retained almost 50% of its strength even after 10 cycles. Also, it can be used at localities that expect relatively less frequent WDFT cycles. Overall, from Tables 2 and 3, it is evident that WDFT is more detrimental when compared with FTWD from both strength and stiffness perspectives, thereby significantly affecting the durability and performance of stabilized soils.
UCS of Cement-Stabilized Clays for WDFT and FTWD Methods
Note: UCS = unconfined compressive strength; WDFT = wetting-drying-freezing-thawing; FTWD = freezing-thawing-wetting-drying; OPC = ordinary Portland cement; PLC = Portland limestone cement; na = not applicable.
Insights from Void Ratio and Volumetric Strain
To further understand the difference between the effects caused by the WDFT and FTWD methods on the performance of stabilized clay, void ratio (e), which is a simple yet very important parameter that is directly related to the pore space and compactness of the soil matrix, was calculated and compared for all specimens. Comparing the void ratios of the specimens right after different durability cycles (0, 3, 7, and 10) and before their UCS testing would give a clear picture of the evolution of pore space that has a strong correlation with the strength. It is worth mentioning that despite the strength and stiffness data not being collected at the later stages of the durability tests, volume and mass changes could still be successfully measured to calculate the void ratio and volumetric strain, unless specimens failed completely. This is reasonable as strength-related tests often require specimens to have a stable and fixed geometry, while such restrictions do not affect the measurement of mass and volume of the stabilized soil specimens. Soil phase relationships were used for void ratio calculations using the mass, volume, and moisture content measurements from the specimens. A reasonable assumption that was made for simplifying these calculations was that the change in mass was solely a result of the ingress and evaporation of water during the durability cycles. While this holds true for up to the 3rd cycle measurements, it might not be completely valid for the 7th and 10th cycles, as the specimen starts to lose solid mass at these stages. Therefore, the void ratios calculated in these cases are estimates. However, the results show that such estimation can still greatly help in getting simple yet meaningful indications on the complex multi-phase/physics involved phenomenon occurring within the specimens during the four-season durability testing.
The evolution of void ratio with the progression of coupled durability cycles is presented in Figure 8 for WDFT and FTWD conditioning. The void ratios for failed specimens (for instance, OPC specimens subjected to 10 cycles of WDFT, and PLC specimens subjected to seven and more WDFT cycles) would be relatively high as a result of the severe internal damage caused by the environmental stressors. As the specimens are extremely fragile at this point, quantifying these is practically challenging, unless nondestructive methods such as X-ray computed tomography (CT) scanning are employed, which are beyond the scope of the present study. Therefore, the void ratios for failed specimens were not reported in the present study, but it should be borne in mind that they are relatively high when compared with stable specimens.
Evolution of void ratio with progression of durability cycles for WDFT and FTWD methods.
Figure 8 strongly supports the observed UCS deterioration for both stabilizers with the WDFT conditioning. It can be seen that the void ratios for both OPC and PLC-stabilized soils have shown an increasing trend with the increased number of environmental stress cycles, which explains the observed continuous strength reduction in Figure 7. For instance, if we consider the case of OPC-treated specimens, the initial average void ratio (0 durability cycles) was found to be 0.517, whereas for the specimens post-WDFT durability, it was found to be 0.554 after three cycles, with further expected rise in the void ratio for seven and 10 cycles. A similar trend can be noticed in PLC-treated specimens too, with the void ratio increasing to 0.525 after three cycles of WDFT from an initial value of 0.498 for 0 cycles, after which a rapid increase in the void ratio is expected as the specimens failed for seven- and 10-cycle scenarios.
Also, in agreement with observed trend of UCS results of OPC-treated specimens subjected to FTWD cycles (Figure 7), corresponding void ratio changes (Figure 8) show that the void ratio slightly decreased at cycle 3 to a value of 0.502 from 0.517 and then increased to 0.560 at cycle 7, after which it did not vary significantly, stabilizing at a value of 0.565 for 10 cycles. This matches the general belief that soils with a higher void ratio tend to be weaker in strength. Whereas for PLC-treated specimens subjected to FTWD, the void ratio trends indicated continuous increment, with a minor variation in void ratio up to three cycles (from 0.498 to 0.510), after which it increased to 0.612 and 0.666 as the cycles progressed to seven and 10, respectively. This, in general, matched the strength reduction of PLC-treated clay specimens after FTWD durability cycles, with the exception at cycle 3, where a slight increase in UCS was observed (Figure 7). This strength increase during the early stage of FTWD test was most likely because the strength improvement caused by cement hydration overcame the loss of strength thanks to the slightly increased void ratio.
Although the void ratio gives insight into UCS behavior, further microstructural investigations are needed to get better insights into the other strength-gaining aspects, such as the continued formation of cementitious reaction products with continuing durability cycles. In addition to the void ratio measurements taken at the end of 0, 3, 7, and 10 cycles, the volumetric strain at each and every phase/step of the durability cycles (both WDFT and FTWD) was also recorded to understand its evolution and provided deeper insights into the differences imposed by WDFT and FTWD cycles.
Volumetric strain evolution at every step for each set of cycles (3, 7, and 10) and different durability methods (WDFT and FTWD) are collectively presented in Figure 9. The volumetric strain in these figures is measured relative to the initial volume of the specimen before the durability testing. Each data point in these figures is the measurement of volumetric strain after the completion of a particular environmental phase. For instance, in Figure 9a for the three cycles of the WDFT method, the first data point at the origin is the initial state, and the measurement at the second data point is the volumetric strain after wetting in the first cycle. Then the third data point is the volumetric strain after drying, then so on and so forth through the cycle. A similar analogy applies to FTWD (Figure 9b), the first data point is the initial state, the second data point is volumetric strain after freezing, then the third point is volumetric strain after thawing, followed by wetting and drying data points as the cycle progresses. In general, drying and freezing cause the specimens to shrink, while wetting and thawing lead to swelling and expansion. This was observed through all the durability cycles, for both WDFT and FTWD. It is clear that drying resulted in the most significant volumetric reduction in the specimens, such that subsequent expansion of specimens caused by wetting and thawing was not enough to overcome such shrinkage, especially during the early stages of the durability tests. This led to an overall negative volumetric strain at the beginning of the durability cycles, as shown in Figure 9, a and b . However, as the durability cycles progressed to seven and 10 cycles, degradation of the specimens aggravated as a result of the damage from continuous environmental stressors, and the specimens exhibited an overall expansion, compared with the original volume before durability as shown in Figure 9, c to f .
Volumetric strain evolution through coupled durability testing for: three cycles of (a) WDFT; (b) FTWD; seven cycles of (c) WDFT; (d) FTWD; and 10 cycles of (e) WDFT; (f) FTWD.
Two key observations can be made from Figure 9 that give important insights to explain the differences between the performance of stabilized clay specimens subjected to WDFT and FTWD cycles. It can be seen that the specimens undergoing WDFT durability cycles started to show an overall expansion (positive volumetric strain) after three or four cycles, whereas for FTWD, specimens remained in the negative volumetric strain zone until after around seven cycles. The volumetric expansion led to the formation of a weaker and open pore structure within the specimens, which allowed increased moisture ingress in subsequent cycles to cause further and more rapid deterioration. This observation of volumetric changes supports the finding from strength and stiffness tests that WDFT is more damaging compared with FTWD. On the other hand, when comparing PLC and OPC-treated specimens, PLC-treated samples tended to deteriorate more quickly as they switched from shrinking to expansion at much earlier stages than OPC during the durability test. This also agrees with the previous findings that the mechanical performance of PLC-treated specimens deteriorates faster.
While other factors such as the continued hydration, pore pressure building up, leaching of the cementitious products, damage to soil fabric, changes to pore network, crack propagation, as well as changes to matric/capillary suction, degree of saturation, and hydraulic conductivity all collectively contribute to the changes in the performance/durability of stabilized soils under environmental stresses, understanding these aspects from a multi-phase/multi-physics framework at multi-scale levels is essential to have a complete picture. Despite this, through its well-planned test program, the present study successfully brought out several important aspects of the influence of coupled durability cycles and highlighted the importance of the sequence in which the environmental stressors act, leading to varying intensities of damage. Also, the two novel coupled durability testing procedures (WDFT and FTWD) described in this paper, with stiffness and strength assessments together, are of immense value to develop robust standards in this direction for making better durability testing protocols. In addition, considering the increased complexity in weather patterns, these findings are of extreme importance and useful in designing resilient transportation infrastructure.
Applications, Limitations, and Future Works
The practicality of the proposed WDFT and FTWD methods applies to a focused geographic region of the United States. Pavements located in the Northern Midwest and Upper Plains regions (including states such as Wyoming, Montana, South Dakota, North Dakota, and Minnesota) experience these sequences of precipitation and temperature fluctuations annually. It is well known that M R and the strength of stabilized subgrade vary with the temperature and moisture content for all soils ( 24 ). With these methods, stiffness degradation could potentially be mapped with seasonal inputs used in pavement design and performance predictions with further research and careful implementation.
There are also some limitations to this work that need to be addressed in future research and studies before the application of these durability methods in practice. For example, a high temperature of 70°C (158°F) during the drying phase adopted from the ASTM D559 might not be the most suitable for subgrade temperatures encountered in four seasonal regions, as it does not fully promote damage-related mechanisms important during the subsequent freezing phase (i.e., more severe ice lens development) because of limited moisture presence after drying. Further research in modifying these temperatures would benefit the development of aggressive and representative coupled durability sequences. Also, in the present study, only two sequences of environmental loading (i.e., WDFT and FTWD) were considered. Different combinations (such as WFTD and others) are also possible, and an appropriate sequence must be selected to suit site-specific seasonal changes.
In addition, more studies on different stabilizers and different soils are needed in the future, as this study includes testing only one soil with two cement stabilizers. Further works should focus on a variety of different geomaterials (such as granular soils, silts, and high plasticity clays), along with different stabilizers (such as hydrated lime, fly ash, and polymers) and dosages, and the effects of curing time and conditions on durability characteristics must be established. These variables need to be explored for a more thorough understanding of the coupled durability methods to optimize and standardize the testing protocols. Future research should also consider microstructural testing to support mineralogical and strength changes that occur through the combined durability cycles. Lastly, correlations should be developed with field data (i.e., falling weight deflectometer for M R ), and this would greatly improve the interpretation of lab results and establish the effects of differences in confining states between the laboratory specimens and the field conditions.
Conclusions
This study highlights the first insights into newly developed coupled durability methods for stabilized geomaterials and brings out the importance of the sequence of environmental stressors on the response. The coupled durability methods included simulating four seasonal stressors, including Wetting (W), Drying (D), Freezing (F), and Thawing (T), in two different sequences (WDFT and FTWD) to track degradation of the stabilized specimens through volumetric strain, resilient modulus (M R ), and unconfined compressive strength (UCS) assessments. Current ASTM standards on uncoupled WD and FT durability were combined to develop the test protocols. The durability of a low-plasticity clay stabilized with ordinary Portland cement (OPC) and Portland limestone cement (PLC) under these coupled WDFT and FTWD cycles (0, 3, 7, and 10) was evaluated to bring out fresh perspectives on four-season durability of stabilized soils. Key conclusions from this study are as follows.
The WDFT method was determined to be most critical in comparison to the FTWD method, which is supported by test results from M R and UCS testing, along with insights from the evolution of void ratio and volumetric strains with durability conditioning, whatever the type of stabilizer.
The WDFT method was found to push the specimens more toward the expansion side, along with drastic changes in the volumetric strains when compared with the FTWD, both of which are known to cause intense damage to the specimen. On the other hand, for the case of FTWD, the changes in the volumetric strains are relatively stable and gradual, and also the specimens were found to spend most of their time on the contraction side, which leads to slow degradation.
PLC-treated specimens exhibited relatively higher initial stiffness (M R ), whereas OPC-treated specimens exhibited relatively higher initial strength (UCS). However, both strength and stiffness degradation with coupled durability cycles are rapid in the case of PLC specimens when compared with OPC specimens for both WDFT and FTWD conditioning.
Footnotes
Author Contributions
The authors confirm contribution to the paper as follows: study conception and design: Kyle Parr, Jianxin Huang, Jeb S. Tingle, Sopharith Chou, Anand J. Puppala; data collection: Kyle Parr; analysis and interpretation of results: Kyle Parr, Jianxin Huang, Balaji Lakkimsetti, Sopharith Chou, Anand J. Puppala; draft manuscript preparation: Kyle Parr, Jianxin Huang, Balaji Lakkimsetti, Sopharith Chou, Anand J. Puppala, Jeb S. Tingle. All authors reviewed the results and approved the final version of the manuscript.
Declaration of Conflicting Interests
The authors declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: The corresponding author Dr. Anand J. Puppala is a Senior Associate Editor of Transportation Research Record’s Editorial Board. All the other author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Financial support was provided by the U.S. Army Engineer Research and Development Center, Vicksburg, Mississippi, for this study. Project #W912HZ-22-BAA-01.
Any opinions, findings, conclusions, or recommendations expressed in this paper are those of the authors and do not necessarily reflect the view of the U.S. Army Engineer Research and Development Center.