Abstract
Subgrade treatment has traditionally been achieved using calcium-based cement. However, it does not necessarily enhance sustainable design. Recently, low-carbon alternatives such as portland limestone cement (PLC) have gained attention as substitutes for traditional cement. In addition, recycled concrete aggregate fines (fRCA), a waste product, have shown potential for application in transportation infrastructure because of their enhancements in pavements. This study investigates the effectiveness of PLC and fRCA in improving soil properties under different environmental stressors. Clayey soil was treated with PLC (10% PLC or 10C) and PLC-fRCA mixtures at different ratios (8% PLC/15% fRCA or 8C_15fRCA and 8% PLC/30% fRCA or 8C_30fRCA). Improvements in strength, stiffness, and volumetric changes were evaluated through unconfined compressive strength and repeated load triaxial tests after exposure to various environmental conditioning cycles (0, 6, and 12 cycles of wet–dry or freeze–thaw) in the laboratory. Results indicated that untreated soil collapsed within two cycles of environmental conditioning. In contrast, treated soils exhibited significant improvements in strength and resilience to environmental stressors. Stiffness also improved with treatment, and despite some reduction after exposure to environmental conditioning, treated specimens maintained relatively higher stiffness values. These enhancements are attributed to the formation of strong binding gels from hydration and secondary reactions among PLC, fRCA, and soil, which exhibit strong resistance to moisture intrusion, helping to preserve their engineering properties. Overall, this study provides a comprehensive understanding of the potential of using fRCA as a co-additive to PLC, offering a more sustainable and durable alternative for the long-term performance of transportation infrastructures.
Keywords
Reliable transportation infrastructure underpins the socio-economic growth of any nation or region. It facilitates the movement of goods, services, and people, enabling various elements of the economy to thrive. ( 1 ). Therefore, an optimally functioning transportation infrastructure is crucial for all stakeholders in a region or community. Despite its significance, transportation infrastructure is a costly endeavor, fraught with complex challenges, and its failure can result in catastrophic social and economic consequences. Extensive research indicates that unstable subgrade soil is a leading cause of pavement failures and various transportation infrastructure issues ( 2 , 3 ). Distresses in transportation infrastructure caused by problematic soils and exacerbated by climatic stressors result in millions of dollars in maintenance, rehabilitation, and reconstruction costs in the U.S.A. alone ( 4 , 5 ). Thus, stable subgrade soil is crucial for the long-term durability and serviceability of transportation infrastructure.
Although transportation practitioners strive to avoid problematic soils, constructing pavements over such soils can sometimes be unavoidable. In these cases, it is essential to improve the weak soil to ensure an optimally functioning transportation system throughout its design life, minimizing failures and excessive rehabilitation. Chemical stabilization is widely recognized for treating problematic soils. Because of its efficiency and effectiveness, it is one of the most commonly used methods. It uses stabilizing agents such as binder materials to form compounds that enhance the engineering properties of the soils ( 6 ). These agents are categorized into two types: traditional and non-traditional stabilizers. Traditional stabilizers include cement, lime, and fly ash, either alone or in combination. Non-traditional stabilizers include geopolymers, silica fume, cement kiln dust, lime kiln dust, gypsum, and blast furnace slag ( 3 , 7 – 15 ). Traditional chemical stabilization leads to the formation of various phases of calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H), which act as binding gels for individual soil particles ( 3 ). These C-S-H and C-A-H gelatinous products significantly enhance soil strength and engineering performance ( 16 ). Consequently, the unconfined compressive strength (UCS), workability, and stiffness of the treated soil are improved, while its plasticity is reduced ( 17 ).
However, traditional chemical stabilizers, particularly cement, have significant environmental drawbacks. The cement production process and its supply chain are associated with substantial greenhouse gas emissions. Studies indicate that cement production contributes approximately 7% of global greenhouse gas emissions in the form of CO2 equivalents ( 18 ). In addition, traditional chemical stabilizers incur high costs with respect to natural resource utilization ( 19 ). As the world moves toward sustainability to address climate change, the use of traditional stabilizers for subgrade soil improvement is becoming increasingly outdated. In response to environmental concerns, the market has introduced alternative blended cement products, such as portland limestone cement (PLC) and portland composite cement. These products are designed to reduce greenhouse gas emissions by incorporating additives that decrease clinker content, presenting a more sustainable option for soil stabilization ( 20 ).
On the other hand, the use of recycled and sustainable materials to replace environmentally costly solutions is gaining traction. The utilization of recycled materials such as reclaimed asphalt pavement (RAP) and recycled concrete aggregate (RCA) is already underway, with research highlighting multiple benefits. RAP, combined with admixtures, has been shown to perform exceptionally well in asphalt mixes, and its application as an alternative base and sub-base material is gaining momentum because of its sustainability ( 21 ). Similarly, RCA derived from construction and demolition (C&D) waste is increasingly gaining popularity as alternative to traditional materials for base and sub-base layers. Research indicates that RCA provides performance comparable to that of natural aggregates as a granular base material. Its use supports resource conservation and waste reduction, aligning with broader sustainability goals in construction ( 22 ). The comparable performance of RCA, coupled with over 60% of global C&D waste still being disposed of in landfills, underscores RCA’s significance as a sustainable alternative. This has brought the utilization of RCA in transportation infrastructure to the forefront of current research and practice ( 23 , 24 ). However, the application of RCA in geotechnical contexts, such as the stabilization of weak subgrade soils, remains largely unexplored. Preliminary studies suggest that incorporating C&D waste powder into weak soils enhances both the UCS and the California bearing ratio (CBR) values ( 25 ). Another study reported that application of sand-sized RCA fines or recycled concrete aggregate fines (fRCA) is effective in enhancing the engineering performance of problematic soils ( 26 ).
Therefore, a comprehensive study is necessary to assess the improvement in engineering and durability properties of soil treated with RCA, along with its associated sustainability benefits, which is crucial for advancing the practical application of RCA in geotechnical engineering and enhancing the sustainability of construction practices. In response to these needs, a research study was designed to evaluate and compare the effectiveness of stabilizing weak subgrade soils using PLC and RCA fines or fRCA as co-additives. A range of engineering tests, including strength, stiffness, and moisture-induced strain assessments, were performed to develop a comprehensive understanding of effective stabilization methods for weak subgrade soils. The study provided valuable insights into the comparative evaluation and benefits of using low-carbon cement stabilizers combined with recycled waste materials to enhance the engineering performance and durability of weak geomaterials. The next section details the materials and testing program used in the study.
Materials and Methods
Geomaterial
In this study, the tested soil was collected from a local pavement construction site. Basic characterization was performed in accordance with the corresponding ASTM International and Texas Department of Transportation (TxDOT) standards, as presented in Table 1. This soil consists of 39.4% sand, 35.7% silt, and 19.6% clay, as illustrated in Figure 1a. Moreover, its liquid limit is about 41%, with a plasticity index (PI) of 25%. Thus, this soil was classified as low plasticity clay (CL) or A-6, as per the Unified Soil Classification System (USCS) and American Association of State Highway and Transportation Officials (AASHTO) soil classification, respectively. The linear shrinkage strain and free swell strain of this soil were found to be 4.1% and 4.2%, respectively. Moreover, the sulfate content of the soil was recorded to be less than 100 parts per million (ppm). In addition, the natural soil is predominantly composed of quartz, illite, chaoite, dolomite, and calcite, as presented in Figure 1b.
Basic Characterization of the Natural Soil
Note: USCS = Unified Soil Classification System; pcf = pounds per cubic foot; ppm = parts per million; NA = not available; Tex = Texas Department of Transportation Standard; AASHTO = American Association of State Highway and Transportation Officials.
Basic characterization of natural soil: (a) grain size distribution curve and (b) X-ray diffractogram.
Recycled Concrete Aggregate Fines
RCA was collected from a local RCA supplier. It was oven-dried in a temperature-controlled chamber at 40°C (104°F) to remove in situ moisture caused by storage conditions. Then it was sieved to remove the particles larger than 425 μm and the fRCA (<425 μm) was stored in sealed containers to be used as sustainable co-additive for this study. Moreover, the specific gravity of the fRCA was recorded as 2.59 in accordance with ASTM D854. According to the X-ray diffraction (XRD) result, this fRCA was predominantly composed of quartz, calcite, and dolomite. Figures 2a and b illustrate the pictorial view and X-ray diffractogram of the fRCA.
Recycled concrete aggregate fines (fRCA): (a) pictorial view of fRCA and (b) X-ray diffractogram.
Chemical Stabilizer
According to the U.S. Army Corps of Engineers (USACE) and the Federal Highway Administration (FHWA), stabilization using a combination of lime and cement is recommended for treating high-plasticity soil (PI > 20) instead of using only a cement stabilizer. In this study, PLC, in accordance with ASTM C595, was sourced from a local supplier and utilized as a calcium-based chemical stabilizer (Figure 3a). PLC is composed of alite, calcite, tricalcium aluminate, and gypsum. Figures 3a and b illustrate a pictorial view and X-ray diffractogram of the PLC.
Portland limestone cement (PLC): (a) pictorial view of PLC and (b) X-ray diffractogram.
Selection of Stabilizer Dosage
National Cooperative Highway Research Program (NCHRP) Report #144 ( 27 ) was adopted as the governing guideline for this study, recommending the use of lime, cement, or a combination of both, as stabilizer for subgrade soils with a PI ranging from 15 to 35. Before optimizing the stabilizer dosage, the guideline stipulates conducting a pH test in accordance with USACE procedures to rule out any potential interference from organic matter with the hydration process, which was subsequently confirmed in the laboratory.
For stabilizer dosage optimization, the guideline recommended an initial cement content of 12% by dry soil weight for the stabilization of A-6 soil, with adjustments of ±2% to achieve a minimum UCS of 250 pounds per square inch (psi; 1724 kPa) after a 7-day curing period. UCS testing was performed in accordance with ASTM D1633. The average UCS values for specimens treated with 10%, 12%, and 14% PLC were 3080 kPa (446.7 psi), 3998 kPa (579.8 psi), and 5199 kPa (754 psi), respectively. Since the 10% PLC-treated specimens achieved a UCS exceeding 1724 kPa (250 psi), as stipulated by the guideline, this dosage was selected as the optimal PLC content for the chemical stabilization of the soil.
For the design of the PLC-fRCA-treated geomaterial mix, a reduced PLC dosage was selected in conjunction with higher dosages of recycled material to enhance the study’s sustainability benefits by decreasing the cement stabilizer content and utilizing a C&D waste byproduct. Accordingly, an 8% PLC content by dry weight was chosen, with RCA incorporated at 15% (8C_15fRCA) and 30% (8C_30fRCA) by dry weight as co-additives. Table 2 lists the various specimen groups used for the study as well as the percentages of different constituents.
Proportions of Constituent Materials in Various Testing Groups
Note: UT = untreated; C = cement; fRCA = recycled concrete aggregate fines; NA = not applicable.
Specimen Preparation
Untreated (UT) soils were prepared by evenly mixing dry natural soil with the optimum moisture content (OMC). For the chemically treated specimens, the target dosage of stabilizer and co-additives was uniformly mixed with dry natural soil. Subsequently, the required amount of molding water was added to achieve OMC, and the mixture was blended thoroughly to form a homogeneous mixture. The treated soils were equilibrated at room temperature for 15 min before being molded into various specimens.
Two types of cylindrical specimens were prepared according to governing standards for the testing regimen, referred to as Specimen No. 1 and Specimen No. 2. Specimen No. 1 was prepared by static compaction in three layers (Figure 4a) using a cylindrical mold with a diameter of 71 mm (2.8 in.) and an aspect ratio of 2:1 (H:D) to achieve the desired maximum dry unit weight (MDUW) and OMC, as shown in Figure 5. These specimens were used for UCS tests, repeated load triaxial (RLT) tests, and for monitoring volume and moisture content changes before and after exposure to moisture and temperature variations. Specimen No. 2 (Figure 4b) was prepared using a standard Proctor compaction mold in three layers, with a diameter of 101.6 mm (4 in.) and a height of 116.4 mm (4.58 in.) and was used for determining soil loss in durability testing. The OMC and corresponding MDUW for the different geomaterial groups were determined as listed in Table 3. It was observed that the moisture–density curve shifted upwards and to the right for the 12% PLC-treated soil, likely caused by the addition of finer cement particles. In contrast, for the fRCA-PLC-treated geomaterial mixes, while the moisture–density curve also shifted upwards and to the right, the change was less pronounced compared to the PLC-only mix. This difference can be attributed to the reduction in finer cement content and its replacement with relatively coarser fRCA in the mixes (8C_15fRCA and 8C_30fRCA).
Specimens for the study: (a) Specimen No. 1 and (b) Specimen No. 2.
Moisture–density relationship curves for various geomaterial groups.
Optimum Moisture Content (OMC) and Maximum Dry Unit Weight (MDUW) Values for Various Geomaterials
Note: UT = untreated; C = cement; fRCA = recycled concrete aggregate fines.
Table 3 shows the OMC and MDUW values for various geomaterial groups.
After compaction, the treated specimens were demolded and cured for 7 days in accordance with NCHRP guidelines. Treated specimens were placed in hermetically sealed chambers at 23 ± 2°C (73 ± 4°F) and approximately 100% relative humidity to ensure adequate moisture for chemical reactions. To comprehensively evaluate the impact of environmental stressors on the engineering properties of both UT and treated specimens, five sets of specimens were prepared and conditioned under various environmental conditions before testing. These sets included 0-cycle, 6-cycle wet–dry (W-D), 6-cycle freeze–thaw (F-T), 12-cycle W-D, and 12-cycle F-T specimens. The W-D specimens were tested according to ASTM D559 (W-D durability), while the F-T specimens followed ASTM D560 (F-T durability). Each W-D cycle involved 5 h of soaking in a water bath followed by oven drying at 71 ± 3°C (160 ± 5°F) for 42 h. Each F-T cycle consisted of freezing in a chamber at ≤−23°C (−9.4 °F) for 24 h, followed by thawing in hermetically sealed chambers at 23 ± 2°C (73 ± 4°F) with approximately 100% relative humidity for 23 h. Figures 6a–d provide pictorial views of the durability testing.
Durability testing of geomaterials: (a) ASTM D559 wet–dry (W-D) cycles, (b) wire brushing on W-D specimens, (c) ASTM D560 freeze–thaw (F-T) cycles, and (d) F-T specimens after wire brushing.
Experimental Program
Engineering tests incorporating environmental stressors were conducted on both UT and chemically treated specimens (PLC and PLC-fRCA) to evaluate the improvement in weak subgrade soils and to understand the effect of fRCA additives on soil performance by comparing the performance of PLC-treated specimens with that of PLC-fRCA-treated specimens through the following tests.
UCS tests were performed to evaluate the strength improvement of treated specimens and to compare the performance between PLC and PLC-fRCA treatments. This test was conducted on triplicate specimens of both UT and treated soils after exposure to various environmental stressors. The specimens were tested in accordance with ASTM D1633 at a strain rate of 1.3 mm/min. UCS values were determined by recording the average peak normal stress from the stress–strain curves of the triplicate specimens. Dimensional measurements to monitor volumetric changes were taken at specific locations throughout the environmental stressor cycles.
RLT tests were conducted to assess the impact of stabilizers on resilient modulus (Mr) values, considering various environmental stressor conditions. The resilient modulus is defined as the ratio of deviatoric stress to the resilient strain experienced by a geomaterial under repeated loads, simulating traffic loading conditions. It is a critical parameter for pavement design and offers insight into the stiffness behavior of the geomaterial. In this study, all specimens were subjected to RLT testing in accordance with AASHTO T307 following exposure to environmental stressors. Initially, each specimen underwent 500 cycles of loading to remove initial plastic strains. This was followed by 15 loading sequences, each consisting of 100 loading cycles. During these sequences, specimens were exposed to varying combinations of confining pressures (41.4, 27.6, and 13.8 kPa or 6, 4, and 2 psi) and deviatoric stresses (13.8, 27.6, 41.4, 55.2, and 68.9 kPa or 2, 4, 6, 8, and 10 psi). The average of the last five cycles from each loading sequence was used to determine the Mr value for each stress state combination.
Results and Discussion
Unconfined Compressive Strength
Figure 7 depicts the stress–strain curves from UCS testing along with mean and standard deviation values for various geomaterial groups. The UT specimen yielded an UCS value of 53.9 psi (372 kPa), which is much lower than the required strength. However, after chemical treatment, the UCS values for the three treated geomaterial groups (10C, 8C_15fRCA, and 8C_30fRCA) at 7-day curing were noted to be 446.7 psi (3080 kPa), 444.1 psi (3062 kPa), and 418.2 psi (2884 kPa), respectively, which were observed to be more than the minimum recommended as per NCHRP guidelines (250 psi or 1724 kPa) as well as showing more improvement in engineering strength. This improvement may be attributed to the products from the hydration reaction caused by PLC and PLC-fRCA treatment, which resulted in binding gels that enhanced the inter-particle bonding, making them more resistant to moisture intrusion. This increase in strength can also be attributed to presence of calcium hydroxide in PLC that helped in pozzolanic reactions, leading to an increase in strength at the end of the curing period.
Stress–strain curves for various geomaterial groups.
Not only did the UCS show improvement but also the elastic modulus of the treated specimens increased more than eight-fold compared to that of the UT specimens. For instance, the elastic modulus of UT soil was 5 kips per square inch (ksi; 34.4 MPa), and after treatment, the elastic modulus of 10C, 8C_15fRCA, and 8C_30fRCA reached 59.5 ksi (410.2 MPa), 88.8 ksi (612.2 MPa), and 41.8 ksi (288.2 MPa), respectively. To further understand these enhancements, a more comprehensive analysis of different environmental stressors is presented in the following paragraph.
Figure 8 presents the UCS values for various geomaterial groups subjected to the W-D environmental stressor. During the W-D process, UT soil specimens failed during the first cycle because of moisture intrusion and a loss of inter-particle bonding, highlighting the necessity of chemical treatment to enhance natural soil performance. In contrast, soil treated with PLC and fRCA exhibited improved resistance to moisture-induced strains. For example, specimens from all three treated geomaterial groups (10C, 8C_15fRCA, and 8C_30fRCA) withstood 12 cycles of the W-D process, with less than 1% volumetric strain recorded at the end of the 12th cycle.
Unconfined compressive strength (UCS) test values for various geomaterial groups after wet–dry (W-D) conditioning.
In addition, UCS values for all treated geomaterial groups (10C, 8C_15fRCA, and 8C_30fRCA) showed significant improvement after six W-D cycles. Specifically, the UCS increased by 374.5 psi (2582 kPa) for 10C (1.8 times), 377.5 psi (2603 kPa) for 8C_15fRCA (1.85 times), and 476.7 psi (3287 kPa) for 8C_30fRCA (2.1 times) compared to values recorded after curing. This enhancement in UCS is likely caused by accelerated chemical reactions under higher temperatures, as each W-D cycle involves drying specimens at 71 ± 3°C (160 ± 5°F) for 42 h. In addition, moisture ingress during the wetting phase may have contributed to the formation of additional cementitious products, resulting in improved bonding between soil particles. These factors collectively contributed to the increased strength of treated soil specimens after W-D cycles ( 28 , 29 ).
At the conclusion of 12 W-D cycles, a marginal improvement in UCS for 10C (7.5%) was observed, whereas UCS values for 8C_15fRCA and 8C_30fRCA showed slight degradation (<2.5%). This degradation may be attributed to moisture-induced damage sustained during the durability testing. The UCS testing regime indicated that 10C demonstrated slightly better resistance to environmental stressors compared to 8C_15fRCA and 8C_30fRCA. Nevertheless, all PLC-fRCA specimens provided substantial improvements over UT soils.
With respect to the F-T environmental stressor, UT soil specimens failed after only two cycles, highlighting the necessity for treatment to enhance the durability of natural soils for long-term serviceability in transportation infrastructure. Conversely, treated specimens (10C, 8C_15fRCA, and 8C_30fRCA) demonstrated significant improvement, as all treated specimens were able to resist the F-T environmental stressor. Figure 9 illustrates the UCS values of UT and treated specimens at 0, 6, and 12 F-T cycles.
Unconfined compressive strength (UCS) test values for various geomaterial groups after freeze–thaw (F-T) conditioning.
An improvement in UCS values was observed for 10C and 8C_15fRCA specimens after six F-T cycles. Specifically, the UCS increased by 32.6 psi (225 kPa) for 10C (1.1 times) and 123.9 psi (854 kPa) for 8C_15fRCA (1.3 times) compared to values recorded after curing. Both treated geomaterial groups continued to show strength improvements even after the sixth F-T cycle. Notably, the improvement for 8C_15fRCA (33.3%) was marginally higher than for 10C (27.5%). Furthermore, the increase in UCS values between 6 and 12 F-T cycles (27.5% for 10C and 33.3% for 8C_15fRCA) was greater than the increase observed between after-curing and six F-T cycles (7% for 10C and 28% for 8C_15fRCA). However, the UCS value for 8C_30fRCA exhibited a slight degradation (6%) over the course of the durability study compared to values observed after curing. This degradation occurred after approximately six F-T cycles (∼7%), with a marginal improvement (1%) observed after 12 F-T cycles.
It was noted that all three treated geomaterial groups generally exhibited higher UCS values during W-D durability testing compared to F-T testing. This relatively lower strength increase (for 10C and 8C_15fRCA) and degradation (for 8C_30fRCA) may be attributed to the deterioration of the internal pore structure of the geomaterials caused by the formation of ice lenses during the freezing phases in F-T testing. In addition, sub-zero temperatures in F-T testing likely impeded the chemical reactions necessary for the formation of cementitious products. The slight degradation in strength observed for 8C_30fRCA may be attributed to the enhanced detrimental effects of the freezing phase caused by increased porosity, resulting from the higher granular material content in this geomaterial group compared with 10C and 8C_15fRCA.
Stiffness Properties (Mr)
RLT tests were conducted on specimens from all four geomaterial groups (UT, 10C, 8C_15fRCA, and 8C_30fRCA) at the end of the curing period and after 6 and 12 cycles of W-D and F-T conditions. These tests were performed to evaluate the improvement in resilient moduli caused by chemical stabilization and to assess the influence of environmental stressors on the performance of the specimens. For a more comprehensive analysis, the Mr values of all soil groups were modeled using a universal model (Equation 1), as recommended by NCHRP Project 1-37A for pavement design:
where Mr is the resilient modulus (MPa), θ is bulk stress (kPa), Pa is atmospheric pressure (kPa), τoct is octahedral shear stress (kPa), and k1, k2, and k3 are constant model parameters. The first parameter, k1, is related to the elastic modulus of the material, while k2 depicts the impact of bulk stress on the resilient moduli values and k3 captures the shear response on deviatoric stresses in the material.
Table 4 presents the values of the three parameters (k1, k2, and k3) for all four geomaterial groups (UT, 10C, 8C_15fRCA, and 8C_30fRCA), along with their corresponding R2 values. Significant improvements were observed in the k1, k2, and k3 parameters for treated geomaterials compared to UT soil at the end of the curing period. Specifically, the k1 parameter values increased by factors of 1.5, 2.4, and 2.7 for the 10C, 8C_15fRCA, and 8C_30fRCA specimens, respectively, compared to the UT soil. In addition, the k3 parameter, which indicates resistance to deviatoric stress or potential softening behavior, was found to be negative for both UT and treated specimens. However, the treated specimens showed improvements in k3 values compared with the UT soil (k3 = −3.259), with increases of approximately 27% for 10C and 8C_15fRCA, and 36% for 8C_30fRCA. The increase in the k1 and k3 parameters indicates that the formation of cementitious gel improved the bonding between soil particles, enhancing stiffness. This improvement effectively transformed weak subgrade soils into a more robust and resilient subgrade layer for transportation infrastructure.
Universal Model Parameter Values for Different Geomaterial Groups
Note: UT = untreated; C = cement; fRCA = recycled concrete aggregate fines; W-D = wet–dry; F-T = freeze–thaw; 0 cycle = after 7-day curing; NA = not applicable.
Environmental stressors were observed to reduce the k1 value (resilient modulus), likely because of internal cracks and changes in moisture content. The k1 value generally decreased with the number of W-D cycles but increased with the number of F-T cycles, with the exception of 8C_30fRCA, which may be attributed to the changes in C-S-H formation at elevated temperatures resulting in coarser and brittle C-S-H bonds, as also observed by Gallucci et al. ( 30 ). In addition, the k3 value for W-D specimens was higher than that for F-T specimens, indicating that F-T specimens exhibited weaker shear resistance to deviatoric stress. This finding was consistent with the UCS test results, which showed higher compressive strength for W-D specimens compared with F-T specimens. The more pronounced degradation in F-T specimens is attributed to the more destructive nature of F-T conditions and increased moisture content. Despite the observed reduction in stiffness, treated geomaterials demonstrated improvements in both elastic moduli and shear resistance properties. Therefore, the treated geomaterials proved to be relatively more resistant to repeated loads and environmental stressors compared to UT soil, highlighting the positive impact of the chemical treatment.
Moisture Content and Volumetric Changes
Figures 10a and b illustrate specimens from all three treated geomaterial groups following W-D and F-T environmental stressor conditioning. Notably, all treated specimens retained their shape and exhibited minimal damage despite exposure to harsh environmental conditions in the laboratory. This suggests that the chemical stabilizer, in combination with co-additives, effectively improved the engineering properties of the treated soils. In addition, the W-D specimens appear significantly drier than the F-T specimens, indicating that different environmental stressors can lead to varying degrees of improvement and degradation in the specimens. Moreover, detailed calculations are provided in the following paragraphs.
Specimens of treated geomaterial groups at the end of environmental conditioning: (a) after 12 wet–dry (W-D) cycles and (b) after 12 freeze–thaw (F-T) cycles.
Figures 11a and b illustrate the changes in volume and moisture content at the end of each cycle of W-D durability testing for different geomaterial groups. The UT specimen collapsed during the initial wetting cycle, whereas the treated specimens exhibited significantly improved performance, surviving all cycles and remaining viable for engineering tests.
Volumetric change and moisture content trend for wet–dry (W-D) conditioning: (a) volume change trend and (b) moisture content variation.
Figure 11a shows that the most substantial volumetric changes occurred during the first two cycles of W-D testing. Despite this, the maximum volumetric change remained below 1%, indicating that the treated soil specimens effectively mitigated moisture-induced strains. This minimal volumetric swell is attributed to the formation of cementitious materials that bind the soil particles together, thereby reducing swell–shrink behavior. Notably, the PLC-treated specimens with fRCA as a co-additive (8C_15fRCA and 8C_30fRCA) demonstrated superior resistance to volumetric changes compared to the 10C specimens during the wetting phases of the W-D testing. This improved performance may be caused by the lower water absorption potential of fRCA compared to clay minerals. Therefore, replacing clay with fRCA reduces water absorption and results in lower volumetric swell.
In addition, Figure 11b reveals that the moisture content during the W-D process consistently remained below the OMC, with values being 5–6% lower by the end of 12 cycles. This suggests that the specimens lost moisture because of drying conditions, and as it remained consistently below the OMC despite subsequent re-soaking, moisture intrusion caused by wetting was minimal as compared to UT specimens. This indicates that the treated specimens exhibit strong resistance to moisture intrusion, which helps preserve their engineering properties, such as strength and stiffness. In addition, soil loss recorded after W-D durability testing was 1.2% for 10C, 5.0% for 8C_15fRCA, and 3.7% for 8C_30fRCA, which are minimal and align with industry standards, including those set by the USACE.
With respect to the F-T environmental stressor, UT specimens exhibited a volume change of 5% after two F-T cycles. In contrast, treated specimens showed minimal volumetric swell, as low as <0.1% by the end of the 12th F-T cycle (Figure 12a). This underscores the effectiveness of the chemical stabilizer in enhancing soil resistance to damage and volumetric strains. A similar trend to the W-D process is observed during the F-T process, with the most substantial volume change occurring during the first two F-T cycles. The moisture content during F-T conditioning showed an increase for all geomaterial groups compared to their OMC (Figure 12b). UT specimens experienced a 9% increase in moisture content over OMC after two F-T cycles, leading to their collapse. In contrast, treated geomaterial groups (10C, 8C_15fRCA, and 8C_30fRCA) exhibited an increase of 3–4% over OMC by the end of the 12th F-T cycle, demonstrating the effectiveness of the chemical stabilizer and co-additives in preventing moisture intrusion and improving performance.
Volumetric change and moisture content trend with freeze–thaw (F-T) conditioning: (a) volume change trend and (b) moisture content variation.
Furthermore, UT specimens showed significant soil loss of 63.7% by the end of the second F-T cycle, resulting in collapse. Treated specimens, however, experienced substantially lower soil loss, with values of 0.6% for 10C, 4.8% for 8C_15fRCA, and 2.9% for 8C_30fRCA. These results not only meet industry requirements but also highlight the superior resilience and sustainability of treated soils as subgrade materials for better transportation infrastructure. It is notable that W-D specimens exhibited greater volumetric swell compared to F-T specimens, likely because of the higher variation in moisture content during W-D conditioning. Despite this, treated specimens outperformed UT ones under both environmental stressors (W-D and F-T). Interestingly, in both W-D and F-T environmental stressor conditioning, soil loss was marginally higher in PLC-fRCA-treated specimens compared to PLC-only treated specimens. This is likely because of the replacement of cohesive soil particles with granular fRCA particles in the PLC-fRCA specimens.
Conclusions
A research study was conducted to investigate the effectiveness of PLC and fRCA in enhancing soil engineering properties under different environmental stressors. Clayey subgrade soil was treated with PLC and a PLC-fRCA mixture to evaluate improvements in strength, stiffness, and volumetric changes through UCS and RLT tests, following exposure to various environmental conditioning cycles in the laboratory. The key findings of the study are summarized as follows.
UT soil collapsed within the first two cycles of environmental stressors. In contrast, both PLC and PLC-fRCA-treated specimens showed significant strength improvements and demonstrated greater resilience to environmental stressors. This enhancement is attributed to the formation of strong binding gel products (C-S-H or C-A-H) resulting from the hydration and pozzolanic reactions among PLC, fRCA, and soil.
Similar to strength, the stiffness of the soils improved with PLC and PLC-fRCA treatment. Despite a reduction in stiffness after exposure to F-T and W-D cycles, treated specimens maintained higher stiffness values compared to UT ones, indicating a positive impact of PLC and PLC-fRCA on soil stiffness.
UT specimens experienced significant weight loss (≈64%). In contrast, soil loss was substantially reduced (<5%) after stabilization with PLC and PLC-fRCA. In addition, volumetric changes in the subgrade soil were reduced to <1% with treatment after both 12 cycles of W-D and F-T conditioning.
F-T cycles proved more destructive compared with W-D cycles for all treated geomaterial groups. Moisture content increased by 3–4% over OMC for specimens subjected to F-T testing, while it decreased by 5–6% below OMC for W-D specimens. This variation in moisture content influenced the stiffness properties of the treated soils.
In conclusion, the study demonstrates that combining fRCA with a reduced dosage of traditional cement stabilizers enhances the engineering properties of weak subgrade soils. This combination yields results comparable to those achieved with full doses of traditional cement stabilizers. Further research is recommended to include microstructural and chemical analyses within a sustainability framework to gain a detailed understanding. Moreover, future investigations should also focus on assessing the long-term performance incorporating a long-term durability study, and a comprehensive sustainability analysis of this treatment across various soil types and dosages of cement and fRCA along with field testing to bridge the gap between laboratory findings and practical applications. This will ensure a comprehensive understanding of its effectiveness and viability in transportation infrastructure projects.
Footnotes
Author Contributions
The authors confirm contribution to the paper as follows: study conception and design: A.J. Puppala, N. Biswas; data collection, testing, analysis, and interpretation of engineering testing results: M. Sanei, S. Chou; draft manuscript preparation: M. Sanei, S. Chou, N. Biswas, A.J. Puppala. All authors reviewed the results and approved the final version of the manuscript.
Declaration of Conflicting Interests
The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: A.J. Puppala is a member of Transportation Research Record’s Editorial Board. All other authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Southern Plains Transportation Center-funded project #CY1-TAMU-UTEP-01 and the NSF Industry-University Cooperative Research Center (I/UCRC) program-funded Center for Integration of Composites into Infrastructure (CICI) site at Texas A&M University, College Station, Award #2017796 (Phase III).
