Resilient modulus of stabilized subgrade soil using commercial products and industrial by-products
Date of Award
Doctor of Philosophy
Soil stabilization is an ancient technique, which has been improvising over the years for different project requirements. The present study is focused on stabilizing the natural bed of soil, on which the pavements are laid. The material to be stabilized is commercially available EPK clay, classified as silt with medium-high plasticity. The study is focused on stabilizing EPK clay with additives/stabilizer and check its resilient behavior using resilient modulus (RM) test, as pavements are laid over natural bed (subgrade) of soil, which are not strong enough to take the traffic load in most cases. The additives used were, calcium sulfoaluminate (CSA) cement (environmentally friendly cement i.e., produces 50% less carbon dioxide than regular ordinary Portland cement), lime sludge (LS-byproduct from a water treatment plant), fly ash (FA-byproduct from a coal fired thermal power plant), and polypropylene fiber (F). Although, materials like cement and lime have proven efficient, the use of these environmentally friendly cement leaves less carbon footprint on the society and use of byproducts solves the problem of their disposal, reduces the cost of the project, and promotes sustainability. Stabilization process of EPK clay with commercial products and industrial byproducts begins with standard Proctor test to find out the optimum moisture content and maximum dry unit weight of each mix proposed in the study. Then, the unconfined compressive strength (UCS) samples were prepared based on the standard Proctor moisture-density relationship. The prepared samples were cured for 7, 14, and 28 days, by wrapping them in plastic film and placing in a controlled water tub. Samples were also tested right after preparing them without any curing as 0 days sample. The results demonstrated an increase in UCS strength with the increase in curing period as well as increase in the percentage of the additives. The use of lime sludge (LS) and polypropylene fiber (F) alone did not improve the UCS strength much, as the strength did not increase more than 50 psi for all mixes and with different curing periods. Therefore, lime sludge and polypropylene fiber were considered soil modifiers instead of stabilizers. Incorporating calcium sulfoaluminate (CSA) cement with lime sludge (LS), polypropylene fiber (F) and fly ash (FA) as an activator with lime sludge, led to a substantial improvement in UCS strength. UCS strengths at 28 days curing period for EPK clay + 20% LS + 20% FA, EPK clay + 7.5% CSA cement, and EPK clay + 8% LS + 8% CSA cement were 201.88, 158.13 and 177.37 psi, respectively. The cured and uncured samples were tested for ultrasonic pulse velocity (UPV) test, before performing the UCS test. UPV is a nondestructive test mainly used with cementitious materials in predicting the strength and check for defects (mainly voids) in a sample. UPV, has been gaining importance recently and has led its path into testing stabilized soil specimens (usually used in testing concrete specimens over a period of time). Results show that UPV has a linear relationship with UCS, as the samples with higher pulse velocity had higher UCS values. Regression analysis between UCS and UPV had reasonable correlations with CSA cement as well as CSA cement mixed with lime sludge and polypropylene fiber. Strength increase in UCS samples were also evaluated based on the microstructure analysis, using the microscopic images at different magnifications, obtained by performing scanning electron microscopy (SEM). With chemical reaction between EPK clay and stabilizer or additives, flocculation and agglomeration happened and an increase in curing period led to a much denser soil matrix, resulting in an increased UCS strength. Consolidation tests were also performed on all the mixes to check their compressibility behavior. The samples were prepared based on their standard Proctor moisture-density relationship and were loaded and unloaded in a specific sequence. The results indicated a decrease in compression index (Cc) values at higher dosages of additive content for all the mixes in lieu of virgin EPK clay. Reduced compression index values were more pronounced with use of CSA cement, CSA cement with fiber, and CSA cement with lime sludge. Finally, the resilient modulus (RM) tests were carried out on all the EPK clay mixes for uncured and cured samples. Resilient modulus is a fundamental input parameter in design of pavements and the test procedure replicates the in-situ condition in a laboratory set up. The test works on a principle of applied stress to recoverable strain, mainly a stiffness measurement. RM values for EPK clay and EPK clay mixed with lime sludge, polypropylene fiber for all curing periods were less than 10,000 psi. Incorporating calcium sulfoaluminate (CSA) cement with Lime sludge (LS), Polypropylene fiber (F) and fly ash (FA) as an activator with lime sludge (LS) led to a substantial improvement in RM values, which had a range of 12,632-74,331 psi. RM test results were evaluated based on effect of increase in curing period as well as additive content. RM values increased but did not follow a trend unlike in UCS test, where the strength increased with increase in curing period and additive content. RM values for 28 days were lower than 7 or 14 days in few cases, demonstrating strain hardening in the sample and indicating that strength and stiffness are not the same. Material constants obtained from the RM test were used to back calculate the moduli using Uzan (1985) model. A plot of calculated versus experimental RM values for different mixes and curing period were plotted to see their relatability based on regression analysis. Reasonable correlations were obtained with an R2 value ranging from 0.714-0.918. Loading mechanism in RM test consists of multiple repetitions of loading and unloading the sample, inducing permanent strain in it, which was calculated by measuring the height of the sample before and after the test. Among all the EPK clay mixes, highest permanent strain of 0.216 inches was observed for EPK clay + 1.5% fiber mix at 14 days curing period and the least permanent strain of 0.0003 inches was observed for EPK + 8% LS + 8% CSAC mix at 0 and 7 days curing period. A strain of 0.5 inches is permissible in real life, anything more than that causes pavement deformation. The present study is primarily focused on using resilient modulus (RM) test, as it yields a fundamental input parameter in designing a pavement, to eliminate the notion of RM test being complicated and initiate IDOT (Illinois Department of Transportation) to adopt RM test in designing their pavements. The other part of the study is to effectively use industrial byproducts (Lime sludge and Fly ash) to promote sustainability and an environment friendly CSA cement (to have a lesser carbon footprint on the society) in stabilizing subgrade for pavement construction.
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