Evaluation of Cement-Modified Soil (CMS) with Microcracking and Its Effects on Flexible Pavement Performance
Tannoury, George Atef
University of Kansas
Civil, Environmental & Architectural Engineering
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Cement modification of subgrade has been widely practiced for the past few decades. Recently, cement has become a more economical binder to modify in-situ subgrade soil since other binders, such as fly ash, have become less available and therefore their prices have increased significantly. In addition, a much higher percentage of fly ash needs be used, when compared with cement to achieve the same subgrade strength and stiffness. In general, cement-modified subgrade is prone to develop shrinkage cracking, which can eventually reflect through asphalt pavement layers to the surface after construction. For some subgrade soils, a high cement content is needed to meet the unconfined compressive strength requirement without jeopardizing durability. A higher cement content will result in higher shrinkage cracking potential. To overcome this problem, a microcracking technology has been developed and adopted in the field. This technology involves re-compaction of cement-modified soil (CMS) with a roller, 24 to 48 hours after initial compaction, to induce microcracks in the CMS and minimize the potential for large shrinkage cracks. Microcracking of CMS is not expected to significantly reduce the strength and stiffness of CMS, but it is expected to increase its hydraulic conductivity and reduce the potential for large shrinkage cracks. Unfortunately, the procedure to simulate microcracking of CMS in the laboratory and to evaluate its effect on properties of CMS has not been established yet. This dissertation documents the development of such a procedure and discusses the effect of microcracking on the properties (strength and modulus) of CMS specimens. The developed procedure utilized unconfined compression (UC) tests to generate micro-cracks in specimens. To generate micro-cracks, the loading stress level was found to be equal to the unconfined compressive strength of the CMS specimen. The laboratory results showed that microcracking increased the hydraulic conductivity of the specimen and reduced its electrical resistivity when the specimen was saturated. To evaluate the effects of microcracking on the field performance of CMS, field and laboratory tests, including Electrical Resistivity (ER) tests, Light Weight Deflectometer (LWD) tests, Falling Weight Deflectometer (FWD) tests, and Resilient Modulus (Mr) tests, were conducted on the CMS at two different locations in the State of Kansas, USA. The ER results from the field did not show a clear correlation between the ER value and the microcracking process because the ER results fluctuated within the device accuracy range. The (LWD) tests conducted in the field showed that adding cement increased the subgrade modulus. However, after applying three passes of roller compaction to generate the microcracks in the CMS in the field, the subgrade modulus dropped to approximately 40% of its original value on average. The back-calculation analyses of the FWD test data from both sites showed that the actual resilient moduli of the microcracked CMS layers in the field were significantly higher than the laboratory resilient moduli of the microcracked and uncracked reconstituted specimens. Also, the laboratory resilient moduli of four cored specimens from one of the sites were approximately 25 to 50 percent higher than those of reconstituted specimens from the same site. However, the laboratory resilient modulus test results showed that the microcracked specimens reconstituted from the soils obtained from the field had slightly higher Mr values than the uncracked specimens. In addition, the performance of an asphalt concrete pavement over the CMS with microcracking was evaluated based on a mechanistic empirical approch. The KENLAYER Computer Program was used to predict the pavement responses under traffic loading. The hot mix asphalt (HMA) and the subgrade were modeled as linearly elastic materials and their stiffness values used in KENLAYER were backcalculated from FWD testing using the ELMOD V.6 software. However, the CMS was either modeled as a non-linearly elastic material with its properties determined from the laboratory resilient modulus tests or as linearly elastic material with the properties backcalculated from the FWD tests. Furthermore, the typical pavement structural distresses, such as permanent deformation (rutting) and fatigue cracking, and the remaining service life were evaluated for the actual pavement thicknesses used in the field.
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