Recommended Practice for Stabilization of Subgrade Soils and Base Materials
Author: Little, Dallas N Texas Transportation Institute Nair, Syam Texas Transportation Institute | Size: 1.02 MB | Format:PDF | Quality:Original preprint | Publisher: Transportation Research Board | Year: 2009 | pages: 67 | ISBN: -
Long-term performance of pavement structures is significantly impacted by the stability of the underlying soils. In situ subgrades often do not provide the support required to achieve acceptable performance under traffic loading and environmental demands. Although stabilization is an effective alternative for improving soil properties, the engineering properties derived from stabilization vary widely due to heterogeneity in soil composition, differences in micro and macro structure among soils, heterogeneity of geologic deposits, and differences in physical and chemical interactions between the soil and candidate stabilizers. These variations necessitate the consideration of site-specific treatment options which must be validated through testing of soil-stabilizer mixtures. This report addresses soil treatment with the traditional calcium-based stabilizers: Portland cement, lime, and fly ash. The report describes and compares the basic reactions that occur between these stabilizers and soil and the mechanisms that result in stabilization. The report presents a straightforward methodology to determine which stabilizers should be considered as candidates for stabilization for a specific soil, pavement, and environment. The report then presents a protocol for each stabilizer through which the selection of the stabilizer is validated based on mixture testing and mixture design. The mixture design process defines an acceptable amount of stabilizer for the soil in question based on consistency testing, strength testing, and in some cases (resilient) modulus testing. Within each additive validation and mixture design protocol, an assessment of the potential for deleterious soil-additive reactions is made.
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Recommended Practice for Stabilization of Sulfate-Rich Subgrade Soils
Author: Little, Dallas N Texas A&M University, College Station Nair, Syam Texas A&M University, College Station | Size: 1.08 MB | Format:PDF | Quality:Original preprint | Publisher: Transportation Research Board | Year: 2009 | pages: 66 | ISBN: -
Calcium-based stabilizers (CaO-based stabilizers) develop a high pH environment when mixed with water during construction. This high pH condition and the presence of lime (CaO) introduce an environment favorable to the formation of two expansive minerals, ettringite and possibly thaumasite, when sulfate-bearing salts are present in soil. The success of a lime stabilization project depends heavily on the accuracy of predictions regarding the potential of the native soils to form these disruptive minerals. This report describes the nature of these minerals and the mechanism of their formation in soils stabilized with lime, cement, or fly ash. The key issue for the engineer in deciding how to properly stabilize sulfate-bearing soils is to quickly and efficiently determine: (1) the sulfate content of the soils and (2) the threshold quantity of sulfates likely to cause damage. This report addresses both issues but focuses on the methods used to quantify sulfate content. Quick and effective field techniques and more precise laboratory tests are compared and evaluated. Recommendations are presented regarding the appropriate testing protocol to use. Specific recommendations for changes to the American Association of State Highway and Transportation Officials (AASHTO) test method T-290-95 (methods A and B) are made. The report also outlines techniques for effective risk assessment including reconnaissance, topographical assessment, pedological and geological assessment, climatic assessment, and soil sampling plans. The report also outlines construction tests that can reduce risks for various levels of sulfate determined using a method such as AASHTO T-290-95 (revised) or equivalent.
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This report documents and presents the results of a study of minimum reinforcement requirements for the design of concrete bridge structures. This study included a review of U.S. and international practice, test data and research findings related to minimum reinforcement requirements and flexural cracking of concrete structures. A total of 4 representative methods of specifying minimum reinforcement were evaluated and compared by performing design calculations on a wide range of concrete bridge members. The findings of this study suggest that in nearly all cases lightly reinforced concrete members can develop the nominal flexural strength and have significant strength and ductility reserves after cracking has occurred. Also, the modulus of rupture over estimates the flexural cracking stress of concrete bridge members. A rational approach to the specification of minimum reinforcement is proposed, where variables are appropriately factored and includes the maximum rather than nominal strength of the section as a true measure of ductile versus brittle response.
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Driveways are private roads that provide access (both ingress and egress) between a public way and abutting properties, and any facilities on those properties. The roadway engineers’ focus is often on a part of the driveway, the area where the driveway intersects the public highway or street. Since these connections form the link or interface between public streets and highways and the activities they serve, driveways are an integral part of the roadway transportation system. There has been relatively little comprehensive research on or national guidance for the geometric design of driveways in recent decades. The objective of this project was to develop recommendations for geometric design of driveways that will be useful to state departments of transportation, local governments, and consultants in preparing driveway design standards and practices. The project included an extensive review of related literature, a survey of transportation agencies, a listing of almost 100 factors that can affect the design of a driveway, a list of needed research topics, and research on issues related to driveway vertical alignment. The project produced two documents, the project report and a driveway design guide.
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This research provides a methodology for evaluation of durability related strength loss of bonded carbon fiber reinforced polymer (CFRP) systems applied to concrete beams. The report addresses test methods to establish a durability strength reduction factor, identification of corresponding field exposure conditions affecting durability, and suggestions for the application of the durability strength reduction factor for design of field applications. The durability strength reduction factor is a measure of the loss in strength over time due to environmental exposure. It is defined as the ratio of the flexural strength of a 4 in. x 4 in. x 14 in. concrete beam reinforced with CFRP exposed at 140°F and submerged in water or 100% relative humidity for 60 days to the flexural strength of a control specimen. The resulting durability strength reduction factor may be used to evaluate CFRP system performance. Two field environments are suggested: Wet and Air. In a Wet environment water accumulates at the bond surface. This is the default condition and corresponds to test results in submerged water at 140°F for 60 days. An Air environment allows drying between wetting episodes so water cannot accumulate on the bond surface. This condition corresponds to test results in 100% relative humidity at 140°F for 60 days.
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The objective of the first two phases of NCHRP Project 3-70 was to develop and test a framework and enhanced methods for determining levels of service for automobile, transit, bicycle, and pedestrian modes on urban streets, in particular with respect to the interaction among the modes. Phase 2 resulted in the multimodal level of service method (MMLOS) described in NCHRP Report 616, "Multimodal Level of Service for Urban Streets." The objective of Phase 3 of NCHRP Project 3-70 was to field test the MMLOS method with various public agencies around the United States. This Final Report presents the results of this third phase of the research. During Phase 3 the MMLOS method was field tested in 10 metropolitan areas of the United States. Public agency staffs were trained on the MMLOS method and it’s implementing software. They assisted in data collection and evaluated the suitability of the MMLOS method for use within their agency. Based on the results of these field tests several revisions were made to the spreadsheet software for implementing MMLOS. Additional guidance was provided to deal with conditions encountered in the field that were not anticipated when the original guide, NCHRP Web-Only Document 128, was written. Finally, a few minor modifications to the pedestrian level of service model are recommended to improve its sensitivity to some of the conditions encountered in the field tests.
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The research focused on: 1) Finalizing the two primary model components [i.e., a viscoelastic continuum damage (VECD) model for crack initiation and a hot mix asphalt fracture mechanics (HMA-FM) model for crack propagation], involving development and integration of sub-models that are relevant to dominant top-down cracking mechanisms into each model component; 2) Verifying the reasonableness of the two enhanced primary model components (i.e., the VECD-based model and the HMA-FM-based model); and 3) Developing for and integrating with the HMA-FM-based crack propagation model a simplified fracture energy-based crack initiation model to illustrate the potential of a completed system and to help formulate a plan for integrating, calibrating, and validating the two enhanced primary model components. In summary, the work performed clearly indicates that the VECD-based model and the HMA-FM-based model developed and evaluated in this project can form the basis for a top-down cracking model suitable for use in the Mechanistic-Empirical Pavement Design Guide (MEPDG). Furthermore, the component models can form the basis for an improved performance model to predict multiple cracking distresses simultaneously, including top-down cracking, bottom-up cracking, and thermal cracking. The project also identified and recommended research efforts to develop calibrated/validated top-down cracking performance models for use in the MEPDG.
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This report presents the results of an interlaboratory study (ILS) to prepare precision estimates for AASHTO T265 test method used for Laboratory Determination of Moisture Content of Soils. The materials for the ILS included two coarse- and two fine-grained soil-aggregate blends that were prepared according to Grading A and Grading E of AASHTO M147, “Materials for Aggregate and Soil-Aggregate Subbase, Base, and Surface Courses.” Each of the four blends had less than 10% soil passing #200 sieve to represent suitable materials for base and subbase. The comparison of the statistics of moisture content data for the four soil-aggregate blends indicated that the variability of moisture content measurement is the same for the blends with clay and silt; however, the variability is different for the blends with fine and coarse gradations. A precision statement for AASHTO T265 that includes the precision estimates developed in this study has been prepared and provided in the report.
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This report presents the results of an interlaboratory study (ILS) to prepare precision estimates for AASHTO T265 test method used for Laboratory Determination of Moisture Content of Soils. The materials for the ILS included two coarse- and two fine-grained soil-aggregate blends that were prepared according to Grading A and Grading E of AASHTO M147, “Materials for Aggregate and Soil-Aggregate Subbase, Base, and Surface Courses.” Each of the four blends had less than 10% soil passing #200 sieve to represent suitable materials for base and subbase. The comparison of the statistics of moisture content data for the four soil-aggregate blends indicated that the variability of moisture content measurement is the same for the blends with clay and silt; however, the variability is different for the blends with fine and coarse gradations. A precision statement for AASHTO T265 that includes the precision estimates developed in this study has been prepared and provided in the report.
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