Laureano R. Hoyos

Characterization of Cement-Fiber-Treated Reclaimed Asphalt Pavement Aggregates: Preliminary Investigation

The use of reclaimed asphalt pavement (RAP) materials in road construction has been proven to reduce both the amount of construction debris disposed of in urban landfills and the rate of depletion of natural resources. However, source-dependent product variability, federal and local environmental regulation, and deficient strength-stiffness characteristics often limit RAP applications in road bases. These limitations have led to new research efforts aimed at exploring novel, cost-effective, chemical and/or mechanical stabilization methods to treat RAP materials before their reuse in pavement construction. In this work, a series of tests were performed on RAP aggregate materials treated with different dosages of portland type I/II cement and with alkali-resistant glass fibers. Tests include permeability, leachate, unconfined compression, and small-strain shear moduli through resonant column testing. Leachate tests include pH, total and volatile dissolved solids, total and volatile suspended solids, and turbidity. Test results confirm the potential of cement-fiber-treated RAP material as an environmentally and structurally sound alternative to nonbonded materials for base and subbase applications in pavement engineering. A companion paper presents the results from a comprehensive repeated-load triaxial test program to investigate the resilient modulus characteristics of cement-treated RAP.

VOLUMETRIC SHRINKAGE STRAIN MEASUREMENTS IN EXPANSIVE SOILS USING DIGITAL IMAGING TECHNOLOGY

Expansive soils undergo large volumetric shrinkage strains, which eventually lead to high heave movements upon hydration of these soils. Current test methods to determine shrinkage strain potentials of soils are restricted by several limitations including small specimen sizes, molds with rigid walls that restrain shrinkage strains in lateral directions, and manual measurement errors. In this paper, a novel methodology to conduct volumetric shrinkage strain test on cylindrical soil specimens and a digital imaging technique to analyze and determine volumetric shrinkage strains of the test are described. As part of the evaluations of this methodology, volumetric shrinkage strains of four types of medium to high expansive soils were researched. Shrinkage tests were conducted on all four soils at three different moisture contents. Volumetric shrinkage strains were then measured using both digital and conventional manual approaches. Test results showed that the test and the developed measurement methodology provided repeatable and realistic shrinkage strain measurements. Digital measurements provided more accurate results than manual measurements by accounting for even minor shrinkage cracks in the soils. The significance of the digital measurements in relation to the current shrinkage strain characterizations is discussed. Potential geotechnical application areas where these test results could be used are also described.

DEVELOPMENT OF A STRESS/SUCTION-CONTROLLED TRUE TRIAXIAL TESTING DEVICE FOR UNSATURATED SOILS

A servo-controlled, true triaxial (cubical) testing apparatus has been modified to test 10-cm cubical specimens of unsaturated soil under suction-controlled conditions and for a wide range of stress paths. The equipment is a mixed-boundary type of device, with the specimen seated on top of a saturated high-air-entry disk and between five flexible membranes on the remaining sides of the cube. This paper describes the development of the device, including details of main apparatus components, specimen preparation, step-by-step assembling procedure, and the corresponding validation of its suitability for testing unsaturated soils. The device features two independent pore-air pressure and pore-water pressure control/monitoring systems. Matric suction states in the specimens are induced and maintained constant during testing by using the axis-translation technique. A companion paper (Macari and Hoyos Jr. 2001) presents the results from a comprehensive experimental study of the stress-strain response of a recompacted silty sand following simple-to-complex stress paths under suction-controlled conditions.

Experimental Studies on Stabilized Clays at Various Leaching Cycles

AbstractThe effectiveness of chemical stabilization and its permanency were investigated by subjecting chemically treated soils to leaching tests by internal water flushing, which simulates moisture ingress/digress into subsoils from rainfall events. Control and chemically stabilized soils from eight different locations were subjected to these leaching tests over 14 cycles in the laboratory. Each leaching cycle represents one pore volume of water flushed through the soil specimen. Leaching samples were collected after 3, 5, 7, and 14 cycles and were chemically analyzed to address pore fluid chemical composition changes and possible leaching of the chemical additives. Unconfined compressive strength tests were conducted on soil specimens after 3, 7, and 14 leaching cycles to study soil strength variations. The effects of soil type, stabilizer type and dosages, and the curing method on the leaching of stabilizers are analyzed. Test results and analyses indicate that leaching of stabilizer did occur, but the...

Mechanical Behavior of an Unsaturated Soil Under Multi-Axial Stress States

A series of drained (constant suction) true triaxial tests has been conducted on several identically prepared, 10-cm side, cubical specimens of recompacted silty sand to study the stress-strain-strength behavior of an unsaturated soil under multiaxial stress states and suction-controlled conditions. The experiments were conducted in a stress/suction-controlled true triaxial (cubical) setup following a multi-stage testing scheme. Matric suction states in the specimens were induced and maintained constant during testing by using the axis-translation technique. Imposed stress paths included an initial hydrostatic compression followed by conventional traxial compression, triaxial compression, or simple shear in the first octant of the octahedral stress plane. Test results are used to evaluate the nature of principal strain response of an unsaturated soil along multi-axial stress paths, and to observe the influence of matric suction on the shape, size, and position of the failure envelopes in the octahedral stress plane.

RANKING OF FOUR CHEMICAL AND MECHANICAL STABILIZATION METHODS TO TREAT LOW-VOLUME ROAD SUBGRADES IN TEXAS

Expansive soils encountered in North Texas exhibit low strength properties, as well as high swell and shrinkage characteristics. These soil properties often result in the poor performance of pavement infrastructures, particularly those built for low-volume traffic conditions. Pavement distress caused by differential heaving leads to pavement cracking and ponding problems. This causes riding discomfort for commuters and induces traffic delays due to the continual repair of the pavements. Hence, it is necessary to explore and develop new and alternate stabilization methods to improve stabilization of expansive soils. Laboratory investigations were designed and conducted on four local expansive soils stabilized with low-calcium Class F fly ash, Type V sulfate-resistant cement, ground granulated blast furnace slag (GGBFS), and lime mixed with fibers. Test results were analyzed and ranked by well-established scales to select the top-performing stabilizers. Results showed that the sulfate-resistant cement, followed by the lime with fibers and the GGBFS stabilization methods, provided effective stabilization of soft and expansive soils. Ranking assessments were performed, and a few important conclusions on stabilization mechanisms were drawn.

Constitutive modeling of unsaturated soil behavior under axisymmetric stress states using a stress/suction-controlled cubical test cell

Abstract Two elasto-plastic critical state-based formulations for modeling the constitutive behavior of an unsaturated soil are presented and briefly discussed. Computational constitutive drivers were implemented to allow for the numerical simulation of suction-controlled conventional triaxial tests. Simulations were obtained from explicit and implicit integration techniques. The algorithms support numerical analyses in a deviatoric stress plane by using a mixed control constitutive driver in conjunction with a generalized cam-clay model, within a constant-suction scheme. The results from a series of suction-controlled conventional triaxial tests conducted on several identically prepared, 10-cm side, cubical specimens of compacted silty sand, using a recently developed stress/suction-controlled cubical testing device, were used for validation of the models and the enhanced features proposed in the present work. Matric suction states in the specimens were induced and maintained constant during testing by using the axis-translation technique.

Fiber and fly ash stabilization methods to treat soft expansive soils

This paper presents a summary of test results from two separate studies conducted on expansive soil stabilization using recycled waste materials. Fly ash and polypropylene fibers were evaluated in these studies. Two different expansive soil types were used as control soils. Both methods increased strength and decreased shrinkage strains of raw expansive soils. Fly ash method also reduced plasticity and free swell characteristics. Both stabilizers are recycled waste products and therefore their use in soil stabilization will reduce landfilling costs and enhance recycling efforts.

Evaluation and Mix Design of Cement-Treated Base Materials with High Content of Reclaimed Asphalt Pavement

Reclaimed asphalt pavement (RAP) and granular base materials were collected from stockpiles throughout Texas to evaluate the feasibility of using mixes containing high RAP content for base course applications. Mixes containing 100%, 75%, and 50% RAP treated with 0%, 2%, 4%, and 6% of portland cement were evaluated in a full-factorial laboratory experiment. For mixes of 75% and 50% RAP, both virgin and salvage base materials were used. Experimental results indicated that besides the cement content, the RAP content and finer aggregate content significantly affected the properties of the RAP mixes, but the effects of RAP type and asphalt content in RAP were limited. To achieve a 300-psi unconfined compressive strength as required by the Texas Department of Transportation, the optimum cement contents were statistically about 4%, 3%, and 2% for mixes of 100%, 75%, and 50% RAP, respectively. Because the achievement of any specified strength or stiffness might not always ensure the durability of a mix, other parameters that might be relevant to performance and long-term durability were evaluated through laboratory testing. These parameters included modulus, indirect tensile strength, and moisture susceptibility as well as cement leaching.

SWRC Modelling Framework for Evaluating Volume Change Behavior of Expansive Soils

The soil-water Retention curve (SWRC) has been used as a tool by geotechnical researchers and practitioners to determine the properties such as the shear strength, coefficient of permeability, bearing capacity and the modulus of elasticity of unsaturated soils. Such studies are valuable to the practicing engineers as they alleviate the use of time consuming and elaborate testing techniques required for determining the unsaturated soil properties. In this paper, an attempt is made to study the relationship between the SWRC and the swelling pressure including the one dimensional swell behavior for two different expansive soils using statically compacted specimens. Test results were analyzed to evaluate the clay mineralogy and influence of initial compaction moisture content or matric suction on both swell strain and swell pressure properties in one-dimensional test conditions. Comparisons of swell pressures and swell strain potentials and related SWRC properties of these soils are also made.