Anand J. Puppala

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.

Effects of Fiber Reinforcement on Strength and Volume Change in Expansive Soils

The results of a research study to investigate the influence of discrete and randomly oriented polypropylene fiber reinforcement on expansive soil stabilization are presented. Two expansive soils were used as control soils in the testing program. Two types of fibers and four fiber dosages (0,0.3,0.6, and 0.9 percent by dry weight of soil) were considered. Both raw and fiber-reinforced clayey samples were prepared and subjected to unconfined compressive strength (UCS), volumetric shrinkage, three-dimensional free swell, and swell pressure tests. Test results were statistically analyzed to investigate the effectiveness of fiber reinforcement on strength, swell, and shrinkage characteristics of expansive clays. Results indicated that the fiber reinforcement enhanced the UCS of the soil and reduced both volumetric shrinkage strains and swell pressures of the expansive clays. The fiber treatment also increased the free swell potential of the soils. Practical implications of the findings and future research directions are discussed.

Field Investigations on Performance of T-Shaped Deep Mixed Soil Cement Column–Supported Embankments over Soft Ground

The soil cement deep mixing method has been used to improve soft clayey soils under embankment loading conditions. A compacted granular fill layer or geosynthetic reinforcement layer is placed over the top of soil cement deep mixed (DM) columns to reduce differential settlement between DM soil and the surrounding untreated soil, which, in turn, increases embankment stability. Typically, in conventional deep mixing methodology, the soil cement columns are closely spaced, indicating large area replacement ratios in the con- struction projects. Such practice could increase construction costs substantially. In this research, a new type of DM column, called a T-shaped DM (TDM) column, was designed and used as an alternative to the large-area-replacement-ratio DM columns employed in the field. Unlike in the conventional column, the cross section of the new column varies along the installation depth. Large amounts of cement slurry are injected and thoroughly mixed with the native shallow soil using specially designed mixing blades. At greater depths, deep mixing methodology is applied only to smaller-diameter columns, resulting in large-diameter columns near the surface and smaller-diameter columns deeper. Field trials were conducted to investigate the performance of TDM column-supported soft ground under embankment loading. For comparison, performance of conventional DM column-supported soft ground under similar embankment loading is presented. Differences in quality control studies and in situ plate loading tests on TDM and conventional DM columns are discussed. Under field embankment loading con- ditions, stress concentration ratio, excess pore water pressures generated in the soft clays, total monitored settlement, and lateral soil dis- placement near embankment toes are analyzed and discussed for both treatments. It is concluded that TDM columns have considerable advantages over conventional DM because they both mitigate settlement and enhance the performance of the embankments while reducing construction costs. DOI: 10.1061/(ASCE)GT.1943-5606.0000625.

Regression Model for Resilient Modulus of Subgrade Soils

Subgrade soil is an important part in both flexible and rigid pavement structures. AASHTO recommends the use of bulk stress and deviatoric stress models to characterize granular and cohesive soils, respectively. However, these models oversimplify the fundamental behavior of subgrade soils. Being proposed is the use of an octahedral stress-state model to characterize the resilient modulus of different soils. Eight different soils representing major soil types in Louisiana were selected to validate the model and to calibrate the model parameters. Additional analysis was then performed to develop correlations between the model parameters and other soil properties. Three types of correlation were produced: (a) model parameter with soil properties, (b) model parameters with California bearing ratio, and (c) model parameters with unconfined compressive strength. Statistical analysis indicated that the correlation between model parameter and soil properties yielded the best results.

Quantitative estimation of clay mineralogy in fine-grained soils

Stabilization design guidelines based on soil plasticity properties have certain limitations. Soils of similar plasticity properties can contain different dominant clay minerals, and hence, their engineering behavior can be different when stabilized with the same chemical additive and dosage. It is essential to modify stabilizer design guidelines by including clay mineralogy of the soil and its interactions with chemical additives used. Chemical properties of a soil including cation exchange capacity (CEC), specific surface area (SSA) and total potassium (TP) are dependent on clay mineral constituents, and an attempt is made in this study to develop a rational and practical methodology to determine both clay mineralogy distribution and dominant clay mineral in a soil by using three measured chemical soil properties and their analyses. This approach has been evaluated by determining and evaluating clay minerals present in artificial and natural clayey soils of known and unknown clay mineralogy. A total of twenty natural and six artificial soils were considered and used in the chemical analyses. Test results and subsequent analyses including the development of artificial neural network (ANN) based models are evaluated and described in this paper.

Semi-empirical model for the prediction of modulus of elasticity for unsaturated soils

A semi-empirical model is proposed in this paper to predict the variation of modulus of elasticity with respect to matric suction for unsaturated sandy soils using the soil-water characteristic curve (SWCC) and the modulus of elasticity under saturated conditions. Using this model, comparisons are provided between the predicted and measured moduli of elasticity and elastic settlements from model footing test results on three different sandy soils. The results of this study are encouraging as there is good agreement between the predicted and measured moduli of elasticity and settlements.

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.

Engineering Behavior of Lime-Treated Louisiana Subgrade Soil

Lime stabilization is often used to treat subgrade soils when they are soft and cohesive in nature. A study was conducted to investigate the engineering behavior, including the resilient and strength behaviors, of a lime-treated subgrade soil. The lime treatment procedure was adapted from the specifications of the Louisiana Department of Transportation and Development. Silty clay, a soil often found in Louisiana subgrades, is used as a base soil. A summary of various engineering properties of a lime-treated soil from resilient modulus, unconfined compression strength, and California bearing ratio (CBR) tests conducted at five moisture content and dry density levels is provided. Tests were also performed on the raw soil without lime treatment, and these results were compared with those of tests with the lime-treated soil. The comparisons indicate that the present lime treatment method results in an increase in strength and resilient modulus properties and a decrease in plasticity characteristics and plastic strains. A regression model with three constants was used to analyze the resilient modulus test results. The model constants are presented as functions of soil properties. Resilient modulus correlations that use either CBR or unconfined compression strength, moisture content, dry density, degree of compaction, and stresses as dependent attributes are developed.

Comprehensive life-cycle cost analysis for selection of stabilization alternatives for better performance of low-volume roads

Low-volume roads (LVRs), such as rural, farm-to-market, and less-used local and city roads, are an important part of the worlds transportation infrastructure. LVRs have been credited as a direct cause of the socioeconomic development of rural communities. It has been estimated that 60% of the road network in the United States is made up of low-volume roads. The construction, maintenance, and rehabilitation of these roads are major tasks that result in about 54% of the total annual expenditure of transportation agencies in the United States. Better design and construction methods will lead to lower maintenance and rehabilitation costs of LVRs. Stabilization of weak subgrade soils to support LVRs is a widely accepted method of improving their performance. However, the selection of a stabilization alternative on the basis of cost–benefit analysis is a crucial task for a transportation agency and one that has not been addressed in a systematic manner. In this paper, a new conceptual engineering economics tool–based life-cycle cost analysis (LCCA) is developed to optimize and to select the best stabilizer and the stabilization technique for a given subgrade soil and given traffic conditions. In this analysis, agency, user, and externality costs are addressed. Two case studies are analyzed for European and U.S. road conditions to validate the LCCA model. Results demonstrate that, under specific boundary conditions, soil stabilization can play an important role, merging the environmental and mechanical effectiveness of low-volume roads.

MECHANISTIC EVALUATION OF HYDRATED LIME IN HOT-MIX ASPHALT MIXTURES

Permanent deformation and moisture damage are common distresses found in pavements today. The use of mineral fillers such as hydrated lime is known to provide a decrease in moisture susceptibility. In many cases, mineral fillers will also increase the mixture stiffness. Conventional asphaltic concrete mixtures and mixtures modified with hydrated lime were evaluated for their fundamental engineering properties as defined by indirect tensile strength and strain, permanent deformation characteristics, resilient modulus, and fatigue resistance. A typical Louisiana low-volume dense-graded mixture was used. The test factorial included two aggregate types (limestone and gravel) and two asphalt cement types (a conventional AC-30 and one modified with styrene-butadiene polymer). The results indicated that the addition of hydrated lime as mineral filler improved the permanent deformation characteristics and fatigue endurance of the asphaltic concrete mixtures. This improvement was particularly apparent at higher testing temperatures with mixes containing polymer-modified asphalt and limestone aggregate.