Thomas D White

Correlation of fine aggregate imaging shape indices with asphalt mixture performance

This study addresses the relationship between fine aggregate shape properties and the performance of hot-mix asphalt. Aggregate shape was expressed as three independent properties: form, angularity, and texture. Image analysis procedures and indices were developed to capture these properties. The indices were measured for 22 aggregate samples. The samples were related to the hot-mix asphalt rutting resistance measured under wet and dry conditions in the Purdue wheel-tracking device. Among the different aggregate shape properties, texture had the strongest correlation with rutting resistance. Resistance to rutting increased exponentially with an increase in aggregate texture.

MULTILAYER ELASTIC PROGRAM FOR BACKCALCULATING LAYER MODULI IN PAVEMENT EVALUATION

Multilayer solutions that calculate stresses, strains, and displacements in pavement structures caused by surface loading have been in existence for several years. However, most elastic layer computer software applications have been developed for mainframe computers. Recently, computational capabilities of the practicing engineer have been greatly enhanced by recent developments in personal computers. Results of this study involved development of an accurate and efficient solution to the multilayer problem. In addition, software was developed for use on a personal computer that included routines for backcalculating pavement layer moduli from measured surface deflections. A convergence technique, employing a combination of closed form and numerical solutions, has been developed to provide accuracy of results at the surface of the pavement and near the load. Special methods of solving the system of boundary equations were developed to obtain the terms needed for the closed form part of the solution. Maximum efficiency has been obtained through an in-depth analysis of the parameters affecting accuracy and their relationship with the numerical integration step. In its current form, the computer solution WESLEA is a fully competent three-dimensional solution that will handle up to five layers with varying interface conditions and a maximum of 20 loads. The fifth layer is semi-infinite and can be made stiff based on the choice of modulus of elasticity. A personal computer-based backcalculation program, WESDEF, has been developed utilizing the multilayer program. An evaluation of the new program is made through comparison of identical deflection data using the BISAR n-layered computer program.

Dynamic Properties of Fiber-Modified Overlay Mixture

Cores and slabs of fiber-modified asphalt overlay were taken from a series of test sections constructed on two highways in Indiana. Extensive laboratory investigations were conducted to evaluate the fiber-modified asphalt mixtures. Laboratory tests included physical properties, beam fatigue, and complex modulus. Because of a lack of a standard method for fiber extraction, three fiber extraction methods were evaluated. Test results show that more than 95 percent of fiber can be extracted from paving mixtures. Actual fiber contents in test sections were found to be different from target fiber contents by a significant amount. Physical property tests show that the addition of fibers is related to a higher percentage of air voids in the in situ pavement. Results of fatigue tests indicate that fibers increase the fatigue life of overlay mixture. There are inconsistencies in the significance of fiber on both dynamic modulus and phase angle. However, the results do indicate that the addition of fibers will chang...

Hot-Mix Asphalt Segregation: Measurement and Effects

Segregation of hot-mix asphalt (HMA) has been associated with poor pavement performance. As a result, a nondestructive test method that could quickly identify segregation and be easily included in field testing would be useful to agencies. A study was conducted of four HMA mixtures with five levels of segregation. All levels of segregation for each mix were characterized by gradation, asphalt content, density, and air voids. The segregated mixes were tested with an air permeameter and a nuclear moisture/density gauge. In addition, thermal imaging equipment was field-tested on an existing pavement, at a hot-mix asphalt plant, and at a paving project to determine its overall effectiveness in detecting segregation. Preliminary wheel-tracking tests with the PURWheel tracking device were used to evaluate segregated mixture performance.

Effect of Segregation on Fatigue Performance of Asphalt Paving Mixtures

The main objective of this research was to investigate the effect of segregation on fatigue performance of asphalt paving mixtures. Segregation in asphalt concrete pavements occurs when coarse material is concentrated in some areas of the pavement and fine materials in others. Segregation has been one problem that has resulted in poor performance in many pavements. Materials used in the study included gravel, natural sand, and an AC-20 asphalt cement. Aggregates were selected to meet the Indiana Department of Transportation specification for a No. 8 binder with a maximum aggregate size of 1 in. (25 mm). To simulate various levels of coarse and fine segregation, four artificially segregated mixtures were prepared in addition to the control asphalt mixture. Slabs were compacted for the five mixtures using the Purdue linear compactor. These slabs were cut into beams that were used to test for the fatigue properties of the five segregated asphalt concrete mixtures. Limits of fatigue were examined when flexural stiffness was one-third and one-half of the initial stiffness at 200 load applications. Results of the investigation indicated that the fatigue life of segregated asphalt concrete mixtures was significantly affected.

Gradation Effects on Hot-Mix Asphalt Performance

The effect of gradation on hot-mix asphalt (HMA) performance has long been a contentious issue. One objective of the National Pooled Fund Study No. 176 was to evaluate the impact of gradation on the rutting performance of HMA. To this end, 21 Superpave mixtures were designed that used a range of materials, nominal maximum size aggregates, and gradations typical of those used throughout the United States. Tests that included both laboratory and prototype-scale loading were used to evaluate the permanent deformation characteristics of the mixtures. Analysis of the data revealed that adequate performance could be obtained with mixture gradations plotting above (ARZ), through (TRZ), and below (BRZ) the restricted zone. Laboratory tests suggested that ARZ and TRZ gradations might provide better deformation resistance than BRZ gradations. However, prototype-scale accelerated pavement testing did not show any clear trends in performance relative to gradation alone with respect to the restricted zone. This means that the restricted zone alone is not adequate to characterize gradation to ensure acceptable rutting performance and should therefore be omitted from Superpave specifications.

Alternatives for Railroad Traffic Simulation Analysis

In North America, the process for determining appropriate railroad infrastructure for new service or an increased volume of existing service usually includes the use of simulation software. Decisions are generally based on statistical analysis of the simulation output. The simulation and analysis that are commonly conducted, however, may not provide an accurate assessment of the adequacy of the infrastructure. Furthermore, the output data comparisons commonly used to describe the effect of infrastructure on traffic may not be easily associated with traffic conditions. These shortcomings can be mitigated with appropriate care in developing the simulation input data and changing the output analysis methodology.

Structural Adequacy of Rubblized Portland Cement Concrete Pavement

The objective of rubblization is to eliminate reflection cracking in hot-mix asphalt (HMA) overlay by the total destruction of the existing slab action. Rubblization is applicable when there is little potential for retaining slab integrity and structural capacity of the original jointed reinforced concrete pavement. Rubblization has been used successfully for rehabilitation of other portland cement concrete (PCC) pavement types. Typically, the slab is reduced to pieces less than 300 mm in size. Subsequently, the slab is converted to a high-strength granular base. Restoration of the structural capacity is accomplished with an overlay of HMA. In 1991, the Indiana Department of Transportation (INDOT) awarded a contract to apply the rubblization technique to a portion of US-41 in Benton County. The pavement sections were evaluated before and after rubblizing and overlay to estimate the AASHTO layer coefficient of rubblized concrete pavements. The layer coefficient determined in this study (a2 = 0.25) represents a value of two standard deviations less than that reported by PCS/Law. Currently, INDOT uses a layer coefficient of 0.20 for rubblized PCC pavements. On the basis of the results of this study, the layer coefficient can be set within two standard deviations of the mean (i.e., 0.22). This layer coefficient of 0.22 represents a conservative value that is recommended for rubblized PCC pavements with similar conditions. If INDOT continues to use the rubblization technique in pavement rehabilitation, a study encompassing slabs of different thicknesses on different subgrades should be undertaken for confirmation of the values that are reported herein.

EFFECT OF GRADATION ON ASPHALT MIXTURE PERFORMANCE

Phase I of National Pooled Fund Study No. 176, “Validation of Strategic Highway Research Program (SHRP) Asphalt Mixture Specifications Using Accelerated Testing,” has been completed. One of the topics addressed in this project was the effect of gradation on asphalt mixture performance. The purpose of Phase I was to evaluate sensitivity of tests to changes of gradations within the framework of Superpave aggregate specifications. Tests included in the study were prototype scale accelerated pavement tests, laboratory scale wheel-tracking tests, and triaxial tests. Mixtures reported on here included one asphalt (PG 64-22), one coarse aggregate (limestone), and one limestone sand (fine aggregate angularity value of 44). Mixtures in the study have gradations above, through, and below the restricted zone. Results indicate that tests used are sensitive to changes in gradation and aggregate types.

LABORATORY AND FIELD MOISTURE CONDITIONS FOR FLEXIBLE PAVEMENT

The primary source of water in pavements is from surface infiltration. Water from surface infiltration and other sources increases pavement maintenance costs and shortens pavement life. These effects can be mitigated by including a subdrainage system in the pavement design. A study of pavement subdrainage systems has been conducted that included a phase for laboratory tests for the hydraulic characteristics of materials used in three test sections built on I-469 at Fort Wayne, Indiana. The purpose of building these test sections was to evaluate the subdrainage performance and materials used in the sections. The sections differ in the permeable drainage layer as well as the filter (separation) layer. The sections were instrumented with different types of sensors. Instrumentation installed in the sections included time domain reflectometry (TDR) probes, moisture suction blocks, temperature probes, resistivity tree, and rainfall and outflow tipping buckets. Other features of the instrumentation site included an air temperature probe and access tube for a nuclear moisture content probe. The laboratory hydraulic characteristic test results on the materials are presented. Field moisture condition in the layers is also presented. Calibration equations for the TDR are developed for the field materials. A summary of three rainfall events and pavement outflow volumes recorded for the tests is presented. Recommendations are made for materials to be used in subdrainage systems.