Hasan Kazmee

Characterization of Railroad Ballast Behavior Under Repeated Loading

Characterizing railroad ballast behavior under repeated train loading is of significant importance for evaluating field settlement or permanent deformation potentials of unbound aggregate ballast layers. For the proper characterization of ballast behavior under dynamic loading, a new triaxial test setup was recently developed at the University of Illinois at Urbana–Champaign. Capable of accommodating cylindrical specimens with a diameter of 305 mm (12 in.) and a height of 610 mm (24 in.), this closed-loop servohydraulic test setup used a load cell and four displacement transducers mounted on the specimen to quantify deformation behavior under loading. Preliminary test results evaluating effects of different applied stress states as well as geogrid reinforcement on ballast behavior established the consistency and repeatability of this new test equipment. Laboratory findings are presented from an ongoing research study aimed at investigating the effects of different ballast types and field degradation trends on permanent deformation accumulation. The ballast type with the highest mill abrasion value was found to accumulate the highest permanent deformation under repeated load triaxial testing. Permanent deformation trends observed for four other ballast types showed direct correlations to the degrees of particle degradation observed in track sections constructed with these ballast materials and trafficked for approximately 18 months with a total track usage of 320 million gross tons.

Investigation of Geogrid-Reinforced Railroad Ballast Behavior Using Large-Scale Triaxial Testing and Discrete Element Modeling

Geogrids are well known for improving the performance of unbound aggregate layers in transportation applications by providing confinement and restraining movement through interlock between individual aggregate particles and geogrid apertures. Geogrid reinforcement offers an effective remedial measure when railroad track structures are susceptible to track geometry defects resulting from excessive movement and particle reorientation within the ballast layer. This paper presents findings from an ongoing research study at the University of Illinois aimed at quantifying the effects of geogrid reinforcement on the shear strength behavior of railroad ballast. The effects of two geogrid types on ballast shear strength were evaluated through laboratory testing and numerical modeling. An imaging-based discrete element method (DEM) modeling approach was used to identify the optimal position for geogrid reinforcement to achieve the maximum shear strength gain in cylindrical triaxial specimens. Geogrids were installed at five depths within the cylindrical specimen and tested for shear strength properties with a large-scale triaxial test setup to evaluate the effectiveness of both geogrid aperture shape and reinforcement depth. Placing two layers of geogrids in the middle of the specimen was found to result in the maximum shear strength gain. Such placement of the geogrid ensured the intersection of the shear failure plane with the reinforcement layer, ultimately leading to significant shear strength gains. The DEM simulations were observed to capture accurately the ballast shear strength behavior with and without geogrid reinforcement.

Using Accelerated Pavement Testing to Evaluate Reclaimed Asphalt Pavement Materials for Pavement Unbound Granular Layers

AbstractUnbound aggregate layers are commonly used as subgrade replacement and subbase over weak subgrade soils. With the recent focus on sustainable construction practices, ever-increasing transportation cost, and scarcity of natural resources, nontraditional and locally available recycled materials have become viable for the construction of unpaved and low volume roads. To this end, a research study was recently undertaken at the Illinois Center for Transportation to evaluate the engineering applications and field performances of reclaimed asphalt pavement (RAP) materials used in pavement unbound base/subbase and foundation layers. Twelve different full-scale test sections were constructed for the field performance investigations with accelerated pavement testing. Construction quality control was achieved through in-place density and modulus measurements using field testing. Periodic rut measurements were carried out on pavement surfaces throughout the accelerated loading process. Observed rutting trend...

Pavement Working Platforms Constructed with Large-Size Unconventional Aggregates

As a sustainable construction practice, recent research efforts of the Illinois Department of Transportation (DOT) have been focused on an engineered approach for developing construction specifications and performance validation of the use of large-size virgin and recycled unconventional aggregates in soft subgrade remediation. The Illinois DOT Bureau of Design and Environment has issued a special provision that specifies certain gradation bands for improved aggregate subgrade to allow enhanced use of large rocks from both primary crusher type virgin and recycled sources. Nevertheless, laboratory tests are usually inadequate for characterizing the properties and engineering behavior of large-size unconventional aggregates. Therefore, the recent field performance evaluation study using accelerated pavement testing was initiated at the University of Illinois with 12 full-scale pavement working platforms constructed with selected large-size virgin and recycled materials, including reclaimed asphalt pavement, recycled concrete aggregate, and demolition waste. For in-depth performance testing and evaluation, this paper describes test section construction and quality control sequences conducted at different phases of the study, including state-of-the-art field aggregate and pavement layer imaging techniques. The rutting performance trends of test sections were investigated in light of innovative test devices and techniques, including variable energy penetration testing, geoendoscopy, transverse ground-penetrating radar scans, as well as postfailure trenching images. Penetration of large rocks into the soft subgrade was effective in achieving a stable construction platform. Moreover, reclaimed asphalt pavement as a granular pavement layer was prone to large permanent deformations.

Sustainable Alternatives in Low Volume Road Base Course Applications Evaluated through Accelerated Pavement Testing

Depletion of good quality aggregate sources combined with strict environmental regulations has forced transportation agencies to consistently seek unconventional and sustainable alternatives for pavement construction materials. Accordingly, the Illinois Department of Transportation (IDOT) uses a wide variety of recycled materials for pavement construction platform as well as base/subbase course applications. However, no established design standards are available to govern the design and construction of pavement systems incorporating such recycled materials. This paper presents findings from an ongoing research study at the University of Illinois focused on the performance evaluation of different unconventional aggregates in low volume road base/subbase course applications through accelerated pavement testing (APT). Seven different aggregate materials incorporating different proportions of recycled concrete and reclaimed asphalt pavement materials were used to construct twenty four full-scale pavement test sections. Performance under loading of each pavement section is currently being studied through accelerated pavement testing and periodic rut-depth measurements after different number of load applications. Results from accelerated testing of four such pavement sections are presented in this paper.

Use of a Variable Energy Penetrometer and Geo-Endoscopic Imaging in the Performance Assessment of Working Platforms Constructed with Large Size Unconventional Aggregates

Transportation agencies commonly use large size aggregates, often referred to as rock cap or aggregate subgrade, e.g., by Illinois Department of Transportation (IDOT), for stabilizing weak subgrades at wet of optimum moisture states. Adequate characterization of these large rocks is not possible in the laboratory with the use of standard tests. Accordingly, a cone penetration based strength index is the best field assessment tool since shear strength profile is closely linked to unbound aggregate or aggregate subgrade layer performance. To this end, an innovative variable energy dynamic cone penetration (DCP) device, popularly known as PANDA in France, was utilized in a recent Illinois Center for Transportation (ICT) research study involving the performance assessment of large size aggregates over soft subgrades. Twelve full scale working platform sections were constructed with six different types of virgin and recycled large size aggregate materials. Accelerated pavement testing (APT) was carried out on these sections to monitor the rutting progression with number of passes of a certain wheel load assembly. To evaluate layer properties and adequately relate them to rutting performance, PANDA tests were conducted along with traditional DCP soundings on the loading applied pavement test section centerlines. A geo-endoscopic probe was also used in the holes opened by the PANDA tests to identify layer interfaces and visually document subsurface moisture conditions. The PANDA and geo-endoscopy testing has proven very beneficial in the performance assessment of the large size aggregate subgrade materials under simulated traffic loading. This paper presents current detailed technical knowledge on the PANDA and geo-endoscopy test equipment and highlights field results associated with the recent ICT project soundings conducted in the pavement working platform test sections.

Field Performance Evaluations of Large Sized Unconventional and Recycled Aggregates for Subgrade Improvement

Illinois Department of Transportation has recently introduced new gradation bands to accommodate large sized unconventional and recycled materials in subgrade remedial applications. Existing standardized test protocols cannot characterize these materials properly. To this end, an accelerated pavement testing study was undertaken to evaluate rutting performance trends of pavements constructed with such aggregate materials. An advanced field image segmentation technique was implemented for the in situ characterization of aggregate size and morphological properties, i.e. shape, texture and angularity, texture. Four pavement working platform test sections were constructed with railway ballast or riprap sized virgin aggregates as well as large-sized concrete demolition waste capped with densely graded dolomite and reclaimed asphalt pavement (RAP) materials. Construction quality control was achieved through nuclear gauge density checks and modulus measurements, the latter utilizing lightweight deflectometer and soil stiffness gauge. Next, the full scale test sections constructed over weak engineered subgrades were tested with unidirectional wheel loading in accelerated pavement testing. Periodic rut measurements were taken to quantify the rutting progression. Contributions of the subsurface layers to surface rut accumulation were assessed by the use of the following field equipment: variable energy lightweight penetrometer, ground penetrating radar (GPR), and geo-endoscopic probe for visualization of layer compositions and interfaces. Despite higher modulus properties obtained, RAP capped sections typically accumulated higher permanent deformation. Geo-endoscopic imaging revealed that presence of shallow water table led to early failure in one of the test sections. According to the study results, current Illinois subgrade stability design framework was found to be adequate for utilizing large sized unconventional and recycled materials in subgrade remedial applications.

Performance of Aggregate Subgrade Layers in Low Volume Roads Constructed with Unconventional Recycled Aggregates

With transportation sector accounting for approximately 27% of total greenhouse gas emissions, transportation agencies all over United States have made sincere efforts to reduce such carbon footprint by incorporating more recycled materials in design mixes. In Illinois, recycled materials commonly include large sized unconventional aggregates used to stabilize weak subgrade soils that are abundantly found in wet of optimum conditions and prone to frost effects. To facilitate increased use of such “aggregate subgrade” materials, IDOT has recently introduced new gradation bands. However, performance of such recycled aggregate subgrade materials within the bounds of current design framework is largely unknown. To this end, a full scale accelerated pavement test study was undertaken to study six different aggregate subgrade materials involving construction of twelve full scale flexible pavement sections over weak engineered subgrade. Results of accelerated pavement testing on six test sections and performance of three different aggregate subgrade materials are highlighted in this paper including data from quality control tests such as nuclear density gauge, GeoGauge for composite layer modulus, and lightweight and falling weight deflectometers. Current pavement design framework was adequate when designing with two out of three aggregate subgrade materials that constituted different proportions of recycled materials. One noteworthy finding was that asconstructed hot mix asphalt thickness variation was found to be quite large due to reclaimed asphalt pavement subbase sinkage observed during paving operations.

Performance Evaluations of Pavement Working Platforms Constructed with Large-Sized Unconventional Aggregates

With recent focus on sustainable construction practices and the ever-increasing transportation cost and scarcity of natural resources, use of unconventional aggregate materials, such as primary crusher run and concrete demolition waste, have become viable for the construction of pavement working platforms over very weak and often wet subgrade soils. To this end, a research study was undertaken at the Illinois Center for Transportation to evaluate the adequacy and field performances of such large-sized aggregate materials and validate new material specifications. A state-of-the-art image analysis technique was utilized to characterize the size and morphological properties, e.g. shape, texture and angularity of two large-sized aggregates, referred to herein as primary crusher run and crushed concrete. For the field evaluation, full-scale test sections were constructed with these large-sized aggregate materials over a very weak engineered subgrade and subjected to accelerated pavement testing. Construction quality control was achieved through in-place density and modulus measurements on conventional aggregate capping surface layers using nuclear gauge, lightweight deflectometer and soil stiffness gauge type devices. Periodic rut measurements were carried out on the pavement surface throughout the accelerated loading process using an Accelerated Transportation Loading Assembly (ATLAS). Contributions of the underlying pavement layers to the total rut accumulation was evaluated through innovative applications of ground penetrating radar (GPR), a light weight penetrometer device, known as the French Panda, as well as a geoendoscopy probe. Layer intermixing and material migration at the aggregate subgrade and subgrade interface was found to improve the layer stiffness and pavement performance results significantly.

Behavior of Geogrid Reinforced Ballast at Different Levels of Degradation

Geogrids have been successfully used in highway applications for stabilization and reinforcement purposes. Recent research efforts have also been dedicated to study geogrids in railway applications, especially for ballast reinforcement. Ballast, typically containing large-sized aggregate particles with uniform gradation, is an essential layer in railway substructure to facilitate load distribution and drainage. However, the reinforcement effect of geogrids in ballast has not yet been thoroughly investigated. Especially with the accumulation of tonnage, ballast will increasingly get fouled due to aggregate degradation or contamination by other materials, affecting aggregate-geogrid interlock mechanism at different levels of degradation. In this study, monotonic triaxial strength tests - performed on both clean and fouled ballast specimens - have been reinforced by geogrids and tested using a large-scale triaxial test device recently developed at the University of Illinois specifically for ballast size aggregate materials. Two different geogrids with square- and triangular-shaped apertures are used in comparison. To further investigate the geogrid reinforcement mechanisms, an imaging based discrete element modeling (DEM) approach was also adopted with the capability to create actual ballast aggregate particles as three-dimensional polyhedron blocks having the same particle size distributions and imaging quantified average shapes and angularities. By addressing adequately the particulate nature of different sized and shaped ballast aggregate particles and their interactions with each other at contact points, and through the innovative use of membrane elements surrounding the cylindrical ballast specimen, the ballast DEM model accurately captured the strength behavior of both clean and degraded ballast specimens reinforced by geogrids with different aperture shapes.