Sanghyun Chun

Effect of Gradation Characteristics on Performance of Superpave Mixtures in the Field

Previous studies have indicated that existing mix design criteria, including voids in mineral aggregates, gradation control points, and effective asphalt content, do not capture all critical aspects of gradation and mixture volumetric properties found to be strongly related to rutting and cracking performance. The performance of Superpave® mixtures varied significantly in mixtures that met all existing criteria. Therefore, there was a need to identify and verify additional criteria that could ensure better and more consistent performance of Superpave mixtures. Four parameters from the dominant aggregate size range (DASR)– interstitial component (IC) model have been determined to relate well to field performance of Superpave mixtures: DASR porosity, disruption factor (DF), effective film thickness (EFT), and the ratio of coarse portion to fine portion of fine aggregate (CFA/FFA). These four parameters were used in a field performance evaluation of rutting and cracking that was conducted to identify performance-related criteria. Results indicated that DASR porosity, which reflected the characteristics of coarse aggregate structure, was the most dominant parameter to control rutting performance. IC characteristics including DF, EFT, and CFA/FFA could not overcome the problems associated with a mixture that had DASR porosity outside the acceptable range. However, IC characteristics were more important than DASR porosity to differentiate clearly field cracking performance. The DF criteria should be considered in conjunction with EFT and CFA/FFA criteria to distinguish effectively the field cracking performance. Therefore, the introduction of DASR-IC criteria into current mix design guidelines and specifications will help to ensure good field rutting and cracking performance of Superpave mixtures.

Long-term field evaluation and analysis of top-down cracking for Superpave projects

A field evaluation of cracking performance was undertaken for 10 pavement sections as part of a long-term Superpave monitoring project. It was found that top-down cracking was the major type of cracking. Also, moisture damage was identified in four of these sections. An analysis based on the energy ratio (ER) parameter showed that the mixtures that were affected by moisture generally exhibited a much faster reduction in the ER (indicating reduced fracture resistance) than those that were not. Further analysis of cracking performance was conducted using the enhanced hot mix asphalt fracture mechanics-based pavement performance model (HMA-FM-E). The HMA-FM-E is capable of predicting the entire process of top-down cracking from the onset of cracking until pavement failure, and thus provides more valuable information that may better assist material and pavement engineers to optimise their designs. The results showed that the predictions for sections not affected by moisture generally agreed well with field observations. More importantly, it was identified that the key to further improve the accuracy of the performance model is to more accurately predict changes of mixture properties affected by moisture in addition to oxidative ageing.

Evaluation and Implementation of PG 76-22 Asphalt Rubber Binder in Florida

The Florida Department of Transportation (DOT) has had a long history of recycling ground tire rubber (GTR) from waste tires in highway projects. The Florida DOT first experimented with asphalt rubber binder (ARB) more than 35 years ago. In 1988, the Florida legislature mandated the investigation of using recyclable materials such as GTR in roadway construction. For more than 25 years, the Florida DOT has effectively satisfied the intent of the legislature. However, the ARB has often been associated with unpredictable performance and handling issues, such as the separation of the GTR from the binder. These performance and handling issues have resulted in higher construction costs and have made ARB less appealing because a more reliable alternative, PG 76-22 polymer modified asphalt (PMA), is available. As a result, a task team of the Florida DOT and industry members was formed with the goal of finding a way to make ARB handle and perform similarly to PG 76-22 (PMA), Floridas gold standard binder. This paper documents an accelerated pavement testing (APT) study to compare hot-mix asphalt mixtures constructed with PG 76-22 (PMA) and PG 76-22 (ARB). This APT study represents the final stage of the implementation of a new PG 76-22 (ARB) specification.

Development and Evaluation of Laboratory Conditioning Procedures to Simulate Mixture Property Changes Effectively in the Field

The study reported here involved the development and evaluation of laboratory conditioning methods and testing protocols to consider heat oxidation and moisture that would simulate more effectively asphalt mixture aging in the field and thereby help to assess asphalt mixture property changes properly over time. In this study, “aging” was defined as any detrimental effect on asphalt mixture properties during the pavement life. Three laboratory conditioning procedures were identified and further developed to evaluate the effects of heat oxidation and moisture: heat oxidation conditioning (HOC), cyclic pore pressure conditioning (CPPC), and a combination of HOC and CPPC. Cores from aged field pavements were used in the energy ratio approach, which could integrate various factors that affected cracking performance, including key mixture properties and pavement structure, to evaluate the effectiveness of the conditioning procedures. HOC was accomplished with the Superpave® long-term oven aging (LTOA) procedure, and CPPC was employed to induce moisture damage to the asphalt mixtures. Results indicated that asphalt mixtures subjected only to oxidative aging did not exhibit drops in mixture fracture energy (FE) comparable to the level observed in the field. The inclusion of CPPC was able to induce additional damage effectively through simulation of long-term moisture intrusion and repeated internal water pressure under traffic load. Further, the combination of LTOA followed by CPPC led to the most relevant changes in fracture properties with a level of reduction in FE consistent with field observation.

Predictive relationships for HMA fracture properties based on mixture component characteristics

This study was mainly focused on identifying appropriate relationships between asphalt mixture component characteristics and asphalt mixture properties known to control cracking performance. The current lack of material property models that can accurately describe the changes in material properties over time in the field is probably the greatest deficiency in our ability to accurately predict pavement performance. Therefore, there is a need to evaluate existing material property models, and develop improved models for use in the prediction of pavement performance. Relationships able to predict initial fracture energy and creep rate, which are the properties known to govern the change in material property over time and are also required for performance model predictions, were developed in this study. In addition, conceptual relationships were identified to describe changes in these properties over time (aging) by including the effect of the non-healable permanent damage related to load and moisture. This can serve as the foundation for further development of improved models to predict mixture properties as a function of age in the field based on additional field data and laboratory studies using more advanced laboratory conditioning procedures. The verified relationships will also serve to provide reliable inputs for prediction of service life using pavement performance prediction models.

Enhanced Gradation Guidelines to Improve Asphalt Mixture Performance

In 2010 a theoretical approach to the evaluation and specification of aggregate gradations to resist rutting was evaluated by the Florida Department of Transportation by accelerated pavement testing. This approach, known as the dominant aggregate size range (DASR) gradation model, provides a framework to ensure that the coarse aggregate of the resulting mixture has sufficient aggregate interlock to resist permanent deformation. Further research by the University of Florida and the Department of Transportation found that the properties of the interstitial components (ICs) within the DASR voids were strongly related to the durability and fracture resistance of asphalt mixtures. Parameters that made up the combined DASR-IC model included the DASR porosity, disruption factor (DF), effective film thickness (EFT), and fine aggregate ratio (FAR). The original evaluation of the DASR model was recently extended to include DASR gradations that might have had marginal aggregate interlock (i.e., marginal DASR porosity) and the effect of IC properties on mixture fracture resistance. The results confirmed that the rutting performance of the asphalt mixture was primarily controlled by the DASR porosity and that mixtures with marginal DASR porosity might still have had significantly better rutting performance than mixtures with poor DASR porosity. In addition, it was shown that the DASR porosity, DF, EFT, and FAR parameters played a critical role in mixture cracking performance. The validation of the combined DASR–IC model was documented, and the acceptable range of each parameter for improved mixture rutting and cracking performance was confirmed.

Cracking performance characterisation of asphalt mixtures containing reclaimed asphalt pavement with hybrid binder

This study investigated the effect of reclaimed asphalt pavement (RAP) on the rheological properties of a hybrid binder and the fracture properties and cracking performance of resultant mixtures. Fracture properties of the same mixtures containing up to 40% RAP with a standard polymer-modified asphalt (PMA) binder were used as benchmarks. RAP mixtures with hybrid binder exhibited lower resilient modulus, higher failure strain and higher fracture energy density than PMA RAP mixtures. Results indicated that rubber particles might not dissolve in the base binder; therefore, hybrid binder had more “free” soft binder to blend with the RAP whereas the more integrated polymer network present in PMA binder resulted in stiffer mixtures. Furthermore, PMA RAP mixtures exhibited lower damage accumulation rate than hybrid RAP mixtures. Eventually, hybrid and PMA RAP mixtures yielded comparable energy ratio values, which indicate both binder types can be interchangeably used in mixtures with up to 40% RAP for satisfactory cracking performance.

Evaluation of top-down cracking potential for asphalt pavements with 4.75 mm nominal maximum aggregate size mixture layer using full-scale field tests and finite element analysis

This study primarily focused on evaluating top-down cracking resistance for pavement structures with 4.75 mm nominal maximum aggregate size asphalt mixtures (i.e. SP-4.75) that will help determine more effective application of these mixes for layered flexible pavement system. Crack evaluation and pavement surface response measurements were conducted using full-scale field test sections. In addition, a three-dimensional (3-D) viscoelastic finite-element (FE) pavement model was developed to further validate the results of field tests using analytical solution. The results indicated that the pavement structure with SP-4.75 mixture at the top becomes more prone to top-down cracking when a thin layer with unmodified asphalt binder is applied. It was found that both layer thickness and binder type are key parameters to control top-down cracking potential. Also, the comparative observations between analytical and experimental results confirm that the FE model developed was able to provide accurate, reliable, and realistic prediction for change in pavement surface strain responses due to different conditions of SP-4.75 layer application. It was concluded that adequate thickness and binder type should be considered for proper application of SP-4.75 mixture as a surface course of layered flexible pavement system to effectively resist top-down cracking.

An approach to mitigate effects of colour variation, specularity and pores on microtexture analysis of aggregates

ABSTRACT Previous research indicated that color variation, specularity, and porosity, which are not related to surface roughness or texture of aggregate, affect the value of Texture Index (TI) determined using standard procedures associated with Aggregate Image Measurement System (AIMS). In response, a new technique called Photometric Stereo-Independent Component Analysis (PS-ICA) was developed and found to effectively mitigate color variation effect. Reduced light intensity was found to partially, but not completely, mitigate specularity effect. However, effect of pores could not be mitigated with measurements obtained from existing AIMS procedures. An extensive evaluation was conducted on a broad range of aggregates to determine whether TI values obtained before and after polishing using PS-ICA and reduced light intensity resulted in appropriate assessment of polishing resistance. For aggregates exhibiting strong specularity, it was found that specularity effect increased after initial polishing, which increased TI, thereby confounding and sometimes overwhelming the reduction in TI associated with reduced roughness. The specularity effect appeared to remain constant after initial polishing, such that further polishing resulted in a more consistent reduction in TI. For aggregates not exhibiting specularity, reduction in PS-ICA TI after initial polishing was found to be an excellent parameter to assess polishing resistance.

Evaluation of soil insulation effect on thermal behavior of drilled shafts as mass concrete

Abstract This study focused on investigating the early-age thermal behavior of drilled shafts under different surrounding soil’s thermal properties. Four 1.8 m (6 ft) diameter drilled shafts were constructed using two different concrete mixes and two different soil conditions. A finite element (FE) model was developed to estimate the temperature development of drilled shafts at early-age and validated using temperatures measured from full-scale drilled shafts constructed in the field. The validated analytical model was then used to perform a parametric analysis to evaluate the effects of the surrounding soils at different moisture conditions on change in thermal behavior of drilled shafts at early-age. Results indicated that the FE model developed was capable of accurately predicting temperature development of drilled shafts at early-age. A drier surrounding soil (i.e., gravimetric moisture content of 0% through 6%) was able to serve as a better insulating material that leads to reduced temperature difference in the drilled shafts. Also, it was identified the use of high-volumefly ash concrete mix in conjunction with relatively low heat of hydration can reduce the temperature difference in the drilled shaft.