Gilbert Y. Baladi

ENGINEERING PROPERTIES OF POLYMER-MODIFIED ASPHALT MIXTURES

A large research program sponsored by the Michigan Department of Transportation was designed and completed to evaluate the effect of polymer modification on the various properties of asphalt mixtures. These include the micro- and macrostructural, morphological, chemical, and engineering properties. Some of the engineering properties of the styrene-butadiene-styrene and styrene-etylene-butylene-styrene polymer-modified asphalt mixtures are presented and discussed. The elastic, fatigue, tensile, and permanent deformation properties were investigated at 60, 25, and –5°C. It was found that, for some polymer systems, the fatigue life and the indirect tensile strength increased considerably at 25°C while the elastic properties at -5°C were not affected by the addition of polymer. The implication of this is that the use of some polymer systems in asphalt mixtures enhances their fatigue cracking and rutting resistance without affecting the low temperature cracking potential.

MECHANISTIC ANALYSIS OF TOP-DOWN CRACKS IN ASPHALT PAVEMENTS

Top-down cracks (TDCs) in flexible and rubblized pavements constitute a distress that has been reported in the United States and other countries. Researchers have reached different conclusions regarding the causes of TDCs. In this study, field and laboratory investigations were conducted on conventional flexible and rubblized pavements exhibiting TDCs. The engineering characteristics of the pavement layers were obtained from the analysis of field data and laboratory test results. Detailed mechanistic analyses were then conducted using these characteristics to determine the potential for TDCs. The results of such analyses were compared with field data and are presented and discussed in this paper. The results and field data show that (a) surface radial tensile stress induced by wheel loads and enhanced by differential stiffness due to construction, temperature, and aging can cause TDCs, (b) aging of the asphalt binder decreases the tensile strength and the tensile strain at failure of the asphalt mix, and (c) the locations of the maximum surface tensile stress predicted by the mechanistic analysis correspond very well to the locations of the TDCs observed in the field.

Fatigue and Permanent Deformation Models for Polymer-Modified Asphalt Mixtures

For the past 2 decades, significant research has been conducted on polymer-modified asphalt (PMA) mixtures. Polymers can successfully improve the performance of asphalt pavements at low, intermediate, and high temperatures by increasing mixture resistance to fatigue cracking, thermal cracking, and permanent deformation. Most of the research has been concentrated on the characterization and relative comparison of neat and PMA mixtures, and little work has been done toward the development of fatigue and permanent deformation models for PMA mixtures. A 3-year study that was sponsored by the Michigan Department of Transportation was conducted at Michigan State University to characterize PMA mixtures. It was found that the rheological and engineering properties of PMA mixtures largely depend on the polymer type and content. The improvements in the fatigue lives and resistance to permanent deformation are mainly due to the improvements in the rheological properties of the binders. Fatigue life and permanent deformation models were developed. These models show that the laboratory fatigue life and permanent deformation are strongly related to the rheological properties of binders and the engineering properties of PMA mixtures.

Backcalculated and Laboratory-Measured Resilient Modulus Values

The resilient modulus (MR) of roadbed soils is a necessary parameter in pavement design. The MR of roadbed soils is dependent on the soil type, water content, dry density, particle gradation, Atterberg limits, and stress states. Several procedures can be used for the determination of the MR: laboratory testing, backcalculation with nondestructive deflection testing (NDT) data, and correlations to other soil parameters such as the California bearing ratio, density, and water content. The first procedure is time-consuming and expensive and requires substantial resources to cover the various roadbed soils under the road network. The second is relatively inexpensive and fast and can be designed to cover representative soils under the pavement network. The third procedure is approximate and may be used for the Level 2 design of the Mechanistic–Empirical Pavement Design Guide. The Michigan Department of Transportation (MDOT) sponsored a research project for the development of a systematic procedure to determine the MR values for all soil types encountered under the pavement network in the state of Michigan. The procedure includes laboratory testing of representative disturbed and undisturbed soil samples, NDT with the MDOT falling weight deflectometer, and developing correlations between the MR values obtained from the two tests and between the MR values and other soil parameters that can be obtained using simple tests. The developed correlation equations between the MR measured values and the other soil parameters are presented elsewhere, whereas the correlations between the laboratory-obtained and the backcalculated MR values are discussed in this paper.

Review of Louisiana's Pavement Management System: Phase I

“Louisiana–Vision 2020” serves as a benchmark for improving highway pavements over a 20-year period. Cost-effective pavement preservation is emphasized in current Louisiana state law and federal law. The Louisiana Department of Transportation and Development (LADOTD) and the FHWA strategic plans also emphasize cost-effective pavement preservation. In an effort to improve the pavement management system (PMS), the Louisiana Transportation Research Center initiated a two-phase research study to evaluate the overall performance and effectiveness of the system. This paper focuses on the Phase I study and addresses the state-of-the-art practice of LADOTDs PMS and the results of a departmental survey to assess the needs of the various districts. The use of various location reference systems by different units in the department makes linking to different database sets difficult. Although many of the districts’ engineers have no concerns about all the referencing systems, most would prefer to have a unified reference location system. All the districts have access to the PMS data and most of them use it; however, the degree of use varies from one district to another. This paper also discusses the types of PMS output and reports, the degree to which the outputs are analyzed, the accuracy of the information currently available, and the degree to which the current PMS data track and differentiate between different preservation actions.

COST-EFFECTIVE PREVENTIVE MAINTENANCE: CASE STUDIES

With most of the highway systems in place, emphasis has shifted from design and construction to preservation and expansion. Unfortunately, the engineering skills, knowledge, and experience required to preserve the systems are significantly different than those required to originally design and build the systems. The experience gained in the initial phase, although important, is not in itself sufficient to preserve the systems. Pavement preservation is defined as the sum of all activities undertaken to provide and maintain serviceable roadways. This includes corrective maintenance and preventive maintenance, as well as minor rehabilitation projects. A cost-effective pavement preservation program requires a systematic and comprehensive engineering management of, and a solution to, pavement network problems. Preventive maintenance (PM) is defined by AASHTO as the “planned strategy of cost-effective treatments to an existing roadway system and its appurtenances that preserves the system, retards future deterioration, and maintains or improves the functional condition of the system (without increasing the structural capacity).” Hence, PM actions must be taken on pavements in relatively good condition. The development and implementation of a cost-effective PM program faces several obstacles: political debate, budget constraints, lack of education, and the existing practice of “worst pavements are first.” Some state highway agencies have overcome these obstacles, and they are harvesting the success of their PM programs. The state of the practice of three agencies—those of Arizona, Montana, and Pennsylvania—is presented and discussed.

USE OF DISTRESS AND RIDE QUALITY DATA TO DETERMINE ROUGHNESS THRESHOLDS FOR SMOOTHING PAVEMENTS AS A PREVENTIVE MAINTENANCE ACTION

In this discussion, 462 pavement sections from 37 projects in Michigan were analyzed to investigate the interaction between pavement surface roughness and distress. The main hypothesis of this research is that an increase in roughness leads to higher dynamic axle loads, which in turn can lead to a tangible acceleration in pavement distress. If this relationship is established, then it will be possible to plan a preventive maintenance (PM) action to smooth the pavement surface. Such a PM action is bound to extend the service life of the pavement by several years. The objective of the research presented was to develop such roughness thresholds. The selected projects include 13 rigid, 15 flexible, and 9 composite pavements. The ride quality index (RQI) and distress index were used as measures of surface roughness and distress, respectively. To get a good relationship between distress caused by dynamic loading and surface roughness, dynamic-load-related distress types were extracted. The analysis showed very good relationships between distress and roughness for rigid and composite pavements (R2 = 0.739 and 0.624). However, for flexible pavements there was significant scatter (R2 = 0.375), reflecting the higher variability in flexible pavement distresses. A logistic function was used to fit the data, and roughness thresholds were determined as the RQI-values corresponding to peak acceleration in distress. These were determined to be 64 [international roughness index (IRI) = 1.99 m/km or 124 in./mi] for rigid pavements and 51 (IRI = 1.43 m/km or 89 in./mi) for composite pavements.

RUBBLIZATION OF CONCRETE PAVEMENTS: FIELD INVESTIGATION

Substantial resources are required to preserve aging highway systems. Various alternatives have been used for the rehabilitation of concrete pavements, including bonded and unbonded overlays, full-depth repair, crack and seat and asphalt overlay, joint and crack repairs, asphalt overlay, and rubblization with asphalt surface overlay. Presented are the results of extensive field investigation of rubblized concrete pavements. Various trenches were made in the rubblized pavements before the asphalt surface was placed. Some trenches were made at midslab, and others were made at old joints and cracks. In some trenches, permeability tests were conducted. Advantages and shortcomings of the rubblization procedures are presented and discussed. It is shown that well-executed rubblization procedures lead to durable pavements and that, for certain concrete pavements, rubblization is not a viable option because it may lead to inadequate pavement performance.

DETECTING SEGREGATION IN BITUMINOUS PAVEMENTS

Field and laboratory studies were conducted to determine the feasibility of detecting and quantifying segregation in hot-mix asphalt pavements. The study involved extensive testing at 19 field sites in Michigan. Areas of apparent segregation were classified as light, medium, and heavy. At the selected sampling areas, 1-min nuclear density measurements were made, cores were taken and incinerated, asphalt contents and air voids were determined, and gradation tests were performed. Statistical comparisons were made by using t-tests to characterize the differences between the measured nuclear density and percent passing various sieves. More than 80% of the nuclear density measurements supported the visual observations of a heavy degree of segregation when the significance of the differences (the p-value) between the areas compared was less than 10-3. Moreover, for areas with medium or heavy segregation, the probability that significant differences in nuclear density between segregated and nonsegregated areas with p-values less than 10-3 corresponded to significant differences (p-values less than 10-2) in aggregate gradation was .88. On the basis of the results, a spreadsheet, mbitseg2, was developed to perform statistical tests with the nuclear density measurements obtained during the paving operation. After 6 months of trial testing, the program has been accepted and implemented by the Michigan Department of Transportation, the Michigan Asphalt Paving Association, and the asphalt industry. The program is available on a web page (http://www.egr.msu.edu/prce) in Excel (mbitseg2.xls) and QuattroPro (mbitseg2.wb3) formats.

Characterizing Temperature Susceptibility of Asphalt Binders Using Activation Energy for Flow

The temperature-viscosity relationships for twenty two neat and modified asphalt binders were evaluated using conventional tests (CT) such as penetration, kinematic and absolute viscosities, and ring and ball softening point; rotational viscometer (RV); and dynamic shear rheometer (DSR). The temperature ranged from 25 to 135 °C, 100 to 185 °C, and 7 to 82 °C for conventional, RV, and DSR tests, respectively. Results from several binders studies have revealed that binders having similar penetration, viscosity and performance grade (PG) could show dissimilar rheological behaviors. Hence, there is a need to fully characterize asphalt binders to capture their rheology over a wide range of temperatures and loading frequencies. The activation energy (AE) concept was utilized in this study to characterize the temperature susceptibility of the asphalt binders. Correlations between AE and binder rheological properties (e.g., G*/sinδ and G* sinδ) were evaluated. In addition, a relationship between AE and useful temperature range (UTR) determined from the performance grades (PG) was developed. The advantages of characterizing the relative temperature susceptibility of the binders using AE are discussed. Finally, discussion on mixing and compaction temperatures, especially for polymer modified binders, by using AE is also included in this paper.