Ahmed Faheem

Effect of Mineral Filler Characteristics on Asphalt Mastic and Mixture Rutting Potential

This study, part of the NCHRP 9–45 Project, analyzed the effect of mineral filler properties on asphalt mastic and the rutting potential of asphaltic mixture. The mineral filler properties were characterized by four tests: Rigden voids (RV), fineness modulus (FM), calcium oxide (CaO) content, and methylene blue value. The rheological properties of asphalt binder and mastic were characterized with the use of apparent viscosity and multiple stress creep recovery tests. Dynamic modulus and flow number tests were conducted to examine the asphaltic mixture rutting potential. The tested mixtures included several variables: four asphalt binder types, including virgin and polymer modified; two aggregate gradations; and a selected group of fillers. The study concluded that asphalt mastic performance was significantly affected by the fractional voids in the filler and possibly by the CaO content and FM. This effect, however, depended on binder type. On the one hand, the styrene–butadiene–styrene modified binder showed the strongest effect as a result of the mineral filler inclusion when tested as mastic. On the other hand, RV and CaO content showed relatively greater correlation with the mixture rutting potential, as compared with other filler properties. Addition of RV improved the prediction models for dynamic modulus and flow number. The effect of RV on the mixture rutting potential was more pronounced for the coarse mixture than for the fine mixture.

Modelling of Asphalt Mastic in Terms of Filler-Bitumen Interaction

ABSTRACT Many studies have focused on modelling the stiffening effect of mineral filler on asphalt binder. However, the interaction between both constituents was always a challenge to address. Current European as well as North American specifications includes limits on stiffening effects of fillers that are mostly empirically driven and hard to include in a mixture design process. This study offers a model that allows estimation of stiffening effects based on primary binder and filler properties. This study builds on a conceptual model provided in an earlier publication for understanding the mechanism by which the filler stiffens the asphalt mastic. The model hypothesizes that change of mastic complex modulus with filler content follows two phases; a diluted phase and a concentrated phase. In the diluted phase, the stiffening effect of the filler on the binder follows a linear filling trend where interaction between filler and binder is minimal. On the other hand the effect of the filler departs from the linear trend once its concentration enters the concentrated phase. The deviation from the linear trend indicates the start of the significant interaction between the filler particles and the asphalt binder. In this study a model is proposed for the effect of mineral filler on asphalt binder complex modulus at given temperature and frequency. The model is defined using three main parameters, (a) Initial Stiffening Rate, (b) Terminal stiffening rate, and (c) Critical filler concentration. The results presented in this study provide measurable filler and binder properties that can be used to estimate these three parameters, which are indicators of the mineral filler interaction with asphalt binder. The model provides an effective tool to determine the role of basic filler and binder physical and chemical properties on the overall complex modulus of the mastic.

Estimating results of a proposed simple performance test for hot-mix asphalt from Superpave gyratory compactor results

This study intended to use the Superpave® gyratory compactor (SGC) as a basis for estimating the stability of asphalt mixtures as a surrogate for proposed method for the simple performance test. Several asphalt mixtures were produced with varying aggregate sources, asphalt contents, and gradations. Every mixture was compacted with the SGC and evaluated with the repeated compression test procedure for rutting measurements recommended by NCHRP Project 9-19 and the AASHTO 2002 pavement design manual to evaluate whether the results from the SGC can be related to the rutting of mixtures. Densification curves produced by the SGC were used to determine the volumetric properties besides the calculation of the traffic densification index (TDI), which represents the densification experienced by traffic loading during pavement service life. The traffic force index (TFI) was also calculated with a special accessory added to the SGC during compaction (the pressure distributor analyzer). The TFI represents the work done by the traffic to densify the mixture. Results from the mixture rutting tests were used to estimate the flow number (FN). The FN, an important mixture property, is shown to have a strong correlation to the TFI. The TFI was also found to be strongly correlated with the TDI and gives an opportunity to estimate the mixture resistance to compaction forces with the use of its volumetric behavior. The main finding of the study is that the SGC appears to give information that can be used to characterize the stability of the mixtures. Such information could be used as an initial screening criterion to select mixtures for various traffic levels.

Measuring Effects of Warm-Mix Additives: Use of Newly Developed Asphalt Binder Lubricity Test for the Dynamic Shear Rheometer

An emphasis on environmental stewardship has prompted the use of warm-mix technologies aimed at allowing for production of conventional asphalt mixtures at reduced temperatures. Successful use of warm-mix asphalt (WMA) in field demonstrations has created a need for development of mix design procedures. A major impediment in development of these procedures is the evaluation of the effect of WMA technologies on asphalt binder and mixture workability. The objective of this study was to introduce a new test methodology for estimating asphalt binder workability by measuring the lubricity effects of a surfactant-based additive as well as binder foaming processes through novel use of the dynamic shear rheometer with a new testing fixture. The new test allows measuring the coefficient of friction of binders at various temperatures, loading rates, and normal force. Asphalt binder lubricity measurements were correlated mixture to workability tests defined by the compactive effort required to densify a mixture to 8% air voids. Mixture testing was conducted at temperatures ranging from 90°C to 135°C. Results of asphalt binder workability testing demonstrated a significant reduction in coefficient of friction due to the use of a surfactant-based WMA additive and identified a need for revised procedures for evaluation of foamed asphalts. Both warm-mix processes demonstrated enhanced mixture workability relative to the hot-mix asphalt; however, significant differences were not realized until compaction temperatures were below those normally used in production.

Influence of Filler Fractional Voids on Mastic and Mixture Performance

This study proposed the fractional voids test for mineral fillers introduced by Rigden in 1947 and currently adopted as the European norm, as part of asphalt mixture design and quality control. Laboratory testing conducted in this study included a large collection of natural and manufactured fillers currently used in various regions of the United States. Results showed that filler fractional voids influenced mastic viscosity and nonrecoverable compliance. These mastic properties were also found to be highly correlated with mixture performance measures. This study used mixture performance limits to derive mastic limits based on mixture-to-mastic correlations. Regression models developed to predict mastic performance as a function of filler fractional voids and asphalt binder properties were proposed as a means for incorporating fractional voids into the mix design procedure as a quality control measure. This paper discusses the tested materials and the analysis approach and summarizes the data justifying the proposed limits.

Development of Predictive Models for Initiation and Propagation of Field Transverse Cracking

The development of field transverse cracking prediction models is highly complicated because of several factors, including the difficulty in differentiating thermal cracking from reflective cracking in the field, the high variability of field conditions, and the potential variability in crack initiation and crack propagation mechanisms. As a result, a statistical-based approach is preferred to a mechanical-based prediction model. In this study, statistical methods, partial least squares regression, and binary logistic regression were used to establish prediction models for field transverse cracking. Results indicated that crack initiation and crack propagation were controlled by predictor variables. Material properties (mixture creep compliance, work density, and percentage passing the No. 200 sieve), pavement structure (overlay thickness), climate (low temperature hour), and traffic (average annual daily truck traffic) were found to be key indicators for transverse crack propagation. Low temperature hour, percentage passing No. 200 sieve, indirect tensile strength, and service life were critical predictor variables for crack initiation. In particular, the crack initiation model, developed by the binary logistic regression, predicted the probability of crack initiation. Both models show good predictability and are well validated. These models appear to work for hot-mix and warm-mix asphalt pavements.

Resilient Modulus of Fine-Grained Soils for Mechanistic–Empirical Pavement Design

The research objectives were to establish a resilient modulus test result database and to develop correlations for estimating the resilient modulus of Wisconsin fine-grained soils from basic soil properties. Laboratory testing was conducted on representative Wisconsin fine-grained soils to evaluate their physical and compaction properties. The resilient modulus of each investigated soil was determined from the repeated load triaxial (RLT) test according to the AASHTO T307 procedure. The laboratory testing program produced a high-quality, consistent database of test results. The resilient modulus constitutive equation of mechanistic–empirical pavement design was selected to estimate the resilient modulus of Wisconsin fine-grained soils. Material parameters of the constitutive equation were evaluated from RLT test results. Then statistical analysis was performed to develop correlations between basic soil properties and constitutive model parameters. The resilient modulus values obtained from RLT tests and estimated from the constitutive equations were in agreement. The correlations developed could estimate the resilient modulus of compacted subgrade soils with reasonable accuracy. The proposed material parameter correlations could be used to estimate the resilient modulus of Wisconsin fine-grained soils as Level 2 input parameters. Statistical analysis of the test results also provided resilient modulus values for the investigated soil types that can be used as Level 3 input parameters.

Characterization of Damage and Aging Resistance of Asphalt Mastics with Coal Combustion By-Products

In this study, two asphalt binders are blended with four particulate types; one natural filler and three Coal Combustion By-Products (CCPs), to make mastics at different concentrations. This initial study aims to evaluate the compatibility of different asphalt binders with these particulates in terms of damage and aging resistance. High and intermediate temperature testing is conducted while varying the aging level. Results show that the particulates, at all concentrations, improve high temperature rutting resistance of mastics. At intermediate temperature, the results show significant dependency on particulate, binder and polymer modification types. In general, results show that CCP mastics with unmodified binders increase damage resistance at high temperature, with no determinate effect on aging related stiffness. For the polymer modified binder, Spray Dryer Absorber (SDA) appears to enhance its properties significantly as noticed by increased damage resistance, recovery, and significant retardation in aging related stiffness.

Influence of Aggregate Base Layer Variability on Pavement Performance

The objective of this research was to investigate the variability of base layer construction in hot-mix asphalt (HMA) pavements and to assess the impact of base layer variability or nonuniformity on pavement performance. Subjective acceptance criteria of base layer construction may lead to variable pavement performance and may result in early pavement distresses and failure. Eleven existing HMA pavement projects with aggregate base course layers constructed in the past few years were selected for falling weight deflectometer (FWD) testing and visual distress surveys. In six of these projects, issues with aggregate base performance (stability and uniformity) had been observed and reported during paving of the HMA surface layer. Later, these pavements exhibited various levels of early distresses, including cracking (longitudinal, transverse, and alligator), aggregate base failure, and pavement surface roughness or irregularities (for ride quality). The remaining five HMA pavement projects, for which no issues related to aggregate base layer behavior were reported during construction, performed well after construction. A test section of approximately 1 mi on each project was subjected to FWD testing. The existing HMA pavements that showed early distresses exhibited high levels of spatial variability and nonuniformity in aggregate base course layers, as demonstrated by FWD testing and backcalculated base layer modulus values and distributions. The existing HMA pavements that performed well exhibited low levels of spatial variability and good uniformity in aggregate base course layers, as shown by the FWD test results and the backcalculated base layer modulus values and distributions.

Evaluation of Compacted Aggregate Base Course Layers

Field and laboratory tests were conducted on 10 projects during base course layer construction to evaluate the quality of the constructed base layers. Base aggregates were also collected from these sites for laboratory testing. The field testing program consisted of the in place density by the sand cone method, the dynamic cone penetration (DCP) test, the light weight deflectometer (LWD) test, and the GeoGauge test. Laboratory tests conducted are the particle size analysis, the standard compaction test (AASHTO T 99), and the repeated load triaxial test (AASHTO T 307) for determining the resilient modulus. Analyses were conducted on field and laboratory test results. High spatial variability in field density and moisture content exists in base course layers under construction, as demonstrated by the relative compaction test results. High variability exists along the depth of base course layers, as demonstrated by the dynamic cone penetrometer test results and the estimated profile of California Bearing Ratio (CBR) along the depth of the investigated base layers. Spatial variability and non-uniformity were also demonstrated by the results of the light weight deflectometer and GeoGauge, in which the layer modulus varies within a large range of values.