Jack Youtcheff

Three-dimensional image processing methods to identify and characterise aggregates in compacted asphalt mixtures

X-ray computed tomography (CT) is a novel tool to quantify the aggregate characteristics in asphalt pavements. This tool can potentially be used in QA, acceptance, design and forensic applications in pavement engineering. However, there have been challenges associated with the processing of the 3D X-ray CT images, including: (1) segmentation of aggregates that are in close proximity and (2) processing noisy or poor contrast images. This paper describes image processing methods to overcome these challenges and describes methods for computation of size, location, contact points and orientation of the aggregates in HMA. Validations of the algorithms as well as example computations of contact points and orientation have been presented. A significant increase in the number of contact points with increasing compaction level and preferred orientation perpendicular to the direction of compaction in the gyratory compactor were some of the findings presented in this paper.

Use of small samples to predict fatigue lives of field cores: Newly developed formulation based on viscoelastic continuum damage theory

Fatigue cracking is one of the major distresses in asphalt pavements. Accurate prediction of fatigue life of asphalt pavements can be extremely important both during the design stage and for prediction of remaining service life of in-service pavements. Traditional fatigue life predictions based on bending beam tests can be costly and time-consuming. The uniaxial push–pull (tension–compression) tests run on cylindrical samples have been a novel alternative. However, the traditional sample size for the push–pull tests may prevent its use for thin in-service pavements. This paper presents the results of a study investigating the possibility of using smaller sample sizes for push–pull tests. The viscoelastic continuum damage (VECD) characteristics of regular and small-size samples are compared, and the difference is observed to be negligible. In addition, a practical fatigue life formulation is derived on the basis of VECD theory. Uniqueness of the derived fatigue life (Nf) equation, differing from previously derived VECD-based Nf equations, stems from the fact that it does not force a certain form of equation to fit the damage characteristic curve. Finally, the differences in fatigue lives of different layers of the field sections at FHWAs accelerated loading facility are investigated.

Low-Temperature Physical Hardening of Hot-Mix Asphalt

A study was conducted to determine if low-temperature physical hardening, which has been reported to be exhibited by asphalt binders, also affects hot-mix asphalt. Two asphalt binders, Designation AAM-1 and AAM-2 of the Strategic Highway Research Program, which are known to show the effects of physical hardening, were used to prepare asphalt mixtures with different mineral fillers. The asphalt mixtures were compacted into slabs from which cores were obtained and tested at low temperatures in the thermal stress restrained specimen test. Before testing, the cores were cooled unrestrained to –15°C and held isothermally for 1 h or 24 h. After conditioning, the specimens were restrained and held at a constant length while the temperature was dropped at a rate of 15°C per hour until the specimens fractured. The temperature at which the specimens fractured, the stress at the time of fracture, and the slope of the temperature-stress curve were measured and analyzed using several statistical techniques. The results showed that the mixture made with asphalt binder AAM-2 was affected by conditioning time. The mixture made with asphalt binder AAM-1 was not affected by conditioning time. Other factors, such as mineral fillers and air voids of the mixtures, had a greater influence on the results than did physical hardening.

Characterization of Asphalt Mixture Homogeneity Based on X-ray Computed Tomography

The mechanical performance and life span of asphalt pavement depend largely on the homogeneity of the asphalt mixture. Inhomogeneity of asphalt mixtures is due to the non-uniform segregation of air voids and aggregates in mixtures. In order to evaluate the homogeneity of an asphalt mixture during lab compaction, computed tomography (CT) images of the specimen were obtained based on x-ray CT. The image was virtually cut into slices of various sizes, and the vertical air void distributions and aggregate gradations were quantified. Research results reveal that the air void content in the middle part of the specimen was lower than those on both ends. Also, the homogeneity was better under high compaction levels. The results from aggregate gradations were in good agreement with air void distribution calculations. Therefore, it is feasible to characterize an asphalt mixture’s homogeneity based on the air void distributions and aggregate gradations as determined via x-ray CT.

Performance of Modified Asphalt Binders with Identical High-Temperature Performance Grades but Varied Polymer Chemistries

The rutting resistances of mixtures that contain polymer-modified asphalt binders with identical performance grades but varied polymer chemistries were evaluated. Eleven asphalt binders were obtained for this study: two unmodified asphalt binders, an air-blown asphalt binder, and eight polymer-modified asphalt binders. Five binders used in a prior study also were included. Asphalt binder properties were measured with a dynamic shear rheometer. Mixture rutting resistance was measured by (a) the shear modulus, G*, and the Superpave® binder rutting factor, G*/sinδ; (b) cumulative permanent shear strain (CPSS); (c) the rut depths from the French pavement rutting tester (French PRT); and (d) the creep slopes from the Hamburg wheel-tracking device. CPSS and rut depths from the French PRT were the primary mixture tests because they were specifically developed to measure rutting resistance under dry conditions. The high-temperature properties of the 11 asphalt binders had a high correlation to mixture rutting resistance as measured by the CPSSs. A weak correlation was found using the French PRT. Both correlations were high when the data from all 16 asphalt binders and mixtures were analyzed. The number of discrepancies between the high-temperature properties of the asphalt binders and mixture rutting resistance was low. A change in high-temperature performance grade from 70 to 76 significantly increased rutting resistance based on both mixture tests.

New Possibilities and Future Pathways of Nanoporous Thin Film Technology to Improve Concrete Performance

Aggregates are often considered as inexpensive inert filler material in concrete. However, the mixture of the aggregate with the cement paste creates one of the most vulnerable areas of concrete, the interface of aggregate and cement paste. The judicious application of nanoporous thin films (NPTFs) on the aggregates surface is an effective way to improve those interfaces. The most recent work on concrete shows that the use of different types of NPTF can induce changes in different properties of concrete or in an aggregates mineralogy. In particular, the observed changes in mechanical properties such as compressive, flexural, and tensile strengths; modulus of elasticity; and drying shrinkage can ameliorate longitudinal and transverse cracking, corner breaks, punchouts, and D-cracking. Several mechanisms are proposed to explain the changes observed in concrete and the implications for its ultimate performance. These new results open doors for new applications of NPTFs and indicate possible future research in this field.

EFFECT OF LIME ON IGNITION FURNACE CALIBRATION

This study investigated mix components, other than aggregate source, that were perceived to affect the ignition furnace mixture calibration. Four sets of experiments with one aggregate were designed and run to evaluate the effects of the amounts of lime, sulfur, calcium carboxylates, and fines. Five asphalt binders with different chemistries were evaluated. Two binders had high and low sulfur contents, respectively; two other binders contained relatively high concentrations of carboxylates. Various concentrations of hydrated lime (0 to 3 percent by weight) were added to mixtures and were found to have a significant effect on the ignition furnace correction factor for all five binders. The magnitude of the effect was large enough to cause the quality control tests to fail the tolerances established by the Virginia Department of Transportation. The lime appears to react with the sulfur dioxide (SO2) formed from the combustion of organic sulfur to generate calcium sulfate. The amount of sulfur present in the asphalt can significantly affect the ignition furnace correction factor. Initial thoughts that calcium carboxylates are reacting with carbon dioxide (CO2) to produce some form of calcium carbonate were not substantiated. The influence of carboxylic acid groups, however, does come into effect in the presence of lime. This suggests that lime reacts with the combustion gases to produce some form of carbonate. Basalt fines and portland cement did not have a significant effect on the ignition furnace correction factor. Additional testing should be conducted to assess the effects of fines representing other aggregates, particularly carbonates.

Forensic investigation of the cause(s) of slippery ultra-thin bonded wearing course of an asphalt pavement: influence of aggregate mineralogical compositions

The cause(s) of slippery ultra-thin bonded wearing course (UTBWC) of an asphalt pavement was investigated. Petrographic analysis showed that the aggregate used in the UTBWC is mainly limestone with an average acid-insoluble residue of 5.1%. Coefficient of friction tests were performed both on a comparative UTBWC from Virginia with a different aggregate mineralogy VA-UTBWC and on slabs extracted from the slippery UTBWC pavement overlay. The tests clearly showed that the slippery UTBWC overlay sharply declined throughout the polishing process, consistent with the aggregate mineralogical composition and its low amount of acid-insoluble residue. In contrast, the comparative VA-UTBWC mix showed a gradual increase and then decrease in friction with continued polishing. This investigation clearly showed that the cause of the slippery asphalt pavement problem of the road mainly attributed to limestone aggregate polishing.

Use of artificial neural networks to detect aggregates in poor-quality X-ray CT images of asphalt concrete

Different laboratory compactors and protocols are employed to simulate field compaction using a reduced representative asphalt mixture specimen. Studies show that the mixture density, air voids, and mechanical properties vary within the results of each compaction method and between different compaction protocols, which may yield to estimates that mislead the design and performance prediction of the asphalt pavement. X-ray Computed Tomography (X-ray CT), a non-destructive technique for generating three-dimensional (3D) imaging of the internal structure of opaque materials, has commonly been used to quantify the air void distribution of asphalt mixtures. However, aggregate location, orientation and aggregate-to-aggregate contact points have rarely been successfully quantified using the same technique mainly because of image noise and poor contrast between the coarser aggregates and the other phases of the asphalt mixture (i.e. the air voids and the blend of the finer fraction aggregates and the asphalt binder). To overcome these shortcomings, an advanced tool has been developed utilizing 3D X-Ray CT image processing and artificial neural networks (ANN) to perform image segmentation and identify the coarser aggregates even in poor contrast X-ray CT images. This paper presents the details of the ANN tool and its application in determining the approximate size and location of the coarse aggregates in asphalt specimens.

Practical Method to Determine the Effect of Air Voids on the Dynamic Modulus of Asphalt Mixture

This study presents a practical method for estimating the effect of air voids on the dynamic modulus of asphalt mixture. Dynamic modulus was predicted for mixes with a large range of air void contents using the construction mix volumetric and binder rheological data from 10 accelerated loading facility (ALF) lanes, following the Witczak and Hirsch methods. A large variety of plant-produced and laboratory-prepared mixtures, including hot- and warm-mix asphalt (HMA and WMA), reclaimed asphalt pavement, and recycled asphalt shingles, was tested for dynamic modulus at different air void contents. The experimentally measured and normalized correction factors were found to be more dependent on test temperature than the frequency. The predicted correction factors were found to match with the experimental data at lower temperature but to be clearly lower at high temperature. A set of correction factors for each test temperature is recommended to practitioners correcting dynamic modulus with variation in air voids in asphalt pavement.