Aggregate Degradation Research Articles

Degradation Behavior and Mechanism of HMA Aggregate

Abstract The degradation behavior and mechanism of aggregate used for hot mix asphalt (HMA) mixtures has been investigated in this study. Laboratory tests, including the crushing test, Marshall compactor test, and Superpave gyratory compactor test, were used to evaluate the degradation behavior of single-size aggregate and/or graded aggregates. In order to verify the laboratory test results, the degradation of field HMA materials extracted from asphalt pavements after long-time service was also evaluated. The stress-transfer model for an aggregate skeleton was analyzed in order to provide a fundamental understanding of the stress distribution and degradation mechanism of the aggregate system. The results indicate that the degradation of the aggregate gradation is not random and has a fixed internal trend. The 4.75 mm aggregate proved to be the demarcation point between coarse and fine aggregate. During the degradation process, coarse aggregate tends to break into graded aggregate to form a well-balanced skeleton to bear an external load, and the 4.75 mm aggregate plays a key role in the graded aggregate. The crushing test and Superpave gyratory compactor test proved to be easily done and promising for capturing the degradation of aggregate and the gradation of HMA. The variation in the retained percentage of 4.75 mm aggregate and the breakdown ratio of the original aggregate and/or gradation can quantitatively depict the degradation of HMA aggregate.

On the Road to Sustainability

Most state departments of transportation (DOTs) adopted the Superpave® mix design method in the late 1990s. The Superpave mix design method replaced the Marshall method and became mainstream. Superpave came with aggregate, binder, and mix requirements to control rutting and cracking. With a life expectancy of about 15 years, many Superpave roads are now becoming candidates for resurfacing or recycling. Can Superpave mixes undergo hot in-place recycling with success? In fact, Superpave mixes contain expensive and high-quality aggregates, which meet the consensus properties, and high-quality binder, which meets the performance grade (PG) requirements. If these mixes can be recycled in place and meet the original Superpave properties, then substantial savings can be realized. The potential savings are in the 30% to 50% range of the cost to mill and resurface. In addition, emissions could be reduced by 70%, while 100% of the existing material could be reused. This paper reports on an experiment conducted in Gainesville, Florida, in 2010 at the Florida DOT State Materials Office, in which a Superpave mix underwent the hot in-place recycling process. The properties of the aggregate, binder, and mix were compared to answer certain questions about aggregate degradation, binder rejuvenation, and mix properties. The answers to these questions shed light on the possibility of the 100% recycle of Superpave mixes. The results showed that minimal aggregate degradation took place. The recycled mixes met Superpave gradation requirements for the most part. The rejuvenated binder met or exceeded the PG requirement. The mix could be engineered to meet the air voids requirement of Superpave.

Induction of autophagy with catalytic mTOR inhibitors reduces huntingtin aggregates in a neuronal cell model

Huntington's disease is a progressive neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in the huntingtin gene. This expansion produces a mutant form of the huntingtin protein, which contains an elongated polyglutamine stretch at its amino-terminus. Mutant huntingtin may adopt an aberrant, aggregation-prone conformation predicted to start the pathogenic process leading to neuronal dysfunction and cell death. Thus, strategies reducing mutant huntingtin may lead to disease-modifying therapies. We investigated the mechanisms and molecular targets regulating huntingtin degradation in a neuronal cell model. We first found that mutant and wild-type huntingtin displayed strikingly diverse turn-over kinetics and sensitivity to proteasome inhibition. Then, we show that autophagy induction led to accelerate degradation of mutant huntingtin aggregates. In our neuronal cell model, allosteric inhibition of mTORC1 by everolimus, a rapamycin analogue, did not induce autophagy or affect aggregate degradation. In contrast, this occurred in the presence of catalytic inhibitors of both mTOR complexes mTORC1 and mTORC2. Our data demonstrate the existence of an mTOR-dependent but everolimus-independent mechanism regulating autophagy and huntingtin-aggregate degradation in cells of neuronal origin.

FAT10 Protein Binds to Polyglutamine Proteins and Modulates Their Solubility

Expansion of polyglutamine (pQ) chain by expanded CAG repeat causes dominantly inherited neurodegeneration such as Huntington disease, dentatorubral-pallidoluysian atrophy (DRPLA), and numbers of other spinocerebellar ataxias. Expanded pQ disrupts the stability of the pQ-harboring protein and increases its susceptibility to aggregation. Aggregated pQ protein is recognized by the ubiquitin proteasome system, and the enzyme ubiquitin ligase covalently attaches ubiquitin, which serves as a degradation signal by the proteasome. However, accumulation of the aggregated proteins in the diseased brain suggests insufficient degradation machinery. Ubiquitin has several functionally related proteins that are similarly attached to target proteins through its C terminus glycine residue. They are called ubiquitin-like molecules, and some of them are similarly related to the protein degradation pathway. One of the ubiquitin-like molecules, FAT10, is known to accelerate protein degradation through a ubiquitin-independent manner, but its role in pQ aggregate degradation is completely unknown. Thus we investigated its role in a Huntington disease cellular model and found that FAT10 molecules were covalently attached to huntingtin through their C terminus glycine. FAT10 binds preferably to huntingtin with a short pQ chain and completely aggregated huntingtin was FAT10-negative. In addition, ataxin-1,3 and DRPLA proteins were both positive for FAT10, and aggregation enhancement was observed upon FAT10 knockdown. These findings were similar to those for huntingtin. Our new finding will provide a new role for FAT10 in the pathogenesis of polyglutamine diseases.

The WD40 Repeat PtdIns(3)P-Binding Protein EPG-6 Regulates Progression of Omegasomes to Autophagosomes

PtdIns(3)P plays critical roles in the autophagy pathway. However, little is known about how PtdIns(3)P effectors act with autophagy proteins in autophagosome formation. Here we identified an essential autophagy gene in C.elegans, epg-6, which encodes a WD40 repeat-containing protein with PtdIns(3)P-binding activity. EPG-6 directly interacts with ATG-2. epg-6 and atg-2 regulate progression of omegasomes to autophagosomes, and their loss of functioncauses accumulation of enlarged early autophagic structures. Another WD40 repeat PtdIns(3)P effector, ATG-18, plays a distinct role in autophagosome formation. We also established the hierarchical relationship of autophagy genes in degradation ofprotein aggregates and revealed that the UNC-51/Atg1 complex, EPG-8/Atg14, and binding of lipidated LGG-1 to protein aggregates are required foromegasome formation. Our study demonstrates that autophagic PtdIns(3)P effectors play distinct roles in autophagosome formation and also providesa framework for understanding the concerted action of autophagy genes in protein aggregate degradation.

Mechanical Degradation of Aggregate by the Los Angeles-, the Micro-Deval- and the Nordic Test Methods

The quality of aggregate used as buildings materials is defined by European Standard test methods. According to the agreement within the European Economic Area, each individual country decides test methods of current interest. Among the Nordic countries, the Los Angeles-, micro-Deval- and the Nordic test are the most common methods used to decide the mechanical properties of the aggregate. The three test methods are all drum test where the degradation of the material occur by rotation between the test material and steel balls together with, or without water. The mechanical test methods are empirical and are believed to express either resistance to fragmentation or wearing. The results of this study show that analysing the particle size distribution of a material after the drum testing give indication of which type and degree of degradation the test material is exposed to. Knowledge of the type of degradation for the test methods is important compared to the understanding of the real breakdown of the aggregate for instance used in road construction.

Influence of particle breakage on the resilient modulus of railway ballast

Numerous research studies have been carried out in the recent past to determine the influence on the resilient modulus of railway ballast of parameters such as stress history, number of loading cycles, density, grading, fines content, maximum grain size, aggregate type, particle shape and moisture content. However, the influence of particle breakage on the resilient modulus has not been described in depth. The current study highlights the influence of particle breakage on the resilient modulus of latite ballast (commonly used in railway track in New South Wales, Australia). Aggregate degradation is represented by the ballast breakage index (BBI). Laboratory test results indicate that the BBI has a profound influence on the resilient modulus of ballast. The resilient modulus increases with BBI, irrespective of the effective confining pressure. In addition, a simple hyperbolic relationship has been proposed between the resilient modulus MR and the bulk stress φ for latite ballast.

Quantitation of selective autophagic protein aggregate degradation in vitro and in vivo using luciferase reporters

The analysis of autophagy in cells and tissue has principally been performed via qualitative measures. These assays identify autophagosomes or measure the conversion of LC3I to LC3II. However, qualitative assays fail to quantitate the degradation of an autophagic substrate and therefore only indirectly measure an intact autophagic system. “Autophagic flux” can be measured using long-lived proteins that are degraded via autophagy. We developed a quantifiable luciferase reporter assay that measures the degradation of a long-lived polyglutamine protein aggregate, polyQ80-luciferase. Using this reporter, the induction of autophagy via starvation or rapamycin in cells preferentially decreases polyQ80-luciferase when compared with a nonaggregating polyQ19-luciferase after four hours of treatment. This response was both time- and concentration-dependent, prevented by autophagy inhibitors and absent in ATG5 knockout cells. We adapted this assay to living animals by electroporating polyQ19-luciferase and polyQ80-luciferase expression constructs into the right and left tibialis anterior (TA) muscles of mice, respectively. The change in the ratio of polyQ80-luciferase to polyQ19-luciferase signal before and after autophagic stimulation or inhibition was quantified via in vivo bioluminescent imaging. Following two days of starvation or treatment with intraperitoneal rapamycin, there was a ~35% reduction in the ratio of polyQ80:polyQ19-luciferase activity, consistent with the selective autophagic degradation of polyQ80 protein. This autophagic response in skeletal muscle in vivo was abrogated by co-treatment with chloroquine and in ATG16L1 hypomorphic mice. Our study demonstrates a method to quantify the autophagic flux of an expanded polyglutamine via luciferase reporters in vitro and in vivo.

Intranuclear Degradation of Polyglutamine Aggregates by the Ubiquitin-Proteasome System

Huntington disease and its related autosomal-dominant polyglutamine (pQ) neurodegenerative diseases are characterized by intraneuronal accumulation of protein aggregates. Studies on protein aggregates have revealed the importance of the ubiquitin-proteasome system as the front line of protein quality control (PQC) machinery against aberrant proteins. Recently, we have shown that the autophagy-lysosomal system is also involved in cytoplasmic aggregate degradation, but the nucleus lacked this activity. Consequently, the nucleus relies entirely on the ubiquitin-proteasome system for PQC. According to previous studies, nuclear aggregates possess a higher cellular toxicity than do their cytoplasmic counterparts, however degradation kinetics of nuclear aggregates have been poorly understood. Here we show that nuclear ubiquitin ligases San1p and UHRF-2 each enhance nuclear pQ aggregate degradation and rescued pQ-induced cytotoxicity in cultured cells and primary neurons. Moreover, UHRF-2 is associated with nuclear inclusion bodies in vitro and in vivo. Our data suggest that UHRF-2 is an essential molecule for nuclear pQ degradation as a component of nuclear PQC machinery in mammalian cells.

Effects of rising temperature on the formation and microbial degradation of marine diatom aggregates

Effects of elevated temperature on the formation and subsequent degradation of diatom aggregates were studied in a laboratory experiment with a natural plankton community from the Kiel Fjord (Baltic Sea). Aggregates were derived from diatom blooms that developed in indoor mesocosms at 2.5 and 8.5 degrees C, corresponding to the 1993 to 2002 mean winter in situ temperature of the Western Baltic Sea and the projected sea surface temperature during winter in 2100, respectively. Formation and degradation of diatom aggregates at these 2 temperatures in the dark were promoted with roller tanks over a period of 11 d. Comparison of the 2 temperature settings revealed an enhanced aggregation potential of diatom cells at elevated temperature, which was likely induced by an increased concentration of transparent exopolymer particles (TEP). The enhanced aggregation potential led to a significantly higher proportion of particulate organic matter in aggregates at 8.5 degrees C. Moreover, the elevated temperature favoured the growth of bacteria, bacterial biomass production, and the activities of sugar- and protein-degrading extracellular enzymes in aggregates. Stimulating effects of rising temperature on growth and metabolism of the bacterial community resulted in an earlier onset of aggregate degradation and silica dissolution. Remineralization of carbon in aggregates at elevated temperature was partially compensated by the formation of carbon-rich TEP during dark incubation. Hence, our results suggest that increasing temperature will affect both formation and degradation of diatom aggregates. We conclude that the vertical export of organic matter through aggregates may change in the future, depending on the magnitude and vertical depth penetration of warming in the ocean.

Exact solution for stochastic degradation and fragmentation processes in arbitrary chain and ring aggregates with multiple bonds

This paper presents a statistical theory of stochastic evaporation and degradation processes in complex polymerlike ring and chain aggregates with multiple degrading bonds between the primary particles (monomers). The exact kinetic solution fully describing fragmentation processes is obtained for such aggregates with arbitrary number of primary particles (monomers) and bonds between them. The effects of additional interaction of multiple bonds with each other is shown to have a drastic impact on the predicted kinetic processes and time-dependent particle size distributions during aggregate degradation. Structural effects associated with different distributions of multiple bonds and bonding configurations in the aggregates are also investigated and shown to have a significant impact on typical fragmentation time and accumulation of fragmenting aggregates in intermediate modes. The developed theory and its results will be important for degradation of multistranded polymers, polymer networks, self-assembling structures, surface nanoclusters and nanotechnology, and formation and evolution of aerosol aggregates resulting from transport and industry emissions.

Thermochemical degradation of limestone aggregate concrete on exposure to sodium fire

Limestone aggregate concrete blocks were subjected to sodium fire conforming to a realistic scenario in order to qualify them as protective sacrificial layers over structural concrete flooring in liquid metal-cooled fast breeder reactors. Mid infrared absorption measurements were carried out on these sodium fire-exposed samples as a function of depth from the affected surface. Definite signatures of thermochemical degradation indicating dehydration and structural modification of the limestone concrete have been obtained. Control runs were carried out to delineate the thermal effects of sodium fires from that of the chemical interaction effects. Measurements on limestone aggregate samples treated with fused NaOH provided direct evidence of the exact mechanism of the sodium attack on concrete. The observed degradation effects were correlated to the mechanical strength of the concrete blocks and to the intensity of the sodium fire experienced.

Mechanism of proteoglycan aggregate degradation in cartilage stimulated with oncostatin M

To investigate the potential synergistic and differential effects of cytokine combinations on proteoglycan aggregate catabolism in cartilage. Bovine articular cartilage explants were maintained in organ culture and subjected to stimulation with cytokine combinations including interleukin-1alpha (IL-1alpha), IL-1beta, IL-6, IL-17, tumor necrosis factor-alpha (TNFalpha) and oncostatin M (OSM). Aggrecan, link protein and hyaluronan (HA) release and degradation were analyzed, and the effect of the hyaluronidase inhibitor apigenin was investigated. For all cytokine mixtures studied cleavage of aggrecan only by aggrecanase action was apparent. However, OSM acting synergistically with IL-1 or TNFalpha produced a rapid release of all proteoglycan aggregate components due to both aggrecan and HA degradation. This was abolished by the hyaluronidase inhibitor, apigenin. In addition, in the presence of OSM a low molecular weight aggrecan G3 product was observed, suggesting altered aggrecanase cleavage activity is induced by this cytokine. Under cytokine stimulation, aggrecan release from cartilage may take place via proteolysis of the aggrecan core protein or via depolymerization of HA, with the latter mechanism being induced by OSM. OSM is associated with joint inflammation and its participation may account for the more rapid loss of aggrecan from articular cartilage in the inflammatory arthritides, compared to osteoarthritis.

Multi-technique and multi-scale approach applied to study the structural behavior of heterogeneous materials: natural SiO2 case

A multi-technique and multi-scale approach have been initiated to study the structural properties of a natural heterogeneous material. This approach has led us to introduce the concepts of long-range order LRO and short-range order SRO. The correlation between these concepts has enabled us to advance in the comprehension of the mechanisms of degradation of natural SiO2 aggregate submitted to the process of the alkali–silica reaction (ASR). On LRO scale, the structural changes of the aggregate are revealed by the reduction of the size of grains, which means that the attack starts by affecting amorphous and poorly crystallized zones. This phenomenon generates the formation of very unstable amorphous and nanocrystallized products, which take part in the degradation of the aggregate. On SRO scale tetrahedral environment of silicon atoms is preserved in spite of the introduction of OH- ions with a relaxation phenomenon. The second layer of coordination around silicon remains almost unchanged. These results add new information showing that ASR must be considered as a multi-scale process. The methodology itself may be extended to other problems.

Soil physical properties and their relations to organic carbon pools as affected by land use in an alpine pastureland

Land disturbances (such as cultivation and overgrazing) and global warming have been decreasing soil organic C stocks in alpine regions of China. This study characterized changes in soil aggregation, bulk density, particle density, porosity and water holding capacity in relation to changes in total organic C and carbohydrate-C fractions under a long-term (28 years) annually-cultivated pasture (oats), and a short-term (8 years) introduced perennial pasture (cultivated once at establishment), compared with those in an adjacent native pasture. In annually-cultivated pasture, total soil organic C decreased by 29–41% and various fractions of carbohydrate-C decreased by 33–49% (concentrated acid extract), 14–45% (diluted acid extract) and 15–40% (hot water extract) in 0–30 cm depths. To a similar extent, introduced perennial pasture significantly decreased total soil organic C and various carbohydrate-C fractions only at 0–10 cm depth. Upon cultivation of native pasture for hay, soil aggregate stability (expressed as mean weight diameter, MWD) significantly decreased by 27–54% at 0–30 cm depth, with macro-aggregates (> 0.5–5 mm) tending to form micro-aggregates (< 0.25 mm) when subjected to wet sieving of various duration. Perennial pasture induced a similar magnitude of aggregate degradation in soil. The induced decrease in carbohydrate-C was responsible for the aggregate degradation mainly at 0–10 cm depth but not at 10–30 cm. Soil particle density was significantly affected by land use at all depths, whereas bulk density and porosity were significantly affected by land use only when averaged across all depths. Annual and perennial pastures significantly decreased soil water holding capacity in comparison with the native pasture. Generally, soil bulk density and particle density increased with depth, and porosity and water holding capacity decreased with depth. Significant correlations were found between total organic C and bulk density, particle density, porosity and water holding capacity. This study suggests that, besides depleting soil organic C pools, the current land use and management changes had adversely affected important soil physical properties, potentially accelerating erosion and reducing soil infiltration and water retention, thus threatening alpine soil's ecological functions in the region.

The effect of asphalt mixture gradation and compaction energy on aggregate degradation

This paper investigates the influence of asphalt mixture type (aggregate gradation) and compaction energy level on the degradation of aggregate during laboratory compaction. The most severe form of laboratory compaction in terms of potential aggregate degradation, i.e. Marshall (impact) compaction has been used together with a standard UK aggregate and two asphalt mixture gradations consisting of a continuous and gap-graded mixture design. The results of aggregate degradation have been determined by comparing the aggregate gradation in terms of retained aggregate per individual sieve size before and after compaction. The aggregate gradation after compaction has been determined by recovering the aggregate from compacted asphalt mixture specimens and then re-grading using the series of sieve sizes used to batch the original material. The results show that asphalt mixture type and therefore aggregate gradation has a far greater influence on aggregate degradation under compaction than alterations to the level of compaction energy within the normal compaction levels experienced in the laboratory. The gap-graded asphalt mixture experienced a greater amount of aggregate degradation after compaction compared to the continuously graded mixture. The effect of increased compaction energy level did not alter the pattern of aggregate degradation for the continuously graded asphalt mixture and only increased the degree of degradation by a small amount.

Effect of Aggregate Degradation on Volumetric Properties of Georgia's Hot-Mix Asphalt

Various degrees of degradation of aggregate particles can occur in hot-mix asphalt (HMA) production during the mixing and compaction process. This breakdown may significantly increase the dust fraction of the aggregate gradation relative to that of laboratory specimens. This increased dust fraction could cause a loss of voids in the mineral aggregate (VMA) and air voids. After evaluation of 22 variations of HMA mixtures during production and construction, it was determined that aggregate degradation did result from plant mixing and field compaction activities. Aggregate degradation varied somewhat with respect to where the mixture was sampled within the production and construction processes. Aggregate degradation resulting from binder extraction that uses the ignition method, relative to the solvent extraction method, was also examined. Mixture volumetric properties (i.e., air voids and VMA) were significantly affected by aggregate degradation. On the basis of this study, before a volumetrics specification for mixture acceptance is investigated for future implementation in Georgia, a change in the Georgia Department of Transportation's mix design specification is recommended. The minimum VMA values for Superpave® mix design should be increased by 1% for all dense-graded mixes to compensate for the amount of aggregate degradation and loss of VMA during HMA production and construction. It is believed that this research-based policy change for mix design will better maintain the balance between proper durability and rut resistance of asphalt pavements in Georgia.

Link protein can retard the degradation of hyaluronan in proteoglycan aggregates

Loss of articular cartilage and intervertebral disc function in arthritis or disc degeneration is associated with degradation of the proteoglycan (PG) aggregates by either proteolysis of aggrecan or hyaluronan (HA) degradation. The aim of this work was to determine whether degradation of HA in PG aggregate degradation is influenced by link protein (LP) stabilization of the PG aggregates. Aggrecan and LP were prepared from fetal bovine epiphyseal cartilage, and PG aggregates were formed in the presence or absence of LP. The PG aggregates were exposed to hyaluronidase or free radicals to promote HA degradation. Degradation of HA, aggrecan and LP were assessed by gel filtration chromatography and polyacrylamide gel electrophoresis. High concentrations of hyaluronidase cleaved both PG aggregates between each aggrecan molecule, whereas low concentrations gave much less cleavage of the LP-stabilized aggregate. High free radical concentrations gave extensive cleavage of all components of both PG aggregates, whereas low concentrations are more selective for HA damage and to a much lesser extent in the LP-stabilized aggregates. Thus the presence of LP caused a diminution in the capacity of both catabolic agents to degrade HA as long as levels of the degradative agents were not excessive. In addition to stabilizing the PG aggregates towards dissociation, LP may also help protect the PG aggregates from degradation under conditions where tissue catabolism is promoted.

Discrete Element Method Analysis of Railtrack Ballast Degradation during Cyclic Loading

Ballast materials forming part of railway structures are subjected to cyclic loads. As a result of these loads, ballast densification, aggregate degradation, and lateral spread of the ballast material underneath the ties takes place inducing permanent deformations on the railways. Maintenance and rehabilitation costs of railtracks due to problems related with ballast performance are substantial, and millions of dollars are annually spent around the world in these activities. Understanding the crushable behavior of railtrack ballast could lead to the design of better railways that will reduce these costs. This paper presents the results of two discrete element method simulations intended to study the effect of crushing on the behavior of a simulated track ballast material forming part of a simulated track section. Even though the two simulations consider the same idealized material, crushing was allowed only in one simulation. The simulated track sections were subjected to a cyclic load, and the values of permanent deformation as a function of number of cycles were recorded. The obtained results showed that the induced permanent deformation strongly increased when considering particle crushing even though only a few particles were broken. Moreover, it was found that crushing concentrated underneath the simulated sleepers. Snap shots of the track sections are presented allowing a visualization of the evolution of crushing.

Characterization of Aggregate Resistance to Degradation in Stone Matrix Asphalt Mixtures

Stone matrix asphalt (SMA) mixtures rely on stone-on-stone contacts among particles to resist applied forces and permanent deformation. Aggregates in SMA should resist degradation (fracture and abrasion) under high stresses at the contact points. This study uses conventional as well as advanced imaging techniques to evaluate aggregate characteristics and the resistance of aggregates to degradation. Aggregates from different sources and aggregate types with various shape characteristics were used in this study. The micro-Deval test was used to measure aggregate resistance to abrasion. The aggregate imaging system was used to examine the changes in aggregate characteristics caused by abrasion forces in the micro-Deval. The resistance of aggregates to degradation in the SMA was evaluated through the analysis of aggregate gradation before and after compaction with conventional mechanical sieve analysis and nondestructive X-ray computed tomography. The findings of this study led to the development of an approach for evaluating aggregate resistance to degradation in SMA. This approach measures aggregate degradation in regard to abrasion, breakage, and loss of texture.