Yongli Zhao

Thermal oxidative aging characterization of SBS modified asphalt

Both macro and micro-methods were introduced to study the physical and chemical properties of thermal oxidative aging of SBS (styrene-butadiene-styrene) modified asphalt. The physical properties of SBS modified asphalt before and after aging were analyzed by normal tests. The structure and quality variation of SBS modified asphalt during the aging process was analyzed by FTIR (Fourier transform infrared spectrum). FTIR result shows that the degeneration of SBS modified asphalt is mainly caused by oxidative reaction and rupture of C=C double bond. The molecular weight variations of asphalt function groups and SBS polymer were studied by GPC (Gel Permeation Chromatography). GPC result shows that small molecules transform into larger one in asphalt and SBS polymer molecule degrade during the aging process. SBS polymer may lose its modifying function after long time aging.

Investigation of micro-mechanical response of asphalt mixtures by a three-dimensional discrete element model

The micro-mechanical response of asphalt mixtures was studied using the discrete element method. The discrete element sample of stone mastic asphalt was generated first and the vehicle load was applied to the sample. A user-written program was coded with the FISH language in PFC3D to extract the contact forces within the sample and the displacements of the particles. Then, the contact forces within the whole sample, in asphalt mastic, in coarse aggregates and between asphalt mastic and coarse aggregates were investigated. Finally, the movement of the particles in the sample was analyzed. The sample was divided into 15 areas and a figure was drawn to show how the balls move in each area according to the displacements of the balls in each area. The displacements of asphalt mastic balls and coarse aggregates were also analyzed. The experimental results explain how the asphalt mixture bears vehicle load and the potential reasons why the rutting forms from a micro-mechanical view.

Property Characterization of Asphalt Binders and Mixtures Modified by Different Crumb Rubbers

AbstractThis study focused on the property characterization of asphalt and asphalt mixtures modified by different types of crumb rubber (normal and desulfurized). This characterization was based on...

Influences of Preheating Temperature of RAP on Properties of Hot-Mix Recycled Asphalt Mixture

This study analyzed the agglomerate characteristics of reclaimed asphalt pavement (RAP) material and evaluated the influences of the preheating temperature of RAP on the properties of recycled asphalt mixture. Specific surface-area analysis was conducted to reveal the agglomerate characteristics of RAP. A triaxial shear test was conducted for RAP with different preheating conditions to evaluate the dispersion characteristics of RAP and its influences on RAP property. Freeze–thaw splitting test, wheel tracking test, and low-temperature bending-beam test were conducted to evaluate the influences by preheating temperature of RAP on the properties of recycled asphalt mixture. It is found that, because of the agglomerates of RAP, the specific surface area of RAP is much lower than that of real aggregates. Sufficient preheating of RAP can promote the dispersion of RAP by activating the flowability of aged asphalt in RAP. The properties of recycled asphalt mixture keep improving with the increase of the preheating temperature of RAP. There is a minimum-temperature limit for RAP preheating to guarantee the properties of recycled asphalt mixture.

Aging Behaviour and Mechanism of SBS-Modified Asphalt

The aging behaviour and mechanism of styrene-butadiene-styrene (SBS)-modified asphalt has been investigated based on laboratory tests in this study. Rolling thin-film oven (RTFO) test and pressure-aging-vessel (PAV) test were conducted to prepare aged SBS-modified asphalt. Conventional property tests, including penetration, soften point, and ductility, as well as elastic recovery and SHRP property tests, including viscosity, dynamic shear rheometor (DSR) and bending-beam rheometer (BBR) tests, were mainly used to analyse the aging behaviour of SBS-modified asphalt. Then Fourier transform infrared spectroscopy (FTIR) and gel-permeation chromatography (GPC) tests were conducted to capture the aging mechanism of SBS-modified asphalt. The testing results indicated that SBS-modified asphalt has similar aging behaviour as base asphalt but better performance and anti-aging ability because of the existence of the SBS modifier. However, it is still inevitable that SBS-modified asphalt becomes aged because of thermal oxidation. The aging of SBS-modified binder consists of two parts: (i) the oxidative aging of the asphalt components, and (ii) the degradation of the SBS modifier. Because of the protective effect between asphalt and SBS polymer, the aging degrees of asphalt and SBS polymer were lower than those aged independently.

Strength Mechanism and Influence Factors for Cold Recycled Asphalt Mixture

This study focused on the key factors affecting the tensile strength of cold recycled asphalt mixture with cement and emulsified asphalt. The specific surface areas and strength of RAP were analyzed. The interaction between the emulsified asphalt and cement was observed. Comprehensive laboratory testing was conducted to evaluate the influences of RAP, emulsified asphalt, and cement on the tensile strength of cold recycled asphalt mixture. It is found that although RAP is used as aggregates, its inner structure and strength are much different from real aggregates. The strength of RAP has decisive effect on the strength of cold recycled asphalt mixture. New aggregates and fine gradation design can help improve the bonding between RAP and binder. For emulsified asphalt, slow setting of asphalt can give sufficient time for cement to hydrate which is helpful for strength formation in the cold recycled asphalt mixture. The high viscosity of asphalt can improve the early strength of cold recycled asphalt mixture that is important for traffic opening in the field. Cement is an efficient additive to improve the strength of cold recycled asphalt mixtures by promoting demulsification of emulsified asphalt and producing cement hydrates. However, the cement content is limited by RAP.

Using RAP Material in High Modulus Asphalt Mixture

With the purpose of improving the high temperature rutting resistance of recycling asphalt mixture, the design concept of high modulus asphalt mixture (HMAM) was adopted to design recycling asphalt mixture in this study. Control HMAM was designed to prepare recycling high modulus asphalt mixtures (RHMAMs) with different contents of reclaimed asphalt pavement (RAP). Performance tests including simple performance test (SPT) test, wheel tracking test, low-temperature bending beam test, immersion Marshall test, freeze-thaw splitting test, and bending beam fatigue test were conducted to evaluate the dynamic modulus, high temperature rutting resistance, low-temperature cracking resistance, moisture stability, and anti-fatigue performance of RHMAMs. This paper proves that the addition of RAP can improve the high temperature rutting resistance of HMAM while jeopardizing its low temperature cracking resistance, moisture stability, and anti-fatigue property. With RAP content increasing, the dynamic modulus and dynamic stability of RHMAMs increase while the failure strain, tensile strength ratio, and fatigue life of RHMAMs decrease. With 20%-30% of RAP, RHMAMs have similar performances to HMAM without RAP. With 40%-50% of RAP, while the dynamic stability and moisture stability of RHMAMs can meet the requirements by Chinese specification, the low-temperature failure strain of RHMAMs was already lower than the requirements by Chinese specification and the fatigue life of RHMAMs degraded obviously compared to that of the HMAM. It is indicated that RHMAMs can provide satisfied high temperature rutting resistance and moisture stability while its low-temperature cracking resistance and anti-fatigue property needs to be paid attention to.

Numerical Study on the Effect of Coarse-Aggregate Morphology on Shear Performance

The effect of coarse-aggregate morphology on shear performance was investigated by the discrete element modeling in this paper. An algorithm was developed first to compact aggregates without the need to know the void content. This algorithm uses four steps (aggregate generation, gravity compaction, wall accelerated compaction, and wall uniform compaction) to compact a coarse-aggregate sample. The triaxial shear test was then simulated to evaluate the shear performance of coarse aggregates. Effects of the aggregate angularity, surface texture, flat and elongated ratio, and flat and elongated particle content on the shear performance of coarse aggregates were finally studied based on the results of the triaxial shear test.

Degradation Behavior and Mechanism of HMA Aggregate

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.

Gradation Design of the Aggregate Skeleton in Asphalt Mixture

Rutting is one of primary distresses in asphalt pavements. The asphalt mixtures with strong aggregate skeleton have a good rutting resistance. The gradation design method of coarse aggregate skeleton and asphalt mixture was studied. A three-dimensional spheroid model was used to analyze the mechanical characteristics of the aggregate skeleton. The aggregate crushing test and the Marshall compact test were conducted to verify the theoretical analysis. The results show that the stability of aggregate skeleton was improved by increasing the number of contact points. And a gradation design model was developed for the coarse aggregate skeleton. 4.75 mm was defined as the dividing size between coarse and fine aggregates. The asphalt content and performance-related volume parameters were introduced into the design method. It ensures the aggregate skeleton formation and the desired performance of asphalt mixtures. The aggregate gradation is very similar to that of SMA, and that the designed asphalt mixture has good high-temperature performance. It is concluded that the proposed design method a have good practical applicability.