Study on Influencing Factors of Asphalt-Aggregate Stripping Mechanism

In order to study the stripping mechanism of asphalt aggregate comprehensively, the conditions of the stripping of asphalt aggregate are divided into two types, which are anhydrous environment and water environment. The stress generation and release of asphalt film under anhydrous environment and the differences in stripping mechanism of asphalt film under tensile and pressure stress were analyzed. The existence of water in the mixture and its harmfulness to stability were also described in this paper. Moreover, the transport behavior of water in asphalt was studied by the principles of electrochemical testing. The test results show that the diffusion rate of water in modified asphalt film is one half of that of base asphalt, so the blocking water ability of modified asphalt is better than that of matrix asphalt. Moreover, the condition of water spalling the asphalt-aggregate interface is characterized by a change in the mass of asphalt film before and after boiling. It can be concluded that the mass loss of asphalt film is minimal with limestone and modified asphalt, which shows that it has the best spalling resistance.

Effect of Superplasticizer and Wetting Agent on Volumetric and Mechanical Properties of Cold Recycled Mixture with Asphalt Emulsion

Cold recycled mixture with asphalt emulsion (CRME) has gained more appreciation due to its environmental and economical advantages. Surfactant greatly affects the interaction between asphalt emulsion and cement, which can greatly affect the volumetric and mechanical properties of CRME. If the surfactant can greatly improve the volumetric and mechanical properties of CRME, that could be of great attraction. In this study, a polycarboxylate-based superplasticizer and wetting agent (DN500, polymers containing high-pigment groups) were employed to improve the volumetric and mechanical properties of CRME. Results indicate that the addition of superplasticizer and DN500 can reduce the void content of CRME and increase the indirect tensile strength (ITS) and stiffness modulus as well as critical strain energy density (CSED) of CRME. Besides, the failure strain of CRME is also increased by adding superplasticizer and DN500. This phenomenon is probably due to that superplasticizer can decrease the viscosity of cement asphalt emulsion paste (CAEP) and help to form a better asphalt film, and DN500 can moderately decrease the viscosity of CAEP and increase the wetting ability of asphalt emulsion as a wetting agent. CRME with superplasticizer has the best mechanical properties among all CRMEs. Compared to reference CRME, the ITS, stiffness modulus, and CSED of CRME with superplasticizer can increase by 33.7%, 8.0%, and 17.5% at the optimum water content, respectively. It is recommended to improve the volumetric and mechanical properties of CRME by adding superplasticizer.

Improvement of Asphalt-Aggregate Adhesion Using Plant Ash Byproduct

The adhesion bonding between asphalt and aggregate significantly influences field performance and durability of asphalt pavement. Adhesion promoters are typically used to improve asphalt-aggregate bonding and minimize moisture-related pavement damage, such as cracking and raveling. This study evaluated the effectiveness of plant ash byproduct as adhesion promoter to improve asphalt-aggregate adhesion performance. Three commonly used aggregate types (granite, basic rock, and limestone) and two asphalt binder types were used in laboratory testing. A modified stripping test method was developed to evaluate test results with image analysis and measurement of asphalt film thickness. The contact angle test and scanning electron microscopy (SEM) with energy disperse spectroscopy (EDS) were conducted. Test results showed that plant ash lixivium significantly improved asphalt-aggregate adhesion. Among three aggregate types, granite yielded the worst asphalt-aggregate adhesion for both control and treated specimens. The effectiveness of adhesion promotion varied depending on the type of asphalt or aggregate and temperature. The SEM/EDS observations showed that the mesh-like crystalline was formed at the interface between asphalt binder and aggregate in the treated specimen, which was believed to enhance the interfacial bonding and prevent asphalt film peeling off from aggregate.

Effect of Mix Design Variables on Thermal Cracking Performance Parameters of Asphalt Mixtures

To address asphalt pavement thermal cracking, researchers have developed performance-based evaluation tools for asphalt mixtures. A minimum fracture energy obtained from a disc-shaped compact tension test and Black space parameters determined by the stiffness and relaxation properties of asphalt mixtures are two such methods to ensure good thermal cracking resistance. Mix specifiers and producers strive to meet the requirements set by these performance-based criteria by adjusting their mix designs. However, there is a lack of information and consensus on the effect of mix design variables (such as binder grade and mix volumetrics) on thermal cracking performance of mixtures as it relates to fracture energy and Black space location. This study strives to fill this gap by quantifying the effect of: (1) recycled asphalt content, (2) effective binder content, (3) air voids, (4) asphalt film thickness, (5) voids in mineral aggregates, and (6) PG low and high temperature grades on thermal cracking resistance. A large dataset, 90 mixtures from the Minnesota Department of Transportation and 81 mixtures from University of New Hampshire database, was used for the study. The results indicate a strong correlation between binder related properties (binder content, asphalt film thickness, PG spread) and fracture energy. The correlation coefficients obtained from this study for PG spread, effective binder content, and air void can be confidently employed to achieve targeted fracture energy thresholds. The same can be achieved for the Glower-Rowe parameter at 15ºC by employing the correlation coefficients obtained for PG low temperature, virgin asphalt content, and voids in the mineral aggregate.

Nano-Scale Binder Phase Identification in Asphalt Concrete

Asphalt concrete (AC) consists of asphalt binder and aggregate. Aggregate consists of: coarse aggregate and fines. Asphalt binder creates a coating or film around the aggregate, which is defined as the binder phase of AC. Fines are believed to be trapped inside an asphalt film or mixed with asphalt binder, creating a composite material called mastic. Thus, AC has three phases: mastic, asphalt film binder, and coarse aggregate. All these phases play major roles in performance of AC. Researchers have performed various tests on asphalt binder at micro scale to understand the macro scale behavior of AC. However, test methods developed and performed on binders, to this day, are mostly rheological shear and bending beam tests. No studies have been conducted on the compression stiffness or modulus and hardness of and binder, rather than shear and binders stiffness. In addition, the existing tests used in the asphalt area cannot be performed on binder and mastic while they are an integral part of AC. Nanoindentation tests can be performed on aggregate and asphalt binder while they are integral parts of AC. Because, in nanoindentation test, a nanometer size tip, which is smaller than binder film thickness as well as other phases. In the study, Performance Grade (PG) 64–28 was used for the study, same binder had been used afterwards to characterize asphalt and AC. A loading rate of 0.005 mN/sec, a dwell time of 200 sec and a maximum load 0.055 mN were employed in the study. In the current study 20 indentations were done on the asphalt binder sample and 100 indentations were done on AC sample, due to heterogeneity of the sample. However, to identify a specific phase in AC sample, the current study adopts the depth range technique for as same loading protocol. The depth rage of binder phase was acquired by independent indentation on same asphalt binder sample. As, asphalt is known to be a viscoelastic material that exhibits creep behavior, the creep compliance of asphalt binder was used for validation of the depth range assumption. The validation of phase identification was done by comparing the asphalt binder phase creep response while they are integral part of AC with creep response of independent asphalt binder sample under nanoindenter. The comparison shows depth resolution technique can successfully identify the binder phase of AC.