Interaction nonlinearity in asphalt binders

Asphalt mixtures are complex composites that comprise aggregate, asphalt binder, and air. Several research studies have shown that the mechanical behavior of the asphalt mixture is strongly influenced by the matrix, i.e. the asphalt binder. Characterization and a thorough understanding of the binder behavior is the first and crucial step towards developing an accurate constitutive model for the composite. Accurate constitutive models for the constituent materials are critical to ensure accurate performance predictions at a material and structural level using micromechanics. This paper presents the findings from a systematic investigation into the nature of the linear and nonlinear response of asphalt binders subjected to different types of loading using the Dynamic Shear Rheometer (DSR). Laboratory test data show that a compressive normal force is generated in an axially constrained specimen subjected to torsional shear. This paper investigates the source of this normal force and demonstrates that the asphalt binder can dilate when subjected to shear loads. This paper also presents the findings from a study conducted to investigate the source of the nonlinearity in the asphalt binder. Test results demonstrate that the application of cyclic shear loads results in the development of a normal force and a concomitant reduction in the dynamic shear modulus. This form of nonlinear response is referred to as an “interaction nonlinearity”. A combination of experimental and analytical tools is used to demonstrate and verify the presence of this interaction nonlinearity in asphalt binders. The findings from this study highlight the importance of modeling the mechanical behavior of asphalt binders based on the overall stress state of the material.     

Effect of Cationic Surfactant Head Groups on Synthesis, Growth and Agglomeration Behavior of ZnS Nanoparticles

Colloidal nanodispersions of ZnS have been prepared using aqueous micellar solution of two cationic surfactants of trimethylammonium/pyridinium series with different head groups i.e., cetyltrimethylammonium chloride (CTAC) and cetyltrimethylpyridinium chloride (CPyC). The role of these surfactants in controlling size, agglomeration behavior and photophysical properties of ZnS nanoparticles has been discussed. UV–visible spectroscopy has been carried out for determination of optical band gap and size of ZnS nanoparticles. Transmission electron microscopy and dynamic light scattering were used to measure sizes and size distribution of ZnS nanoparticles. Powder X-ray analysis (Powder XRD) reveals the cubic structure of nanocrystallite in powdered sample. The photoluminescence emission band exhibits red shift for ZnS nanoparticles in CTAC compared to those in CPyC. The aggregation behavior in two surfactants has been compared using turbidity measurements after redispersing the nanoparticles in water. In situ evolution and growth of ZnS nanoparticles in two different surfactants have been compared through time-dependent absorption behavior and UV irradiation studies. Electrical conductivity measurements reveal that CPyC micelles better stabilize the nanoparticles than that of CTAC.     

Comprehensive analysis of surface free energy of asphalts and aggregates and the effects of changes in pH

Moisture damage is a major factor in the deterioration of asphalt pavements. In order to combat this problem, it is essential to understand the effects of moisture on the adhesive and cohesive bonds in asphalt concrete mixtures. These effects can be quantified through the use of surface free energy, which is a thermodynamic material property that has been successfully used to select asphalt binders and aggregates that have the necessary compatibility to firm strong bonds and resist moisture damage.This study aimed at understanding the effects of material characteristics and additives on surface energy and the resulting bond between asphalt binders and aggregates. As such, the study involved measuring the surface free energy of 37 neat and polymer modified asphalt binders and 11 aggregates were measured. In addition, the surface free energy was measured for three asphalt binders after two anti-strip agents were added separately (six binder-anti-strip agent combinations) and for nine asphalt binders that were both short and long-term aged. The study also examined the effect of water pH on surface energy and water-aggregate adhesive bond. It was found that anti-strip agents, in general, reduced the cohesive bond energy of asphalt binder, allowing better wetting and adhesion to aggregates and increase in resistance to moisture damage. Aging of the asphalt either increased or decreased the cohesive bond depending upon the chemical composition of the unaged asphalt binder. Statistical analysis was conducted to rank the moisture resistance of asphalt binders and asphalt–aggregate combinations, respectively. The results showed that the pH of the water increased slightly due to contact with the aggregates, but did not significantly alter the total surface tension of the water or surface free energy components of the asphalt binder.     

Three dimensional distribution of the mastic in asphalt composites

The microstructure of the fine aggregate matrix has a significant influence on the mechanical properties and evolution of damage in an asphalt mixture. This paper presents the findings from a study conducted to identify a quantitative method to characterize the three-dimensional microstructure of the matrix in an asphalt mixture. The influence of binder content, coarse aggregate gradation, and fine aggregate gradation on the microstructure of the matrix was also investigated. Results indicate that for a given aggregate type, binder content and aggregate gradation influence the degree of anisotropy whereas gradation of the coarse aggregate has the most influence on the direction of anisotropy of the asphalt mastic. Addition of asphalt binder or adjustments to the fine aggregate gradation also resulted in a more uniform spatial distribution of the asphalt mastic.     

Characterisation of crack tip stresses in elastic-perfectly plastic material under mode-I loading

Asymptotic crack tip stress fields are developed for a stationary plane strain crack in incompressible elastic-perfectly plastic material under mode-I loading. Detailed investigations have revealed that in between the two extreme conditions of crack tip constraint, that is, between the fully plastic Prandtl [1] field and the uniform stress field the most general elastic-plastic crack tip fields can be completely described by the 5-sector stress solution proposed in this article. The 3-sector stress field proposed by Li and Hancock [2] and the 4-sector field proposed by Zhu and Chao [3] are subsets of the general elastic-plastic field proposed in this work. This study has revealed that cases arise where the severe loss of crack tip constraint can lead to compressive yielding of crack flank. This particular situation leads to 5-sector stress field. Detailed studies have revealed that, in the most general case of elastic-plastic crack tip fields, the T_π parameter proposed by Zhu and Chao [3] cannot be used as a constraint parameter to represent a unique state of stress at the crack tip. A new constraint-indexing parameter T_CS-2 is proposed, which along with T_p is capable of representing the entire elastic-plastic crack tip stress fields over all angles around a crack tip. Excellent agreement is obtained between the proposed asymptotic crack tip stress field and the full-field finite element results for constraint levels ranging from high to low. It is demonstrated that the proposed constraint parameters are adequate to represent the crack tip constraint arising due to specimen geometry and loading conditions as well as the additional constraint that arises due to weld strength mismatch.     

Numerical modelling of block size effects and influence of joint properties in multiply jointed rock

Two-dimensional numerical experiments are conducted for investigating the effects of block size and joint properties on the behaviour of multiply jointed rock. The paper explains how the size of the individual blocks controls both the shear strength of the assembly (rock mass) and its deformational characteristics. A closely, jointed rock mass, in which block rotations occur, exhibits a lower stiffness but a higher strength than a rock mass with widely spaced joints. These numerical results are similar to those from reported physical model tests on jointed slabs of a rock model material. The paper shows how parametric studies on a single joint using the Barton-Bandis (BB) formulation are useful for providing information about the response of a jointed rock mass.