Effect of warm mix additives on the interfacial bonding characteristics of asphalt binders

The quality of the interfacial bonding between asphalt binder and aggregates plays a significant role in determining the durability of asphalt mixtures. Warm mix asphalt (WMA) modifiers have been used extensively in the last decade primarily to reduce production and compaction temperatures as well as to improve workability of asphalt mixtures. This study aimed to provide better understanding of the effects of these WMA modifiers on the interfacial bonding between asphalt binders and aggregates. The evaluation focused on measuring surface energy of binders in unaged and aged states and aggregates and then calculating energy parameters that describe the potential of a given asphalt-aggregate combination to resist fatigue cracking and moisture damage. Results show that the combination of asphalt-WMA additive, as well as the content applied of WMA additive has a significant impact on the fatigue cracking and moisture damage resistance. The results suggest that it is poor practice to use a given type and percentage of WMA modifier without regard for binder type. Instead, test methods are recommended to evaluate the compatibility of asphalt binder, WMA additive type/content, and aggregates for improved performance at different conditions.     

Linear and nonlinear viscoelastic and viscoplastic analysis of asphalt binders with warm mix asphalt additives

Warm-Mix Asphalt (WMA) is a widely used product, which proved a contribution to the reduction in asphalt mixing and compaction temperatures. This reduction leads to lower fuel consumption and smoke emission in asphalt plants. Most of the characterisation of binders used in WMA has focused in the past on measuring linear viscoelastic properties and associated Superpave parameters. Several studies have shown that the average stresses and strains of the asphalt mixture remain mostly within the linear viscoelastic response. However, localised strains in the binder phase of the mixture could reach values high enough to induce nonlinear viscoelastic and viscoplastic deformations. Therefore, this study focuses on an experimental and analytical evaluation of linear, nonlinear viscoelastic and viscoplastic responses of selected binders modified for use in WMA. The first part of the paper analyses the linear viscoelastic material properties and their ability to evaluate permanent deformation resistance. Then, the non-recoverable creep compliance parameter obtained from the Multiple Stress Creep Recovery (MSCR) test is analysed to assess the nonlinear response and permanent deformation of asphalt binders. The paper utilises a nonlinear plasto-viscoelastic (NPVE) approach to assess and quantify the nonlinear plasto-viscoelastic response of binders by separating the recoverable and irrecoverable strains measured in the MSCR test. Two WMA additives were included in this study by mixing them with polymer-modified and unmodified asphalt binders. Analysis of results showed that the NPVE approach captured a higher percentage of recovery than the NLVE approach. However, binder’s performance evaluation and ranking did not change by adopting the NPVE approach. The nonlinear viscoelastic parameters provided insight on the behaviour of asphalt binders mixed with WMA additives during loading cycles. Sasobit showed higher influence than Advera on binders in resisting permanent deformation by increasing the recoverable strain during the unloading phase.     

Viscoplastic modeling of asphalt mixes with the effects of anisotropy, damage and aggregate characteristics

This paper presents an approach for constitutive modeling of the viscoplastic behavior of asphalt mixes. This approach utilizes an anisotropic non-associated flow rule based on the Drucker–Prager yield surface. The selection of this yield surface is motivated by the field stress paths and material properties associated with permanent deformation at high temperatures. The efficacy of the model is demonstrated by analyzing data from compressive triaxial tests conducted at different confining pressures and strain rates for three different mixes. The model parameters are related to the experimental measurements of aggregate shape characteristics, aggregate surface energy, inherent anisotropic distribution of aggregates, and microstructure damage measured using X-ray computed tomography and image analysis techniques. Establishing the relationship between the model parameters and material properties is important in order to optimize the mix properties, and achieve desirable mix performance.     

The Cognitive Benefit of Dynamic Representations on Procedural Skill Acquisition: A Computational Modeling Approach

Cognitive computational modeling is a viable methodology for further investigation of the hitherto inconclusive findings on the cognitive benefits of dynamic versus static visualization components of instructions. This is more so as contemporary cognitive architectures such as the Adaptive Control of Thought–Rational (ACT–R) 6.0 are increasingly applied to traditional cognitive psychology research problems. The application of this methodology is, however, restricted by the limited capability of existing architectures for implementing detailed atomic motor actions such as those involved in complex skill acquisition and performance. This article presents a 2-component computational modeling methodology for investigating the cognitive processes involved in the acquisition and performance of skilled motor tasks. The approach specifies a novel combination of a sequence-of-point technique with a movement control mechanism to implement variously acquired cognitive mental task representations and their intertwined role in postlearning performance as evident in the atomic control of motor actions. This paradigm is validated for 2 experiments using incrementally developed cognitive models developed in ACT–R 6.0. The model's quantitative outputs correlate significantly with equivalent empirical human data. This has implications for multimedia instructional design, especially where rapid, transferrable skill acquisition is desired on initial exposure.     

Finite-Element Analysis of Hot Mix Asphalt Microstructure Using Effective Local Material Properties and Strain Gradient Elasticity

This paper presents the development of a methodology for microstructure analysis and modeling of hot mix asphalt (HMA). This methodology relies on using effective local material properties and strain gradient theory in the finite element analysis of HMA microstructure. The effective local properties are calculated using an analytical micromechanical model that captures the influence of percent of particles on the microscopic response of HMA. Strain gradient elasticity is used in order to account for the effect of particle size in the finite element analysis. The autocorrelation function and the moving window technique are used to determine the microstructure characteristic length scales that are used in strain gradient elasticity. A number of asphalt mixes with different aggregate types and size distributions are analyzed in this paper.     

Thermomechanical Framework for the Constitutive Modeling of Asphalt Concrete

This study is concerned with the constitutive modeling of asphalt concrete. Unlike most constitutive models for asphalt concrete that do not take into account the evolution of the microstructure of the material, this study incorporates the evolution of the microstructure by using a framework that recognizes that a body’s natural configurations can evolve as the microstructure changes. The general framework, on which this study is based, is cast within a full thermomechanical setting. In this paper, we develop models within the context of a mechanical framework that stems from the general framework for models based on the full thermodynamic framework and the resulting equations represent a nonlinear rate type viscoelastic model. The creep and stress relaxation experiments of Monismith and Secor are used for validating the efficacy of the model, and it is found that the predictions of the theory agree very well with the available experimental results. The advantages of using such a framework are many, especially when one wants to model the diverse mechanical and thermodynamic response characteristics of asphalt and asphalt concrete.     

Development of an Elastoviscoplastic Microstructural-Based Continuum Model to Predict Permanent Deformation in Hot Mix Asphalt

Permanent deformation in hot mix asphalt is caused by a combination of densification (decrease in volume and hence increase in density) and shear deformation. The primary objective of this paper is to develop an elastoviscoplastic model that accounts for the influence of important microstructure properties such as anisotropy and damage on permanent deformation. The model incorporates a yield surface based on the Drucker-Prager function that is modified to capture the influence of stress state on the material response. Also, parameters that reflect the directional distribution of aggregates and damage density in the microstructure are included in this yield surface model. The elastoviscoplastic model is converted into a numerical formulation and is implemented in finite element (FE). The FE model is used in this study to simulate experimental measurements under different confining pressures and strain rates.     

Evaluation of ageing in asphalt cores using low-field nuclear magnetic resonance

This paper introduces an innovative methodology for estimating the ageing of asphalt concrete cores without extracting the binder. Asphalt concrete samples at different ageing stages (unaged, 3-month and 6-month aged) and with different percent air voids (4%, 7% and 10%) were analysed with low-field nuclear magnetic resonance (NMR). The transverse relaxation time T₂ and relative hydrogen index (RHI) obtained from NMR measurements were related to the viscosity of the asphalt binder. The samples were analysed during cooling from 70°C to room temperature, showing increase in viscosity with decreasing temperature. There was a clear trend indicating higher viscosities in samples that were aged for a longer period and samples with higher percent air voids. The RHI and T₂ values obtained from low-field NMR measurements and the viscosity data calculated from measurements using a dynamic shear rheometer were correlated to develop a model that relates viscosity with RHI.     

A Framework for the Verification of Certifying Computations

Formal verification of complex algorithms is challenging. Verifying their implementations goes beyond the state of the art of current automatic verification tools and usually involves intricate mathematical theorems. Certifying algorithms compute in addition to each output a witness certifying that the output is correct. A checker for such a witness is usually much simpler than the original algorithm—yet it is all the user has to trust. The verification of checkers is feasible with current tools and leads to computations that can be completely trusted. We describe a framework to seamlessly verify certifying computations. We use the automatic verifier VCC for establishing the correctness of the checker and the interactive theorem prover Isabelle/HOL for high-level mathematical properties of algorithms. We demonstrate the effectiveness of our approach by presenting the verification of typical examples of the industrial-level and widespread algorithmic library LEDA.