Use of fine aggregate matrix for computational modeling of low temperature fracture of asphalt concrete

In this study, a discrete element computational model is applied to simulate the fracture behavior of asphalt mixtures at low temperatures. In this model, coarse aggregates are explicitly represented by rigid spherical particles. The bonds that connect these particles represent the fine aggregate matrix (FAM), which is defined as the combination of asphalt binder and fine aggregates. The bending beam rheometer (BBR) tests are performed to determine the strength and Young’s modulus of FAM at low temperatures. The model is then used to simulate the semi-circular bend (SCB) tests on the mixtures. The model is verified by a series of BBR and SCB tests on both conventional and graphite nano-platelet modified asphalt materials. The comparison between the experimental and simulated results indicates that the peak load capacity of the SCB specimens is primarily governed by the tensile strength of the FAM. However, in order to capture the entire load–displacement curve of the SCB specimens, one needs to employ a softening constitutive model of the FAM, which requires the information on its fracture energy. Several experimental methods for measuring the fracture energy of FAM are discussed for future prediction of the complete load–displacement response of asphalt mixtures at low temperatures.     

Cohesive Modeling of Fracture in Asphalt Mixtures at Low Temperatures

Low temperature cracking is the major distress observed in asphalt pavements in the northern US and Canada. In the past years fracture mechanics concepts were introduced to investigate the fracture properties of asphalt mixtures at low temperatures. In this paper the cohesive zone model (CZM) is used to describe the fracture behavior of asphalt mixtures at low temperatures and the interface element is used to numerically simulate the material response under monotonic loading. The simulation is calibrated with the experimental results from a newly proposed semi circular bend (SCB) test. A parametric analysis of the input material properties indicates that the tensile strength has a significant effect on the peak load in the SCB configuration, the modulus has a strong effect on the calculated stiffness of the SCB specimen, and the fracture energy influences the post-peak behavior of the asphalt mixtures. The calibrated numerical model was applied to simulate the low temperature cracking in a simplified asphalt pavement and to study the influence of these material parameters on the performance of asphalt pavements.     

Using recycled taconite as alternative aggregate in asphalt pavements

Current demand from transportation agencies for construction and maintenance of their pavement network requires the use of large amounts of aggregate. Diminishing reserves of aggregate and environmental regulations for new aggregate extraction sites restricts the use of standard aggregates and encourages the use of recycled materials. This paper investigates the suitability of using taconite aggregates, a byproduct of taconite mining industries, for construction of asphalt pavements by evaluating the mechanical performance of asphalt mixtures prepared with taconite aggregates. The experimental work is performed by means of the semi-circular bending test (SCB), the indirect tensile test (IDT), dynamic modulus (E¹) test, and the thermal stress restrained specimen test (TSRST) with acoustic emission monitoring. The results indicate that, in general, the mixtures prepared with taconite aggregates perform slightly better than the mixtures prepared with standard aggregates.     

On the representative volume element of asphalt concrete at low temperature

The feasibility of characterizing asphalt mixtures’ rheological and failure properties at low temperatures by means of the Bending Beam Rheometer (BBR) is investigated in this paper. The main issue is the use of thin beams of asphalt mixture in experimental procedures that may not capture the true behavior of the material used to construct an asphalt pavement.For the rheological characterization, three-point bending creep tests are performed on beams of different sizes. The beams are also analyzed using digital image analysis to obtain volumetric fraction, average size distribution, and spatial correlation functions. Based on the experimental results and analyses, it is concluded that representative creep stiffness values of asphalt mixtures can be obtained from testing at least three replicates of the thin (BBR) mixture beams.Failure properties are investigated by performing strength tests using a modified Bending Beam Rheometer (BBR), capable of applying loads at different loading rates. Histogram testing of BBR mixture beams and of larger beams is performed and the failure distribution is analyzed based on the size effect theory for quasibrittle materials. Different Weibull moduli are obtained from the two specimens sizes, which indicates that BBR beams do not capture the representative volume element (RVE) of the material.     

The fracture process zone in asphalt mixture at low temperature

The fracture process zone (FPZ) is a key factor to mechanistically characterize material fracture. This study investigates the FPZ of asphalt mixture at low temperature. The fracture process under a semi-circular bend (SCB) test of seven asphalt mixtures that represent a combination of different factors was monitored using an acoustic (AE) system with eight piezoelectric sensors. The size of FPZ was estimated by locating micro-cracks that correspond to 95% AE energy before peak load in the vicinity of the initial crack tip. The experimental data illustrates the significant influence of test temperature on the behavior of the asphalt mixture. Comparison results showed that the size of the FPZ significantly depends on air voids and aggregate type, but is less depend on the asphalt content. It was found that at a very low temperature, different loading rates produced very close FPZ, both for the width and length. No obvious difference was observed on the width of the FPZ for the three different initial notch lengths, whereas the length of the FPZ was found significantly increases with the decrease of the notch length. The size of FPZ was also numerically estimated for one case with the cohesive zone model (CZM) calibrated by experimental data from the same SCB test. The FPZ size obtained with both methods agrees reasonably with each other.     

Indirect determination of size effect on strength of asphalt mixtures at low temperatures

Low temperature cracking of asphalt pavements is a major distress in cold regions. Accurate assessment of strength of asphalt mixtures at low temperatures is of great importance for ensuring the structural integrity of asphalt pavements. It has been shown that asphalt mixtures behave in a quasibrittle manner at low temperatures and consequently its nominal strength strongly depends on the structure size. The size effect on the strength of asphalt mixtures can be directly measured by testing geometrically similar specimens with a sufficiently large size range. Recent studies have shown in theory that for quasibrittle structures, which fail at the macrocrack initiation from one representative volume element, the mean size effect curve can also be derived from the scaling of strength statistics based on the finite weakest link model. This paper presents a comprehensive experimental investigation on the strength statistics as well as the size effect on the mean strength of asphalt mixtures at ?24 °C. It is shown that the size effect on mean structural strength can be obtained by strength histogram testing on specimens of a single size. The present study also indicates that the three-parameter Weibull distribution is not applicable for asphalt mixtures.     

Parameter Identification Procedure for Heterogeneous Viscoelastic Composites Using Iterative Functions

This paper presents a new numerical procedure for the determination of the viscoelastic compliance properties of a matrix phase from a simple three-point bending test on a composite beam. The composite is modeled as elastic inclusions randomly dispersed throughout a viscoelastic matrix. It is also assumed that the spatial distribution of the inclusions in the composite is known or can be determined. Zevin’s method of iterative functions is proposed for the determination of the matrix properties. Following a detailed explanation of the proposed scheme, a numerical verification is performed using three-dimensional finite-element (FE) analysis simulations. The proposed scheme was applied to the experimentally obtained creep compliance of the asphalt concrete beam. The obtained viscoelastic properties of the asphalt binder matrix phase were used as input into the FE model to simulate the behavior of the composite beam. An excellent comparison between the experimental data and the predicted beam deflections was observed. This shows that the proposed method is robust and it can be implemented to solve identification problems for viscoelastic composite materials.     

Obtaining asphalt binder rheological properties from BBR strength test—the effect of loading rate

Mechanics of Time-Dependent Materials - Selecting asphalt binders that have good cracking resistance at low temperatures is the first step in designing asphalt mixtures for durable asphalt...     

Thermal conductivity of reclaimed asphalt pavement (RAP) and its constituents

Up to the present, most work on the use of reclaimed asphalt pavement (RAP) has been empirical in nature. Very recent advances have demonstrated that finite-element techniques can be effectively used for modelling asphalt mixing drums in order to optimise the relative proportions of new and recycled materials and to determine the amount of time required to achieve full melting inside of the drum. A necessary prerequisite for the modelling is a definitive knowledge of the thermal conductivities of RAP and its components. This need motivated the present experimental work which encompassed RAP particles, RAP particles with the asphalt binder removed, and pure asphalt binder of different degrees of ageing. Also evaluated were taconite tailings, residual rock from the processing of iron-containing ore, and sand. The tailings have been mentioned as a candidate aggregate. The conductivity results for the solid media were related to three metrics: (a) the size ranges of the solids, (b) the density of the sample as a whole and (c) the porosity of the sample. All of the conductivity results for the investigated solid media fell in the range from 17 to 30 W/m °C. The measured conductivities of the binder ranged from 0.17 to 0.19 W/m °C.