Using the surface free energy method to evaluate the effects of liquid antistrip additives on moisture sensitivity in hot mix asphalt

The objectives of this research are to evaluate the susceptibility of aggregates and asphalt binder with and without liquid antistrip (LAA) additives to moisture damage based on the properties that affect the adhesion bond between the aggregate and asphalt binder and the cohesion strength of the asphalt binder using the surface free energy (SFE) concept and laboratory testing. The percentage of the aggregate surface area that was exposed to water (P) due to each cycle was used as a screening parameter for evaluating the compatibility of the asphalt binder and aggregates in terms of the resistance to moisture damage. The results show that adding LAA causes the total SFE of the asphalt binder to increase, which results in a decrease in stripping between the aggregate and asphalt binder in the presence of water. Similar results were obtained from a dynamic modulus test. From the data obtained, we conclude that LAA caused a reduction of the magnitude of P that improves its resistance to moisture damage.     

Microstructure and fracture morphology of carbon nano-fiber modified asphalt and hot mix asphalt mixtures

This paper focuses on the microstructure and fracture surface morphology of neat and carbon nanofibers (CNF) modified asphalts and hot mix asphalt (HMA) mixtures using scanning electron microscopy (SEM). Asphalt binder was modified with 1.5 % of CNF by weight of binder. The modified asphalt was used to construct HMA mixtures at various CNF dosages, mixed with aggregate, using the Superpave Gyratory compactor. Small rectangular specimens extracted from the center of large HMA samples were tested under direct tension and the fracture surface was examined under SEM. The SEM analysis developed a fundamental understanding of the role that the CNF modification plays in the performance enhancement of asphalt and HMA mixtures. It was found that CNF not only possess good adhesion characteristics but also exhibits high connectivity and were evenly distribution throughout the binder. The fracture surface morphology also revealed that CNF exhibited crack bridging at micro/nano scale which may enhance the resistance to cracking due to repeated traffic loads.     

Investigation into three-layered HMA mixtures

Hot-mix asphalt (HMA) mixtures consist of three phases: aggregate, asphalt binder (mastic) and air voids, of which the first two (aggregate and asphalt binder) provide the structure that withstands various kinds of loading.

Due to the nature of high inhomogeneity between aggregate and asphalt binder, significant stress and strain concentration occurs at the interface between the two phases, which causes adverse effect to HMA mixtures and potentially contributes to pavement distresses/failure.

This paper presents a novel idea to mitigate the stress and strain concentration by introducing an intermediate layer between aggregate and asphalt binder in HMA mixture. Microstructural analyses of layered system indicated that the three-layered composite HMA mixture would greatly improve the performance of asphalt mixture. The composite mixture showed more than 10% reduction in internal stress and strain and consequently its performance could be potentially improved. To validate the theoretical analyses, a laboratory experiment was conducted to compare the performance of a conventional mixture to that of a conceptual three-layered composite HMA mixture, which was formed by incorporating a stiff natural asphalt (gilsonite) as the intermediate layer. The results of the limited laboratory experiment confirmed the findings from the theoretical analyses.     

Investigating the effect of nanoparticles on the rutting behaviour of hot-mix asphalt

Rutting is considered as one of the major damages in asphalt mixtures. In this study, different types of nanoparticles such as TiO₂, Al₂O₃, Fe₂O₃ and ZnO in different percentages were added to the base asphalt binder in order to decrease the rutting potential of hot-mix asphalt (HMA). In the first step, asphalt binder tests for characteristics such as penetration grade, ductility, softening point and viscosity were performed on the asphalt binder modified by the nanoparticles. Then, after preparing HMA samples, the static creep test was done at two stress levels at a specific temperature. Results of this study showed that using the nanoparticles improved the behavioural properties of the asphalt binder and decreased rutting in asphalt mix samples. Furthermore, scanning electron microscope images taken from the asphalt binder samples modified by the nanoparticles demonstrated that these nanoparticles were properly distributed in the asphalt binder space and had a positive effect on the rutting performance of the asphalt mixes.     

Adhesive properties of asphalts and aggregates in tropical climates

In Central and South America, pavement deterioration due to moisture is high. The deterioration is directly related to the compatibility between the asphalt and aggregates, as well as the cohesiveness of the asphalt matrix. The affinity between these materials affects how well the bond will behave in the presence of water, and therefore the susceptibility of the asphalt mixture to moisture in the long term. It is well accepted that traditional tests for assessing moisture damage are not necessarily representative of high moisture conditions, such as those present in Colombia and Costa Rica. Therefore, it is imperative that methods to quantify the actual moisture susceptibility of hot-mix asphalt be adopted and implemented in local specifications. In order to characterise the true adhesion properties of regional materials, both physicochemical and mechanical analysis has been implemented to determine the moisture susceptibility of different binder–aggregate combinations typically used in Costa Rica and Colombia. The effect of antistrip additives on the water resistance of such combinations was also evaluated. The asphalt bond strength test was applied to mechanically determine the adhesive and cohesive strength of the binder–aggregate pairs. In addition, the measurement of physicochemical properties such as surface free energies of aggregates and binders allowed the determination of work of adhesion, cohesion and debonding of asphalt from the aggregate surface in the presence of water. A correlation between the physicochemical and the mechanical properties was found for most of the cases.     

Effects of high friction aggregate and PG Plus binder on rutting resistance of hot mix asphalt mixtures

The rutting resistance of hot mix asphalt (HMA) Superpave? mixes in surface course materials was investigated using asphalt material characterisation tests and a digital imaging processing (DIP) technique. The effects of the type of aggregate, the type of binder and the binder content on rutting resistance were quantified. Two types of aggregate were examined: Superpave? SP12.5 and high friction SP12.5 FC2. Both a modified (PG Plus) and an unmodified binders were considered at the optimum binder content and the optimum content plus an additional 0.5%. To accurately identify the effect of each variable, the shear upheave of these mixes was also quantified. The DIP technique involved estimating the number of aggregate contacts, the total contact length and internal structure index of two-dimensional images of the experimentally tested samples. The results showed that both the rutting resistance and stiffness of HMA surface mixes were sensitive to aggregate type, binder type and binder content. A high friction aggregate provided a better internal structure characteristic, as well as superior rutting resistance and stiffness for HMA mixes. The use of PG Plus and the addition of 0.5% to the optimum binder content negatively affected HMA stiffness and rutting resistance. However, the levels of rutting resistance for all mixes were acceptable (rut depth < 12.5 mm), even when the shear upheave was considered. Internal structure indices measured by DIP were effective for capturing changes in the internal HMA structure with respect to aggregate type and asphalt cement content.     

Analytical modeling and experimental study of tensile strength of asphalt concrete composite at low temperatures

In this study, analytical modeling of the tensile strength of hot-mix asphalt (HMA) mixtures at low temperatures was developed. To do this, HMA mixtures were treated as a two-phase composite material with aggregates (coarse and fine) dispersed in an asphalt mastic matrix. A two-phase composite model, which was similar to Papanicolaou and Bakos's [J. Reinforced Plast. Compos. 11 (1992) 104] model with a particle embedded in an infinite matrix, was proposed. Unlike Papanicolaou and Bakos's model, an axial stress was introduced to the fiber end to consider the load transferred from the asphalt mastic the aggregate. Efforts were also made to consider the effect of aggregate gradation, asphalt mastic degradation, and interfacial damage between the aggregates and asphalt mastic matrix on the tensile strength of the HMA mixtures. Experimental investigations were conducted to validate the developed theoretical relations. A reasonable agreement was found between the predicted tensile strength and the experimental results at low temperatures. Parameters affecting the tensile strength of asphalt mixtures were discussed based on the calculated results.     

Investigating the effect of filler types on thermodynamic parameters and their relationship with moisture sensitivity of asphalt mixes

Filler plays a significant role in mastic cohesion and adhesion between aggregate–asphalt binder in asphalt mixes. In the majority of research on investigating moisture damage based on thermodynamic concepts, little attention has been given to the role of filler. In the present study, 20 different combinations of asphalt mixes made with 4 filler types (stone powder, hydrated lime, calcium carbonate and portland cement), with two types of asphalt binder (60–70 and 85–100), and two types of aggregate (limestone and granite) were used. Then thermodynamic parameters (with and without considering the effect of filler) were calculated and the relationship between these parameters and test results of moisture sensitivity of asphalt mixes was investigated using statistical analyses. Results obtained by thermodynamic parameters show that only stone powder filler caused an increase in free energy of adhesion between base asphalt binder and aggregates, and other fillers reduced free energy of adhesion. The maximum amount of debonding energy in samples made by asphalt binder 60–70, was related to mastics containing calcium carbonate and hydrated lime fillers, and in asphalt binder 85–100, mastics containing portland cement and calcium carbonate had the maximum amount of debonding energy. However, the minimum amount of debonding energy was related to the mastic containing stone powder. In addition, the results of moisture sensitivity mechanical tests show that samples containing calcium carbonate and hydrated lime fillers had the maximum amount of tensile strength ratio. Finally, the amount of adjusted coefficient of correlation between debonding energy and modified Lottman test results increased from 0.553 in 4 base compounds (without filler) to 0.701 in 16 compounds with filler. The difference in correlation coefficients show the necessity to use the effect of filler on calculating thermodynamic parameters in investigating moisture sensitivity of various asphalt mixes.     

Fatigue life and endurance limit prediction of asphalt mixtures using energy-based failure criterion

Fatigue cracking is one of the major types of distress in asphalt mixtures and is caused by the accumulation of damage in pavement sections under repeated load applications. The fatigue endurance limit (EL) concept assumes a specific strain level, below which the damage in hot mix asphalt (HMA) is not cumulative. In other words, if the asphalt layer depth is controlled in a way that keeps the critical HMA flexural strain level below the EL, the fatigue life of the mixture can be extended significantly. This paper uses two common failure criteria, the traditional beam fatigue criterion and the simplified viscoelastic continuum damage model energy-based failure criterion (the so-called G^R method), to evaluate the effect of different parameters, such as reclaimed asphalt pavement (RAP) content, binder content, binder modification and warm mix asphalt (WMA) additives, on the EL value. In addition, both failure criteria are employed to investigate the impacts of these parameters in terms of the fatigue life of the study mixtures. According to the findings, unlike an increase in RAP content, which has a negative effect on the mixtures’ fatigue resistance, a higher binder content and/or binder modification can significantly increase the EL value and extend the fatigue life as was proved before by other researchers, whereas WMA additives do not significantly affect the mixtures’ fatigue behaviour. A comparison of the model simulation results with the field observations indicates that the G^R method predicts the field performance more accurately than the traditional method.     

Optimisation of hot mix asphalt performance based on aggregate selection

Researchers over the last four decades have identified and demonstrated the effects of aggregate morphological properties (particularly shape, size distribution, angularity and texture) on the mechanical properties of hot mix asphalt (HMA). Rare studies, however, have clearly established the relationships between the aggregate properties and pavement performance. Therefore, they have not provided methods to optimise aggregate properties at the design stage to improve that performance. This study focuses on understanding the effects of aggregate gradation and type on moisture damage resistance of HMA and on pavement performance as indicated by stiffness and rutting. Results show that basalt aggregate achieves higher moisture susceptibility resistance and stiffness than limestone aggregate. Coarser gradation has the highest permanent deformation, while open gradation 2C provides the lowest moisture damage resistance. Furthermore, dense gradation 4C provides the lowest rutting and the highest stripping resistance. It is indicated that suitable selection of aggregate type and gradation can improve pavement performance and reduce the moisture damage problem of HMA.     

Effects of long-term aging on moisture sensitivity of foamed WMA mixtures containing moist aggregates

Long-term aging of an asphalt mixture is complicated, but can be simulated in the laboratory. The objective of this study was to investigate the influence of long-term aging on moisture susceptibility of foamed warm mix asphalt (WMA) mixtures containing moist aggregate. Weight loss, indirect tensile strength (ITS) of dry and conditioned specimens, and deformation (flow) were measured for all mixtures. The experimental design included two aggregate moisture contents (0 and ~0.5% by weight of the dry mass of the aggregate); two lime contents (1 and 2% lime by weight of dry aggregate) and one liquid anti-stripping agent (ASA); one foaming WMA additive (Asphamin) and two foaming water contents (2 and 3%); and two aggregate sources. A common long-term aging procedure was used in this study. A total of 64 mixtures were evaluated and 256 specimens were made and tested in this study. The test results indicated that long-term aging improved the moisture resistance of WMA mixtures regardless of the ASA and moisture conditioning. In addition, aggregate source significantly affected the moisture resistance regardless of the foaming technology, ASA, and aggregate moisture content. The mixture with various hydrated lime contents exhibited similar moisture resistance under dry and wet conditions. The liquid ASA used in the WMA mixture showed a weaker resistance to the moisture damage in comparison with hydrated lime.     

Numerical analysis of moisture vapor diffusion in asphalt mixtures using digital images

The presence of moisture in asphalt mixtures is detrimental to their performance, e.g., softening the asphalt binder and weakening the aggregate-binder bond. One of the mechanisms of moisture transport, and the focus of this study, is molecular diffusion. Moisture diffusion occurs in response to a concentration gradient. The objective of this study was to estimate the diffusion coefficient of moisture vapor in asphalt mixtures by using finite element (FE) and finite difference (FD) numerical algorithms that employ digital images to discretize the composite. X-ray computed tomography was used to characterize the microstructure of laboratory-prepared specimens and provide the required three-dimensional digital images, which were segmented into three phases: air voids, a mixture of asphalt binder and the fine aggregate fraction, and coarse aggregates. Individual diffusion coefficients were assigned to each phase and the effective diffusion coefficient for the composite was computed using the numerical algorithms. The outcome was compared against experimental values. The effective diffusion coefficient for the asphalt mixtures obtained using the FD method showed closer agreement with the experimental data, while the FE results overestimated the experimental measurements in all cases.     

Effects of recycled asphalt pavements on the fatigue life of asphalt under different strain levels and loading frequencies

Fatigue lives of Hot Mix Asphalt (HMA) and binder have been studied separately for a long time. However, fatigue lives of HMA containing Recycled Asphalt Pavement (RAP) and the binder extracted from the same HMA containing RAP have not been studied yet. This study examines the effects of RAP, loading frequency and strain level on the fatigue lives of asphalt mixtures and binders. In addition, the relationship between the fatigue lives of asphalt mixture and binder is determined. Beam fatigue tests were conducted to determine the fatigue behaviors of two asphalt mixtures: one with 35% RAP and the other without RAP. To evaluate binder’s fatigue behavior, binders were extracted and recovered from these two mixtures. Then, fatigue lives of these two binders were determined using time sweep and Linear Amplitude Sweep (LAS) tests. Results show that presence of RAP in mixture causes a decrease in the mixture’s fatigue life, whereas it causes an increase in the fatigue life of binder. As expected, an increase in loading frequency results in an increase in the fatigue lives of asphalt mixture as well as binder. In addition, increase in strain level causes a decrease in the fatigue lives of both mixtures and binders. Fatigue lives of binders from time sweep and LAS tests show a good correlation with the mixture’s fatigue life by the beam fatigue test.     

Influence of different sources of microstructural heterogeneity on the degradation of asphalt mixtures

The response and degradation of the hot mix asphalt (HMA) materials used in pavement structures are affected by their inherent heterogeneity. The objective of this work is to study the impact of two different sources of HMA heterogeneity in the uncertainty of the mechanical moisture degradation of HMA. The first source of heterogeneity is the spatial variability of the properties of the bulk fine aggregate matrix (FAM) of the mixture, and the second is the location and shape of the coarse aggregate particles. The heterogeneity of the bulk FAM phase was modelled using a random field technique, while that of the coarse aggregates was accounted for by randomly generating realistic probable sets of aggregate particles. Thus, ‘computational replicates’ of HMA microstructures were generated and subjected to moisture diffusion and mechanical loading using a finite element approach. In the mechanical simulations, a non-linear viscoelastic moisture damage constitutive relationship based on continuum damage mechanics theory was selected to characterise the response of the bulk FAM phase. The results show that conducting computational simulations with realistic HMA microstructures that properly capture the heterogeneity of the material is useful to quantify the mean values and dispersion (i.e. uncertainty) associated with the response and degradation of the mixture. This information, which cannot be easily obtained in the field or in the laboratory due to the difficulty of acquiring a sufficient amount of data, is useful to conduct structural reliability analysis and to predict the life cycle behaviour of the material.     

Evaluation of the performance of warm mix asphalt in Washington state

Warm mix asphalt (WMA) is a relatively new and emerging technology for the asphalt industry. It offers potential construction and environmental advantages over traditional hot mix asphalt (HMA). However, WMA must perform at least as well as HMA before it can be used extensively. This study evaluates the performance of WMA mixtures and their corresponding HMA control mixtures obtained from various field sites in the state of Washington. Four WMA technologies are examined, including Sasobit^® and three water-foaming technologies, Gencor^®, Aquablack? and ALmix Water Injection. Performance tests are conducted on the field cores to evaluate and compare the rutting, moisture susceptibility, fatigue and thermal resistance of WMA and HMA, respectively. Also, the extracted binders from the field cores are evaluated. In addition, the early-age field performance of WMA and HMA control pavements are compared.     

The effect of aggregate gradation on creep and moisture susceptibility performance of warm mix asphalt

It is clear that the purpose of mixture design is to select optimum asphalt content for a desired aggregate structure to meet the prescribed criteria. Aggregate makes up high proportion of volume and mass of mixtures; hence, it is considered as an important constituent of asphalt concrete. This study postulates that the gradation is an important characteristic of the aggregate in adoption of the optimum mixture. One aggregate source, three gradations and different percentages of Sasobit^® was used to manufacture hot mix asphalt and warm mix asphalt. The test results indicated that the aggregate gradation affects the rutting resistance and especially the moisture susceptibility of the introduced mixtures, differently. Rutting resistance was evaluated using the flow number parameter, and in order to determine the moisture sensitivity mechanism, a mechanical and visual inspection tests were carried out. At the end, it is concluded that the optimum aggregate gradation for these two types of mixtures is different.     

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.     

Examination of moisture sensitivity of aggregate–bitumen bonding strength using loose asphalt mixture and physico-chemical surface energy property tests

In this study, the moisture sensitivity of different kinds of aggregates and bituminous binders is examined by comparing the performance between five empirical test methods for loose mixtures – static immersion test, rolling bottle test (RBT), boiling water test (BWT), total water immersion test and the ultrasonic method – with more fundamental surface energy-based test data. The RBT and BWT results showed that limestone aggregates perform better than granite aggregates and that, for unmodified binders, stiffer binders provide better moisture resistance compared with softer binder. Both tests were sensitive to aggregate type, binder type and anti-stripping agent type. Ranking of the mixtures by RBT and BWT was in general agreement with the surface energy-based tests, especially for mixtures that performed worst or best in RBT and BWT. The magnitude of the work of debonding in the presence of water was found to be aggregate type dependent which suggests the physico-chemical properties of aggregates may play a fundamental and more significant role in the generation of moisture damage, than bitumen properties.     

Laboratory investigation of utilizing high percentage of RAP in rubberized asphalt mixture

The utilization of crumb rubber and reclaimed asphalt pavement (RAP) has proven to be economical, environmentally sound and effective in increasing the performance properties of the asphalt mixtures. The objective of this research was to investigate the laboratory engineering behavior characteristics of the rubberized asphalt binders and mixtures made with PG 64-22 and a softer binder (PG 52-28) containing a high percentage of RAP (30%). Some of the testing used for this research included viscosity, dynamic shear rheometer (DSR), bending beam rheometer (BBR), indirect tensile strength (ITS), resilient modulus, and fatigue life evaluations. The experimental design included the use of two aggregate and RAP sources, two virgin binder grades (PG 64-22 and PG 52-28), two types of crumb rubber (ambient and cryogenic), and four rubber contents (0%, 5%, 10%, and 15%). The results indicated that: (1) the crumb rubber improved the aging resistance of the aged binder and prolonged the fatigue life of the mixtures containing 0% RAP, in addition, results indicated a decrease of ITS and resilient modulus values was found as the rubber content increased, regardless of rubber type; (2) the utilization of softer binder decreased the influence of aged binder and decreased the resilient modulus values of the mixtures. In most cases, regardless of rubber types, the rubberized mixtures containing 30% RAP made with PG 52-28 binder did not show a significant increase in fatigue life with those made with PG 64-22 binder.     

Reconsideration of the fatigue tests for asphalt mixtures and binders containing high percentage RAP

When applying reclaimed asphalt technology in a flexible pavement project, most performance concerns are related to low temperature and fatigue cracking since the stiffness of the HMA mixture could dramatically increase through adding a high percentage of reclaimed asphalt pavement (RAP) material. The purpose of this study is to evaluate asphalt mixtures with high RAP contents, prepared using two RAP addition methods, for their performance based on fatigue-cracking resistance rather than relying on volumetric properties. Asphalt mixture samples were prepared with three RAP binder content replacement percentages (30, 40 and 50%) using two preparation methods: the as-is RAP gradation (traditional method) and the splitting of the RAP gradation into coarse and fine fractions (fractionated method). Asphalt mixture beam fatigue and binder fatigue time-sweep tests were performed. Beam fatigue samples also underwent freeze–thaw cycling for freeze–thaw damage evaluation. Rather than basing the performance based solely on S–N_f curves to illustrate the fatigue performance, the beam fatigue test data was analysed through a dissipated energy approach. Faster fatigue degradation was observed for the 40% RAP binder and beam mixture when subjected to repeated loading. From a morphology aspect, this can be explained by the binder’s phase separation and physical hardening effects.