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.     

Performance evaluation of warm mix asphalt containing reclaimed asphalt mixtures

The objective of this study is to expose the effect of a variety of variables including three reclaimed asphalt pavement (RAP) contents, two warm mix asphalt (WMA) additives and a rejuvenating agent (or lack of) on the performance of WMA containing (WMA–RAP) materials. A laboratory study was conducted to evaluate the performance of WMA–RAP mixtures through rutting, bending and freeze-thaw splitting tests. Analysis of variance (ANOVA) was performed to analyse the significant effect of the variables on the performance. The tests results showed that the increased RAP content led to an increased rutting resistance and the decreased resistance to low-temperature cracking and moisture damage. The addition of the rejuvenating agent into the WMA–RAP mixtures can significantly improve the low-temperature cracking and moisture resistance. The ANOVA results showed that the RAP content had a significant effect on the rutting and low-temperature cracking resistance, and moreover, the rejuvenating agent (or lack of) had a large effect on the low-temperature cracking and moisture resistance.     

Effect of warm mix asphalt chemical additives on the mechanical performance of asphalt binders

Warm mix asphalt (WMA) has become very popular in asphalt pavement construction because it allows reducing both energy consumptions and carbon emissions. WMA can be obtained by using different types of additives and can be produced, applied, and compacted at temperatures 20–40 °C lower than hot mix asphalt. WMA additives allow reducing the working temperatures without compromising the final performance of the asphalt concrete. Many WMA additives are available on the worldwide market and some of them reduce the viscosity of asphalts binder (organic additives or foam) whereas others do not act on this sense (chemical additives). This study focuses on the effect of chemical additives on the performance of asphalt binders for WMA production. To this purpose, a neat bitumen, a polymer modified bitumen (PMB) and two different chemical additives were selected. All the binders were characterized through conventional tests, DSR, MSCR, FTIR and microscopic analysis. The result clearly showed that the influence of the chemical additives on the neat bitumen is negligible or non-existent. On the contrary, significant changes were observed in the modified bitumen properties. Specifically, chemical additives reduce the viscosity temperature susceptibility of PMBs in the temperature range between 80 and 140 °C, increase the rutting resistance potential and the elastic response of PMBs at high temperatures. Moreover, a morphological inspection supported the modifications observed in the rheological properties of PMBs.     

Field performance of top-down fatigue cracking for warm mix asphalt pavements

As a result of repeated rehabilitation efforts over the past few decades, often asphalt pavements have become deep-strength pavements. Consequently, top-down cracking has become a primary distress type. In particular, the top-down cracking performance of warm mix asphalt (WMA) pavements, i.e. how does it compare with similar hot mix asphalt (HMA) pavements is largely unclear mainly due to the lack of field performance data. This paper presents an effort of monitoring the top-down cracking performance of 28 pavement projects including WMA pavements and their corresponding HMA control pavements with service lives ranging between 4 and 10 years. These pavements cover different climate zones, WMA technologies, service years, pavement structures and traffic volume levels. Two rounds of distress surveys were conducted at a two-year interval, and the material (asphalt binder and mixture) properties of the pavements were determined using field cores. The top-down cracking performance of the HMA and WMA pavements was compared based on the first and second round distress surveys. It was found that the HMA and WMA pavement in general exhibited comparable performance. The significant determinants (material properties) for top-down cracking were determined, which were vertical failure deformation of mixes measured at 20 °C from indirect tension test.     

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.     

Impact of nanosized additives on the fatigue damage behaviour of asphalt mixtures

The study presented in this paper aimed at evaluating the impact of different nanosized additives, including an organophilic nanoclay and multiwall carbon nanotubes, on the fatigue properties of dense‐graded asphalt mixtures. Cyclic direct tension fatigue tests were carried out, and the corresponding results were interpreted by means of a simplified version of the visco‐elastic continuum damage model. The experimental investigation also included linear viscoelastic characterization of the considered materials. Results derived from tests carried out on the mixtures containing nanosized additives were compared with those obtained for a reference standard mixture. It was found that the use of the abovementioned additives can give a substantial contribution to the enhancement of the fatigue damage resistance of asphalt mixtures. Moreover, when comparing the two types of additives, it was observed that organophilic nanoclays can outperform multiwall carbon nanotubes.     

An overview of the emerging warm mix asphalt technology

The asphalt industry is constantly attempting to reduce its emissions as concerns are growing on global warming. This is done by decreasing the mixing and compaction temperatures of asphalt mixtures without affecting the properties of the mix which is possible through numerous available technologies in the industry. The production of asphalt mix is done by warm mix asphalt (WMA) technology at considerably lower temperatures (120°C or lower). Less energy consumption, lower mixing and compaction temperatures, early site opening, reduced ageing, fewer emissions, cool weather paving, better workability and, finally, an extended paving window could be mentioned as some of the benefits obtained by using the WMA. This paper presents the WMA techniques and technologies such as foaming techniques, wax and chemical additives techniques. Additionally, the performance of WMA popular technologies such as Sasobit^®, WAM^®-Foam, Evotherm^®, Low energy asphalt, Rediset^® WMX and REVIX? are fully described.     

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.     

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.     

温拌剂种类及掺量对不同沥青流变性能的影响

为了研究温拌剂种类及掺量对沥青流变性能的影响,通过动态剪切流变仪对分别掺加RH和Evotherm温拌剂的SBS改性沥青和基质沥青进行温度扫描试验,分析了它们的复数剪切模量、相位角和车辙因子。结果表明,Evotherm温拌剂在28~52℃能够提升两种沥青抗车辙性能,52℃后效果减弱;并且在28~40℃间可以提高两种沥青弹性恢复性能,40℃后会使SBS改性沥青弹性恢复性能减弱,对基质沥青无影响。增加Evotherm温拌剂掺量,在28~52℃会使温拌SBS改性沥青抗车辙性能提高,使温拌基质沥青抗车辙性能有所减弱,52℃后影响不明显。RH温拌剂在28~46℃对两种沥青抗车辙性能略有不利,但负面作用随温度升高逐渐减弱甚至消失;RH温拌剂能够显著增大两种沥青高温时弹性恢复性能。RH温拌剂掺量增大会使两种沥青抗车辙性能降低,但会使弹性恢复性能增强。     

Finite element modelling of field compaction of hot mix asphalt. Part I: Theory

This paper presents the theoretical background for the development of a constitutive model that is used in the simulation of the compaction of asphalt mixtures. The constitutive model is developed to comply with the principles of thermodynamics, and is derived to represent the macroscopic behaviour of an asphalt mixture as a highly compressible viscoelastic material. The paper presents the details of the mathematical formulation and the computational implementation of the model in the finite element package computer-aided pavement analysis 3D. The capabilities of the compaction model and its sensitivity to changes in model's parameters are illustrated using simple numerical applications. In a companion publication, (Masad et al., Finite element modelling of field compaction of hot mix asphalt. Part II: Application, International Journal of Pavement Engineering, Accepted, 2014), the model is verified against field compaction measurements which demonstrate the ability of the model to capture the general trends of the compaction observed in the field.     

Comparison between the fatigue response of hot and warm mix asphalts based on the dissipated energy approach

This study focused on the fatigue behaviour of warm mix asphalt (WMA) based on the dissipated energy (DE) approach. Two conventional binders consisting of 60/70 and 85/100 penetration-grade bitumens were used to prepare the control mix. WMA was prepared by incorporating 2% Sasobit by weight of bitumen. First, the basic properties of WMA containing optimum bitumen content were compared with those of control hot mix asphalt (HMA). The main laboratory programme included four-point flexural fatigue test that was accomplished at different strain levels of 250, 500, 750 and 1000 microstrain. The studied WMA had comparable Marshall stability and indirect tensile strength to those of control HMA. Furthermore, based on the resilient modulus test results, the temperature susceptibility of WMA was slightly more than that of the control mix. Fatigue lives of studied mixes were evaluated using the conventional fatigue curves that were developed based on the initial strain level. Comparison between these curves revealed the predominant fatigue behaviour of WMAs at different examined strain levels. The initial DE and the cumulative DE of WMAs were lower than those of HMAs. The latter issue justifies the predominant fatigue response of WMA. By considering the DE curve, the ratio of dissipated energy change (RDEC) was calculated. Afterward, the plateau value (PV) was determined using the moving average of fatigue data in the plateau stage of the RDEC curve. Finally, PV-based fatigue models were developed which could precisely estimate the fatigue life regardless of the mix type and testing condition.     

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.     

Numerical computation in the viscoelastoplastic continuum damage of hot mix asphalt concrete

The objective of this paper is to develop an accurate and advanced material characterization of hot mix asphalt concrete using an existing viscoelastoplastic constitutive model that accounts for rate of loading, temperature and stress state with growing damage. The modelling strategy of viscoelastoplastic continuum damage is based on modelling strain components separately and then combining the resulting models to obtain a final integrated viscoelastoplastic model. According to this model, the initial-boundary value problem is numerically solved using the constitutive relationship expressed in the convolution integral form. The model is successful in predicting responses up to localization when microcracks start to coalesce.     

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.     

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 characterisation of asphalt mixes containing RAP and RAS

Due to its economic and environmental benefits, using reclaimed asphalt pavement (RAP) and reclaimed asphalt shingles (RAS) in new hot-mix asphalt (HMA) has become an integral part of today's asphalt industry. The advantages of using RAP and RAS in HMA are not limited to economic and environmental benefits, and may result in improving a number of mix performance characteristics including rutting and resistance to moisture-induced damage. Despite aforementioned benefits, concerns over premature pavement distresses resulting from using RAP and RAS limit their usage in HMA. Furthermore, because of the lack of mechanistic performance data, use of new mixes containing RAP and RAS remains limited. Therefore, the present study was undertaken to investigate the effects of using different amounts of RAP and RAS on laboratory performance of HMA, and to generate valuable input design parameters for implementation of the mechanistic-empirical pavement design guide (M-EPDG), using local materials. Four types of base course mixes containing 0% RAP, 25% RAP, 40% RAP and 20% RAP+5% RAS, and three types of surface course mixes containing 0% RAP, 25% RAP and 20% RAP+5% RAS were tested. Laboratory tests were conducted to evaluate stiffness, low-temperature cracking, fatigue life, rut and moisture-induced damage potential of the mixes. It was found that dynamic modulus and creep compliance of the asphalt mixes increase and decrease, respectively, with an increase in the amount of RAP and/or RAS used in the mix. Fatigue life was found to increase with increasing RAP content up to 25%, and to decrease when the RAP and/or RAS content exceeded 25%, or when RAS was used in the mix. It should be noted that this conclusion was drawn based on a 15% increment in RAP content. Hamburg wheel tracking (HWT) test results showed increased resistance to rutting and moisture-induced damage, with an increase in the amount of RAP and/or RAS. However, the tensile strength ratio test results were not confirmed by HWT. The findings of this study are expected to be helpful in understanding the effects of using different amounts of RAP and RAS on the performance of asphalt mixes produced using local materials. Furthermore, valuable design input parameters, developed in this study for new mixes containing RAP and RAS, may be used for calibration of the M-EPDG input parameters, with local materials.     

老化对改性沥青微观结构及疲劳性能的影响

为了研究老化对改性沥青微观结构及疲劳性能的影响,通过沥青常规指标测试试验、动态剪切流变（DSR）试验和AFM测试试验,从宏细观角度分析了老化前后苯乙烯-丁二烯-苯乙烯三嵌段共聚物（SBS）改性沥青和胶粉改性沥青的针入度、延度和软化点及疲劳性能、微观结构的变化情况。结果表明：随着老化程度的加深,两种改性沥青的稠度、硬度增加,高温性能得到改善,而温度敏感性变低,低温抗裂性能变差。综合分析针入度比、延度比和软化点比得出胶粉改性沥青的抗老化性能优于SBS改性沥青;随温度的降低、频率的增大和老化程度的加深,两种改性沥青的抗疲劳性能变差;微观结构观测结果显示,SBS改性沥青具有"蜂型结构",老化后"蜂型结构"的体积增大、高度增加、数量减少;胶粉改性沥青没有"蜂型结构",胶粉颗粒与沥青在共混共融过程中发生溶胀、脱硫和降解等行为会影响沥青中"蜂型结构"的形成;老化前后胶粉改性沥青表面形貌粗糙度和高度变化不大;胶粉改性沥青的抗疲劳、抗老化和高温性能总体优于SBS改性沥青。     

基于劲度模量分析的橡胶沥青混合料疲劳寿命研究

变幅加载下沥青的疲劳损伤累积具有明显的非线性特征,传统的Miner’s线性疲劳损伤累积准则无法表征不同变幅加载次序下沥青的非线性疲劳损伤累积(NLFDA)。该研究旨在建立考虑加载次序影响的NLFDA模型,准确表征加载次序对沥青疲劳损伤累积的影响。通过开展应力控制的沥青恒幅加载疲劳试验,采用耗散伪应变能(DPSE)表征沥青疲劳损伤,分析恒幅加载下沥青的疲劳损伤累积规律;采用低-高和高-低两种加载次序,开展应力控制的沥青变幅加载疲劳试验,分析变幅加载下沥青的疲劳损伤累积规律;基于损伤等效准则,建立考虑加载次序影响的NLFDA模型,分析加载次序对疲劳损伤累积的影响。结果表明：应力控制模式下的沥青疲劳损伤,呈先缓慢后急剧的非线性增加演化趋势;恒幅加载下沥青疲劳损伤服从Miner’s准则发生线性累积,且累积寿命分数等于1;变幅加载下沥青疲劳损伤不服从Miner’s准则而发生非线性累积。低-高和高-低变幅加载次序下,沥青累积疲劳寿命随一级寿命分数的增大而分别增加和减小,累积寿命分数分别大于1和小于1;建立的NLFDA模型可克服Miner’s准则缺陷,并较为准确地表征加载次序对沥青疲劳损伤累积的影响。

     

Fatigue resistance investigation of warm‐mix recycled asphalt binder,mastic, and fine aggregate matrix

Fatigue cracking is one of the primary distresses in warm‐mix recycled asphalt pavements. This paper evaluates the fatigue resistance evolution of warm‐mix recycled asphalt materials in different scales during the service period. The strain sweep test and time sweep test were performed, respectively, by dynamic shear rheometer to determine the linear viscoelastic limits and to characterize the fatigue behavior of warm‐mix recycled asphalt binder, mastic, and fine aggregate matrix with different ageing levels and recycling plans. The dissipated energy method was used to define the failure criterion and to construct the fatigue model. Effects of ageing levels and recycling plans on stiffness and fatigue resistance were investigated. Performance correlations among warm‐mix recycled asphalt binder, mastic, and fine aggregate matrix were developed, respectively, by the statistical method to determine the critical material scale for stiffness and fatigue resistance.