Prediction models of mixtures’ dynamic modulus using gene expression programming

The dynamic modulus (E*) among asphalt mixtures’ mechanical property parameters not only is important for asphalt mixtures’ pavement design but also in determining asphalt mixtures’ pavement performance associated with pavement response. Based on the principle of gene expression programming (GEP) algorithm, this paper explored two different GEP approach models, namely: GEP-I and GEP-II to predict the E* of hot mix asphalt (HMA) and mixtures containing recycled asphalt shingles, respectively. In this paper, The GEP-I was developed from a large database containing 2750 test data points from 205 unaged laboratory-blended HMA mixtures including 34 modified binders, and the GEP-II model was developed using the E* database containing 1701 sets of experimental data from 4 different demonstration projects. Both the GEP-I model and GEP-II model were compared with other E* prediction models. A sensitivity analysis of each model parameter was conducted by correlating these parameters with dynamic modulus. Both the GEP-I model and GEP-II model showed significantly higher prediction accuracy compared with the existing regression models and could easily be established. It is expected that these two GEP models could lead to more accurate characterisation of the asphalt mixtures’ E*, resulting in better performance prediction.     

Emergency paving using hot-mixed asphalt incorporating warm mix technology

This paper presents results of a study on hot-mixed and warm-compacted asphalt incorporating warm mix technologies for use in emergency construction following a natural disaster. Case studies were first reviewed to investigate feasibility of the concept. Next, an overall emergency paving framework was developed, complemented by a two-component laboratory investigation. Component one developed a series of short-term ageing protocols for use in preparation of test specimens; component two evaluated those specimens for compactability and rut resistance. Results indicated that (1) material could be hauled up to 6 h and still be effectively used in emergency paving, (2) the two warm mix additives studied improved compactability of hot-mixed and warm-compacted asphalt and (3) rut resistance was acceptable for emergency applications. A discussion on the post natural disaster permanent residual value of the hot-mixed and warm-compacted material is also provided.     

基于表面能理论的老化温拌SBS改性沥青结合料的粘附性

在SBS改性沥青中添加Sasobit与Evotherm温拌剂制备得到温拌沥青,对3种SBS改性沥青进行旋转薄膜烘箱老化(RTFOT)与压力老化(PAV),采用接触角法检测了老化前后温拌SBS改性沥青与蒸馏水、甘油和甲酰胺等3种液体的接触角,探讨了短期老化与长期老化对温拌SBS改性沥青接触角的影响。基于表面自由能理论,得到了温拌SBS改性沥青的表面能及其色散分量和极性分量,研究了老化对温拌SBS改性沥青表面能的影响。结果表明:随着老化程度的加强,温拌SBS改性沥青与水的接触角逐渐增大,疏水性变化明显;原样与Evotherm温拌SBS改性沥青的总表面自由能及其分量均随着老化程度的加强呈降低趋势,其在短期老化阶段的表面自由能下降较为严重;经长期老化,Sasobit温拌SBS改性沥青的极性分量有增大趋势。     

Influence of selected WMA additives on viscoelastic behaviour of asphalt mixes and pavements

Warm mix asphalt additives are effective in decreasing production, laying and compaction temperatures of asphalt mixes. However, there are still questions concerning influence of warm mix additives on properties of asphalt mixes and pavement performance. This paper presents results of the comprehensive research of viscoelastic behaviour of asphalt mixes and pavement structures with layers made with warm mix asphalt additives at high temperatures. Two additives of significantly different effects on mixes at higher temperatures were selected for analysis, namely aliphatic synthetic wax produced with the use of Fisher–Tropsch method and formulation of surfactant- based molecules (ionic and non-ionic). Viscoelastic properties of mixes with these two additives and, as a reference mix, with neat unmodified asphalt binder were determined in uniaxial compression with sinusoidal loading using Asphalt Mixture Performance Test. The viscoelastic analysis of pavement structures was performed with use of the VEROAD software and data from laboratory testing. Two different pavement structures were analysed, for light and heavy traffic. The temperature distribution in pavement structure during the hottest summer day in northern Poland in 2012 was taken into account. The model of pavement was loaded with moving wheel at different speeds. The analysis has shown that two tested warm mix additives had different effect on viscoelastic transient response at high temperatures. One of them (Fischer–Tropsch wax) evidently caused an increase in resistance of asphalt mix and pavement structure to loading at high temperature. The second additive (formulation of surfactant-based molecules) slightly reduced resistance of asphalt mix and pavement to loading at high temperatures as compared with the reference mix.     

沥青混合料抗剪性能研究综述

总结并评价了沥青混合料抗剪试验方法、抗剪机理及抗剪设计研究现状,对9种剪切试验方法的分析和评价有助于设计、施工和科研人员正确选用这些试验方法。混合料剪切强度由沥青胶泥的性质、集料和沥青界面的剪切强度和集料之间的嵌挤情况共同决定,为提高混合料抗剪性能指出了方向。抗剪设计在应用于工程前尚需深入研究容许剪应力法中相关系数的确定、基于抗剪性能的车辙预估模型的标定等问题。     

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.     

Verification of the new viscoelastic method of thermal stress calculation in asphalt layers of pavements

The new viscoelastic method of thermal stress calculations in asphalt layers has been developed and published recently by the author. This paper presents verification of this method. The verification is based on the comparison of the results of calculations with results of testing of thermal stresses in Thermal Stress Restrained Specimen Test. The calculations of thermal stresses according to the new method were based on rheological parameters of the Burgers model. The parameters were measured in laboratory at different low temperatures, at long time creep under constant loading. Five asphalt mixes were tested. Three of them were high modulus asphalt concretes and two conventional asphalt concretes. Specimens were prepared in exactly the same way both for rheological creep tests and for the Thermal Stress Restrained Specimen Test. The results of measured thermal stresses were compared with thermal stresses calculated from the new viscoelastic method developed by the author and in most cases a good agreement was found. For comparison, the measured stresses were compared with results of calculations according to the existing methods. The viscoelastic Monismith method failed in prediction of thermal stresses. The prediction from the quasi-elastic Hills and Brien method was underestimated, but better than from the Monismith method and worse than from the new viscoelastic method. The reasons of discrepancies were discussed.     

A Finite-Element Approach to Predict Permanent Deformation Behaviour of Hot Mix Asphalt Based on Fundamental Material Tests and Advanced Rheological Models

Abstract:  An ongoing research project, undertaken at the Christian Doppler Laboratory for 'performance-based optimisation of flexible road pavements', focuses on the evaluation of advanced rheological models (i.e. Power Law, Huet and Huet–Sayegh) to describe permanent deformation behaviour of hot mix asphalt (HMA) and their implementation in a finite-element (FE) code. To accomplish this, an appropriate algorithm was developed for the summation of the different contributions of the stress history to determine the resulting creep strain tensor to simulate real-life stress/strain situations in flexible road pavements. Furthermore, the mathematical background for parameter identification from dynamic stiffness tests (four-point bending beam and dynamic tension compression tests) has been developed and a straightforward programme for data fitting is presented. For the validation of the implemented constitutive equations derived for the selected rheological models, a FE simulation of triaxial tests on cylindrical HMA specimens was carried out.     

Fracture mechanics of idealised bituminous mixes

The durability of asphalt pavements is strongly impaired by cracks, caused primarily by traffic loads and environmental effects. In this work, fracture behaviour of idealised asphalt mixes is investigated. Experiments on idealised asphalt mixes under pure-tension mode (mode I cracking) were performed and fracture parameters were evaluated. In these three-point bend fracture tests, the test variables were temperature and load rate. The test data were stored in an asphalt materials database and special-purpose tools were implemented to analyse and handle the laboratory data automatically. Fracture mechanism maps were constructed, showing the conditions associated with ductile, brittle and ductile–brittle transition regimes of behaviour. The mechanism maps show the failure response of the material in terms of the stress intensity factor, strain energy release rate and J-integral as a function of the temperature-compensated crack mouth opening strain rate. Fracture behaviour of asphalt mix specimens was simulated by cohesive zone model in conjunction with a novel material constitutive model for asphalt mixes. The finite element model agrees well with the experimental results and provides insights into fracture response of the notched asphalt mix beam specimens.     

δ25 Crack opening displacement parameter in cohesive zone models: experiments and simulations in asphalt concrete

Recent work with fracture characterization of asphalt concrete has shown that a cohesive zone model (CZM) provides insight into the fracture process of the materials. However, a current approach to estimate fracture energy, i.e., in terms of area of force versus crack mouth opening displacement (CMOD), for asphalt concrete overpredicts its magnitude. Therefore, the δ₂₅ parameter, which was inspired by the δ₅ concept of Schwalbe and co‐workers, is proposed as an operational definition of a crack tip opening displacement (CTOD). The δ₂₅ measurement is incorporated into an experimental study of validation of its usefulness with asphalt concrete, and is utilized to estimate fracture energy. The work presented herein validates the δ₂₅ parameter for asphalt concrete, describes the experimental techniques for utilizing the δ₂₅ parameter, and presents three‐dimensional (3D) CZM simulations with a specially tailored cohesive relation. The integration of the δ₂₅ parameter and new cohesive model has provided further insight into the fracture process of asphalt concrete with relatively good agreement between experimental results and numerical simulations.     

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.     

Fatigue resistance potential for hot mix asphalt using viscoelastic continuum damage analysis

This paper describes testing and evaluation of the fatigue resistance potential of hot‐mix asphalt mixtures using viscoelastic continuum damage analysis, which is based on dynamic modulus determination, a state‐variable approach and damage calculation. The dynamic modulus test for stiffness characterization and the direct tension test for fatigue resistance characterization were used in the testing procedure. The state‐variable approach can be used for numerical computation of a viscoelastic convolution integral. A Nelder–Mead simplex search was used in this study to determine the damage parameter of a stiffness reduction function. The fatigue resistance was evaluated as a function of loading rate, asphalt binder content, modifier (e.g. usage of hydrated lime), and temperature, and was found experimentally to have a strong dependence on these factors.     

环氧沥青混凝土桥面铺装材料研究与应用进展

随着桥梁设计理念、结构分析、施工技术等不断更新发展与完善,目前桥梁正逐渐朝着大跨度、高强度、长寿命、高耐久等方向发展,同时,未来桥梁也将面临更复杂的建设环境、更多的功能需求,因此必须加强研发与新型桥梁设计体系相匹配的特种桥面铺装材料。环氧沥青混凝土桥面铺装材料以高强度、耐高温、抗疲劳、抗老化等优异路用性能脱颖而出,逐渐受到关注。然而,环氧沥青混凝土制备工艺复杂、施工条件严苛、耐久性不足等问题日益凸显,这些缺点使得其在桥面铺装领域的推广受到一定阻碍。为此,研究者们针对如何提升环氧沥青混凝土使用品质及耐久性进行了深入研究并取得了一定成果。这一系列成果先后在大量桥面铺装实体工程中得以应用,良好的使用效果也为环氧沥青混凝土的进一步推广奠定了基础。环氧沥青混凝土在桥面铺装领域的研究成果可以概括为三个方面:铺装结构组合优化、制备工艺优化、混凝土原材料优化。其中铺装结构组合从早期的单质单层结构逐渐过渡到单质双层、异质双层结构,趋于合理的铺装结构组合使环氧沥青混凝土材料的性能得以充分发挥。制备工艺从热拌法发展到温拌甚至冷拌,在保证环氧沥青混凝土使用性能的同时减少了对环境的污染,并且在一定程度上降低了施工难度。而在原材料应用方面,研究者们不断对双组分环氧沥青与三组分环氧沥青的性能进行对比分析;同时,环氧沥青的改性方式趋于多样化,从单独使用改性沥青发展到同步使用改性环氧树脂,改性剂从纤维发展到高分子聚合物、超支化聚合物等。这些措施不仅改善了沥青与环氧树脂的相容性,也增强了环氧沥青混凝土的相关性能。此外,为使环氧沥青混凝土具有更好的稳定性,在级配优化方面也进行了深入研究。然而目前环氧沥青混凝土的一系列研究成果较为散乱,缺乏对其系统的总结与梳理,且环氧沥青混凝土的性能评价指标及要求仍需深入研究与完善。为进一步确定环氧沥青混凝土桥面铺装材料科学合理的性能评价指标及要求,本文全面梳理了国内外环氧沥青混凝土相关规范,系统调查了大量实体工程及研究动态,对比分析了不同主要原材料的环氧沥青混凝土对其路用性能的影响,最终推荐了环氧沥青混凝土桥面铺装结构组合、原材料类型、级配范围和性能评价指标及要求,为环氧沥青混凝土桥面铺装材料规范完善与质量控制奠定了基础。     

沥青砂混合料粘弹塑力学特性研究

在0.1MPa、0.15MPa、0.2MPa、0.25MPa和0.3MPa下进行了沥青砂试样单轴压缩和蠕变实验,分析了其压缩和蠕变性质,根据变形机理提出了粘弹塑本构模型可由粘弹性和粘塑性的两个子模型串联构成,通过对粘塑性子模型中粘性系数进行改进,理论推导了模型蠕变本构方程,确定了模型参数,并求得模型参数与加载应力函数关系。进行模型预测与实验结果对比,结果表明:该模型能够描述沥青砂试样在不同应力下蠕变变形的3个阶段,反映了沥青砂混合料粘弹塑变形特点。     

Nonlinearly viscoelastic analysis of asphalt mixes subjected to shear loading

This study presents the characterization of the nonlinearly viscoelastic behavior of hot mix asphalt (HMA) at different temperatures and strain levels using Schapery’s model. A recursive-iterative numerical algorithm is generated for the nonlinearly viscoelastic response and implemented in a displacement-based finite element (FE) code. Then, this model is employed to describe experimental frequency sweep measurements of two asphalt mixes with fine and coarse gradations under several combined temperatures and shear strain levels. The frequency sweep measurements are converted to creep responses in the time domain using a phenomenological model (Prony series). The master curve is created for each strain level using the time temperature superposition principle (TTSP) with a reference temperature of 40°C. The linear time-dependent parameters of the Prony series are first determined by fitting a master curve created at the lowest strain level, which in this case is 0.01%. The measurements at strain levels higher than 0.01% are analyzed and used to determine the nonlinear parameters. These parameters are shown to increase with increasing strain levels, while the time–temperature shift function is found to be independent of strain levels. The FE model with the calibrated time-dependent and nonlinear material parameters is used to simulate the creep experimental tests, and reasonable predictions are shown.     

Comparison of response for three different composite pavement sections to environmental loads

Composite pavement structures are constructed mainly either as Portland cement concrete (PCC)-over-PCC or hot mix asphalt (HMA)-over-PCC. Several successful in-service projects have been reported in Europe. The design and construction of these sections in the United States, however, still require effort. The current study includes the analysis of the response of three different composite pavement sections to the environmental loads. These sections were constructed in May of 2010 at the Minnesota Road Research Facility. The sections are constructed in three individual cells, Cell 70, a HMA-over-PCC with recycled concrete aggregate (RCA), Cell 71, exposed aggregate concrete (EAC)-over-RCA and Cell 72, EAC-over-economical concrete. All cells were heavily instrumented with thermocouples, moisture sensors, and static and dynamic strain gauges. This study characterises the structural response of HMA-over-PCC pavements and also PCC-over-PCC to the environmental loads.     

The characterisation of foamed asphalt cement using a rotational viscometer

Warm mix asphalt using foaming technology is a widely used alternative to traditional Hot Mix Asphalt in the USA. However, there has been relatively limited research exploring the behaviour of the foamed asphalt cement, especially using traditional asphalt cement testing equipment. This research used the rotational viscometer to develop four new metrics that quantify the behaviour of foamed asphalt cement. These four metrics showed that increasing the foaming temperature increased the observed viscosity, but the initial observed viscosity decreased with lower asphalt binder grades. However, the point at which the observed viscosity crossed the actual viscosity increased with lower asphalt binder grades. Overall, the Wirtgen foamer had higher observed viscosity vs. the PTI foamer and provided a more robust foaming material. However, it is recommended that the AccuFoamer also be explored in future research, along with comparing laboratory produced foamed asphalt cement with field produced foamed asphalt cement.     

Air voids reduction phenomena of asphalt concrete – A continuum approach

Asphalt concrete used in flexible highway pavement construction has 5–8 percent air voids immediately after laying of the roadway. Constitutive laws for asphalt concrete developed till now have modelled the mix as a linear elastic or viscoelastic material and have not taken into account the effect of void concentration on the mechanical behaviour of the material. In this paper, the theory of linear elastic material with voids is used to model asphalt concrete under isothermal conditions. Two cases of void reduction behaviour are studied, one in which the void volume reduces asymptotically under a constant load and the other in which it reaches the refusal air void content. The model is used to predict the creep behaviour under constant compressive stress as well as to obtain the hysteretic stress-strain behaviour. Solutions for the case of uniaxial deformation are derived and the strains are simulated for a constant compressive stress. Use of the air voids reduction measure as a possible damage parameter is also examined. This revised version was published online in July 2006 with corrections to the Cover Date.     

The German segmented steel roller compaction method – state-of-the-art report

Out of the European methods, the steel roller method (EN 12697-33) is preferred in Germany. A segmented roller is used, as specified in the German Standards in detail (acc. to TP Asphalt-StB, part 33; in German only). It is appropriate for laboratory compaction of any hot rolled asphalt mix types (such as asphalt concrete AC, or stone mastic asphalt SMA) either manufactured in the laboratory or in a mixing plant. And it realizes properties that are similar to those of specimen taken from the road. The segmented roller compaction method as described in the German Standards has been developed in the frame of several studies conducted at the Braunschweig Pavement Engineering Centre (see, e.g. Arand & Renken, 1990, 1999; Renken, 2001, 2002).     

Study on the void reduction behaviour of porous asphalt pavement based on discrete element method

The objective of this study was to analyse the void reduction behaviour of porous asphalt mixture under load. A three-dimensional discrete element model of porous asphalt mixture based on aggregate gradation and void gradation was built in PFC3D software. The parameter of the model was obtained from creep test. The rutting test was simulated using this discrete element model. And a new method was developed to obtain and analyse the void structure in discrete element model. The simulation results were compared with one of the laboratory test. The comparative analysis indicates that, the discrete element method can be used to simulate the creep response and void reduction behaviour of porous asphalt mixture. Further research shows that porosity, effective porosity, number of connected components and section pores have a good correlation with strain of porous asphalt mixture. With the increase in strain, the proportion of section pores with diameter less than 2 mm increases. In the initial stage of loading, the void reduction is the main reason for rut increment of porous asphalt mixture. In the later stage, the void structure is almost incompressible; the lateral deformation of mixture becomes the domination factor.