Laboratory investigations on the mechanical properties degradation of granite under freeze-thaw cycles

To understand the deteriorate characteristics of granite in cold regions, the influences of freeze-thaw cycles on the mechanical properties need to be investigated. The rock specimens (biotite granite from Tibet in China) were frozen and thawed in a temperature and humidity controlled container where the temperature varied from + 40 to − 40 °C and the humidity was kept at 100% continuously. The freeze-thaw tests were conducted for one cycle including 4 h of freezing and 4 h of thawing, and the number of cycles was from 0 to 150. The deterioration of the specimens was examined by the changes of strength, deformation characteristics, elastic modulus, cohesive strength and internal frictional angle in a series of uniaxial and triaxial compression tests. The experimental results show that (1) the axial strain corresponding to the peak stress increases with an increasing of confining pressure and freeze-thaw cycles; (2) The compressive strength decayed exponentially with the number of freeze-thaw cycles, as well as elastic modulus and cohesive strength; (3) the internal frictional angle remained constant in the process of freeze-thaw cycles; (4) The interrelation among compressive strength, confining pressure and number of cycles can be described by Mohr-Coulomb strength criterion.     

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.     

盐冻融循环条件下沥青高温流变性能及微观结构

利用动态剪切流变仪(DSR)和原子力显微镜(AFM)对冻融循环前后的基质沥青和胶粉改性沥青进行测试,比较分析了不同盐浓度和冻融循环次数下两种沥青的动态黏弹参数及微观结构,并测试了冻融前后两种沥青的基本指标。结果表明:经历冻融循环后,基质沥青储存模量和车辙因子均增加,疲劳因子减小,而胶粉改性沥青没有明显变化,冻融循环增加了基质沥青的高温弹性性能,但显著降低了抗疲劳性能,而冻融循环对胶粉改性沥青的高温性能影响不大;AFM观测表明基质沥青出现"蜂形"结构,冻融循环后"蜂形"变大、变长,而胶粉改性沥青的结构几乎没有变化,因此北方寒冷地区使用胶粉改性沥青作为路面材料更具优势。     

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.     

Impact of asphalt concrete fatigue endurance limit definition on pavement performance prediction

Many well-constructed Hot Mix Asphalt pavements have been in service for 40 or more years without any evidence of fatigue cracking. This field experience suggests that there exists a strain level, known as the fatigue endurance limit (FEL), below which an asphalt concrete pavement will not exhibit fatigue cracks. Several studies have been conducted to define and verify this limit. Each of these methods is associated with certain assumptions regarding the nature of the FEL and heretofore a comprehensive comparison of each has not been made using a consistent set of mixtures. Likewise, the impact of any observed differences in FEL on the predicted pavement performance has not been made. This paper investigates and compares six different methods for identifying the FEL: NCHRP 9–44A approach, simplified viscoelastic continuum damage model, smeared-healing with continuum damage model, plateau value approach, pseudo-strain analysis method, and reduced cycles method. Each method is found to yield different values ranges from approximately 30–170 microstrains at 21.1 °C. The predicted FEL from each of the six methods are then used with the mechanistic empirical design algorithm to evaluate their effects on predicted pavement performance. Simulation outputs show different pavement performance and perpetual pavement structural design thicknesses from each of the methods. The study outcomes are expected to benefit future field verification research of FEL as it provides comprehensive analyses using six different methods. This future verification research may indicate the method that best represents actual perpetual pavement design and performance.     

粉煤灰／海泡石纤维复合沥青混合料的制备与性能研究

从海泡石纤维和粉煤灰纤维的微观结构特性出发，进行粉煤灰，海泡石复合纤维增强沥青复合材料的制备。通过路用性能试验，研究了海泡石纤维和粉煤灰纤维对沥青混合料性能的影响以及结合机理。结果表明，添加适量海泡石和粉煤灰纤维可以制备性能优良的纤维复合沥青混合料。海泡石纤维对沥青表现极强吸持能力，有效调节沥青与胶浆的含量。粉煤灰纤维在沥青中主要起加固和改善混合料的作用。两种纤维的添加，使沥青混合料的高温变形性、水稳定性、低温抗裂性和抗疲劳性等显著提高。     

两种粘结料对泡沫沥青冷再生混合料性能的影响

通过加速加载试验、低温弯曲试验、飞散试验研究了泡沫沥青和水泥两种粘结材料对泡沫沥青冷再生混合料(CRMFA)长期高温抗变形能力、低温抗裂性以及抗松散性能的影响。研究结果表明:对CRMFA的长期高温抗变形能力而言,存在一个最佳泡沫沥青用量;增加水泥掺量有助于提高CRMFA的长期高温抗变形能力。CRMFA的低温柔韧性随泡沫沥青用量的增大而提升,但随水泥掺量增加呈抛物线变化规律,为保证CRMFA具有良好的低温抗裂性能,水泥掺量宜小于2wt%。CRMFA的抗松散性能较差,增加泡沫沥青用量可有效降低其松散性;水泥掺量在1.5wt%~2.5wt%时,对CRMFA抗松散性的改善作用不显著。     

基于动态蠕变实验的沥青混合料本构关系研究

沥青混合料应变在一定温度和外荷载作用下会随着时间增长而逐渐增加的现象称为蠕变;重复加载蠕变实验反映了路面荷载和变形的响应关系,常用于分析路面高温抗车辙性能。为了真实模拟路面的力学状况和描述沥青混合料的动态蠕变性能,对四种沥青和两种沥青混合料进行动态蠕变实验;研究发现沥青混合料的动态蠕变呈非线性,同时采用经典Burgers模型与由Maxwell体和分数阶Kevin体串联组合的分数阶导数Burgers模型对沥青混合料蠕变曲线进行拟合,并对比分析其拟合结果。结果表明,分数阶模型参数a代表了材料的记忆特性,分数阶导数Burgers模型可定量的描述沥青混合料动态蠕变性能的非线性,能更好的分析其高温性能。     

盐冻融循环对沥青混合料性能的影响因素研究

为了研究沥青混合料在冻融循环及除冰盐溶液共同作用下的抗裂性能,在断裂力学理论基础上,采用半圆试件弯拉实验,并通过J积分来评价沥青混合料的抗裂性能;利用极差分析法原理和灰色关联熵分析法原理深入分析沥青种类、冻融循环次数和除冰盐浓度对沥青混合料抗裂性能的影响。结果表明,盐冻融循环作用下,对断裂韧度影响程度的大小关系为沥青种类冻融循环次数除冰盐浓度;与基质沥青混合料、SBS改性沥青混合料和复合胶粉改性沥青混合料相比,橡胶粉改性沥青混合料具有更好的抗裂性;随着冻融循环次数及盐浓度的增加,混合料的抗裂性逐步降低。     

Mechanical and structural assessment of laboratory- and field-compacted asphalt mixtures

Compaction forms an integral part in the formation of the aggregate orientation and structure of an asphalt mixture and therefore has a profound influence on its final volumetric and mechanical performance. This article describes the influence of various forms of laboratory (gyratory, vibratory and slab-roller) and field compaction on the internal structure of asphalt specimens and subsequently on their mechanical properties, particularly stiffness and permanent deformation. A 2D image capturing and image analysis system has been used together with alternative specimen sizes and orientations to quantify the internal aggregate structure (orientation and segregation) for a range of typically used continuously graded asphalt mixtures. The results show that in terms of aggregate orientation, slab-compacted specimens tend to mimic field compaction better than gyratory and vibratory compaction. The mechanical properties of slab-compacted specimens also tend to be closer to that of field cores. However, the results also show that through careful selection of specimen size, specimen orientation and compaction variables, even mould-based compaction methods can be utilised with particular asphalt mixtures to represent field-compacted asphalt mixtures.     

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.     

Aggregate distribution influence on the indirect tensile test of asphalt mixtures using the discrete element method

The purpose of this study is to investigate the effect of horizontal aggregate distribution, i.e. aggregate distribution in horizontal cross sections, on the indirect tensile (IDT) test of asphalt mixtures. An index of aggregate homogeneity, used to evaluate the aggregate distribution in a two-dimensional (2D) cross section, was comprehensively described; the horizontal aggregate distribution was evaluated by the index. A microstructure-based discrete element model for predicting the IDT test results was established by a discrete element program called particle flow code in two dimensions (PFC2D). Based on this model and by loading horizontal cross sections of asphalt mixtures along different directions, the effects of horizontal aggregate distribution on the splitting strength and maximum horizontal stress with regard to an IDT test were numerically simulated by means of the discrete element method; the obtained results were verified by performing an actual IDT test. Results reveal that the splitting strengths and maximum horizontal stresses in the IDT test exhibit anisotropy. Furthermore, it is revealed that there is an insignificant correlation between the horizontal aggregate distributions and the average splitting strengths and average maximum horizontal stresses, as well as a significant correlation between the horizontal aggregate distributions and the variations in the splitting strengths and maximum horizontal stresses.     

盐分对橡胶沥青及橡胶沥青混合料高温性能的影响

为研究盐分对橡胶沥青胶结料及橡胶沥青混合料高温性能的影响,将橡胶沥青试件在不同浓度的盐水中浸泡不同的时间。通过软化点、fail temperature和蠕变劲度的粘性成分Gv3个不同指标来评价盐分对橡胶沥青高温性能的影响,通过车辙实验评价盐分对橡胶沥青混合料高温性能的影响。结果表明,3个指标均升高,说明经过盐水浸泡之后的橡胶沥青高温性能都有所提升,盐分条件的存在没有对橡胶沥青胶结料的高温性能造成不利影响。在相同浸泡时间7 d的条件下,随着盐分浓度的增加,软化点、fail temperature和Gv值总体上呈现先增大后减小趋势,升高最大值均出现在盐分浓度为3%时,说明橡胶沥青的高温性能先升高后降低,在3%盐分浓度时达到最佳;在相同浸泡盐分浓度3%的条件下,随着浸泡时间的增加,3个指标呈升高趋势,说明橡胶沥青胶结料的高温性能得到提升。在4种不同浓度的盐水中浸泡后,橡胶沥青混合料的动稳定度值出现了不同程度的降低,混合料高温抗车辙能力变差。     

盐溶液干湿循环对CFRP-混凝土界面粘结性能的影响

为了考察盐溶液干湿循环条件对碳纤维增强复合材料（Carbon fiber reinforced polymer,CFRP）-混凝土界面粘结性能的影响,本文采用5%的NaCl溶液来模拟海水环境,经过不同次数的干湿循环后,利用单面剪切试验对48个试件界面性能的变化情况进行了研究,分析了上述环境对界面破坏形式、界面承载力及界面剪应力等参数的影响。结果表明：在盐溶液干湿循环作用下,界面粘结性能发生显著地退化,具体表现为,随着干湿循环次数的增加,界面承载力会不断降低,且下降程度与混凝土强度和CFRP的粘贴尺寸有关,界面剪应力在不同环境下的分布具有相似性,即荷载的不断增加会使剪应力逐渐由加载端向自由端传递,但在传递过程中,有效传递区域的长度不会发生改变。     

盐冻条件下水泥砂浆受风沙冲蚀磨损性能及损伤机制

通过气流挟沙喷射法对水泥砂浆进行了复合盐-冻融循环后气-固两相流冲蚀磨损试验研究。基于SEM、激光共聚焦显微镜（LSCM）和XRD,观测了水泥砂浆试件冲蚀磨损后表面微观形貌,分析了盐冻前后其表面化学成分,探究其冲蚀损伤机制。结果表明：相同冲蚀试验条件下,水泥砂浆试件冲蚀率随冲蚀角度的增加而增大,在高冲蚀角下的增长趋势较为缓慢;冲蚀率随复合盐-冻融循环次数的增加出现先减小后增大的趋势,在冻融10次时冲蚀率最小;同等条件下,8%浓度下的冲蚀率均大于10%浓度下的冲蚀率。冲蚀损伤过程中动能法向分量起决定性作用,明显特征表现为冲蚀坑和微破坏区的出现。复合盐-冻融循环对冲蚀磨损性能的影响主要体现在两个方面：物理方面,表现为降温过程中溶液过饱和晶体析出以及冰胀压和渗透压对内部孔隙的作用;化学方面,溶液中的阴离子与水泥砂浆发生化学反应,生成钙矾石和石膏晶体填充在水泥砂浆内部,提高了其致密性,但后期晶体压力的增大会促进裂缝的产生,对水泥砂浆抗冲蚀磨损能力产生不利影响。     

盐冻循环对沥青性能及其微观结构的影响

对基质沥青、SBS改性沥青和胶粉改性沥青在盐冻循环前后的3个基本指标进行了测试,采用极差分析法和灰色关联熵分析法对盐冻融循环前后影响沥青高低温性能的各因素进行了对比分析,并通过扫描电子显微镜对冻融循环前后的3种沥青试样的微观结构进行观测。结果表明,冰冻温度、盐溶液浓度和冻融循环次数都会影响沥青的温度敏感性,使沥青的高低温性能降低,并对沥青微观结构产生巨大影响,但是研究也发现沥青自身性能指标的影响更为显著,特别是沥青自身的针入度值。因此在寒冷地区修建道路时,沥青材料的选用要严格控制其基本指标。     

Effect of aggregate types on the performance of neat and EVA-modified asphalt mixtures

The objective of this study is to determine the effect of bitumen modification with varying percentage of ethylene vinyl acetate (EVA) for preparing the hot-mix asphalt containing various aggregates (marble, granite and quartzite) and compare the results with mixes prepared with neat viscosity grade 30. The selection of aggregates has been done based on their acidic and basic nature. The physical and mechanical properties of EVA-modified bitumen and neat bitumen aggregate mixes were evaluated. Moisture susceptibility tests and wheel tracking test were carried out on the samples. Brookfield viscometer and dynamic shear rheometer were used to determine the mixing and compacting temperatures of neat and modified bitumen. The results indicate that mixes prepared with aggregates that are basic in nature, i.e. with higher calciumcontent, show better bonding with both neat bitumen and EVA-modified bitumen compared with acidic aggregates, i.e. aggregates with higher silica content.     

Experimental determination of the thermal conductivity of molten pure salts and salt mixtures

The thermal conductivity of some molten salts was measured at atmospheric pressure, using the coaxial cylinder method. The pure compounds NaCO₃, KNO₃, and NaNO₂, the equimolar mixture NaNO₃-KNO₃, and HITEC, which is a three-component mixture, NaNO₃-NaNO₂-KNO₃ (0.07-0.40-0.53 in weight), were investigated. For mixtures, it was found that the experimental thermal conductivity coefficients are in agreement with calculated values using a simple linear mixing law. The thermal diffusivity was calculated and compared with experimental data.     

Thermodynamic behavior of mixtures of methane and ethane in the critical region

An isomorphic equation of state for near-critical binary mixtures is used to represent equation of state and specific heat data for mixtures of methane and ethane in a substantial range of densities and temperatures around the critical line.