摘要
沥青路面以其众多的优点在我国高等级公路中得到了越来越广泛的应用。然而,随着公路运输量的日益增大和大型重载车辆的增多,沥青路面往往在使用早期就出现大量的开裂现象,严重损害路面的使用性能,增加路面维护成本和缩短路面的使用寿命。开裂对沥青路面结构的应用、设计和分析提出了严峻的挑战,提高沥青路面的抗开裂能力已成为我国高等级公路建设中一个急需解决的问题。沥青混合料是沥青路面的组成材料,为了揭示沥青路面的损伤破坏机理,为沥青路面结构设计提供可靠的理论和技术支持,研究沥青混合料在不同载荷和温度条件下的开裂行为显得非常必要。
论文针对沥青混合料的随机和异质的材料特征,发展了一种随机骨料快速生成和投放算法来建立比拟真实的沥青混合料二维和三维细观结构模型,提出了将随机骨料模型与粘聚带模型相结合的细观异质断裂模型建模框架,并开展了一系列的实验室断裂测试,通过数值模拟与实验测试相结合的方法研究了沥青混合料在不同加载条件下的裂纹扩展机理,讨论了加载方式、材料组成和细观结构等因素对沥青混合料开裂行为的影响。主要创新性研究工作包括：
(1)开展了沥青混合料单边切口梁试件的三点弯曲开裂实验,通过改变预切口位置构造了Ⅰ型和混合型加载条件,研究了不同加载条件下的裂纹开裂特征。实验结果表明,在Ⅰ型载荷作用下裂纹呈现出竖直向上扩展的趋势,而在混合型载荷作用下裂纹呈现出向梁中心扩展的趋势。
(2)设计并制作了沥青混合料直接剪切实验装置,基于长方体试件开展了纯Ⅱ型载荷条件下的开裂试验,验证了装置的可靠性,研究了温度对混合料开裂特征和抗开裂能力的影响。实验结果表明,温度会显著影响沥青混合料力学性能,包括初始刚度、剪切强度和载荷峰值过后的软化行为。
(3)提出和发展了一种骨料快速生成和随机投放算法来创建沥青混合料的细观结构模型。将沥青混合料看作有粗骨料和沥青砂组成的两相复合材料,研究了骨料快速生成、投放、振荡和内接技术,形成了考虑骨料含量、级配和分布等细观组成因素的高效的沥青混合料二维和三维随机骨料模型创建方法。
(4)提出了一种宏细观相结合的多尺度建模框架。对结构中需要重点关心的区域采用骨料快速生成和投放算法建立异质的细观力学模型,其他区域采用均匀化的宏观模型,来实现计算效率和计算结果可靠性之间的平衡。建立了沥青混合料单边切口梁的多尺度模型对三点弯曲实验进行了数值模拟,验证了多尺度模型的可行性和有效性,并进一步研究了预裂纹位置和骨料分布对开裂方向的影响。
(5)提出了随机骨料模型与粘聚带模型相结合的细观异质断裂模型创建方法。编制了在随机骨料有限元网格模型中生成粘聚界面单元的程序,将粘聚界面单元作为潜在裂纹插入到骨料-沥青砂界面、沥青砂单元之间,来模拟裂纹的扩展过程。采用二维异质随机骨料断裂模型对三点弯曲实验进行了数值模拟,模拟结果与实验结果的对比验证了模型的可靠性。进一步采用二维细观异质断裂模型研究了无预制裂纹条件下沥青混合料在单轴拉伸和直接剪切实验中从微裂纹萌生、起裂和汇聚,宏观裂纹发生、发展,直至试件完全破坏的全过程,分析了沥青混合料的细观起裂特征和开裂机制,评估了骨料级配、含量、分布等因素对开裂行为的影响。
(6)将二维断裂模型扩展到三维,开展了三维异质断裂模型的单轴拉伸模拟,讨论了三维条件下的计算规模和计算效率,评估了沥青混合料内部细观结构因素对沥青混合料开裂行为的影响。模拟结果表明三维断裂模型能够预测更加真实的沥青混合料开裂特征,但计算规模更大、计算效率较低。细观结构内部粗骨料分布、数量和沥青砂的断裂参数都是影响沥青混合料开裂性能的重要因素。
Asphalt mixture is widely used in high-grade highway and airport pavement construction due to its high strength, lower noise, easy construction and maintenance and smooth and comfortable for driving. With rapid increase of traffic volume and heavy vehicle amount in recent years, asphalt pavements often fail before reaching its design life due to cracking. As one of the most common distresses, cracking directly reduces the road service capacity and deteriorates the road performance. Crack phenomena pose a severe challenge to application and design of asphalt pavement structures. Understanding the fundamental mechanisms behind crack initiation and propagation is an important issue in development of accurate, mechanistic design procedures based upon pavement performance and life-cycle costs.
In the paper, random aggregate generation and packing algorithm is developed to create two-and three-dimensional heterogeneous models of asphalt mixture and a heterogeneous fracture modeling approach by combining the mesostructural modeling technique and the bilinear cohesive zone model is proposed to predict fracture evolution behavior in asphalt mixture. Then, an integrated frame by coupling experiments and heterogeneous fracture simulation is presented to study the crack behavior in asphalt mixture under different load modes. The main creative achievements are shown as follows.
(1) The three-point bending tests are performed based on the single-edged notch beams, and mode I and the mixed mode load conditions are designed by offsetting the notch location. The experimental results show that a crack propagates approximately along the initial crack direction under mode I load condition, but tends to propagate towards the top midspan point of beam under mixed mode load condition.
(2) Using the self-manufactured direct shear test setup, a series of direct shear test are performed on the cuboid specimens to evaluate the influence of temperature on shear cracking behavior of asphalt mixture. It is revealed from the test results that the temperature significantly influences the mechanical properties of asphalt mixture including initial stiffness, shear strength and post-peak softening behavior.
(3) The random aggregate generation and packing algorithm is developed to create a two-or three-dimensional heterogeneous model of asphalt mixture. Asphalt mixture is treated as a composite consisting of randomly distributed coarse aggregates and asphalt mastic, and some mesoscale factors including aggregate content, gradation, shape and distribution are considered as variable design parameters. The techniques of aggregate generation, packing, perturbation and conversion involved in the modeling algorithm are studied.
(4) A multiscale modeling frame combining macroscale and mesoscale is proposed to achieve the balance between the computation efficiency and the accuracy. In this frame, the mesostructural models are created in some concerned local regions and the homogeneous models are done in the other regions. As examples, a series of multiscale models are created to investigate the cracking behavior of pre-cracked asphalt mixture under three-point bend loading. It is proved that the mutiscale modeling frame is feasible and reasonable by comparing the numerical simulation results with the experimental results. Then the effects of crack location and aggregate distribution on cracking behaviors are evaluated.
(5) A mesoscale heterogeneous fracture modeling approach is proposed by combining the mesostructural model and the cohesive zone model to predict complex cracking behavior in asphalt mixture. The mesostructural model is firstly discretized by finite elements, then, the cohesive elements are inserted into the boundaries of initial elements to be as potential microcrack sources by a self-compiled computer program. The two-dimensional heterogeneous fracture models are created to study the cracking behaviors under three point bending and the comparisons between the numerical simulation results and experimental results prove that the heterogeneous fracture model is feasible and reliable. Then the heterogeneous fracture models are used to study the microcrack evolution characteristics and mechanisms under uniaxial tension and direct shear, respectively. Additionally, the effects of some mesoscale factors such as aggregate gradation, distribution and content on cracking behaviors are evaluated.
(6) The heterogeneous fracture simulation is extended from a two dimensional case to a three dimensional one and the complex three dimensional crack propagation in asphalt mixture is simulated. The simulation results show that the three dimensional fracture model predicts more realistic crack path, but its computational efficiency is lower. The aggregate distribution, aggregate amount and the cohesive fracture parameters are important factors influencing the crack performance of asphalt mixture.
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