Discrete element modeling of asphalt concrete cracking using a user-defined three-dimensional micromechanical approach

We established a user-defined micromechanical model using discrete element method (DEM) to investigate the cracking behavior of asphalt concrete (AC). Using the “Fish” language provided in the particle flow code in 3-Demensions (PFC3D), the air voids and mastics in asphalt concrete were realistically built as two distinct phases. With the irregular shape of individual aggregate particles modeled using a clump of spheres of different sizes, the three-dimensional (3D) discrete element model was able to account for aggregate gradation and fraction. Laboratory uniaxial complex modulus test and indirect tensile strength test were performed to obtain input material parameters for the numerical simulation. A set of the indirect tensile test were simulated to study the cracking behavior of AC at two levels of temperature, i e, −10 °C and 15 °C. The predicted results of the numerical simulation were compared with laboratory experimental measurements. Results show that the 3D DEM model is able to predict accurately the fracture pattern of different asphalt mixtures. Based on the DEM model, the effects of air void content and aggregate volumetric fraction on the cracking behavior of asphalt concrete were evaluated.     

粗骨料粒径对混凝土弯拉强度尺寸效应影响的试验研究

为获取粗骨料粒径对混凝土材料弯拉强度及尺寸效应的影响,完成27组素混凝土梁四点弯曲试验,获得粗骨料最大粒径10、20、30 mm,尺寸为100 mm×100 mm×400 mm、150 mm×150 mm×550 mm、200 mm×200 mm×700 mm混凝土试件的弯拉强度,采用Ba?ant尺寸效应理论对试验数据进行分析,提出指数型弯拉强度尺寸效应经验公式。研究结果表明：相同骨料体积率、水灰比和构件尺寸下,混凝土材料弯拉强度随着骨料粒径的增大而降低;各骨料粒径下混凝土弯拉强度均存在尺寸效应现象,小尺寸时尺寸效应现象更显著,骨料粒径30 mm时,混凝土弯拉强度随几何尺寸增大而降低趋势最平缓;Ba?ant尺寸效应率与试验结果吻合较好,但不适用于全尺寸范围,提出的指数型尺寸效应经验公式,在全尺寸范围内能够较好地反映混凝土弯拉强度,可用于混凝土材料弯拉强度尺寸效应预测。     

Discrete Element Modeling of Asphalt Concrete Cracking Using a User-defined Three-dimensional Micromechanical Approach

We established a user-defined micromechanical model using discrete element method (DEM) to investigate the cracking behavior of asphalt concrete (AC). Using the Fish language provided in the particle flow code in 3-Demensions (PFC3D), the air voids and mastics in asphalt concrete were realistically built as two distinct phases. With the irregular shape of individual aggregate particles modeled using a clump of spheres of different sizes, the three-dimensional (3D) discrete element model was able to account ...     

应力吸收层贯入试验数值分析与试验方法评价

为了评价应力吸收层沥青混合料的抗剪性能,对应力吸收层沥青混合料进行了不同温度下抗压回弹模量试验和贯入试验的数值分析,分析了试件规格、压头大小、矿料级配等对试验结果的影响.根据数值分析和室内试验结果,验证了应力吸收层沥青混合料贯入试验的可行性,得到适宜的试验条件.分析表明:采用直径和高度均为100 mm的圆柱形静压试件和直径19 mm的压头进行贯入试验时,试验数据稳定,可有效区分应力吸收层矿料级配的优劣.研究成果为应力吸收层沥青混合料的质量评价提供了理论和试验依据.     

基于细观结构特征的沥青混合料空隙三维分布特征研究

细观结构特征对沥青混合料宏观性能有着重要影响,其中细观结构的空隙率特征是影响沥青混合料车辙、水损害的关键因素之一.通过工业CT分别对最大公称粒径分别为13 mm与16 mm的AC、SAC、SMA的6种沥青混合料试样进行无损扫描,获取其内部细观结构特征信息,并对其试样进行三维重构.提取试样的内部空隙体积参数并计算其三维空隙率;选取部分断面图像计算其二维空隙率,并对两种方法计算出的空隙率进行对比分析.研究结果表明:试样内部空隙数量随着空隙体积增大呈递减趋势,且0～50 mm3体积区间内空隙数量最多;相同级配下,SAC的空隙率小于AC和SMA;最大公称粒径为13 mm的空隙率小于16 mm的空隙率;空隙率由试样两端向中间呈逐渐减小的趋势,且SMA较SAC表现得明显,最大公称粒径为16 mm的试样比最大公称粒径13 mm的表现得更明显;3D计算空隙率比2D计算空隙率更接近实测值.     

Effect of aggregate gradation on low temperature performance of asphalt paving mixtures

Low temperature cracking has become one of the important factors that diminish asphalt pavement’s ride quality and service life.Especially in cold region,cracking caused by low temperature is the main distress form.This paper discussed the effect of aggregate gradation on the low temperature performance in asphalt paving mixtures.A total of 11 asphalt mixtures with 11 different aggregate gradations and one asphalt binder content were studied.Volumetric properties of the coarse aggregate and asphalt mixtures showed aggregate grading has a significant impact on the degree of aggregate interlock in asphalt mixtures.A trend is existed in the low temperature performance with the change of gradation.With the aid of mathematic statistics,it indicates gradation affects the low temperature performance significantly.The findings also indicate the relationship between the degree of aggregate interlock in asphalt mixtures and the low temperature performance:With the stone-to-stone contact developed,the mixture has a high energy to resist contract and deformation at low temperature.The properties of fine aggregate and asphalt play an important part in resisting low temperature cracking in floating structure.But it provides lower energy to resist low temperature cracking compared to the skeleton structure.     

基于超短栓钉的钢-超薄UHPC组合桥面性能

为了满足对自重敏感的大跨桥梁钢桥面的翻修与加固需求,提出采用超短栓钉作为连接件的钢-超薄UHPC轻型组合桥面结构（简称“新超薄体系”）. 通过钢-超薄UHPC组合板负弯矩试验,研究关键设计参数对超薄UHPC层抗裂性能的影响. 试验结果表明：当UHPC最大裂缝宽度小于0.15 mm时,裂缝宽度的增长近似呈线性,在钢筋屈服以后,裂缝宽度迅速增大;配筋率和钢筋直径对名义开裂应力的影响较大. 基于试验结果,分析已有的裂缝宽度计算公式,确定钢-超薄UHPC组合板裂缝宽度的建议计算公式. 以某特大跨径悬索桥为工程背景,进行整体和局部有限元分析,论证了方案应用于实际工程的可行性. 计算结果表明：钢-超薄UHPC组合桥面的自重与常规60 mm厚的钢桥面铺装基本持平,主缆和吊索内力变化小于3.0%;钢桥面（OSD）各典型疲劳细节的应力幅值降低了10.1%~52.0%,且均小于200万次疲劳强度;UHPC层中最大拉应力为8.4 MPa,远小于试验得到的名义开裂应力.     

纤维沥青混合料抗裂性能的试验研究

采用MTS 810对未切缝及预切缝两种小梁试件进行低温3点弯曲试验,并利用临界应变能密度及材料的临界应力强度因子评价纤维长度及纤维力学性能对低温抗裂性能的影响.结果表明,掺加纤维能有效阻滞裂纹的进一步发展,提高沥青混合料的低温抗裂能力,且随纤维力学强度的增大及长度的适当增长,纤维的增强效果进一步改善.     

应力吸收层沥青混合料的低温抗裂性能

通过对自主研发的Sampave等3种改性沥青结合料和不同级配的应力吸收层沥青混合料进行低温弯曲和低温蠕变试验,分析沥青结合料和矿料级配对应力吸收层沥青混合料低温抗裂性能的影响.结果表明:自主研发的Sampave特种改性沥青混合料具有优良的低温抗裂性能,但单一改性剂SBR改性沥青并不能达到应力吸收层沥青结合料的性能要求;矿料4.75 mm、0.075 mm筛孔通过率对其低温抗裂性能影响较敏感.     

试件尺寸对锈蚀钢筋与混凝土间黏结性能的影响

为研究试件尺寸(立方体试件边长为10D,钢筋直径D=14、20、25 mm)对锈蚀钢筋与混凝土间黏结性能的影响,通过电化学加速锈蚀方法,获得了7组不同钢筋锈蚀率(0%、0.5%、1.0%、2.0%、5.0%、8.0%、10.0%)的中心拉拔试件.分析了试件尺寸、锈蚀率等影响因子作用下钢筋与混凝土间黏结性能的变化规律.结果表明:随着锈蚀率的增加,试件尺寸越大其锈胀裂纹出现越早且最大锈胀裂纹越宽;各试件的黏结强度、黏结刚度均随混凝土的恶化及锈蚀率的增加呈现出先增长后逐渐削减的趋势,黏结能量则随锈蚀率的增加而逐渐下降,且较大尺寸试件(D=25 mm)的黏结性能受锈蚀率的影响更为显著;锈胀裂纹出现前,试件自由端峰值滑移、黏结能量下降较为缓慢;当锈胀裂纹出现后,其自由端峰值滑移、黏结能量急剧下降,而后随着锈胀裂纹的加宽,其下降速率趋于平缓.基于试验结果,建立了不同锈蚀率下残余黏结应力与试件尺寸的关系式.     

地铁管片抗裂度及裂缝宽度试验和计算方法

为了研究钢筋混凝土衬砌管片的开裂性能,对2片 1:1 不同配筋率的钢筋混凝土衬砌管片进行了试验。研究发现,配筋率的提高对钢筋混凝土衬砌管片抗裂度的影响十分有限,随着配筋率的提高平均裂缝间距、最大裂缝宽度减小,但减小幅度不大。根据试验结果,分析了不同配筋率对钢筋混凝土衬砌管片抗裂度和裂缝宽度的影响,给出了考虑衬砌管片高度影响的抗裂度及考虑配筋率影响的最大裂缝宽度实用计算公式,理论计算结果与试验数据吻合良好。     

级配碎石混合料及其基层的抗剪性能

为了研究级配碎石基层在路面结构中能否发生剪切破坏,对级配碎石基层材料抗剪强度及在路面结构中的剪应力进行了对比研究;应用高精度静三轴仪对选定的级配碎石进行振动成型和击实成型的抗剪强度研究,揭示不同成型方法对抗剪强度的影响,采用有限元分析典型路面结构在标准荷载作用下剪应力的大小,明确级配碎石基层发生剪切破坏的可能性;三轴试验表明振动成型级配碎石过渡层的抗剪强度为0.17 MPa,击实成型的为0.14 MPa,振动成型试样比击实的抗剪强度提高20%;剪应力分析表明:水平荷载在级配碎石基层中产生的最大剪应力在0.04~0.05 MPa;级配碎石的抗剪强度是最大结构剪应力的3.5倍,级配碎石基层在高等级沥青路面结构中产生剪切破坏的可能性很小.     

基于数字图像处理和有限元建模方法的沥青混合料劈裂试验数值模拟

应用数字图像处理技术及工业CT扫描获得试件图像,结合有限元建模方法,建立了包含集料、空隙和胶浆在内的沥青混合料有限元模型,并模拟研究了沥青混合料劈裂试验。结果表明:通过定义应力集中因子,可用来描述沥青混合料内部应力的不均匀分布,级配类型、模量比、有无空隙及加载位置都对模拟结果有较大影响。采用DIP-FEM方法能够很好地将沥青混合料的微观结构和宏观力学性能结合起来。     

Design method and performance for large stone porous asphalt mixtures

Design method for large stone porous asphalt mixtures (LSPM) was analyzed to avoid the early distresses of semi-rigid asphalt pavements. Based on stone-to-stone skeleton structure concept, processes of LSPM gradation design was given. The gradation composite design for LSPM shows that the LSPM nominal maximum size (NMS) should be larger than 26.5 mm, and the NMS sieve passing percentage should be greater than 50%. Through experiments and calculations on the volume properties of the aggregate, the range of aggregate gradation curve of LSPM was given. In terms of asphalt binder’s normalized test results, MAC-70 and SBS modified asphalt were selected as the asphalt binders. The applicability of large scale Marshall Method and gyratory compaction method to shape specimens was investigated. Based on the asphalt mixture performance evaluation, the optimum asphalt content range (3.1%–3.6%), the bitumen film’s thickness range (13–16 μm) and the air void range (135–18%) were recommended. Finally, LSPM was tested by the laboratory performance tests including rutting resistance test, fatigue test and water stability test. The theoretic and practical analysis shows that LSPM has a good performance on water permeability, rutting resistance and reflection crack resistance.     

Degradation behavior of aggregate skeleton in stone matrix asphalt mixture

The degradation behavior of aggregate skeleton in stone matrix asphalt mixture was investigated based on theoretical analysis, laboratory test and field materials evaluation. A stress-transfer model was established to provide the fundamental understanding of the stress distribution and degradation mechanism of stone matrix asphalt (SMA) aggregate skeleton. Based on the theoretical analysis, crushing test and superpave gyratory compactor (SGC) test were used to evaluate the degradation behavior of aggregate skeleton of SMA. To verify the laboratory test results, gradation analysis was also conducted for the field materials extracted from SMA pavements after long-time service. The results indicate that the degradation of SMA aggregate skeleton is not random but has fixed internal trend and mechanism. Special rule is found for the graded fine aggregates generated from coarse aggregate breakdown and the variation of 4.75 mm aggregate is found to play a key role in the graded aggregates to form well-balanced skeleton to bear external loading. The variation of 4.75 mm aggregate together with the breakdown ratio of aggregate gradation can be used to characterize the degradation behavior of aggregate skeleton. The crushing test and SGC test are proved to be promising in estimating the degradation behavior of SMA skeleton.     

高弹桥面铺装表面冰层破坏特性的断裂力学分析

提出以破冰率Rbreak作为桥面冰层破坏的定量评价指标,根据强度理论及能量法计算确定了高弹性桥面冰层破坏形式及破冰率大小,通过在冰层底部不同方向植入初始裂纹,采用断裂力学方法计算了裂纹扩展特征,首次提出了基于断裂力学方法的破冰率修正系数,得到主要结论:采用橡胶颗粒沥青混合料高弹桥面铺装层可实现桥面冰层破坏,且冰层破坏形式为受压破坏;由断裂力学方法计算得到的冰层底部裂纹扩展方式为滑开型开裂;提出以滑开型开裂对受压型破坏的破冰率修正系数为1.81.     

级配对Superpave沥青混合料性能的影响

为了研究级配对沥青混合料路用性能的影响,根据Superpave法选取粗细程度不同的两种沥青混合料,用贝雷法分析级配,并进行旋转压实、浸水马歇尔、车辙、低温劈裂室内试验,结果表明：贝雷参数满足要求的级配对沥青混合料的高温性能影响显著,对体积指标、低温抗裂性以及水稳定性有一定影响,尤其是粒径0．3～2．36mm的颗粒含量对沥青混合料性能影响较大;粗级配沥青混合料的路用性能较优。     

ATB柔性基层与半刚性基层的层间抗剪规律

以ATB-25沥青混合料与水泥稳定碎石成型复合试件,控制复合试件层间沥青和碎石用量,采用自主研发的剪切仪进行抗剪强度试验,分析影响层间抗剪强度的因素及变化规律。通过设置试验温度、碎石和沥青用量等参数,制定不同的试验方案进行试验。试验结果表明:对层间抗剪强度影响最大的因素是温度,从5 ℃上升到35 ℃时最大剪应力从1.36 MPa下降到0.16 MPa,其次是沥青用量；碎石用量影响最小,而且不同温度下沥青和碎石的最佳用量也不同。在实际工程中,在考虑沥青和碎石的用量时,应考虑工程所在的地区,高温地区沥青的用量应比低温地区稍低一些,同时严格控制碎石用量,提高层间联结质量,明确层间抗剪强度与层间联接各因素的规律。     

沥青混合料回收料变异性

为了控制沥青混合料回收料(reclaimed asphalt pavement,RAP)的变异性,保证再生沥青混合料的性能,通过现场取料,测试了不同来源RAP的矿料级配、沥青含量和沥青性能,分析了各项指标的变化规律和差异性;结合普通热拌沥青混合料质量控制要求,提出了厂拌热再生沥青混合料RAP最大添加比例控制模型;分析了RAP中油石比与粒径的分布特性;最后建立了混合沥青针入度波动范围模型.结果表明:RAP的矿料级配、油石比和沥青性能变异性均较大;计算得到的RAP最大添加比例能有效保证再生混合料矿料级配和油石比的质量要求;RAP中的油石比与粒径呈指数分布,粒径越小,油石比越大;混合沥青的针入度波动范围与旧沥青的油石比和针入度密切相关,且RAP添加比例越大,波动范围越大.     

ATB柔性基层与半刚性基层的层间抗剪规律

以ATB-25沥青混合料与水泥稳定碎石成型复合试件,控制复合试件层间沥青和碎石用量,采用自主研发的剪切仪进行抗剪强度试验,分析影响层间抗剪强度的因素及变化规律。通过设置试验温度、碎石和沥青用量等参数,制定不同的试验方案进行试验。试验结果表明:对层间抗剪强度影响最大的因素是温度,从5 ℃上升到35 ℃时最大剪应力从1.36 MPa下降到0.16 MPa,其次是沥青用量;碎石用量影响最小,而且不同温度下沥青和碎石的最佳用量也不同。在实际工程中,在考虑沥青和碎石的用量时,应考虑工程所在的地区,高温地区沥青的用量应比低温地区稍低一些,同时严格控制碎石用量,提高层间联结质量,明确层间抗剪强度与层间联接各因素的规律。