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.     

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.     

Laboratory study of the effect of RAP conditioning on the mechanical properties of hot mix asphalt containing RAP

This paper evaluates the effect of reclaimed asphalt pavement (RAP) laboratory conditioning on the rheological properties of recycled hot-mix asphalt. Four different conditioning processes were used on a single RAP source before mixing: unheated RAP, RAP heated at 110 °C in a microwave, RAP heated in a covered pan at 110 °C in a draft oven, and RAP heated in a non-covered pan at 110 °C in a draft oven. Dense graded 20 mm HMA was designed using a PG 64-28 binder and mixed with 25 % of the four different conditioned RAPs. Thermal stress restrained specimen test (TSRST) and complex modulus test were used to characterize RAP conditioning effect. Test results showed that the complex modulus of the four mixes has no different rheological behaviour, and did not affect TSRST results as much.     

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。     

Experimental investigation of basic oxygen furnace slag used as aggregate in asphalt mixture

Chinese researchers have commenced a great deal of researches on the development of application fields of basic oxygen steel making furnace slag (BOF slag) for many years. Lots of new applications and properties have been found, but few of them in asphalt mixture of road construction engineering. This paper discussed the feasibility of BOF steel slag used as aggregate in asphalt pavement by two points of view including BOF steel slag's physical and micro-properties as well as steel slag asphalt materials and pavement performances. For the former part, this paper mainly concerned the mechanochemistry and physical changes of the steel slag and studied it by performing XRD, SEM, TG and mercury porosimeter analysis and testing method. In the second part, this paper intended to use BOF steel slag as raw material, and design steel slag SMA mixture. By using traditional rutting test, soak wheel track and modified Lottman test, the high temperature stability and water resistance ability were tested. Single axes compression test and indirect tensile test were performed to evaluate the low temperature crack resistance performance and fatigue characteristic. Simultaneously, by observing steel slag SMA pavement which was paved successfully. A follow-up study to evaluate the performance of the experimental pavement confirmed that the experimental pavement was comparable with conventional asphalt pavement, even superior to the later in some aspects. All of above test results and analysis had only one main purpose that this paper validated the opinion that using BOF slag in asphalt concrete is feasible. So this paper suggested that treated and tested steel slag should be used in a more extensive range, especially in asphalt mixture paving projects in such an abundant steel slag resource region.     

Laboratory investigation of the properties of epoxy asphalt rubber (EAR)

Crumb rubber is preliminarily mixed with asphalt in wet process to produce ductile and elastic asphalt rubber (AR), which has been extensively used in high performance bituminous mixtures for road pavement. Epoxy asphalt is a thermosetting polymer modified asphalt with excellent performance and has been widely applied on the pavement of steel bridge decks. Epoxy asphalt rubber (EAR) was prepared by mixing AR with the epoxy (EP). The effect of AR concentration on the phase-separated morphology, viscosity, thermal stability and mechanical properties of the neat EP were compared with that of asphalt. Laser scanning confocal microscopy observations revealed that AR particles disperse in the continuous epoxy phase with co-continuous phase-separated structures in EARs with 40 and 50 wt% AR. However, the phase inverts to continuous asphalt structures with dispersed spherical and co-continuous epoxy phase as AR concentration reaches 60 wt%. The addition of AR increases the viscosity of the neat EP. The thermal stability of the neat EP is improved with the incorporation of AR. The presence of AR decreases the tensile strength of the neat EP, while the elongation at break of the neat EP increases with the increase of AR concentration. The viscosity and mechanical properties of EAR with 50 wt% AR completely satisfy the technical requirement of hot-mix epoxy asphalt binder for steel bridge deck pavements.     

基于VIC-3D技术的沥青混合料界面力学特性实验研究

为研究沥青混合料开裂及裂缝发展规律,通过SBS改性沥青混合料半圆试件三点弯曲实验,基于数字图像相关技术,利用VIC-3D(video image correlate,3D)采集系统对试件从加载到破坏全过程进行图像采集,并利用VIC-3D软件计算得到试件在受弯拉过程中三维位移场、应变场的变化及变化速率,分析并得出裂缝出现及发展规律;利用扫描电子显微镜对开裂试件断口进行观察,结合位移场、应变场及其变化速率揭示了试件开裂及裂缝发展的机理。结果表明,利用水平方向应变随时间变化规律可探究裂缝的发展轨迹;沥青混合料的开裂和裂缝发展沿沥青砂浆与粗集料的界面区发生,且趋向于表面情况相对差的集料的界面区;裂缝发展速率与界面区的厚度有关,可根据界面区厚度的代表值预测裂缝发展速率。     

Thermal conductivity of reclaimed asphalt pavement (RAP) and its constituents

Up to the present, most work on the use of reclaimed asphalt pavement (RAP) has been empirical in nature. Very recent advances have demonstrated that finite-element techniques can be effectively used for modelling asphalt mixing drums in order to optimise the relative proportions of new and recycled materials and to determine the amount of time required to achieve full melting inside of the drum. A necessary prerequisite for the modelling is a definitive knowledge of the thermal conductivities of RAP and its components. This need motivated the present experimental work which encompassed RAP particles, RAP particles with the asphalt binder removed, and pure asphalt binder of different degrees of ageing. Also evaluated were taconite tailings, residual rock from the processing of iron-containing ore, and sand. The tailings have been mentioned as a candidate aggregate. The conductivity results for the solid media were related to three metrics: (a) the size ranges of the solids, (b) the density of the sample as a whole and (c) the porosity of the sample. All of the conductivity results for the investigated solid media fell in the range from 17 to 30 W/m °C. The measured conductivities of the binder ranged from 0.17 to 0.19 W/m °C.     

高温持续荷载作用下复合纤维对沥青混合料稳定性的影响

为了研究复合纤维沥青混合料在高温持续荷载作用下的稳定性,根据工程中常用纤维和已有研究成果,选用了三种复合纤维,对沥青混合料的变形发展规律及内部空隙结构特征开展试验研究。首先采用加速加载试验,对不掺加纤维和掺加复合纤维的沥青混合料在高温持续荷载作用下的横断面变化和车辙深度进行分析;再利用X-CT扫描技术与VG软件三维重构功能,研究不同纤维沥青混合料的空隙组成及形态特征演变规律。结果表明:高温持续荷载作用下,掺加纤维混合料横断面变形呈现"W"字型,不同纤维在沥青混合料中发挥"加筋"、"增黏"、"吸油"的协同作用,能显著减小沥青混合料的流动变形,且掺加复合纤维III的混合料流动变形最小;掺加不同复合纤维的沥青混合料可采用y=a-b·ln(x+c)进行车辙深度预估。加速加载试验后,掺加三种复合纤维的沥青混合料空隙率最大仅增加7%,分形维数仅降低不到0.5%,表明掺加三种复合纤维基本不改变沥青混合料的空隙结构特征,而且在高温持续荷载作用下,可较好地保持沥青混合料内部空隙的三维形态特征,进而改善沥青混合料的抗变形能力。     

反应性弹性体三元共聚物改性沥青及其混合料性能与机制

采用黏度试验和动态剪切流变试验研究了反应性弹性体三元共聚物（RET）对基质沥青与苯乙烯-丁二烯-苯乙烯嵌段共聚物（SBS）改性沥青性能的影响,通过原子力显微镜（AFM）分析了SBS改性沥青和RET-SBS改性沥青的表面形貌特征,并采用车辙试验、低温弯曲试验、弯曲疲劳试验及加速加载试验评价了RET改性沥青混合料的各项技术性能,最后通过Weibull分布,分析了不同RET改性沥青混合料在不同失效概率下的疲劳性能。结果表明：RET的掺入提高了沥青的黏度和抗车辙因子,对沥青的高温性能有较大改善;通过掺入RET-SBS,增加了改性沥青中的黏性成分;相较于SBS改性沥青,RET-SBS改性沥青的表面粗糙度显著增大;RET改性剂能够明显改善沥青混合料的高温稳定性;RET与SBS改性剂复配,可有效弥补RET对沥青混合料低温性能的不足,明显改善沥青混合料的疲劳性能和高温长期稳定性。     

Effect of crumb rubber gradation on a rubberized cold recycled mixture for road pavements

Cold recycling technique represents the most environmental friendly solution for pavement rehabilitation nowadays. In fact, this technique allows the use of the highest percentage of reclaimed asphalt avoiding the energy consumption related to aggregates heating required by the traditional hot mix asphalt design. The mix design represents a key phase of the cold mix production. The study of workability and compactability properties combined with a deep laboratory investigation is required. The idea of introducing crumb rubber in the cold mixtures was developed based on the concept of maximizing the valorization of recycled materials together with the goal of achieving high performance. In the present research project, two different gradations of crumb rubber, processed with the traditional grading method, have been adopted for the production of a cold recycled mixture stabilized with bitumen emulsion and cement. The spring-back effects of the rubber particles, which occur after compaction, together with the Indirect Tensile Strength and the Indirect Tensile Stiffness Modulus have been studied. The results show that the gradation of the adopted crumb rubber sensibly affects the compaction and mechanical properties of the cold recycled mixture.     

浇筑式沥青混凝土性能影响因素研究

浇筑式沥青混凝土具有较高的变形随从性而在钢桥面铺装上得到了很多的应用,为研究浇筑式沥青混凝土的性能影响因素,采用4种合成级配、3种沥青用量,系统评价了各因素对浇筑式沥青混凝土性能的影响。通过流动度试验和贯入度试验确定最佳含油量,由高温车辙试验和低温弯曲试验进行路用性能检验。研究表明,级配形式和沥青含油量变化对混合料的性能指标的影响较大,因此,实际施工时应严格控制混合料的级配和沥青用量。     

Performance of porous asphalt mixture with various additives

The objective of this study is to evaluate the approaches to improve the durability and strength of the porous asphalt through laboratory testing. Porous asphalt specimens were prepared using three types of binders: high-viscosity binder (HVB), PG76-22 and PG70-22. Various additives: fibre, hydrated lime and DBS polymer, were utilised in the porous asphalt. Comprehensive laboratory tests, including strength test, binder draindown test, Cantabro abrasion test, moisture susceptibility test, rutting test, thermal stress restrained sample test, and permeability test, were conducted. It is found that HVB significantly improved the overall performance of the porous asphalt; DBS additive improved its high-temperature performance, but lowered the cracking resistance at low temperature as well as the durability; fibre enhanced its durability and anti-cracking performance at low temperature; hydrated lime improved its moisture stability while weakening its durability. It is concluded that HVB and polyester fibre should be used in all porous asphalt; DBS additive is good for porous asphalt in high-temperature areas, and hydrated lime can be added to porous asphalt in rainy areas.     

低温成型抗车辙功能沥青混合料路用性能研究

我国寒冷地区热拌沥青混合料的施工季节短,为了减少低温条件下压实成型对沥青路面使用性能的不利影响,开展掺加低温施工高性能添加剂的沥青混合料路用性能的研究工作。研究了掺加低温高性能添加剂的沥青混合料的压实成型工艺,通过室内车辙实验、水稳定性实验和力学性能实验分析其路用性能特点。结果表明,掺加PRLT添加剂的沥青混合料能够在相对较低的温度条件下成型,提高了沥青混合料的可压实性,可以延长热拌沥青混合料的施工季节,适用于寒冷地区的沥青路面建设;该添加剂显著提高沥青混合料高温抗车辙性能,并一定程度上改善低温抗裂性能、抗水损害性能和力学性能。     

聚乙二醇对沥青及其混合料储热性能的影响

利用相变材料物相转变时吸收或释放大量热量而本身温度保持不变的特性,将相变材料聚乙二醇应用于沥青及沥青混合料中。考察了聚乙二醇对沥青性质、相变沥青混合料降温性能和高温稳定性的影响。结果表明,聚乙二醇的掺入可降低沥青的温度敏感性,制备的相变沥青与石灰质的集料粘附性良好;相变沥青混合料具有良好的储热能力,掺量为15%和20%时,相变沥青混合料在自然光照条件下的能主动降温1.3℃和1.4℃,其高温稳定性能满足当前规范要求。此外,通过红外光谱分析得出聚乙二醇与沥青的相互作用形式仅仅是物理共混。     

沥青路面导热系数测试及其对路面温度场影响的模拟

沥青路面温度场是沥青路面车辙计算和沥青混合料粘弹性分析的重要参数,路面温度场的精确预估涉及到面层各层材料热物理参数的精确测试,因此沥青混合料热物理参数的准确测试变得越来越重要.基于对测试结果重现性和可比性的考虑,采用瞬态热线法进行沥青混合料导热系数的测试,选取几种有代表性的混合料进行了导热系数测试,分析了影响导热系数的因素.基于ABAQUS模拟计算路面温度场分布,通过改变路面上、中、下面层的导热系数,计算路面温度场分布变化的情形,对计算结果的分析发现,上面层导热系数对面层温度场分布的影响更显著,可以预期通过改变上面层导热系数能达到主动改善沥青路面工作温度的目的.     

复合多聚磷酸改性沥青混合料疲劳性能

采用四点小梁弯曲疲劳试验方法,考虑不同试验温度和不同应变水平等因素的影响,研究多聚磷酸（PPA）-苯乙烯-丁二烯-苯乙烯嵌段共聚物（SBS）复合改性沥青混合料疲劳性能的变化规律,并与SBS改性沥青混合料进行对比。结果表明：相同条件下,PPA-SBS改性沥青混合料比SBS改性沥青混合料残留劲度模量比大,损伤因子小,稳定阶段耗散能变化率的平均值小,疲劳损伤演变小,抗疲劳性能好;同一温度下,应变水平越大,残留劲度模量比越小,损伤因子越大,耗散能变化率维持稳定阶段的平均值越大,疲劳损伤演变越剧烈,抗疲劳性能越差;同一应变水平下,15℃时沥青混合料试件的抗疲劳性能优于10℃时。     

The fracture process zone in asphalt mixture at low temperature

The fracture process zone (FPZ) is a key factor to mechanistically characterize material fracture. This study investigates the FPZ of asphalt mixture at low temperature. The fracture process under a semi-circular bend (SCB) test of seven asphalt mixtures that represent a combination of different factors was monitored using an acoustic (AE) system with eight piezoelectric sensors. The size of FPZ was estimated by locating micro-cracks that correspond to 95% AE energy before peak load in the vicinity of the initial crack tip. The experimental data illustrates the significant influence of test temperature on the behavior of the asphalt mixture. Comparison results showed that the size of the FPZ significantly depends on air voids and aggregate type, but is less depend on the asphalt content. It was found that at a very low temperature, different loading rates produced very close FPZ, both for the width and length. No obvious difference was observed on the width of the FPZ for the three different initial notch lengths, whereas the length of the FPZ was found significantly increases with the decrease of the notch length. The size of FPZ was also numerically estimated for one case with the cohesive zone model (CZM) calibrated by experimental data from the same SCB test. The FPZ size obtained with both methods agrees reasonably with each other.