Low-temperature Cracking Research Articles

Pavement Performance Investigation of Asphalt Mixtures with Plastic and Basalt Fiber Composite (PB) Modifier and Their Applications in Urban Bus Lanes Using Statics Analysis.

A new type of plastic and basalt fiber composite (PB) modifier, which is composed of waste plastic and basalt fiber using a specific process, was used for bus lanes to address severe high-temperature deformation diseases due to the heavy loads of buses. The dense gradations of asphalt mixture with a nominal maximum aggregate size of 13.2 mm (AC-13) and 19 mm (AC-20) were selected to fabricate asphalt mixtures. The impact of the modifier PB on the high-temperature rutting resistance, low-temperature crack resistance, and water damage resistance was investigated experimentally. The experimental results showed that adding the modifier PB could enhance the rutting resistance and water damage resistance of asphalt mixtures significantly while maintaining the low-temperature crack resistance. Then, PB-modified asphalt mixtures of AC-13 and AC-20 were employed into a typical pavement structure of a bus lane in Yangzhou city, China, and three types of designed pavement structures were proposed. On this basis, statics analyses of all of the designed structures were performed using the finite element method. The statics analyses revealed that, compared with the standard axle load, the actual over-loaded axle made the pavement structure of the bus lane suffer a 30% higher stress and vertical deformation, leading to accelerated rutting damage on the bus lanes. The addition of the modifier PB could make the pavement structure stronger and compensate for the negative effect caused by the heavy axle load. These findings can be used as a reference for the pavement design of urban bus lanes.

Influence of Fiber Mixing Process on the Cracking Resistance of Cold Recycled Asphalt Mixture

Fiber reinforcement is often used to improve the road performance of cold recycled asphalt mixture (CRAM). The purpose of this research is to evaluate the impact of fiber mixing process on the cracking resistance of CRAM from multiple perspectives. Four kinds of fiber mixing processes, named A, B, C, and D, were designed by changing the order of fiber addition during the mixing process. Then, semicircle bending tests and indirect tensile tests were conducted to characterize the low-temperature cracking behavior of fiber CRAM. Freeze–thaw cycle tests under both dry and water-saturated conditions were performed to investigate the freeze–thaw damage behavior of fiber CRAM. Furthermore, the fiber dispersion in CRAM was observed using scanning electron microscopy (SEM). The results show that the fiber mixing process has a significant effect on the cracking resistance of CRAM. The CRAM specimens prepared by process C have the largest fracture energy, splitting strength, and fracture work, while the specimens made by process D have the smallest value. Specially, the fracture energy of the specimens prepared by process C is 77.23% larger than that of the specimens prepared by process A, while the fracture energy of the specimens prepared by process D is 5.6% smaller than that of the specimens prepared by process A. The reason for this phenomenon is that the fiber is well dispersed in the specimens prepared by process C, which contributes to obtain a better crack resistance. For all CRAM specimens, with the increase of freeze–thaw cycles, splitting strength and fracture work of fiber CRAM decrease. However, there is an obvious difference in the reduction rate of splitting strength and fracture work, especially for the specimens under the water saturation condition. The specimens made by process C have the smallest reduction rate, which indicates a better resistance to freezing and thawing damage. According to the analysis of fiber macro-distribution state in loose CRAM, the fiber dispersion is affected by the humidity conditions in the mixing environment. The best humidity conditions are obtained for fiber dispersion in process C. Based on the SEM observation, the overlapping bridging network structure can be observed in the microstructure of the specimens prepared by process C, allowing the mixture to better transfer and disperse stress.

A case study of the comparison between rubberized and polymer modified asphalt on heavy traffic pavement in wet and freeze environment

Ground tire rubber (GTR) usage in asphalt pavement with the dry process has gained more prominence in recent times. The objective of this work is to investigate the pavement performance of GTR-modified asphalt pavement and polymer-modified asphalt pavement on heavy volume of traffic conditions in Michigan's wet and freeze environment. A suite of laboratory tests was done to evaluate the pavement performance of GTR-modified and polymer-modified asphalt mixtures. To reveal the strain and stress relationship under different frequencies and temperatures, the dynamic modulus test was applied. The Hamburg wheel tracking device (HWTD) was used to assess the high-temperature deformation resistance. The disc-shaped compact tension (DCT) test was used to evaluate the low-temperature cracking characteristics. The characteristics of the asphalt binder were assessed by the dynamic shear rheometer (DSR) for high-temperature properties and the asphalt binder cracking device (ABCD) for low-temperature properties. After the construction, a field noise test was conducted. The experimental results stated that the polymer-modified asphalt mixture and GTR-modified asphalt mixture showed higher dynamic modulus and better ability to prevent cracking than the conventional asphalt mixture at low temperatures, as well as better permanent deformation and stripping resistance than the conventional asphalt mixture. The fracture energy of the GTR-modified hot mix asphalt (HMA) is 13–16 % larger than the polymer-modified HMA. The number of passes to the stripping point of GTR-modified was 510–518 % higher than the conventional HMA. When compared to the field core, the lab-compacted HMA offers superior pavement performance. The extracted asphalt binder test results show the GTR-modified asphalt has better rutting resistance and cracking resistance than polymer-modified asphalt, and the results in the noise test demonstrated that the rubber-modified asphalt pavement mitigated the noise level by 2–3 dB on the road at different vehicle speeds. Moreover, the pavement condition was noticeably enhanced after the reconstruction of the surface course. The total number of passenger tires to be used in this project is about 2270. To summarize, better rutting and cracking properties in asphalt pavement are shown by the project's utilization of rubber technology. And the GTR-modified HMA is comparable to polymer-modified HMA. Therefore, it may be appropriate to utilize rubber technology on high-traffic volume asphalt pavement in Michigan's wet and freeze climate.

Principle analysis of the mix design and performance evaluation of the asphalt-filler volume equivalent substitution method

The asphalt mixture designed with the Marshall mix design method has stable quality and convenient technical management, but the design range of the asphalt-aggregate ratio is small and difficult to adjust, and is therefore difficult to adapt to the design of functional pavement materials with different requirements. In order to solve these problems, the asphalt-filler volume equivalent substitution method (the volume equivalent substitution method) has been proposed. Based on the optimum asphalt-aggregate ratio of the Marshall method, the asphalt-aggregate ratio in the mixture is increased by the equal volume substitution of asphalt and filler. The increase in the asphalt volume is equal to the decrease in the filler volume, and vice versa. With the condition of constant aggregate gradation, the modified asphalt of asphalt mixtures with filler contents of 6.0%, 4.5%, 3.0%, 1.5%, and 0% is designed based on the AC-13 gradation. Based on this, a high-temperature stability test, a low-temperature crack resistance test, a four-point bending fatigue test, and other tests are carried out for the asphalt mixtures. The results show that the dynamic stability of each mix proportion designed with the volume equivalent substitution method is 4924 times/mm even under the most unfavorable conditions (the filler dose is 0%). The maximum bending tensile strain of the specimen with a 0% filler dosage can reach 9667 μ, which is more than twice that of the specimen with a 6% filler dosage. The fatigue test shows that increasing the asphalt content in the mixture is beneficial for improving fatigue durability. The research results show that the material designed by the volume equivalent substitution method has excellent road performance in each proportion. This paper provides a method for the flexible adjustment of the asphalt-aggregate ratio and the asphalt mixture comprehensive road performance. Using this method, the optimum asphalt-aggregate ratio needs to be determined comprehensively according to the stress characteristics and functional requirements of the pavement and the pavement performance index of the mixture, and further research is still needed.

Research on the Influence of Different Warm-Mix Modifiers on Pavement Performance of Bitumen and Its Mixture

Recently, bitumen pavements have been widely used for road paving. Various scholars have used modified bitumen in road paving to improve pavement performance, thus increasing the service life. In this study, Sasobit, XT-W3, and Evotherm were selected as warm-mix modified bitumen and various investigations including conventional performance, Brookfield rotational viscosity, dynamic shear rheology, Marshall stability, high-temperature rutting, water stability, low-temperature beam bending, fatigue, and compaction performance tests of modified bitumen mixtures were conducted to evaluate the pavement performance of modified bitumen and its mixtures. The effect of different warm-mix modifiers on high-temperature performance, temperature sensitivity, low-temperature performance, and water stability were compared and analyzed. The results indicated that incorporating Sasobit, XT-W3, and Evotherm modifiers enhanced the compressive properties, high-temperature rutting resistance, water stability, and fatigue resistance of the bitumen mixtures, whereas the Evotherm modifier reduced the deformation resistance of the bitumen mixtures. XT-W3 and Evotherm modifiers can effectively improve low-temperature cracking resistance, but Sasobit modifiers have a negative impact.

Extraction and Characterisation of Maltene from Virgin Asphalt as a Potential Rejuvenating Agent

The wide application of reclaimed asphalt pavement (RAP) is hindered due to the highly brittle nature of the material, which contributes a major factor towards cracking-related distresses. While the utilisation of rejuvenating agents has been shown to enhance the flexibility of RAP, they also trigger certain negative effects on the performance of asphalt mixtures. In view of this, potential rejuvenators should be able to alter the rheological properties of asphalts to limit fatigue issues and enhance the potential of low-temperature cracking. Therefore, this study aimed to investigate the possibility of extraction and characterisation of maltene from virgin asphalt (VA) as a potential rejuvenating agent in RAP. Several physicochemical characteristics were examined, including density, viscosity, gas chromatography–mass spectrometry (GC–MS), Fourier-transform infrared (FTIR) spectroscopy, CHNS elemental analysis, and energy dispersive X-ray (EDX) analysis. Finally, the stiffness modulus characteristics of the different types of asphalt binders were evaluated at low and high temperatures. The results demonstrated that maltene was successfully extracted from VA using petroleum ether. In addition, the GC–MS showed that the extracted maltene contained polar and non-polar compounds with low molecular weights compared to VA. Furthermore, the spectra curve of maltene was very similar to that of asphalt, indicating its compatibility with asphalt binder and prospective use. Finally, adding maltene to aged asphalt decreased stiffness values to 0.0063, 0.0499, and 0.0108 MPa, which are equivalent to VA values (0.0061, 0.0481, and 0.0104 MPa) at loading times of 1.0, 0.1, and 0.55 s, respectively. Meanwhile, the stiffness modulus characteristics at low temperature were restored with the addition of maltene.

Assessing the impact of ultra-thin diamond nanothreads on the glass transition temperature of a bituminous binder.

Low-temperature cracking and rutting are the most destructive problems of bitumen that hinder the application of high-performance bitumen engineering, which is dependent on its glass transition temperature (Tg). Through in silico studies, this work has systematically investigated the Tg of a bituminous binder with the addition of diamond nanothread (DNT) fillers with varying filler content, alignment, distribution, and functional groups. In general, the glass transition phenomenon of the bitumen is determined by the mobility of its constituent molecules. Tg is found to increase gradually with the increase in the weight percentage of DNT and then decreases when the weight percentage exceeds 5.05 wt%. The enhancement effect on Tg is weakened when DNTs are distributed vertically or functionalized with functional groups. Specifically, DNT fillers induce inhomogeneity, which promotes the motion of small molecules while hindering the motion of large molecules. The aggregation of DNTs and the molecular environment in the vicinity of DNTs directly affect Tg. In summary, aggregation and adhesion are the dominant mechanisms affecting the mobility of the constituent molecules in the DNT/bitumen system and thus its glass transition temperature. This work provides in-depth insights into the underlying mechanisms for the glass transition of a bituminous binder, which could serve as theoretical guidance for tuning the low-temperature performance of the bituminous binder.

Comprehensive Evaluation of Pavement Waterproofing Layer Materials for Bridge and Tunnel Projects

In order to achieve the preferential selection of pavement waterproofing layer materials, so that the waterproofing layer material selection is more scientific and objective, eliminating the impact of subjective factors, this paper uses hierarchical analysis to build a hierarchical evaluation model of pavement waterproofing layer. Based on the literature review method, a two-tier evaluation index of the layered evaluation model of pavement waterproofing layer materials was constructed, mainly three first-tier indexes of performance assessment, service life assessment, and cost assessment, under which the second-tier indexes of waterproofing performance, durability, weather resistance, impermeability, crack resistance, thermal stability, aging resistance, low temperature crack resistance, fatigue resistance, material cost, construction cost, and maintenance cost were established. And through the hierarchical analysis method of calculation, test the consistency by deriving the weight of each indicator, where the most important first layer of indicators for performance, the weight of 0.5390, the second layer of indicators is more important for waterproof performance (0.5303), thermal stability (0.3550), low-temperature crack resistance (0.3550), material cost (0.6333). The study provides a scientific and objective evaluation model for the preferential selection of pavement waterproofing layer materials.

Improvement of the Low-Temperature Cracking and Shear Resistance of Semi-Flexible Mixture Modified by Polymers

Improvement of the Low-Temperature Cracking and Shear Resistance of Semi-Flexible Mixture Modified by Polymers

Design and experiment of KH550 modified high transparency clear asphalt and light colored asphalt pavement

In this study, clear asphalt (CA) modified by a silane coupling agent (KH550) was prepared. A light-colored asphalt mixture (LCAM) was prepared by mixing CA and limestone aggregate. The properties of CA were analyzed by performing the asphalt conventional test, pull-off test, optical test, Fourier-transform infrared spectroscopy, dynamic shear rheometer examinations, and contact angle test. The road performance, chromatic and reflectivity properties of the LCAM specimens were evaluated. The results indicated that the transparency of CA was 76.5%, and the modification of KH550 improved the adhesion of CA. The thermal stability and low-temperature crack resistance of CA were better than those of conventional asphalt. After modification with KH550, the dynamic stability of LCAM decreased, and the moisture resistance and lowtemperature performance increased. The LCAM had higher RGB values and higher reflective performance. Overall, the CA and LCAM materials that we prepared can be used effectively for future road paving.

Reutilization of different bamboo residue fiber as sustainable asphalt modifier: performance evaluation and parameter recommendation.

The large-scale application of bamboo has led to the production of enormous amounts of waste in the form of bamboo residue. In order to reuse the bamboo residue, three types of bamboo fiber (sinocalamus affinis fiber (SAF), green bamboo fiber (GBF), and phyllostachys pubescens fiber (PPF)) extracted from bamboo residues were studied. The properties of bamboo fiber modified asphalt were evaluated by ductility test, cone penetration test, rheologicaltest, and low-field nuclear magnetic resonance (LF-NMR) test. Furthermore, the dispersion properties of bamboo fiber were analyzed by dispersion uniformity test. The results show that the mechanical and high-temperature properties of the bamboo fiber modified asphalt are obviously improved while the low-temperature crack resistance is only slightly weakened. Meanwhile, the signal intensities of SAF modified asphaltand GBF modified asphalt are basically the same before and after aging. It proved the viscosity is not changed much, which verified the good anti-aging properties of SAF modified asphaltand GBF modified asphalt. However, PPF is the best dispersed uniformly in the asphalt. The maximum allowable length and dosage of bamboo fibers is recommended as follows: SAF (9mm, 2.0%), GBF (6mm, 1.5%), and PPF (6mm, 2.0%). The application of bamboo residue in asphalt not only emphasizes the recycling value of bamboo residue waste but also provides an optional natural fiber material for asphalt pavement construction, which meets the requirements of sustainable development.

Optimization of stone matrix asphalt mixture design based on fracture mechanics approach

Low-temperature cracking is one of the most common pavement distresses that occurred in a short period of service life and is known as premature distress. Stone matrix asphalt (SMA) with gap gradation, which has high strength and rutting performance, should also perform well against low-temperature cracking. So in this research, the criteria of SMA mixture design based on the National Cooperative Highway Research Program (NCHRP) 9–8 were optimized in a way so that SMA has the best performance against low-temperature cracking. Therefore, the optimization process was carried out using the response surface method (RSM) considering the combined effect of compaction energy (Ndesign) and binder content as mixture design factors. The fracture evaluation was done based on stress and energy. So critical stress intensity factor (KIC) and critical fracture energy (Gf) were used as fracture evaluation parameters. After conducting the tests and analyzing the results, it was found that 6.09 % binder with Ndesign = 78 leads to a design with a desirability of 0.869, which passed all the NCHRP 9–8 criteria of the mixture design in line with the maximum fracture performance.

Evaluation of long-term fracture behavior of amorphous poly alpha olefin (APAO)-modified Hot Mix Asphalt (HMA) under modes I and II at low and intermediate temperatures

Opening and in-plane sliding cracks are among the most important cracks in flexible pavements, resulting from tensile and sliding deformations, respectively. Loading and freeze–thaw damage (FTD) are the two main factors for the initiation and growth of these types of cracks, which may lead to two types of cracks, intermediate-temperature cracks (ITCs) and low-temperature cracks (LTCs), depending on the ambient temperature. Therefore, in order to simulate the tensile and sliding deformations of mixtures containing ITCs and LTCs, two geometries called symmetric sample SCB (containing vertical cracks) and classical-modified sample SCB-2 (containing angular crack) resulting from mode I loading, as well as two geometries named modified sample SCB-1 (containing vertical crack) and modified sample SCB-2 (containing vertical crack) resulting from mode II loading at temperatures of + 15 and −15 °C were evaluated. In order to control and reduce the cracking potential of Hot Mix Asphalt (HMA), the amorphous poly alpha olefin (APAO) additive was used in percentages of 3, 6, and 9 % and compared with the mixture without additives. Also, all specimens were under 0 and 3 FTD cycles. In order to measure and evaluate the fracture behavior results of unmodified and APAO-modified specimens, fracture toughness indicators (KIC and KIIC) and fracture energy (GF) were calculated. Eventually, to investigate the fracture behaviors of mixtures before the maximum loading, two indicators of tensile stiffness index (TSI) and tensile strength (TS) were evaluated. The results showed that at −15 °C and from the perspective of KIC (or KIIC) and GF (under modes I and II) indicators, the fracture resistance in specimens comprising 6 and 9 % APAO under 0 and 3 FTD cycles was higher compared to the base specimen. At + 15 °C and from the perspective of KIC (or KIIC) indicator, the fracture resistance of mixtures containing 6 and 9 % APAO in 0 and 3 FTD cycles was higher than the unmodified specimen; however, from the point of view of GF (under modes I and II) indicator, the mixture containing 9 % APAO represented greater fracture resistance. Also, it was indicated that mixtures modified with APAO represented higher TSI as well as TS values; therefore, APAO additive increased the resistance to elastic deformation by increasing the fracture performance before the maximum loading. Finally, mixtures containing 6 and 9 % APAO were chosen as the best mixture; however, the use of these mixtures in areas with temperatures lower than −15 °C should be done with caution due to the relative reduction in flexibility of the mixture containing APAO additive.

Performance and Verification of High-Modulus Asphalt Modified by Styrene-Butadiene-Styrene Block Copolymer (SBS) and Rock Asphalt

High asphalt grade and poor high-temperature performance are the primary reasons for the permanent rutting deformation of asphalt pavement. However, the low grade of asphalt and the poor low-temperature performance and fatigue life of the mixture can easily lead to the low-temperature cracking of asphalt pavement. With the rapid increase in road traffic, volume, and traffic load, the performance requirements of road asphalt materials are becoming higher and higher. High-modulus asphalt has excellent temperature stability and good fatigue resistance. However, high-modulus asphalt is expensive, so its use can greatly increase the pavement cost, restricting its wide application in road engineering. It is necessary to find an economical way to produce modified asphalt to meet the current road requirements. The aim of this study is to investigate the effects of styrene-butadiene-styrene block copolymer (SBS) and rock asphalt on the road performance of modified high-modulus asphalt, in which the replacement level of SBS and rock asphalt below 8 wt.% are compared. Apart from the conventional performance measurements, such as softening point, penetration, ductility and viscosity, thermal storage stability and rheological properties are also measured. The test results show that the composite modification of SBS and North American rock asphalt can effectively improve the high-temperature resistance and reduce the temperature sensitivity of 50# matrix asphalt, but it has no obvious improvement on its low-temperature performance. The preferred ternary blending system containing 4~6 wt.% SBS and 6~8 wt.% rock asphalt was obtained by performance analysis. It was verified that the performances of high-modulus asphalt mixture with the ternary blending asphalt above all meet the requirements of high-modulus asphalt mixture performance index.

Effect of Ultraviolet Aging on Rheological Properties and Microstructure of Rubber Asphalt in High Altitude Area

High altitude and strong ultraviolet radiation in Qinghai Tibet Plateau induces the brittleness and hardening of asphalt, which leads to temperature cracks, pits and other diseases of asphalt pavement. In this paper, the accelerated UV aging test of rubber asphalt was carried out by simulating the UV radiation intensity in Xining, Qinghai Province, and the rheological properties and microstructure changes of rubber asphalt were analyzed. The results show that the complex modulus of rubber asphalt increases exponentially with the extension of UV aging time. The phase angle of rubber asphalt decreases linearly with UV aging, and the deformation recovery performance of rubber asphalt increases; UV radiation has the most significant effect on the micro morphology of rubber asphalt. With the further extension of UV aging time, micro cracks are formed in rubber asphalt, and the low-temperature crack resistance and fatigue failure resistance of rubber asphalt are gradually reduced.

Experimental investigation of the rheological behaviors of bitumen and stone matrix asphalt mixtures using waste material

The mechanical performance characteristics of bitumens and mixtures can be improved by the utilization of various modifiers, such as liquid rubber and natural bitumen. In the current study, the effect of liquid rubber and natural bitumen on the rheological behaviors of natural bitumen and liquid rubber/natural bitumen modified samples and Stone Matrix Asphalt (SMA) mixtures was evaluated. In this regard, 0, 5, 15, 25 and 35% natural bitumen as well as 7, 9, 11, and 13% liquid rubber by weight of binder were used to modify virgin binder. Also, to evaluate asphalt binder characteristics, rotational viscosity (RV), dynamic shear rheometer (DSR), multiple stress creep recovery (MSCR), and linear amplitude sweep (LAS) tests were applied. Furthermore, modified natural bitumen and liquid rubber/natural bitumen mixtures were evaluated by four-point beam fatigue (FPBF), resilient modulus (Mr), indirect tensile strength (ITS), dynamic creep, and wheel track experiments. The results indicated that the use of natural bitumen reduces the low-temperature cracking resistance of bitumens, while adding liquid rubber into natural bitumen modified bitumens, the low-temperature cracking of modified bitumens enhances. Moreover, application of natural bitumen resulted in a reduction in the resistance of bitumens against fatigue cracking, while using liquid rubber in natural bitumen modified samples, the fatigue life of bitumens increases. Also, natural bitumen enhances the resistance of bitumens to permanent deformations, while liquid rubber decreases it. The mixtures’ test results indicated that natural bitumen increases Mr, permanent deformation resistance, tensile strength, fracture energy, and intermediate temperature performance of mixtures. Moreover, liquid rubber has a negative effect on performance of natural bitumen-modified specimens.

Optimization of induction heating parameters for improving Self-healing performance of asphalt mixture through partial least square model

The self-healing capacity of asphalt mixture based on induction heating, determined by induction heating parameters, has been the key issue regarding asphalt mixture’s self-healing. This study tried to investigate how induction heating parameters determine asphalt mixture’s self-healing concerning parameters’ coupling effect. AC-13 asphalt mixtures consisting of steel fiber and steel slag were prepared for induction heating. Dynamic stability, low-temperature cracking resistance, and moisture stability were studied. Single-parameter (SP) method and multi-parameter (MP) method considering the coupling effect were separately used to achieve the optimum parameter combinations for improving the healing ratio (HR). A functional relationship by the MP method between HR and induction heating parameters was established based on partial least square through Data Processing System software (DPS). Results verified the coupling effect among parameters. Asphalt mixture with 8 % steel fiber content showed optimum pavement performance. The optimum parameter combination of SP method for HR was: 8 % steel fiber content, 5 mm, 180 s, and 16 kW for induction distance, time, and power respectively, and the optimum HR based on SP method was 69 %. The optimum parameter combination of MP method was: 8.8 %, 5 mm, 107 s, and 24 kW, while its predicted HR was 77 % and the actual HR was 75 %. Therefore, the MP method based on partial least square showed a greater improvement of HR than SP method, which proved that coupling effects of induction heating parameters affect asphalt mixture’s self-healing capacity.

Self-healing behaviors and its effectiveness evaluations of fiber reinforced shape memory polyurethane/SBS modified asphalt mortar

To further understand self-healing behaviors of cracks in fiber reinforced asphalt mortar, shape memory polyurethane/styrene butadiene styrene modified asphalt mortar (PSAM) was first prepared. Then shape memory properties of lignin fiber reinforced PSAM (LFAM) and polyester fiber reinforced PSAM (PFAM) were characterized. The effects of filler-asphalt ratio and fiber type on the multi-cycle self-healing results of LFAM and PFAM were discussed. The self-healing effectivenesses in each crack-healing cycle and whole multi-cycle self-healing process were evaluated. Results indicate that shape memory properties of LFAM and PFAM are lowered as their filler-asphalt ratios are increased. LFAM and PFAM show satisfactory shape memory properties, but PFAM has better shape memory performance than LFAM, and its higher shape recovery force further improves its self-healing effectivenesses. Also, when the filler-asphalt ratio is raised, both creep deflection and creep slope of LFAM and PFAM are decreased, but their creep stiffnesses are increased. The self-healing mostly restores the mechanical properties of LFAM and PFAM. PFAM shows higher low-temperature crack resistance than LFAM. Meanwhile, the creep deflection and creep slope of PFAM are higher than those of LFAM as the filler-asphalt ratio is the same one, while the creep stiffness of PFAM is smaller than that of LFAM. LFAM shows a brittle failure at low temperature, but PFAM shows a flexible failure. Additionally, the self-healing effectiveness of LFAM and PFAM in each crack-healing cycle is decreased as the crack-healing cycle is increased. Finally, PFAM shows a higher self-healing rate than LFAM. The mechanical and self-healing characteristics of PFAM are more stable than those of LFAM. The multi-cycle self-healing test indexes can assess the multi-cycle self-healing effectiveness of LFAM and PFAM.

Study on the mixing process improvement for hot recycled asphalt mixture

Existing studies have shown that the mixing process of hot recycled asphalt mixture has a considerable effect on its performance. The conventional mixing process applied widely in pavement industry, mixed the RAP and virgin aggregate first. Then asphalt binder was added for the second mixing. This conventional mixing process, however, ignored the asphalt-adsorbing capacity of different aggregates, may resulting in the heterogeneity of recycled asphalt mixture after mixing. Adjusting the mixing sequence of RAP, virgin asphalt binders and aggregate is confirmed as an effective way to solve the above-mentioned issue. However, the change in the demand for virgin asphalt binders for each mixing step, which is raised from aggregate gradation and different RAP content, is not taken into consideration in this improved method. In view of this weakness, this study proposed an improved mixing technique for hot recycled asphalt mixture, which comprehensively considered the mixing sequence of materials and determined the amount of asphalt binders added for different RAP content based on the specific surface area of aggregates. And the prediction formula for the amount of virgin asphalt added each step is further proposed from the perspective of aggregate specific surface area. Several laboratory experiments were conducted to verify the feasibility of this improved technique. The results indicated that, the improved mixing technique could contribute to enhancing the water stability, low temperature cracking resistance and fatigue performance of hot recycled asphalt mixtures. And the higher the content of the RAP used in recycled mixture, the stronger the improvement of the improved mixing process.

Study on the Low Temperature Cracking Mechanism of Steel Slag Asphalt Mixture by Macroscale and Microscale Tests

Steel slag’s physical and mechanical properties are close to natural gravel, which can replace natural aggregate with paving asphalt mixture pavement. Based on macro- and microscales, it is necessary to study the low-temperature cracking characteristics of steel slag asphalt mixture (SAM). This paper aims to study the low-temperature cracking characteristics and influence mechanism of steel slag asphalt mixture by macro- and microscale tests. This article analyzes the strain field distribution and cracks propagation law of SAM at low temperatures by the three-point bending test combined with digital image correlation (DIC). The microscopic pore characteristics of SAM were studied by nuclear magnetic resonance (NMR). Based on fracture mechanics and weight function theory, the variation law of the stress intensity factor at the crack tip of the asphalt mixture with crack propagation is derived. The correlation between microscopic pores and low-temperature cracking characteristics of asphalt mixture was analyzed based on the grey correlation theory. The results show that SAM has more small and less large pores, and its total porosity is lesser than that of the basalt asphalt mixture (BAM). The initial elastic stage of SAM resisting bending deformation is longer than that of BAM, the bending strain energy density (dW/dV) is larger, and the crack penetration is slower. The horizontal strain energy density (DE) of SAM is greater than BAM, and the crack tip stress is greater. The porosity correlates well with the low-temperature crack resistance of the asphalt mixture.