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In cold regions with high daily temperature gradients (>20 °C), the durability of cement-stabilized macadam (CSM) base materials is poor and prone to cracking. To effectively reduce the cracking of semi-rigid base layers in cold regions with high daily temperature gradients and extend fatigue life, this study focused on cracking and fatigue characteristics of CSM with a 10% commercial early strength agent (ESA) added by the external mixing method under different curing conditions. The ESA was manufactured by Jiangsu Subote New Materials Co., Ltd. (Nanjing, China). The curing conditions were divided into variable temperature (0–20 °C) and standard temperature (20 °C). CSM curing was carried out through a programmable curing box. The research results indicated that the variable temperature curing conditions reduced the strength and fatigue resistance of CSM and accelerated the modulus attenuation rate of CSM. At the same time, the drying shrinkage of CSM was greater. The temperature shrinkage coefficient and strain of CSM under variable temperature conditions were smaller than those under standard temperature conditions. The effect of variable temperature conditions on the cracking and durability of CSM could not be ignored in cold regions. Compared to standard temperature curing conditions, the indirect tensile strength of CSM reduced by 31.04% under variable temperature conditions, the coefficient of variation increased by 2.97 times, and the discrete type significantly increased. Compared with CSM without ESA, the dry and temperature shrinkage strains of CSM with 10% ESA were reduced by 24.65% and 26.10%, respectively. At a stress level of 0.6, compared to standard temperature curing conditions, the fatigue life of CSM decreased by 97.19% under variable temperature conditions. Under variable temperature conditions, the fatigue life of CSM with 10% ESA increased by 196 times compared to 0% ESA. Adding ESA enhanced the anti-shrinkage cracking, strength, and durability of CSM under variable temperatures. ESA incorporation effectively compensated for the weakened characteristics of CSM under variable temperature conditions. The study proposed a practical approach for boosting the durability of CSM in cold environments. [ABSTRACT FROM AUTHOR]

With increasing societal attention being directed to resource and environment problems, the research focus on high reclaimed asphalt content mixtures has become pertinent. The degree of asphalt fusion in the thermal regeneration process of a high RAP content reclaimed asphalt mixture has a great influence on its performance. In order to explore the development process of hot in-place recycling mixture performance along with internal asphalt fusion, this study conducted research on a geothermal regeneration mixture with 80% RAP content. Dynamic shear rheology (DSR), infrared spectroscopy, and scanning electron microscopy were used to investigate the fusion of recycled mixture under different placement times (1 day, 4 days, and 7 days), and the road performance and fatigue life of the recycled mixture under different placement times were then studied. The results showed that the fusion degree of old asphalt and new asphalt in a recycled asphalt mixture reached 100%, and gradually increased with the extension of placement time. With the increase in placement time, the high temperature performance of the regenerated mixture gradually decreased, the water stability gradually increased, and the low-temperature performance and fatigue life significantly increased from 1 day to 7 days, by 19% and 32%, respectively. [ABSTRACT FROM AUTHOR]

Fatigue cracking is a common form of flexible pavement distress, which generally starts and spreads through bitumen. To address this issue, self-healing elastomer (SHE) modified bitumens were elaborated to assess whether these novel materials can overcome the neat asphalt (NA) fatigue performance and whether the current failure definition, failure criterion, and fatigue-restoration criteria can fit their performance. All bitumens were subjected to short-term and long-term aging. Linear amplitude sweep (LAS) test, LAS with rest period (LASH), and simplified viscoelastic-continuum-damage (S-VECD) model were utilized to appraise the behavior of the mentioned bitumens. The results showed that maximum stored pseudo-strain energy (PSE) and tau (τ) × N (number of loading cycles) failure definitions exhibited high efficiency to accommodate the fatigue life of NA and SHE-modified bitumens. Both failure criteria identified that SHE-modified bitumen (containing 1% of SHE) showed the highest increment of fatigue performance (67.1%) concerning NA. The failure criterion based on total released PSE, in terms of the area under the released PSE curve, was the only failure concept with high efficiency (R2 up to 0.999) to predict asphalt binder fatigue life. As well, the current framework to evaluate bitumen self-restoration failed to fully accommodate asphalt binder behavior, because bitumen with higher restoration could not exhibit greater fatigue performance. Consequently, a new procedure to assess this property including fatigue behavior was proposed, showing consistent results, and confirming that SHE-modified bitumen (containing 1% of SHE) exhibited the highest increment of fatigue performance (154.02%) after application of the rest period. Hence, the optimum SHE content in NA was 1%. Furthermore, it was found that a greater number of loading cycles to failure ( N f ) did not ensure better fatigue performance and stored PSE influenced the bitumen fatigue behavior. [ABSTRACT FROM AUTHOR]

The mechanical performance of cement-stabilized macadam (CSM) is affected by the dosage of cement and age, while strength and fatigue life are two important mechanical indicators in pavement structure design. This study conducted uniaxial compression, four-point bending, indirect tensile strength, and fatigue tests on CSM, considering various cement dosages and ages. Equivalent stress was used to characterize the strength under a three-dimensional (3D) stress state. On the basis of the results of strength tests, the strength model parameters under 3D stress states were calculated. The characteristics of the tensile meridian, compression meridian, and the failure envelope surface under different conditions (different cement dosages and ages) were comparatively analyzed. The fatigue life data were processed by using the Weibull function and reliability theory to eliminate the dispersion. And the relationship between fatigue life and three-dimensional fatigue stress ratio (∆) was established. It turns out that the tensile meridian of CSM with older ages is above that of younger ones, and the same rule applies to the compression meridian. The failure envelope surface of CSM exhibits variations in both shape and size under different conditions. As the cement dosage and age increase, the failure envelope surface expands outward. The fatigue life curves under the three stress states processed by the Weibull function are very close and can be uniformly characterized. The fatigue life of CSM with 4 % cement dosage is more sensitive to ∆ than at other cement dosages, which with an age of 28 days changes faster with ∆ than that with an age of 90 days in the study. This work reveals the strength and fatigue characteristics of CSM under the 3D stress state at different cement dosages and ages. The findings serve as a valuable reference for the design of CSM base layers in diverse conditions. • The strength failure envelope surface of cement-stabilized macadam under different cement dosages and ages was established. • Based on Weibull function and reliability theory, the fatigue performance of cement-stabilized macadam under different stress states was uniformly characterized. • The variation characteristics of the shape and size of the strength failure envelope surface of cement-stabilized macadam were revealed under different cement dosage and age. [ABSTRACT FROM AUTHOR]

Asphaltic material is a kind of viscoelastic material, and its performance varies with test and materials conditions, such as stress states, temperatures, aging degree, gradations, and binders. The anti fatigue design of asphalt pavement has been stifled by these interferences. The objectives of this study are to reveal and compare the effects of loading frequency and aging degree on the fatigue performance of asphalt concretes. A series of tests for tensile fatigue on aged and unaged asphalt concretes were conducted at different loading frequencies. The theory of stress ratio considering different test conditions was proposed based on the conclusion that the stress ratio varies with stress loading rates. Then, the S-N fatigue equation was modified based on the theory. The fatigue test results were analyzed with the modified S-N fatigue equation to characterize the fatigue performance of asphalt concretes aged to different degrees under different loading frequencies. The different effects of these two factors on the fatigue characteristics of asphalt concretes were analyzed and compared. It could be found from the results that the influences of loading frequencies and aging degrees on fatigue characteristics of asphalt concrete were different. The fatigue test results of asphalt concrete under different loading frequencies could be characterized by one fatigue curve fitted by the modified S-N fatigue equation. It meant that the effects of loading frequency on fatigue were equivalent to the stress level. The fatigue test results of asphalt concrete with different aging degrees were fitted into different fatigue curves, respectively. The asphalt concretes aged 1 day had better fatigue life and more stable stress sensitivity than unaged asphalt concrete. When the aging degree was over 1 day, the fatigue performance deteriorated with the increase of aging degree. Moreover, it could be observed that the strength increased with the aging degrees when it was measured by the standard test loading rates. However, when the loading rates increased to a certain range, the contrary conclusion could be obtained because the viscoelasticity of asphalt concretes changed due to aging. [ABSTRACT FROM AUTHOR]

• The variation of strength with loading rate under various loading modes were revealed for cement-stabilized macadam. • The fatigue properties of cement stabilized macadam under different loading modes were uniformly characterized. • The new strength structure coefficient associated with the loading rate was proposed for cement-stabilized macadam. Cement-stabilized macadam is widely employed as a base-layer of pavement in China, because of its high strength and stiffness. However, fatigue cracking of the cement-stabilized macadam based pavement is prone to occur under multiple traffic loads, which affects the service life of the pavement. The main objective of this work is to reveal the fatigue characteristics of cement-stabilized macadam under various loading modes by developing a unified fatigue characteristics model. To this end, an unconfined compressive strength test, indirect tensile strength test, and four-point bending strength test were conducted for the cement-stabilized macadam samples under various loading rates. A power function was employed to the pattern of variation of strength with loading rates. The stress ratio of the fatigue test associated with the loading rate was obtained and defined as the rate-dependent stress ratio. Also, the fatigue tests under various loading modes were implemented based on the rate-dependent stress ratio and nominal stress ratio. The preliminary laboratory experimental results showed that it is challenging to compare fatigue performance under different loading modes based on nominal stress ratio. However, the problems of uncertainty and non-uniqueness of fatigue characteristics under various loading modes were resolved by employing the idea of rate-dependent stress ratio in the correlation analyses. Based on that, the new strength structure coefficient associated with the loading rate was proposed in this study. Therefore, the proposed research results provided the fundamental understanding of the fatigue resistance design of pavement with semi-rigid base-layer. [ABSTRACT FROM AUTHOR]

• Study fatigue based on modulus decay considering variance of compressive-tensile. • A synchronous test method of modulus was used to monitor modulus during fatigue test. • The modulus decay mode of tensile and compressive were established and compared. The cement stabilized macadam base experiences tensile and compressive stresses simultaneously in the actual pavement structures, resulting in the influence of stress states on fatigue performance. In this paper, the fatigue performance of cement stabilized macadam was analyzed based on modulus decay considering the difference between compressive and tensile modulus. A synchronous measurement method of tensile and compressive modulus, which based on convention four-point bending test, was utilized to measure modulus of cement stabilized macadam during a fatigue test. The four-point bending fatigue test was conducted upon four different stress levels of 0.25 MPa, 0.5 MPa, 1.0 MPa, and 1.5 MPa. The average of 10 closest modulus values nearby the first 1% of fatigue life was adopted to determine the initial modulus. The last 5 cycles before the failure of the specimen in the fatigue test was determined as the critical modulus value. It was observed from the tests results that the critical value of compressive fatigue modulus was 8 times higher than that of tensile fatigue modulus. The initial and critical values of both tensile and compression decreased with the increase of stress levels. Under different stress levels, the modulus decay mode of tensile and compressive modulus was similar, but the decay rates of tensile modulus were higher than that of compressive modulus. Based on the modulus decay mode, the concept of critical failure point was put forward to characterize the fatigue behavior of cement stabilized macadam. During the fatigue process, both tensile and compressive modulus decay presented three stages, i.e. preliminary stage, stable stage, and rapid attenuation stage. The stable stage of tensile modulus was shorter than that of compressive modulus, which means that the primary fatigue damage occurred in the tensile zone. [ABSTRACT FROM AUTHOR]

Highlights • The strength and fatigue tests of asphalt mixtures were implemented. • The evolution patterns of strength with loading rate were revealed. • The unified fatigue model of asphalt mixtures was established. • The behavior of thermal fatigue and strength failure was integrated. Abstract In order to reveal the thermal fatigue behavior of asphalt mixtures under low temperature conditions, the four-point bending beam tests of strength and fatigue were carried out. The asphalt mixtures are typical viscoelastic materials whose fatigue properties are significantly influenced by low loading frequency. Thus, the fatigue tests under low loading frequency were used to simulate the thermal fatigue of asphalt mixtures. Based on the nominal stress ratio (not considering the effect of loading rate on the strength value) and the real stress ratio (the strength value corresponding to the loading rate of fatigue test), the fatigue equations of asphalt mixtures under low temperature and low frequency conditions were established. The flexural strengths of the four-point bending beam tests at different loading rates at 0 °C, −10 °C and −20 °C were studied. The results show that the flexural strength of asphalt mixtures expresses a power function on the pattern of variation with the loading rates. The intersection of fatigue curve characterized by the nominal stress ratio and the abscissa axis is much larger than 1. The fatigue curve based on the nominal stress ratio cannot be extended to fatigue failure point (1,1). But the fatigue curve characterized by the real stress ratio can be extended to the strength failure point, whose fatigue life is 1. So, the behavior of strength failure and fatigue failure was integrated. The uniform characterization model of fatigue equations for asphalt mixture under low temperature and low loading frequency conditions was realized, which means that the fatigue equations and curves under different loading frequencies could be characterized by one equation and one curve. The results can provide a theoretical basis for predicting the thermal fatigue performance of asphalt mixtures under low temperature conditions more accurately. [ABSTRACT FROM AUTHOR]

• The effect of load differences and tension-compression modulus differences were considered. • The mechanical response of the pavement was calculated based on the bimodulus theory. • Equivalent stress design index was used to calculate pavement fatigue life. The conventional semi-rigid base asphalt pavement (SRBAP) construction is designed with the classical theory of linear elastic mechanics. There is a significant discrepancy between the calculation results of the pavement structure load response and the measured results, which leads to inaccurate structural resistance and load response calculations. For this reason, this work depended on the actual structure of the SRBAP, took the linear elasticity theory as a contrast, and considered the difference between dynamic and static loads or the difference between the compressive and tensile modulus of pavement materials. Thus, the pavement structure response to load under three conditions is calculated, and five mechanical indexes are selected for comparative analysis. Furthermore, based on the calculated mechanical response, the structural fatigue life of the cement-stabilized macadam (CSMB) was then calculated, replacing the design index in the fatigue cracking model with the underside tensile stress or equivalent stress. The results demonstrated 20–30% variations between the calculated mechanical responses to dynamic and static stresses. Additionally, there is a substantial discrepancy between the strain computed using the bimodulus theory and the traditional linear elasticity theory, with a maximum divergence of 40%. In the meantime, the fatigue life of the CSMB produced using various design indices is noticeably different, and the fatigue life achieved using equivalent stress as the design index is significantly higher than that obtained using underside course tensile stress. Following is a relationship between the CSMB's fatigue life calculated by underside course tensile stress: viscoelastic theory (dynamic load) > bimodulus theory (tension–compression modulus difference) > classical linear elastic theory; The relationship of fatigue life represented by equivalent stress is as follows: viscoelastic theory > classical linear elastic theory > bimodulus theory. [ABSTRACT FROM AUTHOR]

The fatigue test methods under different test conditions have strong impact on the test results, which obstructs a precise evaluation of fatigue characteristic for asphalt mixtures. Different test conditions means different stress states. Fatigue characteristic of a material is an objective attribute, and it should not vary with test methods and conditions. In order to accurately and objectively evaluate the fatigue properties of asphalt mixtures, a normalization model of fatigue characteristics for asphalt mixtures under different stress states was established based on the yield criterion in 3 dimensional stress states and the patterns of variation between the strength and the loading rate. The compressive, tensile and indirect tensile strength and fatigue tests were carried out respectively in different loading rates. The yield surface of strength for asphalt mixture was put forward based on the strength tests results and the yield criterion. Then, the normalization model of fatigue characteristics for asphalt mixtures under different stress states was established based on the yield criterion in 3 dimensional stress states. The results show that the differences of the fatigue tests results of asphalt mixtures under different test condition are large. It is difficult to evaluate the fatigue performance in different stress states of asphalt mixtures by using the conventional S-N fatigue equation. However, these differences can be reduced or even eliminated by using the normalization model, which considered the influence of the stress state and the strength corresponding to the fatigue loading frequency and stress level. The unified expression of fatigue characteristics under different test conditions for asphalt mixtures was realized. The fatigue behavior of asphalt mixture under different stress conditions can be objectively evaluated by using the normalized model. [ABSTRACT FROM AUTHOR]

• The strength properties of asphalt mixture with different gradation curves were evaluated. • A unified fatigue model was developed based on the fatigue stress strength ratio. • The gradation curve of asphalt mixture with the optimal strength and fatigue performance was determined. The gradation design has an essential influence on the mechanical properties of asphalt mixture. Strength and fatigue are two critical mechanical properties of asphalt pavement structure. The purpose of this paper is to reveal the strength and fatigue properties of asphalt mixture with four gradation curves within the range of AC-16, namely Types A, B, C, and D. Strength and fatigue tests were conducted under indirect tensile, direct tensile, and unconfined compressive loading modes. The strength of asphalt mixture with various gradation curves shows a power function relationship with the loading rate. The fatigue performance of asphalt mixture with different gradation curves was first analyzed based on the conventional S-N fatigue model. Then, the unified fatigue models under three different loading modes were established based on the fatigue stress strength ratio, which reduced the difference of S-N fatigue equation parameters. Furthermore, the unified fatigue equations achieved the unity of strength failure and primary fatigue failure. The analysis results indicate that the mixture with Type D gradation curve realizes the highest strength and most extended fatigue life. This research contributes to the gradation design of asphalt mixture. And the unified fatigue equation provides a reference for the durability design of pavement. [ABSTRACT FROM AUTHOR]

• A new fatigue model of asphalt mixture is established, which can normalize the fatigue characteristics under various temperatures and several loading frequencies. • Time-temperature equivalence of fatigue life is demonstrated and fatigue master curves was formed. • The time-temperature equivalence happens on fatigue and strength at the same time in the new model. This work presents a new method for evaluating the time-temperature equivalence and unified characterization of the fatigue performance for asphalt mixtures. For this purpose, direct tensile strength tests and direct tensile fatigue tests were carried out on asphalt mixtures at different temperatures and loading rates. A time-temperature-dependent stress ratio was established. By exploring the correlation between stress ratio and fatigue life, a new fatigue model considering the effect of temperature and loading time was built. The time-temperature equivalent of fatigue performance was confirmed by analysis of the fatigue dates under several temperatures and loading frequencies. The results showed that the direct tensile strength of the asphalt mixture changed non-linearly with the variation of the loading rate or temperature. The stress ratio was also related to temperature and loading rate. The time-temperature stress ratio was linearly related to fatigue life in a double logarithmic coordinate system, and a new S N fatigue curve was formed. The fatigue life under several temperatures and various loading frequencies can be described in one fatigue model and the time-temperature equivalent can be explored. The master curve of fatigue life or stress ratio was built. The time-temperature superposition principle was applicable for fatigue life estimation and stress ratio calculation. The fatigue life under different loading frequencies and various temperatures of the pavement in service can be completely estimated. The relative error between the test results and the calculated values is less than 30%. [ABSTRACT FROM AUTHOR]

• The strength variation of HMAC with loading rate under various loading patterns was revealed. • The fatigue properties of HMAC measured by diverse experiments were normalized. • The accuracy of fatigue resistances design parameters for high modulus asphalt pavements was improved. As a new material, high modulus asphalt concrete (HMAC) with good durability is widely used in the pavement engineering. One of the most critical performance indexes of HMAC is the fatigue cracking. However, in the experiment of evaluating the fatigue properties, different anti-fatigue effects are obtained only by adding different modifiers. In addition, using only one loading mode to determine the fatigue properties of HMAC neglects the fact that there is a difference among the fatigue test results in various loading modes, which artificially leads to the non-uniqueness and unscientific design of high modulus asphalt pavements for the fatigue resistance. Therefore, four internationally accepted mainstream strength tests at different loading rates, including direct tension, indirect tension, unconfined compression and four-point bending tests, were carried out to enhance the veracity of structural resistance design of high modulus asphalt pavements. It is revealed that the fitted curve for the variation of strength with loading rate is a power function with velocity dependence, and the models for direct tension, indirect tension, unconfined compression, and four-point bending strengths established as a function of loading rate are S D = 3.218v0.1425 , S T = 3.833v0.1768 , S C = 14.391v0.1130 and S F = 7.839v0.1489 respectively. Besides, it is shown that the conventional S-N fatigue equation does not take into account the velocity dependence of the strength of HMAC in the stress ratios determined by the tests, and the velocity-dependent stress ratios defined in this paper are determined by the strength model and therefore take into account the viscoelasticity of HMAC. In addition, it is found that extending the fatigue curve backwards, when the fatigue life of HMAC is also close to 1, the velocity-dependent stress ratio is 1. Meanwhile, the fatigue model N f = t v -5.318 was developed by unifying the characterization of HMAC and considering the effect of loading rate in various loading modes. It eliminated the difference of fatigue test results caused by the specimen sizes and loading modes, and effectively improved the completeness of structural design of high modulus asphalt pavements, which provided a theoretical basis for designing the high modulus asphalt pavements. [ABSTRACT FROM AUTHOR]

• The feasibility of the Miller yield criterion in rubberized asphalt mixture was discussed. • A fatigue model based on the von mises stress was obtained. • The design criterion under three-dimensional stress state was established. With the promotion of sustainable development globally, it is a positive and feasible measure to put waste tires into road engineering, such as rubber asphalt pavement. Therefore, it is very significant to improve the accuracy of fatigue resistance design of rubber asphalt pavement structures. This work aims to truly characterize the fatigue resistance of rubberized asphalt pavement structure, and provides favorable conditions for the efficient utilization of waste rubber powder in asphalt pavement engineering. First, the strength tests under different stress status and loading rates were carried out, which set up the strength yield surface on account of the Miller yield criterion. The fatigue tests of rubberized asphalt mixture in various stress levels and stress state were carried out. Von mises stress was taken as the characteristic parameter of the fatigue equation, and a unified fatigue characteristic equation of rubberized asphalt mixture affected by individual stress states was developed through the routine fatigue test. Finally, combined with Miller yield criterion and fatigue unified equation, the fatigue resistance model of rubberized asphalt pavement structure under the three-dimensional stress state was conducted. The results indicate that: the established Miller strength yield surface can reasonably describe the three-dimensional failure characteristics of rubberized asphalt mixture. Besides, the unified model of fatigue properties of rubberized asphalt mixture only needs to be characterized by single parameter, which is a power function coefficient in the new fatigue equation, and the value is −6.021. Moreover, the fatigue resistance model of the rubberized asphalt pavement structure sufficiently considers the difference between the tension–compression strength and fatigue characteristics of rubberized asphalt mixture and the stress state correlation. This study improves the scientificity and reliability of structural resistance design, and provides help for predicting the fatigue life of rubberized asphalt pavement. [ABSTRACT FROM AUTHOR]

• The mechanical response between the field measured and the designed modulus was compared. • The optimization for modulus combinations of pavement structural was proposed. • The fatigue properties of material and structure were unified. The early damage of the semi-rigid base asphalt pavement is related to the pavement structure modulus's unreasonable matching. In this study, three typical pavement structures were selected to analyze the pavement structures' influence on the pavement service life. A three-dimensional finite element pavement structure model was established. The independent variables are subgrade modulus, base course modulus, and subbase modulus. The deflection, the bottom tensile stress, and maximum shear stress were chosen as the evaluation indexes. The effect of the modulus on the mechanical response of the pavement structure was analyzed. The optimal modulus combination of the pavement structure was determined through multi-factor range analysis. The mechanical response and fatigue life before and after the optimization pavement structure were compared. The results showed that the field measured modulus of Structure 1 and 2 was higher than the design modulus. Moreover, while the modulus of base course and subbase course was increased, the deflection gradually reduced. The base course's bottom tensile stress and the subbase were increased, and the maximum shear stress was basically unchanged. After the modulus combination optimized pavement structure, the mechanical response was significantly reduced. The fatigue life based on the deflection and bottom tensile stress, and the laboratory normalized fatigue equation were significantly increased. By the combination of fatigue performance of pavement materials and pavement structure, it was possible to provide an effective optimization method for the design of semi-rigid base asphalt pavement in this research work. [ABSTRACT FROM AUTHOR]

Due to the high viscosity of high-content rubber modified asphalt, maintaining a high temperature for a long time is required when preparing rubber modified asphalt mixtures. This problem limits the attainment of ideal conditions in experimental research. Using the bio-oil could reduce the viscosity of rubber modified asphalt and thus reduced the mixing temperature of its mixtures. The objective of the manuscript is to propose production protocol of bio-oil rubber modified asphalt mixture (BRMAM) and evaluate its performance. The rubber (20 % mass ratio of modified asphalt) and bio-oil (5 % and 10 % mass ratio of modified asphalt) were used to prepare two types of the BRMAM under lower mixing temperature. The high-temperature stability, low-temperature anti-cracking, water stability and other mechanical performance were appraised. Based on the test results, the total deformation of the BRMAM decreased by 39.4 %, the Marshall modulus increased by 45.5 % compared to the neat asphalt mixture, and their fracture energy increased by 36.27 %. Under the condition of high stress ratio, the fatigue life of modified asphalt mixture rose by 32.4 %. The addition of crumb rubber could enhance the elastic recovery of BRMAM. The synergism of the bio-oil and crumb rubber could lower the mixing temperature without sacrificing the performance. This study provides a basis for BRMAM in the laboratory and field applications or similar scenarios. The study could contribute to the recycling of renewable resources and the reduction of carbon emission. • Proposed lower temperature bio-oil rubber modified asphalt mixture production method. • Evaluated the performance of the bio-oil rubber modified asphalt mixture. • Get the optimal bio-oil and rubber content combination. [ABSTRACT FROM AUTHOR]

• The Ayyagari model was applied to semi-rigid layers materials. • The unified fatigue model was obtained based on yield criterion. • A new design index of structural resistance was proposed. As the primary high-grade pavement structure, asphalt pavement with semi-rigid layers has been extensively applied in China. Among the semi-rigid layers materials, cement-treated aggregates (CTA) are an essential pavement element, and its fatigue life significantly affects the durability of the asphalt pavement structure. Various fatigue tests have been adopted to evaluate the fatigue properties of CTA. However, the fatigue resistance of the CTA measured under various methods is different, which seriously affects the accuracy of the structural design of the semi-rigid layers. Hence, this work aims to establish a design criterion for CTA to characterize the structural resistance of semi-rigid layers. The uniaxial compression (UC), four-point bending (4 PB), and indirect tensile (IDT) strength and fatigue tests for CTA were carried out. According to the traditional S - N fatigue model, the curves of fatigue performance and stress ratio of CTA under various stress states are quite different. To intuitively compare the structural resistance of CTA under various test types, the mean stress and effective stress of CTA under different loading rates were obtained. The yield surface was then established based on the Ayyagari model, considering the multiaxial yield criterion of CTA. The effective stress ratio was defined, and its relationship with the fatigue life of CTA was established. Based on the Ayyagari model and the normalized equation, the fatigue resistance criterion of semi-rigid layers under the three-dimensional stress state was proposed. [ABSTRACT FROM AUTHOR]

• A connection between strength and fatigue was revealed. • The established new fatigue model eliminates the influence of test methods. • A quantitative analysis was made on the difference in fatigue properties. At present, many test methods and models have been used to evaluate the fatigue performance of pavement materials. However, different test methods and material types have a significant impact on the fatigue result evaluation. Thus, to evaluate the fatigue characteristics of different pavement materials under diverse test methods, the strength and fatigue performance of three pavement materials were characterized through direct tension, uniaxial compression, and indirect tensile tests at multiple loading rates and stress levels. In this paper, three pavement materials, such as SBS-modified asphalt mixture (SBS-MAM), crumb rubber asphalt mixtures (CRAM), and cement-stabilized macadam (CSM), were tested. On this basis, the strength yield surface models related to loading rates and the fatigue normalized unified model of three pavement materials under various stress states were established. Comparing the differences among the three pavement materials' fatigue performance, the results show that the fatigue results of the three pavement materials were completely different under dissimilar test methods. The conventional fatigue equation cannot evaluate the influence of specimen size and stress state on fatigue performance. However, the fatigue normalization model established in this paper can settle these problems. It is also clear that the fatigue performance of CSM is superior to those of the other two mixtures when the stress level is small. However, the fatigue life of CSM is significantly more sensitive to the stress level than those of the other two mixtures. Based on the establishment and comparative analysis of the normalized models of fatigue characteristics of different pavement materials, the scientific conversion of pavement material fatigue to structural fatigue is realized, which provides sights into the anti-fatigue design of pavement structures. [ABSTRACT FROM AUTHOR]

• A suitable yield criterion for rubberized asphalt mixtures was proposed. • A strength yield surface and unified fatigue equation were determined. • The impact of the test method on the results of the fatigue test was eliminated. Under the action of traffic load, the stress and failure state of pavement structure and materials are complex and diverse. Moreover, pavement materials are multi-phase, multi-component, and multi-scale mixtures. Their mechanical properties have significant differences in tension behavior, compression characteristics, and correlations with stress states. In view of the above problems, different testing approaches were carried out in this paper. A suitable yield criterion for rubberized asphalt mixtures was proposed. Then, the strength yield surface was determined under different loading rates. Based on the fatigue test results, the concept of fatigue stress ratio was further proposed. The relationship between the rubberized asphalt mixtures fatigue life and the fatigue stress ratio was established. The results showed that the yield criterion covers the impact of the loading rates and tensile-compression strength difference on the rubberized asphalt mixtures' mechanical behavior. The strength yield surface's shape is elliptical in the σ m - σ e stress space and expands outward as the loading rate increases. This paper unified the fatigue equations under different testing approaches. The established fatigue equations reflect the viscoelastic characteristics and strength characteristics of materials. The study effectively solves the difficult problem of fatigue equation selection in the structural design of asphalt rubber pavement. [ABSTRACT FROM AUTHOR]

• The fatigue performance of asphalt mixtures was investigated by the model mobile load simulator (MMLS3). • The fatigue damage model of asphalt mixtures was developed using resilient modulus ratio and loading repetitions. • The stiffness reduction process of asphalt mixtures with different gradation was compared. As the primary surface type for high-grade Chinese highways, the long-term performance of asphalt pavement can affect significantly transportation efficiency. It has been confirmed that fatigue cracking is one of the major failure modes for asphalt pavement, which can cause a great reduction of asphalt pavement durability. Currently, the fatigue properties of asphalt mixtures are evaluated mainly based on traditional laboratory fatigue tests, such as bending fatigue tests and indirect tensile fatigue tests. However, none of these fatigue test methods can realistically evaluate the fatigue performance of asphalt pavement under the repeated wheel load. To resolve this issue, the model mobile load simulator (MMLS3), which is a small-scale accelerated pavement test device, was utilized to evaluate the fatigue life of six kinds of asphalt mixtures. The fatigue damage variable was defined as stiffness reduction. Based on the small-scale accelerated pavement test, a stiffness reduction model for asphalt mixtures was determined. A stiffness reduction law for asphalt mixtures during the fatigue process was proposed. The stiffness evolution process could be divided into three phases, which are the adaptation phase, the quasi-stationary phase, and the failure phase. A maximum value of the rate of stiffness reduction from the quasi-stationary phase into the failure phase was proposed as the failure threshold. Besides the fatigue life obtained from the failure criteria had a good agreement with the test results. The results from this research can be used as a theoretical reference for evaluating the fatigue life of asphalt mixtures and improving the durability of asphalt pavement. [ABSTRACT FROM AUTHOR]

Strength and fatigue life are essential parameters of pavement structure design. To accurately determine the pavement structure resistance of rubber asphalt mixture, the strength tests at various temperatures, loading rate, and fatigue tests at different stress levels were conducted in this research. Based on the proposed experiments, the change law of rubber asphalt mixture strength with different temperatures and loading rates was revealed. The phenomenological fatigue equation of rubber asphalt mixture was established. The genetic algorithm optimized backpropagation neural network (GA-BPNN) is highly reliable for optimizing production processes in civil engineering, and it has a remarkable application effect. A GA-BPNN strength and fatigue life prediction model was created in this study. The reliability of the prediction model was verified through experiments. The results showed that the rubber asphalt mixture strength decreases and increases with the increase of temperature and loading rate, respectively. The goodness of fit of the rubber asphalt mixture strength and fatigue life prediction model based on the GA-BPNN could reach 0.989 and 0.998, respectively. The indicators of the fatigue life prediction model are superior to the conventional phenomenological fatigue equation model. The GA-BPNN provides an effective method for predicting the rubber asphalt mixture strength and fatigue life, which significantly improves the accuracy of the resistance design of the rubber asphalt pavement structure. [ABSTRACT FROM AUTHOR]

• The variation patterns of fatigue parameters with aging degree were established. • Sensitivity analysis of fatigue parameters for aging asphalt mixture was carried out. • Statistical models of the relation between PV and fatigue life have been derived. The objective of this paper is to reveal the effects of aging on the fatigue performance of asphalt mixture based on variation patterns of fatigue parameters with aging degrees. Specimens with different aging degrees were prepared by modified SHRP's Long-Term-Oven Aging (LTOA) test method, in which five different aging durations, included 0, 1, 3, 5, 7 days, were adopted to substitute the fixed 5 days. The fatigue parameters, including stiffness modulus, fatigue life, phase angle, Plateau Value (PV), and cumulative dissipation energy of specimens with different aging degrees under different stress ratios were obtained by the tensile fatigue test. The relationship of fatigue parameters with aging degrees and stress ratios were established. It could be observed from the results that the aging degree and stress ratio had dramatic influences on the fatigue performance of asphalt mixture. Sensitivity analysis showed that aging had the most significant effect on the initial stiffness modulus of asphalt mixture, and should be used as a control index in fatigue design. Statistical models of the relation between (PV) and fatigue life (N f) established by the Rate of Dissipated Energy Change (RDEC) approach has an effective correlation. The Plateau Value model eliminated the effects of aging degrees and reflected the uniqueness of material parameters. Also, it could provide novel ideas for effective prediction of the fatigue life of asphalt mixtures with different aging degrees based on plateau value. [ABSTRACT FROM AUTHOR]

• The stress ratio was redefined based on the strength failure envelope line under multi-field coupling conditions. • The unified model of fatigue performace under various stress states, temperatures, and loading frequencies were proposed. • The dispersion caused by different methods and test conditions can be eliminated by the unified fatigue model. For a scientific and reliable design of asphalt mixture subjected to fatigue damage, a comprehensive understanding and realistic prediction of asphalt mixture fatigue life under complex loading conditions with variable loading rates or temperature were a major concern. In this study, direct tensile test, uniaxial compression test, indirect tensile test, and four-point bending test were used to study the fatigue behavior of asphalt mixtures with a particular focus on the action of temperature and frequency. Strength and fatigue were investigated separately under varied loading rates at several temperatures. A new idea for determining the stress ratio based on the strength failure envelope line considering temperature and loading rate was proposed. The connection between strength and fatigue was established by the stress ratio. The test conditions for strength and fatigue tests were set to be the same leading to the fatigue data distributed regularly. Experimental data were analyzed and normalized by studying the evolution of the number of cycles with the stress ratio. A new analytical approach was introduced to simulate the fatigue performance. The fatigue performance under multi-field coupling conditions was uniformly characterized. The dispersion of fatigue equation caused by the test method and conditions is eliminated. Engineering application showed that the design deviation of the pavement fatigue resistance was reduced to less than 6%. [ABSTRACT FROM AUTHOR]

Rubber activation provides a feasible approach for the preparation of high-content rubber modified asphalt without increasing the complexity of the production process. The electromagnetic thermal activation method was used for crumb rubber. The high dosage (30 % mass ratio of neat asphalt) activated crumb rubber and SBS (1.2 % mass ratio of neat asphalt) modified asphalt (ARMA) was prepared. The neat asphalt (NA) and 20 % non-activated crumb rubber modified asphalt (RMA) constituted the control group. Storage stability, rheological behavior, and microscopic properties tests were conducted to evaluate the performance differences of them. The rheological behavior of RMA and ARMA was superior to NA. The electromagnetic thermal activation method fragmented rubber particles into numerous disordered small particles, thereby expanded the contact area with asphalt. This resulted in the storage stability of ARMA significantly better than that of RMA. The cross-linked wrinkles were generated by the combined effect of SBS modifier and activated crumb rubber, which enhanced the rheological, anti-fatigue, and elastic behavior. Both RMA and ARMA exhibited high-temperature PG grades of 82˚C. At 70°C, ARMA retained the capability to endure Extremely Heavy Traffic Grade conditions. At a strain level of 2.5 %, ARMA exhibited a 178 % increase in fatigue life compared to RMA. The crumb rubber was dispersed within the RMA and ARMA. Both ARMA and RMA did not exhibit the obviously bee-structure. However, ARMA exhibited a stronger polymer network. Moreover, both the carbonyl index and sulfoxide index of ARMA were lower than those of RMA and NA. The activation process improved the compatibility between rubber and asphalt, and realized the high value utilization of waste rubber. • Rubber activation provides a feasible approach for the preparation of high-content rubber modified asphalt. • The electromagnetic thermal activation method was used for crumb rubber. • The high value utilization of waste rubber realized the development of green and sustainable transportation construction. [ABSTRACT FROM AUTHOR]

The objective of this study is to delve into the nonlinear damage evolution mechanism of hot central plant recycling asphalt mixture under the influence of creep-fatigue interaction. A predictive model for lifespan, considering the interaction of temperature and load, was developed. Dynamic creep tests were conducted at different temperatures to assess the creep effect of the asphalt mixture under actual road temperature conditions. The investigation began by acknowledging the material's viscoelastic properties, thereby deriving the loss compliance value - a key indicator of temperature effects - using an equivalent conversion factor and a viscoelastic mechanics model. This allowed for the incorporation of creep behavior into the load system. Furthermore, to analyze the nonlinear damage mechanism and fatigue prediction mechanism, direct tensile fatigue tests were performed at different temperatures and different stress levels. The fatigue damage evolution law of hot central plant recycling asphalt mixture containing high RAP considering the creep effect was revealed, and the fatigue life evaluation method under the combined action of temperature and stress level was proposed. The results show that the fitting degree of the time-temperature conversion equation and the viscoelastic mechanics model to the test data is more than 0.9, and the derived loss compliance value better captures the creep effect. Based on the nonlinear creep-fatigue damage model, the damage law of high-RAP hot central plant recycling asphalt mixture was identified. The optimized fatigue life prediction model fully considers the combined effect of temperature and stress level and analyzes the influence parameters of the real state of pavement fatigue. The prediction accuracy is within the requirements of engineering applications. The model's predictive accuracy meets the criteria for engineering applications, enhancing the multifaceted damage coupling analysis approach for hot central plant recycling asphalt mixture and predict the service life of recycled pavements under complex conditions, establishing a groundwork for the prediction and assessment of long-lasting recycled pavements. • The damage model of creep-fatigue interaction effectively describes the nonlinear damage evolution trend of HRAP-HR. • The optimized fatigue life prediction model more effectively accounts for the creep effect. The creep effect is well reflected by the loss compliance value. [ABSTRACT FROM AUTHOR]

In the actual service process of asphalt pavement, the dynamic response of the pavement structure layer is induced by vehicle loads. The dynamic modulus is a crucial parameter for analyzing the mechanical response of asphalt pavement. Due to significant influences from temperature and loading frequency on test measurements, along with its time-temperature equivalence, it becomes imperative to investigate the behavior of dynamic modulus under different temperature frequencies. Furthermore, laboratory loading modes also exert substantial effects on dynamic modulus. Employing different loading modes during measurement often leads to significantly varied stress states in specimens, yielding distinct values for dynamic modulus. Therefore, this study conducts dynamic modulus tests using direct tension, uniaxial compression, and indirect tension under five test temperatures and six loading frequencies while examining its variations under different stress states and temperature frequencies. The corresponding master curves and prediction models of dynamic modulus are derived, providing a valuable reference for predicting the dynamic modulus under wide-area test conditions. Through comparative analysis of dynamic modulus values at different stress levels, the stress level is incorporated into the master curve to establish corresponding master curves of dynamic modulus. By applying relevant theories and research on time-temperature equivalence, we obtained dynamic modulus master curves for three loading modes by fitting and analyzing dynamic modulus test data at various temperatures and loading frequencies. Combining the Sigmoid model and Williams-Landel-Ferry(WLF) equation, we derived prediction models for dynamic modulus under three loading modes. Comparative analysis of dynamic modulus values under these three loading modes allowed us to establish transformation equations for the prediction models in different loading scenarios. Finally, representative dynamic modulus values for permanent deformation in typical pavement structure layers were predicted based on laboratory test results and the derived models. The research findings presented in this paper have significant implications in eliminating the influence of loading modes on dynamic modulus predictions, establishing a unified prediction model, and providing essential parameters for predicting permanent deformation and fatigue life equivalent temperature. • Introducing stress level variables into the dynamic modulus master curve. • Extending the two-dimensional dynamic modulus master curve to three-dimensional master surface. • Derived the conversion equations between prediction models for different loading methods. [ABSTRACT FROM AUTHOR]

This study aims to supplement the degraded SBS components and improve the road-related properties of Hot in-place recycling (HIR) asphalt mixtures. Under the direct-to-plant process, a composite rejuvenating agent (SBS-CRA) composed of instant SBS, regenerant, and coupling agent is used to regenerate the HIR asphalt/asphalt mixture. Explore the effects of SBS-CRA on the properties and chemical composition of extracted asphalt using rheological and Fourier transform infrared spectroscopy tests. Evaluate the road-related properties and microstructure of SBS-CRA modified HIR asphalt mixtures under the direct-to-plant process through high and low temperature, water stability, fatigue, and scanning electron microscopy tests. And conduct performance verification by paving the test road. The research results indicate that the SBS-CRA can promote extracted asphalt's low-temperature behavior and viscosity while maintaining excellent high-temperature behavior. Under the direct-to-plant process, the SBS-CRA modified HIR asphalt mixture's high and low-temperature behavior and water stability comply with the specified requirements. After SBS-CRA regeneration, the asphalt on the surface of RAP material is smooth. Considering the road-related properties of SBS-CRA modified HIR asphalt mixture, the optimal recommended dosage of SBS-CRA is 5%, and the optimal mixing time of SBS-CRA and RAP material is recommended to be 120 s. At a 5% SBS-CRA content, the fatigue life of the HIR asphalt mixture is 407.9 times higher than that of the neat asphalt mixture. The on-site test results show that the freeze-thaw splitting strength ratio of the HIR asphalt mixture is 88.76%, which meets the specification requirements. The laboratory and on-site test results have proven that applying SBS-CRA in HIR technology is feasible. • Revealed the behavior and mechanism of SBS-CRA recycled asphalt. • Explored the effects of SBS-CRA dosage and mixing time on HIR asphalt mixture under a direct-to-plant process. • Evaluate road performance through laboratory and on-site testing. [ABSTRACT FROM AUTHOR]

• The Desai three-dimensional strength criterion was improved by including the effects of loading velocities. • A new approach for fatigue characterization under three-dimensional stress state was proposed. • The fatigue life values obtained under distinct stress states were uniformly characterized. To improve the durability of asphalt pavement, high modulus asphalt mixtures were developed and widely utilized to construct heavy trafficked pavements to resist rutting. Except for rutting, serving under repeated heavy loads, pavements with high modulus asphalt mixtures are also vulnerable to fatigue cracking. To prevent fatigue cracking, the fatigue resistance of high modulus asphalt mixtures is a crucial indicator for pavement structural design and checking computations, which is generally obtained by laboratory tests. However, the current test methods are generally based on maximum tensile stress or strain theory and neglecting the effects of loading velocities, so the fatigue resistance obtained cannot match that under actual pavement serving conditions. Moreover, the fatigue performance of high modulus asphalt mixtures obtained by different test methods are quite different, which caused random and disunity when selecting fatigue parameters for pavement structural design. Therefore, the objective of this paper is to accurately and uniformly characterize the fatigue resistance of high modulus asphalt mixtures. To this end, the indirect tensile, direct tensile, unconfined compressive strength and fatigue tests of high modulus asphalt mixtures were implemented under distinct loading velocities. Based on the Desai yield criterion, the strength yield surfaces under diverse loading velocities and the fatigue path of high modulus asphalt mixtures in three-dimensional stress state were established. Finally, the fatigue life values of high modulus asphalt mixtures obtained under distinct test methods and loading velocities were uniformly characterized by the relationship between fatigue life and equivalent stress ratio Δ , which was N f = Δ - 5.42 . With the uniform characterizing model, the disunity of fatigue parameters can be eliminated, and the fatigue resistances of high modulus asphalt mixtures under three-dimensional stress states and different loading velocities can be evaluated. [ABSTRACT FROM AUTHOR]

• Flexural tensile strengths calculated with tensile and compressive differences are reduced by nearly 20 % when compared to those calculated without tensile and compressive differences. • A power function decay model of the tension–compression moduli of cement stabilized macadam was established in the four-point bending fatigue test. • The fatigue damage model of cement stabilized macadam was established and its accuracy was verified. Cement stabilized macadam (CSM) is a mixture created by combining cement as a binding material, water, and various particle sizes of minerals. The cement stabilized macadam material exhibits different moduli in tension and compression due to both its mechanical properties and the mechanical response in the asphalt pavement structure. To explore the strength, modulus and fatigue damage characteristics of CSM under different tensile and compressive modulus, this paper carried out the research. The calculation formulas for flexural tensile strength and tension–compression modulus were derived based on the difference in tension–compression modulus. A four-point bending strength test of CSM was carried out. Four-point bending fatigue tests with varying stress ratios were performed. It reveals deterioration laws of tension–compression modulus during fatigue process. The double logarithmic fatigue equation of CSM, as well as its nonlinear fatigue damage evolution equation, are established based on fatigue life results. The results of an analytical solution for fatigue life of rectangular section beams based on damage mechanics were obtained and compared to the results of four-point bending fatigue tests. The results reveal that the modulus of CSM differs in tension and compression. The calculated flexural tensile strength with the tension and compression difference is nearly 20 % lower than without the difference. The calculation approach that does not account for the difference in tension and compression overestimates the flexural tensile strength of CSM. The tension–compression moduli of CSM exhibit a power function decay law in the four-point bending fatigue processes. In the first stage, the dynamic modulus decays at a steady rate, accounting for 90 % of total fatigue life. The second stage in the life ratio of 0.9 occurs when the decay rate of the modulus rapidly increases and enters the stage of sharp decay. Based on the results of the four-point bending test, the modulus decay model was created. The decay parameter m has an exponential relationship with the stress ratio. The relative errors between the mean values of fatigue life tests and the predicted values of the analytical solutions range from 10.8 to 30.6 percent. The established fatigue damage equation for CSM can better reflect its fatigue damage evolution law. The research findings have great significance to solve the determination of pavement materials parameters based on the difference in tension and compression modulus. It also has a specific reference value for asphalt pavement design theory and calculation method when the varied modulus of tension and compression are considered. [ABSTRACT FROM AUTHOR]

• A damage model considering the interaction between fatigue and creep was established. • The model could reflect the effect of stress level on the damage evolution process. • The nonlinear characteristics of damage evolution could be well-reflected. To describe the interaction of fatigue-creep damage of an asphalt mixture under the action of semi-sine cyclic loading reasonably, a direct tension fatigue test was carried out under four different stress levels (0.5, 1, 1.5, and 2 MPa) based on the fact that semi-sine fatigue stress can be decomposed into creeping stress and simple fatigue stress. In addition, tension-compression fatigue and static creep tests were carried out under four stress levels: 0.25, 0.5, 0.75, 1 MPa. For each test, five parallel tests were carried out at the same stress level. The Chaboche fatigue damage model and Kachanov creep damage model were employed to evaluate the damage evolution of the mixture both in the fatigue and creep tests. According to the test results, the damage evolution trends were revealed under different test processes and stress levels, and the damage evolution models of three different test methods were obtained. Then, based on the principle of semi-sine fatigue stress decomposition, a fatigue-creep damage interaction model considering the interaction of fatigue and creep damage was established using normalized parameters (X = N / N f = t / t R) as the intermediate variable. The results indicated that the damage of asphalt mixture under the action of the semi-sine cyclic load was not a simple linear accumulation of fatigue damage and creep damage, but an overall result of the interaction of fatigue damage, creep damage, and fatigue-creep damage. The fatigue-creep damage interaction model provided a good fit with the experimental data and showed a high degree of consistency with the classical damage model. The model could be used to describe the damage evolution trend of asphalt mixture under the action of semi-sine cycle load and the interaction effect of fatigue damage and creep damage. [ABSTRACT FROM AUTHOR]