Cracking Resistance Of Asphalt Mixtures Research Articles

Interfacial adhesion properties and debonding mechanisms in rejuvenated asphalt mixtures

The interfacial strength between asphalt and aggregate significantly affects the macroscopic properties of rejuvenated asphalt mixtures. Investigating the interfacial properties of rejuvenated asphalt mixtures contributes to understanding the adhesion mechanisms and debonding behavior, enhancing the service life of rejuvenated asphalt mixtures. In this study, molecular dynamics methods were utilized to construct interface models of virgin, aged, and rejuvenated asphalt-aggregate systems, subsequent to which asphalt-aggregate interface pull-out tests were conducted. The findings revealed that the exponential-type cohesive zone model (CZM) effectively characterize the stress-strain correlations during asphalt-aggregate interface pull-out tests. The combination of adhesive and cohesive failure is the typical damage modes at asphalt-aggregate interface. Cohesive failure tended to be the main failure mode between aged asphalt-aggregate interface, while asphalt oxidation aging notably reduced the interface tensile strength and hence the cracking resistance of asphalt mixtures. The interface failure patterns between rejuvenated and virgin asphalt were similar, demonstrating an adhesive failure at lower temperatures and a shift towards cohesion failure at higher temperatures. The interface failure usually occurred at lower temperatures especially for aged asphalt, while the incorporation of rejuvenator could effectively mitigate the cracking potential asphalt mixtures. This study simulated the interfacial failure behavior for rejuvenated asphalt mixtures, which can be a theoretical basis of using rejuvenator in recycled asphalt mixtures.

Evaluating the fracture characterization of asphalt mixtures under freeze-thaw damage based on full-field measurements

Understanding the fracture characterization of asphalt mixtures is important to improve pavement performance in seasonal frozen regions. It is necessary to analyze the cracking characteristics and mechanism of asphalt mixtures under the action of freeze–thaw damage. The semi-circular bending (SCB) crack test combined with digital image correlation (DIC) was conducted to evaluate the fracture characterization of asphalt mixtures under freeze–thaw cycles. The X-ray scanning measurement, bending beam rheology (BBR) test, oblique shear test, the generalized Maxwell model (GM), and the cohesive zone model (CZM) were utilized to reconstruct the 3D heterogeneous finite element (FE) model and reveal the damage mechanism. The results indicated that the freeze–thaw cycles negatively affected the cracking resistance of asphalt mixtures. After 20 freeze–thaw cycles, the fracture energy, stiffness, flexibility index, stress intensity factor, and crack evolution rate decreased by 22.96%, 19.53%, 63.93%, 63.93%, and 61.15%, respectively. The FE model was able to reflect the mechanical properties of asphalt mixtures with different numbers of freeze–thaw cycles well. The FE model can well reflect the mechanical properties of asphalt mixtures under different numbers of freeze–thaw cycles. It also reveals the deterioration of asphalt mortar properties and interfacial adhesion under the action of freeze–thaw damage, which leads to the reduction of internal stress transfer efficiency and attenuation of cracking resistance of asphalt mixtures. This study contributed to the damage mechanism evaluation of asphalt mixtures in seasonal frozen regions and may help with pavement design, construction, and maintenance.

Effects of Thermo-Reversible Aging on the Cracking Resistance of Asphalt Mixtures in the Semi-Circular Bend Test

The semi-circular bend (SCB) test has been widely used to evaluate the cracking resistance of asphalt mixtures. However, uncertainty has arisen with respect to the accuracy of the test method in cold regions where thermo-reversible aging of the asphalt binder plays a significant role in determining the cracking resistance of asphalt pavement. This study aimed to optimize the current SCB protocol by including an evaluation of thermo-reversible aging phenomena. Asphalt mixtures were collected from different regions in North America. Two groups of SCB samples were prepared, with one tested at 0°C after 2 h of conditioning at the test temperature, and the other tested at the same temperature after 72 h of conditioning at −20°C. The SCB tests were conducted at 50 mm/min and the force was recorded versus both the vertical load line displacement and horizontal crack mouth opening displacement (CMOD). To quantify the effects of thermo-reversible aging on cracking resistance, indicators including the flexibility index (FI), fracture energy (FE), cracking resistance index (CRI), balanced cracking index (BCI), and cracking initiation index (CII) were calculated. The results show that the FI and BCI are greatly affected by cold conditioning, decreasing by 34.6 % and 22.3 %, respectively. The post-peak slope of the force–CMOD curve deteriorates to varying degrees, which provides guidance for optimizing the current SCB protocol to better rank the low-temperature cracking resistance of asphalt mixtures. In contrast, the CII is affected to only a minor degree by thermo-reversible aging and is also a preferred indicator from a repeatability perspective. Important variations in stress relaxation ability (e.g., m-value, phase angle), and how these respond to cold conditioning, are missed in rapid SCB mixture tests and therefore best determined on recovered asphalt binder.

Research on Fracture Behavior of Fiber-Asphalt Mixtures Using Digital Image Correlation Technology.

Many researchers use fiber to improve the cracking resistance of asphalt mixtures, but research concerning the effects of fiber on fracture behavior is limited. The fracture behavior of asphalt mixtures with various fiber types (basalt fiber, glass fiber, and polyester fiber) and contents (0.1%, 0.2%, 0.3%, 0.4%, and 0.5%) has been studied using the indirect tensile asphalt cracking test (IDEAL-CT) in conjunction with digital image correlation (DIC) technology. The evaluation indexes used in the test included crack initiation energy (Gif), crack energy (Gf), splitting tensile strength (RT), cracking tolerance index (CTindex), and the real-time tensile strain (Exx) obtained using digital image correlation technology. The results showed that despite the fiber type, the increase of fiber content resulted in first, an increase, and then, a decrease of the cracking resistance of asphalt mixtures, indicating the presence of optimum fiber content-specifically, 0.4%, 0.3%, and 0.3% for basalt fiber, glass fiber, and polyester fiber, respectively. The development of real-time tensile strain, obtained based on digital image correlation technology, could be divided into two stages: slow-growth stage and rapid-expansion stage. In addition, asphalt mixture with basalt fiber presented the best cracking resistance at both the slow-growth and rapid-expansion stages. This research is helpful in understanding the effects of fiber type and content on the fracture behavior of asphalt mixtures and has certain reference significance for the application of fiber in asphalt mixtures.

An Evaluation of the Cracking Resistance of Steel- and Glass-Fiber-Reinforced Asphalt Mixtures Produced at Different Temperatures

This study focuses on the effects of the production temperatures, warm mix asphalt (WMA) additive, and fiber content on the cracking resistance of steel- and glass-fiber-reinforced asphalt mixtures. By using three different approaches, which included different mixing and compaction temperatures, along with the incorporation of a WMA additive, the samples were produced utilizing the Marshall mix design method. The low-temperature cracking resistance and bottom-up fatigue cracking resistance of the asphalt mixture samples were assessed through indirect tensile (IDT) tests performed at two different test temperatures: −10 °C and 20 °C, respectively. According to the fracture work density values, glass fibers significantly improve the low-temperature cracking performance of asphalt mixtures. Furthermore, it was found that the low-temperature cracking resistance of the hot mix asphalt (HMA) mixtures containing fibers was similar to that of the mixtures prepared using the WMA additive at 15 °C lower mixing and compaction temperatures than the HMA mixtures. To conclude, the WMA additive improved the compactability of the steel- and glass-fiber-reinforced asphalt mixtures without compromising the low temperature cracking performance, despite the low mixing and compaction temperatures.

Characterization on the mesostructural distribution of asphalt mixtures via digital image processing and relation to macroscopic performances

The distribution characteristics of the mesostructure of asphalt mixtures have an important influence on the macroscopic performance of asphalt mixtures. In this study, by means of industrial computed tomography and voxel analysis, the distribution of voids and skeleton microstructure of asphalt mixture is characterized from multiple perspectives. The relationship between mesostructural characteristic parameters and macroscopic properties of asphalt mixtures was established by the indirect tensile test, uniaxial compression test, and digital image correlation test. The results show that the mesostructural distribution of the voids of different types of asphalt mixtures shows a certain degree of transverse isotropy. Air voids content and fractal dimension of voids are the mesostructural characteristic parameters that affect the cracking resistance of asphalt mixtures at low temperatures. The air voids content has a negative exponential relationship with the strength of asphalt mixture. When the void fractal dimension is 1.3, the splitting tensile strength of asphalt mixture reaches the maximum value. Skeleton structure is a mesostructure characteristic parameter that affects the high temperature stability of asphalt mixture. The lower voids in coarse aggregate skeletons and degree of anisotropy are positively correlated with the uniaxial compressive strength. After voids in coarse aggregate skeletons are lower than 35 %, the uniaxial compressive strength of asphalt mixture is significantly improved. The proposed skeleton evaluation index can effectively characterize the material composition differences of different types of asphalt mixtures. The strain cloud diagram of the digital correlation experiment shows the place where the void distribution first reached the maximum strain of about 0.003 during loading, and the high strain area corresponds to the void centroid distribution. The air voids centroid distribution is the mesostructural characteristic parameter of the indirect tensile strain distribution.

Investigating the cracking performance of asphalt mixture containing RAP using different cracking indices

Due to environmental benefits and economic issues, the use of reclaimed asphalt pavement (RAP) in asphalt mixtures has become a necessity. However, mixtures containing RAP have poor performance against cracking at different temperatures due to excessive stiffness and aged binder. Therefore, evaluating the cracking performance of RAP is of great importance. In this research, the semicircular bending (SCB) test was used to evaluate fracture characteristics. For this purpose, the indicators of peak load (Pmax), fracture energy until failure (FEUF), fracture energy after failure (FEAF), fracture strain tolerance (FST), and power (P) are used. The SCB test was performed at 25 °C and − 12 °C, and all indicators were calculated at intermediate and low temperatures. Three different percentages of RAP (0%, 50%, and 100%) were used. Also, three different rejuvenators, Waste Engine Oil (WEO), Oleic Acid (OA), and Vacuum Bottom (VB), were used with four different doses. Then recycled high-density polyethylene (r-HDPE) in four different amounts (0%, 3%, 5%, and 7%) was used separately and in combination with rejuvenators to prepare the samples. Also, to expand the application of SCB tests worldwide, and due to the availability and simplicity of the Marshall device, non-standard samples with a diameter of 100 mm made by this device were used. Experimental results showed that the use of a rejuvenator alone, as well as in combination with the recycled polymer, could improve the cracking resistance of asphalt mixtures at medium and low temperatures. Results showed that VB performed best at 25 °C and OA at − 12 °C. An increase of 53% was seen in the FST index for the VB rejuvenator and 62% in the OA rejuvenator. Also, VB (at 25 °C) and OA (at −12 °C) improved crack propagation resistance by 62% and 94%, respectively. To evaluate the results, statistical analyzes were used. The statistical results showed that using the Marshall compaction device to make the samples has a suitable accuracy. In addition, the statistical results showed that the fracture behavior of mixtures containing RAP at a temperature of 25 °C differs from a temperature of − 12 °C.

Experimental Study on the Cracking Resistance of Asphalt Mixture with Different Degrees of Aging

The cracking resistance of asphalt mixture is a non-negligible issue. However, the cracking resistance evolution law, motivated by two factors (thermos-oxidative aging degree and test temperature), is not yet well understood. The aim of this investigation is to gain more insight into the effect of thermos-oxidative aging and test temperature on the cracking resistance of asphalt mixture. Asphalt mixture (AC-13) and stone mastic asphalt mixture (SMA-13) were selected and exposed to different thermo-oxidative aging degrees (unaging (UA); short-term thermo-oxidative aging (STOA); long-term thermo-oxidative aging for 2/5/8 days (LTOA2d/LTOA5d/LTOA8d)). A direct tension test at different test temperatures (10 °C, 20 °C, 30 °C, 40 °C) was adopted to obtain their stress–strain curves and evaluation indexes (tensile strength, ultimate strain, pre-peak strain energy density, and post-peak strain energy density). The comprehensive index-cracking resistance index (CRI) was established by the entropy weight method combined with the technique to order preference by similarity to ideal solution (TOPSIS) method and the corresponding aging coefficient was determined. The results showed that STOA can increase the aging coefficient of asphalt mixture, thereby boosting the cracking resistance. Additionally, the effect can be weakened by elevations in the test temperature. Meanwhile, LTOA can decrease the aging coefficient and thereby weaken the cracking resistance. This effect becomes more prominent with elevations in the test temperature. SMA-13 possesses a superior cracking resistance to AC-13, with a gap in CRI value of 3–69%, regardless of the aging degree and test temperature. A good relationship exists between the aging coefficient and the two factors (aging degree and test temperature).

Another look at the semi-circular bend test for the performance ranking of hot mix asphalt

Semi-circular bend (SCB) testing has been widely used to evaluate the cracking resistance of asphalt mixtures. However, it remains unresolved which of the various SCB protocols can provide the most sensitive and precise ranking of different asphalt mixtures. This study aims to improve the current SCB protocol, emphasizing mixture preparation, testing condition and evaluation indices. The effect of aging methods in terms of reversible aging and irreversible aging and loading rates on the intermediate and low temperature cracking resistance of asphalt mixtures was analyzed. Subsequently, the response of different cracking evaluation indices was analyzed. Finally, the repeatability of different cracking evaluation indices was discussed. The results show that higher loading rates are slightly more sensitive to mix composition, possibly due to the toughening effect of polymer modifiers. However, whether higher or lower rates are more accurate remains to be investigated with ongoing field studies. Cracking initiation and resistance indices (CII and CRI) were superior from a repeatability and dynamic range perspective in comparison to flexibility and balanced cracking indices (FI and BCI). Thermoreversible aging over three days of cold conditioning at − 20 °C was found to significantly affect binder properties in selected samples. However, none of the SCB parameters after testing appropriately cold conditioned samples at room temperature reflected this deterioration. Hence, it remains to be investigated whether mixture testing at ambient temperatures properly ranks superior quality Alberta, Canada and Venezuelan sourced materials that are low in wax.

Use of Random Forest to Predict Intermediate Temperature SCB Jc Parameter of Long-Term Aged Asphalt Mixtures

With the growing use of reclaimed asphalt pavement (RAP), recycled asphalt shingles (RAS), and other innovative additives in asphalt mixtures, there are concerns about the accuracy of Superpave volumetric-based mixture design. Performance tests in mix design procedures are needed to ensure desirable pavement performance. The Louisiana Department of Transportation and Development (LADOTD) has adopted the semi-circular bend (SCB) critical strain energy release rate parameter, Jc, to evaluate the cracking resistance of asphalt mixtures. To address practical shortcomings, this study aims to develop a predictive model to estimate SCB Jc at intermediate temperature of long-term aged (LTA) asphalt specimens from volumetric properties of plant-produced asphalt mixtures, and rheological and chemical properties of unaged asphalt binder by using a machine-learning model, random forest. Asphalt mixture SCB test and asphalt binder rheological and chemical properties tests were conducted. Stepwise correlation analysis was used to determine the most influential variables to SCB Jc. Random forest model was optimized using grid search with hyperparameter combinations. Results show that the developed random forest model was able to predict the SCB Jc fracture parameter of asphalt mixtures and a good agreement was observed between predicted and measured SCB Jc values. Variable importance scores based on mean decrease in impurity (MDI) showed that the Δ Tc had the most influence on SCB Jc prediction accuracy, and other inputs such as coarse aggregate type, LTA days, Fourier transform infrared spectroscopy test carbonyl index, and linear amplitude sweep test ALas also had significant effects on SCB Jc.

Summarizing the Effect of Mixture Variables and Test Parameters on Fatigue Performance of Asphalt Mixtures

Abstract Fatigue cracking performance of asphalt mixtures is significantly affected by mixture variables and environmental and loading conditions in the field. Thus, there is a need to understand the effect of critical factors on the fatigue cracking resistance of asphalt mixtures through an extensive study. For this purpose, the present research is focused on characterizing fatigue cracking resistance of asphalt mixtures by considering different factors such as aggregate gradations (two dense and two gap gradations), binder types (two unmodified and three modified binder types), binder content (optimum binder content and optimum binder content ±0.5 %), test temperatures (15°C, 25°C, and 35°C), and three stress levels for different asphalt layer thicknesses (40, 80, 120, 160, 200, and 240 mm). An indirect tensile fatigue test was performed to characterize fatigue cracking resistance of asphalt mixtures, and a Kaplan–Meier survival analysis was carried out to examine whether the factors that were considered significantly affected the fatigue performance. Pavement sections with varying thicknesses of asphalt layers were analyzed using IITPAVE, a pavement analysis software, to compare the mixtures’ fatigue performance. All of the variables taken into consideration in this study were shown to have a statistically significant effect (p value < 0.05). However, the binder type had a stronger influence than the other variables on the resistance to fatigue cracking. Finally, a fatigue life equation (adjusted R2 = 0.77) was developed considering all the mixture combinations, which can be used for determining the laboratory fatigue life of asphalt mixtures.

Field Validation on the Use of Poker-Chip Test to Predict Cracking in Flexible Pavements

Cracking in flexible pavements is one of the most prevalent forms of distress. Two types of cracking ubiquitous to most regions are fatigue or alligator cracking, and thermal or low-temperature cracking. Cracking characteristics of a flexible pavement depend on a combination of factors that include the structural design of the pavement, asphalt mixture properties, component material properties, and construction quality. The cracking resistance of asphalt mixtures used in flexible pavements may be improved by use of asphalt binders with properties that are suitable to the application. Asphalt binder relies on its stiffness, tensile strength, ductility, and ability to relax to resist cracking due to repeated loading or changes in temperature. In current performance grade specifications of asphalt binders, cracking resistance is primarily characterized by asphalt rheology. Over the years, several researchers have considered this to be a gap. Several studies have been conducted to close this gap by considering various properties including strength and ductility. This study focuses on ductility of asphalt binders as measured by the poker-chip test to resist cracking. Field performance data measured on several pavement sections in Texas were compared with ductility of asphalt binders recovered from field cores. This study finds the poker-chip test to be a promising means of measuring ductility as well as characterizing ability of asphalt binders to resist fatigue and thermal cracking.

Performance characterization of asphalt mixtures under different aging levels: A fracture-based method

This paper proposes a straightforward approach to characterize the fracture properties and fatigue life of asphalt mixtures over the service time. Since the serviceability of flexible pavements varies as they age, assessing the performance of asphalt mixes under aged conditions is essential. To investigate the sensitivity of fracture-related parameters to aging, accelerated laboratory aging, which is conditioning asphalt concrete samples in a force draft oven for different aging times (0, 3, 5, 7, and 9 days), was used. The Semi-Circular Bending (SCB) test at 25 °C was carried out on neat and activated carbon-modified samples to examine the fracture responses of asphalt mixes. First, force–displacement curves at failure were plotted. Then, fracture-related parameters (e.g., peak load, fracture toughness, strain energy, and J-integral) and damage levels corresponding to each fracture criteria were measured versus aging levels. In addition, the fatigue life ratio was determined to measure the influence of aging on the long-term performance of asphalt mixtures. To incorporate the effect of aging on the overall cracking resistance of asphalt mixtures, different fracture-related parameters versus service time were studied. Experimental findings suggest that the area under the fracture property versus service time can provide a reliable index for evaluating the long-term performance of asphalt mixtures. In addition, statistical analysis was performed to assess the impact of aging on fracture-related parameters obtained from the SCB fracture test. The proposed method was able to simultaneously factor in the fracture resistance and the rate of damage development during flexible pavement service life.

Using Artificial Neural Networks to Predict the Cracking Resistance Change Due to Asphalt Binder Content Variation

This paper presents the results of a comprehensive laboratory testing program that was conducted to evaluate the effect of variations in various asphalt mixture properties on changes in the cracking resistance of asphalt mixtures. To this end, a testing factorial consisting of 13 mixes and three asphalt binder content protocols, that is, optimum asphalt binder content (OAC), OAC+0.3%, and OAC−0.3%, were considered. Indirect tension asphalt cracking tests (IDEAL-CT) was conducted on the considered mixtures to examine the interaction effects between mixture characteristics and variations in the binder content on changes in cracking resistance as measured by cracking tolerance index (CTI). Statistical analyses were performed to evaluate the significance of the interaction effects with respect to asphalt mixture type. Regression and artificial neural network models were then developed to predict changes in CTI based on asphalt binder content variation. The results suggested that changes in CTI due to the variation in binder content were significantly influenced by reclaimed asphalt pavement (RAP) content, mixture type, and the type of aggregate used in the production of the asphalt mixture. A good correlation was obtained between mixture characteristics and changes in the CTI values of the asphalt mixtures. The correlation results were further enhanced with the use of artificial neural networks (ANNs).

A phase-field cohesive zone model for fracture simulation of asphalt mixture

The traditional cohesive zone model (CZM) based on interfacial damage mechanics has been widely used in quasi-brittle fracture simulation of asphalt mixture at low temperature. However, the traditional CZM is mesh dependent in the cohesive fracture simulation. This study aims to introduce a phase field method (PFM) to simulate brittle failure of asphalt mixture. The model was implemented in a framework of finite element method (FEM). The uniaxial tensile test was used to simulate the sensitivity of parameters that may affect the results. Furthermore, typical laboratory tests for evaluating the crack resistance of asphalt mixtures, such as the single-edge notched beam (SEB) test and the semi-circular bending (SCB) test, were simulated to validate the suitability of the PFM. Moreover, the numerical results and computational efficiency of the PFM-based cohesive fracture simulation within FAM was also discussed and compared with those based on CZM. The results indicate that the numerical results of the typical failure tests based on PFM are in good agreement with the experimental results at both macro and micro scales. In addition, the numerical results of cohesive fracture simulation within FAM based on PFM and CZM are slightly different. The brittle behavior of the asphalt mixture predicted based on PFM was more pronounced compared to the results predicted by CZM. Moreover, the cohesive fracture simulation based on PFM requires less computational memory and time. In general, this paper successfully introduced a PFM to study the quasi-brittle failure of asphalt mixture.

Evaluation of the Factors Affecting the Performance of Fiber-Reinforced Asphalt Mixtures

This paper summarizes the results of the first comprehensive laboratory study that was conducted to evaluate the effect of using different aramid fiber properties (types, lengths, and dosages) with different combinations on the performance of dense-graded asphalt mixtures reinforced with aramid fibers. Asphalt mixtures with two different aramid fibers (Fiber A and Fiber B) were prepared. For each type of fiber, two lengths and four doses were considered. The semicircular bend (SCB) test, indirect tensile asphalt cracking (IDEAL-CT) test, and the overlay tester (OT) were conducted to evaluate fracture resistance and reflection cracking resistance of prepared mixtures, along with the asphalt concrete cracking device (ACCD) test for evaluating the low-temperature cracking resistance, the Hamburg wheel-tracking (HWT) test for evaluation of rutting susceptibility. Cost analysis was also done to evaluate the feasibility of adding aramid fibers to the asphalt mixtures. Results of the conducted tests indicated that the effects of adding aramid fibers to asphalt mixtures vary depending on the type of aramid fiber and its properties. In general, the addition of the Type A and Type B aramid fibers to the unmodified performance grade (PG) 64-22 mixtures resulted increasing their fatigue lives by 104% and 65%, respectively. Adding Type B fiber [short aramid/polyolefin fiber blend with 12.5 mm (0.75-in.) length] or long Type A fiber (wax-treated aramid fiber with a length of 1.5 in.) to the PG 64-22 mixtures resulted in improving the cracking resistance of reinforced mixtures. In addition, doubling the suppliers’ recommended dosage of each fiber type resulted in significantly lower cracking resistance values. The fibers did not improve the resistance to low-temperature cracking for asphalt mixtures. However, the rutting and moisture susceptibility results for fiber-reinforced mixtures were satisfactory compared with the unreinforced mixtures. Results of cost analysis indicated that the addition of aramid fibers to the mixtures resulted in a significant reduction of construction cost for these mixtures to the values between 27% and 37%. The optimum formulation of both aramid fibers for the purpose of improving cracking resistance of asphalt mixtures was found as 1.5-in. length at the supplier recommended dosage for Type A aramid fiber, and 0.75-in. length and the supplier recommended dosage for Type B aramid fiber.

Study on the cracking behavior of asphalt mixture by discrete element modeling with real aggregate morphology

This research aimed to investigate the crack resistance of asphalt mixtures via laboratory tests and discrete element modelling. For the construction of virtual cracking model, the interactive 2D modeling method based on various software and the establishment method of 3D aggregate are proposed. The description method of crack path is also improved.When establishing the 2D model, considering the mesostructural characteristics of an AC-20 asphalt mixture, the aggregate-asphalt mortar-void three-phase structure was established based on PFC2D software and image processing. Taking into account the gradation and form of coarse aggregate, a 3D model incorporating real aggregate shapes was constructed by PFC3D. Based on the numerical model, semi-circular bending and tensile tests of specimens with pre-cut slits of different lengths were conducted to analyse their stress levels, cracking process and crack locations under low-temperature conditions. Taking the fracture performance parameters as an evaluation index, the feasibility of the numerical simulations was verified in comparison to laboratory test data. In the early phase of model loading, the stress distribution has the characteristics of a compression-tension zone. Crack formation undergoes a process of smooth expansion and rapid penetration, and cracks in the compression zone pass through the aggregate. The failure state presented by the particle flow program was in general agreement with the crack evolution results obtained from the tests. Additionally, the fracture parameters of the simulations had a favourable overlap with the experimental observations, indicating that the discrete element simulations can adequately represent the crack propagation process in asphalt mixes. This approach is effective for in-depth and efficient exploration of the cracking performance of asphalt mixtures.

Influence of fiber-asphalt interface property on crack resistance of asphalt mixture

This paper aims to analyze the interface property between fibers and asphalt, and study its influence on the crack resistance of the asphalt mixture. Four fibers (basalt fiber A (BF-A), basalt fiber B (BF-B), polyester fiber (PEF) and polyacrylonitrile fiber (PAF)) were selected in the study. The interface adhesion parameters were obtained by conducting the novel fiber-asphalt pullout test, and the micromorphology of the fiber-asphalt interface area was also observed. The crack resistance of the mixture was determined using the indirect tensile asphalt cracking test (IDEAL-CT) and the semi-circular bending test (SCB). Basic crack resistance indicators such as CTIndex and flexibility index (FI) were used for analyzing. Meanwhile, digital image correlation (DIC) technology was adopted to quantitatively characterize the crack inhibition mechanism of fibers in the fiber-asphalt mixtures. The results showed that the interface adhesion strength between BF-A and asphalt is the largest at 25 ℃, followed by BF-B, PEF and PAF. The addition of fibers can effectively improve the CTIndex and FI of asphalt mixtures, and it is found that the interface adhesion characteristics between fibers and asphalt have a great influence on the crack propagation process. The crack opening velocity curve obtained from the DIC analysis can quantitatively characterize the crack development law of the asphalt mixture. The coefficient of determination R2 between the interface adhesion strength and the DIC index (stage I velocity change rate and timing point 1 time) exceeded 0.85. The results can provide scientific references for the design and application of fiber reinforced asphalt mixture.

Analysis of fracture morphology of asphalt concrete under different temperatures

By studying the fracture morphology of splitting failure and bending failure of asphalt mixture under different temperatures, it is confirmed that temperature has an impact on the micro mechanism of asphalt mixture fracture. The fracture at low temperature is relatively flat, the fracture of aggregate and asphalt aggregate interface account for a certain proportion, and the failure of asphalt mixture is mainly brittle fracture; With the increase of temperature, the fracture surface becomes rough, and the fracture of asphalt mortar layer is mainly; The addition of rubber powder improves the viscosity of asphalt and the bonding strength of asphalt mortar, greatly improves the fracture ratio of aggregate and interface area, and improves the crack resistance of asphalt mixture; The fracture morphology of asphalt mixture specimen better reflects its mechanical characteristics.

Investigation of low-temperature cracking resistance of asphalt mixtures by conducting Disc-Shaped Compact Tension (DC(T)) and Semi-Circular Bend (SCB) tests

This research was focused to investigate the low-temperature cracking resistance of five plant produced SP12.5 surface course asphalt mixtures by conducting the Disc-Shaped Compact Tension (DC(T)), Semi-Circular Bend (SCB) and Bending Beam Rheometer (BBR) tests. The DC(T) and SCB testing were conducted at three temperatures, −18, −24 and −30℃, to characterize the cracking resistance and determine the mixture-based low-temperature Performance Grade (PG) of asphalt mixtures by using the suggested minimum threshold values of DC(T) fracture energy. Moreover, BBR testing was conducted on the recovered asphalt binders from the same asphalt mixtures. The analysis of tests results showed that the DC(T) and SCB fracture energies at three temperatures were statistically comparable and characterized the low-temperature cracking resistance similarly. Even though, the low temperature PG of the recovered asphalt binders were within the Superpave specifications, in some cases, they were mismatched with the mixture-based low-temperature PG. Dissimilar to the SCB fracture energies, the DC(T) fracture energies had better relationships with the low-temperature properties of the recovered asphalt binders. Considering the low-temperature PG of the recovered asphalt binders and the mixture-based low-temperature PG, the DC(T) and SCB tests are essentially needed to reliably evaluate the low-temperature cracking resistance of asphalt mixtures. Moreover, the relationships between the DC(T) fracture energies and the recovered asphalt binder properties revealed that there was a better correlation between the DC(T) fracture energies at three testing temperatures and the recovered asphalt binders’ temperatures corresponding to the stiffness value of 300 MPa.