Disk-shaped Compact Tension Research Articles

Performance Evaluation of Asphalt Mixtures with Reclaimed Asphalt Pavement and Recycled Asphalt Shingles in Missouri

This study investigates the performance of eighteen different dense-graded asphalt mixtures paved in Missouri. The sections contain a wide range of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS), and different types of additives. The large number of sections investigated and the associated breadth of asphalt mixtures tested provided a robust data set to evaluate the range, repeatability, and relative values provided by modern mixture performance tests. As cracking is one of the most prevalent distresses in Missouri, performance tests such as the disk-shaped compact tension test (DC[T]) and Illinois flexibility index test (I-FIT) were used to evaluate the cracking potential of the sampled field cores. In addition, the Hamburg wheel tracking test (HWTT) was employed to assess rutting and stripping potential. Asphalt binder replacement (ABR) and binder grade bumping at low temperature were found to be critical factors in low-temperature cracking resistance as assessed by the DC(T) fracture energy test. Six sections were found to perform well in the DC(T) test, likely as a result of binder grade bumping (softer grade selection) or because of low recycling content. However, all of the sections were characterized as having brittle behavior by the I-FIT flexibility index. Service life and ABR were key factors in the I-FIT test. Finally, a performance-space diagram including DC(T) fracture energy and HWTT rut depth was used to identify mixtures with higher usable temperature interval (UTImix), some of which contained significant amounts of recycled material.

New machine learning-based prediction models for fracture energy of asphalt mixtures

This paper presents innovative machine learning methods called gene expression programming (GEP) and hybrid artificial neural network/simulated annealing (ANN/SA) to predict the fracture energy of asphalt mixture specimens. The GEP and ANN/SA models are developed using an experimental database including a number of disk-shaped compact tension (DC(T)) test results for fracture energy. The fracture energy is formulated in terms of various predictor variables such as asphalt binder performance grading (PG), asphalt content, aggregate size, aggregate gradation, reclaimed asphalt pavement (RAP) content, reclaimed asphalt shingles (RAS) content, crumb rubber content, and test temperature. A calculation procedure is presented to interpret the models and transform them into practical design equations. A sensitivity analysis is conducted to evaluate the effect of these predictor variables on the fracture energy. Based on the results, the proposed design equations accurately characterize the fracture energy of asphalt mixtures. The GEP model appears to be more practical than the ANN/SA model because of its better generalization and simpler functional structure. The models are recommended for pre-design purposes or as a means to determine asphalt mixture fracture energy when testing is not feasible.

Effect of Mix Design Variables on Thermal Cracking Performance Parameters of Asphalt Mixtures

To address asphalt pavement thermal cracking, researchers have developed performance-based evaluation tools for asphalt mixtures. A minimum fracture energy obtained from a disc-shaped compact tension test and Black space parameters determined by the stiffness and relaxation properties of asphalt mixtures are two such methods to ensure good thermal cracking resistance. Mix specifiers and producers strive to meet the requirements set by these performance-based criteria by adjusting their mix designs. However, there is a lack of information and consensus on the effect of mix design variables (such as binder grade and mix volumetrics) on thermal cracking performance of mixtures as it relates to fracture energy and Black space location. This study strives to fill this gap by quantifying the effect of: (1) recycled asphalt content, (2) effective binder content, (3) air voids, (4) asphalt film thickness, (5) voids in mineral aggregates, and (6) PG low and high temperature grades on thermal cracking resistance. A large dataset, 90 mixtures from the Minnesota Department of Transportation and 81 mixtures from University of New Hampshire database, was used for the study. The results indicate a strong correlation between binder related properties (binder content, asphalt film thickness, PG spread) and fracture energy. The correlation coefficients obtained from this study for PG spread, effective binder content, and air void can be confidently employed to achieve targeted fracture energy thresholds. The same can be achieved for the Glower-Rowe parameter at 15ºC by employing the correlation coefficients obtained for PG low temperature, virgin asphalt content, and voids in the mineral aggregate.

Evaluation of Viscoelastic and Fracture Properties of Asphalt Mixtures with Long-Term Laboratory Conditioning

Aging affects the properties of asphalt mixtures in different ways; increase of stiffness, decrease of relaxation capability, and the increase of brittleness, resulting in changes in cracking behavior of asphalt mixtures. In this study, ten plant-produced, lab-compacted mixtures with various compositions (recycled materials, binder grades, binder source, and nominal maximum aggregate size) are evaluated at different long-term aging levels (24 hours at 135°C, 5 days at 95°C, and 12 days at 95°C on loose mix and 5 days at 85°C on compacted specimens). The asphalt mixture linear viscoelastic properties (|E*| and δ) and master curve shape parameters measured from complex modulus testing and fracture properties (measured from disc-shaped compact tension and semi-circular bending fracture testing) are compared at different levels of aging. The results indicate that the mixture exposure time to aging is proportional to the dynamic modulus and phase angle changes. Generally, the fracture parameters of mixtures become worse when aging level changes from 5 to 12 days aging. In spite of the similar viscoelastic properties, the mixtures with 24 hours at 135°C and 12 days at 95°C aging do not show similar fracture parameters.

Microwave-healing performance of modified asphalt mixtures with flake graphite and exfoliated graphite nanoplatelet

This paper computationally and experimentally investigated the microwave healing performance of graphite (flake graphite and exfoliated graphite nanoplatelet (xGNP)) modified asphalt mixture. The original fracture energy and strength were first measured for the modified asphalt mixture through the disk-shaped compact tension test. Then the micro-wave healing performance (recovered fracture energy and strength) were further examined. All these were enhanced with the added carbon materials. The recovered fracture strengths were also compared with FE cohesive zone model (CZM) simulation with digital image correlation (DIC) calibrated parameters. The predicted recovered fracture strength had good agreement with the experimental measurement.

Correlating laboratory and full-scale reflective cracking tests for airfield pavements

Asphalt overlays minimize moisture infiltration and help in restoring the smoothness and structure of existing airfield concrete pavements, but due to reflective cracking, the new asphalt overlay often fails before achieving its design life. In airfields, the potential for reflective cracking presents a major challenge for rigid pavement rehabilitation involving asphalt overlays. To assist airport engineers and others concerned with the temperature-induced reflection cracks, several full-scale test pavements have been constructed, instrumented, and tested at the Federal Aviation Administration (FAA) National Airport Pavement Test Facility (NAPTF). The main objective of this study was to identify and develop correlations between full-scale and laboratory reflective cracking tests. A customized Overlay Tester (COT) and Cyclic Disc-Shaped Compact Tension (CDCT) Test were used evaluate the fracture and fatigue performance of hot mix asphalt (HMA) materials at low temperatures. The COT was conducted at the same conditions as the full-scale tests and test results were analyzed using several parameters. Temperature effect was pronounced in strain parameters (initial strain, failure strain, and strain at Nf(NLC)). Further fatigue parameters (Nf(crack) and Nf(NLC)) indicated a swifter deterioration of HMA mixture at a higher displacement (cooling) rate. Fracture parameter (Gfini) was found to decrease with the increase of displacement rate due to the brittleness of HMA at low temperatures as excepted. A two-term exponential function from the CDCT tests demonstrated the fracture behavior of HMA under cyclic loading. The released energy rate factor (R2) correlated well with the fatigue life. Finally, a set of shift factors were derived between the full-scale, OT, and CDCT tests. Valuable data obtained from this study provides immediate support to the future FAA reflective cracking research.

Challenges in using the Disc-Shaped Compact Tension (DCT) test to determine role of asphalt mix design variables in cracking resistance at low temperatures

ABSTRACTThere have been many fracture tests – the Thermal Stress Restrained Specimen Test, the Single-Edge Notched Beam test, the Semi-Circular bend test, the Indirect Tensile Test and the Disc-Shaped Compact Tension (DCT) test – developed to understand thermal cracking in asphalt pavement. Among these tests, the DCT test is the most recent test method developed that has gained significant interest. This paper includes a laboratory study to measure the effect of different mixture design parameters on the DCT test results. The parameters include per cent binder replacement from recycled asphalt pavement, binder modification, low-temperature binder grade, oxidative ageing and mix design traffic level. To investigate the significance on the factors controlled, ANOVA and multi-linear regression analyses are used to show that only a few factors can be considered significant in terms of their effects on DCT parameters, and the significance of those factors could not explain the range in DCT response variables. Some of the trends in change in the DCT test responses with mixture ageing and some other factors are also found to be illogical. This paper does not offer solutions, but highlights some of the challenges experienced when applying the DCT test to performance specifications.

Effect of specimen thickness on the fracture resistance of hot mix asphalt in the disk-shaped compact tension (DCT) configuration

In this study, the effect of specimen thickness on the fracture resistance of a dense-graded hot mix asphalt (HMA) is investigated. The fracture toughness, Kc and fracture energy, Gf in triplicate DCT specimens with width-to-thickness ratios ranging from 1.46 to 4.4 is measured at 27 °C. The coefficient of variation (COV) of Kc and Gf is calculated to determine reliability as a function of thickness. Photos of cracked specimens and 3D surface scans are employed to study how the crack path and fracture surface changed with thickness. The ASTM specifications and linear-elastic fracture mechanics theory are applied to show that Kc does not reach a plane-strain condition, Kc≠KIc, for the given ratios. The average Kc increases with thickness while the COV is inconsistent with thickness. The average Gf is independent of thickness but the COV decreases with thickness; thus, Gf is thickness-dependent. The crack path and fracture surface cannot be used to identify the plane-stress or plane-strain condition due to large aggregates dominating the fracture process. A method for estimating the specimen thickness required for the plane-strain condition and a method to estimate the thickness at which fracture toughness is maximized is demonstrated.

Evaluation of the Low Temperature Properties of Asphalt Mixtures Using a Digital Image Correlation Approach

Abstract Asphalt concrete performance hinges on its cracking resistance. Numerous test methods have been used to consider fracture of asphalt concrete at intermediate and low temperatures. All focus on global mixture measurements of load and displacement at a discrete location on the test specimen. Digital image correlation (DIC) measurements provide a means to evaluate the full-field response of asphalt concrete undergoing fracture. In particular, several studies have employed DIC to quantify the size of strain localization as a potential definition of the fracture process zone (FPZ). The current study evaluated the low temperature fracture properties of asphalt concrete using the disk-shaped compact tension (DC(T)) test (ASTM D7313-13) and in-plane DIC-measured FPZ size. Tests were completed to compare the sizes of the FPZ with respect to the test specimen thickness, aging of neat and polymer-modified asphalt mixtures, maximum aggregate size, and reclaimed asphalt pavement (RAP) content. Results in the current study found that DIC measurements differentiated the size of the FPZ in terms of polymer modification, aging, and RAP content.

Application of laboratory asphalt cracking tests to cold in-place recycled mixtures

Cold in-place recycling (CIR) is an asphalt pavement rehabilitation technology which is rapidly gaining popularity. This technique consists in milling the top 75–100 mm of an existing asphalt pavement, mixing the crushed material with a recycling agent, and reusing it to repave the roadway. Hence, there is a need to identify laboratory tests for developing CIR mixtures with improved resistance to cracking. This study investigates the applicability of semi-circular bend (SCB) and disk-shaped compact tension (DCT) tests to CIR mixtures. Tests were performed on five similar laboratory and field prepared mixtures. Since the CIR mixtures were relatively softer and more brittle than typical hot mix asphalt, minor modifications to the testing parameters and data analysis approach were found necessary. Fracture energies computed from SCB and DCT data resulted in being statistically similar and detected considerable differences among the five mixtures. Overall, both fracture tests showed good repeatability and potential applications in mix design engineering and optimisation of CIR mixtures, quality control and field validation usages.

Comprehensive evaluation of low-temperature fracture indices for asphalt mixtures

For performance-based specifications and design processes, a number of cracking-related index parameters have been proposed for asphalt mixtures in recent years. A number of these parameters have been developed to utilise results from fracture tests. This study conducted a comprehensive evaluation of various fracture index parameters including fracture energy, Illinois flexibility index, stress intensity factor and toughness index. Over 200 tests from 61 distinct test data sets representing 21 asphalt mixtures are included. The focus of this study is on low-temperature cracking and all indices were evaluated using test results from the disk-shaped compact tension fracture tests conducted at low temperatures. The objective of this study was to determine if there is a relationship between various fracture index parameters as well as to determine typical measurement variability associated with each parameter. Comparisons were made between different indices and correlations were determined for the mix rankings provided by individual indices. The results indicate that fracture energy successfully captured the mix rankings and showed a strong correlation with other indices. In order to better capture post-peak softening behaviour of asphalt mixtures, the flexibility and toughness indices have been utilised; however, these parameters were found to have high variability. A new index called fracture strain tolerance has been proposed that was shown to provide the same level of distinction between mixtures as the flexibility and toughness indices while having considerably lower variability. Finally, several areas were identified for future extension of this research.

Comparison of asphalt binder and mixture cracking parameters

Cracking is one of the most prevalent types of distresses in asphalt pavements. There are different cracking index parameters that are determined from tests conducted on binders and mixtures to assess cracking potential. The objective of this study is to compare binder and mixture parameters and evaluate the similarities and differences between the rankings and values obtained. This study includes binder and mixture testing on 14 plant-produced mixtures including 3 different binder grades, 3 binder sources, 3 aggregate gradations, and mixtures containing a range of reclaimed asphalt pavement and/or recycled asphalt shingles contents. Testing included PG grading and 4 mm dynamic shear rheometer testing on the extracted and recovered binders that were long-term aged. Mixture testing included complex modulus, simplified viscoelastic continuum damage (SVECD) fatigue, and disc-shaped compact tension testing on short-term-aged mixtures. Parameters evaluated included high and low PG temperatures, ΔTcr, Glover–Rowe parameter (binder and mix-based), R value, dynamic modulus, phase angle, number of cycles to failure from SVECD and layered viscoelastic critical distresses analysis, and fracture energy. The results show that generally the binder parameters correlate well with each other but the mixture parameters do not. Good correlation was observed between binder and mixture stiffness-based parameters, but there was generally low correlation observed between binder and mixture cracking parameters for the mixtures evaluated in this study, possibly a result of differences in ageing level. Recommended future work includes non-linear statistical analysis, incorporation of field performance, and testing on long-term-aged mixtures.

Fracture toughness of a hot work tool steel-TiC composite produced by mechanical milling and spark plasma sintering

The effect of the processing route and the addition of 20vol% TiC particles on the fracture toughness of the hot work tool steel AISI H13 was investigated. To this end, as-atomized, mechanically milled and mechanically alloyed powders were consolidated by spark plasma sintering. Nearly dense materials were produced. The fracture toughness was measured with modified disk-shaped compact tension specimens. Acoustic emissions and the fracture surfaces were analyzed to study the damage evolution. The consolidation of the as-atomized powder resulted in the highest relative density and fracture toughness. In contrast, relatively low fracture toughness was achieved for the mechanically milled powder which was attributed to the lower relative density after sintering. The lowest fracture toughness was achieved for the steel-TiC composite material that also exhibited the lowest density. Furthermore, crack path deflection was lowest for this material. This was attributed to the TiC particles which served as a weak crack path. Early cleavage fracture due to stress concentration at TiC particles/agglomerations was proved by means of acoustic emission analysis.

Comparison of fracture toughness values of normal and high strength concrete determined by three point bend and modified disk-shaped compact tension specimens

The modified disk shaped compact tension test is a configuration derived from standard compact tension test that is used for measuring fracture mechanical properties of primarily metallic materials. The compact tension configuration is commonly used for measurement fracture mechanical properties as e.g. fracture toughness, Young’s modulus, work of fracture etc. The modified compact tension tests imply significant modifications of the specimen morphology in order to avoid premature failure. The modified compact tension test is not proper for quasi-brittle materials due to its complicated shape (steel-concrete interface), but it is easily extracted from drill core and we do not need large amount of material for obtaining fracture properties as we need for e.g. three- or four- point bend test. Since it is a new test method, a wide range of tests is needed to be done before it can be applied. In the paper the selected outputs of the experiments performed on normal and high strength concrete will be processed and the values of fracture mechanical parameters will be discussed.

Comparison of fracture toughness values of normal and high strength concrete determined by three point bend and modified disk-shaped compact tension specimens

The modified disk shaped compact tension test is a configuration derived from standard compact tension test that is used for measuring fracture mechanical properties of primarily metallic materials. The compact tension configuration is commonly used for measurement fracture mechanical properties as e.g. fracture toughness, Young’s modulus, work of fracture etc. The modified compact tension tests imply significant modifications of the specimen morphology in order to avoid premature failure. The modified compact tension test is not proper for quasi-brittle materials due to its complicated shape (steel-concrete interface), but it is easily extracted from drill core and we do not need large amount of material for obtaining fracture properties as we need for e.g. three- or four- point bend test. Since it is a new test method, a wide range of tests is needed to be done before it can be applied. In the paper the selected outputs of the experiments performed on normal and high strength concrete will be processed and the values of fracture mechanical parameters will be discussed.

Methods and criteria for evaluation of asphalt mixture resistance to low temperature cracking

In cold regions and areas where there is a huge difference between high and low temperatures asphalt pavements are subject to low temperature cracking. The appeared cracks form pavement discontinuities, through which water penetrates into pavement structure. It reduces the bearing capacity of the whole pavement structure, weakens adhesion between bitumen and aggregate, affects bonding between layers and increases the development of frost heaves. A sealing of cracks deals with these issues. However, additional inspections after each winter have to be carried out to identify both cracks that have newly appeared and cracks that need to be resealed. These activities significantly increase road maintenance cost. Selection of the appropriate asphalt mixture by its performance at low temperatures reduces or even prevents low temperature cracking of asphalt pavements. A number of methods such as the Indirect Tensile Test, the Bending Beam Rheometer Test, the Thermal Stress Restrained Specimen Test, Asphalt Thermal Cracking Analyser, the Single-Edge-Notched Beam Test, the Disc-Shaped Compact Tension Test, the Semi-Circular Bend Test, the Fenix Test, Asphalt Concrete Cracking Device and Spectral Analysis of Acoustic Emission are developed to evaluate asphalt mixture resistance to low temperature cracking. This paper presents an analysis of these tests, emphasizes their advantages and disadvantages and gives limiting criteria to evaluate asphalt mixture resistance to low temperature cracking. The test advantages and disadvantages are deciding factors in a test selection. Some tests such as the Thermal Stress Restrained Specimen Test and Spectral Analysis of acoustic emission can directly reveal the lowest temperature at which asphalt mixture can withstand induced thermal stresses.

Improving fatigue and low temperature performance of 100% RAP mixtures using a soybean-derived rejuvenator

One of the major obstacles towards higher mixture percentages of reclaimed asphalt pavements (RAP) is its greater susceptibility to failure under low temperatures and fatigue loading. Rejuvenators offer a very attractive solution by partially or fully restoring the aged properties of the RAP binder. In this research, a soybean-derived rejuvenator is used to modify a PG58-28 binder at 6% and 12% by weight. The soybean-modified PG58-28 and a neat PG58-28 are blended with an extracted RAP binder. Changes in the rheological properties of the different blends are assessed using performance grading (PG), temperature-frequency sweep and linear amplitude sweep (LAS) testing. RAP mixtures made of 100% RAP with the addition of neat PG58-28 or soybean-modified PG58-28 were used to prepare dynamic modulus and disk-shaped compact tension (DCT) specimens. The binder testing results clearly indicate that the soybean-derived rejuvenator has a significant impact on both the low and high temperature properties of the RAP binder. Such improvement was not attainable by using the neat PG58-28 alone. The soybean-derived rejuvenator also shows sustained durability with aging. The LAS results indicate a significant increase in the fatigue life of the soybean rejuvenated RAP binder. Results of dynamic modulus testing do not reveal significant differences between the various mixtures. The fracture energy of the mixtures prepared with the soybean rejuvenator is higher than the control mixtures as revealed by DCT test results.

Characterization of temperature- and rate-dependent fracture properties of fine aggregate bituminous mixtures using an integrated numerical-experimental approach

This paper employs an integrated numerical-experimental approach to evaluate the temperature- and rate-dependent fracture characteristics of four fine aggregate bituminous matrices. Two bending tests (semicircular bending, and single-edge notched beam) and one tension test (disk-shaped compact tension) were performed in the laboratory at three temperatures (−10°C, 10°C, and 25°C) and three loading rates (0.5mm/min, 1.0mm/min, and 2.0mm/min). The fracture tests were further simulated using a computational model based on the finite element method that is incorporated with material viscoelasticity and cohesive zone fracture. Two cohesive zone fracture parameters, i.e., cohesive strength and fracture energy, were determined via a calibration process between experimental and numerical results. The results obtained indicated that temperature- and rate-dependent fracture characteristics are obvious in viscoelastic bituminous mixtures, and an accurate identification of such characteristics is a key step towards the implementation of successful computational microstructure predictive models. This would lead to core insights into the effects of constituents on the overall mixture performance, with significant savings in experimental costs and time.

Modified Disk-Shaped Compact Tension Test for Measuring Concrete Fracture Properties

A new approach for measuring the specific fracture energy of concrete denoted modified disk-shaped compact tension (MDCT) test is presented. The procedure is based on previous ideas regarding the use of compact tension specimens for studying the fracture behavior of concrete but implies significant modifications of the specimen morphology in order to avoid premature failures (such as the breakage of concrete around the pulling load holes). The manufacturing and test performance is improved and simplified, enhancing the reliability of the material characterization. MDCT specimens are particularly suitable when fracture properties of already casted concrete structures are required. To evaluate the applicability of the MDCT test to estimate the size-independent specific fracture energy of concrete (GF), the interaction between the fracture process zone of concrete and the boundary of the MDCT specimens at the end of the test is properly analyzed. Further, the experimental results of GF obtained by MDCT tests for normal- and high-strength self-compacting concrete mixes are compared with those obtained using the well-established three-point bending test. The procedure proposed furnishes promising results, and the GF values obtained are reliable enough for the specimen size range studied in this work.

A J-integral approach using digital image correlation for evaluating stress intensity factors in fatigue cracks with closure effects

Digital image correlation (DIC) techniques are used to obtain the stress intensity factor (SIF) via experimental J-integral evaluations in an AISI 4340 steel disk-shaped compact-tension (DC(T)) specimen subjected to mode I loading conditions. The combined analytical and experimental methodology is based on the computation of the J-integral over an arbitrary but elastic contour path enclosing the crack tip. The DIC displacement field around the crack is used to directly determine the displacement gradients, the stress components, and the total strain energy for points along the selected path employing stress-strain linear elastic relations. The proposed analytical-experimental hybrid approach is then applied to investigate the influence of crack closure on SIF measurements under constant amplitude loading conditions, after and before the application of overloads. The effects of the closure-induced nonlinearities observed in the experimental results are analyzed and discussed.